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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Neuron | 3/8 | https://en.wikipedia.org/wiki/Neuron | reference | science, encyclopedia | 2026-05-05T07:31:09.374333+00:00 | kb-cron |
Neurons vary in shape and size and can be classified by their morphology and function. The anatomist Camillo Golgi grouped neurons into two types; type I with long axons used to move signals over long distances and type II with short axons, which can often be confused with dendrites. Type I cells can be further classified by the location of the soma. The basic morphology of type I neurons, represented by spinal motor neurons, consists of a cell body called the soma and a long thin axon covered by a myelin sheath. The dendritic tree wraps around the cell body and receives signals from other neurons. The end of the axon has branching axon terminals that release neurotransmitters into a gap called the synaptic cleft between the terminals and the dendrites of the next neuron.
=== Structural classification ===
==== Polarity ====
Most neurons can be anatomically characterized as:
Unipolar: single process. Unipolar cells are exclusively sensory neurons. Their dendrites receive sensory information, sometimes directly from the stimulus itself. The cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function, so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system. Bipolar: 1 axon and 1 dendrite. They are found mainly in the olfactory epithelium, and as part of the retina. Multipolar: 1 axon and 2 or more dendrites Golgi I: neurons with long-projecting axonal processes; examples are pyramidal cells, Purkinje cells, and anterior horn cells Golgi II: neurons whose axonal process projects locally; the best example is the granule cell Anaxonic: where the axon cannot be distinguished from the dendrite(s) Pseudounipolar: 1 process which then serves as both an axon and a dendrite
==== Other ==== Some unique neuronal types can be identified according to their location in the nervous system and distinct shape. Some examples are:
Basket cells, interneurons that form a dense plexus of terminals around the soma of target cells, found in the cortex and cerebellum Betz cells, large motor neurons in primary motor cortex Lugaro cells, interneurons of the cerebellum Medium spiny neurons, most neurons in the corpus striatum Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar neuron Pyramidal cells, neurons with triangular soma, a type of Golgi I Rosehip cells, unique human inhibitory neurons that interconnect with Pyramidal cells Renshaw cells, neurons with both ends linked to alpha motor neurons Unipolar brush cells, interneurons with unique dendrite ending in a brush-like tuft Granule cells, a type of Golgi II neuron Anterior horn cells, motoneurons located in the spinal cord Spindle cells, interneurons that connect widely separated areas of the brain
=== Functional classification ===
==== Direction ==== Afferent neurons convey information from tissues and organs into the central nervous system and are also called sensory neurons. Efferent neurons (motor neurons) transmit signals from the central nervous system to the effector cells. Interneurons connect neurons within specific regions of the central nervous system. Afferent and efferent also refer generally to neurons that, respectively, bring information to or send information from the brain.
==== Action on other neurons ==== A neuron affects other neurons by releasing a neurotransmitter that binds to chemical receptors. The effect on the postsynaptic neuron is determined by the type of receptor that is activated, not by the presynaptic neuron or by the neurotransmitter. Receptors are classified broadly as excitatory (causing an increase in firing rate), inhibitory (causing a decrease in firing rate), or modulatory (causing long-lasting effects not directly related to firing rate). The two most common (90%+) neurotransmitters in the brain, glutamate and GABA, have largely consistent actions. Glutamate acts on several types of receptors and has effects that are excitatory at ionotropic receptors and a modulatory effect at metabotropic receptors. Similarly, GABA acts on several types of receptors, but all of them have inhibitory effects (in adult animals, at least). Because of this consistency, it is common for neuroscientists to refer to cells that release glutamate as "excitatory neurons", and cells that release GABA as "inhibitory neurons". Some other types of neurons have consistent effects, for example, "excitatory" motor neurons in the spinal cord that release acetylcholine, and "inhibitory" spinal neurons that release glycine. The distinction between excitatory and inhibitory neurotransmitters is not absolute. Rather, it depends on the class of chemical receptors present on the postsynaptic neuron. In principle, a single neuron, releasing a single neurotransmitter, can have excitatory effects on some targets, inhibitory effects on others, and modulatory effects on others still. For example, photoreceptor cells in the retina constantly release the neurotransmitter glutamate in the absence of light. So-called OFF bipolar cells are, like most neurons, excited by the released glutamate. However, neighboring target neurons called ON bipolar cells are instead inhibited by glutamate, because they lack typical ionotropic glutamate receptors and instead express a class of inhibitory metabotropic glutamate receptors. When light is present, the photoreceptors cease releasing glutamate, which relieves the ON bipolar cells from inhibition, activating them; this simultaneously removes the excitation from the OFF bipolar cells, silencing them. It is possible to identify the type of inhibitory effect a presynaptic neuron will have on a postsynaptic neuron, based on the proteins the presynaptic neuron expresses. Parvalbumin-expressing neurons typically dampen the output signal of the postsynaptic neuron in the visual cortex, whereas somatostatin-expressing neurons typically block dendritic inputs to the postsynaptic neuron.
==== Discharge patterns ==== Neurons have intrinsic electroresponsive properties like intrinsic transmembrane voltage oscillatory patterns. So neurons can be classified according to their electrophysiological characteristics:
Tonic or regular spiking. Some neurons are typically constantly (tonically) active, typically firing at a constant frequency. Example: interneurons in neurostriatum. Phasic or bursting. Neurons that fire in bursts are called phasic. Fast-spiking. Some neurons are notable for their high firing rates, for example, some types of cortical inhibitory interneurons, cells in globus pallidus, retinal ganglion cells.
==== Neurotransmitter ====