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 discuss 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 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 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 so than multipolar ones. Unlike the latter, which act both as motor neurons and as interneurons, bipolar neurons mainly fulfil 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 unipolar ones, which are more rounded, and multipolar ones, which in many cases have been compared to stars.

In addition to being relatively common in some regions of the human body, particularly the sensory pathways, bipolar neurons 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 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 to the optic nerve, which sends the signals to the brain. Therefore, the action of bipolar neurons is essential for sight.

2. In the vestibulocochlear nerve

The vestibular and cochlear branches of the eighth cranial pair 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 into the semicircular canals.

3. In the olfactory epithelium

Bipolar neurons play the role of olfactory receptors in the olfactory epithelium , located on the roof of the nasal cavity. The dendrites of these neurons have cilia, which retain the odour 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 cord . In some cases the dendrite and axon are located at 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 based on the structural division of these cells according to the number of poles (made up of axons and dendrites) they have. From this point of view we can distinguish bipolar from unipolar, pseudounipolar, multipolar and anaxonic neurons.

1. Unipolars

In unipolar neurons the axon and dendrites start from the same prolongation of the soma or cell body; this structure is known as “neurite”. This is a type of neuron not found in humans.

2. Pseudounipolars

In some bipolar neurons we find an axon divided into two branches; one of them is directed towards the spinal cord and the other towards the peripheral nervous system. These neurons are known as “pseudo-unipolar” because they appear to have a single polysaccharide 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 form a large part of the central nervous system and have mainly a motor function, although many multipolar neurons allow communication between the peripheral and central nervous system; therefore, this category also includes interneurons.

4. Anaxonics

In anaxonic neurons, which are found in the brain and the retina, there is either no true axon or it is indistinguishable from dendrites. These cells act as interneurons.