Ramesh Rajan; Week 2 MED1022; Physiology
State of things in external world is exteroreception, about the relationship of our body to the world is proprioception, state of things within the body is interoception. Nociceptors respond to pain stimuli. Every species has different range for each sensory modality, allows it to fit in niche. Peripheral components have three sections- accessory structures which translate stimulus to receptors, receptor cells which transduce stimulus to a response and afferent fibres which transmit information to the CNS.
Receptors also have an RMP. The inside of the cell is negative compared to the outside. Receptor cells tend to be less polarised than neurons (vision -30mV, hearing -30, olfaction -60mV). Change in RMP creates receptor potential which indicates presence of stimulus. A receptor may be a specialised ending of a nerve cell or a specialised cell in its own right. If the former, the RP causes an AP at some point along the neuron. In the latter, RP modulates transmitter release from the receptor cell to the next cell and this leads to APs in the neuron.
Each sensory modality has a set of codes to signal different parts of the stimulus. Intensity coding, location coding, coding of special object properties and coding of change. Coding for intensity can a) activate each receptor more, that is as stimulus intensity increases a receptor sensitive to that stimulus will respond more strongly (open more ion channels, activate more molecules) which will mean a larger change in ionic movement and a larger change in RMP; b) activate more neurons, as stimulus intensity increases more neurons reach threshold.
Coding for location can send information in a systematic order to the CNS to form a 'map' of the receptor surface.
Coding for quality is receptor tuning- each receptor 'tuned' to only a part of the stimulus range. Differently tuned receptors allow for discrimination between parts of the stimulus range.
Coding for change- adaptation is decreasing responsiveness despite a continued stimulus, all sensory receptors adapt. Different receptors adapt at different rates. Slow adaptation allows signalling of duration and other static elements of a stimulus. Rapid adaptation allows signalling of dynamic aspects.