Division of Nervous System : Central & Peripheral Nervous System

DIVISION OF NERVOUS SYSTEM 

Division of Nervous System


Division of Nervous System is into two major parts:
I. Central Nervous System (CNS) : All the parts of the nervous system which are present along the median longitudinal axis of the body constitute the central nervous system. It consists of brain and spinal cord.
II. Peripheral Nervous System ( PNS ) : It is formed by all the whitish thread-like nerves that connect the various body parts with the CNS.

The PNS is again divided into 2 subdivisions :
1. Somatic nervous system (SNS) : It is the voluntary part of PNS. It conducts impulses to and from skeletal (voluntary) muscles, bones, ligaments, joints and skin receptors.
2. Autonomic nervous system (ANS) It is the involuntary part of nervous system that controls the involuntary activities of various body parts.
It includes all those nerves, nerve fibres, ganglia (aggregates of cell bodies of neuron) that carry impulses from viscera to CNS which controls involuntary actions of smooth muscles, cardiac muscles and glands.

CENTRAL NERVOUS SYSTEM ( CNS )

It consists of two parts :

  1. Brain 
  2. Spinal cord 

The Brain 

Position, Protection and Meninges The human brain is highly developed. It is largest amongst all the animals. In an average it approximately weighs about 1350 g. It is made-up of about 1000 billion neurons.

The brain is placed and well protected in the cranial cavity or brain box of skull, It is surrounded by three protective membranous covering called meninges which continue backwards on the spinal cord.

These are Piamater (innermost, very thin, tough and highly vascular membrane), Arachnoid (thin delicate middle layer and non-vascular) and Duramater (outermost, thick, non-vascular, tough fibrous membrane), from inner to outerside. Between the meninges, spaces are present. These are filled with cerebro spinal fluid (CSF). The same fluid is also filled in the central spaces of brain and central canal of spinal cord.

Functions of CSF : 


  1. It serves as shock absorbing medium. It protects the brain and the rest of CNS against jerk and jolts. 
  2. It maintains constant pressure in and around the brain.

Structure of Brain

Human brain is a whitish, bilaterally symmetrical structure. It is divisible into three regions :

  1. Fore-brain 
  2. Mid-brain 
  3. Hind-brain

1. Fore-Brain 

It is differentiated into three parts :
(a) Olfactory lobes
(b) Cerebrum
(c) Diencephalon

(a) Olfactory lobes : The olfactory lobes of human brain are a pair of poorly developed, club-shaped, widely separated bodies. They are visible from the ventral surface only. The olfactory lobes are concerned with the sense of smell.
(b) Cerebrum : It is the largest part and forms 4/5th weight of the brain. It consists of right and left lobes called cerebral hemispheres. They cover all the parts of the brain superiorly and lie side by side. Their surface is highly folded or convoluted with ridges and grooves to increase the surface area.

Each cerebral hemisphere is hollow from inside and their walls have 2 regions, outer cortex and inner medulla. The outer portion of cerebrum contains grey coloured cell bodies and hence called grey matter, which is folded to form convolutions. The folds (gyri) and grooves (sulci) increase the surface area to accommodate more neurons and hence believed to have more intelligence.

The inner portion consists of axons of neurons which is white in colour, hence, called white matter. The two cerebral hemispheres are separated from each other by a very deep groove known as cerebral fissure. The two hemispheres are held together by a transverse band of nerve fibres called corpus callosum. Each cerebral hemisphere is divided into four lobes called frontal lobe, parietal lobe, occipital lobe and temporal lobe.

Functions of Cerebrum : 

  • (i) It is the seat of memory, will power, emotions, experiences, intelligence, reasoning, learning, invent and plan. 
  • (ii) It initiates and controls the movements of striped muscles. 
  • (iii) It controls the conscious sensations and all voluntary actions. 
  • (iv) It perceives the sensory impulses, such as, pain, touch, taste, smell, hearing and sight. 
  • (v) What is called subconscious or uncon- scious mind is also located in the cerebrum. Many past experiences are covered up by more recent impressions which dominate conscious activity. Past experiences may be recalled in dreams or when hypnotized or when skilfully questioned. 
(c) Diencephalon : It is a small rhomboidal lobe. It is completely covered superiorly by large cerebrum but visible from the inferior surface as a small area. It lies between cerebrum and midbrain. It is distinguishable into two parts- the thalamus and hypothalamus.

Functions of Diencephalon : Diencephalon of human brain controls many body functions, such as :

  • Hypothalamus temperature, control of pituitary gland and blood pressure. controls body 
  • Thalamus acts as a relay centre for pain pressure impulses to cerebrum.

2. Mid-brain 

Mid-brain of humans is significantly small, thick stalk, about 2 cm long. It connects the fore-brain with cerebellum and pons of hind brain.

Functions of Mid-brain: 

  • It connects the hind brain with fore brain and conveys impulses. 
  • It controls the sight and auditory impulses (hearing).

3. Hind-brain 

It is the posterior, small part of the brain. It is differentiated into three parts :
(a) Pons varolii
(b) Cerebellum
(c) Medulla oblongata.

(a) Pons varolii : It is centrally located below cerebellum.
Functions : 
It carries impulses from medulla oblongata to fore-brain. It also carries impulses from one hemisphere of the cerebellum to other and coordinates muscular movements on both sides.

(b) Cerebellum : It is the largest part of hind-brain. It is present below the cerebrum and above the medulla oblongata, behind the pons. It consists of two large lateral lobes called cerebellar hemispheres. Its surface is marked by a large number of transverse fissures which give it a laminated appearance core of white matter It has an inner surrounded by grey matter (cortex) which is greatly folded. It has no convolutions.
Functions : 
It helps to maintain the balance or equilibrium during movements and it controls and coordinates muscular activity (balance of the body).

(c) Medulla oblongata : It is about 2.5 cm long that extends from pons to spinal cord. It is almost triangular in shape. It is also hollow from inside.
Functions : 
It controls the involuntary activities of internal organs like beating of heart, movement of breathing, sneezing, swallowing, vomiting, coughing, hiccupping and peristaltic movement of alimentary canal.

The Spinal Cord 

The spinal cord is a long, unsegmented, cord like structure extending from medulla oblongata of brain through the neural canal of the vertebral column to the lumbar region. It is about 45 cm in length and 35 g in weight. It is also covered by three membranes called meninges. Spaces present between these meninges are full of cerebro-spinal fluid (CSF).

Walls of vertebrae, meninges and CSF protect the spinal cord. Like-brain, spinal cord is also made-up of grey matter and white matter. But in the spinal cord the arrangement of white and grey matter is reverse i.e white matter is external and grey matter is internal in position. The spinal cord is hollow from inside containing a cavity called central canal.

Functions of Spinal Cord: 

The spinal cord performs two important functions:

  • The spinal cord tracts serve as two-way conduction paths between peripheral nerves and the brain. Ascending tracts conduct sensory impulses up the cord to the brain while descending tracts conduct motor impulses down the cord from the brain. 
  • The spinal cord functions as reflex centre which controls all the spinal reflexes. 
  • It controls reflexes below neck.

PERIPHERAL NERVOUS SYSTEM ( PNS ) 

All the whitish, thread-like nerves, which connect the various body parts with the central nervous system, collectively constitute the peripheral nervous system. It is divided into two :

  1. Somatic Nervous System
  2. Autonomic Nervous System

Somatic nervous system 

It consists of Cranial and Spinal nerves.

Cranial nerves : Those nerves which arise from the brain are called cranial nerves. There are 12 pairs of cranial nerves. The cranial nerves olfactory nerves may be sensory, example (nose), optic nerves (eye) and auditory nerves (ear), motor, example - occulomotor nerves (going to eye muscles) and mixed in nature, example - facial nerves (going to and coming from face).

Spinal nerves : Spinal nerves arise from the spinal cord and these are 31 pairs in man. These are classified into the following five groups :

  1. Cervical spinal nerves in the neck - 8 pairs 
  2. Thoracic spinal nerves in thorax - 12 pairs 
  3. Lumbar spinal ne -5 pairs rves in abdomen 
  4. Sacral spinal nerves in hip region 5 pairs 
  5. Coccygeal spinal nerves in tail region -1 pair 
All spinal nerves are mixed nerves.

Autonomic nervous system (ANS) 

That part of the nervous system which controls all the involuntary activities of various body parts is called autonomic nervous system. It is divided in two parts :
(A) Sympathetic
(B) Parasympathetic

Emotions and Autonomic Nervous System: 

Autonomic nervous system is strongly influenced by emotions, such as, sorrow, happiness, anger, fear, sexual stimulation, etc. Long continuous emotional stresses may cause certain disorders like high blood pressure, stomach ulcers, etc.


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