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8/3/2019 physiology ....final material .... CNS .... Receptor Physiology
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5/5/12
Receptor Physiology
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Sensations &Perceptions
Sensation is an awareness of sensorystimuli in brain
Perception meaningful interpretation orconscious understanding of sensory data
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Visceral afferents vs. sensoryafferents
q Visceral afferent the incoming pathway for subconsciousinformation derived from the internal viscera (organs)
q Sensory afferent : the incoming pathway that carriesperipheral sensory information and does reach the level ofconscious awareness
v Sensory information:
Somaticsensation, from the body surface
Special senses, including vision, hearing, taste, andsmell
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Components of sensoryphysiology
q Modality: form of energy of the stimulus, e.g., heat, light,sound, pressure..
q Reception: ability of receptor to absorb energy of a stimulus
q Transduction: conversion of stimulus energy into membrane
potential
q Transmission: receptor potentials transmitted via AP's toCNS
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Receptor physiology
q Classes of receptors according to their adequatestimulus:
Photoreceptors
Mechanoreceptors
Thermoreceptors
(warm & cold receptors)
Osmoreceptors
Chemoreceptors
Nociceptors, or pain receptors
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From receptor to CNS
Stimulation of a receptoralters its membrane
permeability
nonselectiveopening of all small ion
channels
local depolarizing change inpotential
(or receptor potential)
action potential in the afferent neuronmembrane
next to the receptor (Na+ influx)
AP self-propagates along the
afferent fiber to the CNS
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Stronger stimulus larger receptor potential greaterfrequency of AP
q Receptor potential:
Graded potential (amplitude and duration depend on strength/rate ofstimulus )
No refractory period, (so summation in response to rapidly successivestimuli is possible)
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No generation ofaction potential in thereceptor itself (veryhigh threshold)
InsteadGradedreceptor
Potentialhere
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Receptor Adaptation(desensitization)
q Receptor adaptation:
The decrease in the extent of receptor depolarizationdespite sustained stimulus strength
Persistent stimulus gives no response
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Tonic receptors vs. Phasicreceptors
q Two types of receptors based on their speed of adaptation:
1. tonic receptors (slowly adapting)
2. Phasic receptors (rapidly adapting)
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AdaptationAdaptation in a sensory receptoris related to a decline in thegenerator potential with time.A,The generator potential ismaintained without decline, andthe action potential frequencyremains constant.
B, A slow decline in the
generator potential is associatedwith slow adaptation.
C, In a rapidly adaptingreceptor, the generator
potential declines rapidly.
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Coding of sensory information
The action potentials in the nerve from a touch
receptor, for example, are essentially identical to
those in the nerve from a warmth receptor
This raises the question of
Why stimulation of a touch
receptor causes a sensation of
touch and not of warmth?
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Coding of sensory information
q MODALITY
q LOCALITY
q
INTENSITY
C di f S
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Coding of SensoryInformation
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Labeled line principle
A particular sensory input is projectedto a specific region of the cortex.
A particular sensory
modality
Detected by a specializedreceptor type
Sent over a specific afferent and
ascending pathway (a neural pathwaycommitted to that modality)
Excites a defined area in the
somatosensory cortex
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Receptive field
q The receptive field of a somatosensory neuron is the area fromwhich a stimulus produces a response in that unit.
q The size of a receptive field varies inversely with the
density of receptors in the region:
The more closely receptors of a particular type arespaced, the smaller the area of skin each monitors
The smaller the receptive field in a region, the greater itsacuity or discriminative ability
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Comparison of discriminative ability ofregions with small versus large
receptive fields
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RECEPTOR PHYSIOLOGY
q Functions of receptors:
respond to stimuli
Transduction: convert stimulus forms of energy intoelectrical signals (action potentials)
q Stimulus Modality: various energy forms of stimuli, such asheat, light, sound, pressure, and chemical changes
q Receptors have differential sensitivities to various stimuli,however, Some receptors can respond weakly to stimuliother than their adequate stimulus, e.g., mechanicalpressure.
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