A unipolar neuron is a neuron in which only one process, called a

neurite A neurite or neuronal process refers to any projection from the cell body of a neuron. This projection can be either an axon or a dendrite. The term is frequently used when speaking of immature or developing neurons, especially of cells in culture ...

, extends from the cell body. The neurite then branches to form dendritic and axonal processes. Most neurons in the central nervous systems of

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s, including insects, are unipolar. The cell bodies of invertebrate unipolar neurons are often located around the edges of the

neuropil Neuropil (or "neuropile") is any area in the nervous system composed of mostly unmyelinated axons, dendrites and glial cell processes that forms a synaptically dense region containing a relatively low number of cell bodies. The most prevalent anat ...

, in the so-called cell-body rind. Most neurons in the central nervous systems of

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...

s, including mammals, are multipolar. In multipolar neurons, multiple processes extend from the cell body including dendrites and axons. Some neurons in the vertebrate brain have a unipolar morphology: a notable example is the

unipolar brush cell Unipolar brush cells (UBCs) are a class of excitatory glutamatergic interneuron found in the granular layer of the cerebellar cortex and also in the granule cell domain of the cochlear nucleus. Structure The UBC has a round or oval cell body w ...

, found in the

cerebellum The cerebellum (Latin for "little brain") is a major feature of the hindbrain of all vertebrates. Although usually smaller than the cerebrum, in some animals such as the mormyrid fishes it may be as large as or even larger. In humans, the cerebel ...

and granule region of the

dorsal cochlear nucleus The dorsal cochlear nucleus (DCN, also known as the "tuberculum acusticum"), is a cortex-like structure on the dorso-lateral surface of the brainstem. Along with the ventral cochlear nucleus (VCN), it forms the cochlear nucleus (CN), where all a ...

. A third morphological class,

bipolar neuron A bipolar neuron, or bipolar cell, is a type of neuron that has two extensions (one axon and one dendrite). Many bipolar cells are specialized sensory neurons for the transmission of sense. As such, they are part of the sensory pathways for smell ...

s, extend just one axon and dendritic process from the cell body. Examples of bipolar neurons include most invertebrate

sensory neuron Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded potentials. This process is called sensory transduction. The cell ...

s and bipolar cells of the vertebrate retina. Some vertebrate

s are classified as pseudo-unipolar. Pseudo-unipolar neurons initially develop as bipolar cells, but at some point the two processes that extend from the cell body fuse to form a single neurite. The axon then splits into two branches. Sensory neurons with cell bodies in the

dorsal root ganglia A dorsal root ganglion (or spinal ganglion; also known as a posterior root ganglion) is a cluster of neurons (a ganglion) in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the do ...

of the vertebrate spinal cord are pseudo-unipolar: one branch projects to the periphery (to sensory receptors in the skin, joints, and muscle), the other to the spinal cord.

References

* * {{DEFAULTSORT:Unipolar Neuron Neurons