Nervous System In Human Body | Parts of Nervous System

Nervous System In Human Body

The nervous system consists of the central and the peripheral nervous system. It generates, modulates and transmits information in the human body. Nervous System in humans can also be divided into following parts:

1. CNS or Central nervous system (it consists of brain and spinal cord).

2. PNS or Peripheral nervous system (consists of cranial and spinal nerves) and

3. ANS or Autonomic nervous system (consists of branches from brain and spinal cord).

1. Central Nervous System (CNS)

The central nervous system (CNS) is made up of : (A) Brain and (B) Spinal cord

(A) Brain

Brain is the most vital organ of the body. It is the centre of all voluntary actions. However it also regulates many involuntary actions (like salivation, heart beating, blood pressure regulation etc.). It is the Centre of all nervous activities. It is a highly complicated structure. Brain is located in the head region. 

The brain is enclosed in a hard bony cover, called brain box or cranium (formed of 8 bones). Brain is protected by three membranous layers, called meninges.  Brain is a hollow structure and a liquid, called CSF (cerebro-spinal fluid) is present in its cavities.

Parts of the Brain

(a) Fore brain : It consists of following parts :

• Olfactory lobes : These are hollow structures, which are responsible for the sense of smell.
• Cerebral hemispheres (cerebrum) : These are two half-spherical structures. These are highly developed and enlarged in man, These are the centre of intelligence, memory, speech, vision and reasoning etc.
• Diencephalon : It is a small, unpaired, 3rd part of the fore brain. Its roof is called epithalamus, while floor is called floor hypothalamus. From hypothalamus, pituitary gland is attached. Diencephalon is responsible for controlling hunger, thirst, temperature of body, pH of body fluids, emotions, anger etc.

(b) Mid brain : It consists of four solid optic lobes. These optic lobes are helpful in vision as well as in hearing.

(c) Hind brain : It consists of following parts :

(i) Cerebellum : The cerebellum, like the cerebrum, is large and well developed. The cerebellum consists of two large lobes and a central small fold, called vermis. Cerebellum is the centre of coordination. It coordinates movement of body parts and maintains body posture and balance. It helps in activities like walking in a straight line, picking up a pencil and riding a bicycle etc.

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Voluntary and involuntary actions

Actions which are under the control of our will are called voluntary actions. Example: walking, speaking, hand movement etc. On the other hand those actions, which are not under the control of our will, are called involuntary actions. Example: heart-beating. etc.

(ii) Pons varolii : it is a band of nerve fibres that lies between the medulla and the mid brain on the ventral side. It serves to carry impulses from one part of the cerebellum to the other.

(iii) Medulla oblongata : It is the posterior most and simplest part of the brain. It is conical in appearance. The lower end of medulla extends backwardly and forms the spinal cord. Medulla oblongata mainly controls the involuntary activities of the body such as peristalsis (movement of alimentary canal). vomiting, coughing, respiratory movements etc.

Brain Parts And Functions

Parts of the brain	Functions
Olfactory lobes	Sense of smell
Cerebral hemisphere	Memory, thinking, reasoning, speech, vision, hearing, learning and centre of intelligence.
Diencephalon and hypothalamus	Homeostatic centre, controls thirst and hunger, maintains temperature, pH, controls emotions and behaviour.
Optic lobes	Vision (deep analytical) and concentrate hearing
Cerebellum	Coordination of skeletons, maintains body posture.
Medulla oblongata	Bridge between spinal cord and brain, maintains many Involuntary activities.

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(B) Spinal Cord

• Spinal cord is an extension of medulla oblongata, i.e., it arises from the rear part of the brain. It comes out from the cranium (skull) through an aperture, called foramen magnum.
• Spinal cord is a long, cylindrical and very thick nerve. It is hollow from inside and the cavity is called neurocoel.
• Spinal cord is about 45 cm long and extends from brain to waist region.
• Spinal cord is a highly delicate structure, therefore, it is protected by the vertebral column. The vertebral column internally has a long tubular space. The spinal cord is present in this space. (This is similar to the presence of a refill inside a pen).
• For extra protection, spinal cord is also protected by meninges.
• The transverse section (T.S.) of spinal cord shows the presence of an inner zone of 'butterfly shaped' gray matter. The outer zone consists of white matter.
• A pair of spinal nerves arises, one from each side of spinal cord.
• A total of 31 pairs of spinal nerves arise from the spinal cord.
• All spinal nerves are "mixed" nerves because each contains both sensory and motor neurons.
• Each spinal nerve originates by a dorsal nerve root and a ventral nerve root. The dorsal root is sensory in nature, while ventral root is motor, Immediately after its formation, the spinal nerve bifurcates to form two distinct branches : (a) sensory nerve and (b) motor nerve.
• The sensory nerve brings impulses from sense organs whereas the motor nerve sends impulses muscles.
• The dorsal and ventral roots of spinal cord and spinal nerve, together form reflex arc.

Functions of Spinal Cord

The spinal cord performs following important functions are :

• It conducts nervous impulses to and from the brain.
• It is concerned with reflex actions.

What are Reflex Actions?

These are sudden responses of the body to some external stimuli. In these responses, the brain is not directly involved. Therefore, these are carried out by spinal cord only, with the help of a reflex arc.

Types of Reflex Actions

(a) Innate reflex actions : These actions are instantaneous. These are not previously learned by the person. These are present since the birth. 

Example: sneezing, coughing, eye-blinking, knee jerk, movement of diaphragm etc.

(b) Habitual reflex actions: Some actions require constant learning in the beginning, but when they are perfectly learned, they become habitual. In habitual actions, brain is not directly involved. 

Example: driving, walking etc.

(c) Conditional reflex actions : These actions occur automatically when certain specific condition is satisfied. 

Example: salivation on smelling a delicious food is an example of conditional reflex action.

What Is Reflex Arc? 

The path, taken by the sensory and motor neurons in the spinal nerves, form the reflex arc.

• A reflex arc includes 5 components.

1. a sense organ (like skin),
2. sensory neuron of spinal nerve,
3. spinal cord,
4. motor neuron of spinal nerve and,
5. an effector (muscle).

• Reflex arc is responsible for the quick execution of the reflex action.

2. Peripheral Nervous System (PNS)

• This system includes nerves that arise from CNS i.e., the nerves arising from brain and spinal cord.
• Those nerves which originate from brain are called cranial nerves, and those originate from spinal cord are called spinal nerves.
• There are 12 pairs of cranial nerves in man.
• The number of spinal nerves in man is 31 pairs.
• The peripheral nervous system acts like a bridge between CNS and different parts of the body.

3. Autonomic Nervous System (ANS)

Autonomic nervous system controls the involuntary activities of the body. It is called autonomic, because its activities can not be controlled by the will of the individual. For example: contraction and dilation of pupil (in eyes), slow and fast peristalsis, slow and fast heart-beat etc.

Autonomic nervous system (ANS) consists of two subsystems :

a. Parasympathetic nervous system and

b. Sympathetic nervous system

Both these systems work in opposite direction and thereby regulate many autonomic activities with in the body.

Difference between Sympathetic and Parasympathetic System

Sympathetic System Increases heart-beat	Parasympathetic System Decreases heart-beat
Constricts blood vessels and raise blood pressure	Dialate blood vessels and lowers blood pressure
Dialates pupil	Constricts pupil
Dialates bronchi	Constricts bronchi
Inhibits gastric secretions	Stimulates gastric secretion
Relaxes urinary bladder	Constricts urinary bladder