Human Nervous System

Human Nervous System

The complexity of nervous system can be observed during the course of evolution. We the human beings differ from other animals in our ability to think and take actions, which is due to the well developed nervous system. Human nervous system is differentiated into central nervous system (CNS), peripheral nervous system (PNS) and autonomic nervous system (ANS).

The CNS acts as centre for information processing and control. It consists of the brain and the spinal cord. The PNS is made up of the nerves which connect the brain and spinal cord to all parts of the body. The ANS is formed of sympathetic and parasympathetic nerves.

Central Nervous System

The brain and the spinal cord being delicate vital structures are well protected in bony cavities of the skull and the vertebral column respectively. CNS is formed of two types of matter such as white matter or grey matter with respect to the presence or absence of myelin sheath which we have discussed earlier.

The brain is the controlling centre of all the body activities. It is covered by three connective tissue membrane or meninges :

(i) Duramater (dura: tough; mater: membrane)is the outermost thick fibrous membrane

(ii) Arachnoid membrane (arachnoid: spider) is the middle, thin vascular membrane providing web like cushion

(iii) Piamater (Pia: soft or tender) is the innermost, thin delicate membrane richly supplied with blood.

Meningeal membranes protect the brain from mechanical injury.

A human brain is formed of three main parts: (a) forebrain (b) midbrain and (c) hindbrain.

Forebrain: The forebrain is formed of cerebrum and diencephalon. The latter consists of dorsal thalamus and ventral hypothalamus.

Cerebrum

It is the largest portion forming nearly two-third of the brain. The cerebrum is longitudinally divided into two halves as right and left cerebral hemispheres by a deep cleft called median cleft. Two cerebral hemispheres are interconnected by thick band of nerve fibres called corpus callosum. The outer portion of each cerebral hemisphere is formed of grey matter and is called cerebral cortex. The inner or deeper part is formed of white matter and is called cerebral medulla. The cortex is extremely folded forming elevations called gyri with depressions between them termed as sulci that increase its surface area.

Each cerebral hemisphere is divisble into a frontal lobe, a parietal lobe, a temporal lobe and an occipital lobe. These lobes are also known as cerebral lobes and are associated with specific functions. Any damage in specific lobe inturn affects its function.

The cerebrum is responsible for the thinking, intelligence, consciousness, memory, imagination, reasoning and willpower.

Thalamus

Thalamus present in cerebral medulla is a major conducting centre for sensory and motor signalling. It acts as a relay centre.

Hypothalamus

It lies at the base of the thalamus. It controls involuntary functions like hunger, thirst, sleep, sweating, sexual desire, anger, fear, water balance, blood pressure etc. It acts as a thermoregulatory (temperature control) center of the body. It controls the secretion of hormones from anterior pituitary gland and is an important link between nervous system and endocrine system.

Midbrain

It is located between thalamus and hind brain. The dorsal portion of the mid brain consists of four rounded bodies called corpora quadrigemina that control visual and auditory (hearing) reflexes.

Hindbrain

It is formed of three parts cerebellum, pons and medulla oblongata.

Cerebellum

It is second largest part of the brain formed of two large sized hemispheres and middle vermis. It coordinates voluntary movements and also maintains body balance.

Pons

‘Pons’ a latin word meaning bridge. It is a bridge of nerve fibre that connects the lobes of cerebellum. It relay signals between the cerebellum, spinal cord, midbrain and cerebrum. It controls respiration and sleep cycle.

Medulla Oblongata

Medulla oblongata is the posterior most part of the brain that connects spinal cord and various parts of brain. It has cardiac centres, respiratory centres, vasomotor centres to control heart beat, respiration and contractions of blood vessels respectively. It also regulates vomiting and salivation.

Spinal Cord

Spinal cord is a cylindrical structure lying in the neural canal of the vertebral column. It is also covered by meninges. It extends from the lower end of medulla oblongata to the first lumbar vertebra. The posterior most region of spinal cord tapers into a thin fibrous thread like structure called filum terminale.

Internally, the spinal cord contains a cerebrospinal fluid filled cavity known as the central canal. The grey matter of spinal cord is ‘H’ shaped. The upper end of letter ‘H” forms posterior horns and lower end forms anterior horns. A bundle of fibres pass into the posterior horn forming dorsal or afferent root. Fibres pass outward from the anterior horn forming ventral or efferent root. These two roots joins to form spinal nerves. The white matter is external and have bundle of nerve tracts. Spinal cord conducts sensory and motor impulses to and from the brain. It controls reflex actions of the body.