 Outlining the mechanism of the Nervous System

o Basic structural features of neurons

o Describing the structure and function of the neurons
o Propagation of the nerve impulse in non-myelinated and myelinated axons
(resting and action potential)
o External and internal ion concentration in a neurone
o Synaptic terminal
o Synaptic transmission
o Components of the brain and spinal cord
o Distinction between spinal reflexes and cranial reflexes

Neuron structure.

The cells of nervous system are called nerve cells or neurons. These are the structural and
functional units of nervous system. Neurons are excitable,i.e.they function by using electrical
stimulation and this electrical message is known as action potential.

Each neuron contains a nerve cell body with a nucleus and organelles such as mitochondria,
endoplasmic reticulum, and Golgi apparatus. Branching off the nerve cell body are the dendrites,
which act like tiny antennae picking up signals from other cells and the axon. The following
diagram illustrate a typical neuron.

A Nerve cell with all its processes is called a neurone. It is the structural and functional unit of
the nervous system. A neurone has cell body called soma and two types of processes called
axons and dendrites.

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 1. CELL BODY (SOMA)

The nucleated cytoplasmic portion of a neurone is termed cell body or soma. Cell body
contains a large spherical central nucleus along with large number of Nissl's granules( groups of
ribosomes) within the cytoplasmic matrix called neuroplasm. The Nissl's granules contain
ribonucleoprotein and are involved in protein synthesis.

The neuroplasm also contains mitochondria, golgi bodies,melanin , lipochrome pigment

granules. The amount of these cell organelles varies with the functional activity of the cell. The
cell body is with non functional centrosome because of which neurones cannot divide.

Function: The cell body (soma) is the factory of the neuron. It produces all the proteins for the
dendrites, axons and synaptic terminals and contains specialized organelles such as the
mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes and
polysomes to provide energy and make the parts, as well as a production line to assemble the
parts into completed products

2. DENDRITES

Delicate cytoplasmic threads called neurofibrils are present throughout the entire length of axon
and dendrites arising from cell body. The cell body and its processes are surrounded externally
by a thin membrane, the neurone membrane.The cell body is present in grey matter of the central
nervous system-brain and spinal cord.

The short cytoplasmic processes of cell body which receives stimulus from other neurone are
called dendrites. The dendrites conduct nerve impulses induced by stimuli towards the cell
body..

Function: Dendrites receives impulses from axon of another neurone through synapse and
conducts the impulse towards the cell body therefore it is called the receptive organ.

3. AXON
The long cytoplasmic process of cell body which transmits impulse from cell body to other
neurone is termed axon. Axon is considerably longer than dendrites. The axon arises from the
cell body and its length varies depending on the functional relationship of the neurone. The
cytoplasm of axon known as axoplasm contains mitochondria, neurofibrils but no nissl's
granules. Axon is present in white matter of central nervous system and peripheral nervous
system. The nerve fibres or axon are covered by a lipid rich membrane called myelin sheath.
The myelin sheath is formed by schwann cells and each schwann cell covers a part of the nerve
fibre. The region where axon is not covered by myelin sheath is the junction of adjacent
myelinated segments called node of ranvier.

Note: The axon hillock is where the axon is joined to the cell. It is from here that the electrical
firing known as an action potential usually occurs.

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Function : Axon transmits impulse from cell body of one neurone to dendron of another neurone
through synapse.

TYPES OF NERVE CELLS

Neurons are classified into three categories by location and four by shape.

Location

(1) Afferent neurons, whose cell bodies lie outside the CNS, (2) interneurons, contained
entirely within the CNS, and (3) efferent neurons, whose cell bodies are in the CNS but
whose axons lead away to other organs.
Shape

Unipolar neurons has a nerve cell body with a single process or fiber which acts as both as axon
and dendron.; common in insects (3) bipolar neurons, which have one axon and one dendrite;
they are found in the retina of the eye, roof of nasal cavity and inner ear .As such they are
pathway for smell, sight, taste and hearing. and (3) multipolar neurons, have one axon and
multiple dendrites. Many dendrites which allows the integration of information from other
neurons.The cerebral cortex contains multiple neurons.

Note

Sensory neurons (afferent neurons) are unipolar, bipolar, or multipolar shaped cells that
conduct action potentials toward or into the central nervous system. They carry somatic nervous
system signals from the skin, joints, skeletal muscles, sensory organs (eyes, ears, mouth, and
nose). They also carry autonomic nervous system signals from the visceral organs (heart, lungs,
vessels, etc

Motor neurons are (efferent neurons; lower motor neurons) are multipolar shaped cells that
conduct action potentials out of the central nervous system.

Their cell bodies and dendrites are located in the central nervous system and their axons run
inside the nerves to the peripheral organs.
Autonomic neurone cells are nerve cells in the spinal cord characterized by an axon that leaves
the central nervous system to establish a functional connection with an effector (muscle or
glandular) tissue

Key concepts

A neuron is an individual cell, whereas, a group of neurons form a nerve.

2.There are two types of neurons ‘“ sensory and motor neurons; while there are three types of
nerves ‘“ afferent, efferent and mixed nerves.
3.Nerves are found in the peripheral nervous system, while neurons are found in the brain, spinal
cord and the peripheral nerves.
4.A neuron can also be called a neurone or a nerve cell.

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5. Neurons conduct nerve impulses, while nerves transmit information to various parts of the
body.

THE CENTRAL AND PERIPHERAL NERVOUS SYSTEMS

The nervous system can be divided into two major regions: the central and peripheral nervous
systems. The central nervous system (CNS) is the brain and spinal cord, and the peripheral
nervous system (PNS) is everything else, which consist of bundles of axons (which are also
called fibers). The brain is contained within the cranial cavity of the skull, and the spinal cord is
contained within the vertebral cavity of the vertebral column.
Nerve Fiber:

Nerve fiber is a thread like extension of a neuron, which is formed by the axon and its covering.
Thus each nerve fiber is an axon with its coverings. Larger axons are covered by a myelin sheath
and are termed myelinated or medullated fibers. Thinner axons, of less than one micron
diameter, do not have the myelin sheath and are therefore termed non-myelinated or non-
medullated.

Myelinated Fibers:
Myelinated fibers form the bulk of the somatic nerves. The somatic nervous
system (SoNS or voluntary nervous system) is the part of the peripheral nervous
system associated with the voluntary control of body movements via skeletal
muscles. The SoNS consists of afferent nerves or sensory nerves, and efferent
nerves or motor nerves.
Non-Myelinated Fibers:
Non-myelinated fibers comprise the smaller axons of the CNS, in addition to
peripheral autonomic fibers, several types of fine sensory fibers (C fibers of skin,
muscle and viscera), olfactory nerves are part of non-myelinated fibers etc.

PROPAGATION OF THE NERVE IMPULSE IN NON-MYELINATED AND

MYELINATED AXONS (RESTING AND ACTION POTENTIAL)

Neurons send messages electrochemically. This means that chemicals cause an electrical signal.
Chemicals in the body are "electrically-charged" -- when they have an electrical charge, they are
called ions. The important ions in the nervous system are sodium and potassium (both have 1
positive charge, +), calcium (has 2 positive charges, ++) and chloride (has a negative charge, -).
There are also some negatively charged protein molecules. It is also important to remember that
nerve cells are surrounded by a membrane that allows some ions to pass through and blocks the
passage of other ions. This type of membrane is called semi-permeable.

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RESTING MEMBRANE POTENTIAL

When a neuron is not sending a signal, it is "at rest." When a neuron is at rest, the inside of the
neuron is negative relative to the outside. Although the concentrations of the different ions
attempt to balance out on both sides of the membrane, they cannot because the cell membrane
allows only some ions to pass through channels (ion channels). At rest, potassium ions (K+) can
cross through the membrane easily. Also at rest, chloride ions (Cl-) and sodium ions (Na+) have a
more difficult time crossing. The negatively charged protein molecules (A-) inside the neuron
cannot cross the membrane. In addition to these selective ion channels, there is a pump that uses
energy to move three sodium ions out of the neuron for every two potassium ions it puts in.
Finally, when all these forces balance out, and the difference in the voltage between the inside
and outside of the neuron is measured, you have the resting potential. The resting membrane
potential of a neuron is about -70 mV (mV=millivolt) - this means that the inside of the neuron is
70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron
and more potassium ions inside that neuron.

The concentration of ions in neuron cell (resting membrane potential)

ACTION POTENTIAL

The resting potential tells about what happens when a neuron is at rest. An action potential
occurs when a neuron sends information (stimulus) down an axon, away from the cell body.
Neuroscientists use other words, such as a "spike" or an "impulse" for the action potential. The

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action potential is an explosion of electrical activity that is created by a depolarizing current.
This means that some event (a stimulus) causes the resting potential to move toward 0 mV.
When the depolarization reaches about -55 mV a neuron will fire an action potential. This is the
threshold. If the neuron does not reach this critical threshold level, then no action potential will
fire. There are no big or small action potentials in one nerve cell - all action potentials are the
same size. Therefore, the neuron either does not reach the threshold or a full action potential is
fired - this is the "ALL OR NONE" principle

Action potentials are caused when different ions cross the neuron membrane. A stimulus first
causes sodium channels to open. Because there are many more sodium ions on the outside, and
the inside of the neuron is negative relative to the outside (due to some negative charged
proteins), hence sodium ions rush into the neuron. Remember, sodium has a positive charge, so
the neuron becomes more positive and becomes depolarized (excess positive ions).

It takes longer for potassium channels to open. When they do open, potassium rushes out of the
cell, reversing the depolarization. Also at about this time, sodium channels start to close. This
causes the action potential to go back toward -70 mV (a repolarization). The action potential
actually goes past -70 mV (a hyperpolarization) because the potassium channels stay open a bit
too long. Gradually, the ion concentrations go back to resting levels and the cell returns to -70
mV. The graph below is the summary of the process of the action potential.

PROPAGATION OF THE NERVE IMPULSE IN NON-MYELINATED AND

MYELINATED AXONS

Myelinated axon impulse propagation

Nerve impulse “jumps” from neurofibril node to neurofibril node .This process is called
salutatory conduction. The movement of the impulses in the myelinated axon requires less

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energy (ATP) and the impulses move fast because the nerve impulse has to be generated at only
at the Ranvier nodes and not repeatedly.

Non Myelinated axon impulse propagation

Nerve impulses must travel the entire length of the axon and this process is called continuous
conduction. .Hence nerve impulses takes longer to reach the end of the axon.

Synaptic terminal
Nerve impulses (action potential) are usually carried to the neighboring cell by chemicals called
neurotransmitters, which are released by the nerve cell and are taken up by another cell on the
other side of the synapse. In a neuron, synaptic vesicles store various neurotransmitters that are
released at the synapse

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Chemical synapses are biological junctions through which neurons' signals can be exchanged to
each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses
allow neurons to form circuits within the central nervous system. They are crucial to the
biological computations that underlie perception and thought. They allow the nervous system to
connect to and control other systems of the body.

At a chemical synapse, one neuron releases neurotransmitter molecules into a small space (the
synaptic cleft) that is adjacent to another neuron. The neurotransmitters are kept within small
sacs called vesicles, and are released into the synaptic cleft by exocytosis. These molecules then
bind to receptors on the postsynaptic cell's side of the synaptic cleft. Finally, the
neurotransmitters must be cleared from the synapse through one of several potential mechanisms
including enzymatic degradation or re-uptake by specific transporters either on the presynaptic
cell or possibly by neuroglia to terminate the action of the transmitter.

Synaptic transmission

1. Synthesis of the neurotransmitter e.g adrenaline, dopamine etc. This can take place in the
cell body, in the axon, or in the axon terminal.
2. Storage of the neurotransmitter in storage granules or vesicles in the axon terminal.
3. Calcium enters the axon terminal during an action potential, causing release of the
neurotransmitter into the synaptic cleft.
4. After its release, the transmitter binds to and activates a receptor in the postsynaptic
membrane, causing ionic channels on the membrane to either open or close. When these

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 channels open, depolarization occurs, resulting in the initiation of another action
potential
5. Deactivation of the neurotransmitter. The neurotransmitter is either destroyed
enzymatically, or taken back into the terminal from which it came, where it can be
reused, or degraded and removed

THE BRAIN AND SPINAL CORD

The brain is a complex organ made up of specialized nerve and supportive tissues. It’s
surrounded by many bones that together form the skull. The part of the skull where the brain sits
is called the cranium. The base, or lower part, of the brain is connected to the spinal cord.
Together, the brain and spinal cord are known as the central nervous system (CNS). Many nerves
send electrical signals to and from the brain and spinal cord.

Structure and function of the brain

The brain is the body’s control centre. It constantly receives and interprets nerve signals from the
body and sends new signals based on this information. Different parts of the brain control
movement, speech, emotions, consciousness and internal body functions, such as heart rate,
breathing and body temperature.

The brain has 3 main parts: cerebrum, cerebellum and brain stem (medulla oblongata).

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PARTS OF THE BRAIN

The cerebrum is the largest part of the brain. It is divided into 2 halves called the left and right
cerebral hemispheres. The 2 hemispheres are connected by a bridge of nerve fibres called the
corpus callosum.

The right half of the cerebrum (right hemisphere) controls the left side of the body. The left half
of the cerebrum (left hemisphere) controls the right side of the body.

The cerebral cortex is the outer, folded part of the brain. It is also called the grey matter. The
cerebral cortex is mostly made up of the cell bodies and dendrites of nerve cells (neurons). Cell
bodies contain the nucleus and other main parts of the cell. Dendrites are the short branching
fibres that receive signals from other nerve cells. The inner part of the cerebrum is called the
white matter. It is mostly made up of the long fibres of a nerve cell (called axons) that send
signals to and from the brain to the rest of the body. The fatty coating that surrounds axons
(called myelin) gives this part of the brain a whitish appearance.

Each hemisphere is divided into 4 sections called lobes. These include the frontal, parietal,
temporal and occipital lobes.

Each lobe has different functions:

The frontal lobe controls movement, speech, behaviour, memory, emotions and intellectual
functions, such as thought processes, reasoning, problem solving, decision-making and planning.

The parietal lobe controls sensations, such as touch, pressure, pain and temperature. It also
controls the understanding of size, shape and direction (called spatial orientation).

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 The temporal lobe controls hearing, memory and emotions. The dominant (left side in most
right-handed people) temporal lobe also controls speech.

The occipital lobe controls vision.

Cerebellum

The cerebellum is located under the cerebrum at the back of the brain. It is divided into 2 parts or
hemispheres and also has grey and white matter.

The cerebellum is responsible for: movement, posture, balance, reflexes, complex actions
(walking, talking) and collecting sensory information from the body

Brain stem

The brain stem is a bundle of nerve tissue at the base of the brain. It connects the cerebrum and
cerebellum to the spinal cord.

The brain stem has 3 areas:

 midbrain (also called the mesencephalon)

 pons
 medulla oblongata

The brain stem sends information to and from the other parts of the brain to the rest of the body
and controls:

 breathing
 body temperature
 blood pressure
 heart rate
 hunger and thirst
 digestion of food

Cerebrospinal fluid (CSF)

The cerebrospinal fluid (CSF) is a clear, watery liquid that surrounds, cushions and protects the
brain and spinal cord. The CSF also carries nutrients in the blood to (and removes waste products
from) the brain. It circulates through chambers called ventricles and over the surface of the brain
and spinal cord.

Meninges

The brain and spinal cord are covered and protected by 3 layers of tissue (membranes) called the
meninges:

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Other parts of the brain
Corpus callosum
The corpus callosum is a bundle of nerve fibres that allows communication between the 2
cerebral hemispheres. It is the largest fibre bundle in the brain.
Thalamus
The thalamus is a structure in the middle of the brain that has 2 lobes or sections. It acts as a
relay station for almost all information that comes and goes between the brain and the rest of the
nervous system in the body.
Hypothalamus
The hypothalamus is a small structure in the middle of the brain below the thalamus. It plays a
part in controlling body temperature, hormone secretion, blood pressure, emotions, appetite and
sleep patterns.
Pituitary gland
The pituitary gland is a small, pea-sized organ in the centre of the brain. It is attached to the
hypothalamus and makes a number of different hormones that affect other glands of the body’s
endocrine system. It receives messages from the hypothalamus and releases hormones that
control the thyroid and adrenal gland, as well as growth and physical and sexual development.
Pineal gland
The pineal gland is a very small gland in the third ventricle of the brain. It produces the hormone
melatonin, which influences sleeping and waking patterns and sexual development.

DISTINCTION BETWEEN SPINAL REFLEXES AND CRANIAL REFLEXES

spinal reflexes involve more than one synapse ; an example is the withdrawal reflex of the hand
from a painful stimulus (such as fire). Cranial reflexes are mediated by pathways in the cranial
nerves and brain; examples are the blinking and swallowing reflexes. Below is An example of
the spinal reflex

An example of the spinal reflex

The end!
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