Central Nervous System

 The Central Nervous System of Human

The central nervous system includes the brain and the spinal cord. The function of central nervous system is to direct the incoming messages to the motor neurons that are connected to the specific parts of the body, which in turn responds to a stimulus. In other words, the function of the central nervous system is to collect all the information from all the receptors of our body and send them to the effetors.

Let us now describe the two organs of the central nervous system, namely brain and spinal cord in detail.

The Human Brain

Brain

The brain is also called encephalon, the uppermost part of central nervous system. It is said to be the highest coordinating centre of the body. The brain is protected by a bony box in the skull called the cranium. It is situated in the cranial cavity of the skull in the head region. The brain is soft, whitish organ surrounded by three membranes, known as meaninges. The space between these membranes is filled with a fluid, called the cerebrospinal fluid, which protects the brain from mechanical shocks.

Brain divided into three main regions, namely,

(a) Fore brain

(b) Mid brain

(c) Hind brain

Fore brain

The forebrain forms the greater part of the brain. It consists of three parts namely,

(i) Cerebrum or cerebral hemispheres

(ii) Olfactory lobes

(iii) Diencephalon

(i) Cerebrum: Cerebrum forms the largest part of the brain. It is divided into two hemispheres, right hemisphere, and left hemisphere by a deep longitudinal fissure. But, the base of two hemispheres is connected by a sheet of nerve fibres called corpus callosum. The surface of the cerebrum has several folds called gyri. The gyri help to increase the surface area of the cortex. The outer surface of cerebrum is called the grey matter or cortex. The inner surface of the cerebrum is called the white matter or medulla. The depressions or shallow grooves found in-between the gyri are called sulci or sylci. Each of the cerebral hemispheres is divided into four lobes namely, frontal lobe, parietal lobe, occipital lobe, temporal lobe.

Frontal lobe is at the front of the head (anterior), parietal lobe is at the top of the head (middle), occipital lobe is at rear (posterior) and temporal lobe is at the side (lateral). Each cerebral hemisphere posses a fluid-filled cavity called the lateral ventricle.

(ii) Olfactory lobes: These are a pair of club-shaped, small structures fully covered by the cerebral hemispheres. These lobes receive impulses from olfactory receptors and relay sense of smell to the temporal lobe of the cerebrum. It is visible only in ventral view of the brain.

(iii) Diencephalon: It is located on the inferior side of the brain. It is visible only in the ventral only in the ventral view of brain. It is the region between cerebrum and brain stem. It consists of three regions namely, Thalamus, Hypothalamus, and Epithalamus.

Diencephalon

The roof of diencephalon is called epithalamus. The sides of diencephalon are called thalamus and the floor of diencephalon is called hypothalamus. The thalamus has cluster of nuclei which acts as relays, for particular sensory pathways.

The hypothalamus contains reflex centres, linked to the autonomic system. There is a funnel shaped stalk called infundibulum, which extends from its floor. It has a narrow cavity, called the third ventricle.

Functions of Fore Brain

(i) The cerebrum is the main thinking centre of brain.

(ii) The cerebrum is the site of learning, reasoning, intelligence and memory.

(iii) The cerebrum controls our sensations, actions, movements and thoughts.

(iv) The cerebrum has different areas to form different functions.

(v) The cerebrum has associated areas to control the thinking and memory, to do various activities.

(vi) The cerebrum has sensory areas to gather information from sense organs like skin, tongue, nose, eyes and ears and give sensation or feeling.

(vi) The cerebrum controls all the voluntary actions.

(b) Mid Brain

The mid brain is also called mesencephalon. It is a small region consisting of two fibre tracts and four swellings. The fibre tracts connect the hind brain with the fore brain. The four swellings are collectively called, corpora quadrigemina.

Functions of Mid Brain

(i) It controls the reflex movements of the head, neck and trunk in response to visual and auditory stimuli.

(ii) It also controls the reflex movements of the eye muscles, like the changes in pupil's size and shape of the eye lens.

(c) Hind Brain

The hind brain is also called Rhombencephalon. It consists of three parts namely,

(i) Cerebellum

(ii) Pons Varoli

(iii) Medulla oblongata

Hind brain

(i) Cerebellum: The cerebellum is the second largest parts of the brain. It lies behind the cerebrum and above medulla oblongata. The cerebellum consists of two hemispheres. It surface has many ridges called folia. It has three parts namely,

• Small anterior flocco nudular lobe
• A narrow ventral vermis
• Two large lateral hemispheres

(ii) Pons Varoli

Pons varoli is located superior to medulla oblongata. It contains ascending and descending nerve tracts.

(iii) Medulla Oblongata

The Medulla oblongata is continuous with spinal cord. It acts as a bridge between the brain and spinal cord. It contains a fluid filled cavity called the fourth ventricle. It connects spinal cord to the brain.

Functions of Hind Brain

(i) Cerebellum maintains posture, equilibrium and muscle tone.

(ii) Pons varoli controls some activities of respiration and act as a relay centre for different parts of the brain.

(iii) Medulla oblongata controls the rate of heart beat, breathing, contraction and expansion of blood vessels, swallowing, coughing, sneezing and vomiting.

Spinal Cord

The spinal cord is a cylindrical structure which begins in conti-nuation with medulla oblongata of brain and extends downwards to the lumbar region. The spinal cord is shorter than the vertebral column. There are two enlargements in the spinal cord namely,

(a) Cervical enlargement

(b) Lumbar enlargement

Below the lumbar enlargement there is a cone like region called conus medullaris. The spinal cord extends inferiorly from this cone-region to the end of the vertebral column as a connective tissue filament, called the filum terminale. All these together resemble a horse's tail and so, are called cauda equina. The spinal cord is enclosed in a vertebral column, which protects it. Internally, the spinal cord possesses a narrow, fluid filled cavity called central canal. The spinal cord is also surrounded by meninges. The 31 pairs of spinal nerves arise from the spinal cord.

Function of Spinal Cord

(i) It conducts sensory and motor impulses to and from the brain.

(ii) It acts as a reflex centre in order to reduce brain's work.

Function of Brain

(i) The brain receives information in the form of nerve impulses from all sensory organs.

(ii) The brain sends the instructions to the effectors, to function properly.

(iii) The brain correlates with various stimuli.

(iv) The brain coordinates the mechanism and chemical reaction of the body activities to work efficiently.

(v) The brain stores information to make itself organ of thought and intelligence.