![]() In multipolar neurons, multiple processes extend from the cell body including dendrites and axons. Most neurons in the central nervous systems of vertebrates, including mammals, are multipolar. The cell bodies of invertebrate unipolar neurons are often located around the edges of the neuropil, in the so-called cell-body rind. Most neurons in the central nervous systems of invertebrates, including insects, are unipolar. The neurite then branches to form dendritic and axonal processes. Cell bodies are usually grouped into separate bundles called ganglia.A unipolar neuron is a neuron in which only one process, called a neurite, extends from the cell body. Most nerves contain both sensory and motor fibers. The following terms apply to neurons and groups of neurons:Ī nerve is a bundle of nerve fibers in the peripheral nervous system (PNS). Unipolar neurons are mostly sensory neurons. The trigger zone in a unipolar neuron is located at the junction of the axon and dendrites. Dendrites emerge from one of the terminal ends of the axon. Both processes function together as a single axon. Unipolar neurons have one process of emerging from the cell body that branches, T‐fashion, into two processes. Bipolar neurons are found only as specialized sensory neurons in the eye, ear, or olfactory organs. They emerge from opposite sides of the cell body. Most neurons are of this type.īipolar neurons have one axon and one dendrite. Multipolar neurons have one axon and several to numerous dendrites. Neurons are structurally classified into three groups, as shown in Figure 1: More than 90 percent of the neurons of the body are association neurons. ![]() Effectors include muscles, sweat glands, and many other organs.Īssociation neurons ( interneurons) are located in the CNS and transmit impulses from sensory neurons to motor neurons. Motor neurons ( efferent neurons) transmit nerve impulses from the CNS toward effectors, target cells that produce some kind of response. Sensory neurons ( afferent neurons) transmit sensory impulses from the skin and other sensory organs or from various places within the body toward the central nervous system (CNS), which consists of the brain and spinal cord. Functionally, they fall into three groups: Neurons can be classified by function or by structure. The synaptic knobs contain neurotransmitters, chemicals that transmit nerve impulses to a muscle or another neuron. At its end, each axon or axon collateral usually forms numerous branches ( telodendria), with most branches terminating in bulb‐shaped structures called synaptic knobs (synaptic end bulbs, also called terminal boutons). The cytoplasm of the axon, the axoplasm, is surrounded by its plasma membrane, the axolemma. A few axons branch along their lengths to form axon collaterals, and these branches may return to merge with the main axon. Nerve impulses arise in the trigger zone, generally located in the initial segment, an area just outside the axon hillock. ![]() It emerges from the cell body at the cone‐shaped axon hillock. The axon is typically a long, slender process of the cell body that sends nerve impulses. The dendrite is typically a short, abundantly branched, slender process (extension) of the cell body that receives stimuli. There are clusters of rough endoplasmic reticulum (not shown in Figure 1) that are called Nissl bodies or are sometimes referred to as chromatophilic substances. The cell body (soma or perikaryon) contains the nucleus and other cell organelles. It consists of the following parts, shown in Figure 1:
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