Nociception and Spinal Reflexes
John Tuthill
Summary: Nociception and Spinal Reflexes
Nociceptors are sensory neurons that respond to physical stimuli strong enough to cause tissue damage. They innervate every tissue of the body except, ironically, the brain. Nociceptors are classified by the type of physical stimuli (i.e., modality) that they respond to, such as pressure, heat, or cold. Their activity gives rise to the perception of pain.
A spinal reflex is a rapid activation of muscle produced by the stimulation of sensory receptors. Spinal reflexes are mediated by nerve cells in the peripheral nervous system (sensory neurons) and the spinal cord (spinal interneurons, cells confined to the spinal cord and motor neurons, the cells that innervate skeletal muscle). Unlike planned, intentional movements, reflexes are involuntary and can be evoked without the involvement of the brain.
This spinal reflex is mediated by actions of just two populations of neurons: the primary sensory receptors of a sense organ called the muscle spindle and motor neurons, the cells that activate skeletal muscle fibers.
Application of a noxious stimulus to the skin results in widespread and powerful activation of ipsilateral flexor muscles that generally produces a withdrawal of the limb away from the stimulus, called the flexion reflex. The opposite reaction, called the crossed-extension reflex, is often observed in the contralateral limb.
Lesions in the brain or spinal cord and several disease processes can be manifested in excessive spinal reflexes and spasticity.
Learning Objectives
- To know how nociceptors are activated and sensitized.
- To understand the difference between reflexes and other types of movements.
- To differentiate the neural circuits underlying the flexion and stretch reflexes.
- To understand how reflex testing can be used clinically to diagnose neuro-pathologies that affect motor and sensory function.
(Unless otherwise noted, all figures are from: Kandel ER, Schwartz JH, Jessell TM 2012, Siegelbaum SA, Hudspeth AJ. ‘Principles of Neural Science, 5th ed. McGraw-Hill, New York.)
Nociceptors are classified by the type of physical stimuli (i.e., modality) that they respond to. Some nociceptors respond to more than one modality and are referred to as polymodal nociceptors. Others, called sleeping or silent nociceptors, normally do not respond to any type of stimulus, but become activated under conditions of inflammation or tissue damage.
Thermal nociceptors are activated by noxious (i.e., tissue damaging) hot or cold temperatures within the receptive field acting on TRP channels. TRPV1 channels are expressed on thermal nociceptors that respond to heat/pain temperatures above 42 °C. Other temperatures in the warm–hot range are transduced by different thermal nociceptors that express a different TRP channel. Cold stimuli are sensed by thermal nociceptors that express TRPM8 channels. An interesting finding related to cold stimuli is that tactile sensibi