Classification of Nerve Fibers

Classification of Nerve Fibers

Nerve fibers are classified by their diameter, degree of myelination, and speed of conduction. There are three primary groups of nerve fibers:

■■ Group A fibers: These mostly serve the joints, skeletal muscles, and skin, and are primarily somatic sensory and motor fibers, with the largest diameter of all types of fibers and thick myelin sheaths. These fibers conduct impulses at speeds as high as 300 miles per hour.

■■ Group B fibers: Of intermediate diameter, with light myelination, group B fibers conduct impulses at speeds averaging approximately 30 miles per hour.

■■ Group C fibers: These fibers are nonmyelinated with the smallest diameter and cannot create saltatory conduction; they conduct impulses at 2 miles per hour or less.

Both B and C fibers include motor fibers of the ANS that serve the smaller somatic sensory fibers that transmit sensory impulses from the skin (including small touch and pain fibers), visceral sensory fibers, and those that serve the visceral organs.

Structural Classification of Neurons

Neurons are classified based on the number of pro-cesses that extend from their cell bodies. The three major structural categories of neurons are multipolar, bipolar, and unipolar:

■■ Multipolar neurons: They make up most of the neurons whose cell bodies lie within the brain or spinal cord. They have three or more processes that arise from their cell bodies, with only one process being an axon and the rest being den-drites. Multipolar neurons are the most common, and more than 99% of neurons in the human body are multipolar (FIGURE 11-6). They are also the most common type in the CNS.

■■ Bipolar neurons: These neurons exist only in specialized parts of the eyes and nose. They have only two processes arising from their cell bodies. Only one process of each neuron is an axon and the other is a dendrite. Bipolar neurons are very rare in the body. They are located in the retinas of the eyes and in the nasal cavity.

■■ Unipolar neurons: Often aggregated in specialized ganglia located outside the brain and spinal­ cord, these neurons have a single short process­ extending from the cell body that divides into two T-like branches that function more like a single axon. One branch of the more distal peripheral­ process is associated with dendrites near a peripheral body part and the other branch (the central process) enters the brain or ­spinal cord. Unipolar neurons originate as bipolar neurons and are more accurately described as pseudounipolar­ neurons.

Functional Classification of Neurons

The functional classification of neurons is based on the direction in which action potentials are conducted:

■■ Sensory neurons (afferent neurons): They carry nerve impulses from the peripheral body parts into the CNS. They may have receptor ends at the tips of the dendrites or receptor cells that are associated with the dendrites in the sensory organs or the skin (FIGURE 11-7). Somatic sen-sory neurons monitor the external environment, whereas visceral sensory neurons monitor the body’s internal environment. Sensory receptors are classified as interoceptors, exteroceptors, and proprioceptors. Interoceptors provide sen-sations of deep pressure,­ distension, and pain and are found in the digestive, cardiovascular, respiratory, reproductive, and urinary systems. Exteroceptors provide perception of tempera-ture, touch, pressure, smell, taste, equilibrium, hearing, and sight. Proprioceptors provide per-ception of skeletal muscle and joint movement and position.

■■ Interneurons: They conduct action potentials from one neuron to another within the CNS. The cell bodies of some interneurons form masses called nuclei in the CNS, which are similar toganglia.

■■ Motor neurons (efferent neurons): They conduct action potentials away from the CNS toward mus-cles or glands. Somatic motor neurons innervate skeletal muscles. Their cell bodies lie within the CNS, whereas their axons extend outward within peripheral nerves, innervating skeletal muscle fibers at the neuromuscular junctions. ­Visceral motor neurons innervate smooth and cardiac muscle, glands, and adipose tissue. The axons of these neurons that lie within the CNS innervate other visceral motor neurons in the peripheral autonomic ganglia. Visceral motor neurons that have cell bodies in these ganglia innervate and control the peripheral effectors. Axons extend-ing from the CNS to an autonomic ganglion are known as preganglionic fibers. Axons that connect ganglion cells to peripheral effectors are known as postganglionic fibers.

1. Describe the structures of neurons, dendrites, and axons.

2. Identify the differences between sensory and motor neurons.

3. What are the three major structural categories ofneurons?

4. Differentiate between multipolar and bipolar neurons.