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 2. 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. They also have more specialized structures, including dendrites and axons.
Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections.įigure 1. Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. 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 1 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.
But neurons are also highly specialized-different types of neurons have different sizes and shapes that relate to their functional roles. Most neurons share the same cellular components. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors-from basic reflexes to more complicated behaviors like finding food and courting mates. A human brain contains around 86 billion neurons. This number compares to 75 million in the mouse and 300 million in the octopus. The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. List and describe the four main types of neurons.List and describe the functions of the structural components of a neuron.
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