Bipolar neurons: morphology, location and functions
Neurons are the cells that allow the transmission of electrical impulses through the nervous system. They can specialize in the reception of sensory stimuli or in the contraction of muscles, among other functions, and they are divided into several classes depending on 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 the rest of cell types: the unipolar, the pseudounipolar, the multipolar and the anaxonic.
- Related article: "Types of neurons: characteristics and functions"
Morphology of bipolar neurons
The soma of bipolar neurons has two cytoplasmic extensions, which in turn are also branched. One of these extensions acts like a dendrite , which allows receiving electrochemical impulses sent by presynaptic neurons, and the other as an axon, transmitting stimulation generated by the neuronal body to other cells.
Bipolar neurons are more frequent in the central nervous system of human beings than unipolar neurons, although much less than multipolar neurons. Unlike the latter, which act as motor neurons and as interneurons, the bipolar they mainly fulfill the function of sensory neurons .
In addition to being characterized by the separation between its two extensions, as a consequence of this fact the bipolar neurons have a particularly elongated form in comparison with that of the unipolar ones, which are more rounded, and that of the multipolar ones, which in many cases have been compared with stars.
In addition to being relatively common in some regions of the body of humans, particularly 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.
Locations and main functions
Bipolar neurons specialize in the transmission of sensory stimulation; this means that they are sensory neurons. Regardless of whether they are located in the visual system, in the auditory-vestibular or in the olfactory, their function is always related to this task.
1. In the retinas
The middle layer of the retina is formed by 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 with the optic nerve, which sends the signals to the brain. Therefore, the action of bipolar neurons is fundamental for the eyesight.
- Related article: "Types of neurons: characteristics and functions"
2. In the vestibulocochlear nerve
The vestibular and cochlear branches of the eighth cranial nerve They are composed of bipolar cells. While the vestibular branch transmits information about balance to the brain, the cochlear is related to the sense of hearing. The bipolar neurons are located in the vestibular ganglion and their axons extend to the semicircular canals.
3. In the olfactory epithelium
Bipolar neurons fulfill the function of Smell receptors in the olfactory epithelium , located on the roof of the nasal cavity. The dendrites of these neurons present cilia, which retain odor molecules in the mucosa. By joining these, the neuron transmits electrical impulses to the olfactory bulb through the cribriform 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 medulla . In some cases the dendrite and the axon are located in opposite poles of the cell body, while in others both extensions are very close.
Other types of neurons
The classification of some neurons as "bipolar" is framed in the structural division of these cells according to the number of poles (constituted by axons and dendrites) that they present. From this point of view we can distinguish bipolar neurons from unipolar neurons, from pseudounipolar neurons, from multipolar neurons and from anaxonic neurons.
1. Unipolar
In unipolar neurons the axon and the dendrites start from the same extension of the soma or cellular body; This structure is known as "neurite". It is a type of neuron that is not found in humans.
2. Pseudounipolar
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 known as "pseudounipolar" because they seem to have a single polypore because the axons and dendrites are joined, although in reality they have two.
3. Multipolar
Multipolar neurons have two or more dendrites located at a point separate from the axon. They make up a good part of the central nervous system and they mainly have a motor function, although many multipolar neurons allow communication between the peripheral and central nervous systems; therefore, this category also includes interneurons.
4. Anaxonic
In the anaxonic neurons, which are found in the brain and in the retina, there is no true axon or it is indistinguishable from the dendrites. These cells act as interneurons.
