A-delta and C-fibers carry pain information as an electrical action potential to the dorsal horn of the spinal cord.
"Their function is to transmit pain information from the peripheral tissues at the site of injury up to the spinal cord to
synapse with the second-order neuron," states Smith.
They release substance P, glutamate, calcitonin and gene-related peptide (CGRP) at the synapse and that signals the second-order
neuron about the pain in the periphery. A-delta sensory neurons are myelinated, fast-conducting, "first-pain" responders to
intense noxious mechanical stimulation. C-fiber primary afferent neurons are non-myelinated, slower-conducting, dull, burning,
aching "second-pain" responders, but polymodal, i.e., responding to noxious mechanical, chemical and thermal stimuli. The
stimuli are noxious in that they are strong and tissue-damaging, whereas nonnociceptive receptors are below the tissue-damaging
Once the pain signals reach the brain, the various CNS components (i.e., reticular formation, thalamus, hypothalamus) are
involved in processing pain.
"We can't say that descending modulation is the same in horses as in people," explains Smith, "but the information after processing
in the thalamus, hypothalamus, cortex, etc., converges again in the brainstem and is sent via the dorsolateral funiculus to
the spinal cord segment of concern, where inhibitory (e.g., endorphins, serotonin, gaba) and/or excitatory neurotransmitters
(e.g., glycine) are released."
The balance of each that is released depends on the degree of windup and the amount of modulation that occurred before the
information ascended as well as in the cortex and other higher CNS centers.
Gating is the normal regulatory mechanism that affects the way pain is perceived. "Presumably the same mechanisms (as noted
in humans) apply in horses," Smith says.
The first gating mechanism is in the spinal chord, where sensory fibers linked to pain fibers can act to suppress each other.
A non-pain sensation, such as massaging or icing-down an injured area, can inhibit the transmission of pain.
A second gating mechanism is found in the brain, where endorphin release may suppress pain signals, reducing the pain perception.
Knowledge improves control
"When it comes to looking at therapeutics for pain control, we look at those different types of processes (of the physiology
of pain)," states Pettifer. "We look at applying a local anesthetic peripherally to prevent transmission of the stimulus,
or giving non-steroidal drugs like phenylbutazone that decrease inflammation at the periphery, and that decreases the activity
of the receptors in the periphery that effect pain stimuli," Pettifer explains.
Drugs like opioids and NMDA-antagonists function at the level of the spinal cord. There are sedatives that, in combination
with analgesics, reduce anxiety around painful situations. "We attack the problem from many points – a multimodal approach
to pain control, with the notion that you actually achieve much better control if you look at these different facets of the
pain system and target your therapy toward interrupting their function," Pettifer explains.
"The more we understand about pain, the more we are able to manage it," he says. "Once you learn more about the physiology
of pain and the various neurological processes involved, the development of a pain-management strategy largely becomes an
attempt to modify the activity of these systems to the benefit of the animal."
Ed Kane is a Seattle author, researcher and consultant in animal nutrition, physiology and veterinary medicine, with a background
in horses, pets and livestock.