Cell Body Segment

The cell body segment (perikaryon) is the trophic or metabolic center of the neuron. It consists of the nucleus and the cytoplasm with its organelles around the nucleus. The location of the cell body is not critical to the functioning of neurons that are pseudounipolar in shape because the cell body is not involved in the generation, conduction, or transmission of nerve impulses.

Receptive Segment

The receptive segment of the neuron, also referred to as the dendritic zone, varies in shape and location. It is the site on a neuron where a stimulus results in a change in the cell membrane potential that will generate a nerve impulse in the trigger zone. In primary afferent neurons the receptive segment is in the receptor (the ending in a sense organ). In the olfactory system, the receptive segment is the distal end of the cell body of the neuron that is acted on by the chemical odor. In interneurons and LMNs the dendrites and cell bodies are the receptive segment and are acted on by chemical neurotransmitters released by synaptic terminals of other neurons. In some instances, the receptive segments may be on the synaptic end bulbs (the transmissive segment) of a neuron (axoaxonic synapses; see Fig. 14-7D). Receptive segments of neurons function by developing a nonpropagated change in the membrane potential of the neuron that is spread decrementally along the neuron to affect the trigger zone segment of the neuron (see Fig. 14-1).

Trigger Zone

The trigger zone segment is the site of generation of nerve impulses in the conductile segments of the neuron. The trigger zone of receptor cells of the olfactory system, interneurons, and LMNs is the initial segment of the axon just distal to the axon hillock of the cell body. The trigger zone of primary afferent neurons is the part of an axon that is the closest to the receptive segment of the neuron (the receptor).

Conductile Segment

The conductile segment of the neuron is commonly referred to as the neuronal fiber or axon. In the case of bipolar, or pseudounipolar, neurons, the conductile segment is divided into two parts, depending on the relationship of the segment to the cell body. The conductile segment of a primary afferent neuron that begins at a receptor in the periphery is referred to as the peripheral conductile segment. (As stated earlier, in many histology textbooks this is erroneously called a dendrite.) The conductile segment of these cells that carries the nerve impulses away from the cell bodies and into (and within) the CNS is called the central conductile segment. The conductile segments of olfactory neurons, interneurons, and LMNs begin at a specialized part of the cell body referred to as the axon hillock, and the conductile segments, the axons, extend away from the cell bodies to their termination at the transmissive segments, which are the location of the synapses with other neurons or effector organs. In alpha, beta, and gamma LMNs, the axon begins at the axon hillock and has a long peripheral conductile segment within the PNS that conducts nerve impulses to the transmissive segment. Spinal cord autonomic lower motor neurons have preganglionic axons that have synapses with ganglionic autonomic neurons (postganglionic) (see Fig. 14-11). Ganglionic autonomic neurons have postganglionic axons that activate smooth or cardiac muscle or glands or, in the case of the adrenal medulla, release the neurotransmitter directly into the blood stream.

The conductile segments of all neurons are capable of conducting impulses in either direction, depending on the site on the neuron where the neuronal impulses are generated (see Fig. 14-4). In some primary afferent neurons that have branching peripheral conductile segments, each with a receptor associated with them, a nerve impulse generated at the trigger zone of one branch can travel toward the CNS via the conductile segment, then travel away from the CNS via another branch of the conductile segment. Those nerve impulses traveling away from the trigger zone are referred to as being conducted orthodromically; those traveling away from the CNS are referred to as being conducted antidromically (in human primary afferents, anterograde). Dendrites of interneurons and LMNs are not included in the conductile segments of these neurons because dendrites of these neurons do not conduct nerve impulses, but the bioelectrical change initiated in them by synapses spreads decrementally (not regenerated as a nerve impulse) to the cell body (orthodromically) or away from the cell body (antidromically).

Transmissive Segment

The transmissive segment of a neuron is located at the terminal end of the conductile segment, the axon, called the telodendron of the neuron. The transmissive segment of primary afferent neurons, interneurons, lower motor neurons and projection neurons of the CNS is modified to form a synaptic end bulb (see Fig. 14-7), where neurotransmitters are released on the receptive segments of interneurons or efferent neurons. The transmissive segments of alpha, beta, and gamma motor neurons are modified to form neuroeffector junctions (motor end plates), where neurotransmitters are released to act on the effector (muscle fibers).

The peripheral terminal ends of sensory (afferent) axons are modified to form specialized structures, called receptors (receptori) (Figs. 14-5 and 14-6). These may, or may not, have connective tissue capsules (corpusculi nervosi capsulata) surrounding them. Receptors are specialized to generate neuronal impulses in their axons when a specific stimulus or sensory modality is applied to them. These include chemical, mechanical, thermal, or electromagnetic energy. In humans, each receptor type has a low threshold for a specific modality thus is most sensitive to that modality. Free nerve endings (terminationes nervi libera) (see Fig. 14-6A) are responsive to warm or cold temperatures, noxious stimuli that are potentially or actually damaging to tissue and may be associated with perception that is exhibited as pain, and crude mechanical stimulation. End bulbs of Krause (corpora bulboidei) (Fig. 14-6B and C) respond to slow movement (flutter). Pacinian corpuscles (corpusculi nervosi capsulata) (Fig. 14-6D) respond to rapid vibration. Muscle spindles (fusi neuromusculari) (Fig. 14-6E) detect muscle length and the velocity at which a muscle is being stretched. Golgi tendon organs (fusi neurotendini) (Fig. 14-6F) detect muscle tension. Ruffini corpuscles (corpora tacti) (Fig. 14-6G) respond to tangential stretching. Hair follicle receptors (terminationes folliculi pili) (Fig. 14-6H) respond to movement but not to continual pressure, and thus are referred to as mechanical motion detectors. Merkel discs (epithelioidocyti tacti) (Fig. 14-5) and tactile corpuscles (toruli tactiles) (Fig. 14-5) respond continuously to sustained perpendicular pressure.

Other afferent as well as efferent axons may have specialized branches (telodendria) at their terminal endings. These telodendria end by forming a synaptic end bulb (synapsis vesicularis), in which synaptic vesicles (vesicula synaptica) containing a neurotransmitter are concentrated (Fig. 14-7). The synaptic end bulbs contact another neuron (dendrites, cell body, or occasionally an axon). Efferent axons ending on muscle form motor end plates (terminationes neuromusculi). The telodendria form synapses where they release neurotransmitters that affect the membrane channels of the other neuron or the muscle fiber.