Nervous system

Nervous system
By, K. P. Komal, Assistant Professor, Govt. Science College, Chitradurga. 1
Lecture Notes
On
Nervous system
By,
K. P. KOMAL
ASSISTANT PROFESSOR
DEPARTMENT OF BIOCHEMISTRY
GOVERNMENT SCIENCE COLLEGE, CHITRADURGA. 577501
KARNATAKA STATE.

Nervous system
The Nervous System
 The nervous system is a complex, highly coordinated network of tissues that
communicate via electro chemical signals.
 It is responsible for receiving and processing information in the body and is
divided into two main branches:
 The central nervous system and
 The peripheral nervous system.
The Central Nervous System
 The central nervous system receives and processes information from the senses.
 The brain and the spinal cord make up the central nervous system.
 Both organs lie in a fluid called the cerebrospinal fluid, which cushions and
nourishes the brain.
 The blood-brain barrier protects the cerebrospinal fluid by blocking many drugs
and toxins. This barrier is a membrane that lets some substances from the blood
into the brain but keeps out others.
 The spinal cord connects the brain to the rest of the body. It runs from the brain
down to the small of the back and is responsible for spinal reflexes, which are
automatic behaviors that require no input from the brain.
 The spinal cord also sends messages from the brain to the other parts of the body
and from those parts back to the brain.

Nervous system
 The brain is the main organ in the nervous system. It integrates information from
the senses and coordinates the body’s activities. Different parts of the brain do
different things.
The Peripheral Nervous System
 All the parts of the nervous system except the brain and the spinal cord belong to
the peripheral nervous system.
 The peripheral nervous system has two parts: the somatic nervous system and the
autonomic nervous system.
The Somatic Nervous System
The somatic nervous system consists of nerves that connect the central nervous system
to voluntary skeletal muscles and sense organs. Voluntary skeletal muscles are muscles
that help us to move around. There are two types of nerves in the somatic nervous
system:
 Afferent nerves carry information from the muscles and sense organs to the
central nervous system.
 Efferent nerves carry information from the central nervous system to the muscles
and sense organs.
The Autonomic Nervous System
The autonomic nervous system consists of nerves that connect the central nervous
system to the heart, blood vessels, glands, and smooth muscles. Smooth muscles are
involuntary muscles that help organs such as the stomach and bladder carry out their
functions. The autonomic nervous system controls all the automatic functions in the
body, including breathing, digestion, sweating, and heartbeat. The autonomic nervous
system is divided into the sympathetic and parasympathetic nervous systems.
 The sympathetic nervous system gets the body ready for emergency action. It is
involved in the fight-or-flight response, which is the sudden reaction to stressful
or threatening situations. The sympathetic nervous system prepares the body to

Nervous system
meet a challenge. It slows down digestive processes, draws blood away from the
skin to the skeletal muscles, and activates the release of hormones so the body can
act quickly.
 The parasympathetic nervous system becomes active during states of relaxation. It
helps the body to conserve and store energy. It slows heartbeat, decreases blood
pressure, and promotes the digestive process.
Neurons: Cells of the Nervous System
There are two kinds of cells in the nervous system: glial cells and neurons. Glial cells,
which make up the support structure of the nervous system, perform four functions:
 Provide structural support to the neurons
 Insulate neurons
 Nourish neurons
 Remove waste products
The other cells, neurons, act as the communicators of the nervous system. Neurons
receive information, integrate it, and pass it along. They communicate with one
another, with cells in the sensory organs, and with muscles and glands.
Each neuron has the same structure:
 Each neuron has a soma, or cell body, which is the central area of the neuron. It
contains the nucleus and other structures common to all cells in the body, such as
mitochondria.
 The highly branched fibers that reach out from the neuron are called dendritic
trees. Each branch is called a dendrite. Dendrites receive information from other
neurons or from sense organs.
 The single long fiber that extends from the neuron is called an axon. Axons send
information to other neurons, to muscle cells, or to gland cells. What we call
nerves are bundles of axons coming from many neurons.

Nervous system
 Some of these axons have a coating called the myelin sheath. Glial cells produce
myelin, which is a fatty substance that protects the nerves. When an axon has a
myelin sheath, nerve impulses travel faster down the axon. Nerve transmission
can be impaired when myelin sheaths disintegrate.
 At the end of each axon lie bumps called terminal buttons or synaptic bulbs.
Terminal buttons release neurotransmitters, which are chemicals that can cross
over to neighboring neurons and activate them. The junction between an axon of
one neuron and the cell body or dendrite of a neighboring neuron is called a
synapse.
Role of Myelin
People with multiple sclerosis have difficulty with muscle control because the
myelin around their axons has disintegrated. Another disease, poliomyelitis, commonly
called “polio,” also damages myelin and can lead to paralysis.
Types of Neurons:
There are three major types of neurons: sensory neurons, motor neurons, and
interneurons. All three have different functions, but the brain needs all of them to
communicate effectively with the rest of the body (and vice versa).
Sensory Neurons
 Sensory neurons are neurons responsible for converting external stimuli from the
environment into corresponding internal stimuli.

Nervous system
 They are activated by sensory input, and send projections to other elements of
the nervous system, ultimately conveying sensory information to the brain or
spinal cord.
 Unlike the motor neurons of the central nervous system (CNS), whose inputs
come from other neurons, sensory neurons are activated by physical modalities
(such as visible light, sound, heat, physical contact, etc.) or by chemical signals
(such as smell and taste).
 Most sensory neurons are pseudounipolar, meaning they have an axon that
branches into two extensions—one connected to dendrites that receive sensory
information and another that transmits this information to the spinal cord.
Motor Neurons
 Motor neurons are neurons located in the central nervous system, and they
project their axons outside of the CNS to directly or indirectly control muscles.
 The interface between a motor neuron and muscle fiber is a specialized synapse
called the neuromuscular junction. The structure of motor neurons is multipolar,
meaning each cell contains a single axon and multiple dendrites. This is the most
common type of neuron.
Interneurons
 Interneurons are neither sensory nor motor; rather, they act as the "middle men"
that form connections between the other two types.
 Located in the CNS, they operate locally, meaning their axons connect only with
nearby sensory or motor neurons.
 Interneurons can save time and therefore prevent injury by sending messages to
the spinal cord and back instead of all the way to the brain.
 Like motor neurons, they are multipolar in structure.

Nervous system
Structural diversity in neurons:
1) Multipolar neurons: usually have
several dendrites and one axon.
Mostly present in brain and spinal
cord.
2) Bipolar neurons: usually have one
main dendrite and one axon. They
are found in the eye, inner ear and
in the olfactory area of the brain.
3) Unipolar neurons: are sensory
neurons that originate in the
embryo as the bipolar neurons.
During development the axon and
dendrites fuse into a single process that devides into two branches a short
distance from the cell body.
Interneurons are often named for the histologists who first describe them.
4) Purkinje cells: usually found in cerebellum.
5) Pyramidal cells: found in brain, have a cell body shape like pyramid.
Neuroglia: in Central nervous system and peripheral nervous system:

Nervous system
1) Astrocytes are the most numerous type of glial cell. In fact, they are the most
numerous cells in the brain. Astrocytes come in different types and have a variety
of functions.
 They help regulate blood flow in the brain,
 maintain the composition of the fluid that surrounds neurons, and
 Regulate communication between neurons at the synapse.
 During development, astrocytes help neurons find their way to their
destinations and contribute to the formation of the blood-brain barrier,
which helps isolate the brain from potentially toxic substances in the blood.
 Take up excess neurotransmitter.
 Participate in metabolism of neurotransmitters.
 Maintain poper balace of Ca 2+ &K+.
2) Microglia is related to the macrophages of the immune system and act as
scavengers to remove dead cells and other debris.
3) Oligodendrocytes: The oligodendrocytes of the CNS and the Schwann cells of the
PNS share a similar function. Both of these types of glial cells produce myelin, the
insulating substance that forms a sheath around the axons of many neurons.
Myelin dramatically increases the speed with which an action potential travels
down the axon, and it plays a crucial role in nervous system function.
4) Ependymal cells, which line the ventricles of the brain and the central canal of
the spinal cord, have hair like cilia that beat to promote circulation of the
cerebrospinal fluid found inside the ventricles and spinal canal.
5) Satellite glial cells cover the cell bodies of neurons in PNS ganglia. Satellite glial
cells are thought to support the function of the neurons and might act as a
protective barrier, but their role is still not well-understood.

Nervous system
Communication between Neurons
In 1952, physiologists Alan Hodgkin and Andrew Huxley made some important
discoveries about how neurons transmit information. They studied giant squid, whose
neurons have giant axons. By putting tiny electrodes inside these axons, Hodgkin and
Huxley found that nerve impulses are really electrochemical reactions.
Neurons: Cells of the Nervous System
The Resting Potential
Nerves are specially built to transmit electrochemical signals. Fluids exist both
inside and outside neurons. These fluids contain positively and negatively charged atoms
and molecules called ions. Positively charged sodium and potassium ions and negatively
charged chloride ions constantly cross into and out of neurons, across cell membranes.
An inactive neuron is in the resting state. In the resting state, the inside of a neuron
has a slightly higher concentration of negatively charged ions than the outside does. This
situation creates a slight negative charge inside the neuron, which acts as a store of
potential energy called the resting potential. The resting potential of a neuron is about
–70 millivolts.

Nervous system
Graded potential:
When stimulus causes ligand gated or mechanically gated ion channel to open or
close in an excitable cell’s plasma membrane that, cell produces a graded potential.
When a response is more negative polarization, it is termed a hyperpolarizing graded
potential when the response is less negative polarization, it is termed a depolarizing
graded potential.
Usually ligand gated and mechanically gated ion channels are present in the
dendrites of sensory neurons and ligand gated ion channels are most numerous in the
dendrites and cell bodies of interneurons and motor neurons. Graded potential is useful
for short distance communication.
The Action Potential
When something stimulates a neuron, gates, or channels, in the cell membrane
open up, letting in positively charged sodium ions. For a limited time, there are more
positively charged ions inside than in the resting state. This creates an action potential,
which is a short-lived change in electric charge inside the neuron. The action potential
zooms quickly down an axon. Channels in the membrane close and no more sodium ions
can enter. After they open and close, the channels remain closed for a while. During the
period when the channels remain closed, the neuron can’t send impulses. This short
period of time is called the absolute refractory period, and it lasts about 1–2
milliseconds. The absolute refractory period is the period during which a neuron lies
dormant after an action potential has been completed.

Nervous system
The Synapse
The gap between two cells at a synapse is called the synaptic cleft. The signal-
sending cell is called the presynaptic neuron, and the signal-receiving cell is called the
postsynaptic neuron.
Neurotransmitters are the chemicals that allow neurons to communicate with
each other. These chemicals are kept in synaptic vesicles, which are small sacs inside the
terminal buttons. When an action potential reaches the terminal buttons, which are at
the ends of axons, neurotransmitter-filled synaptic vesicles fuse with the presynaptic
cell membrane. As a result, neurotransmitter molecules pour into the synaptic cleft.
When they reach the postsynaptic cell, neurotransmitter molecules attach to matching
receptor sites. Neurotransmitters work in much the same way as keys. They attach only
to specific receptors, just as certain keys fit only certain locks.
When a neurotransmitter molecule links up with a receptor molecule, there’s a
voltage change, called a postsynaptic potential (PSP), at the receptor site. Receptor sites
on the postsynaptic cell can be excitatory or inhibitory.
There are two types of synapses that differ both structurally and functionally,
1) Electrical synapse:
In an electrical synapse ionic current spreads directly between adjucent cells
through gap junctions. Each gap junction contains a hundred or so tubular

Nervous system
proteins called connexons that form tunnels connecting the cytosol of the two
cells. Both ions and molecules are able to flow back and forth through the
connexons between adjucent cells. In the case of ions, this provides a path for
flow of current. Gap junctions are common in visceral smooth muscle, cardiac
muscle and developing embryo.
Electrical synapse have three advantages. They are,
a) Faster communication: comparing to chemical synapse it is faster
communication because action potential conduct directly across gap
junction.
b) Synchronization: electrical synapse can synchronise the activity of group of
neurons or muscle fibers.
c) Two way transmission: it allows two way transmission of action potential.
2) Chemical synapse:
Presynaptic and postsynaptic neurons of a chemical synapseare in close proximity,
plasma membrane do not touch. They are separated by a synaptic cleft, a 20-50nm
space filled with intestinal fluid. The presynaptic neuron releases a neurotransmitters
that diffuse across the synaptic cleft and acts as receptor in plasma membrane of the
post synaptic neuron to produce a postsynaptic potential, a type of graded potential. In
essence the presynaptic electrical signal is converted into chemical signal. The post
synaptic neuron receives the chemical signal and in turn generates an electrical signal.
The time required at the process of chemical synapse , the synaptic delay is about 0.5
msec.
A typical chemical synapse transmits a signal as follows:
1) An action potential arrives at a synaptic bulb of presynaptic axon.
2) The depolarising phase of action potential opens voltage gated Ca2+ channels.in
addition to this Na+ are also opened.

Nervous system
3) An increase in concentration of Ca2+ inside presynaptic neuron triggers exocytosis
of some synaptic vesicles, which inturn releases neurotransmitters. Single vesicle
contains thousand molecules of neurotransmitters.
4) The neurotransmitters diffuse across the synaptic cleft and bind to
neurotransmitter receptor in a post symnaptic neuuron’s plasma membrane.
5) Binding of neurotransmitter molecules to their receptors on ligand gated ion
channels opens the channel and allows ions to flow across the membrane.
6) Depending on which ions the channel admit, the ionic flow causes depolarization
or hyperpolarization of post synaptic membrane. Opening of Na+ which causes
depolarization, however opening of Cl- channel allows hyperpolarization.
7) If the depolarization reaches threshold, one or more action potentials are
generated. It is only way of transmission of action potential.

Nervous system
Neurotransmitters
So far, researchers have discovered about 15–20 different neurotransmitters,
and new ones are still being identified. The nervous system communicates accurately
because there are so many neurotransmitters and because neurotransmitters work only
at matching receptor sites. Different neurotransmitters do different things.
Neurotransmitter Major functions
Excess is
associated
with
Deficiency is
associated
with
Acetylcholine
Muscle movement, attention, arousal,
memory, emotion
Alzheimer’s
disease
Dopamine
Voluntary movement, learning, memory,
emotion
Schizophren
ia
Parkinsonism
Serotonin
Sleep, wakefulness, appetite, mood,
aggression, impulsivity, sensory perception,
temperature regulation, pain suppression
Depression
Endorphins Pain relief, pleasure
Norepinephrine
Learning, memory, dreaming, awakening,
emotion, stress-related increase in heart
rate, stress-related slowing of digestive
processes
Depression
GABA
Main inhibitory neurotransmitter in the
brain
Glutamate
Main excitatory neurotransmitter in the
brain
Multiple
sclerosis

Nervous system
Agonists and Antagonists
Agonists are chemicals that mimic the action of a particular neurotransmitter. They
bind to receptors and generate postsynaptic potentials.
Nicotine and Receptors
Nicotine is an acetylcholine agonist, which means that it mimics acetylcholine closely
enough to compete for acetylcholine receptors. When both nicotine and acetylcholine
attach to a receptor site, the nerve fibers become highly stimulated, producing a feeling
of alertness and elation.
Antagonists are chemicals that block the action of a particular neurotransmitter. They
bind to receptors but can’t produce postsynaptic potentials. Because they occupy the
receptor site, they prevent neurotransmitters from acting.
Homeostatic Control Systems
 Homeostasis is maintained by the concerted effort of body systems
communicating via both electrical (nervous) and chemical (hormonal) systems.
 Both nerves and hormones are specific in their actions - nerves terminate in
specific parts of the organism, while hormones only produce activity in specific
target cells.
 The actions of both nerves and hormones involve chemical substances - hormones
are chemicals themselves, while nerves use chemicals called neurotransmitters to
facilitate electrical signalling.
 Nerves tend to bring about a response very rapidly, while hormonal responses are
much slower but tend to be longer lasting.
 The initiation of homeostatic responses results from an external or internal
stimulus, which is detected by a specific type of receptor.
Types of Receptors:

Nervous system
Receptors are either proteins or glycoproteins, that binds ligands with high
affinity cells or components of cells that responds to sensory stimuli are also refered as
receptors. Recep;tors forms the interface between their surroundings and nervous
system. They usually detect the perticular change in the environment known as the
adiquote stimulus and transduce that into electrical activity. Sensory receptors are the
link by whhich all information about the internal and external environment enters the
nervous system. They are the structures which convert different forms of stimulus
energy into nerve impulse. There are different types of sensory receptors they are,
Photoreceptors are also known as electromagnetic receptors. There are one more
receptors that is Nocireceptor or pain receptor which sences damage occuring in tissues
whether physical or chemical damage.
Homeostatic Control via the Nervous System
Thermoregulation
Animals capable of temperature regulation within a given range are called
homeotherms and maintain a constant body temperature through a negative feedback
loop
 The hypothalalmus acts as a control centre in thermoregulation by detecting
fluctuations in body temperature

Nervous system
 The skin also possesses thermoreceptors and relays this information to the
hypothalamus, which coordinates corrective measures
When body temperature rises, the following cooling mechanisms may occur:
 Vasodilation: The skin arterioles dilate, bringing blood into closer proximity to the
body surface and allowing for heat transfer (convective cooling)
 Sweating: Sweat glands release sweat, which which is evaporated at the cost of
latent heat in the air, thus cooling the body (evaporative cooling)
When body temperature falls, the following heating mechanisms may occur:
 Vasoconstriction: The skin arterioles constrict, moving blood away from the
surface of the body, thus retaining the heat carried within the blood
 Shivering: Muscles begin to shake in small movements, expending energy through
cell respiration (which produces heat as a by-product)
Other mechanisms through which homeotherms may regulate their body temperature
include:
 Piloerection: Animals with furry coats can make their hair stand on end
(piloerection), trapping pockets of warm air close to the body surface
 Behavioural responses: Animals may physically respond to environmental
conditions in a bid to regulate temperature (e.g. bathing, burrowing, etc.)

Nervous system

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