Question
What are the phases of a pain sensation?
Answer
There are four physiological phases of nociception including transduction, transmission, modulation and perception. Transduction occurs after the sensation is perceived by the nociceptors. It is the process by which the free nerve endings participate in the compression of the mechanical or thermal stimulus into electrical impulses which are then transported by the nervous system up the spinal cord. Transduction is the transformation of a stimulus from the initial input or energy form into an electrical impulse that can be processed by the nervous system. The nociceptors usually get transformed in this process. Potassium gets released from the nerve endings and certain substances like prostaglandins and histamines are also released. Substance P is also released from nearby cells causing certain action potentials which are transmitted by the neurons.
The next phase is called transmission which is the process of the impulse traveling to the dorsal horn of the spinal cord, then up the sensory spinothalamic tract to the brain.
Modulation is the process of either dampening or amplifying a signal. It typically occurs at the dorsal horn of the spinal cord as the signal ascends to the cortex. However, there are other areas involved with the descending pathway from the central level. There are descending inhibitory inputs that come from the cortex, hypothalamus, the midbrain or the medulla that may decrease or block nociceptor signals at the dorsal horn.
The last step in the process is perception which is the conscious awareness of pain. The signal has reached the brain and is interpreted as a sensation. Perception also involves the integration of transduction and transmission, as well as personal factors such as experience of pain, psychological factors, motivation, thoughts and ideas, stress level and emotions. All of these factors come together to form the impression and perception of the pain and then the body responds to the sensation.
Editor’s note: This Ask the Expert was adapted from the article ‘Evaluation and Management of Pain’. The complete article can be accessed here.
