UNIT 2:

NERVOUS SYSTEM

What is the Nervous System?

The nervous system or the neural system is a complex network
of neurons specialized to carry messages. The complexity of the
nervous system increases as we move towards higher animals.
For instance, cnidarians such as jellyfish have relatively simple
nerve nets spread throughout their body. Crabs have a more
complicated nervous system in the form of 2 nerve centers
called dorsal ganglion and ventral ganglion.
As we move further up the ladder, higher organisms such as
vertebrates have a developed brain. Moreover, it is one of the
most complicated structures in the animal kingdom, containing
billions of neurons, all intricately connected.
In the human body, the neural system integrates the activities of
organs based on the stimuli, which the neurons detect and
transmit. They transmit messages in the form of electrical
impulses and convey messages to and from the sense organs.
Thus, the nervous coordination involves the participation of the
sense organs, nerves, spinal cord, and brain.
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One of the most complex organ system to ever evolve, the
human nervous system consists of two parts, namely:
 1. Central Nervous System (consists of the brain and spinal
cord)
2. Peripheral Nervous System (includes all the nerves of the
body)
Central Nervous System
Central Nervous System (CNS) is often called the central
processing unit of the body. It consists of the brain and the
spinal cord.
Brain
The brain is one of the important, largest and central organ of
the human nervous system. It is the control unit of the nervous
system, which helps us in discovering new things, remembering
and understanding, making decisions, and a lot more. It is
enclosed within the skull, which provides frontal, lateral and
dorsal protection. The human brain is composed of three major
parts:
1. Forebrain
2. Midbrain
3. Hindbrain
Forebrain
The Cerebrum: Also known as the cerebral cortex, the cerebrum
is the largest part of the human brain, and it is associated with
higher brain function such as thought and action. Nerve cells
make up the gray surface, which is a little thicker than our
thumb. White nerve fibers beneath the surface carry signals
between nerve cells in other parts of the brain and body. Its
wrinkled surface increases the surface area, and is a six-layered
structure found in mammals, called the neocortex. It is divided
into four sections, called “lobes”. They are; the frontal lobe, the
parietal lobe, the occipital lobe and the temporal lobe.

Functions Of The Lobes:

Frontal Lobe – The frontal lobe lies just beneath our forehead and
is associated with our brain’s ability to reason, organize, plan,
speak, move, make facial expressions, serial task, problem solve,
control inhibition, spontaneity, initiate and self-regulate
behaviors, pay attention, remember and control emotions.
Parietal Lobe – The parietal lobe is located at the upper rear of
our brain, and controls our complex behaviors, including senses
such as vision, touch, body awareness and spatial orientation. It
plays important roles in integrating sensory information from
various parts of our body, knowledge of numbers and their
relations, and in the manipulation of objects. Portions are
involved with our visuospatial processing, language
comprehension, the ability to construct, body positioning and
movement, neglect/inattention, left-right differentiation and self-
awareness/insight.
Occipital Lobe – The occipital lobe is located at the back of our
brain, and is associated with our visual processing, such as visual
recognition, visual attention, spatial analysis (moving in a 3-D
world) and visual perception of body language; such as postures,
expressions and gestures.
Temporal Lobe – The temporal lobe is located near our ears, and
is associated with processing our perception and recognition of
auditory stimuli (including our ability to focus on one sound
among many, like listening to one voice among many at a party),
comprehending spoken language, verbal memory, visual memory
and language production (including fluency and word-finding),
general knowledge and autobiographical memories.

Midbrain
The midbrain is located below the cerebral cortex, and above the
hindbrain placing it near the center of the brain. It is comprised
of the tectum, tegmentum, cerebral aqueduct, cerebral peduncles
and several nuclei and fasciculi. The primary role of the midbrain
is to act as a sort of relay station for our visual and auditory
systems. The midbrain is the smallest region of the brain, and is
located most centrally within the cranial cavity.
Thalamus – the primary role of the thalamus is to relay sensory
information from other parts of the brain to the cerebral cortex
Hypothalamus – the primary role of the hypothalamus is to
regulate various functions of the pituitary gland and endocrine
activity, as well as somatic functions e.g.body temperature,
sleep, appetite.
Pituitary Gland – the primary role of the pituitary gland is an
important link between the nervous system and the endocrine
system. It releases many hormones which affect growth,
metabolism, sexual development and the reproduction system. It
is connected to the hypothalamus and is about the size of a pea.
It is located in the center of the skull, just behind the bridge of
the nose.

Hindbrain
The Cerebellum – The cerebellum, or “little brain”, is similar to
the cerebrum with its two hemispheres and highly folded surface.
It is associated with regulation and coordination of movement,
posture, balance and cardiac, respiratory and vasomotor centers.
Brain Stem – The brain stem is located beneath the limbic
system. It is responsible for vital life functions such as breathing,
heartbeat, and blood pressure. The brain stem is made of the
midbrain, pons, and medulla.
Pons – The primary role of the pons is to serve as a bridge
between various parts of the nervous system, including the
cerebellum and cerebrum. Many important nerves that originate
in the pons, such as the trigeminal nerve, responsible for feeling
in the face, as well as controlling the muscles that are responsible
for biting, chewing, and swallowing. As part of the brainstem, a
section of the lower pons stimulates and controls the intensity of
breathing, while a section of the upper pons decreases the depth
and frequency of breaths. The pons is also associated with the
control of sleep cycles, and controls respiration and reflexes. It is
located above the medulla, below the midbrain, and just in front
of the cerebellum.
Medulla – The primary role of the medulla is regulating our
involuntary life sustaining functions such as breathing, swallowing
and heart rate. As part of the brain stem, it also helps transfer
neural messages to and from the brain and spinal cord. It is
located at the junction of the spinal cord and brain.

Spinal Cord
The spinal cord is a cylindrical bundle of nerve fibers and
associated tissues enclosed within the spine and connect all parts
of the body to the brain. It begins in continuation with the
medulla and extends downwards. It is enclosed in a bony cage
called vertebral column and surrounded by membranes called
meninges. The spinal cord is concerned with spinal reflex
actions and the conduction of nerve impulses to and from the
brain.

Function Of Spinal Cord

Important functions of Spinal Cord are mentioned below:

 Forms a connecting link between the brain and the PNS

 Provides structural support and builds a body posture
 Facilitates flexible movements
  Myelin present in the white matter acts as an electrical
insulation
 Communicates messages from the brain to different parts of
the body
 Coordinates reflexes
 Receives sensory information from receptors and
approaches towards the brain for processing.
A column of nerve tissue that runs from the base of the
skull down to the centre of the back. It is covered by three
thin layers of protective tissue called membranes. The
spinal cord and membranes are surrounded by the vertebrae
(back bones). The spinal cord and the brain make up the
central nervous system (CNS).

Peripheral Nervous System

Peripheral Nervous System (PNS) is the lateral part of the
nervous system that develops from the central nervous system
which connects different parts of the body with the CNS. We
carry out both voluntary and involuntary actions with the help of
peripheral nerves.
Also refer: Peripheral Nervous System
PNS includes two types of nerve fibers:
1. Afferent nerve fibers – These are responsible for
transmitting messages from tissues and organs to the CNS.
 2. Efferent nerve-fibers – These are responsible for
conveying messages from CNS to the corresponding
peripheral organ.
Classification of the peripheral nervous system:
Somatic neural system (SNS): It is the neural system that
controls the voluntary actions in the body by transmitting
impulses from CNS to skeletal muscle cells. It consists of the
somatic nerves.
Autonomic neural system (ANS): The autonomic neural
system is involved in involuntary actions like regulation of
physiological functions (digestion, respiration, salivation, etc.).
It is a self-regulating system which conveys the impulses from
the CNS to the smooth muscles and involuntary organs (heart,
bladder and pupil). The autonomic neural system can be further
divided into:
1. Sympathetic nervous system
2. Parasympathetic nervous system
Neuron

A Neuron is a structured and functional unit of the nervous

system and unlike other cells, neurons are irregular in shape and
able to conduct electrochemical signals. The different parts of a
neuron are discussed below.
 Dendrite stretches out from the cell body of a neuron, and it
is the shortest fibre in the cell body.
  Axon is the longest thread on the cell body of a neuron and
has an insulating and protective sheath of myelin around it.
 Cell body consists of cytoplasm and nucleus.

 Synapse is the microscopic gap between a pair of adjacent

neurons over which nerve impulses pass, when moving

from one neuron to the other.
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Nerves
Nerves are thread-like structures that emerge from the brain and
spinal cord. It is responsible for carrying messages to all the
parts of the body. There are three types of nerves. Some of these
neurons can fire signals at speeds of over 119 m/s or above 428
km/h.
1. Sensory nerves send messages from all the senses to the
brain.
2. Motor nerves carry messages from the brain to all the
muscles.
3. Mixed nerves carry both sensory and motor nerves.
Also read: Nerves
Cranial nerves begin from the brain as these nerves carry
impulses to start from the central nervous system. Certain
cranial nerves belong to the group of mixed nerves while certain
ones fall under sensory nerves. Spinal nerves originate from the
spinal cord. All the spinal nerves carry impulses to and from the
central nervous system and these are part of mixed nerves. The
above nervous system diagram depicts the various nerves arising
from various parts of the body.
What are the parts of the nervous system?

The nervous system has two main parts:

 The central nervous system is made up of the brain and
spinal cord.
  The peripheral nervous system is made up of nerves that
branch off from the spinal cord and extend to all parts of
the body.
The nervous system transmits signals between the brain and the
rest of the body, including internal organs. In this way, the
nervous system’s activity controls the ability to move, breathe,
see, think, and more.1
The basic unit of the nervous system is a nerve cell,
or neuron. The human brain contains about 100 billion neurons.
A neuron has a cell body, which includes the cell nucleus, and
special extensions called axons (pronounced AK-sonz)
and dendrites (pronounced DEN-drahytz). Bundles of axons,
called nerves, are found throughout the body. Axons and
dendrites allow neurons to communicate, even across long
distances.
Different types of neurons control or perform different activities.
For instance, motor neurons transmit messages from the brain to
the muscles to generate movement. Sensory neurons detect light,
sound, odor, taste, pressure, and heat and send messages about
those things to the brain. Other parts of the nervous system
control involuntary processes. These include keeping a regular
heartbeat, releasing hormones like adrenaline, opening the pupil
in response to light, and regulating the digestive system.
When a neuron sends a message to another neuron, it sends an
electrical signal down the length of its axon. At the end of the
axon, the electrical signal changes to a chemical signal. The
axon then releases the chemical signal with chemical
messengers called neurotransmitters (pronounced noor-oh-
TRANS-mit-erz) into the synapse (pronounced SIN-aps)—the
space between the end of an axon and the tip of a dendrite from
another neuron. The neurotransmitters move the signal through
the synapse to the neighboring dendrite, which converts the
chemical signal back into an electrical signal. The electrical
signal then travels through the neuron and goes through the
same conversion processes as it moves to neighboring neurons.
The nervous system also includes non-neuron cells,
called glia (pronounced GLEE-uh). Glia perform many
important functions that keep the nervous system working
properly. For example, glia:
 Help support and hold neurons in place
 Protect neurons
 Create insulation called myelin, which helps move nerve
impulses
 Repair neurons and help restore neuron function
 Trim out dead neurons
 Regulate neurotransmitters
The brain is made up of many networks of communicating
neurons and glia. These networks allow different parts of the
brain to ―talk‖ to each other and work together to control body
functions, emotions, thinking, behavior, and other activities

AUTONOMIC NERVOUS SYSTEM

The autonomic nervous system is a network of nerves that
regulates unconscious body processes. The autonomic system is
the part of the peripheral nervous system responsible for
regulating involuntary body functions, such as heartbeat, blood
flow, breathing, and digestion.
Because of this, the autonomic nervous system is also
sometimes known by another name: the involuntary nervous
system
This system is further divided into three branches: the
sympathetic system, the parasympathetic system, and the enteric
nervous system.1
 Parasympathetic nervous system: This part of the
autonomic nervous system helps maintain normal body
functions and conserves physical resources. This division
also performs such tasks as controlling the bladder, slowing
heart rate, and constricting eye pupils.
 Sympathetic nervous system: Ths division regulates
the flight-or-fight responses. The sympathetic system also
performs such tasks as relaxing the bladder, speeding up
heart rate, and dilating eye pupils.
 Enteric nervous system: This is the part of the autonomic
nervous system that controls the gastrointestinal tract and
the digestion of food.
How the Autonomic Nervous System Works

The autonomic nervous system operates by receiving

information from the environment and from other parts of the
body. The sympathetic and parasympathetic systems tend to
have opposing actions in which one system will trigger a
response whereas the other will inhibit it.2
Traditionally, stimulation has been thought to take place through
the sympathetic system, while inhibition was thought to occur
via the parasympathetic system. However many exceptions to
this have been found.
Today, the sympathetic system is viewed as a quickly
responding system that mobilizes the body for action where the
parasympathetic system is believed to act much more slowly to
dampen responses.
For example, the sympathetic nervous system will act to raise
blood pressure while the parasympathetic nervous system will
act to lower it. The two systems work in conjunction to manage
the body’s responses depending upon the situation and need.
If, for example, you are facing a threat and need to flee, the
sympathetic system will quickly mobilize your body to take
action. Once the threat has passed, the parasympathetic system
will then start to dampen these responses, slowly returning your
body to its normal, resting state.
Autonomic Nervous System Functions

Internal organs regulated by the autonomic nervous system

include the heart, blood vessels, stomach, intestine, liver,
bladder, lungs, pupils, genitals, digestive glands, and kidneys.
The autonomic system controls a variety of internal processes,
including:1
 Blood pressure
 Body temperature
 Breathing (respiratory) rate
 Circulatory system
  Digestion
 Electrolyte balance
 Emotional responses3
 Glands of the mouth, nose, and eyes
 Immune system
 Heart rate
 Liver function
 Metabolism
 Pancreas function
 Pupillary response
 Production of body fluids, including sweat and saliva
 Sexual response
 Skin, including sweating and the muscles that cause hair on
the body to stand up
 Urination and defecation
Neurotransmitters in the Autonomic Nervous System
The autonomic nerve pathways connect different organs to the
brain stem or spinal cord. There are also two key
neurotransmitters, or chemical messengers, that are important
for communication within the autonomic nervous system:
 Acetylcholine is often used in the parasympathetic system
to have an inhibiting effect.
 Norepinephrine often works within the sympathetic system
to have a stimulating effect on the body.