![]() The processes of a neuron are classified on a structural basis as an axon ( axon) or as dendrites ( dendriti), based on their shape, length, and cytoplasmic contents. Neurons are classified on the basis of the shape of the cell body and the number, shape, and ramifications of the processes into unipolar ( neuronum unipolare), pseudounipolar ( neuronum pseudounipolare), bipolar ( neuronum bipolare), or multipolar ( neuronum multipolare) cells. The cell body contains the nucleus and organelles and is located at the origin of the axon or at some point along its course. This will be at a synapse with another neuron in the CNS or with an effector organ in the PNS. The axon or conductive segment conducts the impulse that is generated from the dendritic zone to the terminal portion of the axon referred to as the telodendron or transmissive segment. The dendrites or dendritic zone or receptive segment are located where a neuron receives a stimulus from another neuron in the CNS or from its environment in the PNS. ![]() A neuronal cell ( neurocytus), whether in the CNS or PNS, consists of a cell body ( corpus neurona), an axon ( neuritum), and dendrites. The basic anatomic, genetic, and functional unit of the nervous system is the neuron, a highly specialized cell. A nucleus is a collection of neuronal cell bodies in the CNS. A ganglion is a collection of neuronal cell bodies ( corpora neurona) in the PNS and their surrounding supporting cells and connective tissue. These nerves connect the CNS with the structures in the other systems that compose the body. In addition to the axons, a nerve contains their supporting Schwann cells (lemmocytes) and connective tissue arranged as endoneurium, perineurium, and epineurium. The term nerve ( nervus) is applied to a collection of neuronal processes, axons, located in the PNS external to the CNS. The divisions are arbitrary because, as we shall see in this chapter, parts of the same neuron can be in both the CNS and the PNS. The gross nervous system, formed by these two types of cells and connective and vascular tissue, is subdivided into the central nervous system (CNS) ( systema nervosum centrale), consisting of the brain ( encephalon) and spinal cord ( medulla spinalis), and the peripheral nervous system (PNS) ( systema nervosum periphericum), composed of cranial and spinal nerves. The transmissive segments end on other neurons, or on muscle or gland cells in the case of motor neurons. The cell body may be in the receptive segment (olfactory afferent neurons motor and interneurons), in the conductile segment (auditory and vestibular afferent neurons), or branching off of the conductile segment (all other afferent neurons). The location of the cell body is not critical to the functioning of the various neurons. ![]() Olfactory neurons are unipolar, auditory neurons are bipolar, muscle spindle and cutaneous afferents are pseudounipolar, and lower motor neurons and interneurons are multipolar in shape. All neurons, regardless of their shape, have receptive segments (dendritic zones), trigger zones, conductile (axons) and transmissive segments (telodendrons), and trophic areas (cell bodies). Cells composing the nervous system are of two types: neurons ( neurona) that receive and transmit impulses and supporting cells that regulate the rate of axonal conduction of impulses, hold the neurons in place, and prevent the neuronal impulses from spreading between neurons that are not linked by synapses.įIGURE 14-1 Diagram of various kinds of afferent and efferent neurons. The cells constituting the nervous system are highly specialized for receiving stimuli, by means of receptors and highly specialized sense organs, for transmitting and storing information, and for initiating responses to stimuli (Fig. The most extensive treatment of the nervous system can be found in Nieuwenhuys, Donaklaar and Nicholson (1998) The Central Nervous System of Vertebrates 3 volumes, Springer, Berlin (ISBN: 3-0), and the best source for standard terms is to be found in Nomina Anatomica Veterinaria (NAV). Other body systems are specialized to perform various life-sustaining functions, such as locomotion, digestion, respiration, and circulation, and it is essential that these functions be regulated and coordinated by the nervous system. The nervous system ( systema nervosum) informs an animal about its environment, both internal and external, and initiates responses to that environment.
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