A reflex arc is the neural pathway that mediates a reflex. It generally does not involve the brain. In higher animals it is composed of a spinal "reflex integration center" composed of interneurons to connect affector (sensory stimulation) and effector (response) signals. While the reflex generation may be initiated by nociceptive input, extensive processing takes place within the spinal cord. The neural connection from the primary sensory neurons to the motor neurons is a poly-synaptic pathway (Andersen).

In lower animals reflex interneurons not necessarily inolve the spinal cord, see e.g., lateral giant neuron of crayfish.

A reflex to a stimulus is almost simultaneous, as the reflex arc doesn't involve the brain at all.

External link

• Ole K. Andersen, SMI, Aalborg University - Physiological and Pharmacological modulation of the human nociceptive withdrawal reflex

Reflexes | Neurophysiology Correction: The diagram showing the classic "knee jerk" muscle stretch reflex incorrectly depicts a three-cell reflex arc with an interneuron in the spinal cord connecting the sensory and motor neurons (i.e., two synapses). According to Ganong (2001), stretch reflexes “are the best known and studied MONOSYNAPTIC reflexes in the body. Impulses originating in the muscle spindle are conducted to the CNS by fast sensory fibers that pass directly to the motor neurons which supply the same muscle. The neurotransmitter at the central synapse is glutamate.”

Reference: Ganong, W.F. 2001. Review of Medical Physiology. McGraw-Hill Publishing, New York, p. 123.

Reflexbogen (Physiologie) | 反射弧