There are four subdivisions of group A nerve fibers: alpha (α) Aα; beta (β) Aβ; , gamma (γ) Aγ, and delta (δ) Aδ. These subdivisions have different amounts of myelination and axon thickness and
therefore transmit signals at
different speeds. Larger diameter axons and more myelin insulation lead to faster signal propagation. Group A nerves are found in both motor and sensory pathways. Type Aδ fibers are the
afferent fibers of
nociceptors. Aδ fibers carry information from peripheral mechanoreceptors and thermoreceptors to the dorsal horn of the spinal cord. This pathway describes the first-order neuron. Aδ fibers serve to receive and transmit information primarily relating to
acute pain (sharp, immediate, and relatively short-lasting). This type of pain can result from several classifications of stimulants: temperature-induced, mechanical, and chemical. This can be part of a
withdrawal reflex—initiated by the Aδ fibers in the
reflex arc of activating withdrawal responses. These are the
type III group. Aδ fibers carry cold, pressure, and acute pain signals; because they are thin (2–5 μm in diameter) and
myelinated, they send impulses faster than unmyelinated
C fibers, but more slowly than other, more thickly myelinated group A nerve fibers. Their
conduction velocities are moderate. Their
cell bodies are located in the
dorsal root ganglia and axons are sent to the periphery to innervate target organs and are also sent through the dorsal roots to the spinal cord. Within the spinal cord the axons reach the
posterior grey column and terminate in Rexed laminae I to V. ==References==