tousif, role of receptors

SENSORY PERCEPTION, ROLE OF

RECEPTORS

DR Syed Tousif Ahmed Professor

Department of Physiology

Sequence of Events in a ReceptorStimulus

Receptor Protein Activated

Enzyme Cascade (in some cases)

Receptor Ion Channels opened (or closed)

Receptor Current

Receptor Potential

Modulated Impulse Frequency in Second

Order Neuron

Basic Function

Reception

Amplification

Transduction

Transmission Modulated Impulse Frequency in Receptor

Cell Axon

Modulated Transmitter Release from Receptor Cell

Integration, perception

Sensory Receptors

Sensory Receptor Types

Which receptor?

Peripheral Sensory ReceptorsPeripheral Sensory Receptors

• Sensory receptors also classified according to: – Location– Type of stimulus detected– Structure

• Receptors are structurally classified as either simple or complex

• Most receptors are simple and include encapsulated and unencapsulated varieties

• Complex receptors are special sense organs

Receptor Classification by Structural Complexity

Sensory Receptor Types

Somatic Senses

• Touch

• Temperature

• Pain

• Itch

• Proprioception

• Pathway

Encapsulated Nerve EndingsEncapsulated Nerve Endings

• Meissner’s corpuscles

• Pacinian corpuscles

• Ruffini’s corpuscles

• Proprioceptors

Receptor Potentials

• All sensory receptors have one feature in

common.

• Whatever the type of stimulus excites

• Change in electrical potential of the receptor.

• This change in potential is called a Receptor

potential.

Receptor potentials: Changes in the transmembrane potential of a receptor caused by the stimulus.

Generator Potential: A receptor potential that is strong enough (reaches threshold) to generate an action potential.

Remember that APs are all-or-none. The stronger the sitmulus (above threshold) the more APs are fired over a given time period; this is translated by the CNS as a strong sensation.

Receptor/Generator Potential

Pacinian Corpuscle

• Central nerve fiber extending through its core.

• Surrounding – multiple concentric capsule layers

• Compression anywhere on the outside of the corpuscle will

– Elongate,

– Indent or

– Deform the central fiber

• Central fiber of the pacinian corpuscle• The tip of the central fiber - unmyelinated• The fiber - Myelinated• Deformed by compression• Sodium influx - a local circuit of current flow• Node of Ranvier, typical action potentials

transmitted

Mechanisms of Receptor Potentials.

1. By mechanical deformation– Stretches the receptor membrane – Opens ion channels

2. By application of a chemical – Opens ion channels

3. By change of the temperature of the membrane– Alters the permeability of the membrane

4. By the effects of electromagnetic radiation, such as light– Allows ions to move

• Basic cause of the change in membrane potential is a change in membrane permeability of the receptor

Maximum Receptor Potential Amplitude.

• The maximum amplitude of most sensory receptor

potentials

– 100 millivolts

• Change in voltage when the membrane - maximally

permeable to sodium ions.

• When the receptor potential rises above the threshold

for eliciting action potentials

– Action potentials occur

• More the receptor potential rises above the threshold

level - Greater - action potential frequency

Classification by Modality

• Mechanoreceptors – respond to mechanical forces

• Thermoreceptors – respond to temperature changes

• Chemoreceptors – respond to chemicals in solution

• Photoreceptors – respond to light – located in the eye

• Nociceptors – respond to harmful stimuli that result in pain

Touch (pressure)

• Mechanoreceptors

• Free nerve endings

• Pacinian corpuscles

• Ruffini corpuscles

• Merkel receptors

• Meisaner's corpuscles

• Barroreceptors

Temperature

• Free nerve endings

• Cold receptors

• Warm receptors

• Pain receptors

• Sensory coding:– Intensity– Duration

Pain and ItchingPain and Itching

• Nociceptors

• Reflexive path

• Itch

• Fast pain

• Slow pain

Special Senses – External Special Senses – External StimuliStimuli

• Vision

• Hearing

• Taste

• Smell

• Equilibrium

Three Types of ProprioceptorsThree Types of Proprioceptors

– Monitor stretch in locomotory organs– Three types of proprioceptors

• Muscle spindles – measure the changing length of a muscle– Imbedded in the perimysium between muscle

fascicles

• Golgi tendon organs – located near the muscle-tendon junction– Monitor tension within tendons

• Joint kinesthetic receptors – Sensory nerve endings within the joint capsules

Classification by LocationClassification by Location

• Exteroceptors – sensitive to stimuli arising from outside the body– Located at or near body surfaces– Include receptors for touch, pressure, pain, and

temperature

• Interoceptors – (visceroceptors) receive stimuli from internal viscera– Monitor a variety of stimuli

• Proprioceptors – monitor degree of stretch– Located in musculoskeletal organs

Receptor Class by Location: Exteroceptors

• Respond to stimuli arising outside the body

• Found near the body surface

• Sensitive to touch, pressure, pain, and temperature

• Include the special sense organs

Receptor Class by Location: Interoceptors

• Respond to stimuli arising within the body

• Found in internal viscera and blood vessels

• Sensitive to chemical changes, stretch, and temperature changes

Receptor Class by Location: Proprioceptors

• Respond to degree of stretch of the organs they occupy

• Found in skeletal muscles, tendons, joints, ligaments, and connective tissue coverings of bones and muscles

• Constantly “advise” the brain of one’s movements

Simple Receptors: Unencapsulated

• Free dendritic nerve endings– Respond chiefly to temperature and pain

• Abundant in epithelia and underlying connective tissue

• Monitor affective senses• Two specialized types of free nerve

endings– Merkel (tactile) discs– Hair follicle receptors

Simple Receptors: Encapsulated

• Encapsulated nerve endings: dendrites with special supporting structures

• Consist of one or more end fibers of sensory neurons

• Enclosed in connective tissue• Mechanoreceptors • Meissner’s corpuscles (tactile corpuscles) • Pacinian corpuscles (lamellated corpuscles)• Muscle spindles, Golgi tendon organs, and

Ruffini’s corpuscles• Joint kinesthetic receptors

Transduction of Sensory Receptors

Transduction: The process by which an environmental stimulus becomes encoded as a sequence of nerve impulses in an afferent nerve fiber is called sensory transduction

–Sense orgrans transduce sensory energy into neural (bioelectrical) energy

–Converting one type of energy into another type is the process of transduction

–Your brain only deals with bioelectrical impulses so transduction must occur; what cannot be transduced cannot be a stimulus

Each type of receptor is highly sensitive to one type of stimulus for which it is designed and yet is almost nonresponsive to normal intensities of other type of stimuli.

The stimulus to which a given receptor has the lowest threshold is termed the adequate stimulus of the sensory receptor.

.

Adequate Stimulus of Sensory Receptors

Processing at the Receptor Level

• The receptor must have specificity for the stimulus energy

• The receptor’s receptive field must be stimulated

• Stimulus energy must be converted into a graded potential

• A generator potential in the associated sensory neuron must reach threshold

SENSORY REPRESENTATIONS

Labeled line principle

• Receptor

• Stimulation

• Impulse

• Nerve

• Brain

Touch Pain Temp

STIMULUS INENSITY

STIMULUS DURATION

STMULUS LOCATION

Sensory Transduction• Sensory transduction converts stimuli into

graded potentials.• Such changes in receptor membrane potential

are known as• the receptor potential and the generator

potential.

Relation Between Stimulus Intensity and the Receptor Potential.

• Very intense stimulation

• Progressively less and less additional increase in numbers of action potentials

• Allows the receptors to have an extreme range of response,

– From very weak

– To very intense

Mechanism of Receptor adaptation

• Readjustments - structure of the receptor

– Pacinian corpuscle is a viscoelastic structure

– Distorting force - transmitted by the viscous

component

– Elicit a receptor potential.

– Within a few 100ths of a second- fluid within the

corpuscle redistributes

• Electrical type of accommodation

– Progressive “inactivation” of the sodium channels

Adaptation of different types of receptors

• Some sensory receptors

adapt to a far greater

extent than others

• Mechanoreceptors adapt

almost completely

• Nonmechanoreceptors

– Chemoreceptors

– Pain receptors,

never adapt completely.

Slowly Adapting Receptors

• Detect Continuous Stimulus Strength

• The “Tonic” Receptors.

• Continue to transmit impulses to the brain as long as the stimulus is present/hours.

• Keep the brain constantly apprised of the status of the body and its relation to its surroundings.

– Golgi tendon apparatuses

– Macula in the vestibular apparatus

– Baroreceptors and chemoreceptors

Rapidly Adapting Receptors -• Detect Change in Stimulus Strength

• The Rate / Movement / Phasic Receptors

• Cannot be used to transmit a continuous signal

• React strongly while a change is actually taking place.

• Pacinian

– Sudden pressure applied to the tissue excites this receptor for a few milliseconds

– Transmits a signal again when the pressure is released….

– Importance

Sensory Adaptation is one form of Integration

Phasic receptors quickly adapt. The frequency of action potentials diminishes or stops if the stimulus is unchanging.

Tonic receptors adaptslowly or not at all.

Most exteroreceptors (receptors that monitor the external environment) are phasic receptors.

Rapidly Adapting “ The Rate Receptors”

• Also called as “Movement Receptors” or “Phasic Receptors”.

• Detect change in stimulus strength

• They react strongly when a change is actually taking place there fore called as rate receptors

Slowly Adapting “The Tonic Receptors”

• Macula in the vestibular apparatus

• Pain receptors

• Baroreceptors

• Chemoreceptors of the carotid and aortic bodies

Encoding of Sensory Receptor

The quality of the stimulus is encoded in the frequency of the action potentials transmitted down the afferent fibre and the number of sensory receptors activated.

Stretch Receptors:

Weak stretch causes low impulse frequency on neuron leaving receptor.

Strong stretch causes high impulse frequency on neuron leaving receptor. Time

Membrane potential

Frequency Code

Tonic Receptors vs. Phasic Receptors• Slow or no adaptation

• Continuous signal transmission for duration of stimulus

• Monitoring of parameters that must be continually evaluated, e.g.: barorecptors __________ ?

• Rapid adaptation

• Cease firing if strength of a continuous stimulus remains constant

• Allow body to ignore constant unimportant information, e.g.:

___________?

Adaptation of Receptors

• When a continuous sensory stimulus is applied , the receptor responds at a high impulse rate at first and then at a progressively slower rate until finally the rate of action potential decreases to very few or often to none at all

• Some sensory receptors adapt to a far greater extent than others

• While other sensory receptors adapt in hours or days to do so and are called non-adapting receptors

•

Receptive Field

• Is area of skin whose stimulation results in changes in firing rate of sensory neuron– Area varies inversely with density of receptors – E.g. back, legs have low density of sensory

receptors• Receptive fields are large

– Fingertips have high density of receptors• Receptive fields are small

10-22

Two-Point Touch Threshold

• Is minimum distance at which 2 points of touch can be perceived as separate– Measure of tactile acuity or distance between receptive fields

10-25

THE ENDTHE END