Nervous tissue is the fourth major class of vertebrate tissue. The function of the nervous tissue is in communication between parts of the body. It is composed of neurons, which transmit impulses, and the neuroglia, which assist propagation of the nerve impulse as well as provide nutrients to the neuron.
All nervous tissue of an organism makes up its nervous system, which may include the brain, spinal cord, and nerves throughout the organism.
Nervous tissue is made of nerve cells that come in many varieties, all of which are distinctly characteristic by the axon or long stem like part of the cell that sends action potential signals to the next cell.
All living cells have the ability to react to stimuli. Nervous tissue is specialized to react to stimuli and to conduct impulses to various organs in the body which bring about a response to the stimulus. Nerve tissue (as in the brain, spinal cord and peripheral nerves that branch throughout the body) are all made up of specialized nerve cells called neurons. Neurons are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibers (neurons) bound together by connective tissue. A sheath of dense connective tissue, the epineurium surrounds the nerve. This sheath penetrates the nerve to form the perineurium which surrounds bundles of nerve fibers. blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers.
There are three main types of neurons, which are classified according their function:
- Those that conduct impulses from the sensory organs to the central nervous system (brain and spinal cord) are called sensory (or afferent) neurons.
- Those that conduct impulses from the central nervous system to the effector organs (such as muscles and glands) are called motor (or efferent) neurons.
- Those that connect sensory neurons to motor neurons are called interneurons (also known as connector neurons or association neurons)
Structure of a Motor Neuron
A motor neuron has many processes (cytoplasmic extensions), called dendrites, which enter a large, grey cell body at one end. A single process, the axon, leaves at the other end, extending towards the dendrites of the next neuron or to form a motor endplate in a muscle. Dendrites are usually short and divided while the axons are very long and does not branched freely. The impulses are transmitted through the motor neuron in one direction, i.e. into the cell body by the dendrites and away from the cell body by the axon . The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules called Nissl bodies are found in the cytoplasm of the cell body. Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon. The axon is surrounded by the myelin sheath, which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells. The myelin sheath together with the neurilemma is also known as the medullary sheath. This medullary sheath is interrupted at intervals by the nodes of Ranvier.
Nerve cells are functionally connected to each other at a junction known as a synapse, where the terminal branches of an axon and the dendrites of another neuron lie in close proximity to each other but normally without direct contact. Information is transmitted across the gap by chemical secretions called neurotransmitters. It causes activation in the post-synaptic cell.
Classification of Neurons
On the basis of their structure, neurons can also be classified into three main types:
- Unipolar Neurons
- Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar neurons.
- Multipolar Neurons
- Motor neurons, which have numerous cell processes (an axon and many dendrites) are often referred to as multipolar neurons. Interneurons are also multipolar.
- Bipolar Neurons
- Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other . An example can be found in the light-sensitive retina of the eye.