What Is the Structure of a Neuron?

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What Exactly Is a Neuron?

The parts of a neuron
BSIP/UIG / Universal Images Group / Getty Images

Neurons are the basic building blocks of the nervous system. These specialized cells are the information-processing units of the brain responsible for receiving and transmitting information. Each part of the neuron plays a role in communicating information throughout the body.

Neurons carry messages throughout the body, including sensory information from external stimuli and signals from the brain to different muscle groups in the body. In order to understand exactly how a neuron works, it is important to look at each individual part of the neuron. The unique structures of the neuron allow it to receive and transmit signals to other neurons as well as other types of cells.

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Dendrites

Dendrites
Dendrites serve as the input region of the neuron and receive information from other cells. Henning Dalhoff / Science Photo Library / Getty Images

Dendrites are treelike extensions at the beginning of a neuron that help increase the surface area of the cell body. These tiny protrusions receive information from other neurons and transmit electrical stimulation to the soma. Dendrites are also covered with synapses.

Dendrite Characteristics

  • Most neurons have many dendrites
  • However, some neurons may have only one dendrite
  • Many are short and highly branched
  • Transmits information to the cell body

Most neurons possess these branch-like extensions that extend outward away from the cell body. These dendrites then receive chemical signals from other neurons, which are then converted into electrical impulses that are transmitted toward the cell body.

Some neurons have very small, short dendrites, while other cells possess very long ones. The neurons of the central nervous systems have very long and complex dendrites that then receive signals from as many as a thousand other neurons.

If the electrical impulses transmitted inward toward the cell body are large enough, they will generate an action potential. This results in the signal being transmitted down the axon.

Reference

Lodish, H., Berk, A., & Zipursky, S.L., et al. (2000). Molecular Cell Biology, 4th edition. New York: W.H. Freeman.

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Soma

Soma
Ed Reschke / Oxford Scientific / Getty Images

The soma, or cell body, is where the signals from the dendrites are joined and passed on. The soma and the nucleus do not play an active role in the transmission of the neural signal. Instead, these two structures serve to maintain the cell and keep the neuron functional.

Characteristics of the soma:

  • Contains numerous organelles which are mostly made of up endoplasmic reticulum.
  • Contains a cell nucleus that produces RNA that supports important cell functions.
  • Supports and maintains the functioning of the neuron.

Think of the cell body as a small factory that fuels the neuron. The soma produces the proteins that the other parts of the neuron, including the dendrites, axons, and synapses, need to function properly.

The support structures of the cell include mitochondria, which provide energy for the cell, and the Golgi apparatus, which packages products created by the cell and secretes them outside the cell wall.

Reference:

Squire, L., Berg, D., Bloom, F., du Lac, S., Ghosh, A., & Spitzer, N., eds. (2008). Fundamental Neuroscience (3rd ed.). Academic Press.

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Axon Hillock

Axon Hillock
The axon hillock is responsible for neural firing. M.aljar3i / Wikimedia Commons

The axon hillock is located at the end of the soma and controls the firing of the neuron. If the total strength of the signal exceeds the threshold limit of the axon hillock, the structure will fire a signal (known as an action potential) down the axon.

The axon hillock acts as something of a manager, summing the total inhibitory and excitatory signals. If the sum of these signals exceeds a certain threshold, the action potential will be triggered and an electrical signal will then be transmitted down the axon away from the cell body. This action potential is caused by changes in ion channels which are affected by changes in polarization.

In a normal resting state, the neuron possesses an internal polarization of approximately -70mV. When a signal is received by the cell, it causes sodium ions to enter the cell and reduce the polarization.

If the axon hillock is depolarized to a certain threshold, an action potential will fire and transmit the electrical signal down the axon to the synapses. It is important to note that the action potential is an all-or-nothing process and that signals are not partially transmitted. The neurons either fire or they do not.

Reference

Lodish, H., Berk, A., & Zipursky, S.L., et al. (2000). Molecular Cell Biology, 4th edition. New York: W.H. Freeman.

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Axon

Axon
The axon transmits information away from the cell body. Encyclopaedia Britannica/UIG Universal Images Group/Getty Images

The axon is the elongated fiber that extends from the cell body to the terminal endings and transmits the neural signal. The larger the axon, the faster it transmits information. Some axons are covered with a fatty substance called myelin that acts as an insulator. These myelinated axons transmit information much faster than other neurons.

Axon Characteristics

  • Most neurons have only one axon
  • Transmit information away from the cell body
  • May or may not have a myelin covering

Axons can range dramatically in size. Some are as short as 0.1 millimeters, while others can over 3 feet long.

The myelin surrounds the neurons protects the axon and aids in the speed of transmission. The myelin sheath is broken up by points known as the nodes of Ranvier or myelin sheath gaps. Electrical impulses are able to jump from one node to the next, which plays a role in speeding up the transmission of the signal.

Axons connect with other cells in the body including other neurons, muscle cells, and organs. These connections occur at junctions known as synapses. The synapses allow electrical and chemical messages to be transmitted from the neuron to the other cells in the body.

Reference

Debanne, D., Campana, E., Bialowas, A., Carlier, E., Alcaraz, G. (2011). Axon physiology. Psychological Reviews, 91(2), 555-602. DOI: 10.1152/physrev.00048.2009.

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Terminal Buttons and Synapses

Terminal Buttons
Science Photo Library KTSDESIGN / Brand X Pictures / Getty Images

The terminal buttons are located at the end of the neuron and are responsible for sending the signal on to other neurons. At the end of the terminal button is a gap known as a synapse. Neurotransmitters are used to carry the signal across the synapse to other neurons.

The terminal buttons contain vesicles holding the neurotransmitters. When an electrical signal reaches the terminal buttons, neurotransmitters are then released into the synaptic gap. The terminal buttons essentially convert the electrical impulses into chemical signals. The neurotransmitters than cross the synapse where they are then received by other nerve cells.

The terminal buttons are also responsible for the reuptake of any excessive neurotransmitters released during this process.

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