Human nervous system

                               Human nervous system

Human nervous system is the most complex. It is divided into two main parts:-

(1) Central nervous system

(2) Peripheral nervous system

(1) Central nervous system-

It is Hollow and lies on the mid dorsal part along the main axis of the body it is covered externally by part of axial skeleton. The CNS, in turn, consists of two parts:-

(A) Brain or encephalon situated in the head

(B) spinal cord or myelon located in the neck and trunk.

(A) Brain (Encephalon)-

The brain is the widest and the the uppermost part of the central nervous system. It is the highest coordinating centre in the body. Brain is situated in the cranial cavity of the skull in the head region of the body. The bones of cranium or brainbox protect this delicate organ from mechanical injury. Inside the box, the brain is contained in a fluid- filled baloon which provides further shock absorption. The study of brain in all aspects is called encephalogy.

Morphology-

The brain is soft, whitish organ. It weighs 1.2 - 1.4 Kg and forms about 98% of the weight of the whole central nervous system. It has about 100 billion neurons ( nerve cells). Brain is surrounded by three membranes called meninges which provides protection to it. The space between these three meninges is filled with cerebro- spinal fluid which protects the brain from mechanical shocks. 

Brain is divisible into three main regions:

(1) Forebrain

(2) Midbrain

(3) Hind brain

(1) Forebrain (Prosencephalon)-   It forms the greater part of the brain. It further consists of three regions:  Olfactory lobes, Cerebral hemispheres (cerebrum) and diencephalon.

(a) Olfactory lobes:-  These are a pair of widely separated club shaped small structures. These are fully covered by the cerebral hemispheres and are, thus, visible only in the ventral view of the brain. Each olfactory lobe consists of an anterior olfactory bulb and a posterior narrow olfactory stalk. These lobes receive impulses from olfactory receptors present in the olfactory epithelium in the nose and relay sense of smell to the temporal region of the cerebrum.

(b) Cerebral hemispheres (cerebrum):-   It formed the largest part of the brain. It is also the most complex and specialised part of the brain. The two cerebral hemispheres lie side by side being separated from each other by a deep longitudinal cerebral fissure.  The surface of these cerebral hemispheres is greatly folded to accommodate large number of nerve cells. The folds are called gyri (singular gyrus) and the depressions between them are termed sulci (singular sulcus).  Each cerebral hemisphere is divided by 3 deep fissures into four lobes:  anterior frontal lobe, middle parietal lobe, posterior occipital lobe and lateral temporal lobe. Different areas of cerebrum have different functions. For instance, it has sensory areas which receive impulses from the sense organs or sensory receptors, e.g., eyes, ears, tongue, nose and skin of the body (e.g., muscles). The latter then respond to stimuli. These are specific regions in cerebrum for each kind of stimulus and its response. For instance-

* Frontal lobe is the region for speech, facial muscular activities as well as higher mental activities.

*  Temporal lobe is the region for auditory reception (hearing).

* Occipital lobe is the region for visual reception (sight).

* Parietal lobe is the reason for touch, taste, smell, temperature and conscious association.

Each lobe also has some areas called association areas which store information and experiences. These association areas control thinking, memory, learning and emotions. Internally each cerebral hemisphere possesses a fluid- filled cavity called lateral ventricle.

(c) Diencephalon:-   It lies on the inferior side of the cerebrum and thus is visible in the ventral view of the brain. Its roof is called epithalamus, sides are called thalamai and it's floor is termed hypothalamus. Diencephalon has a narrow cavity called third ventricle. Hypophysis ( pituitary) is attached by a stalk or infundibulum to the hypothalamus region. Hypothalamus has control centres for hunger, thirst, fatigue, sleep, body temperature, sweating and emotions. It secretes neurohormones which regulate the secretions of anterior lobe of pituitary.

(2) Midbrain ( Mesencephalon)-

It is significantly small region. It consists of two fibre tracts called crura cerebri and two swellings called superior and inferior colliculi on each side. The fibrous tracts,i.e., crura cerebri connect hind brain with the fore brain. The four swellings of both sides are together known as corpora quadrigemina. The 2 superior colliculi have centres for sight reflexes while the two inferior colliculi  have centres for auditory reflex.

The mid brain controls reflex movements of :-

 (a) The head, neck and trunk in response to visual and auditory stimuli, and 

 (b) The eye muscles ; changes in pupil size as well as shape of the eye lens.

(3)Hindbrain(Rhombencephalon)-

The hind brain consists of three parts- cerebellum, pons varolii and medula oblongata. The cerebellum is the second largest part of the brain, constituting nearly 12.5% of it. It has two large, lateral cerebellar hemispheres and a central vermis. Cerebellum maintains the posture, equilibrium and muscle tone. Pons varolii controls some aspects of respiration. Medulla oblongata is the posterior most part of the brain which lies below the cerebellum. It continuous posteriorly into the spinal cord. It contains a fluid- filled cavity called fourth ventricle. Medulla oblongata controls:- rate of heartbeat, breathing movements, expansion and contraction of blood vessels to regulate blood pressure, and swallowing, coughing, sneezing and vomiting.

(B) Spinal cord -

Spinal cord is a a cylindrical structure and is about 45 cm long. It begins in continuation with the medulla oblongata of brain and extends downwards upto early part of lumbar region. It then extend to the end of vertebral column as fibrous connective called filum terminale. Internally , the spinal cord possesses a narrow fluid- filled cavity called Central canal. Spinal cord is enclosed in the vertebral column or backbone which protects it. Like brain, spinal cord too is surrounded by meninges. 31 pairs of spinal nerves arise from the spinal cord. Spinal cord performs two important functions:

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

(b) It acts as a centre for the reflex actions. Thus, it reduces brain's work.

(2) Peripheral nervous system-

it connects CNS with different parts of the body. It has two components voluntary and involuntary.

voluntary peripheral nervous system is under the control of will. It consists of nerves that arise directly from CNS connecting different body parts for voluntary (conscious) control of the brain.

Involuntary peripheral nervous system (autonomic nervous system), on the other hand is not under the control of human will. It develops from branches of some cranial and spinal nerves called visceral nerves.

       peripheral nervous system, thus, consists of all the three types of nerves namely,

(1) Cranial nerves 

(2)Spinal nerves and

(3) Visceral nerves.

(1) Cranial nerves -

Cranial nerves arise from the brain and spread to various parts of the head. They are 12 pairs in number. Cranial nerves 1th, 2nd and 8th are sensory nerves; cranial nerves 3rd, 4th, 5th, 6th, 11th and 12th are motor nerves; and cranial nerves 5th, 7th, 9th and 10th are mixed nerves (containing both sensory and motor nerve fibres).

(2) Spinal nerves -

31 pairs of spinal nerves arise from the spinal cord along most of its length and spread throughout the body (except head region).  They are all mixed nerves as they carry both sensory and motor nerve fibres.

(3) Visceral nerves -

Apart from regulating normal functions of the body, many activities of the internal organs such as heart, kidney, lungs, urinary bladder, blood vessels, glands etc. are controlled by specific set of nerves called visceral nerves which mostly arise from spinal cord but a few from the brain also. These form the autonomic nervous system.