Neurons and Nerves

Neurons and Nerves

Various neurons differ in structure, size and shape of their cell bodies. They also vary in the length and size of their axons and dendrites and in the number of synaptic knobs by which they communicate with other neurons.

Neurons also vary in function Some carry impulses into brain or spinal cord, other carry impulses out from the brain or spinal cord, and still other conduct impulses from neuron to neuron within the brain or spinal cord. 

Classification of neurons

On the basis of structural differences, neurons can be classified into three major groups.
 

Multipolar neurons

Multipolar neurons have many processes arising from their cell bodies. Only one process of each neuron is an axon; the rest are dendrites. Most of the neurons whose cell bodies lie within the brain or spinal cord are of this type.
 

Bipolar neurons

The cell body of a bipolar neuron has only two processes, one arising from either end. Although these processes have similar structural characteristics, one serves as an axon and the other as a dendrite. Such neurons are found within specialized parts of the eyes and ears.

Unipolar neurons

Unipolar neurons occur in specialized masses of nerve tissue called ganglia, which are located outside the brain and spinal cord.
 

Each of these neurons has a single process extending from its cell body. A short distance from the cell body, this process divides into two branches; one branch is connected to some peripheral body part and serves as a dendrite; the other enters the brain or spinal cord and serves as an axon.
 

Neurons can also be classified on the basis of their functional differences into the following groups –
 

Sensory neurons

Sensory neurons (afferent neurons) are those that carry nerve impulses from peripheral body parts into the brain or spinal cord. Either these neurons have specialized receptor ends at the tips of their dendrites, on their dendrites are closely associated with receptor cells located in the skin or various sensory organs.
 

Changes that occur inside or outside the body are likely to stimulate receptor ends or receptor cells, triggering nerve impulses. The impulses travel along the sensory neuron fibers, which lead to the brain or spinal cord, and are processed in these parts by other neurons. Most sensory neurons are unipolar.
 

Interneurons

Interneurons (association or internuncial neurons) lie within the brain or spinal cord. They are multipolar and form links between other neurons. Inter-neurons function to transmit impulses from one part of the brain or spinal cord to another. That is, they may direct incoming sensory impulses to appropriate parts for processing and interpreting. Other incoming impulses are transferred to motor neurons.
 

Motor Neurons

Motor neurons (efferent neurons) are multipolar and carry nerve impulses out from the brain or spinal cord to effectors – parts of the body capable of responding, such as muscles or glands. When motor impulses reach muscles, for example- these effectors are stimulated to contract; when they reach glands, the glands are stimulated to release secretions.
 

Two specialized groups of motor neurons supply impulses to smooth and cardiac muscles. One type, the accelerator neurons, cause an increase in muscular activities. The second type, inhibitory neurons, cause such actions to decrease.
 

While a nerve fibers is an extension of a neuron, a nerve is a cordlike bundle (or group of bundles) of nerve fibers held together by layers of connective tissues.

Like nerve fibers, nerves that conduct impulses into the  brain or spinal cord are called sensory nerves, while those that carry impulses out to muscles or glands are termed motor nerves. 

Most nerves, however, include both sensory and motor fibers and they are called mixed nerves.