Multipolar neurons can be found in the central nervous system (brain and spinal cord). Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons are the most common type of neuron. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain. A bipolar neuron has one axon and one dendrite extending from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. Unipolar neurons have only one structure that extends away from the soma. Figure 7.7 illustrates these four basic neuron types. While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Examples include (a) a pyramidal cell from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and (c) olfactory cells from the olfactory epithelium and olfactory bulb. There is great diversity in the size and shape of neurons throughout the nervous system. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.įigure 7.6. Dendrites from a single neuron may receive synaptic contact from many other neurons. It is important to note that a single neuron does not act alone-neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon. Along the axon there are periodic gaps in the myelin sheath. The myelin sheath is not actually part of the neuron. This insulation is important as the axon from a human motor neuron can be as long as a meter-from the base of the spine to the toes. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Chemicals released at axon terminals allow signals to be communicated to these other cells. These terminals in turn synapse on other neurons, muscle, or target organs. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. Once a signal is received by the dendrite, it then travels passively to the cell body. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Neurons also contain unique structures, illustrated in Figure 7.5 for receiving and sending the electrical signals that make neuronal communication possible. Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components.
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