Neural mechanisms of muscle pain

Action potentials originating from nociceptors carry information about noxious stimuli, but the perception of pain from muscles is the end product of information processing in the central nervous system (CNS).

Nociceptors

In skeletal muscles, there are three types of nociceptors that encode the intensity of noxious stimuli:

1. (a) Specific mechanical nociceptors responding only to high-intensity stimuli.

2. (b) Polymodal nociceptors encoding innocuous and nociceptive, mechanical and chemical stimuli.

3. (c) The free nerve endings in muscle tissue concentrated around small arterioles and capillaries between the muscle fibres and not activated by normal muscle movement or increasing muscle tension.

As in other tissues, nociceptor information in muscles is transduced and carried to the CNS by Aδ- and C-afferent fibres. C-fibres are mainly excited during ischaemic contractions and are sensitized following tissue lesion and inflammation.

Dorsal horn neurones

When muscles are healthy, most dorsal horn (DH) neurones receive projections from Aδ-afferent fibres, sometimes in combination with C-fibres. DH neurones receiving exclusive projections from C-afferent fibres are quite rare. The effect of C-afferent fibres on DH neurones increases greatly following inflammation. Thus, it has been suggested that, in the absence of peripheral muscle pathology, acute pain is mainly due to Aδ-fibres, while chronic muscle pain is related to C-fibres.