Cellular Biology

The Structure and Function of Neurons



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A neuron is a nerve cell, a specialized cell that conducts nerve impulses. Its function is to transmit information throughout the body and brain. Sensory neurons transmit information from the senses to the spinal cord and brain. Motor neurons send information in the other direction, from the brain to the muscles. Neurons can also be categorized as afferent, if they send messages toward the central nervous system, or efferent, if they send information away form it. Interneurons transmit information between neurons.

Neurons are like ordinary animal cells in many ways. Both types have a nucleus containing DNA, a protective membrane, and organelles, subunits of the cell. Organelles include the mitochondria, which produce energy for the cell's use and the Golgi Apparatus, which modifies proteins.

There are differences between neurons and other cells, however. Neurons do not divide and reproduce. In general, they cannot be replaced. (However, one Harvard study appears to show that in limited circumstances they may be, by neural stem cells previously held in a dormant state in the brain.) Neurons are capable of making new connections throughout life, and it appears to be the connections, rather than the neurons themselves, which are responsible for brain activity.

Neurons also have synapses and neurotransmitters, which other cells do not. These form the pathways along which neurons transmit information.

Structure

Neurons are not rounded shapes like many cells. They are many-branched blobs, and may have a long tail, perhaps with branches at its end. The branches that surround the soma (body) of the cell are the dendritic tree. They gather the information that the cell will transmit. The (sometimes very long) tail is the axon. At its end are the branching terminals that transmit the signal onward. The axon is sometimes covered by a myelin sheath, which insulates it.

Dendrites are treelike extensions of the neuron. The name comes form the Greek for tree. They are spotted with synapses, the junctions where electrochemical information enters the cell. They receive information and transmit it to the soma as electrical information. They are not passive conduits, but can consolidate and modulate the information they receive. Most neurons have many dendrites.

The soma, the cell body, is where messages from the dendrites are passed on. The nucleus and the soma do not modify the messages. Their job is the maintenance of the cell.

The axon hillock is at the end of the soma closest to the axon. It serves as an electrical gate. A central fact about neurons is that they do not fire a graded amount. They cannot send a weak message or a strong one. Either they fire, send a message, or they don't. Whether there is enough information to send a message is decided at the axon hillock. If the charge, the amount of input, exceeds the hillock's threshold, the neuron will fire.

The axon is the fiber that transmits the signal away from the soma and on to other neurons. Most neurons have only one. Its fiber is thicker and often longer than the dendrites'. It extends from the cell body to the terminal endings.

Some axons are enclosed in a myelin sheath. Myelin is an insulator, composed mostly of fat. It forms the white matter of the brain. Axons with myelin insulation send their impulses faster, and by a different method that those without it. In addition, if the axon is damaged, it can use the sheath as a sort of framework to rebuild, while axons without sheaths cannot repair themselves.

A myelin sheath is formed of Schwann cells with gaps between them called nodes of Ranvier. Schwann cells are glial cells, that is, cells evolved to provide support and nutrition to neurons. If an axon is damaged, the Schwann cell can form a tunnel to guide its repair. Schwann cells are shorter than most axons, and curl around them to look like long beads along a chain. The gaps between the Schwann cells are called nodes of Ranvier. The message a neuron carries travels by jumping from node to node in a myelinated axon, and travels faster that way, using less energy than an axon without myelin requires.

The terminal buttons at the end of the axon pass the signal on. Terminal buttons abut a gap called the synapse. Sometimes a message can jump this gap electrically, but usually it is assisted by neurotransmitters that help it on its way.

Function

The message that a neuron transmits is an electrochemical impulse. The impulse is an action potential received in the dendrites, integrated, and transmitted by the terminal buttons of the axon.

This action potential travels through the neuron electrically. At the synaptic cleft, which is the gap between one neuron and another, or between the neuron and the destination of its message, the impulse is usually transmitted chemically. The chemicals that receive and transmit the action potential are called neurotransmitters.

Neurotransmitters are chemicals made and modified by neurons that carry messages across the synaptic cleft. They gather inside the terminal buttons of the axon, are released to carry a message, and bind to receptors in the cell on the other side of the gap. The neurotransmitters are fast, do not weaken the signal, and generally only pass a message in one direction.

An action potential traveling in a neuron can be thought of as a voltage pulse, one that travels up a nerve and leaps a gap to its target in another neuron, a muscle, or a gland.

Conclusion

This has been a very rough sketch of a fascinating subject. To see an excellent schematic of a neuron, click here. For one reliable source to explore the subject further, click here.

Neurons carry messages to and from the brain. These messages are transmitted electrochemically. Neurons relay information from the senses to the brain, and responses from the brain to muscles and glands.

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