Unit 1 nervous system

UNIT 1: THE
NERVOUS SYSTEM
(Campbell et.al, 2010)

1. Introduction
Two systems enables animals to
respond to the environment:
• Nervous system
• Endocrine system
What is the difference between the
two systems?

• Each unicellular organism can
respond to stimuli in its environment.
• Animals are multicellular and most
groups respond to stimuli using
systems of neurons.
• The simplest animals with nervous
systems, the cnidarians, have
neurons arranged in nerve nets.

• A nerve net is a series of
interconnected nerve cells.

• More complex animals have nerves
• Nerves are bundles that consist of
the axons of multiple neurons.
AXON

• Bilaterally symmetrical animals
exhibit cephalization.
• Cephalization is the clustering of
sensory organs at the front end of
the body.

• Relatively simple cephalized
animals, such as flatworms, have a
central nervous system (CNS).
• The CNS consists of a brain and
longitudinal nerve cords.

• Annelids and arthropods have
segmentally arranged clusters of
neurons called ganglia.

• In vertebrates:
• The CNS is composed of the brain
and spinal cord
• and the peripheral nervous
system (PNS) is composed of
nerves and ganglia.

• In vertebrates:
• The spinal cord conveys
information from the brain to the
PNS.
• The spinal cord also produces
reflexes independently of the
brain.
• A reflex is the body’s automatic
response to a stimulus.
• For example, a doctor uses a
mallet to trigger a knee-jerk reflex

2. The Human Nervous system
The human nervous system has 3
specific functions:
• Receives sensory input
• It performs integration
• It generates motor output

The human nervous system
CENTRAL NERVE
SYSTEM (CNS)
PERIPHERAL NERVE
SYSTEM (PNS)
Cranial and Spinal
nerves)
BRAIN SPINAL
CORD SOMATIC/MOTOR
NERVOUS SYSTEM
(Skin, senses and
skeletal muscles)
AUTONOMIC
NERVOUS
SYSTEM
(Cardic- ,
smooth muscles
and glands)
SYMPATHETIC PARASYMPATHETIC

a. Human Brain and spinal cord
Consist of 6 major parts:
• Cerebrum
• Cerebellum
• The Diencephalon
• Medulla Oblongata
• Pons
• Spinal cord

• The cerebrum has right and left cerebral
hemispheres.
• Each cerebral hemisphere consists of a
cerebral cortex (gray matter) overlying
white matter and basal nuclei.
• Grey matter: cell bodies of neurons
• White matter: Myelin sheath around axons
of neurons.
• The cerebral cortex consists of folds and
grooves called sulci and gyri.
• The basal nuclei are important centres for
planning and learning movement
sequences

• A thick band of axons called the corpus
callosum provides communication
between the right and left cerebral
corticals
• The right half of the cerebral cortex
controls the left side of the body, and vice
versa.
• Each side of the cerebral cortex has four
lobes: frontal, temporal, occipital, and
parietal lobe.
• Protected by 3 meninges: Dura mater (outer),
Pia mater(inner), Arachnoid (middle).

The human brain (Cerebrum) viewed
from the rear

FUNCTION S OF FOUR LOBES OF
CEREBRAL CORTEX
1. Frontal lobe - motor control, memory,
reasoning, intelligence, imagination,
thinking, judgment. Initiates voluntary
muscle movement.
2. Parietal lobe – Taste, speech,
somatosensory integration
3. Temporal lobe – Hearing and balance,
smell
4. Occipital lobe - Sight

• The Cerebellum lies under the occipital lobe
of cerebrum.
• Divided into 2 cerebellar hemispheres which
is attached by a vermis.
• Consist of parallel grooves.
• Cross section show the white matter in a “tree-
like” shape, with thin grey matter surrounding it
• Functions:
• 1) Maintain posture and balance.
• 2) Ensures smooth, coordinated voluntary
muscle movement.

• The diencephalon develops into three
regions: the epithalamus, thalamus,
and hypothalamus.
• The epithalamus includes the pineal
gland and generates cerebrospinal
fluid from blood. Secrete melatonin
– believed to be involved in jet lag
and insomnia. Also starts puberty.

• Thalamus has grey matter on inside
and white matter on outside.
• Thalamus is located on side and roof
of third ventricle.
• Functions of the thalamus:
1. Receives info via nerves from various
parts of body and sends it to the
appropriate part of the cerebrum for
integration.
2. Involved in emotions and memory.

• The hypothalamus : Forms the floor
of the third ventricle.
• Functions of the HYPOTHALAMUS:
1) Helps to maintain homeostasis by
regulating hunger, sleep, thirst, body
temperature and water balance.
2) Controls the pituitary gland (link
between nervous and endocrine
gland)

• The medulla oblongata:
• Below Pons above spinal cord.
• Contains the reflex centres:
1. Cardiac reflex centre: regulate
heartbeat
2. Respiratory- : regulate breathing
3. Vasomotor- : regulate blood pressure
4. Peristaltic - : regulate peristalsis
5. Regulate reflexes like vomiting,
sneezing, coughing, hiccups

• The Pons:
• Pons in Latin means bridge.
• Contains bundles of axons traveling
between the cerebellum and the rest
of CNS.
• Help Medulla Oblongata to regulate
breathing – has Pneumotaxic- and
Apneustic centers

• The Spinal Cord:
• The spinal cord is an elongated
cylindrical structure, about 45 cm
long.
• It extends from the medulla
oblongata to the second lumbar
vertebrae of the backbone.
• The terminal part of the spinal cord is
called the conus medullaris.

• The spinal cord consists of long tracts of
myelinated nerve fibres (known as white
matter) arranged around a symmetrical
butterfly-shaped cellular matrix of grey
matter.
• The grey matter contains cell bodies of motor
neuron fibres, and interneurons.
• Protected by the vertebral column.
• The central canal of the spinal cord and the
ventricles of the brain are hollow and filled with
cerebrospinal fluid
• The cerebrospinal fluid is filtered from blood
and functions to cushion the brain and spinal
cord.

• The spinal cord functions primarily in
the transmission of neural signals
between the brain and the rest of
the body.
• Also contains neural circuits that can
independently control numerous
reflexes and central pattern
generators.

• Cross section through spinal cord

• REFLEX ARC REACTION OF SPINAL
CORD

B. PERIPHERAL NERVE SYSTEM
• Lies outside the CNS.
• Contains nerves (bundles of axons)
• Humans have 12 pairs of cranial
nerves and 31 pairs of spinal nerves.
• Cranial nerves originate in the brain and
mostly terminate in organs of the head
and upper body
• Spinal nerves originate in the spinal
cord and extend to parts of the body
below the head

• It consist of two functional
components: Somatic (MOTOR)
and autonomic nerve system.
• The motor system carries signals to
skeletal muscles and is voluntary.
• The autonomic nervous system
regulates the internal environment
in an involuntary manner.

• The autonomic nervous system has
sympathetic, parasympathetic, and
enteric divisions.
• The sympathetic and parasympathetic
divisions have antagonistic effects on
target organs.
• The sympathetic division correlates with
the “fight-or-flight” response
• The parasympathetic division promotes a
return to “rest and digest”
• The enteric division controls activity of the
digestive tract, pancreas, and gallbladder.

C. NERVE TISSUE
Consist of:
• Nerve cells called neurons (receives
and conduct impulses to and from CNS
• And supporting cells called neuroglia
(supply support and nourishment to
neurons).

STRUCTURE OF A NEURON
• Vary in appearance depending on their function
and location.
A neuron consist of 3 major parts:
1. Cell body (Contains nucleus and several other
organelles in neuroplasm e.g. Nissl bodies,
Neurofibrils, ext.)
2. Axon (Nerve fiber that transports impulses away
from cell body to another neuron. Always protected by
a Myelin sheath)
3. Dendrite (Short nerve processes that transports
impulses towards the cell body from other neurons or
from the sensory receptors).

Neurons are classified according to structure or
function.
According to structure there are 3 types of
neurons:
a.UNIPOLAR NEURON (Neuron with one process
extending from cell body)
b. BIPOLAR NEURON (Neuron with 2 processes
extending from the cell body)
c. MULTIPOLAR NEURON (Neuron with many
processes extending from the cell body)

TYPES OF NEURONS ACCORDING TO
FUNCTION:
• SENSORY NEURON: Transport impulses from
the sensory receptors to the central nervous
system (CNS)
• INTERNEURON: Convey impulses between
various parts of the CNS like between sensory
and motor neurons. Integration of impulse.
• MOTOR NEURON: Transport impulses from
the CNS to the effector organ (muscle or
gland)

TYPES OF NEURONS ACCORDING TO FUNCTION:
• SENSORY NEURON: Transport impulses from
the sensory receptors to the central nervous
system (CNS)
• INTERNEURON: Convey impulses between
various parts of the CNS like between sensory
and motor neurons. Integration of impulse.
• MOTOR NEURON: Transport impulses from
the CNS to the effector organ (muscle or
gland)

3. TRANSMISSION OF NERVE IMPULSES ACROSS
AN AXON
• Nerve impulses are conducted along
neurons by means of electrochemical
transmission.
• There has to be a significant electrical
potential difference across a membrane
(membrane potential) for an impulse to
be conducted.

RESTING AND ACTION POTENTIAL OF
THE AXONAL MEMBRANE
RESTING POTENTIAL
• When an axon is not conducting an
impulse the membrane potential is -
65mV
• This indicates that the inside of the
axon is more negative [more K-ions and
less Na+ ions] than the outside [less K-
ions and more Na+ ions])

ACTION POTENTIAL BEGINS
• Is a rapid change in polarity across a part of the
axon membrane as the nerve impulse occurs.
• It uses gated channel proteins in axon membrane
to exchange Na+ and K- ions.
• When the Na+ gates open and Na+ moves inside
the axon, depolarization take place.
• The action potential changes from -65mV to +40mV.
• The Na+ gates close and K- gates open and K- leave
the axon
• Action potential change from +40mV to -65mV,
repolarization occurred

ACTION POTENTIAL BEGINS (DEPOLIRIZATION)

ACTION POTENTIAL ENDS (REPOLIRIZATION)

Threshold
• Is the minimum change in polarity across the axon
membrane that is required to generate an action
potential.

Propagation of Action Potentials –
• In nonmyelinated axons the impulse
travels at 1m/sec.
• In myelinated axons, the gated ion
channels (producing an action potential)
are concentrated at the nodes of Ranvier,
therefore the action potential “jumps”
from node to node – this is called
saltatory conduction. Speed – 200m/sec.

4. Chemical transmission across a synapse
• Synapse – Open space between the axon of
one neuron and the dendrite of the following
neuron.
• The membrane of the first neuron is the
presynaptic membrane and the membrane
of the other neuron is called the postsynaptic
membrane.
• Gap between neurons is the synaptic cleft.

Chemical transmission across a synaptic
cleft
• It is carried out by means of neurotrans-
mitters, found in synaptic vesicles.
• Nerve impulses reaches the axon terminal.
• Gated channels for Ca+ open and Ca+ enter
the terminal.
• The rise in Ca+ stimulates synaptic vesicles to
merge with the presynaptic membrane.
• Neurotransmitter molecules are released into
synaptic cleft.

• They diffuse through to the
postsynaptic membrane.
• Where they bind with specific
receptor proteins.
• Taken up by vesicles of dendrite
• Change impulse to electrical
transmission further.

5. Neurotransmitters and
Neuromodulators
Neurotransmitters:
• Acetylcholine (ACh) - Excites skeletal muscles but
inhibits cardiac muscles. Excitatory or inhibitory
effect on smooth muscles and glands
• Norepinephrine (NE)- Important for dreaming,
waking and mood
• Dopamine - Emotions, learning and attention
• Serotonin - Thermoregulation, sleeping, and
perception

Neurotransmitters:
• Neurotransmitters are present in CNS and
PNS.
• After the effect of neurotransmitters, they are
digested by enzymes in the postsynaptic
membrane or retaken up by presynaptic
membrane.
Neuromodulators:
Block the release of neurotransmitters or modify
a neuron’s response to a neurotransmitter.
E.g. endorphins.

5. Neurodisorders
• Disorders of the nervous system include
schizophrenia, depression,
Alzheimer’s disease, Parkinson’s
disease and ADD.
• Genetic and environmental factors
contribute to diseases of the nervous
system.

Depression
• Two broad forms of depressive illness are
known: major depressive disorder and
bipolar disorder
• In major depressive disorder, patients
have a persistent lack of interest or
pleasure in most activities
• Bipolar disorder is characterized by
manic (high-mood) and depressive
(low-mood) phases
• Treatments for these types of depression
include drugs such as Prozac and
lithium

Drug Addiction and the Brain Reward
System
• The brain’s reward system rewards motivation
with pleasure
• Some drugs are addictive because they
increase activity of the brain’s reward system
• These drugs include cocaine, amphetamine,
heroin, alcohol, and tobacco
• Drug addiction is characterized by compulsive
consumption and an inability to control intake

Drug Addiction and the Brain Reward
System
• Addictive drugs enhance the activity of the
dopamine pathway
• Drug addiction leads to long-lasting changes
in the reward circuitry that cause craving for
the drug.

Alzheimer disease
• Alzheimer’s disease is a mental deterioration
characterized by confusion, memory loss, and
other symptoms
• Alzheimer’s disease is caused by the formation
of neurofibrillary tangles and amyloid
plaques in the brain
• A successful treatment in humans may hinge on
early detection of amyloid plaques
• There is no cure for this disease though some
drugs are effective at relieving symptoms

Unit 1 nervous system

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Unit 1 nervous system