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Coordination - Nervous System

Coordination: With the evolution of multicellularity it was absolutely necessary to develop certain system which may co-ordinate the functions among the different cells and may exchange the information between the changes in external environment and internal cellular environment. For this purpose primarily nervous system developed and as the complexity increased another system of co¬ordination was also evolved which is now known as endocrine system.

NERVOUS SYSTEM

The nervous system co-ordinates the different organs in several ways. It receives the external stimulus, analyses and then interprets them, so as to produce the sensations. It plays an important role in the formation and retention of the memory of the past. It also plays a very great role in the conduction of messages and information between the different organs of the body. It has also the inhibitory and excitatory activites to control the muscles and glands for their proper functions. It also Co-ordinates between the visceral organs. The special sense organs such as eyes, ears, smell, taste etc. are also associated with the nervous system. The unit of nervous system is neurons and they form an integrated network in the body.

Nervous system in man;

(1) Central nervous system (CNS); Brain and spinal cord.

(2) Peripheral nervous system; Nerves from brain (cranial nerves) and spinal Cord (spinal nerves).

(3) Autonomic nervous system; Nerves and ganglion arising from the ventral part of the spinal cord.

Coverings of nervous system:

Brain and spinal cord is covered by membranous meninges. These are from inside to the outside piamater, arachnoid membrane and duramater respectively. The cavities of brain are ventricles, spinal cord is central canal and the cavities present between the meninges are subdural and sub-arachnoid spaces.

Piamater is thin and highly vascular.

Cerebrospinal fluid is present in sub¬arachnoid space but not in subdural space.

THE BRAIN

Brain (encephalon) consists of three parts-

(1) Fore brain (prosencephalon): It includes cerebral hemisphere (telencephalon), olfactory lobe and the diencephalon.

(2) Mid brain (mesencephalon): It bears optic lobes (four lobes) also known as corpora quadrigemina and cerebral peduncle.

(3) Hind brain: It consists of cerebellum (metencephalon) and medulla oblongata (myelencephalon).

Cerebral hemispheres have the folds and grooves respectively known as gyri and sulci. The outer layer of cerebral hemisphere is cerebral cortex and is made up of grey matter which contains the nerve cells in it. This has the highest centres of sensations and activities such as general sensory, visual, auditory, premotor and motor area etc.

Hyphothalamus contains nerve centres for hunger, thirst, temperature regulation, emotional reactions and the autonomic nervous system. It also controls the secretion of adenohypo¬physis (anterior lobes) by the neurohor-mone secretion. The specialized cells known as nuclei preopticus or preoptic nuclei situated in the hypothalamus are neurosecretory cells secreting the hormones for the storage of posterior pituitary or neurohypophysis.

The optic lobes control muscle tone and modify the signals produced by the cerebral cortex.

The cerebellum has the centres for the maintenance of balance, posture and muscle tone.

The posteriormost and the last part of the brain is medulla oblongata and pons varolii which have the centres of vital importance such as respiration, salivation and circulation.

Special features:

1. Internally both the cerebral hemispheres are joined together by a strip of nerve fibres called corpus callosum.

2. The centres for cold; heat, touch etc. are located in the somasthetic area of parietal lobe.

3. Pons varolii is made up of white N.F. It joins both cerebellum.

4. End part of medulla is called brain stem.

5. Medulla controls most of the involuntary activities.

6. Centres for sneezing and coughing, are located in medulla.

Ventricles of brain:

1. 1st ventricle or rhinocoel

2. IInd ventricle or paracoel

3. IIIrd ventricle or diocoel

4. Iter

5. IVth ventricle or cavity of medulla oblongata. in the of factory lobes In the cerebral hemi¬spheres

In the diencephalon

Narrow canal or cavity between crura cere¬brae of midbrain

In the medulla oblon¬gaia'

Spinal cord: The position of grey and white matters is reversed in the spinal cord to that of the brain i.e. white matter surrounds the grey matter. The spinal nuclei are embedded in the grey matter.

Centrally the spinal cord bears central canal. Wing like ridges of grey matter are called dorsolateral and ventrolateral horns respectively. Lateral horns are also found in thoracic and lumbar region.

Dorsal sulcus of spinal cord is joined with central canal by dorsal septum.

Spinal cord is the centre of reflex actions and it forms ink between brain and spinal nerves.

The nerve tracts of spinal cord are of two types-

The ascending tract is the sensory tract and conducts the nerve impulses from the peripheral organs to the brain.

The descending tract is the motor tract and conducts the nerve impulses from the brain to the muscles and glands.

Peripheral nervous system: The peripheral nervous system is made up of nerves with efferent and afferent nerve fibres. These are motor and sensory fibres. There are 31 pairs of spinal nerves in man and 37 pairs in rabbit. The nerves originated directly from cranium of brain are known as cranial nerves. They are 12 pairs in mammals.

The activities of visceral organs are controlled by the specialized nerves and a chain of ganglion which constitute the autonomic nervous system. This system includes two sub-systems-

The sympathetic system: A pair of sympathetic nerves with a chain of ganglions form this system. It extends from the neck to the end of abdomen. These nerves secrete a chemical 'sympathin'. This substance accelerates the action of the organ.

The parasympathetic system: The parasympathetic fibres are associated with the nerves of the head and sacral region. These fibres on stimulation secrete the acetylcholine which inhibits the action accelerated by the sympathin.

SPINAL NERVES

1. Cervical nerves - 8 pairs

2. Thoracic nerves - 12 pairs

3. Lumbar nerves - 5 pairs

4. Sacral nerves - 5 pairs

5. Coccygeal nerves - 1 pair

All the spinal nerves are mixed. Each develops from dorsal and ventral roots of spinal cord.

NEURONS OR NERVE CELLS

Neurons are the morphological and physiological unit of the nervous system. Each neuron consists of a cell body or perikaryon or cyton, afferent fibres, the dendrons or dendrites and efferent fibres the axons. Main characteristic of cyton is the presence of Nissl's granules made up of RNA. The terminal part of the axon has a swollen vesicular structure known as buttons which forms the connection with dendron of another neuron. This connection is known as synapse.

NERVE FIBRES

The aggregation of neuron forms nerve fibres. The nerve fibres are of two types-

(I) Myelinated nerve fibres: These are mainly present in the white matter of the central nervous system, cranial nerves and spinal nerves. These fibres are provided with a core which is surrounded by the myelin sheath. The axons contain cytoplasm known as axoplasm and surface membrane of axoplasm is known as axolemma which is made up of lipoproteins. This membrane is related with the transmission of nerve impulses.

The myelin sheath in the peripheral nervous system is interrupted at certain points. These points are known as node of Ranvier. Due to the presence of this node, the transmission of nerve impulses is of saltatory type.

The myelinated sheath of the cranio-spinal series are also covered by a soft sheath containing highly flattened, nucleated Schwann cells. The

layer of Schwann cells is covered again by an ultrathin membrane the neurolemma or neurilemma.

(ii) Non-myelinated or grey nerve fibres: These are very small fibres which are devoid of myelin sheath. The axons are surrounded by a covering of syncytial, nucleated Schwann cells. e.g. ganglionic cells of autonomic nervous system.

PHYSIOLOGY OF NERVE CONDUCTION

Non-myelinated nerve fibres: At rest the external surface has positive charge and internal surface has negative charge. The outer surface has more Na+ and inner surface has more K+; but due to the presence of CI- and metabolism it is negatively charged. The outer and inner surface of a nerve membrane has a membrane potential or resting potential of - 70 milli volt or - 7 volt. The resting potential depends upon a physiochemical equilibrium or Donnan equilibrium which provides the prior information of exit or entry of Na+, K+ and CI- ions. This is known as depolarization. A continuous inflow of K+ ions causes the inner surface again negatively charged and the outer surface becomes positively charged. This is repolarization. The active transport of Na+ outside and K+ inside re-establishes the neuron in the resting stage. This process is known as sodium pump or sodium-potassium pump or metabolic pump. At a point where sodium ion enters, the outer surface becomes negatively charged whereas the adjoining points are still positively charged. The potential between these two points known as action potential causes depolarization of the forwarding point, Thus nervous stimulation travels forward which produces the nerve impulse. Thus an automatic self reinforcement is produced. So the nerve impulse is a self propagating wave of depolarization and repolarization.

Myelinated nerve fibres: These fibres 'are covered with a high electric resistant myelinated sheath which acts as an effective electric insulator. But this insulation is interrupted at node of Ranvier. Thus action potential transmits from one node to another. This leaping of charge propagates the nerve impulse. This is known as saltatory propagation.

The transmission of nerve impulse in the myelinated nerve fibres is 20 times faster than in the non-myelinated fibres.

The velocity of nerve impulse in the non-myelinated nerve fibres is few metres/second, whereas in myelinated nerve fibres it is 100 metres/second. The highest speed is approximately 200 miles/hour in body balancing and fast reflex movements (in the sensory and motor fibres).

In synaptic buttons, a chemical transmitter substance acetylcholine is packed in the synaptic vesicles. This acetylcholine is released from the vesicles when a nerve impulse reaches in the button. It accumulates in the synaptic cleft causing the depolarization in the dendrite or axon.

Thus nerve impulse is an electro¬chemical transmission.