Neurons are the cells that allow the transmission of electrical impulses through the nervous system. They may specialize in receiving sensory stimuli or contracting muscles, among other functions, and are divided into several classes according to the number of dendrites and axons that form them.
In this article we will talk about the morphology, location and functions of bipolar neurons, Which are characterized by the presence of an axon and a dendrite. We will also describe the main aspects of other cell types: unipolar, pseudounipolar, multipolar and anaxonic.
Morphology of bipolar neurons
The soma of bipolar neurons has two cytoplasmic extensions, which in turn are also branched. One of these extensions acts as a dendrite, Which allows to receive electrochemical impulses sent by presynaptic neurons, and the other in axon form, transmitting stimulation generated by the neuronal body to other cells.
Bipolar neurons are more common in the central nervous system of humans than unipolar neurons, although much less so than multipolar neurons. Unlike the latter, which act as motor neurons and interneurons, bipolar they mainly perform the function of sensory neurons.
In addition to being characterized by the separation between its two extensions, bipolar neurons therefore have a particularly elongated shape compared to unipolar, which are more rounded, and multipolar, which in many cases have been compared to stars.
In addition to being relatively common in certain regions of the human body, particularly the sensory pathways, bipolar neurons they are very numerous in the spinal ganglia of fish. People also have bipolar neurons in this section of the spinal cord during embryonic development.
Main locations and functions
Bipolar neurons specialize in transmitting sensory stimulation; that means they are sensory neurons. Whether they are located in the visual, auditory-vestibular or olfactory system, their function is always linked to this task.
1. In the retinas
The middle layer of the retina is made up of bipolar neurons, Which modulate the impulses received by the photoreceptors (rods and cones), before they reach the ganglion cells; these in turn connect the retina to the optic nerve, which sends signals to the brain. Therefore, the action of bipolar neurons is essential for vision.
2. In the vestibulocochlear nerve
The vestibular and cochlear branches of the eighth cranial pair they are made up of bipolar cells. While the vestibular branch transmits information about balance to the brain, the cochlear is related to the sense of hearing. Bipolar neurons are located in the vestibular ganglion and their axons extend to the semicircular canals.
3. In the olfactory epithelium
Bipolar neurons perform the function of olfactory receptors in the olfactory epithelium, Located on the roof of the nasal cavity. The dendrites of these neurons have cilia, which hold odor molecules in the lining. By binding to these, the neuron transmits electrical impulses to the olfactory bulb through the cribiform plate of the skull.
4. In the spinal ganglia
During embryonic development, it is possible to find bipolar neurons in the spinal ganglia, located in the dorsal roots of the spinal cord. In some cases, the dendrite and axon are located at the opposite poles of the cell body, while in the other two extensions they are very close.
Other types of neurons
The classification of certain neurons as “bipolar” is framed in the structural division of these cells according to the number of poles (made up of axons and dendrites) present. From this point of view, we can distinguish bipolar neurons from unipolar, pseudounipolar, multipolar and anaxonic neurons.
In unipolar neurons axon and dendrites start from the same extension soma or cell body; this structure is known as a “neurite”. It is a type of neuron that cannot be found in humans.
In some bipolar neurons we find an axon divided into two branches; one of them goes to the spinal cord and the other to the peripheral nervous system. These neurons are called “pseudounipolar” because they appear to have only one pole because axons and dendrites are joined, although they actually have two.
Multipolar neurons have two or more dendrites located at a point separate from the axon. They make up a large part of the central nervous system and mainly have a motor function, although many multipolar neurons allow communication between the peripheral and central nervous system; therefore, this category also includes interneurons.
In anaxonic neurons, which are found in the brain and retina, there is no true axon or it is indistinguishable from dendrites. These cells act like interneurons.