• Neural tissue exert the greatest control over the body’s responsiveness to changing conditions.
• It generally develops from the ectoderm but microgliocytes (glial cell type) arise from the mesoderm of the embryo.
• The special properties of the cells of the nervous tissue are, excitability and conductivity. –– Excitability is the ability to initiate nerve impulse in response to stimuli (changes outside and inside the body).
–– Conductivity means the ability to transmit a nerve impulse (potential change in membrane of a nerve cell). Neural tissue consists of nerve cells (neurons) and glial cells
• Neurons are structural and functional units of nervous tissue.
• Each neuron consists of a cell body or cyton and cell processes, called neurites.
• Neurites are of two types, dendrons and axons.
Based on Structure:
Non-polar neurons – Each neuron has several branched processes (projections). These neurons are rare in vertebrates but occur in cnidarians (coelenterates) e.g., Hydra.
Unipolar neurons – The body has only one axon. It is found usually in the embryonic stage.
Pseudounipolar neurons – A single process arises from the cyton and then divides into axon and dendrite. They are found in dorsal root ganglia of spinal nerves.
Bipolar neurons – Each bipolar neuron has one axon and one dendrite. They are present in the retina of the eye.
Multipolar neurons – These neurons have several dendrites and an axon. They are found in cerebral cortex.
Based on function:
Afferent (sensory) neuron – They connect sense organs with the central nervous system (brain and spinal cord). They bring sensory impulse from sense organs to the central nervous system.
Efferent (motor) neuron – They connect the central nervous system to the effectors (muscles and glands). They carry motor impulses from the central nervous system to the effectors.
Interneuron (relaying neuron) – They are present in the central nervous system and occur between the sensory and motor neurons for distant transmission of impulses. They are meant for integrating and analysing the input of information and distributing it to other parts of nervous system.