What Is a Bipolar Neuron?

A bipolar neuron is a nerve cell located in humans and some animals and is defined by two “processes,” or arm-like extensions, that reach out from either side of the cell body. Like all neurons, these play an important role in the nervous system generally, shuttling sensations back and forth from the nerves to the brain where they can be translated and processed. Neurons with bipolar extensions are almost always involved in sensory responses. They’re really important when it comes to interpreting retinal signals from the eyes, for example, and also impact smell and hearing. Their main job is to detect signal changes, then carry these from the nervous system to the appropriate processing center in the brain.

The nervous system is made firstly, of course, of nerves, but specialized cells known as neurons are the main carriers of the signals emitted by those nerves. Neurons are charged with certain electrochemical particles that allow them to pass signals along intensive networks very quickly. They come in many different shapes and sizes and are usually dedicated to certain types of signals. In many ways they are similar to any other cell in the body; their chemistry is about the same, for instance, and they typically carry genetic material and other proteins and energy-based matter. There are some unique elements, too. Neurons are shaped in such as way as to connect quickly with each other and provide “links” or “chains” for nerve transmissions, and they usually have a specialized coating that enables electrical charges to move more quickly.

Determining a neuron’s polarity is usually a matter of looking at how many extensions it has. Neurons usually have extensions called denrites and axons that help carry charges and can also help link the neurons together. How these extensions are oriented and protrude helps determine the cell’s orientation. A neuron that is bipolar is one that has two extensions protruding in opposite directions, giving it the appearance of having arms. Alternatives include multipolar neurons, which have many protrusions, or pseudonuipolar neurons, which have two sets of both an axon and a dendrite running in opposite directions.

Bipolar neurons tend to take on an oval shape when viewed in sections. They typically communicate with other cells in the nervous system through chemical signaling, and are classed as electrically excitable.

Further, these sorts of cells are usually described as either "on" or "off" when it comes to their energy output. Whereas "on" bipolar neurons are excited by chemical signaling and begin transmitting, those that are "off" are not typically in a receiving mode and as such aren’t actively transmitting. Both are important in terms of maintaining sensory energy, though, and most can switch from on to off and back again as circumstances require.

These neurons are primarily involved in transmitting signals related to human senses. Sight is one of their key functionalities, and cells with this sort of specification are usually involved directly with the retina, the vision processing center of the eye. An "on" retinal bipolar neuron typically reacts to glutamate release by expanding in the light. The neuron is considered "off" when light exposure is minimized and the retina returns to its normal size. While "on," these neurons protect the retina from cation-permeable channels.

Smell is another important function. Neurons with this job are usually considered olfactory in nature. Olfactory cells pass between supporting cells and near the surface of the nervous system. Dendrites expand to form rods that assist in creating a layer of fluid from Bowman glands, and processes that properly operate prevent olfactory disorder and other related issues.

Hearing is another important neuron-supported system that relies heavily on bipolar-oriented cells. The cochlear ganglia are cells that carry signals from the inner ear to the brain, for instance. The vestibular ganglia are similarly part of the vestibular system, and these regulate the body's balance. Problems with nerve signals along these pathways can cause people to feel dizzy and to experience vertigo.