| Bipolar neuron | |
|---|---|
 Bipolar nerve cell from thespinal ganglion of the pike. | |
 | |
| Details | |
| Identifiers | |
| Latin | neuron bipolare |
| TH | H2.00.06.1.00050 |
| FMA | 67282 |
| Anatomical terms of neuroanatomy | |
Abipolar neuron, orbipolar cell, is a type ofneuron characterized by having both anaxon and adendrite extending from the soma (cell body) in opposite directions. These neurons are predominantly found in the retina and olfactory system.[1] The embryological period encompassing weeks seven through eight marks the commencement of bipolar neuron development.[2]
Many bipolar cells are specializedsensory neurons (afferent neurons) for the transmission ofsense. As such, they are part of the sensory pathways forsmell,sight,taste,hearing,touch,balance andproprioception. The other shape classifications of neurons includeunipolar,pseudounipolar andmultipolar. Duringembryonic development, pseudounipolar neurons begin as bipolar in shape but become pseudounipolar as they mature.[3]
Common examples are theretina bipolar cell, thespiral ganglion andvestibular ganglion of thevestibulocochlear nerve (cranial nerve VIII),[4] the extensive use of bipolar cells to transmitefferent (motor) signals to control muscles andolfactory receptor neurons in theolfactory epithelium for smell (axons form theolfactory nerve).[citation needed]
Bipolar neurons, classified as second-order retinal neurons, play a crucial role in translating responses to light into a neural code for vision.[5]Often found in theretina, bipolar cells are crucial as they serve as both direct and indirect cell pathways. The specific location of the bipolar cells allow them to facilitate the passage of signals from where they start in the receptors to where they arrive at the amacrine and ganglion cells. Bipolar cells in the retina are also unusual in that they do not fire impulses like the other cells found within the nervous system. Rather, they pass the information by graded signal changes. Bipolar cells convey impulses fromphotoreceptors (rods andcones) to ganglion cells,[6] which in turn transport the visual signals to the brain through the optic nerve. Bipolar cells come in two varieties, having either an on-center or an off-center receptive field, each with a surround of the opposite sign. The off-center bipolar cells have excitatory synaptic connections with the photoreceptors, which fire continuously in the dark and are hyperpolarized (suppressed) by light. The excitatorysynapses thus convey a suppressive signal to the off-center bipolar cells. On-center bipolar cells haveinhibitory synapses with the photoreceptors and therefore are excited by light and suppressed in the dark.[7]
Bipolar neurons exist within the vestibular nerve as it is responsible for special sensory sensations including hearing, equilibrium and motion detection. The majority of the bipolar neurons belonging to the vestibular nerve exist within the vestibular ganglion with axons extending into the maculae of utricle and saccule as well as into the ampullae of thesemicircular canals.[8]
Bipolar cells are also found in thespinal ganglia, when the cells are in anembryonic condition.
Sometimes the extensions, also calledprocesses, come off from opposite poles of the cell, and the cell then assumes a spindle shape.
In some cases where two fibers are apparently connected with a cell, one of the fibers is really derived from an adjoining nerve cell and is passing to end in a ramification around the ganglion cell, or, again, it may be coiled helically around the nerve process which is issuing from the cell.
Von Economo neurons, also known as spindle neurons, found in a few select parts of thecerebral cortex of apes and some other intelligent animals, possess a single axon and dendrite and as such have been described as bipolar.[9][10]
This article incorporates text in thepublic domain frompage 722 of the 20th edition ofGray's Anatomy(1918)
