![]() ![]() Unipolar neurons have only one process emerging from the cell body which causes them to appear T-shaped. Multipolar cells have more than two processes, the axon and two or more dendrites. Bipolar cells have two processes, the axon and a dendrite. Unipolar cells have one process that includes both the axon and dendrite. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron’s polarity (Figure 2). Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. The first way to classify them is by the number of processes attached to the cell body. They can be classified by many different criteria. And there are many different types of neurons. There are many neurons in the nervous system-a number in the trillions. These bulbs are what make the connection with the target cell at the synapse. At the end of the axon is the axon terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic end bulb. The length of the axon between each gap, which is wrapped in myelin, is referred to as an axon segment. Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. It acts as insulation, much like the plastic or rubber that is used to insulate electrical wires. Myelin is a fatty substance that makes the axon appear white. Many axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Because the axon hillock represents the beginning of the axon, it is also referred to as the initial segment. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. This is a tapering of the cell body toward the axon fiber. Where the axon emerges from the cell body, there is a special region referred to as the axon hillock. The major parts of the neuron are labeled on a multipolar neuron from the CNS. Figure 1 shows the relationship of these parts to one another. This gives the neuron a polarity-meaning that information flows in this one direction. Information flows through a neuron from the dendrites, across the cell body, and down the axon. The dendrites are projections that branch many times, forming small, tree-shaped structures protruding from the cell body that provide locations for other neurons to communicate with the cell body. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called synapses. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. That single axon can branch repeatedly to communicate with many target cells. Neurons are usually described as having one, and only one, axon-a fiber that appears as a long cord emerging from the cell body and projects to target cells. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. The cell body contains the nucleus and most of the major organelles. Parts of a NeuronĪs you learned in the first section, the main part of a neuron is the cell body, which is also known as the soma (soma = “body”). The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible. An important part of the function of neurons is in their structure or shape. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. Neurons are the cells considered to be the basis of nervous tissue. Neurons are important, but without glial support they would not be able to perform their function. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. ![]() Glial cells, or glia, are known to play a supporting role for nervous tissue. They are electrically active and release chemical signals to target cells. They are responsible for the computation and communication that the nervous system provides. Neurons are the primary type of cell that most anyone associates with the nervous system. Nervous tissue is composed of two types of cells, neurons and glial cells. 5 Nervous system The introductory part of the chapter is adapted from BC OpenStax Anatomy and Physiology book under the CC BY license. ![]()
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