Any uncomfortable sensation to the body either localized or spread to a specific body surface after repeated exposure of irritant stimulus is referred to as pain. Specialized receptors called the nociceptors to detect pain. Nociceptors are specialized nerve endings present beneath the superficial layer of the skin, which detects a submodality of sensory perception known as pain (Hall, 2010).
Pain can be elicited by various types of stimuli, such as mechanical, thermal. And chemical pain stimuli. However, to perceive pain, there are two types of fibers, Alpha delta fibers, and (non-myelinated) C fibers. In contrast to other fibers, Nociceptors adapt very little. However, if the sensitivity of pain receptors is increased, then it is stated as hyperalgesia (Dubin & Patapoutian, 2010; Hall, 2010).
Fast pain elicited by the thermal and mechanical stimulus only and is localized to the point of action of a stimulus, whereas the slow pain is elicited mostly by the chemical stimulus, which is not localized (Dubin & Patapoutian, 2010; Hall, 2010).
Nociceptors detect pain through pain fibers as the sensory information is carried to the spinal cord entering through the Dorsal horn of the spinal cord.
There are two pathways for the pain.
- Neospinothalamic pathway
- Paleospinothalamic pathway
Neospinothalamic pathway
Pathway for the fast pain conducted by A-delta fibers is elicited by stimulating neurons that travel from the spinal cord to the thalamic or somatosensory cortex of the brain. Pain sensations are perceived by the Nociceptors and transmitted to the spinal cord eliciting two sets of neurons for a complete transmission (Hall, 2010).
First-order neurons
Sensory information through the receptors is carried to the dorsal horn of the spinal cord, which releases a neurotransmitter named Substance P (Hall, 2010).
Second-order Neurons
First-order neurons excite second-order neurons in the spinal cord, and fibers cross contralaterallyto the other side of the white matter of the spinal cord. These second-order neurons travel to the spinal cord to the reticular area of the brain stem. Still, most of the fibers end up in the thalamus, and thalamus directs those fibers to the somatosensory cortex of the body (Dubin & Patapoutian, 2010; Hall, 2010).
Neurotransmitter
It is believed that excitatory neurotransmitter named as glutamate is secreted by the A-delta fibers.
Paleospinothalamic pathway
It is a much older pathway for the pain and conducts slow pain via type C fibers. Sensory information carried through sensory receptors enter the spinal cord, and First-order neurons end up in a specific area named as substantia geletinosa of the spinal cord and releasing a neurotransmitter and exciting the second order neurons that cross to the other side of the spinal cord and travel along the length in the anterolateral column of the spinal cord. The only difference is the termination of fibers. Fibers conducting slow pain (paleospinothalamic pathway) terminates in two more areas other than the reticular system of the brain, tectal area of the mesencephalon, and the periaqueductal area around the duct of Sylvius. Studies reveal that Type c fibers secrete both Substance P and glutamate while traveling along the spinal cord (Dubin & Patapoutian, 2010).
References
Dubin, A. E., & Patapoutian, A. (2010). Nociceptors: the sensors of the pain pathway. The Journal of Clinical Investigation, 120(11), 3760–3772.
Hall, J. E. (2010). Guyton and Hall textbook of medical physiology e-Book. Elsevier Health Sciences.