6.3: Neurons - Structure and Functioning

Components

All the billions of neurons in the nervous system have three basic parts. The nerve cell body contains the nucleus of the cell along with cytoplasm and organelles (e.g., mitochondria and ribosomes) (Figure 6.2a). The nerve cell body supplies the other two parts of the neuron with the materials and energy they need. It can also pick up messages from other neurons.

Figure 6.2a Neuron structure (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

There are three types of nerve cells based on the number of extensions from the nerve cell body (Figure 6.2b). A unipolar neuron has one extension, which branches very close to the nerve cell body into a dendrite and an axon. A bipolar neuron has two extensions, a dendrite and an axon. A multipolar neuron has more than two extensions from the cell body; two or more dendrites and one axon.

Figure 6.2b Neuron types (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Each dendrite can branch up to several hundred times. Like nerve cell bodies, dendrites can pick up messages from other nerve cells. They are also the parts of the sensory cells that monitor conditions. A dendrite being activated by another neuron or by a stimulus starts nerve impulses that travel along the dendrite to the nerve cell body, which passes the impulses to the third part of the neuron: the axon.

Each neuron has only one axon, which extends out from the nerve cell body. Each axon may have up to several hundred branches (axon collaterals). The impulses that are passed to the axon travel the entire length of each of its branches. Each branch then passes the impulses to another structure. Axons can pass impulses to other neurons, muscle cells, and gland cells, although all the branches from one neuron's axon can go to only one of these types of cells.

Operations

Reception

All neurons perform three main functions. Reception involves having impulses generated in response to environmental conditions or messages from other neurons. Dendrites and nerve cell bodies are the parts that usually perform reception (Figure 6.3a).

Figure 6.3a Neuron functioning (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Conduction

The second function - conduction - refers to the movement of impulses along the neuron to the end of the axon (Figure 6.3a). Conduction in longer dendrites and axons occurs through a special mechanism called an action potential. This mechanism involves several activities of the neuron cell membrane that carefully control the inward and outward movement of ions, especially sodium and potassium ions.

Transmission

Once impulses have been conducted to the end of the axon, they are passed to the next structure by the third neuron function: transmission (Figure 6.3a). The place where transmission occurs between neurons is called a synapse. Transmission to muscle cells occurs at neuromuscular junctions, and transmission to gland cells takes place at neuroglandular junctions. The process of transmission is essentially the same in all three cases.

At a synapse, when an action potential reaches the end of an axon, it causes small packets (synaptic vesicles) at the end of the axon terminal to burst like blisters (Figure 6.3b). These packets contain a chemical called a neurotransmitter, which is then released into the small space (synaptic cleft) between the neurons. Most neurons can release only one type of neurotransmitter. The neurotransmitter diffuses to the dendrite or cell body of the next neuron, where it attaches to receptor molecules on the cell membrane. Each type of receptor molecule is designed to bind to only one type of neurotransmitter.

Figure 6.3b Neuron functioning (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Once enough neurotransmitter has been bound to the receptor molecules, the receiving neuron responds. Depending on the type of neurotransmitter and the type of neuron, the receiving neuron will be stimulated to perform reception and start its own impulses or will be inhibited from acting. The nervous system uses stimulatory transmissions to start or speed up an activity; it uses inhibitory transmissions to slow down, stop, or avoid an activity. A neurotransmitter continues to have its effect on the next cell until it is eliminated or counteracted. Neurotransmitters can be counteracted when antagonistic neurotransmitters are sent into the synapse.

Although a few synapses involve one neuron transmitting to one other neuron, synapses often have many neurons converging to transmit messages to a single neuron. The amount and length of the response by the receiving neuron depend on the balance between the amount of stimulatory and inhibitory neurotransmitters it receives at any moment from the many neurons connected to it. Thus, by changing the combinations of neurotransmitters at synapses, the nervous system can provide exquisitely precise adjustments to its impulses and the resulting body activities. The effect of such an interplay of stimulatory and inhibitory transmitters is experienced, for example, by a person whose hands are being burned by a hot beverage but who puts down the cup slowly and carefully to avoid spilling the beverage.

The branching of axons allows for divergence. Thus, impulses in one neuron can spread to many muscle cells, gland cells, or neurons. One can experience the effects of divergence when hearing a frightening sound or noticing a flirtatious glance. The heart pounds, the breathing increases, the stomach tightens, and the legs may become weak and shaky.

Another important function of synapses is to keep order in the nervous system. Since messages can pass only from axons to the next neuron, synapses ensure that impulses move through the system only in the correct direction. Finally, synapses play an essential role in remembering.

Neuroglia

The CNS contains neuroglia cells, which provide a variety of services for the neurons (e.g., support and defense). These cells do not perform reception or conduct or transmit nerve impulses. One type makes a material called myelin, which forms a coating on CNS axons. The myelin coating on an axon resembles beads on a string. It causes impulses to travel faster by making them jump along the neuron (Figure 6.2a). Since myelin is white, it causes the regions that contain it to become white in appearance; these areas are referred to as the white matter of the brain and spinal cord.

The areas of the CNS that do not have myelin possess the pinkish gray color of plain neurons; these regions constitute the gray matter. The gray matter is important because it contains the synapses. All the complicated nervous system functions, including coordination, remembering, and thinking, require these synapses.

Schwann cells

Neurons in the PNS are assisted by Schwann cells. These cells produce myelin on dendrites and axons; this myelin is structurally and functionally similar to CNS myelin (Figure 6.2a).