Pseudounipolar cells embryologically start out as bipolar cells, but the cell body grows out on one side, and the two processes emerge from a tortuous neck of the cell (Fig. 14-2). Both processes are structurally axons. Bipolar cells have two processes, classified structurally as axons. Multipolar cells have many processes, one axon and the rest classified as dendrites (Figs. 14-1 and 14-3). Olfactory neurons are unipolar, auditory neurons are bipolar, muscle spindles and cutaneous afferents are pseudounipolar, and lower motor neurons (LMNs) and interneurons are multipolar. The location of the cell body is not critical to the functioning of the various neurons. The cell body may be in the receptive segment such as olfactory afferent neurons, LMNs, and interneurons. The cell body may be in the conductive segment such as auditory and vestibular afferent neurons or the cell body may branch off the conductive segment such as all other afferent neurons. The transmission segments end on other neurons or on muscle or gland cells in the case of motor neurons. 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). 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.
Introduction to the Nervous System
Structure of Neurons
Functional Segments of Neurons
Receptive Segment
Transmissive Segment
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Introduction to the Nervous System
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