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 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 the cell types: unipolar, pseudounipolar, multipolar and anaxonic.
The soma of bipolar neurons has two cytoplasmic extensions, which in turn are also branched. One of these processes acts as a dendrite which allows it to receive 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 common in the central nervous system of humans than unipolar neurons, although much less than multipolar neurons. Unlike the latter, which act as motor neurons and interneurons, the bipolar ones They mainly serve the function of sensory neurons
In addition to being characterized by the separation between their two extensions, as a consequence of this fact, bipolar neurons have a particularly elongated shape compared to that of unipolar neurons, which are more rounded, and that of multipolar neurons, which in many cases have been compared with stars.
In addition to being relatively common in some regions of the human body, 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, auditory-vestibular or olfactory system, their function is always related 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 with the optic nerve, which sends signals to the brain. Therefore, the performance of bipolar neurons is essential for sight.
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 branch 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 smell receptors in the olfactory epithelium, located on the roof of the nasal cavity. The dendrites of these neurons have 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.
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 the axon are located at opposite poles of the cell body, while in others both processes are very close.
Other types of neuron
The classification of some neurons as “bipolar” is part of the structural division of these cells based on the number of poles (made up of axons and dendrites) they present. From this point of view we can distinguish bipolar neurons from unipolar, pseudounipolar, multipolar and anaxonic neurons.
1. Unipolar
In unipolar neurons the axon and dendrites start from the same extension of the soma or cell body; This structure is known as “neurite”. This 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 appear to have a single pole 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 separate point on the axon. They make up a large part of the central nervous system and they have mainly 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 anaxon neurons, which are found in the brain and retina, there is no true axon or it is indistinguishable from the dendrites. These cells act as interneurons.
