Trigger Point Pain Referral is a result of the way the body is wired. The following
mechanisms of trigger point pain referral have been observed and described (1):-
Axon Branching:
A single nerve cell has pain input from two different areas of the body, one from
a trigger point, and the other from somewhere else (the pain referral zone). The single nerve cell has no
way of "knowing" which incoming axon branch caused it to fire; as far as the nerve
cell is concerned, the pain message that it passes up to the brain could have come from either area of the body.
The brain therefore registers pain in all areas that input to that nerve.
Diagram right:
The arrows represent a pain signal arising from a trigger point.
Convergence Projection:
A nerve cell in the spinal cord has input from two or more pain carrying neurons.
It is stimulated to fire by an incoming pain nerve attached to a trigger point,
but has no way of signalling to the brain which incoming pain nerve has caused
it to fire. Thus the brain registers pain
in all the regions that send pain messages via that nerve cell.
Diagram right:
The final nerve is firing in response to a pain signal from a nerve connected to a Trigger Point.
Convergence Facilitation:
Works like a transistor. The nerve attached to the trigger point excites a nerve
cell in the spinal cord, thus lowering that nerve's stimulus threshold and making
it more willing to pass a pain signal from the pain referral area up to the brain.
Diagram right:
The trigger point associated neuron does not carry a pain message. It does
however amplify any pain signal from the pain referral area.
Activity of the Sympathetic Nervous System:
The brain is wired to carry a message from the trigger point to the sympathetic nervous system.
The sympahthetic nervous system responds by releasing nerve excitatory chemicals
in the referral zone. Pain, changes in blood flow and mucus membrane secretion result.
Diagram right:
The sympathetic nervous system responds to a trigger point pain signal by releasing nerve excitatory chemicals .
