Nerves

A nerve is defined as a collection of nerve fibers beyond the central nervous system. Loose connective tissue holds the nerve together from inside the gathered fibers. This also helps to strengthen the nerve. The individual nerve fibers are then protected within a sheath of connective tissue, called endoneurium. The group of fibers is then protected with individual sheaths known as perineurium. The group of fibers is known as fasciculusis. The entire nerve is then protected via another protective coating of epineurium. The epineurium contains tiny blood vessels and is likely to contain adipose cells. This means that will all the connective tissue protective layers, only about ¼ of the bulk of a nerve is comprised of actual nerve tissue. Over half of a nerve is comprised of connective tissue and about another quarter is taken up by the myelin.

Mixed nerves, which are a mixture of motor nerves and sensory nerves, makes up the majority of the nerves in the human body. Cranial nerves are comprised of either sensory nerves or motor nerves but are rarely made of mixed nerves. Taste, smell, hearing, and sight are directed by the special sensory nerves. Motor nerves send impulses to the muscles for movement and the glands for secretion. Neurons and nerve fibers are classified by the area of the body which they serve.

Somatic sensory refers to the sensory receptors and subsequent nerve systems of the skin, bones muscles, and joints. The receptors receive the stimuli and transmit the information back through the somatic sensory fibers to the central nervous system where integration occurs. The eyes and ears also contain somatic sensory systems.

Somatic motor systems are responsible for muscle contraction. Nerve impulses traverse the somatic motor system from the central nervous system to induce muscle contraction.

Nerves

Visceral sensory fibers are responsible for conveying the impulses from the visceral organs to the central nervous system as well as from the blood vessels to the nervous system where integration takes place. The senses of taste and smell are also known as visceral senses and are part of a visceral sensory system.

Visceral motor fibers are also referred to as autonomic motor fibers. They are considered to be a part of the autonomic nervous system. Information comes from the central nervous system and innervates the smooth muscle tissue of the visceral organs, the cardiac muscle tissue, and the glands.

Neurons are distinguished from each other by structure and function. When classified by function, the classification is based on the direction of the nerve impulses, either going to the brain or going away from the brain. Sensory pulses, which come from the sensory receptors, are conducted via afferent (sensory) neurons to the central nervous system. Efferent (motor) neurons are responsible for the conduction of pulses to the muscles and glands. Motor neurons can be either visceral or nonvisceral, alternatively known as somatic or autonomic. Positioned between the sensory neurons and the motor neurons are the association neurons, also known as the interneurons. Association neurons are located in the brain and the spinal cord.

When classifying neurons by structure, the number of process that extend from the cell body of the neuron distinguishes one type of neuron from another. With processes at both ends, the bipolar neuron can be recognized by its spindle shape as well. These are found within the retina of the eye. Psuedounipolar neurons have just one process which then divides itself in two. These neurons get their name from their original start as bipolar neurons which go through the convergence of the processes and partial fusion during their embryonic stage. The majority of sensory neurons are pseudounipolar neurons, their cell bodies positioned within the sensory ganglia of the cranial or spinal nerves.

The most common type of neuron is the multipolar neuron which can be recognized by their multiple dendrites and one axon which protrudes from the body of the cell. One of the better samples of this type of neuron is the motor neuron.