What is Nervous System? The System engage specialized cells or neurons linked together directly or via the central nervous system, to outline network that attach the cell or organ’ which receive stimuli (receptors) and those which carry out actions or responses (effectors). The neuron has the capacity to produce and carry out impulses which pass through across the synapse and pass from the receptors to the effectors, carry about nervous coordination.
ELEMENTS OF NERVOUS SYSTEM
The elements of nervous system which help in coordination are:
Receptors detect changes in the external and internal environment of the animal. The receptor may be a cell, or neuron ending or a receptor Organ. Classification on the basis of environmental change or energy. On the basis of environmental change or energy receptors are of following types:
a. Chemoreceptors: These are for smell, taste and for blood CO2 (cells of medulla, aortic and carotid bodies), oxygen (carotid bodies), blood glucose, amino acids and fatty acids (receptors in the hypothalamus).
b. Mechanoreceptors (pacinian corpuscles which are encapsulated nerve endings, found in the sensitive pad of each finger): These detect stimuli of touch (free nerve endings + expended (finished) tip ending + stray (wander away) endings), pressure and hearing and equilibrium (cochlear receptors and vestibular receptors respectively).
c. Photoreceptors (electromagnetic receptors),respond to stimuli of light for example in eyes, rods and cones.
d. Thermo-receptors: They respond to cold and warmth.
e. Nociceptors: (Undifferentiated endings) which produce the sensation of pain.
Modality of sensation
Each type of the principal type of sensation that we can experience e.g. pain, touch, sight, sound and so forth is called a modality of sensation. however despite the reality that we experience these different modalities of sensation nerve fibres send out only impulses. How is it that different nerve fibres transmit different modalities of sensation? The answer to this question is:
1 . Particular Reception centers in CNS: Each nerve tract terminates at a specific point in the CNS (Central Nervous System); and the type of sensation is determined by the point in the nervous system to which the fibre leads. So touch stimulus is carried by nerve impulse in the ‘touch’ area of the brain. Similarly fibres from the eyes (retina) terminate in the visual cortese of brain.
2. Receive only specific stimulus; There are many receptors which respond to the mechanical conditions of internal organs. Examples are the receptors of the stomach wall which may concerned with arousal of ‘hunger’ stretch receptors in the carotid and aortic arteries of tetra-pods have important roles in the regulation of blood pressure;, endings with similar properties are found in the bronchial vessels of fishes.
Working of Sensory Receptors with Special Reference to Skin
In the skin there are at least 3 different types of sensory endings involved in to stimulus reception. In skin, the receptors are concerned with at least five different senses: touch, pressure, heat, cold and pain.
1. Hair end organs: Situated at the base of hairs are hairs end organs, receive touch stimulus.
2. Meissner’s ‘ corpuscles (encapsulated endings) which lie in papillae which extend into the ridges of the fingertips. The corpuscle consists of spiral and much twisted endings, each of which ends in a knob. These are touch receptors.
3. Pacinian corpuscles – Situated quite deep in the body. These are also encapsulated neuron endings and receive deep pressure stimulus. Those located in the limbs probably form a basis for vibration sense.
4. Intensity of stimulus: The intensity of stimulus received would either be transmitted in the form of repeated impulses or by more fibres carrying the impulse to the CNS. The detection of vibrations of the ground by terrestrial vertebrates is probably achieved by receptors at the joints.
Relative abundance: The relative abundance of various types of receptors differs greatly e.g., pain receptors are nearly 27 times more abundant than cold receptors. The cold receptors are nearly 10 times more abundant than heat or temperature receptors.
Distribution of receptors: The receptors are not distributed evenly over the entire surface of the body e.g., touch receptors are much more numerous in the finger tips than in the skin of the back, as might be expected in view of the normal functions of those two parts of the body.
Flow of stimulus: The stimulus received by the receptors in the skin which are the endings of sensory neurons is passed to the motor neurons via inter or associative neurons which are present in the brain and via spinal cord and impulse is sent by the motor neurons to the effectors, which are muscles and glands.
The principal structural and functional units of the nervous system are neurons, but there are also other cells, in higher animals, and in humans called neuroglia, which make up as much as half of the nervous system. Neuroglia plays a essential role in the nutrition of neurons and their protection by myelin sheath.
A typical neuron
Cytoplasmic processes: The neuron has protoplasmic processes come up from its cell body containing nucleus and various organelle embedded in the cytoplasm. There are two main types of cytoplasmic processes or fibres.
Dendron or dendrites: The one which carry impulse towards cell body is called dendron, if it is a single fibre but if smaller fibres they are called dendrites (singular: dendrite).
Axons: The processes conducting impulses away from cell body are termed axons. These may be more than a meter long in some neurons. Nissl’s granules which are groups of ribosomes associated with rough endoplasmic reticulum and protein synthesis and golgi apparatus are present in the cell body. Microtubules, neurofibrils, rough endoplasmic recticulum and mitochondria are present throughout the axoplasm (cytoplasm of axon) of the neuron.
Cell body or soma: The cell body or soma is the main nutritional part of the cell and is concerned with the biosynthesis of materials necessary, for the growth and maintenance of the neuron. If the cell body of the -neuron remains intact, it can regenerate axonal and dendrites fibres; but neurons once mature, do not divide any further.
TYPES OF NEURONS
There are three functional types of neurons in mammals i.e., the sensory, associative (intermediate/relay) and motor neurons.
i) Sensory Neurons
These are the neurons which carry nerve impulses from receptors to brain or spinal cord. In the sensory neurons, the single, elongated dendrite is called a dendron because it carries nerve impulses from the receptors to the cell body. The shorter fibre is called axon because it conducts impulses away from the cell body from the cell body.
ii) Motor neurons
These are the neurons which carry nerve impulses from brain and spinal cord to the effectors in all parts of the body. In the motor neuron there is a long axon and a number of small dendrites given out from the cell body.
iii) Associative Neurons
These are the neurons which occur exclusively in the spinal cord and brain. They serve as intermediate links between numerous sensory and motor neurons.
These are the structures which respond when they are stimulated by impulse coming via motor neuron. The principal effectors are glands, which respond by secreting; and muscles which respond by contracting.