Nerve Fiber Classification

Like nerve fibers, nerves that conduct impulses into the brain or spinal cord are called sensory nerves, and those that carry impulses to muscles or glands are termed motor nerves. Most nerves, however, include both sensory and motor fibers, and they are called mixed nerves.

Nerves originating from the brain that communicate with other body parts are called cranial nerves, whereas those originating from the spinal cord that communicate with other body parts are called spinal nerves. The nerve fibers within these structures can be subdivided further into four groups as follows:

1. General somatic efferent fibers carry motor impulses outward from the brain or spinal cord to skeletal muscles and stimulate them to contract.

2. General visceral efferent fibers carry motor impulses outward from the brain or spinal cord to various smooth muscles and glands associated with internal organs, causing certain muscles to contract or glands to secrete.

3. General somatic afferent fibers carry sensory impulses inward to the brain or spinal cord from receptors in the skin and skeletal muscles.

4. General visceral afferent fibers carry sensory impulses to the central nervous system from blood vessels and internal organs.

The term general in each of these categories indicates that the fibers are associated with general structures such as the skin, skeletal muscles, glands, and viscera. Three other groups of fibers, found only in cranial nerves, are associated with more specialized, or special, structures:

1. Special somatic efferent fibers carry motor impulses outward from the brain to the muscles used in chewing, swallowing, speaking, and forming facial expressions.

2. Special visceral afferent fibers carry sensory impulses inward to the brain from the olfactory and taste receptors.

3. Special somatic afferent fibers carry sensory impulses inward to the brain from the receptors of sight, hearing, and equilibrium.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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