Acute pain is a sensation triggered by a threatening (noxious) stimulus to the skin (cutaneous pain), the musculoskeletal system or the internal organs (visceral pain). A signal is then transmitted to the brain so it becomes aware of the threat and can decide how to respond (fight or flight). Axons travel throughout the body back to the spinal cord. Their pathways look like a tree, where the spinal cord is the main trunk with branches extending out into the body, and twigs and side shoots spreading again so all tissues are reached.
Within the spinal column, there are three plexuses (bundle of nerve roots);
- the brachial plexus is a network of nerves formed by the ventral ramus of the lower four cervical nerves and first thoracic nerve. This plexus extends from the spinal cord, through the cervicoaxillary canal in the neck, over the first rib, and into the armpit. It supplies afferent and efferent nerve fibers to the chest, shoulder, arm and hand.
- The lumbar plexus is a web of nerves in the lumbar region of the body which forms part of the larger lumbosacral plexus. It is formed by the divisions of the first four lumbar nerves and from contributions of the subcostal nerve, which is the last thoracic nerve. Additionally, the ventral rami of the fourth lumbar nerve pass communicating branches, the lumbosacral trunk, to the sacral plexus. The nerves of the lumbar plexus pass in front of the hip joint and mainly support the anterior part of the thigh.
- The sacral plexus is a nerve plexus which provides motor and sensory nerves for the posterior thigh, most of the lower leg and foot, and part of the pelvis. It is part of the lumbosacral plexus and emerges from the lumbar vertebrae and sacral vertebrae. A sacral plexopathy is a disorder affecting the nerves of the sacral plexus, usually caused by trauma, nerve compression, vascular disease, or infection.
When a painful event happens;
This creates two painful stimulus which travel up towards the dorsal horn of the spinal cord;
- C fibres – Small diameter, non myelinated
Transmit dull, slow, achy pain (0.5-2m/s)
- Aδ (A delta) fibres- Medium diameter with some myelination
Sharp, acute pain (4-30m/s)
These Pain stimuli travel quickly to the substantia gelatinosa in the dorsal horn of the spinal cord where the “gating” mechanism occurs. Pain impulses then cross over to the opposite side of the spinal cord and ascend to the higher centres in the brain via the spinothalamic tracts and on to the thalamus and higher centres of the brain, including the reticular formation, limbic system, and somatosensory cortex.
- limbic system- variety of functions including emotion, behavior, motivation, long-term memory and olfaction. Emotional life is largely housed in the limbic system, and it critically aids the formation of memories.
- somatosensory cortex- indicative of the degree of sensitivity of tactile stimulation experienced at any body part
- reticular formation- The reticular formation (brainstem) aids the ascending pathways to the cortex in the ascending reticular activating system and descending pathways to the spinal cord.
How does the body inhibit the pain?
Pain gate mechanism
There are cells of the substantia gelatinosa that can have a inhibitory influence on T-cells. Pre-synaptic inhibition of nociceptive afferent terminals at the point where they synapse with transmission cells
Pain can be felt when: SG cells are inhibited when nociceptive afferents are activated reducing pre-synaptic inhibition allowing nociceptive information to pass to the brain
These inhibitory T-cells are generated by a sensory activation (rubbing or vibration of injured limb). These T-cells inhibit the nociceptive signal (painful stimulus), preventing the transmission o painful signal up towards the thalamus of the brain.
This is due to the activation of A-beta fibres which are highly myelinated, meaning that the conduction of the motor nerve fibre will reach the dorsal horn quicker than the nociceptive pain signals received from the peripheral nervous system (PNS) due to the nerve fibres being located in the central nervous system (CNS).
If you’ld like to research more into this interesting topic then give this video by Dr John Campbell a watch as he goes into greater detail;