The living organisms have the unique power to show sensitivity or irritability to the changes in the internal or external environment.
So any physical or chemical change in the environment is known as stimulus. The living organisms react through three main types of structures; such as receptors, effectors and neurons.
The receptors may be the neurons or specialized organs for detecting stimuli. These are organs of special senses like eye (vision), nose (smell), ear (hearing) and skin (touch) seen in higher vertebrates.
The cells, tissues or organs which show the effect in response to the stimuli is known as effect.
It is of two types:
(a) Somatic effectors:
It includes voluntary and skeletal muscles.
(b) Visceral or Autonomic effectors:
These include cardiac and smooth muscles of visceral organs and glands.
The neuron is the structural and functional unit of nervous system. Each one consists of a cyton, dendron and axon; through which nerve impulses are transmitted in one direction only.
Dendron → Cyton → Axon
Types of Neuron:
Functionally neurons can be of three types:
(i) Sensory neurons:
These neurons receive impulses from receptor or sensory cells and carry them to the central nervous system.
(ii) Motor neuron:
They transmit impulses from central nervous system to the effectors.
(iii) Association neuron:
These are multipolar neurons or inter-neurons which are the main building components of the brain and the spinal cord. They integrate the action of sensory and motor neurons and act as relay stations.
It is an electric wave of changes passing through the neurons, involving movement of ions and chemical reaction at a speed of 100 m/sec (approximately).
It is the gap or microscopic space between the branched end of one axon and dendrites and cyton of another neuron, through which impulses are conducted by a chemical mechanism.
According to the function, the nerves are divided into two types:
(a) Sensory or Afferent nerve:
These nerves carry impulses from the receptor organs to the central nervous system.
(i) Somatic sensory nerve:
These nerves transmit stimuli from extero-receptors or somatic receptors like skin, nose, muscles, eyes etc. to the brain and spinal cord.
(ii) Visceral sensory nerve:
These nerves originate and carry stimuli from the visceral receptors or intero-receptors (i.e. wall of alimentary canal and other visceral organs) to the brain and spinal cord.
(b) Motor or Efferent nerve:
The nerves which carry impulses from central nervous system to the effector organs (i.e. muscles and glands) are known as visceral nerve.
(i) Somatic motor nerve:
These nerves carry nerve impulses from brain and spinal cord to the effector organs like voluntary muscles.
(ii) Visceral motor nerve:
These nerves transmit impulses from central nervous system to the visceral effector organs like involuntary muscles and glands of visceral organs.