Neural control n coordination by BNP.pdf

NEURAL CONTROL AND
CO-ORDINATION
By Biswanath prusty

 It is a system that controls and
coordinates the body activities, conducts
& integrates the information and
responds to stimuli.
 It includes brain, spinal cord & nerves.
 It is made up of specialized cells known
as neurons.
NERVOUS (NEURAL) SYSTEM
Brain
Nerves
Spinal
cord

The Central Nervous System
The organ of the central nervous system that is
likely most familiar to you, yet still holds the
greatest mysteries for physiologists, is the
brain. Enclosed completely by the skull, the
brain is composed primarily of nervous tissue.
This remarkable organ consists of about 100
billion cells called neurons, or nerve cells, that
enable everything from the regulation of
breathing and the processing of algebra to
performing in the creative arts.

The Peripheral Nervous System
The peripheral nervous system is made up of the most numerous organs of the nervous system, the nerves, which carry
signals to and from the central nervous system. A nerve consists of a bundle of long neuron “arms” known as axons that are
packaged together with blood vessels and surrounded by connective tissue sheaths. Nerves are classified according to their
origin or destination: Those originating from or traveling to the brain are called cranial nerves, and those originating from or
traveling to the spinal cord are called spinal nerves. There are 12 pairs of cranial nerves and 31 pairs of spinal nerves.
TYPES OF NERVE FIBRES OF THE PNS
The bundle or group of neuron fibres is called the nerves while the bundle of cell-bodies is called
the ganglion in the Peripheral Nervous System. The nerves are of following types:
1. AFFERENT NERVE FIBRES: These nerves are responsible for the transmission of stimuli from
the receptors to the central nervous system. Hence, these are made up of sensory neurons.
2. EFFERENT NERVE FIBRES: These nerves are responsible for the transmission of response
impulses from the central nervous system to the effector organs. Hence, these are made up of
motor neurons.
DIVISION OF THE PERIPHERAL NERVOUS SYSTEM
1. SOMATIC NEURAL SYSTEM: The neurons that innervate the skeletal muscles are kept under this
system. So, this system is concerned with the coordination of the voluntary activities of the body.
2. AUTONOMIC NEURAL SYSTEM (ANS): The neurons that innervate the involuntary performing
organs are kept under this system. It does not involve the conscious control over the responses

Functional Divisions of
the Nervous System
As the nervous system performs its many tasks,
millions of processes may be occurring simultaneously.
However, all of these tasks or functions generally
belong to one of three types: sensory, integrative, or
motor. Sensory functions involve gathering
information about the internal and external
environments of the body. Integrative functions
analyze and interpret incoming sensory information
and determine an appropriate response. Motor
functions are the actions performed in response to
integration.
Sensory input is gathered by the sensory, or afferent,
division (“carrying toward”) of the PNS. Integration is
performed entirely by the CNS, mostly by the brain.
Motor output is performed by the motor, or efferent,
division (“carrying away”) of the PNS.

Neural control n coordination by BNP.pdf

NEURON (NERVE CELL)
✓ Neuron is the structural
and functional unit of
nervous system.
✓ It has 3 main parts:
❖ Cell body (cyton)
❖ Dendron
❖ Axon
Like epithelial tissue, nervous tissue is highly cellular; about 80% of nervous tissue volume consists of
cells. When you look at such a micrograph of nervous tissue, the most obvious type of cell is the neuron,
which is the excitable cell type responsible for sending and receiving signals. The other cell type in
nervous tissue is the smaller and more prevalent neuroglial cell (“nerve glue”), or neuroglia, which
generally does not transmit signals but rather serves a variety of supportive functions.

STRUCTURE OF A NEURON (NERVE CELL)

a) Cell body (cyton)
 It contains cytoplasm, cell
organelles and Nissl’s
granules (granular bodies).
 The most conspicuous part of
a neuron is its large cell body,
or soma, which ranges from 5
to 100 μm in diameter.

b) Dendron
 Extending from all neuron cell bodies
are long “arms,” cytoplasmic
extensions that are called processes.
These processes allow the neuron to
communicate with other cells. Most
neurons have two types of processes,
including one or more dendrites and
one axon.
 Short fibres that project out of the
cell body.
 Sub branches of dendron are called
dendrites.
 They transmit impulses towards cell
body.

c) Axon (Nerve fibre)
 A long fibre which transmit impulses
away from the cell body.
 The branching of axon is called
axonite.
 Each axonite ends as a bulb-like
structure called synaptic knob.
 Although a neuron may have
multiple dendrites, each neuron has
only a single axon, sometimes called
a nerve fiber.

The billions of neurons in nervous tissue are directly responsible for its sensory,
integrative, and motor functions. Neurons are the excitable cell type responsible for
sending and receiving signals in the form of action potentials. Recall that most neurons
are amitotic, meaning that at a certain point in development, they lose their centrioles and
after that lack the ability to undergo mitosis. Luckily, neurons are very long-lived cells, and
some can easily survive the entire lifespan of an organism if given adequate nutrition and
oxygen in a supportive environment.
Neurons vary greatly in size. Some tiny neurons in the CNS are only 1 mm long, whereas
some PNS neurons may be up to 1 m or longer.
Classification of Neurons
As with many topics that we’ve covered, neurons can be classified according to both their
structure and their function.

TYPES OF NEURON
Multipolar neuron
• One axon and 2 or more dendrons
• Found in the cerebral cortex
Bipolar neuron
• One axon and one dendron
• Found in the retina of eye
Unipolar neuron
• One axon only
• Found in the embryonic stage

Functional Classification Functionally, neurons are grouped into three classes based on the
direction in which they carry information. The three classes are as follows, in order of
information flow:
1. Sensory, or afferent, neurons carry information toward the central nervous system. These
neurons receive information from a sensory receptor and transmit this information to their cell
body in the PNS, then down their axon to the brain or spinal cord. Because sensory neurons
receive information from one area, they are generally pseudo-unipolar or bipolar in structure.
Sensory neurons detect the internal and external environments (such as from the skin and
viscera) and facilitate motor coordination (such as in joints and muscles).
2. Interneurons, also called association neurons, relay messages within the CNS, primarily
between sensory and motor neurons, and are the location of most information processing. The
vast majority of neurons are interneurons. Multipolar in structure, interneurons generally
communicate with many other neurons (for example, one Purkinje cell of the cerebellum can
receive as many as 150,000 contacts from other neurons).
3. Motor, or efferent, neurons carry information away from their cell bodies in the CNS to
muscles and glands. As motor tasks are generally complicated and require input from many
other neurons, most motor neurons are multipolar.

Types of Axon
Myelinated
axon
Non-
myelinated
axon
TYPES OF AXON

 It is enveloped with Schwann cells,
which form a myelin sheath
around the axon.
 Found in spinal and cranial nerves.
 White coloured area, formed of
myelinated nerve fibres is called
white matter.
 Gaps between two adjacent
myelin sheaths are called nodes of
Ranvier.
TYPES OF AXON MYELINATED AXON

 Schwann cells present but no
myelin sheath.
 The gray-coloured area without
myelin sheath is called gray
matter.
 Found in autonomous and
somatic neural systems.
TYPES OF AXON NON-MYELINATED AXON

Certain neuroglia wrap themselves around the axons of neurons to create a structure known
as the myelin Sheath Myelin is composed of repeating layers of the plasma membrane of the
neuroglial cell, so it has the same substances as any plasma membrane: phospholipids, other
lipids, and proteins. The main components (70–80%) of myelin are various lipids, including
cholesterol, phospholipids, and other unique lipids.
In the fluids of the body, electric current is the movement of ions. Ions do not easily pass
through the phospholipid bilayer of the plasma membrane, and so the high lipid content of
myelin makes it an excellent insulator of electrical current (akin to rubber tubing around a
copper wire).
Myelin Sheath

Neuroglia, or neuroglial cells, were named for the early scientific idea that these cells “glued
together” the neurons, as the word root glia means “glue.” However, we now recognize that
neuroglia also serve many more functions. Some of their roles include maintaining the
environment around neurons, protecting them, and assisting in their proper functioning.
Unlike the mostly amitotic neurons, neuroglia retain their ability to divide, and they fill in
gaps left when neurons die.
Six different types of neuroglia can be found in the nervous system, four in the CNS and two
in the PNS.
In CNS- Microglial cells, Oligodendrocytes, Astrocytes, Ependyma.
In PNS- Schwann cells, Satellite cells.
Neuroglia/ Glial cells

GENERATION & CONDUCTION OF
NERVE IMPULSES / Electrophysiology
of Neurons

Conduction of Nerve Impulse
A nerve impulse is the electric signals that pass along the dendrites to generate a nerve impulse or
an action potential. An action potential is due to the movement of ions in and out of the cell. It
specifically involves sodium and potassium ions. They are moved in and out of the cell through
sodium and potassium channels and sodium-potassium pump.
Conduction of nerve impulse occurs due to the presence of active and electronic potentials along
the conductors. Transmission of signals internally between the cells is achieved through a
synapse. Nerve conductors comprise relatively higher membrane resistance and low axial
resistance. The electrical synapse has its application in escape reflexes, heart and in the retina of
vertebrates. They are mainly used whenever there is a requirement of fast response and timing
being crucial. The ionic currents pass through the two-cell membrane when the action potential
reaches the stage of such synapse.
The electrical changes across a neuron’s plasma membrane come in two forms: (1) local potentials, which
travel only short distances, and (2) action potentials, which travel the entire length of an axon. Both types of
potentials rely on the same principles of electrophysiology

Ion Channels and Gradients
Ions cannot pass through the hydrophobic portion of the phospholipid bilayer of the plasma
membrane because they are charged particles. For this reason, their movement across the
plasma membrane is dependent on specific protein channels.
There are two main classes of channels:
● Leak channels are always open and continually allow ions to follow their concentration
gradient into or out of the cell.
● Gated channels are closed at rest, and open only in response to certain stimuli. Some gated
channels, called ligand-gated channels, open in response to a certain chemical binding to the
channel (or to an associated receptor). Other channels, called voltage-gated channels, open
or close in response to changes in voltage across the membrane. A third type of gated
channel is the mechanically gated channel, which opens or closes in response to mechanical
stimulation such as stretch, pressure, and vibration.

Impulse transmission is
electrochemical.
GENERATION AND CONDUCTION OF NERVE IMPULSES
3 steps of impulse
transmission
Maintenance of resting
membrane potential
Action potential
Propagation of action
potential

The Resting Membrane Potential
The electrical gradient across the cell membrane is known as a membrane potential, named for the fact
that, like any gradient, an electrical gradient is a source of potential energy for the cell.
The voltage across the membrane may be measured with a voltmeter. Notice that as you measure from
outside to inside the cell with a voltmeter, the voltage becomes more negative. This negative voltage is
present when the cell is at rest (not being stimulated), and for this reason it is called the resting membrane
potential. The cell in this state is said to be polarized, which simply means that the voltage difference across
the plasma membrane of the cell is not at 0 mV, but rather measures to either the positive or the negative
side (or pole) of zero.
A typical neuron has a resting membrane potential of about -70 mV.
Generation of the Resting Membrane Potential:
Two factors work together:
● Ion concentration gradients favour diffusion of potassium ions out of the cell and sodium ions into the cell.
● Potassium ions diffuse through leak channels more easily than do sodium ions.

 Neural membrane contains various
selectively permeable ion channels.
 In a resting neuron, (neuron not
conducting impulse), the axonal
membrane is more permeable to K+
ions and nearly impermeable to Na+
ions. Also, the membrane is
impermeable to negatively charged
proteins in axoplasm.
GENERATION AND CONDUCTION OF NERVE IMPULSES
Maintenance of resting membrane
potential

A local potential may have one of two effects:
● It may cause a depolarization in which positive charges enter the cytosol and make the membrane potential less negative (e.g., a change from -
70 to -60 mV).
● Alternatively, it may cause a hyperpolarization in which either positive charges exit, or negative charges enter the cytosol, which makes the
membrane potential more negative (e.g., a change from -70 to -80 mV).
Local potentials are sometimes called graded potentials because they vary greatly in size—some produce a larger change in membrane potential
than others.
local potentials cannot send signals over great distance but are useful for short-distance signaling only (which is why they’re called local
potentials).
local potential

Action Potentials
An action potential is a uniform, rapid depolarization and repolarization of the membrane
potential of a cell.
Recall that only axons generate action potentials; dendrites and cell bodies generate
local potentials only. Action potentials are generated in a region called the trigger zone, which
includes the axon hillock and the initial segment of the axon.
Events of an Action Potential
Let’s examine the sequence of events of an action potential in a section of axon The
entire sequence takes just a few milliseconds. Neuronal action potentials have three general
phases: the depolarization phase, the repolarization phase, and the hyperpolarization phase.
During the depolarization phase, the membrane potential rises toward zero and
then becomes briefly positive. The membrane potential returns to a negative value during the
repolarization phase, and then becomes temporarily more negative than resting during the
hyperpolarization phase. Each phase occurs because of the selective opening and closing of
specific ion channels.

The Refractory Period
Neurons are limited in how often they can fire
action potentials. For a brief time after a neuron
has produced an action potential, the membrane
cannot be stimulated to fire another one. This
time is called the refractory period. The
refractory period may be divided into two phases:
the absolute refractory period and the relative
refractory period.

Propagation of Action Potentials
A single action potential in one spot can’t perform its main function, which is to act as a method
of long-distance signaling. To do this, it has to be conducted, or propagated, down the length of
the axon. This movement creates a flow of charged particles, a current. Action potentials are
self-propagating, meaning that each action potential triggers another one in a neighboring
section of the axon. You can imagine this process like a string of dominoes—when the first one is
tipped over, the next one falls, which triggers the next to fall, and the process continues until the
end of the line is reached. Only the first domino needs the “push,” and once they start to fall,
the process sustains itself until the end.
Events of Propagation
The action potential is propagated along the axon by the following sequence of events, shown:

Neuronal Synapses
synapse “to clasp or join”)
Neuronal synapses generally occur between an axon and another part of a neuron; they may
occur between an axon and a dendrite, an axon and a cell body, and an axon and another axon.
These types are called axodendritic, axosomatic, and axoaxonic synapses, respectively
Regardless of the type of synapse, we use certain terms to describe the neurons
sending and receiving the message:
• Presynaptic neuron. The presynaptic neuron is the neuron that is sending the message from
its axon terminal.
• Postsynaptic neuron. The postsynaptic neuron is the neuron that is receiving the message
from its dendrite, cell body, or axon.
The transfer of chemical or electrical signals between neurons at a synapse is called synaptic
transmission, and it is the fundamental process for most functions of the nervous system.
Synaptic transmission allows voluntary movement, cognition, sensation, and emotion, as well as
countless other processes.

Electrical Synapses
An electrical synapse occurs between cells that are electrically coupled via gap junctions.
Observe that in these synapses the axolemmas of the two neurons are nearly touching (they are
separated by only about 3.5 nm) and that the gap junctions contain precisely aligned channels
that form pores through which ions and other small substances may travel. This allows the
electrical current to flow directly from the axoplasm of one neuron to that of the next.
This arrangement creates two unique features of electrical synapses:
● Synaptic transmission is bidirectional. In an electrical synapse, transmission is usually
bidirectional, which means that either neuron may act as the presynaptic or the postsynaptic
neuron and that current may flow in either direction between the two cells.
● Synaptic transmission is nearly instantaneous. The delay between depolarization of the
presynaptic neuron and change in potential of the postsynaptic neuron is less than 0.1 ms
(millisecond), which is extraordinarily fast (we will see that transmission at most chemical
synapses requires from one to a few milliseconds).

Chemical Synapses
Most synapses in the nervous system are chemical synapses. These synapses are more common because
they are more efficient—the current in electrical synapses eventually becomes weaker as it dissipates into
the extracellular fluid. A chemical synapse, in contrast, converts an electrical signal into a controlled
chemical signal, so there is no loss of strength. The chemical signal is reconverted into an electrical signal in
the postsynaptic neuron.
Electrical and Chemical Synapses Compared
● Synaptic vesicles. The axon terminal of the presynaptic neuron of every chemical synapse houses synaptic
vesicles. These vesicles contain chemical messengers called neurotransmitters that transmit signals from
the presynaptic to the postsynaptic neuron.
● Synaptic cleft. Whereas the cells of an electrical synapse are electrically connected by gap junctions, the
cells of a chemical synapse are separated by a larger but still microscopic space called the synaptic cleft. The
synaptic cleft measures 20–50 nm and is filled with extracellular fluid.
● Neurotransmitter receptors. In chemical synapses the postsynaptic neuron must have receptors for the
neurotransmitters that the presynaptic neuron releases or it cannot respond to the signal being transmitted.
Receptors are generally linked either directly or indirectly to ion channels.

Events at a Chemical Synapse
The neuromuscular junction is a type of chemical synapse, and although some of the terms are different, the events
occurring at a neuronal chemical synapse are similar to those occurring at
the neuromuscular junction.
Neuronal synapse depicts the following events:
1. An action potential in the presynaptic neuron triggers calcium ion channels in the axon terminal to open. An
action potential reaches the axon terminal of the presynaptic neuron, which triggers the opening of voltage-
gated calcium ion channels in its axolemma.
2. Influx of calcium ions causes synaptic vesicles to release neurotransmitters into the synaptic cleft. Calcium
ions enter the axon terminal, causing synaptic vesicles in the area to fuse with the presynaptic membrane. This
releases neurotransmitters into the synaptic cleft via exocytosis.
3. Neurotransmitters bind to receptors on the postsynaptic neuron. The neurotransmitters diffuse across the
synaptic cleft, where they bind to neurotransmitter receptors on the membrane of the postsynaptic neuron.
4. Ion channels open, leading to a local potential and possibly an action potential. The binding of
neurotransmitters to receptors generally either opens or closes ligand-gated ion channels in the postsynaptic
membrane, resulting in a local potential. Such local potentials may or may not lead to an action potential in the
postsynaptic neuron

Local potentials in the membrane of the postsynaptic neuron, which are called postsynaptic potentials, can move the membrane
either closer to or farther away from threshold. Therefore,
depending on which channels are opened, one of two events may occur:
• The membrane potential of the postsynaptic neuron moves closer to threshold. A small, local depolarization called an
excitatory postsynaptic potential (EPSP) occurs, which brings the membrane of the postsynaptic neuron closer to threshold. If
the membrane potential reaches threshold, an action potential is triggered.
• The membrane potential of the postsynaptic neuron moves away from threshold. A small, local hyperpolarization known as an
inhibitory postsynaptic potential (IPSP) occurs, moving the membrane of the postsynaptic neuron farther away from threshold,
and so tending to inhibit an action potential from firing.
Termination of Synaptic Transmission
• Diffusion and absorption. Some neurotransmitters simply diffuse away from the synaptic cleft through the extracellular fluid,
where they diffuse through the plasma membrane of a neuron or astrocyte and are then returned to the presynaptic neuron.
• Degradation in the synaptic cleft. Certain neurotransmitters are broken down by enzymes that reside in the synaptic cleft. The
components of the destroyed neurotransmitter are often then taken back up by the presynaptic neuron and resynthesized into
the original neurotransmitter.
• Reuptake into the presynaptic neuron. Some neurotransmitters are removed by a process called reuptake, in which proteins in
the axolemma of the presynaptic neuron transport them back into the presynaptic neuron. Depending on their type, these
neurotransmitters may be repackaged into synaptic vesicles or degraded by enzymes.
Postsynaptic Potentials

excitatory postsynaptic potential (EPSP)
inhibitory postsynaptic potential (IPSP)

CENTRAL NERVOUS SYSTEM
The central nervous system consists of brain and spinal
cord.
Brain and spinal cord are surrounded by membranes
called meninge. There are three layers- outer most dura
mater, middle arachnoid and the inner pia mater.
The dura mater and arachnoid mater are separated by a
space called subdural space.
The arachnoid and pia mater are separated by
subarachnoid space, which contains cerebrospinal fluids.
Within the brain are four irregular shaped cavities called
as ventricles. They are named as
1. Right and left lateral ventricle
2 Third ventricle
3. Fourth ventricle

Cerebrospinal fluid (CSF)
CSF is secreted into each ventricle of the brain by
choroid plexus. CSF is secreted continuously at a
rate of 0.5 ml per minute, i.e., 720 ml per day.
It is a clear, slightly alkaline fluid, and consists of
water, mineral salts, glucose, plasma protein, small
amount of creatinine, urea, and leukocytes.
Function of CSF:
1. It supports and protects the spinal cord.
2. It maintains uniform pressure around brain and
spinal cord.
3 It acts as a shock absorber.
An infection of the meninges is called meningitis

Brain
It is the part of the central nervous system that
is present in the head and protected by the
skull, dorsally and laterally. The box that
houses the brain within the skull is called the
cranium. It weighs about 1200-1400g in an
adult. Structure is shown in Fig. 7.6.
It has three main regions-
1 The fore brain,
2. The mid brain and
3 The hind brain.
The three regions have different parts that
have specific functions.

1. Fore Brain
It is made up of cerebrum, hypothalamus and thalamus.
• Cerebrum
The cerebrum is the largest and most prominent part of the brain. Among all vertebrate's cerebrum of humans is
most highly developed. It is divided into left and right hemispheres by a deep median longitudinal cerebral fissure. The two
hemispheres are joined together by a thick band of fibers called the corpus callosum.
The outer (superficial) region of cerebrum contains grey matter, which contains cell bodies of the neuron and forms the
cerebral cortex.
The deeper region of cerebrum contains white matter, which contains nerve fibres or axons of the neurons.
The cerebral cortex has many folds or convolutions which increase the surface area. It is believed that higher the
number of convolutions, higher is the intelligence.
The cerebrum is divided into 4 lobes according to function.
1. Frontal lobe; conduct 3 function;
• motor control
• Speech production
• Thought processes

2. Parietal Lobe: Associated with sensations (processes information about touch, taste, pressure, pain, and heat/cold) and
understanding speech.
3. occipital Lobe: Receives and processes visual information.
4. Temporal Lobe: Receives auditory signals, olfactory signals, processing language 4 and the meaning of words.
The cerebrum has sensory areas, association areas and motor areas.
The sensory areas receive the messages, the association areas associate this information with the previous and
other sensory information and the motor areas are responsible of the
action of the voluntary muscles.
The inner parts of cerebral hemispheres and a group of associated deep structures like amygdala, hippocampus,
etc,, form a complex structure called the limbic lobe or limbic system. Along with the hypothalamus, it is involved in the
regulation of sexual behavior, expression of emotional reactions (e.g, excitement, pleasure, rage and fear) and motivation.
Functions of Cerebrum:
There are three main functions of cerebrum.
• Cerebrum is responsible for mental activities. It is involved in memory, intelligence, sense of responsibility, thinking,
reasoning and learning.
• It is responsible for sensory perception including perception of pain, temperature, touch, sight, hearing, taste and smell.
• It is also responsible for initiation and control of skeletal muscle contraction.

• Thalamus
It is an area which co-ordinates the sensory impulses from the various sense organs eyes, ears and skin and
then relays it to the cerebrum.
• Hypothalamus
Hypothalamus is a small region and is situated below the thalamus. It is an important region of the brain.
It receives the taste and smell impulses, coordinates messages from the autonomous nervous system, controls
the heart rate. Blood pressure, body temperature and peristalsis.
It also forms an axis with the pituitary which is the main link between the nervous and the endocrine systems,
It also has centres that control mood and emotions.
2. Midbrain
The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. A
canal called the cerebral aqueduct passes through the midbrain; The dorsal portion of the midbrain consists
mainly of four round swelings (lobes) called corpora quadrigemina.

3. Hind Brain
It consists of
Cerebellum,
Pons and
Medulla oblongata.
• Cerebellum
Cerebellum is situated behind the pons and below the cerebrum. It is ovoid in shape and has two hemispheres, which are
separated by a narrow median strip called the vermis.
The cerebellum is involved in the following functions:
• Maintenance of balance and posture: The cerebellum is important for making postural adjustments in order to
maintain balance.
• Coordination of voluntary movements: Most movements are composed of a number of different muscle groups
acting together in a temporally coordinated fashion. One major function of the cerebellum is to coordinate the timing
and force of these different muscle groups to produce body movements.
• Motor learning: The cerebellum is important for motor learning. The cerebellum plays a major role in adapting and
fine-tuning motor programs to make accurate movements through a trial-and-error process (e.g, learning to hit a
baseball).
• Cognitive functions: Cerebellum is also involved in certain cognitive functions, such as language.

• Pons
Pons literally means bridge. It serves as a relay station between the lower cerebellum and spinal cord and
higher parts of the brain like the cerebrum and mid brain
• Medulla Oblongata
It is the lowermost part of the brain located at the base of the skull and is continuous with the spinal cord.
Vital centres lie in its deeper structure are as follows;
• cardiac centre: Controls the rate and force of cardiac contractions.
• Respiratory centre: Controls the rate and depth of respiration.
• Vasomotor centre: Controls the diameter of blood vessels.
• Reflex centres: Controls reflex of vomiting, coughing, sneezing and swallowing.
In the medulla decussation of pyramids takes place, i.e., the motor nerve form cerebrum to the spinal cord
cross from one side to other in medulla.
This means that the left hemisphere of the cerebrum controls the right half of the body and vice versa.
Midbrain, pons and medulla together form brain stem.

The spinal cord begins at the base of the brain and extends as a slender cord to the level of the
intervertebral disk between the first and second lumbar vertebrae.
The spinal cord is enclosed by a protective vertebral column (vertebrae) and consists of 31
segments, each of which gives rise to a pair of spinal nerves. A cervical enlargement gives rise to nerves
leading to the upper limbs, and a lumbar enlargement gives rise to those innervating the lower limbs.
At the lower end of vertebral column is a thick bundle of elongated nerve roots called Cauda
Equina ('horses' tail’).
A cross-section of the spinal cord shows it is composed of grey matter in the centre surrounded
by white matter.
Grey matter resembles the letter H (butterfly) and consists of mixture of multipolar neuron cell bodies. It
has two prominent projections:
Posterior dorsal horn
Anterior ventral horn
Dorsal Horn: Groups of afferent fibres carrying impulses from peripheral sensoly receptors enter through the
dorsal horn.
Ventral Horn: Nerve fibres exit from here to skeletal muscles.
Spinal Cord

White matter is made up of bundles of
myelinated nerve fibers (nerve tracts). It is
divided into: Posterior funiculi,
Lateral funiculi, anterior funiculi.
Functions of the Spinal Cord:
The spinal cord has two major functions:
• To transmit impulses to and from the
brain, and to house spinal reflexes
• Tracts carrying sensory information to
the brain are called ascending tracts
descending tracts carry motor
information from the brain.

Reflex
Action and
Reflex Arc
The involuntary response towards any
stimulus is known as Reflex Action.
The pathway of reflex action is known as
Reflex Arch. Reflex arch or reflex
pathway consist of afferent neuron and
efferent neuron. The afferent neuron
receives signal from a sensory organ and
transmits the impulse via a dorsal nerve
root into the CNS. The efferent neuron
then carries signals from CNS to the
effector organ or muscle.

One experiences a sudden withdrawal of a body part which comes in contact with objects that are
extremely hot, cold, pointed or animals that are scary or poisonous.
The entire process of response to a peripheral nervous stimulation, that occurs In-voluntarily, i.e.,
without conscious effort or thought and requires the involvement of a part Of the central nervous
system is called a reflex action.
The reflex pathway comprises;
• One afferent neuron (receptor) and one efferent (effector) neuron appropriately arranged in a
series.
• The afferent neuron receives signal from a sensory organ and transmits the impulse Via a
dorsal nerve root into the CNS (at the level of spinal cord).
• The efferent neuron then carries signals from CNS to the effector.
• The stimulus and response thus forms a reflex arc.

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