What stimulates a nociceptor? The common response to this question is pain. But pain is not a stimulus—it is a perception that results from the stimulation of a nociceptor. So what is the stimulus necessary for causing a nociceptor to send an impulse to the brain? The answer is tissue destruction or the potential for tissue destruction. These specialized mechanoreceptors have a high threshold for stimulus and therefore will be stimulated only when a tissue is injured or is about to be injured.
Once a nociceptor is stimulated, the impulse must be carried to the spinal cord, which requries that each nociceptor be connected to a peripheral neuron. There are two types of peripheral neurons that carry pain to the spinal cord, which are classified by how fast they transmit the impulse. The A delta neuron carries fast pain resulting from the mechanical deformation of a nociceptor.
The C neuron carries slow pain resulting from chemical irritation of a nociceptor. This is the slow, dull, aching pain that results from inflammation to a tissue. Both the A delta and the C fibers will carry the impulse from the nociceptor to the dorsal horn of the spinal cord where it will synapse with a neuron that carries the impulse in spinal cord tracts up to the brain. These synaptic connections can be found throughout lamina 1–6 of the dorsal horn however, most are found in lamina 2—also known as the substantia gelatinosa. Understanding the neurology of these dorsal horn synapses is imperative to the understanding of how electrotherapy is effective in decreasing pain perception.
Spinal cord pain tracts begin at the dorsal horn at each level of the spinal cord and receive impulses from the A delta and C fibers. The A delta fibers will synapse with the neurons that become part of the Spinothalamic tract and the C fibers will synapse with neurons that become part of the Spinoreticular tract. Therefore, the Spinothalamic tract will carry fast types of pain to the brain and the Spinoreticular tract will carry the slower types of pain to the brain. Each of these tracts will specifically terminate at the thalamus, which integrates all sensory stimuli (except smell) before it reaches the cortex. Impulses that reach the cortex will be perceived as pain.
To summarize the pain pathway, we begin at the nociceptor, which when stimulated by tissue destruction or the potential for tissue destruction, will send impulses by way of A delta (fast) and C (slow) neurons to the dorsal horn of the spinal cord. There the impulses may be passed along important synaptic connections to spinal tract neurons that take the impulses to the thalamus. Once in the thalamus, the impulses are integrated and sent to the cortex where they are perceived as pain. These impulses can be modified at the level of the dorsal horn so the cortex never receives the information and thus never perceives the pain. These inherent modifications can be activated through electrotherapy protocols resulting in a decrease in the patients’ perception of pain. What are these inherent modifications and what protocols can be used to activate them?
