The Nervous System – Introduction and Structural Classification

The Nervous System

  • The nervous system as a whole is divided into three subdivisions: the central nervous system (CNS) , the peripheral nervous system (PNS), and autonomic nervous system (ANS).

The Nervous System

Central Nervous System

  • The central nervous system is the processing centre of the body and consists of the brain and the spinal cord.

1. Brain

  • Brain accommodate in the skull while spinal cord is enclosed by the vertebral column. To support brain and protect it from external pressures, it is surrounded by a covering of triple membrane of connective tissue called the meninges.  The three layers of meninges are duramater, arachnoid and piamater. The innermost layer, piamater is thin, delicate and highly vascular. It is firmly adhere to the brain.  The middle layer is arachnoid membrane, which is thin and highly folded structure in front of cranial venous sinus. The villi like folding helps to reabsorb the cerebrospinal fluid (CSF). 
  • The outermost layer is very thick, strong and non-elastic called duramatter, is made up of collagen fibre. The space between duramater and arachnoid is known as sub-dural space which is filled by serous fluid. The space between arachnoid and piamater is known as sub-arachnoid space and is filled by CSF. CSF is lymph like clear and alkaline fluid whose function is to provide support as well as to protect the brain. It also helps in exchange of metabolic substances between the brain and blood capillaries. CSF mainly present in ventricle of brain, sub-arachnoid space and spinal cord.
  • Cerebrum: It is the largest and most advanced part of brain which comprises of two cerebral hemispheres on the dorsal surface. A longitudinal groove is present between both cerebral hemisphere known as median fissure. Both hemispheres are somewhat connected with curved thick nerve fibres called corpus callosum. The outer portion of cerebrum is called the cerebral cortex which is made upcontains numerous cell bodies and relatively few myelinated axons. This gives as overall grey appearance and hence called as grey matter. The surface of cortex is highly folded. The upward folds are called as gyri consecutive with the downward grooves called sulci. 
  • Beneath the grey matter there are millions of myelinated axon tracts and contains relatively very few cell bodies which give an opaque white appearance. Hence they are collectively called as white matter. Each cerebral hemisphere is divided into four lobes: frontal or anterior, parietal or middle, temporal or lateral and occipital or posterior lobe. Central sulcus separates frontal lobes from parietal lobes. Lateral sulcus or sylvian sulcus separate temporal lobe from frontal lobe and parietal lobe. Occipital lobe is separated by parietal lobe by parieto-occipital sulcus
Nervous System

Dorsal surface of cerebral hemisphere

  • Diencephalon: Diencephalon is small and posterior part of fore brain, covered by cerebrum. It consist of thalamus, hypothalamus, epithalamus and metathalamus.
  1. Thalamus: It is a major part of diencephalon and represents upper lateral wall. It accepts all sensory impulses from the all part of body (except olfaction) and send those to the cerebral cortex. Thus it acts as relay centre. In lower animals thalamus act as sensory centre because cerebral cortex is less develop.
  2. Hypothalamus: It is called the master gland. It represents lower lateral wall of diencephalon. Pituitary gland is attached with its middle part. A web like structure is found on anterior surface of hypothalamus known as optic chiasma. In mammalian brain, corpus albicans is found on the posterior part of hypothalamus.
  3. Epithalamus: Epithalamus represents the roof of diencephalon. Pineal gland is found in this region.
  4. Metathalamus: It represents the floor of diencephalon. It consist medial geniculate body (related to hearing) and lateral geniculate body (related to vision).
  • Mid Brain: It is small and contracted part of brain. Two longitudinal myelinated nerve fibres called cerebral penducles or crura cerebri located at the anterior part of mid brain.  Four spherical projections called optic lobe or colliculus are located at the posterior part of mid brain. Inferior optic lobes are related to acoustic reflex action.
  • Hind Brain: The hindbrain is one of the three major regions of our brains, located at the lower back part of the brain. It includes most of the brainstem and a dense coral-shaped structure called the cerebellum.
  1. Cerebellum: It is second largest part of brain. Human cerebellum is made up of 3 lobes. Lateral lobes are large and spherical, called as cerebellar hemisphere. It control regulation and coordination of voluntary muscles.Cerebellumhelps to maintains the body balance of a person.
  2. Medulla Oblongata: It is tubular and cylindrical in shape present at the posterior part of brain. It controls all the involuntary activities of the body e.g. respiration, metabolism, secretory actions of different cells etc.
  3. Pons varolii: It is small spherical projection which is situated below the mid brain and upper to the medulla oblongata. It consists of many transverse and longitudinal nerve fibres. Transverse nerve fibers are joined with cerebellum, whereas longitudinal fibre are join cerebrum to medulla oblongata. It regulates the breathing reaction through pneumotaxic centre.
Nervous System

Side view of human brain

  • Midbrain, medulla oblongata and pons varolii are situated on one axis called brain stem. The side view of a human brain is shown in Figure, which shows major parts of brain.

2. Spinal Cord

  • Spinal cord is continuation of medulla oblongata which comes out from foramen of magnum and continues in neural canal of vertebral column. It is also covered by duramater, arachnoid and piamater. Narrow space between duramater and vertebra is known as epidural space. The outer part of spinal cord is of white matter while inner part is of grey matter. 
  • The grey matter projects outside and forms the dorsal and ventral horn.  Dorsal and ventral horn continues in a tube like structure known as root of dorsal and ventral horn. In the root of dorsal horn, ganglia are present known as dorsal root ganglia. Sensory neurons are found in the dorsal root ganglia whose axon extend and get embedded into grey matter of spinal cord. These sensory neurons make synapse with ventral root neuron. Motor neurons are found in ventral root whose cyton is found in ventral horn while its dendrons are embedded into grey matter of spinal cord. 
  • Both sensory and motor nerve fiber combined and comes out from intervertebral foramen and form spinal nerve. Spinal cord acts as bridge between brain and organ of the body. It regulates and conducts the reflex action as well as it provides relay path for the impulses coming from brain.

Peripheral Nervous System

  • All nerves arise from brain and spinal cord are included in peripheral nervous system. Nerves arises from brain is termed as cranial nerve whereas nerves arises from spinal cord is termed as spinal nerves. All reptiles, birds and mammals have 12 pairs of cranial nerve. 
  • Amphibians and fishes have only 10 pairs of cranial nerves. In human, I, II, and VIII cranial nerve are pure sensory in nature. III, IV, VI, XI and XII cranial nerve are motor nerve and rest others out of 12 cranial nerves are mixed type of nerves. In human, there are 31 pairs of spinal nerves. Each spinal nerve is of mixed type and arise from the roots of the horns of grey matter of the spinal cord. Spinal nerves are divided into 5 groups according to its position:
  • Cranial spinal nerve     – 8 pairs
  • Thoracic spinal nerve  – 12 pairs
  • Lumber spinal nerve     – 5 pairs
  • Sacral spinal nerve       – 5 pairs
  • Coccygeal nerve           – 1 pairs

Autonomic Nervous System

  • The autonomic nervous system controls activities inside the body that are involuntary e.g. heart rate, sweating, peristalsis etc. It consists of motor neurons passing to the smooth muscle of internal organs. Autonomic nervous system plays an important role in maintaining homeostasis. 
  • It is divided into two parts: 1) Sympathetic and 2) Para-sympathetic. Sympathetic system is related with such intuitive reaction which increases the protection of body in adverse atmospheric condition along with energy consumption. Whereas para-sympathetic system is linked with those reactions in which energy is conserved.

Further Readings