The behavior of animal in the environment to preserve itself relies on coordination of organs systems. Without coordination of different organ systems, different physiological procedures would work in haphazard way, without relating together activities. Linking together in time and space of different activities of the animal is known as coordination. Coordination is occurred by nervous system and sense organs, and by means of chemical substances (hormones) secreted by endocrine glands.
In course of evolution, nervous systems have turn out to be more complex. This is mainly as animals have become larger and more mobile, needing more neurons than sedentary animal. Most basic function of nervous system is (i) to get stimulus (ii) transmission of stimulus to central "brain", (iii) interpretation and analysis of stimulus and (iv) proper response by the effector. With evolution of complex nervous system and bilateral symmetry, cephalisation (formation of head) has occurred, and ganglia in head became large sufficient to be called the brain, that is main nervous control centre of body.
Major Divisions of Nervous System:
There are 2 main divisions of nervous system:
Central Nervous System (CNS):
CNS comprises of brain and spinal cord. It is site of information processing (receiving information and responding to it).
Peripheral Nervous System (PNS):
This comprises of all nerves entering and leaving brain and the spinal cord.
The brain is very delicate organ lodged inside cranium of skull. It is protected by 3 coverings, meninges (meninx: membrane): the outer tough duramater (dura: tough; mater: mother), thin delicate web-like middle arachnoid (arachne: spider), and innermost highly vascular piamater (pia: tender) richly supplied with blood vessels. Space between membranes is filled with fluid called cerebrospinal fluid. There are cavities inside brain that are also filled with same fluid.
Brain comprises of 3 major regions:
(a) Forebrain comprising of cerebrum and diencephalon
(b) Midbrain the small tubular part between fore and hindbrain
(c) Hindbrain comprises of cerebellum, pons, and medulla oblongata.
Individual parts of brain are explained below:
This is largest part of brain, separated into 2 (the right and the left) parts known as cerebral hemispheres. Their outer surface is extremely convoluted with ridges and grooves. Every hemisphere is hollow inside and walls contain 2 (an inner and an outer) regions. Outer region (cerebral cortex) has cell bodies of nerve cells and being grayish in color it is known as gray matter. Inner region is made up of whitish axon fibres and is known as white matter. Corpus callosum is sheet of criss-cross nerve fibres joining two cerebral hemispheres. Left side of cerebrum manages right side of body and vice-versa.
Cerebral cortex has 3 major functions:
This is part of forebrain situated below cerebrum. It comprises of given two parts:
This is the egg shaped mass of gray matter, situated in centre below cerebrum. It is relay centre for sensory impulses (like pain and pleasure) going to cerebrum.
This is region of brain situated below thalamus. It manages motivated behavior like drinking, eating and sex. It manages secretions of pituitary gland hanging below it. It also acts as regulation centre of body temperature and body fluids.
The cerebellum is smaller region of brain situated at base and under cerebrum. It has several furrows instead of convolutions. It also contains cortex of gray matter. Two major functions are:
d) Medulla oblongata:
This is last part of brain that is connected to spinal cord. Its functions are as follows:
i) It is centre for coughing, breathing, swallowing, etc.
ii) It manages heartbeat, movement of alimentary canal and several other involuntary actions.
In all, 12 pairs of nerves (cranial nerves) come out of brain, few of these are sensory, and some motor and few are of mixed type.
Spinal cord extends from medulla of brain downward about whole length of backbone. It is also wrapped in same 3 meninges as brain and space between them has same cerebrospinal fluid. Arrangement of white and gray matter is reversed in it that is white matter is outside and gray matter inside.
Functions of spinal cord:
Peripheral Nervous System:
Peripheral nervous system comprises of all nerves arising from brain and spinal cord. In general, it comprises of two types of pathways: afferent (receiving) sensory pathways and efferent (carrying away) motor pathways.
Afferent (receiving/sensory) pathways are comprised of two types of nerves:
(1) Purely sensory nerves, for instance cranial nerves received from eyes, ears, nose, etc.
(2) Mixed cranial nerves similar to fifth (facial nerve) that has sensory fibres bringing sensations from face but it also has motor fibres that carry impulses away to jaw muscles.
Neuron -Structural and Functional Unit of Nervous System:
Nervous tissue is made up of 2 major cell kinds: neurons and glial cells. Neurons convey nerve messages. Glial cells are in direct contact with neurons and frequently surround them. 3 kinds of neurons happen. Sensory neurons usually contain long dendrite and short axon, and take messages from sensory receptors to central nervous system. Motor neurons contain long axon and short dendrites and convey messages from central nervous system to muscles (or to glands).
Cell body (perikaryon) has nucleus and cell organelles in cytoplasm. Dendrites (short branching processes) extend out from cell body. They convey signals (impulses) from receptor or from axon endings of another neuron. There possibly as many as 200 dendrites in single neuron permitting as many connections with axon endings of other neurons. The long nerve fibre or axon takes impulse from cell body towards terminal branches that may either pass on impulse to another neuron, or in muscle or gland to cause required action. Synapse is point of communication in one nerve cell and another or between nerve cell and muscle. The sheath of fatty material (myelin) frequently covers axon, and such nerve fibres are known as medullated or myelinated fibres.
Structure of Neuron:
Conduction of nerve impulse along neuron and over synapse
Conduction of nerve impulse through nerve fibre is electrical in nature and one through synapse is chemical in nature.
i) Along neuron - electrical signaling:
The transmission (moving from one end to another) of nerve impulse through nerve fibre is electrochemical. It is not just a flow of electrons through the electric wire but it travels as wave of depolarisation. In normal resting condition outside of nerve fibre has positive (+) charge. In this situations nerve fibre is said to be polarised. Polarisation is because of presence of more Na+ ions outside cell membrane. Such state is maintained because of sodium ions being constantly pumped out by means of sodium potassium pump and operated by active transport by using ATP for energy.
Changes when the stimulus arrives at nerve fiber are as given below:
Axon membrane at that spot becomes more permeable to Na+ ions, that move inward and bring about depolarization on the spot. This point of depolarization itself becomes stimulus for adjoining area of membrane that in turn becomes depolarized. In the meantime previous area becomes repolarised because of active movement of sodium ions to outside of membrane by means of what is called 'sodium pump' and now fibre is ready for next wave of depolarization. Thus, nerve impulse is self- propagating wave of depolarization and repolarisation
ii) Over synapse - chemical signaling:
Impulse travelling through the nerve fiber may reach either the destination (muscle or gland) for action or dendrites of another neuron for more transmission. Meeting place is known as synapse. Transmission over the synapse is chemical process. As impulse reaches terminal end of axon, the following events occur:
i) The chemical acetylcholine is released by end of axon.
ii) acetylcholine stimulates next neuron to start new impulse.
iii) acteylcholine is soon broken down there to make synapse ready for next transmission.
'All or none' principle:
If stimulus is strong enough (with minimum threshold) to produce impulse, impulse will set up and travel at its own speed. Threshold is minimum strength of the stimulus which can initiate impulse. Increasing intensity of stimulus can't raise speed of transmission.
Reflex action is automatic, quick and involuntary action in body brought about by the stimulus. For instance
1. You immediately withdraw the hand on accidentally touching the hot plate or sharp thorn.
2. Watering (salivation) of mouth takes place on seeing or just smelling familiar tasty food.
Two kinds of reflexes - simple and conditioned
The two examples of reflex action given are fundamentally different. First one is inborn or natural, that didn't need previous learning. These reflexes are known as simple reflexes.
The other example is outcome of repeated experience. Brain really remembers taste of food and works in the unconscious manner- these reflexes are known as conditioned reflexes.
Some examples of reflexes are given below:
(i) Simple reflex:
a) Rapid closing of eyelids on noticing the object rapidly approaching eye.
b) Coughing when food swallowed enters windpipe instead of food pipe.
(ii) Conditioned reflexes:
a) Applying brakes in the vehicle (car or bicycle) on noticing someone unexpectedly coming in front of it.
b) Tying shoe laces during talking to someone, not knowing whether you are initial putting left lace over the right or vice versa.
Sensory Receptors (the sense organs):
Sense organs are organs by which we sense or notice changes in external environment. Every sense organ contains special sensory cells that receive stimuli and transmit impulses produced through concerned nerve to brain or spinal cord. Brain sorts out impulses, interprets them and transmits message for required response. In human there are usually 5 sense receptors, ears for hearing, eyes for seeing, tongue for taste, skin for sensing touch, pain, heat and nose for smelling etc.
Eye (sense of vision):
Eye is almost spherical in shape, bulging little in front, and is capable to rotate freely in bony socket. It is hollow ball having many structures inside. Wall of eyeball is composed of 3 layers: retina, sclera and choroid. Sclera is outermost tough white layer. At the front it is continued as transparent cornea. Choroid is middle layer. It is made up of connective tissue containing the dense network of blood vessels. Its inner surface is dark brown or black. This stops reflection that would otherwise interfere with clarity of image.
Retina is innermost sensitive layer. It has 2 types of sensory cells- cones (sensitive to bright light and colors) and rods (sensitive to dim light).
Yellow spot situated at visual axis is place of best vision in normal eye. It has maximum number of sensory cells and mainly cone. Rest of retina contains fewer cones and more rods. Blind spot is point where nerve fibres (axons) from all sensitive cells of retina converge to form optic nerve that connects eye to brain. There are no sensory cells at blind spot and any image formed here is not perceived.
=> Transmission of light:
Reflected light rays from object enter eyes by transparent structures of eye that is cornea, conjucativa, lens, aqueous humour, and vitreous humour.
=> Formation of image:
Curvature of cornea bends rays to some degree and lens bends them more to form image on retina.
=> Nature of image:
Image is inverted and real.
Perception of body balance:
Static balance because of gravity - Any bending or change in body posture causes fluid inside semicircular canals to move. Semi circular canals are set in different planes. Sensory hairs in ampulla of canal pick up such movements and impulses are transmitted by auditory nerve. Balance during motion - Utriculus and sacculus perceive dynamic equilibrium. Fine particles of calcium carbonate present in endolymph press on sensory hairs at any time body is in motion. Impulses are carried through auditory nerve.
Tongue and Nose (sense of taste and smell):
Tongue perceives taste and nose perceives smell. Perception relies on nature of chemical substance coming in contact with sensory cells. For taste there is direct contact of substance with sensory cells situated in taste buds on tongue. For smell, molecules of chemical are carried inward by air inhaled and they excite sensory epithelium of nose.
Skin (touch and some other miscellaneous senses):
There is diversity of nerve endings in skin. Few of these are concerned with touch (gentle pressure), few with deep pressure and others with cold, heat and pain. Sense of hunger is because of receptors in stomach wall. Sense of thirst is because of stimulation of nerves in pharynx. And sense of fatigue is situated in muscles.
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