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Transmembrane receptors are activated by chemicals called ligands.For example, a molecule in food can serve as a ligand for taste receptors.
This occurs when a stimulus changes the cell membrane potential of a sensory neuron.
The stimulus causes the sensory cell to produce an action potential that is relayed into the central nervous system (CNS), where it is integrated with other sensory information—or sometimes higher cognitive functions—to become a conscious perception of that stimulus.
An exteroceptor is a receptor that is located near a stimulus in the external environment, such as the somatosensory receptors that are located in the skin.
An interoceptor is one that interprets stimuli from internal organs and tissues, such as the receptors that sense the increase in blood pressure in the aorta or carotid sinus.
Receptor cells can be further categorized on the basis of the type of stimuli they transduce.
Chemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell.For humans, the only electromagnetic energy that is perceived by our eyes is visible light.Some other organisms have receptors that humans lack, such as the heat sensors of snakes, the ultraviolet light sensors of bees, or magnetic receptors in migratory birds.Osmoreceptors respond to solute concentrations of body fluids.Additionally, pain is primarily a chemical sense that interprets the presence of chemicals from tissue damage, or similar intense stimuli, through a nociceptor.The central integration may then lead to a motor response.Describing sensory function with the term sensation or perception is a deliberate distinction.Other transmembrane proteins, which are not accurately called receptors, are sensitive to mechanical or thermal changes.Physical changes in these proteins increase ion flow across the membrane, and can generate an action potential or a graded potential in the sensory neurons.Sensation is the activation of sensory receptor cells at the level of the stimulus.Perception is the central processing of sensory stimuli into a meaningful pattern.