Bipolar neuron
Bipolar nerve cell from the spinal ganglion of the pike.
Latinneuron bipolare
Anatomical terms of neuroanatomy

A bipolar neuron, or bipolar cell, is a type of neuron characterized by having both an axon and a dendrite 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 specialized sensory neurons (afferent neurons) for the transmission of sense. As such, they are part of the sensory pathways for smell, sight, taste, hearing, touch, balance and proprioception. The other shape classifications of neurons include unipolar, pseudounipolar and multipolar. During embryonic development, pseudounipolar neurons begin as bipolar in shape but become pseudounipolar as they mature.[3]

Common examples are the retina bipolar cell, the spiral ganglion and vestibular ganglion of the vestibulocochlear nerve (cranial nerve VIII),[4] the extensive use of bipolar cells to transmit efferent (motor) signals to control muscles and olfactory receptor neurons in the olfactory epithelium for smell (axons form the olfactory nerve).[citation needed]

In the retina

Main article: Retina bipolar cell

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 the retina, 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 from photoreceptors (rods and cones) 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 excitatory synapses thus convey a suppressive signal to the off-center bipolar cells. On-center bipolar cells have inhibitory synapses with the photoreceptors and therefore are excited by light and suppressed in the dark.[7]

In the vestibular nerve

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 the semicircular canals.[8]

In the spinal ganglia

Bipolar cells are also found in the spinal ganglia, when the cells are in an embryonic condition.

Sometimes the extensions, also called processes, 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.

In the cerebral cortex

Von Economo neurons, also known as spindle neurons, found in a few select parts of the cerebral cortex of apes and some other intelligent animals, possess a single axon and dendrite and as such have been described as bipolar.[9][10]

Additional images

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  1. ^ Muzio, M. R., & Cascella, M. (2022). Histology, Axon. In StatPearls. StatPearls Publishing.
  2. ^ Ahimsadasan N, Reddy V, Khan Suheb MZ, et al. Neuroanatomy, Dorsal Root Ganglion. [Updated 2022 Sep 21]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from:
  3. ^ Vanderah, Todd W.; Gould, Douglas J.; Nolte, John (2015). Nolte's The Human Brain : An Introduction to Its Functional Anatomy (7th ed.). Philadelphia, Pennsylvania: Elsevier. ISBN 9781455728596. OCLC 895731173.
  4. ^ "bipolar cell". Medical Dictionary for the Health Professions and Nursing. Farlex. 2012. Retrieved 17 January 2024 – via TheFreeDictionary by Farlex.
  5. ^ Thoreson W. B. (2021). Transmission at rod and cone ribbon synapses in the retina. Pflugers Archiv : European journal of physiology, 473(9), 1469–1491.
  6. ^ Nelson, R., & Connaughton, V. (2007). Bipolar Cell Pathways in the Vertebrate Retina. In H. Kolb (Eds.) et. al., Webvision: The Organization of the Retina and Visual System. University of Utah Health Sciences Center.
  7. ^ "David Hubel's Eye, Brain and, Vision". Archived from the original on 2018-07-20. Retrieved 2014-05-06.
  8. ^ Clinically Oriented Anatomy
  9. ^ Allman, John M.; Tetreault, Nicole A.; Hakeem, Atiya Y.; Manaye, Kebreten F.; Semendeferi, Katerina; Erwin, Joseph M.; Park, Soyoung; Goubert, Virginie; Hof, Patrick R. (April 2011). "The von Economo neurons in fronto-insular and anterior cingulate cortex". Annals of the New York Academy of Sciences. 1225 (1): 59–71. Bibcode:2011NYASA1225...59A. doi:10.1111/j.1749-6632.2011.06011.x. ISSN 0077-8923. PMC 3140770. PMID 21534993.
  10. ^ Cauda, Franco; Geminiani, Giuliano Carlo; Vercelli, Alessandro (2014). "Evolutionary appearance of von Economo's neurons in the mammalian cerebral cortex". Frontiers in Human Neuroscience. 8: 104. doi:10.3389/fnhum.2014.00104. ISSN 1662-5161. PMC 3953677. PMID 24672457.

Public domain This article incorporates text in the public domain from page 722 of the 20th edition of Gray's Anatomy (1918)