CorpusHuman Anatomy
NervousCell

Classification of neurons

Also known as: neuron classification, multipolar neuron, bipolar neuron, pseudounipolar neuron, unipolar neuron, interneuron, nerve fibre classification, Erlanger–Gasser classification

Neurons are classified structurally by the number of processes (multipolar, bipolar, pseudounipolar, anaxonic), functionally by the direction of conduction (sensory, motor, interneuron), and by fibre diameter and conduction velocity (A, B and C fibres).

Overview

Fastest fibres (Aα)70–120 m/s
Slowest fibres (C)0.5–2 m/s
Interneurons≈99% of all neurons

Structural classification. Multipolar neurons have one axon and many dendrites. They are by far the most common type and include all motor neurons, most interneurons, cortical pyramidal cells and cerebellar Purkinje cells. Bipolar neurons have one dendrite and one axon on opposite poles of the soma. They are confined to special senses: retinal bipolar cells, olfactory receptor neurons, and the spiral and vestibular ganglion cells. Pseudounipolar (unipolar) neurons have a single short process that splits into a peripheral and a central branch. They are the primary sensory neurons of the dorsal root and cranial sensory ganglia, and develop from bipolar cells whose processes fuse. Anaxonic neurons have no distinguishable axon, for example retinal amacrine cells and some brain interneurons.

Functional classification. Sensory (afferent) neurons carry impulses from receptors toward the CNS; most are pseudounipolar. Motor (efferent) neurons carry impulses from the CNS to effectors: somatic α and γ motor neurons, and autonomic pre- and postganglionic neurons. Interneurons (association neurons) lie entirely within the CNS between sensory and motor neurons. They make up the overwhelming majority (≈99%) of all neurons, and many are inhibitory (GABA or glycine).

Fibre classification. Peripheral nerve fibres are graded by diameter, myelination and conduction velocity. Larger, myelinated fibres conduct faster. As a rule of thumb, velocity in m/s ≈ 6 × diameter in µm for myelinated fibres.

Parts & subdivisions

  1. Aα fibres (group Ia, Ib)
    12–20 µm, heavily myelinated, 70–120 m/s. Carry muscle spindle primary endings (Ia) and Golgi tendon organs (Ib), and are the axons of α motor neurons.
  2. Aβ fibres (group II)
    6–12 µm, myelinated, 30–70 m/s. Carry touch, pressure and vibration (Meissner, Pacinian, Merkel, Ruffini) and spindle secondary endings.
  3. Aγ fibres
    3–6 µm, myelinated, 15–30 m/s. γ motor neurons to intrafusal muscle fibres.
  4. Aδ fibres (group III)
    2–5 µm, thinly myelinated, 12–30 m/s. Carry fast, sharp, well-localised pain and cold.
  5. B fibres
    <3 µm, thinly myelinated, 3–15 m/s. Preganglionic autonomic fibres (white rami).
  6. C fibres (group IV)
    0.4–1.2 µm, unmyelinated, 0.5–2 m/s. Carry slow, burning, poorly localised pain, warmth and itch, and postganglionic sympathetic fibres (grey rami).
  7. Golgi type I versus type II
    Type I neurons have long axons projecting to distant regions (pyramidal cells, Purkinje cells, motor neurons). Type II neurons have short axons ending locally (local-circuit interneurons such as granule, basket and stellate cells).

Function

  • Provide the vocabulary for describing circuits and localising lesions.
  • Fibre classes predict differential sensitivity to anaesthetics, pressure and disease, such as the small-fibre versus large-fibre neuropathies.
Physiology

Differential block: local anaesthetics block small fibres first (C and B, then Aδ), so pain and autonomic function are lost before touch and motor function. Pressure (a limb "going to sleep") blocks large myelinated A fibres first, so proprioception and motor function go before pain. Hypoxia affects B fibres first.

Clinical significance

Small- versus large-fibre neuropathy

Small-fibre neuropathy (Aδ, C; early diabetes, amyloid, Fabry disease) causes burning pain, impaired pain and temperature sensation, and autonomic dysfunction, with normal reflexes and normal nerve conduction studies. Large-fibre neuropathy (Aα, Aβ; B12 deficiency, CIDP) causes loss of vibration and proprioception, sensory ataxia and areflexia.

Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.