Neuron: Structure, function, and types

Neurons are cells that are specialized for the reception, conduction, and transmission of electrochemical signals. Neurons share characteristics with other cells in the body, but they also are special. By generating electrical impulses and sending messages to other cells, neurons function as information units: their activity is meaningful with respect to behavior. Neurons also are plastic: they have the capacity to change, allowing them to serve as memory units. They come in an incredible variety of shapes and sizes; however, many are similar to the ones illustrated in Figures 3.5 and 4.3.

External anatomy of neurons :

Its dendrites feature small protrusions called dendritic spines that greatly increase the cell’s surface area (Figure 3.5). A neuron may have from 1 to 20 or more dendrites, each of which may have one or many branches. Each neuron has but a single axon that may branch into axon collaterals, which usually emerge from it at right angles. The axon collaterals may divide into a number of smaller branches called teleodendria (“end branches”) before contacting the dendrites of another neuron. At the end of each teleodendrion is a knob called an end foot, or terminal button. The terminal button sits very close to a dendritic spine on another neuron, although it does not touch that spine. This “almost connection” between the surface of the axon’s end foot and the corresponding surface of the neighboring dendritic spine plus the space between the two is the synapse. The cell body, or soma, fuels the cell and houses in its nucleus the chromosomes that carry genetic instructions. Extending from the soma, a distinctive enlargement called the axon hillock (“little hill”) forms the beginning of the neuron’s axon. Information flow through a neuron from the dendritic tree to the terminal button. Information flows from the dendrites to the cell body to the axon hillock and through the axon to its teleodendria and their terminal buttons. At each terminal button, information is conveyed to the next neuron. Some synapses are inhibitory: they decrease the neuron’s ability to pass the information along to other neurons. Other synapses are excitatory: they increase the neuron’s ability to pass the information along to other neurons. Information travels through a neuron on a flow of electrical current that begins on the dendrites and travels along the axon to the terminals. In the axon, the summated flow consists of discrete electrical impulses. As each impulse reaches the terminal buttons, they release a chemical message. The message, a neurotransmitter, carries the signal across the synapse to influence the target cell’s electrical activity— to excite it or inhibit it—and pass the information along.

Internal anatomy of neurons :

Neurons, like all living cells, are like miniature factories, and their product is proteins, complex organic compounds that form the principal components of all cells. Each gene in a cell’s DNA contains plans for making one protein, but retooling along the cellular assembly line enables the cell to make many more proteins than there are genes. Figure 4.3 diagrams many parts of a neuron that cooperate to make and ship the proteins it uses to regulate its own activity or exports to regulate the activity of other neurons. As we describe them, you will see that the factory analogy is apt indeed. In our cellular factory, outer walls separate the neuron from the outside world and discourage unwanted intruders. The outer cell membrane separates it from its surroundings and allows it to regulate the materials that enter and leave its domain. The cell membrane envelops the cell body, the dendrites and their spines, and the axon and its terminals, forming a boundary around a continuous intracellular compartment. Unassisted, very few substances can enter or leave a cell because the membrane presents an almost impenetrable barrier. Embedded membrane proteins serve as the factory’s gates, allowing some substances to leave or enter and denying the rest passage. Within the cell shown in Figure 4.3, other membranes divide their interior into compartments. This setup allows the cell to concentrate chemicals where they are needed and otherwise keep them out of the way.

Prominent among the cell’s internal membranes is the nuclear membrane. It surrounds the nucleus, which, like the factory’s executive offices, houses the blueprints—genes and chromosomes—where the cell’s proteins are stored and copied. When needed, genetic instructions or messages are sent to the factory floor, the endoplasmic reticulum (ER), an extension of the nuclear membrane where the cell’s protein products are assembled. The finished proteins are packed in a membrane and addressed in the Golgi bodies that pass them along to the cell’s transportation network. This network is a system of tubules that carries the packaged proteins to their final destinations (much like a factory’s conveyor belts and forklifts). Other tubules, microfilaments, form the cell’s structural framework. Two other vital components of the cellular factory are mitochondria, the cell’s power plants, which supply it energy, and lysosomes, saclike vesicles that not only transport incoming supplies but also move and store wastes. (More lysosomes are found in old cells than in young ones. Cells apparently have trouble disposing of their garbage just as we do.)

Classes of neurons :

Figure 3.8 illustrates a way of classifying neurons based on the number of processes (projections) emanating from their cell bodies. A neuron with more than two processes extending from its cell body is classified as a multipolar neuron; most neurons are multipolar. A neuron with one process extending from its cell body is classified as a unipolar neuron, and a neuron with two processes extending from its cell body is classified as a ccc. Neurons with a short axon or no axon at all are called interneurons; their function is to integrate neural activity within a single brain structure, not to conduct signals from one structure to another. Classifying neurons is a complex task, and neuroscientists still don’t agree on the best method of classification (see Wichterle, Gifford, & Mazzoni, 2013).

Neurons and neuroanatomical structure :

In general, there are two kinds of gross neural structures in the nervous system: those composed primarily of cell bodies and those composed primarily of axons. In the central nervous system, clusters of cell bodies are called nuclei (singular nucleus); in the peripheral nervous system, they are called ganglia (singular ganglion). (Note that the word nucleus has two different neuroanatomical meanings; it is a structure in the neuron cell body and a cluster of cell bodies in the CNS.) In the central nervous system, bundles of axons are called tracts; in the peripheral nervous system, they are called nerves.

Neurons can differ in many ways—in overall size, length, and branching of axons, and complexity of dendritic processes as well as in their biochemistry and activity. Figure 3.7 illustrates some differences in size and shape among the three basic types of neurons from different parts of the nervous system.

1. Sensory neurons. The simplest sensory receptor, a cell that transduces sensory information into nervous system activity, is the bipolar neuron. It consists of a cell body with a dendrite on one side and an axon on the other and is found in the retina of the eye. A somatosensory neuron projects from a sensory receptor in the body into the spinal cord. Its dendrite and axon are connected, which speeds information conduction because messages do not have to pass through the cell body .

2. Interneurons. Interneurons within the brain and spinal cord link up sensory- and motor-neuron activity in the CNS. The many kinds of interneurons all have multiple dendrites that branch extensively, but like all neurons, a brain or spinal-cord interneuron has only one axon (although it can branch). Interneurons include stellate (star-shaped) cells characterized by many branches, pyramidal cells in the cortex that have a pyramid-shaped cell body, and Purkinje cells of the cerebellum. The pyramidal and Purkinje cells are the output cells of their respective structures.

3. Motor neurons. Motor neurons located in the brainstem and spinal cord project to facial and body muscles. Together, motor neurons are called “the final common path” because all behavior (movement) produced by the brain is produced through them.

The various types of glial cells have different functions as well. Five types of glia are described in Table 3.1.

Neurons are not the only cells in the nervous system; there are about as many glial cells, or glia (pronounced “GLEE-a”). It has been reported that there are 10times as many glia as neurons, but this is incorrect (see Lent et al., 2012): In the human brain, there are roughly equal numbers of neurons and glia (see Nimmerjahn & Bergles, 2015). There are several kinds of glia (see Freeman, 2010). oligodendrocytes, for example, are glial cells with extensions that wrap around the axons of some neurons of the central nervous system. These extensions are rich in myelin, a fatty insulating substance, and the myelin sheaths they form to increase the speed and efficiency of axonal conduction (see Long & Corfas, 2014; McKenzie et al., 2014). A similar function is performed in the peripheral nervous system by Schwann cells, the second class of glia. Oligodendrocytes and Schwann. Notice that each Schwann cell constitutes one myelin segment, whereas each oligodendrocyte provides several myelin segments, often on more than one axon. Another important difference between Schwann cells and oligodendrocytes is that only Schwann cells can guide axonal regeneration ( regrowth) after damage. That is why effective axonal regeneration in the mammalian nervous system is restricted to the PNS. microglia make up the third class of glia. Microglia are smaller than other glial cells—thus their name. They respond to injury or disease by multiplying, engulfing cellular debris or even entire cells (see Brown & Neher, 2014), and triggering inflammatory responses (see Smith & Dragunow, 2014). Astrocytes constitute the fourth class of glia. They are the largest glial cells, and they are so named because they are star-shaped (Astron means “star”). The extensions of some astrocytes cover the outer surfaces of blood vessels that course through the brain; they also make contact with neurons. These particular astrocytes appear to play a role in allowing the passage of some chemicals from the blood into CNS neurons and in blocking other chemicals (see Paixão & Klein, 2010), and they have the ability to contract or relax blood vessels based on the blood flow demands of particular brain regions (see Howarth, 2014; Muoio, Persson, & Sendeski, 2014).

Neurotransmission :

Information is transmitted across a chemical synapse in four basic steps. Each step requires a different chemical reaction:

1. During synthesis, either the transmitter is produced by the cell’s DNA and imported into the axon terminal or its building blocks are imported and it is manufactured in the axon terminal.

2. During the release, the transmitter is transported to the presynaptic membrane and released in response to an action potential.

3. During receptor action, the transmitter traverses the synaptic cleft and interacts with receptors on the target cell’s membrane.

4. During inactivation, the transmitter either is drawn back into the presynaptic axon or breaks down in the synaptic cleft. Otherwise, it would continue to work indefinitely.

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