Sensory Coding PDF

Sensory Coding ○ Converting a receptor stimulus to a recognizable sensation is termed sensory coding. ○ All sensory systems code for four elementary attributes of a stimulus: modality, location, intensity, and duration Modality → the type of energy transmitted by the stimulus Location → the site on...

Sensory Coding ○ Converting a receptor stimulus to a recognizable sensation is termed sensory coding. ○ All sensory systems code for four elementary attributes of a stimulus: modality, location, intensity, and duration Modality → the type of energy transmitted by the stimulus Location → the site on the body or space where the stimulus originated Intensity → signaled by the response amplitude or frequency if action potential generation Duration → refers to time from the start to end of a response in the receptor Modality ○ A sensory receptor is specialized to respond to either mechanical, chemical, thermal, or electromagnetic stimuli (receptor specificity). ○ The form of energy to which a receptors is most sensitive is called its adequate stimulus ○ The adequate stimulus for the rods and cones in the eyes, for example, is light ○ Sensory receptors can respond to forms of energy other than their adequate stimuli, but the threshold for those nonspecific responses is much higher ○ Pressure on the eyeball will stimulate the rods and cones, for example, but the threshold of these receptors to pressure is much higher than the threshold of the pressure receptors in the skin Location ○ A sensory unit is a single sensory axon and all its peripheral branches ○ The receptive field of a sensory unit is the spatial distribution from which a stimulus produces a response in the unit ○ Representation of these senses in the skin is punctate ○ If the skin is carefully mapped, millimeter by millimeter, with a fine hair, a sensation of touch is evoked from the intervening areas ○ Similarly, temperature sensations and pain are produced by stimulation of the skin only over the spots where the receptors for these modalities are located. ○ One of the most important mechanisms that enable localization of a stimulus site is lateral ○ Information from sensory neurons whose receptors are at the peripheral edge of the stimulus is inhibited compared to information from the sensory neurons at the center of the stimulus ○ Thus, lateral inhibition enhances the contrast between the center and periphery of a stimulated area and increases the ability of the brain to localize a sensory input ○ Lateral inhibition underlies two point discrimination Intensity ○ The intensity of sensation is determined by the amplitude of the stimulus applied to the receptor ○ As a greater pressure is applied to the skin, the receptor potential in the mechanoreceptor increases, and the frequency of the action potentials in a single axon transmitting information to the CNS is increased ○ Weak stimuli activate the receptors with the lowest thresholds (receptor sensitivity). ○ As the stimulus strength is increased, it also spreads over a large area to “recruit” fibers in the surrounding area. ○ Some of the receptors activated are part of the same sensory unit, and impulse frequency in the unit therefore increases ○ Because of overlap and interdigitation of one unit with another, receptors of other units are also stimulated and more sensory fibers fire. ○ In this way, more afferent pathways are activated, which is interpreted in the brain as an increase in intensity of the sensation Duration ○ If a stimulus of constant strength is maintained on a sensory receptor, the frequency of the action potentials in its sensory nerve declines over time ○ The phenomenon is known as receptor adaptation ○ The degree to which adaptation occurs varies from one sensation to another ○ Receptors can be classified as rapidly adapting (phasic) receptors or slowly adapting (Tonic) receptors. ○ Meissner and Pacinian corpuscles are rapidly adapting receptors, and Merkel cells and ruffini endings are slowly adapting receptors ○ Muscle spindles and nociceptors are also slowly adapting receptors ○ Different types of sensory adaptation have some value to the individual ○ Light touch would be distracting if it were persistent; and, conversely, slow adaptation of spindle input is needed to maintain posture ○ Similarly, input from the nociceptors provides a warning that would be lost if the receptor adapted rapidly.