18.3: Sensory Perception: Organization of the Somatosensory System
The somatosensory system is the central and peripheral nervous system component that senses and processes touch, pressure, pain, temperature, and body position or proprioception. The process of sensation takes place at three levels:
The receptor level:
The receptor level is the first stage of sensation. It involves the detection of a stimulus by specialized sensory receptors. The stimulus must arrive within the receptor's receptive field. Next, the receptor converts the energy of the stimulus into an electrical signal via transduction, which leads to the generation of membrane potential in the receptor cell. When the potential reaches a certain threshold, a nerve impulse is generated
The circuit level:
The circuit level is the second stage of sensation. The generated nerve impulse now travels to the central nervous system (CNS). Several ascending tracts carry the impulses from the receptors to their final destination in the CNS. The stimulus from the facial area is carried and transmitted by cranial nerves, while stimulus from the back of the head and rest of the body travels by spinal nerve. Sensory neurons conducting impulses from the peripheral nervous system (PNS) to the CNS are called first-order neurons.
The perceptual level:
The perceptual level is the third and final stage of sensation. It involves the reception of sensory information by different regions of the CNS, depending on the stimuli. Only the impulses processed in the cerebral cortex are consciously perceived.
