Cell Types of Nervous System

1. In the free nerve ending, the peripheral process branches widely and ends without obvious specialization. These endings respond primarily to intense stimuli, and are thought to play a role in the perception of pain.

2. In the encapsulated ending, the terminal of the peripheral process is enveloped in an accessory structure modifying the stimulus before it reaches the part of the nerve terminal membrane where the actual stimulus transduction occurs. Examples of encapsulated endings are Ruffini and Golgi endings and pacinian and paciniform corpuscles. The muscle spindle and Golgi tendon organ are highly specialized forms of encapsulated endings in which the sensory nerve terminal also performs stimulus transduction.

3. In the taste buds and the cochlear and vestibular systems, sensory fibers end as synaptic terminals on the bodies of specialized receptor cells, which transduce chemical or mechanical stimulation into a shift in membrane potential, which is then synaptically transmitted.

Olfactory and optic afferent neurons do not fit into any of these categories. Olfactory stimuli are detected by specialized receptor cells with axons projecting directly to interneurons of the olfactory bulb. The retina, which is formed by an outgrowth of the brain, contains both receptor cells and several types of interneurons. The optic nerve, therefore, corresponds more to a central sensory tract than a sensory nerve.

MOTOR NEURONS

All neurons sending efferent axons to the periphery can be described as effector, or motor, neurons. These are typically medium-to-large, multipolar cells with long myelinated axons. There are three classes of motor neurons:

1. Motor neurons supplying skeletal muscles are located in the anterior horn of the spinal cord and project to the periphery via the anterior (ventral) spinal roots. Motor neurons supplying muscles of the face and some muscles of the neck and throat are located in the brainstem motor nuclei and project to their target muscles via the fifth, seventh, and ninth to twelfth cranial nerves. Motor neurons supplying skeletal muscles are of two kinds: alpha motor neurons, which supply the main extrafusal muscle fibers, and fusimotor (gamma motor) neurons, which supply the intrafusal fibers of muscle spindles. The alpha motor neurons have conduction velocities ranging from 50 to 100 m/sec; fusimotor axons have velocities of 20 to 40 m/sec.

Skeletal motor neurons are often referred to as the “final common path,” because they integrate all CNS activity controlling a given muscle, from spindle afferent fibers, spinal interneurons involved in spinal reflexes, brainstem nuclei, and cortical pyramidal cells.

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