physiology ....final material .... CNS .... Receptor Physiology

8/3/2019 physiology ....final material .... CNS .... Receptor Physiology

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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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physiology ....final material .... CNS .... Receptor Physiology

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