A bipolar neuron is a nerve cell with exactly two branches, one at each end, so a signal comes in one side and leaves the other. That simple in-and-out design suits a relay job, and the body uses these cells mainly in the sense organs, passing information from the eyes, nose and ears toward the brain. Despite the shared word, they have nothing to do with bipolar disorder.
What Makes a Neuron Bipolar
Every neuron has a cell body, which holds the nucleus and keeps the cell alive. Branches called dendrites collect incoming signals, and a branch called the axon carries the signal away to the next cell. A bipolar neuron has exactly one dendrite and one axon extending from the cell body.1Lumen Learning. What Are Bipolar Neurons and Where Are They Found? The two leave from opposite ends.2Pearson. Human Anatomy & Physiology The cell body is often oval, and the receiving branch can reach a long way from it.3Kenhub. Types of neurons
Signals flow one way, from the receiving branch through the cell body to the sending branch. Under a microscope, these cells look far simpler than the bushy, many-branched neurons of the brain. With one input and one output, the cell passes a signal along a straight line with little mixing, which is useful when a sense organ needs to send a clean signal toward the brain.
The signal itself is not a physical thing traveling like a package. It is a tiny electrical change in the cell’s outer membrane. At the receiving branch the change is small and varies with how strongly the cell is being stimulated. The cell body adds up that input, and the sending branch passes the result on, often as a series of brief electrical pulses. At the far end, the next cell receives it when chemical messengers are released across a tiny gap.
In anatomy, a “pole” is an end of the cell where a branch emerges, so bipolar means two ends with branches. Bipolar disorder refers to mood swinging between two extremes, and these cells do not cause it.
How Bipolar Neurons Compare With Other Neuron Types
Neurons are sorted by how many branches leave the cell body:
- Unipolar: a single branch leaves the cell body.4Lumen Learning. What Are Bipolar Neurons and Where Are They Found?
- Pseudounipolar: one branch leaves the cell body and then splits into two, like a T.5Lumen Learning. What Are Bipolar Neurons and Where Are They Found?
- Bipolar: one receiving branch and one sending branch, from opposite ends.6Pearson. Human Anatomy & Physiology
- Multipolar: one sending branch and many receiving branches, the most common type in the body.7Lumen Learning. What Are Bipolar Neurons and Where Are They Found?
Multipolar neurons make up most of the brain and spinal cord, including the cells that move muscles. Pseudounipolar neurons carry touch, temperature and pain from skin to the spinal cord. True unipolar neurons are rare in humans and more typical of insects and other invertebrates. Bipolar neurons are the specialists of the eye, nose and inner ear, covered in the next sections.
These four types are sorted purely by shape, not by what the cell does. Two neurons of the same shape can do quite different jobs. Function-based names exist too, such as sensory neurons that bring information in and motor neurons that drive muscles, and they overlap with the shape types rather than replacing them. Some books use “unipolar” to mean the pseudounipolar touch and pain cells, so check which naming your course uses.
Bipolar and pseudounipolar cells are closely linked. Touch and pain nerve cells near the spinal cord are mostly bipolar early in development and become pseudounipolar by birth.8PubMed. Dorsal root ganglion neuron development in chick and rat During growth the two branches move together and merge near the cell body. In a dish, nearby support cells are needed for the change from the young bipolar form to the mature shape.9PubMed. Schwann cells induce morphological transformation of sensory neurones in vitro That is why the T shape is called pseudo, meaning false: the cell looks like it has a single branch, but it began with two.
Bipolar Cells in the Eye
The retina is the thin layer of nerve tissue at the back of the eye that turns light into signals. Light-sensing cells called rods and cones pass their signals to bipolar cells, which pass them to ganglion cells whose fibers form the optic nerve. Bipolar cells are the middle link between catching light and sending it to the brain.
Light passes through the front layers of the retina to reach the rods and cones at the back. They change how much chemical messenger they release onto bipolar cells. Each bipolar cell turns that change into its own signal and hands it to ganglion cells, which send the result along the optic nerve toward the brain.
Cones work in bright light and give sharp detail and color, while rods work in dim light and are highly sensitive. Different bipolar types are tied mainly to cones, and a separate type is tied to rods, so the two kinds of vision stay partly separate on the way through.
Some bipolar cells, called ON cells, respond when light gets brighter, while others, called OFF cells, respond when it dims, and this split happens at the very first connection in the retina.10PubMed Central. TRPM1 is required for the depolarizing light response in retinal ON-bipolar cells Light-sensing cells actually release less of their chemical messenger in light, and ON bipolar cells react to that drop by becoming active.11PubMed Central. TRPM1 is required for the depolarizing light response in retinal ON-bipolar cells OFF bipolar cells respond to dimming, and different types of them answer quickly and briefly or slowly and steadily.12PubMed Central. Differential signalling and glutamate receptor compositions in the OFF bipolar cell types in the mouse retina13PubMed Central. Differential signalling and glutamate receptor compositions in the OFF bipolar cell types in the mouse retina
Separate brightening and dimming channels help the eye spot edges, such as a dark letter on white paper. They also help detect movement, since a moving object brightens some spots and darkens others. Several bipolar types work in parallel, each carrying a slightly different version of the scene.
This early processing is needed because the optic nerve has far fewer fibers than the retina has light-sensing cells, so information must be condensed. Bipolar cells help by passing on changes rather than every detail of light.
Bipolar cells were long thought to send only smooth, graded signals that rise and fall with light.14ScienceDaily. The end of a dogma: Bipolar cells generate action potentials But some types in mammals also fire all-or-nothing spikes, the quick pulses typical of most nerve cells.15PubMed Central. A Mammalian Retinal Bipolar Cell Uses Both Graded Changes in Membrane Voltage and All-or-Nothing Na+ Spikes to Encode Light Spikes carry timing more precisely, which helps the fastest visual signals.16ScienceDaily. The end of a dogma: Bipolar cells generate action potentials
Bipolar Neurons in the Nose
The smell-detecting cells in the lining high inside the nose are bipolar neurons.17PubMed Central. The sense of smell, its signalling pathways, and the dichotomy of cilia and microvilli in olfactory sensory cells One thin branch reaches into the nasal cavity and ends in a knob exposed to the air, while a single unbranched fiber runs to the brain.18PubMed Central. The sense of smell, its signalling pathways, and the dichotomy of cilia and microvilli in olfactory sensory cells That makes these among the only nerve cells in the body that directly touch the outside world.
Odor molecules in the air you breathe dissolve in a thin layer of mucus covering the nose lining and reach the knob-like tips. There, a molecule binds a receptor and starts a chain of chemical messages inside the cell.19PubMed. Transduction mechanisms in vertebrate olfactory receptor cells That chain opens channels that let charged particles in, building an electrical signal.20PubMed Central. The sense of smell, its signalling pathways, and the dichotomy of cilia and microvilli in olfactory sensory cells The signal becomes nerve impulses that travel along the cell’s single fiber, through small openings in the bone above the nose, to the smell center at the front of the brain.21PubMed. Transduction mechanisms in vertebrate olfactory receptor cells There the signals are sorted in small clusters before higher brain areas interpret the smell.22PubMed Central. Olfactory receptors
Each smell cell typically carries one kind of odor receptor, out of a large family of different receptors. Signals from cells with the same receptor type gather in the same clusters. A single odor molecule can activate several receptor types, and a single receptor type responds to several odors. The brain reads the overall pattern of active clusters, much as a chord is recognized from several notes together, which is why one smell cell on its own does not signal a whole smell.
Stem cells at the base of the nose lining replace smell neurons throughout adult life.23PubMed Central. Olfactory epithelium: Cells, clinical disorders, and insights from an adult stem cell niche Replacement speeds up after damage to the smell nerve.24PubMed. Neural regeneration and the peripheral olfactory system It is needed because exposure to air means these cells meet viruses, pollutants and toxins directly. Constant replacement lets the sense of smell recover from much ordinary damage, which is unusual, since most brain neurons are not replaced once lost.
This is why smell loss after a cold or virus often improves over weeks to months as new cells grow and connect. Recovery can be slower or incomplete when the stem cells or the connections to the brain are damaged. Persistent loss is worth raising with a doctor, especially an ear, nose and throat specialist.
Bipolar Neurons in the Inner Ear
Sound is detected by hair cells in the cochlea, the snail-shaped hearing organ. Bipolar nerve cells carry that information rapidly to the brain.25PubMed Central. Making connections in the inner ear: recent insights into the development of spiral ganglion neurons and their connectivity with sensory hair cells
Different regions along the cochlea respond best to different pitches, with high pitches at one end and low pitches at the other. Each hearing nerve cell connects to hair cells in a particular region, so it carries information about a particular range of pitch. These cells also differ along the cochlea, with those serving different pitches carrying different mixes of signaling proteins.26PubMed Central. Reciprocal regulation of presynaptic and postsynaptic proteins in bipolar spiral ganglion neurons by neurotrophins
The two-ended shape fits the ear well: one end connects to a hair cell deep in the ear and the other joins the nerve to the brainstem. The single in-and-out path keeps timing sharp, which matters for telling where a sound comes from.
A neighboring cluster of bipolar neurons carries signals from the balance organs. There are at least five distinct types of these balance nerve cells, each wired to particular zones of the balance organs.27PubMed Central. Morphological and functional diversity of spatially resolved vestibular ganglion neuron cell types One type helps sense gravity and steadies the eyes when the head tilts.28PubMed Central. Morphological and functional diversity of spatially resolved vestibular ganglion neuron cell types
Hair cells do not regrow in humans, so hearing loss from their damage is generally permanent. The hearing nerve cells often survive for some time afterward, though they can gradually decline. That leftover nerve supply is what lets a cochlear implant produce hearing, covered below.
Are Bipolar Neurons Found Anywhere Else
Mostly in the senses. Some small brain cells can look two-ended, but the classic, clear examples are in sensory organs, and textbooks usually list the retina, nose lining and inner ear as the locations to remember.
Many touch and pain neurons pass through a bipolar stage before birth, as described above.29PubMed. Dorsal root ganglion neuron development in chick and rat For these cells, being bipolar is a stage rather than a permanent identity, unlike the eye, nose and ear cells that stay bipolar. Bipolar cells also appear outside humans: tadpole-like sea animals called tunicates have bipolar sensory neurons in their tails that resemble the sensory nerve clusters of the vertebrate head.30Springer Nature. Heads or tails: Shared legacy of ancient sensory neurons
Bipolar cells also turn up in the lab. When sensory neurons are grown from human stem cells, most come out bipolar.31PubMed Central. Characterization of human iPSC-derived sensory neurons and their functional assessment using multi electrode array Such cells grown from people with an inherited burning-pain condition fire too often, which lets researchers test drugs on them.32PubMed Central. In vitro programming and pseudounipolarization of human iPSC-derived sensory neurons
When Bipolar Neurons Are Damaged
In retinitis pigmentosa, an inherited disease where rods and then cones die, bipolar cells begin rewiring early in the disease.33PubMed Central. Retinal Remodeling in Human Retinitis Pigmentosa In mouse models they pull back their receiving branches first, then their sending branches.34PubMed Central. Development and Degeneration of Cone Bipolar Cells Are Independent of Cone Photoreceptors in a Mouse Model of Retinitis Pigmentosa Rewiring means connections change shape and position even before many cells are lost, so sight loss is not only about how many rods and cones remain. Some bipolar cells die only slowly, weeks after cones begin to fail.35PubMed Central. Development and Degeneration of Cone Bipolar Cells Are Independent of Cone Photoreceptors in a Mouse Model of Retinitis Pigmentosa
Some vision-restoring approaches, such as an electrical implant or a light-sensitive protein, give surviving cells a new way to respond to light, so they depend on those cells being healthy. If bipolar cells have already rewired or died, such treatments may work less well.
A cochlear implant is a device that bypasses damaged hair cells and stimulates hearing nerve cells directly. So how many of those cells survive affects how well an implant can work. In animal studies, delivering nerve-growth proteins to the deafened cochlea kept more of these cells alive and made them easier to stimulate.36PubMed Central. Spiral ganglion neuron survival and function in the deafened cochlea following chronic neurotrophic treatment That approach is still experimental and not a standard part of implant surgery. Questions about eligibility for trials or implants are best asked of a retina or hearing specialist.