Class X Episode 5 A. P State

HUMAN NERVOUS SYSTEM

The human nervous system can be divided into three parts or systems. They are:

1) The Central Nervous System or C N S2) The Peripheral nervous System or P N S

And 3) The Autonomous nervous system or A N S

What are the components of central nervous system?The central nervous system has two structures. They are:

a) Brain.b) Spinal Cord.

BRAIN:Human brain is the largest in all animals. It is protected by the Cranium

and Meninges.

CraniumThe brain is covered by three layers. They are:

a) The external Duramaterb) The intermediate Archnoid membrane

And c) The internal piamater. Three layers of the brain continuous with spinal cord is known as Meninges.

Meninges Spinal Cord

The space between the external and intermediate layers is filled by a liquid known as cerebrospinal fluid. The layers of the brain with cerebrospinal fluid are continuous with the spinal cord.

Cerebro Spinal Fluid

The cerebrospinal fluid serves as a shock absorbing medium and protects the brain against shocks and jolts.The weight of human brain is approximately 1400gms.

Brain has three divisions. They are:1) Fore brain having cerebrum and Diencephalon2) Mid brain3) Hind brain having cerebellum, pons and medula.

Let us now examine each part of the brain.1. Fore Brain – Cerebrum

The cerebrum is the largest part of brain and is made up of two halves. Each half of cerebrum is known as Cerebral hemispere

The two hemispheres are connected by a sheet of nerve fibres known as Corpus Collusum

The wall of cerebrum has two regions the outer region is called Cortex. The cortex has bodies of neurons and is grayish in colour. In view of the gray colour it is known as Gray Matter.

The gray matter is highly convoluted with ridges. Due to this several ridges or elevations called Gyri and groves or depressions called sulci are formed on the surface of the brain.

The gyri and sulci increase the surface area and gray matter.

The inner region of the cerebrum has white matter which is composed of Axons of the neurons.

Three deep groves divide the cerebral hemisphere into four lobes. They are:

1) Frontal2) Parietal3) Temporal

And 4) Occipital.

What is the importance of Cerebrum?Cerebrum is the center of consciousness, thinking, memory, reasoning,

perception and stimulus interpretation. The sensory centers are those of sight, reading, hearing, smell, taste,

pain, pressure etc.The motor areas are those of speech, movement and locomotion.

There are some areas which are concerned with intellect, memory, imagination, foresight, emotions, moods etc.

Cerebrum is a unique part of the brain and is a storehouse of past experiences. One can remember past events or recall them in dreams or due to hypnosis.

Are you a left handed or a right handed person. More can find the natural hand of your person. Offer any thing to you friend in the mid sagital part of the body. The offer should be sudden. In majority of the cases the person who is offered some thing suddenly will put out his natural hand.

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The right side of the brain controls the left side of the body and the left side of the brain controls the right side of the body. Hence the right side of the brain controls left handedness while the left side controls the right handedness. The control center for handedness is present on one side of the brain either left or right.

Thus cerebrum is the seat of intelligence, thinking and judgement.

On the lower surface of the cerebrum are two Olfactory lobes for taste and smell.

The Other part of fore brain is Diencephalons. It is concealed by the cerebrum. Diencephalon has two constituents. They are:

a) Thallamus.And b) Hypothallamus.

Thallamus relays sensory impulses except those of smell.

The hypothalamus coordinates various autonomous activities like water balance, control of pituitary gland, body temperature, blood pressure etc.

An Anterior choroids plexus present on the roof of diencephalon produces Cerebrospinal fluid.

The optic chiasma and pituitary gland.

Mid BrainThe second part of the brain is MID BRAIN. The mid brain controls the

fore brain and the hindbrain. It controls muscle toning and has reflexes for sight and hearing.

Hind BrainThe hind brain consists of cerebellum and brain stem.

The brain stem has two parts.a) Pons varoli

and b) Medulla oblongata.

Cerebellum:- Cerebellum lies behind the cerebrum and has two lobes. It has gray matter on the surface and white mater in the deeper layers. Cerebellum controls the voluntary movements of the body and maintains equilibrium and posture of the body

.The cerebellum cannot coordinate muscular movement when a person is under the influence of alcohol or drugs. Driving a vehicle under the influence of alcohol may lead to major and fatal accidents.

Pons varoli is a bridge of nerve fibers connecting the two lobes of cerebellum

Medula oblongataMedulla oblongata has white mater on the surface and gray matter in

duper layers. The roof of medulla oblongata contains a choroids plexus where cerebrospinal fluid is formed. Activities of internal organs are controlled ray the posterior region of medulla. The reflex centers such as cardiac center, respiratory center and vasomotor centers and centers for swallowing, coughing, sneezing and vomiting are located in medulla.

Can we record the electrical waves of the brain? E E G or Electroencephalogram records the waves of the brain.

The absence of B waves or any electrical activity from the brain of a patient indicates brain stem death.

The absence of heart beat, breathing and electrical brain activity is the clinical definition of death.

Spinal Cord:- We have learned that the brain continues as a long and them spinal cord.

The spinal and extends from medulla to the lumber region. It passes through the neural canal of vertebral column.

There is a difference between brain and spinal cord in respect of the placement of white and gray maters

In brain gray mater is external and white mater is in deep layers. However in spinal cord the white mater is in the external layers while the gray mater is in deep layers.

The white mater contains axons running longitudinally to and from brain and crossing from one side to other. The gray mater

contains the cell bodies of motor and connecting neuron. The gray mater forms an ‘ H’ shaped area In the center is a central canal which is continuous with the brain canals and continue through out the length of the spinal cord. This Canal is filled with cerebrospinal fluid. As mentioned earlier the meninges of brain is continued in the spinal cord.

The foloowing image gives us a very broad idea of the nervous system.