Nervous system

Human Physiology/The Nervous System 1
Human Physiology/The Nervous System
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Answers
The central nervous system includes the brain and spinal cord. The brain and spinal cord are
protected by bony
structures, membranes, and fluid. The brain is held in the cranial cavity of the skull and it consists of the
cerebrum,
cerebellum, and the brain stem. The nerves involved are cranial nerves and spinal nerves.
Nervous system
Overview of the entire nervous system
The nervous system has three main functions: sensory input,
integration of data and motor output. Sensory input is when the body
gathers information or data, by way of neurons, glia and synapses. The
nervous system is composed of excitable nerve cells (neurons) and
synapses that form between the neurons and connect them to centers
throughout the body or to other neurons. These neurons operate on
excitation or inhibition, and although nerve cells can vary in size and
location, their communication with one another determines their
function. These nerves conduct impulses from sensory receptors to the
brain and spinal cord. The data is then processed by way of integration
of data, which occurs only in the brain. After the brain has processed
the information, impulses are then conducted from the brain and spinal
cord to muscles and glands, which is called motor output. Glia cells are
found within tissues and are not excitable but help with myelination,
ionic regulation and extracellular fluid.
The nervous system is comprised of two major parts, or subdivisions,
the central nervous system (CNS) and the peripheral nervous system
(PNS). The CNS includes the brain and spinal cord. The brain is the
body's "control center". The CNS has various centers located within it
that carry out the sensory, motor and integration of data. These centers can be subdivided to Lower
Centers
(including the spinal cord and brain stem) and Higher centers communicating with the brain via effectors.
The PNS
is a vast network of spinal and cranial nerves that are linked to the brain and the spinal cord. It contains
sensory
receptors which help in processing changes in the internal and external environment. This information is
sent to the
CNS via afferent sensory nerves. The PNS is then subdivided into the autonomic nervous system and the
somatic
nervous system. The autonomic has involuntary control of internal organs, blood vessels, smooth and
cardiac
muscles. The somatic has voluntary control of skin, bones, joints, and skeletal muscle. The two systems
function
together, by way of nerves from the PNS entering and becoming part of the CNS, and vice versa.
General functions of the CNS
Brain, brain stem, and spinal chord.
CNS:
The "Central Nervous System", comprised of brain, brainstem, and
spinal cord.

The central nervous system (CNS) represents the largest part of the
nervous system, including the brain and the spinal cord. Together, with
the peripheral nervous system (PNS), it has a fundamental role in the
control of behavior.
The CNS is conceived as a system devoted to information processing,
where an appropriate motor output is computed as a response to a
sensory input. Many threads of research suggest that motor activity
exists well before the maturation of the sensory systems, and senses
only influence behavior without dictating it. This has brought the
conception of the CNS as an autonomous system.
Structure and function of neurons
Structure
Neurons are highly specialized for the processing and transmission of
cellular signals. Given the diversity of functions performed by neurons in different parts of the nervous
system, there
is, as expected, a wide variety in the shape, size, and electrochemical properties of neurons. For
instance, the soma of
a neuron can vary in size from 4 to 100 micrometers in diameter.
The soma (cell body) is the central part of the neuron. It contains the nucleus of the cell, and therefore is
where most
protein synthesis occurs. The nucleus ranges from 3 to 18 micrometers in diameter. The dendrites of a
neuron are
cellular extensions with many branches, and metaphorically this overall shape and structure is referred to
as a
dendritic tree. This is where the majority of input to the neuron occurs. However, information outflow (i.e.
from
dendrites to other neurons) can also occur (except in chemical synapse in which backflow of impulse is
inhibited by
the fact that axon do not possess chemoreceptors and dendrites cannot secrete neurotransmitter
chemical). This
explains one way conduction of nerve impulse. The axon is a finer, cable-like projection which can extend
tens,
hundreds, or even tens of thousands of times the diameter of the soma in length. The axon carries nerve
signals away
from the soma (and also carry some types of information back to it). Many neurons have only one axon,
but this
axon may - and usually will - undergo extensive branching, enabling communication with many target
cells. The part
of the axon where it emerges from the soma is called the 'axon hillock'. Besides being an anatomical
structure, the
axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium
channels. This
makes it the most easily-excited part of the neuron and the spike initiation zone for the axon: in
neurological terms it
has the greatest hyperpolarized action potential threshold. While the axon and axon hillock are generally
involved in
information outflow, this region can also receive input from other neurons as well. The axon terminal is a
specialized
structure at the end of the axon that is used to release neurotransmitter chemicals and communicate with
target
neurons. Although the canonical view of the neuron attributes dedicated functions to its various
anatomical
components, dendrites and axons often act in ways contrary to their so-called main function.
Axons and dendrites in the central nervous system are typically only about a micrometer thick, while
some in the

peripheral nervous system are much thicker. The soma is usually about 10–25 micrometers in diameter
and often is
not much larger than the cell nucleus it contains. The longest axon of a human motor neuron can be over
a meter
long, reaching from the base of the spine to the toes. Sensory neurons have axons that run from the toes
to the dorsal
columns, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the
entire
length of their necks. Much of what is known about axonal function comes from studying the squids giant
axon, an
ideal experimental preparation because of its relatively immense size (0.5–1 millimeters thick, several
centimeters
long).
Function
Sensory afferent neurons convey information from tissues and organs into the central nervous system.
Efferent
neurons transmit signals from the central nervous system to the effector cells and are sometimes called
motor
neurons. Interneurons connect neurons within specific regions of the central nervous system. Afferent
and efferent
can also refer generally to neurons which, respectively, bring information to or send information from
brain region.
Classification by action on other neurons
Excitatory neurons excite their target postsynaptic neurons or target cells causing it to function. Motor
neurons and
somatic neurons are all excitatory neurons. Excitatory neurons in the brain are often glutamatergic. Spinal
motor
neurons, which synapse on muscle cells, use acetylcholine as their neurotransmitter. Inhibitory neurons
inhibit their
target neurons. Inhibitory neurons are also known as short axon neurons, interneurons or microneurons.
The output
of some brain structures (neostriatum, globus pallidus, cerebellum) are inhibitory. The primary inhibitory
neurotransmitters are GABA and glycine. Modulatory neurons evoke more complex effects termed
neuromodulation. These neurons use such neurotransmitters as dopamine, acetylcholine, serotonin and
others. Each
synapses can receive both excitatory and inhibitory signals and the outcome is determined by the adding
up of
summation.
Excitatory and inhibitory process
Nerve Synapse
The release of a excitatory neurotransmitter
(ACHe) at the synapses will cause an inflow
of positively charged sodium ions (Na+)
making a localized depolarization of the
membrane. The current then flows to the
resting (polarized) segment of the axon.
Inhibitory synapse causes an inflow of Cl-
(chlorine) or outflow of K+ (potassium)
making the synaptic membrane
hyperpolarized. This increase prevents
depolarization, causing a decrease in the
possibility of an axon discharge. If they are
both equal to their charges, then the
operation will cancel itself out. There are

two types of summation: spatial and temporal. Spatial summation requires several excitatory synapses
(firing several
times) to add up,thus causing an axon discharge. It also occurs within inhibitory synapses, where just the
opposite
will occur. In temporal summation, it causes an increase of the frequency at the same synapses until it is
large
enough to cause a discharge. Spatial and temporal summation can occur at the same time as well.
The neurons of the brain release inhibitory neurotransmitters far more than excitatory neurotransmitters,
which helps
explain why we are not aware of all memories and all sensory stimuli simultaneously. The majority of
information
stored in the brain is inhibited most of the time.
Summation
When excitatory synapses exceed the amount of inhibitory synapses there are, then the excitatory
synapses will
prevail over the other. The same goes with inhibitory synapses, if there are more inhibitory synapses than
excitatory,
the synapses will be inhibited. To determine all of this is called summation.
Classification by discharge patterns:
Neurons can be classified according to their electrophysiological characteristics (note that a single action
potential is
not enough to move a large muscle, and instead will cause a twitch).
Tonic or regular spiking: Some neurons are typically constantly (or tonically) active. Example:
interneurons in
neurostriatum.
Phasic or bursting: Neurons that fire in bursts are called phasic.
Fast spiking: Some neurons are notable for their fast firing rates. For example, some types of cortical
inhibitory
interneurons, cells in globus pallidus.
Thin-spike: Action potentials of some neurons are more narrow compared to the others. For example,
interneurons
in prefrontal cortex are thin-spike neurons.
Classification by neurotransmitter released:
Some examples are cholinergic, GABAergic, glutamatergic and dopaminergic neurons.
Central Nervous System
The central nervous system is the control center for the body. It regulates organ function, higher thought,
and
movement of the body. The central nervous system consists of the brain and spinal cord.
Generation & propagation of an action potential
Electrical characteristics of a neurochemical action potential.
The Nerve Impulse
When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure,
electricity,
chemicals, etc. Different neurons are sensitive to different stimuli(although most can register pain). The
stimulus
causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is
called the
"ACTION POTENTIAL." This moving change in polarity has several stages:
Depolarization
The upswing is caused when positively charged sodium ions(Na+) suddenly rush through open sodium
gates
into a nerve cell.The membrane potential of the stimulated cell undergoes a localized change from-65

millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses
its
polarity so that the outside of the membrane is negative relative to the inside. During this change of
polarity
the membrane actually develops a positive value for a moment(+40 millivolts). The change in voltage
stimulates the opening of additional sodium channels (called a voltage-gated ion channel). This is an
example
of a positive feedback loop.
Repolarization
(The downswing) is caused by the closing of sodium ion channels and the opening of potassium ion
channels.
Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open.
Again,
these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to
restore the
localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).
Refractory phase
is a short period of time after the depolarization stage. Shortly after the sodium gates open they close and
go
into an inactive conformation. The sodium gates cannot be opened again until the membrane is
repolarized to
its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and
potassium
ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be
depolarized. This refractory area explains why action potentials can only move forward from the point of
stimulation.
Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. when there is
a deficit
of calcium ions (Ca+2) in the interstitial fluid the sodium channels are activated (opened) by very little
increase of
the membrane potential above the normal resting level. The nerve fiber can therefore fire off action
potentials
spontaneously, resulting in tetany. Could be caused by the lack of hormone from parathyroid glands.
could be caused
by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable.
Speed of
conduction. This area of depolarization/repolarization/recovery moves along a nerve fiber like a very fast
wave. In
myelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the
nodes of
Ranvier (pictured below in 'types of neurons') by jumping from node to node. This is called "saltatory"
conduction.
Damage to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some
poisons and
drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the
end of this
outline.
Brain
A color-coded image of the brain, showing the
main sections.
The brain is found in the cranial cavity. Within it are found the higher
nerve centers responsible for coordinating the sensory and motor
systems of the body (forebrain). The brain stem houses the lower nerve
centers (consisting of midbrain, pons, and medulla),
Medulla

The medulla is the control center for respiratory, cardiovascular and
digestive functions.
Pons
The pons houses the control centers for respiration and inhibitory functions. Here it will interact with the
cerebellum.
Cerebrum
The cerebrum, or top portion of the brain, is divided by a deep crevice, called the longitudinal sulcus. The
longitudinal sulcus separates the cerebrum in to the right and left hemispheres. In the hemispheres you
will find the
cerebral cortex, basal ganglia and the limbic system. The two hemispheres are connected by a bundle of
nerve fibers
called the corpus callosum. The right hemisphere is responsible for the left side of the body while the
opposite is true
of the left hemisphere. Each of the two hemispheres are divided into four separated lobes: the frontal in
control of
specialized motor control, learning, planning and speech; parietal in control of somatic sensory functions;
occipital in
control of vision; and temporal lobes which consists of hearing centers and some speech. Located deep
to the
temporal lobe of the cerebrum is the insula.
Cerebellum
The cerebellum is the part of the brain that is located posterior to the medulla oblongata and pons. It
coordinates
skeletal muscles to produce smooth, graceful motions. The cerebellum receives information from our
eyes, ears,
muscles, and joints about what position our body is currently in (proprioception). It also receives output
from the
cerebral cortex about where these parts should be. After processing this information, the cerebellum
sends motor
impulses from the brainstem to the skeletal muscles. The main function of the cerebellum is coordination.
The
cerebellum is also responsible for balance and posture. It also assists us when we are learning a new
motor skill, such
as playing a sport or musical instrument. Recent research shows that apart from motor functions
cerebellum also has
some emotional role.
The Limbic System and Higher Mental Functions
Image of the brain, showing the Limbic system.
The Limbic System
The Limbic System is a complex set of structures found just beneath
the cerebrum and on both sides of the thalamus. It combines higher
mental functions, and primitive emotion, into one system. It is often
referred to as the emotional nervous system. It is not only responsible
for our emotional lives, but also our higher mental functions, such as
learning and formation of memories. The Limbic system explains why
some things seem so pleasurable to us, such as eating and why some
medical conditions are caused by mental stress, such as high blood
pressure. There are two significant structures within the limbic system
and several smaller structures that are important as well. They are:
1. The Hippocampus
2. The Amygdala
3. The Thalamus
4. The Hypothalamus
5. The Fornix and Parahippocampus

6. The Cingulate Gyrus
Structures of the Limbic System
Hippocampus
The Hippocampus is found deep in the temporal lobe, shaped like a seahorse. It consists of two horns
that
curve back from the amygdala. It is situated in the brain so as to make the prefrontal area aware of our
past
experiences stored in that area. The prefrontal area of the brain consults this structure to use memories
to
modify our behavior. The hippocampus is responsible for memory.
Amygdala
The Amygdala is a little almond shaped structure, deep inside the anteroinferior region of the temporal
lobe,
connects with the hippocampus, the septi nuclei, the prefrontal area and the medial dorsal nucleus of the
thalamus. These connections make it possible for the amygdala to play its important role on the mediation
and
control of such activities and feelings as love, friendship, affection, and expression of mood. The
amygdala is
the center for identification of danger and is fundamental for self preservation. The amygdala is the
nucleus
responsible for fear.
Thalamus
Lesions or stimulation of the medial, dorsal, and anterior nuclei of the thalamus are associated with
changes in
emotional reactivity. However, the importance of these nuclei on the regulation of emotional behavior is
not
due to the thalamus itself, but to the connections of these nuclei with other limbic system structures. The
medial dorsal nucleus makes connections with cortical zones of the prefrontal area and with the
hypothalamus.
The anterior nuclei connect with the mamillary bodies and through them, via fornix, with the hippocampus
and
the cingulated gyrus, thus taking part in what is known as the Papez's circuit.
Image of the brain showing the location of the
hypothalamus.
Hypothalamus
The Hypothalamus is a small part of the brain located just below
the thalamus on both sides of the third ventricle. Lesions of the
hypothalamus interfere with several vegetative functions and
some so called motivated behaviors like sexuality,
combativeness, and hunger. The hypothalamus also plays a role
in emotion. Specifically, the lateral parts seem to be involved
with pleasure and rage, while the medial part is linked to
aversion, displeasure, and a tendency to uncontrollable and loud
laughing. However, in general the hypothalamus has more to do
with the expression of emotions. When the physical symptoms
of emotion appear, the threat they pose returns, via the
hypothalamus, to the limbic centers and then the prefrontal
nuclei, increasing anxiety.
The Fornix and Parahippocampal
These small structures are important connecting pathways for the limbic system.
The Cingulate Gyrus
The Cingulate Gyrus is located in the medial side of the brain between the cingulated sulcus and the
corpus
callosum. There is still much to be learned about this gyrus, but it is already known that its frontal part

coordinates smells and sights, with pleasant memories of previous emotions. The region participates in
the
emotional reaction to pain and in the regulation of aggressive behavior.
Memory and Learning
Memory is defined as : The mental faculty of retaining and recalling past experiences, the act or instance
of
remembering recollection. Learning takes place when we retain and utilize past memories.
Overall, the mechanisms of memory are not completely understood. Brain areas such as the
hippocampus, the
amygdala, the striatum, or the mammillary bodies are thought to be involved in specific types of memory.
For
example, the hippocampus is believed to be involved in spatial learning and declarative learning (learning
information such as what you're reading now), while the amygdala is thought to be involved in emotional
memory.
Damage to certain areas in patients and animal models and subsequent memory deficits is a primary
source of
information. However, rather than implicating a specific area, it could be that damage to adjacent areas,
or to a
pathway traveling through the area is actually responsible for the observed deficit. Further, it is not
sufficient to
describe memory, and its counterpart, learning, as solely dependent on specific brain regions. Learning
and memory
are attributed to changes in neuronal synapses, thought to be mediated by long-term potentiation and
long-term
depression.
There are three basic types of memory:
1. Sensory Memory
2. Short Term Memory
3. Long Term Memory
Sensory Memory
The sensory memories act as a buffer for stimuli through senses. A sensory memory retains an exact
copy of
what is seen or heard: iconic memory for visual, echoic memory for aural and haptic memory for touch.
Information is passed from sensory memory into short term memory. Some believe it lasts only 300
milliseconds, it has unlimited capacity. Selective attention determines what information moves from
sensory
memory to short term memory.
Short Term Memory
Short Term Memory acts as a scratch pad for temporary recall of the information under process. For
instance,
in order to understand this sentence you need to hold in your mind the beginning of the sentence as you
read
the rest. Short term memory decays rapidly and also has a limited capacity. Chunking of information can
lead
to an increase in the short term memory capacity, this is the reason why a hyphenated phone number is
easier
to remember than a single long number. The successful formation of a chunk is known as closure.
Interference
often causes disturbance in short term memory retention. This accounts for the desire to complete a task
held
in short term memory as soon as possible.
Within short term memory there are three basic operations:
1. Iconic memory - the ability to hold visual images
2. Acoustic memory - the ability to hold sounds. Can be held longer than iconic.

3. Working memory - an active process to keep it until it is put to use. Note that the goal is not really to
move the
information from short term memory to long term memory, but merely to put it to immediate use.
The process of transferring information from short term to long term memory involves the encoding or
consolidation
of information. This is not a function of time, that is, the longer the memory stays in the short term the
more likely it
is to be placed in the long term memory. On organizing complex information in short term before it can be
encoded
into the long term memory, in this process the meaningfulness or emotional content of an item may play a
greater
role in its retention in the long term memory. The limbic system sets up local reverberating circuits such
as the
Papez's Circuit.
Long Term Memory
Long Term Memory is used for storage of information over a long time. Information from short to long
term
memory is transferred after a short period. Unlike short term memory, long term memory has little decay.
Long term potential is an enhanced response at the synapse within the hippocampus. It is essential to
memory
storage. The limbic system isn't directly involved in long term memory necessarily but it selects them from
short term memory, consolidates these memories by playing them like a continuous tape, and involves
the
hippocampus and amygdala.
There are two types of long term memory:
1. Episodic Memory
2. Semantic Memory
Episodic memory represents our memory of events and experiences in a serial form. It is from this
memory that we
can reconstruct the actual events that took place at a given point in our lives. Semantic memory, on the
other hand, is
a structured record of facts, concepts, and skills that we have acquired. The information in the semantic
memory is
derived from our own episode memory, such as that we can learn new facts or concepts from
experiences.
There are three main activities that are related to long term memory:
1. Storage
2. Deletion
3. Retrieval
Information for short term memory is stored in long term memory by rehearsal. The repeated exposure to
a stimulus
or the rehearsal of a piece of information transfers it into long term memory. Experiments also suggest
that learning
is most effective if it is distributed over time. Deletion is mainly caused by decay and interference.
Emotional factors
also affect long term memory. However, it is debatable whether we actually ever forget anything or
whether it just
sometimes becomes increasingly difficult to retrieve it. Information may not be recalled sometimes but
may be
recognized, or may be recalled only with prompting. This leads us to the third operation of memory,
information
retrieval.
There are two types of information retrieval:
1. Recall
2. Recognition

In recall, the information is reproduced from memory. In recognition the presentation of the information
provides the
knowledge that the information has been seen before. Recognition is of lesser complexity, as the
information is
provided as a cue. However, the recall may be assisted by the provision of retrieval cues which enable
the subject to
quickly access the information in memory.
Long-term Potentiation
. Long-term potentiation (LTP) is the lasting enhancement of connections between two neurons that
results from
stimulating them simutaneously. Since neurons communicate via chemical synapses, and because
memories are
believed to be stored within these synapses, LTP and it's opposing process, long-term depression, are
widely
considered the major cellular mechanisms that underlie learning and memory. This has been proven by
lab
experiments. When one of the chemicals involved (PKMzeta, it will be discussed later) is inhibited in rats,
it causes
retrograde amnesia with short term memory left intact (meaning they can't recall events from before the
inhibitor was
given).
By enhancing synaptic transmission, LTP improves the ability of two neuron, one presynaptic and the
other
postsynaptic, to communicate with one another across a synapse. The precise mechanism for this
enhancement isn't
known, but it varies based on things like brain region, age and species. This will focus on LTP in the CA1
section of
the hippocampus, because that's what is well known.
The end result of LTP is a well established neural circuit that can be called upon later for memory.
LTP in the CA1 hippocampus is called NMDA receptor-dependent LTP. It has four main properties.
• Rapid induction
LTP can be rapidly induced by applying one or more brief, high-frequency, stimulus to a presynaptic cell.
• Input specificity
Once induced, LTP at one synapse does not spread to other synapses; rather LTP is input specific. LTP
is only
propagated to those synapses according to the rules of associativity and cooperativity.
• Associativity
Associativity refers to the observation that when weak stimulation of a single pathway is insufficient for
the
induction of LTP, simultaneous strong stimulation of another pathway will induce LTP at both pathways.
• Cooperativity
LTP can be induced either by strong tetanic stimulation of a single pathway to a synapse, or cooperatively
via
the weaker stimulation of many. When one pathway into a synapse is stimulated weakly, it produces
insufficient postsynaptic depolarization to induce LTP. In contrast, when weak stimuli are applied to many
pathways that converge on a single patch of postsynaptic membrane, the individual postsynaptic
depolarizations generated may collectively depolarize the postsynaptic cell enough to induce LTP
cooperatively. Synaptic tagging, discussed later, may be a common mechanism underlying associativity
and
cooperativity.
LTP is generally divided into three parts that occur sequentially: Short-term potentiation, early LTP (E-
LTP) and late
LTP (L-LTP). Short-term potentiation isn't well understood and will not be discussed.
E-LTP and L-LTP phases of LTP are each characterized by a series of three events: induction,
maintenance and

expression. Induction happens when a short-lived signal triggers that phase to begin. Maintenance
corresponds to the
persistent biochemical changes that occur in response to the induction of that phase. Expression entails
the
long-lasting cellular changes that result from activation of the maintenance signal.
Each phase of LTP has a set of mediator molecules that dictate the events of that phase. These
molecules include
protein receptors, enzymes, and signaling molecules that allow progression from one phase to the next.
In addition to
mediators, there are modulator molecules that interact with mediators to fine tune the LTP. Modulators
are a bit
beyond the scope of this introductory book, and won't be discussed here.
Early Phase
Induction
E-LTP induction begins when the calcium inside the postsynaptic cell exceeds a threshold. In many types
of LTP,
the flow of calcium into the cell requires the NMDA receptor, which is why these types of LTP are
considered
NMDA receptor-dependent.
When a stimulus is applied to the presynaptic neuron, it releases a neurotransmitter, typically glutamate,
onto the
postsynaptic cell membrane where it binds to AMPA receptors, or AMPARs. This causes an influx of
sodium ions
into the postsynaptic cell, this short lived depolarization is called the excitatory postsynaptic potential
(EPSP) and
makes it easier for the neuron to fire an action potential.
A single stimulus doesn't cause a big enough depolarization to trigger an E-LTP, instead it relies on EPSP
summation. If EPSPs are reaching the cell before the others decay, they will add up. When the
depolarization reaches
a critical level, NMDA receptors lose the magnesium molecule they were originally plugged with and let
calcium in.
The rapid rise in calcium within the postsynaptic neuron trigger the short lasting activation of several
enzymes that
mediate E-LTP induction. Of particular importance are some protein kinase enzymes, including CaMKII
and PKC.
To a lesser extent, PKA and MAPK activation also contribute.
Maintenance
During the maintenance stage of E-LTP, CaMKII and PKC lose their dependence on calcium and become
autonomously active. They then carry out phosphorylation that underlies E-LTP expression.
Expression
CaMKII and PKC phosphorylate existing AMPA receptors to increase their activity, and mediate the
insertion of
additional AMPA receptors onto the postsynaptic cell membrane. This is achieved by having a pool of
nonsynaptic
AMPA receptors adjacent to the postsynaptic membrane. When the appropriate stimulus arrives, the
nonsynaptic
AMPA receptors are brought into the postsynaptic membrane under the influence of protein kinases.
AMPA receptors are one of the most common type of receptors in the brain. Their effect is excitatory. By
adding
more AMPA receptors, and increasing their activity, future stimuli will generate larger postsynaptic
responses.
Late Phase
Late LTP is the natural extension of E-LTP. L-LTP requires gene transcription and protein synthesis in the
postsynaptic cell, unlike E-LTP. Late LTP is also associated with the presynaptic synthesis of
synaptotagmin and an

increase in synaptic vesicle number, suggesting that L-LTP induces protein synthesis not only in
postsynaptic cells,
but in presynaptic cells as well. This is discussed under "retrograde messenger" below.
Induction
Late LTP is induced by changes in gene expression and protein synthesis brought about by persistent
activation of
protein kinases activated during E-LTP, such as MAPK. In fact, MAPK--Specifically the ERK subfamily of
MAPKs--may be the molecular link between E-LTP and L-LTP, since many signaling cascades involved in
E-LTP,
including CaMKII and PKC, can converge on ERK.
Maintenance
Upon activation, ERK may phosphorylate a number of cytoplasmic and nuclear molecules that ultimately
result in
the protein synthesis and morphological changes associated with L-LTP. These chemicals may include
transcription
factors such as CREB. ERK-mediated changes in transcription factor activity may trigger the synthesis of
proteins
that underlie the maintenance of L-LTP. PKMzeta is one such molecule. When this molecule is inhibited
in rats, they
experience retrograde amnesia (where you can't recall previous events but short term memory works
fine).
Expression
Aside from PKMzeta, many of the proteins synthesized during L-LTP are unknown. They are though to
increase
postsynaptic dendritic spine number, surface area and sensitivity to the neurotransmitter associated with
L-LTP
expression.
Retrograde Signaling
Retrograde signaling is a hypothesis that attempts to explain that, while LTP is induced and expressed
postsynaptically, some evidence suggests that it is expressed presynaptically as well. The hypothesis
gets its name
because normal synaptic transmission is directional and proceeds from the presynaptic to the
postsynaptic cell. For
induction to occur postsynaptically and be partially expressed presynaptically, a message must travel
from the
postsynaptic cell to the presynaptic cell in a retrograde (reverse) direction. Once there, the message
presumably
initiates a cascade of events that leads to a presynaptic component of expression, such as the increased
probability of
neurotransmitter vesicle release.
Retrograde signaling is currently a contentious subject as some investigators do not believe the
presynaptic cell
contributes at all to the expression of LTP. Even among proponents of the hypothesis there is controversy
over the
identity of the messenger.
Language and Speech
Language depends on semantic memory so some of the same areas in the brain are involved in both
memory and
language. Articulation, the forming of speech, is represented bilaterally in the motor areas. However, for
most
individuals, language analysis and speech formation take place in regions of the left hemisphere only.
The two
regions involved are:
1. Broca's Area
2. Wernicke's Area

Broca's area is located just in front of the voice control area of the left motor cortex. This region
assembles the motor
of speech and writing. For example, patients with lesions in this area:
1. Understand language perfectly
2. May be able to write perfectly
3. Seldom speak spontaneously
Wernicke's area is part of the auditory and visual associations cortex. This region is responsible for the
analysis and
formation of language content. For example, patients with lesions in this area:
1. Are unable to name objects
2. Are unable to understand the meaning of words
3. Articulate speech readily but usually nonsensically
Diseases of the Limbic System
There are several well known diseases that are disorders of the limbic system. Several are discussed
here.
Schizophrenia
An increased dopamine (DA) response in the limbic system results in schizophrenia. DA may be
synthesized or
secreted in excess, DA receptors may be supersensitive, and DA regulatory mechanism may be
defective. Symptoms
are decreased by drugs which block DA receptors. Symptoms of schizophrenia are:
1. Loss of touch with reality
2. Decreased ability to think and reason
3. Decreased ability to concentrate
4. Decreased memory
5. Regress in child-like behavior
6. Altered mood and impulsive behavior
7. Auditory hallucinations
Symptoms may be so severe that the individual cannot function.
Depression
Depression is the most common major mental illness and is characterized by both emotional and physical
symptoms.
Symptoms of depression are:
1. Intense sadness and despair
2. Anxiety
3. Loss of ability to concentrate
4. Pessimism
5. Feelings of low self esteem
6. Insomnia or hypersomnia
7. Increased or decreased appetite
8. Changes in body temperature and endocrine gland function
10 to 15% of depressed individuals display suicidal behavior during their lifetime.
The cause of depression and its symptoms are a mystery but we do understand that it is an illness
associated with
biochemical changes in the brain. A lot of research goes on to explain that it is associated with a lack of
amines
serotonin and norephinephrine. Therefore pharmacological treatment strategies often try to increase
amine
concentrations in the brain.
One class of antidepressants is monoamine oxidase inhibitors. Mono amine oxidase is a enzyme that
breaks down
your amines like norephinephrine and serotonin. Because the antidepressants inhibit their degradation
they will
remain in the synaptic cleft for a longer period of time making the effect just as if you had increased
theses types of

neurotransmitters.
A newer class of antidepressants is selective serotonin reuptake inhibitors (SSRI's). With SSRI's
decreasing the
uptake of serotonin back into the cell that will increase the amount of serotonin present in the synaptic
cleft. SSRI's
are more specific than the monoamine oxidase inhibitors because they only affect serotonergic synapses.
You might
recognize these SSRI's by name as Prozac and Paxil.
Bipolar Disorder
Another common form of depression is manic depression. Manic is an acute state characterized by:
1. Excessive elation and impaired judgment
2. Insomnia and irritability
3. Hyperactivity
4. Uncontrolled speech
Manic depression, also known as bipolar disorder, displays mood swings between manic and depression.
The limbic
system receptors are unregulated. Drugs used are unique mood stabilizers.
The hippocampus is particularly vulnerable to several disease processes, including ischemia, which is
any
obstruction of blood flow or oxygen deprivation, Alzheimer’s disease, and epilepsy. These diseases
selectively attack
CA1, which effectively cuts through the hippocampal circuit.
An Autism Link
A connection between autism and the limbic system has also been noted as well. URL: http:/ / www.
autism. org/
limbic. html
Case Study
Central Pain Syndrome
I was 42 years old when my life changed forever. I had a stroke. As an avid viewer of medical programs
on
television I assumed that I would have physical therapy for my paralyzed left side and get on with my life.
No one
ever mentioned pain or the possibility of pain, as a result of the stroke. I did experience unusual sensitivity
to touch
while still in the hospital, but nothing to prepare me for what was to come.
The part of my brain that is damaged is the Thalamus. This turns out to be the pain center and what I
have now is an
out of control Thalamus, resulting in Thalamic Pain syndrome, also called Central Pain Syndrome. This
means that
24 hours a day, seven days a week, my brain sends messages of pain and it never goes away. I am
under the care of
physicians, who not only understand chronic pain, but are also willing to treat it with whatever medications
offer
some help. None of the medications, not even narcotic medications, take the pain away. They just allow
me to
manage it so I can function.
The Peripheral Nervous System
The Cranial Nerves
The peripheral nervous system includes 12 cranial
nerves 31 pairs of spinal nerves. It can be subdivided
into the somatic and autonomic systems. It is a way of
communication from the central nervous system to the
rest of the body by nerve impulses that regulate the
functions of the human body.

The twelve cranial nerves are
I Olfactory Nerve for smell
II Optic Nerve for vision
III Oculomotor for looking around
IV Trochlear for moving eye
V Trigeminal for feeling touch on face
VI Abducens to move eye muscles
VII Facial to smile, wink, and help us taste
VIII Vestibulocochlear to help with balance,
equilibrium, and hearing
IX Glossopharengeal for swallowing and gagging
X Vagus for swallowing, talking, and parasympathetic actions of digestion
XI Spinal accessory for shrugging shoulders
XII Hypoglossal for tongue more divided into different regions as muscles
The 10 out of the 12 cranial nerves originate from the brainstem, and mainly control the functions of the
anatomic
structures of the head with some exceptions. CN X receives visceral sensory information from the thorax
and
abdomen, and CN XI is responsible for innervating the sternocleidomastoid and trapezius muscles,
neither of which
is exclusively in the head.
Spinal nerves take their origins from the spinal cord. They control the functions of the rest of the body. In
humans,
there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. The
naming
convention for spinal nerves is to name it after the vertebra immediately above it. Thus the fourth thoracic
nerve
originates just below the fourth thoracic vertebra. This convention breaks down in the cervical spine. The
first spinal
nerve originates above the first cervical vertebra and is called C1. This continues down to the last cervical
spinal
nerve, C8. There are only 7 cervical vertebrae and 8 cervical spinal nerves.
Lateral cord
The lateral cord gives rise to the following nerves:
• The lateral pectoral nerve, C5, C6 and C7 to the pectoralis major muscle, or musculus pectoralis major.
• The musculocutaneous nerve which innervates the biceps muscle
• The median nerve, partly. The other part comes from the medial cord. See below for details.
Posterior cord
diagram showing human dermatoms, i.e., skin regions with respect to
the routing of their nerve connection of their afferent nerves through the
spinal cord.
The posterior cord gives rise to the following
nerves:
• The upper subscapular nerve, C7 and C8, to the
subscapularis muscle, or musculus supca of the
rotator cuff.
• The lower subscapular nerve, C5 and C6, to the
teres major muscle, or the musculus teres major,
also of the rotator cuff.
• The thoracodorsal nerve, C6, C7 and C8, to the
latissimus dorsi muscle, or musculus latissimus
dorsi.
• The axillary nerve, which supplies sensation to the
shoulder and motor to the deltoid muscle or
musculus deltoideus, and the teres minor muscle,

or musculus teres minor.
• The radial nerve, or nervus radialis, which
innervates the triceps brachii muscle, the
brachioradialis muscle, or musculus
brachioradialis,, the extensor muscles of the
fingers and wrist (extensor carpi radialis muscle),
and the extensor and abductor muscles of the
thumb. See radial nerve injuries.
Medial cord
The medial cord gives rise to the following nerves:
• The median pectoral nerve, C8 and T1, to the pectoralis muscle
• The medial brachial cutaneous nerve, T1
• The medial antebrachial cutaneous nerve, C8 and T1
• The median nerve, partly. The other part comes from the lateral cord. C7, C8 and T1 nerve roots. The
first branch
of the median nerve is to the pronator teres muscle, then the flexor carpi radialis, the palmaris longus and
the
flexor digitorum superficialis. The median nerve provides sensation to the anterior palm, the anterior
thumb,
index finger and middle finger. It is the nerve compressed in carpal tunnel syndrome.
• The ulnar nerve originates in nerve roots C7, C8 and T1. It provides sensation to the ring and pinky
fingers. It
innervates the flexor carpi ulnaris muscle, the flexor digitorum profundus muscle to the ring and pinky
fingers,
and the intrinsic muscles of the hand (the interosseous muscle, the lumbrical muscles and the flexor
pollicus
brevis muscle). This nerve traverses a groove on the elbow called the cubital tunnel, also known as the
funny
bone. Striking the nerve at this point produces an unpleasant sensation in the ring and little fingers.
Other thoracic spinal nerves (T3-T12)
The remainder of the thoracic spinal nerves, T3 through T12, do little recombining. They form the
intercostal
nerves, so named because they run between the ribs. For points of reference, the 7th intercostal nerve
terminates at
the lower end of the sternum, also known as the xyphoid process. The 10th intercostal nerve terminates
at the
umbilicus, or the belly button.
The somatic nervous system is that part of the peripheral nervous system associated with the voluntary
control of
body movements through the action of skeletal muscles, and also reception of external stimuli. The
somatic nervous
system consists of afferent fibers that receive information from external sources, and efferent fibers that
are
responsible for muscle contraction. The somatic system includes the pathways from the skin and skeletal
muscles to
the Central Nervous System. It is also described as involved with activities that involve consciousness.
The basic route of the efferent somatic nervous system includes a two neuron sequence. The first is the
upper motor
neuron, whose cell body is located in the precentral gyrus (Brodman Area 4) of the brain. It receives
stimuli from
this area to control skeletal (voluntary) muscle. The upper motor neuron carries this stimulus down the
corticospinal
tract and synapses in the ventral horn of the spinal cord with the alpha motor neuron, a lower motor
neuron. The

upper motor neuron releases acetylcholine from its axon terminal knobs and these are received by
nicotinic receptors
on the alpha motor neuron. The alpha motor neurons cell body sends the stimulus down its axon via the
ventral root
of the spinal cord and proceeds to its neuromuscular junction of its skeletal muscle. There, it releases
acetylcholine
from its axon terminal knobs to the muscles nicotinic receptors, resulting in stimulus to contract the
muscle.
The somatic system includes all the neurons connected with the muscles, sense organs and skin. It deals
with sensory
information and controls the movement of the body.
The Autonomic System
The Autonomic system deals with the visceral organs, like the heart, stomach, gland, and the intestines.
It regulates
systems that are unconsciously carried out to keep our body alive and well, such as breathing, digestion
(peristalsis),
and regulation of the heartbeat. The Autonomic system consists of the sympathetic and the
parasympathetic
divisions. Both divisions work without conscious effort, and they have similar nerve pathways, but the
sympathetic
and parasympathetic systems generally have opposite effects on target tissues (they are antagonistic).
By controlling
the relative input from each division, the autonomic system regulates many aspects of homeostasis. One
of the main
nerves for the parasympathetic autonomic system is Cranial Nerve X, the Vagus nerve.
Figure 1: The right sympathetic chain and its
connections with the thoracic, abdominal, and
pelvic plexuses. (After Schwalbe.)
The Sympathetic and Parasympathetic Systems
The sympathetic nervous system activates what is often termed the
fight or flight response, as it is most active under sudden stressful
circumstances (such as being attacked). This response is also known as
sympathetico-adrenal response of the body, as the pre-ganglionic
sympathetic fibers that end in the adrenal medulla (but also all other
sympathetic fibers) secrete acetylcholine, which activates the secretion
of adrenaline (epinephrine) and to a lesser extent noradrenaline
(norepinephrine) from it. Therefore, this response that acts primarily on
the cardiovascular system is mediated directly via impulses transmitted
through the sympathetic nervous system and indirectly via
catecholamines secreted from the adrenal medulla.
Western science typically looks at the SNS as an automatic regulation
system, that is, one that operates without the intervention of conscious
thought. Some evolutionary theorists suggest that the sympathetic
nervous system operated in early organisms to maintain survival
(Origins of Consciousness, Robert Ornstein; et al.), as the sympathetic
nervous system is responsible for priming the body for action. One
example of this priming is in the moments before waking, in which
sympathetic outflow spontaneously increases in preparation for action.
The parasympathetic nervous system is part of the autonomic nervous system. Sometimes called the rest
and digest
system or feed and breed. The parasympathetic system conserves energy as it slows the heart rate,
increases
intestinal and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.
After high stress situations (ie: fighting for your life) the parasympathetic nervous system has a backlash
reaction

that balances out the reaction of the sympathetic nervous system. For example, the increase in heart rate
that comes
along with a sympathetic reaction will result in an abnormally slow heart rate during a parasympathetic
reaction.
Organization
Sympathetic nerves originate inside the vertebral column, toward the middle of the spinal cord in the
intermediolateral cell column (or lateral horn), beginning at the first thoracic segment of the spinal cord
and
extending into the second or third lumbar segments. Because its cells begin in the thoracic and lumbar
regions of the
spinal cord, the SNS is said to have a thoracolumbar outflow. Axons of these nerves leave the spinal cord
in the
ventral branches (rami) of the spinal nerves, and then separate out as 'white rami' (so called from the
shiny white
sheaths of myelin around each axon) which connect to two chain ganglia extending alongside the
vertebral column
on the left and right. These elongated ganglia are also known as paravertebral ganglia or sympathetic
trunks. In these
hubs, connections (synapses) are made which then distribute the nerves to major organs, glands, and
other parts of
the body. [1]
In order to reach the target organs and glands, the axons must travel long distances in the body, and, to
accomplish
this, many axons link up with the axon of a second cell. The ends of the axons do not make direct
contact, but rather
link across a space, the synapse.
In the SNS and other components of the peripheral nervous system, these synapses are made at sites
called ganglia.
The cell that sends its fiber is called a preganglionic cell, while the cell whose fiber leaves the ganglion is
called a
postganglionic cell. As mentioned previously, the preganglionic cells of the SNS are located between the
first
thoracic segment and the second or third lumbar segments of the spinal cord. Postganglionic cells have
their cell
bodies in the ganglia and send their axons to target organs or glands.
The ganglia include not just the sympathetic trunks but also the superior cervical ganglion (which sends
sympathetic
nerve fibers to the head), and the celiac and mesenteric ganglia (which send sympathetic fibers to the
gut).
Information transmission
Messages travel through the SNS in a bidirectional flow. Efferent messages can trigger changes in
different parts of
the body simultaneously. For example, the sympathetic nervous system can accelerate heart rate; widen
bronchial
passages; decrease motility (movement) of the large intestine; constrict blood vessels; increase
peristalsis in the
esophagus; cause pupil dilation, piloerection (goose bumps) and perspiration (sweating); and raise blood
pressure.
Afferent messages carry sensations such as heat, cold, or pain.
The first synapse (in the sympathetic chain) is mediated by nicotinic receptors physiologically activated by
acetylcholine, and the target synapse is mediated by adrenergic receptors physiologically activated by
either
noradrenaline or adrenaline. An exception is with sweat glands which receive sympathetic innervation but
have

muscarinic acetylcholine receptors which are normally characteristic of PNS. Another exception is with
certain deep
muscle blood vessels, which have acetylcholine receptors and which dilate (rather than constrict) with an
increase in
sympathetic tone. The sympathetic system cell bodies are located on the spinal cord excluding the cranial
and sacral
regions. The preganglonic neurons exit from the vertebral column and synapse with the postganglonic
nerouns in the
sympathetic trunk.
The parasympathetic nervous system is one of three divisions of the autonomic nervous system.
Sometimes called
the rest and digest system, the parasympathetic system conserves energy as it slows the heart rate,
increases intestinal
and gland activity, and relaxes sphincter muscles in the gastrointestinal tract.
Relationship to sympathetic
While an oversimplification, it is said that the parasympathetic system acts in a reciprocal manner to the
effects of
the sympathetic nervous system; in fact, in some tissues innervated by both systems, the effects are
synergistic.
Receptors
The parasympathetic nervous system uses only acetylcholine (ACh) as its neurotransmitter. The ACh
acts on two
types of receptors, the muscarinic and nicotinic cholinergic receptors. Most transmissions occur in two
stages: When
stimulated, the preganglionic nerve releases ACh at the ganglion, which acts on nicotinic receptors of the
postganglionic nerve. The postganglionic nerve then releases ACh to stimulate the muscarinic receptors
of the target
organ.
The three main types of muscarinic receptors that are well characterised are:
• The M1 muscarinic receptors are located in the neural system.
• The M2 muscarinic receptors are located in the heart, and act to bring the heart back to normal after the
actions of
the sympathetic nervous system: slowing down the heart rate, reducing contractile forces of the atrial
cardiac
muscle, and reducing conduction velocity of the atrioventricular node (AV node). Note, they have no
effect on the
contractile forces of the ventricular muscle.
• The M3 muscarinic receptors are located at many places in the body, such as the smooth muscles of
the blood
vessels, as well as the lungs, which means that they cause vasoconstriction and bronchoconstriction.
They are also
in the smooth muscles of the gastrointestinal tract (GIT), which help in increasing intestinal motility and
dilating
sphincters. The M3 receptors are also located in many glands that help to stimulate secretion in salivary
glands
and other glands of the body.
Nervous Tissue
The nervous system coordinates the activity of the muscles, monitors the organs, constructs and also
stops input from
the senses, and initiates actions. Prominent participants in a nervous system include neurons and nerves,
which play
roles in such coordination.Our nervous tissue only consists of two types of cells. These cells are neurons
and

neuroglia cells. The neurons are responsible for transmitting nerve impulses. Neuroglia cells are
responsible for
supporting and nourishing the neuron cells.
Types of Neurons
There are three types of neurons in the body. We have sensory
neurons, interneurons, and motor neurons. Neurons are a major class of
cells in the nervous system. Neurons are sometimes called nerve cells,
though this term is technically imprecise, as many neurons do not form
nerves. In vertebrates, neurons are found in the brain, the spinal cord
and in the nerves and ganglia of the peripheral nervous system. Their
main role is to process and transmit information. Neurons have
excitable membranes, which allow them to generate and propagate
electrical impulses. Sensory neuron takes nerve impulses or messages right from the sensory receptor
and delivers it
to the central nervous system. A sensory receptor is a structure that can find any kind of change in it's
surroundings
or environment.
Structure of a neuron
Neurons have three different parts to them. They all have an axon, a cell body and dendrites. The axon is
the part of
the neuron that conducts nerve impulses. Axons can get to be quite long. When an axon is present in
nerves, it is
called a nerve fiber. A cell body has a nucleous and it also has other organelles. The dendrites are the
short pieces
that come off of the cell body that receive the signals from sensory receptors and other neurons.
Myelin Sheath
Schwann cells contain a lipid substance called myelin in their plasma membranes. When schwann cells
wrap around
axons, a myelin sheath forms. There are gaps that have no myelin sheath around them; these gaps are
called nodes of
Ranvier. Myelin sheathes make excellent insulators. Axons that are longer have a myelin sheath, while
shorter axons
do not. The disease multiple sclerosis is an autoimmune disease where the body attacks the myelin
sheath of the
central nervous system.
Case Study
A 35-year-old male in 1986 had been admitted to a hospital in Florida three weeks previous to being
diagnosed, with
complaints of weakness and spasticity in the right leg, difficulties with balance, and fatigue and malaise.
Tests
performed at the Florida hospital had revealed abnormalities in spinal fluid and MRI brain scan. The
patient
complained of being severely depressed and anxious. He had anger at his circumstances and frequent
crying spells.
One month previously he had noticed aching and loss of vision in the left eye that had since improved.
This man was diagnosed with Multiple Sclerosis. MS is a chronic, degenerative, and progressive disorder
that affects
the nerve fibers in the brain and spinal cord. Myelin is a fatty substance that surrounds and insulates the
nerve fibers
and facilitates the conduction of the nerve impulse transmissions. MS is characterized by intermittent
damage to
myelin (called demyelination) caused by the destruction of specialized cells (oligodendrocytes) that form
the
substance. Demyelination causes scarring and hardening (sclerosis) of nerve fibers usually in the spinal
cord, brain

stem, and optic nerves, which slows nerve impulses and results in weakness, numbness, pain, and vision
loss.
Because different nerves are affected at different times, MS symptoms often worsen (exacerbate),
improve, and
develop in different areas of the body. Early symptoms of the disorder may include vision changes
(blurred vision,
blind spots) and muscle weakness. MS can progress steadily or cause acute attacks (exacerbations)
followed by
partial or complete reduction in symptoms (remission). Most patients with the disease have a normal
lifespan.
There are different types of MS
Multiple sclerosis is classified according to frequency and severity of neurological symptoms, the ability of
the
CNS to recover, and the accumulation of damage.
Treating Depression
Every now and then we all feel a little blue, these feelings can be caused by losing a loved one. Clinical
depression
goes much further than just feeling down. Depression has many symptoms, including lack of energy,
abnormal
eating habits (either too much or too little) and sleeping problems (also too much or too little). Often a
person can
feel worthless and have thoughts of committing suicide. The cause of depression and its symptoms are a
mystery but
we do understand that it is an illness associated with biochemical changes in the brain. A lot of research
goes on to
explain that it is associated with a lack of amines serotonin and norephinephrine. Therefore
pharmacological
treatment strategies often try to increase amine concentrations in the brain.
One class of antidepressants is monoamine oxidase inhibitors. Mono amine oxidase is a enzyme that
breaks down
your amines like norephinephrine and serotonin. Because the antidepressants inhibit their degradation
they will
remain in the synaptic cleft for a longer period of time making the effect just as if you had increased
theses types of
neurotransmitters.
A newer class of antidepressants is selective serotonin reuptake inhibitors (SSRI's). With SSRI's
decreasing the
uptake of serotonin back into the cell that will increase the amount of serotonin present in the synaptic
cleft. SSRI's
are more specific than the monoamine oxidase inhibitors because they only affect serotonergic synapses.
You might
recognize these SSRI's by name as Prozac and Paxil.
Drugs
A drug is, generally speaking, any substance that changes the way your body works. Some drugs have a
medicinal
effect, and some are used recreationally. They have diverse effects, depending on the drug. Drugs can
do anything
from diminish pain, to preventing blood clots, to helping a depressed person.
Different drugs work in different ways, called the mechanism of action, the drugs covered here will all act
on the
nervous system via receptors on different neurons. There are also drugs that change how enzymes work,
but that's
not part of the nervous system (at least directly) and will not be discussed here.

You've probably heard the terms stimulant (excitatory) and depressant (inhibitory). This is a broad way of
classifying
drugs that work on the CNS. Depressants slow down neural function, and stimulants speed it up.
Most of the common depressants (including alcohol, benzodiazepines, barbiturates and GHB) work on
GABA
receptors, although there are others. Opiates, for example, work on mu opioid receptors and also produce
inhibitory
effects, and some antipsychotics block serotonin. See the alcohol section below to see one way this can
work.
Stimulants work mostly with epinephrine, dopamine or serotonin (or a combination of them). Many of
them either
mimic one, or stop them from leaving the synapse, causing more action potentials to be fired.
Methamphetamine,
discussed below, is a fairly typical stimulant drug.
Drug Abuse
Scientists have long accepted that there is a biological basis for drug addiction, though the exact
mechanisms responsible are only now being identified. It is believed that addictive substances
create dependence in the user by changing the brain's reward functions, located in the mesolimbic
dopamine system—the part of the brain that reinforces certain behaviors such as eating, sexual
intercourse, exercise, and social interaction. Addictive substances, through various means and to
different degrees, cause the synapses of this system to flood with excessive amounts of dopamine,
creating a brief rush of euphoria more commonly called a "high”. Some say that abuse begins when the
user begins
shirking responsibility in order to afford drugs or to have enough time to use them. Some say it begins
when a person
uses "excessive" amounts, while others draw the line at the point of legality, and others believe it amounts
to chronic
use despite degenerating mental and physical health in the user. Some think that any intoxicant
consumption is an
inappropriate activity. Here are some drugs that are abused frequently: Acid/LSD, Alcohol, various
tryptamines and
phenethylamines, Cocaine, Ecstasy/MDMA, Heroin, Inhalants, Marijuana, Methamphetamine,
PCP/Phencyclidine,
Prescription Medications, Smoking/Nicotine and Steroids.
Alcohol
Alcohol is, and has been for thousands of years, one of the most commonly used drugs in the world. It is
legal, with
some restrictions and exceptions, nearly everywhere. It is a common misconception that somehow
alcohol is 'better'
or 'safer' than other recreational drugs. This is simply NOT the case. Alcohol is a depressant, and as such
it has the
potential to cause coma, respiratory depression/arrest and possibly death. Compared with some other
(illegal in most
places) drugs of recreational value (such as marijuana, serotonin based hallucinogens like LSD or
psilocybin)
alcohol is far more toxic and has more risk of overdose. That doesn't mean that moderate drinking will
probably hurt
you, though, either.
Short term effects from drinking (listed roughly as they appear, and as dosage goes up) are: decreased
inhibitions and
thusly judgment, flushing of the face, drowsiness, memory problems begin, severe motor impairment,
blurry vision,
dizziness, confusion, nausea, possible unconsciousness, coma, death (due to respiratory arrest or
possibly aspiration
on vomit).

Alcohol produces these effects mainly via the GABA receptors in the brain. When GABA (or in this case
alcohol)
binds to it's receptor, it lets either Cl- ions in, or K+ out. This is called hyperpolarization, or an inhibitory
postsynaptic potential (IPSP). It makes it harder for the neuron to depolarize and hence harder for it to fire
an action
potential, slowing neural function. At higher doses alcohol will start to block NMDA. NMDA is involved in
memory
(see the long-term potentiation section) so this is thought to account for memory blackouts.
Methamphetamine
In the US, medically prescribed methamphetamine is distributed in tablet form under the brand name
Desoxyn®,
generally for Attention Deficit Hyperactivity Disorder (ADHD) but also for narcolepsy or obesity.
Illicit methamphetamine comes in a variety of forms. Most commonly it is found as a colorless crystalline
solid, sold
on the street under a variety of names, such as: crystal meth or crystal. Methamphetamine may also be
referred to as
shards, rock, pony, crissie, crystal, glass, ice, Jib, critter, Tina, tweak or crank. Dope may refer to
methamphetamine
or other drugs, especially heroin or marijuana. The term "speed" can denote any stimulant including other
amphetamines (e.g. adderall), cocaine and methylphenidate (Ritalin).
Methamphetamine can be injected (either subcutaneous, intramuscular or intravenous), smoked, snorted,
swallowed,
or used rectally or sublingually. The latter two being fairly uncommon. After administration,
methamphetamine
takes from a few seconds (smoked or injected IV) to around 30 minutes (oral) for effects to arise, lasting
around
eight hours depending on the route of administration. Effects/side effects include euphoria, anorexia,
increased
energy, clenching of the jaw/grinding of teeth (bruxism), weight loss, insomnia, tooth decay and psychosis
among
others.
Methamphetamine is neurotoxic to at least some areas of the brain, and owes most of it's effects to the
neurotransmitters dopamine, norepinephrine and serotonin it releases. It also blocks the reuptake of
those
neurotransmitters, causing them to stay in the synaptic cleft longer than normal.
Marijuana
Cannabis sativa.
Marijuana contains a myriad of chemicals, called
cannabinoids, that have psychoactive and medicinal
effects when consumed, the major one being
tetrahydrocannabinol (THC). THC serves to mimic the
endogenous neurotransmitter anandamide (also found in
chocolate) at the CB1 receptors in the brain. Other
cannabinoids include Cannabidiol (CBD), cannabinol
(CBN) and tetrahydrocannabivarin (THCV). Although
THC is found in all parts of the plant, the flower of the
female plant has the highest concentration, commonly
around eight percent. The flowers can be used, or they
can be refined. Trichomes contain most of the THC on
the flowers and can be removed by a few different
methods. These removed trichomes are called kief. Kief
can, in turn, be pressed into hashish. By far the most
common way to consume any of these products is by
smoking, but it can be taken orally as well.

Cannabis has a very long, very good safety record.
Nobody on record has ever died because of marijuana,
directly at least. It is estimated that it would take 1-1.8
kilograms of average potency marijuana, taken orally, to
have a fifty percent chance of killing a 68kg human.
Despite this, the possession, use, or sale of psychoactive
cannabis products became illegal in many parts of the
world in the early 20th century. Since then, while some countries have intensified the enforcement of
cannabis
prohibition, others have reduced the priority of enforcement to the point of de facto legality. Cannabis
remains illegal
in the vast majority of the world's countries.
The nature and intensity of the immediate effects of cannabis consumption vary according to the dose,
the species or
hybridization of the source plant, the method of consumption, the user's mental and physical
characteristics (such as
possible tolerance), and the environment of consumption. This is sometimes referred to as set and
setting. Smoking
the same cannabis either in a different frame of mind (set) or in a different location (setting) can alter the
effects or
perception of the effects by the individual. Effects of cannabis consumption may be loosely classified as
cognitive
and physical. Anecdotal evidence suggests that the Cannabis sativa species tends to produce more of
the cognitive or
perceptual effects, while Cannabis indica tends to produce more of the physical effects.

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