C 12 Mechano Reception

8/12/2019 C 12 Mechano Reception

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Laws of Sensory

Perception

Skin receptors-mechanical receptors

Pacinian receptors


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Receptors- Definition and Properties

- Sensory Receptors are special nerve endings,distributed throughout the body (in the skin,muscles, vessels, bones and joints, in lungs,heart, and another organs).

- They Convert Different Forms of Energy intoElectrical Signals. Thus they serve as

t ransducers, changing the particular form ofenergy ( e.g. mechanical, chemical, thermal, orelectromagnetic) into the electrical signal.

- Our body contains20 types of receptors that can

detect e.g.heat, pressure, stretch, acceleration,sound, light, smells, taste, partial pressure,concentration of salts, hormons...and other formsof stimuli (Only receptors for ionizing radiation aremissing)


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Receptors - Classification

I.According to locality: Externoreceptors- are placedwithin the skin, like receptors for touch, pressure, heat, cold or pain

Proprioreceptors-are placed in muscles, in bones and joints -theyinform about the lengt of muscles and ligaments

Internoreceptorsreceptors within the organs (heart, lungs,kidney) They detect plasma osmolarity, partial pressure of O2bloodpressure..

II.According to type of energy: Mechanoreceptors- theytransform mechanic energy into electric signal.E.g. exte-rorecep

tor, baroreceptors, pulmonary stretch receptors).

Fotoreceptors- receptors containing photopigments (rodsand cones at retina

Chemoreceptorstaste receptors in the tongue, smellreceptors within a nose, osmoreceptors in hypothalamus,..

Nociceptors- pain receptors - in skin, in organs ...

III.According to complexity: simple receptors (skin) andcomplex ones (eye, ear)


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Externoreceptors Sensitive to stimuli arising from outside

of the body

Typically located near the surface of the

body

Include receptors for Touch

Pressure

Pain

Temperature

Special sense receptors


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Internoreceptors or visceroceptors Respond to stimuli arising from within the

internal viscera and body organs,

Internoreceptors monitor a variety of internalstimuli Changes in chemical concentration

Taste stimuli

The stretching of tissues

Temperature

Their activation causes us to feel visceralpain, nausea, hunger, or fullness


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Proprioreceptors Located in the musculoskeletal

organs such as skeletal muscles,

tendons, joints and ligaments

Proprioreceptors monitor thedegree of stretch of these

locomotor organs and send input

to the CNS They are Encapsulated Dendritic

Endings


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Mechanoreceptors

general nerve impulses when they, or

adjacent tissues, are deformed by

mechanical forces

Touch

Pressure

Vibration

Stretch

Itch

Thermoreceptors

Sensitive to temperature changes


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Hair follice receptors.

Mechano-receptors like:

Light TouchMeissners

Corpuscle

Krausetouch, light pressure

Pacini-deep pressure, vibration

Rufini-pressure and touch

Strong PressureMerkels Disk

PainNociceptors

Heat/ColdThermoreceptors,

some nociceptors

Skin (cutaneous)

receptors


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Skin

Type

Depth Adaptation

Modality

Touch Pressure

Pacinian

Corpuscles

Glabrous

and Hairy

Deep Rapid Light Vibration

Meissners

Corpuscles

Glabrous Surface Rapid Light Vibration

Merkels

Disk

Glabrous Surface Slow Crude Pressure

Ruffini

Corpuscles

Hairy Deep Slow Crude Pressure

Hair Follice

Receptors

Hairy Deep Rapid Light Neither

Cutaneous Mechanoreceptor Types

Merkels Disk

Ruffini Ending

Mechanical Displacement

Pacinian Corpuscle

Hair Follicle Receptor


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Most mechanoreceptors areencapsulated dendritic endings Encapsulated receptors vary widely in

shape, size, and distribution in the body

The main types are

Meissners corpuscles-tactil, sensing light

touch

Krauses end bulbs-touch, light pressure

Pacinian corpusclesdeep pressure,pressure change, vibration

Ruffinis corpuscles pressure and touch

Proprioceptors (not in the skin but in themuscles


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Meissners Corpuscles In a Meissners

corpuscle (tactilecorpuscle) a few

spiraling dendrites

are surrounded by

Schwann cells,

which in turn are

surrounded by an

egg-shapedcapsule of

connective tissue


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These corpuscles

are found in thedermal papillae

beneath the

epidermis

These corpuscles

are rapidly

adapting receptors

for fine, light touch


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Meissners corpuscles occur insensitive and hairless areas of the

skin, such as the soles of the feet,

palms, fingertips, nipples, and lips Apparently, Meissners corpuscles

perform the same light touch

function in hairless skin that root hairplexuses perform in hairy skin


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Meissners Corpuscles sensing lighttouch

Highly concentrated in areassensitive to light touch.

Fingertips, lips, tongue,

soles, genitals

Located: surface of the skin

highly sensitive Limited. Must

physically be touching.


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Krauses End ulbs Krauses End Bulbs

are a type ofMeissners

corpuscle for fine

touch

Krauses end bulbs

occur in mucous

membranes in the

lining of the mouthand the conjunctiva

of the eye


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Pacinian Corpuscle Pacinian

corpuscles arescattered

throughout the

deep connective

tissues of the body

Occur in the

hypodermis of the

skin Sensing pressure

change, vibration,

deep pressure


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Pacinian Corpuscles Pacinian corpuscles are rapidly adapting

receptors and are best suited to monitor

vibrations which is an on-off stimulus These corpuscles are large enough to be

visible to the naked eye


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Pacinian Corpuscles sensingpressure change, vibrationsIn joints, organs.

Rapidly adaptingresponds

briefly to begining and end of

stimulus

Detects: pressure change, esp.

vibrations

Up to centimeters away

Responds best to sinusoidal

vibrations w/ narrow frequency

range.

~Low pass filter

Result of onion-shaped


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Ruffinis Corpuscle Ruffinis corpuscles

are located in thedermis of the skin

and joint capsules

of the body

The corpuscle

contains an array of

dendritic endings

enclosed in a thinflattened capsule


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Ruffinis corpuscles

respond topressure and touch

They adapt slowly

and thus canmonitor continuous

pressure placed on

the skin


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Ruffini Ending sensing pressure andtouch

Only in glabrous skinSensitive to skin streching.

Contributes to fine motor control.

Contributes significantly to finger positioning.


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Merkels Disk-sensing strong pressureSlow adaptation because of rigid

structure.Sustainable response30 min. in

humans

Irregular firing in sustained.

Large receptive field.Small, sharp pressure: fast firing

rate.

Large, flat pressure: slow rate

Located in hairless skin and in hair

follices.

Not in skin surrounding follicle.


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The Integumentary System Skin)


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Stratum BasaleDeepest of the epidermal strata made of a singlelayer of

cells (consisting of 3 cell types) Keratinocytes

undergo mitosis rapidly to replace the cells that areexfoliated from the surface

as these keratinocytes move up through the moresuperficial strata

Melanocytes

produce the brown pigment melaninto protectagainst UV radiation (sunlight) damage to DNA ofkeratinocytes

Merkel (tactile)cells

touch receptors that provide sensory perception


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Layers of the Dermis: Papillary Layer

Papillary layer

20% of the thickness of the dermis

Its superior surface contains fingerlike projections

called dermal papillaewhich keep it firmly attachedto the superficially located epidermis

Dermal papillae contain sensory receptors such as

tactile corpuscles(touch receptors) and free nerveendings(sense pain)


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Layers of the Dermis: Reticular Layer

Reticular layer

80% of the thickness of the dermis

Elastic fibers provide stretch-recoil properties

Location of:

hair and associated follicles

pacinian corpuscles (pressure receptors)

blood vessels

glands


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The Pacinian Corpuscle: a

Pressure Receptor


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Coding of sensory information on

Paccinian Corpuscle (a skin receptor

for touch and pressure)


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Coding of Stimulus on Paccinian

corpuscle

Mechanical touch-pressure energy affects thereceptor nerve membrane(without myelin)causing its local depolarization, that results in

an appearance of RECEPTOR orGENERATOR POTENTIAL(GP).Whenanother touch pressure stimuli come on thereceptor it causes creation of many local

potentials and their summation . When theamplitude of GP is above10mV then theseries ofACTION POTENTIALS rise uponthe afferent nerve fibre (which is covered with

myelin),that leaves the Paccinian corpuscle.


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Pacinian corpuscles

Most are found in the dermis of the skin Are the largest skin receptors

Consist of a single neurone ending surrounded by a bulb

of lamellae and a capsule, made of connective tissue

Are also found in joints, tendons and in the tissue liningblood vessels and organs

Provide information about how and when we move

Are sensitive to changes in pressure

They are transducers, converting pressure energy into

electrochemical energy of a generator potential

Give a graded response, the greater the pressure the

greater the frequency of nerve impulses along the

neurone


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Photomicrograph showinga Pacinian Corpuscle

Subject to copyright clearance a suitable imqge showing a Pacinian

Corpuscle could be inserted here.

e.g. one similar to that found at:

http://www.montgomerycollege.edu/faculty/~wolexik/public_html/Pacinian

%20Corpuscle-100x.jpg

capsule


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The Pacinian Corpuscle

Directionof impulse

X Y

Transverse section X-Y

Capsule of connective tissue

Lamellae separated by gel

Single neurone ending (naked)

Myelin sheath

Axon of sensory neurone


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The Pacinian corpuscle is a pressurereceptor External pressure causes

the corpuscle to deform The greater the pressure

the more deformation

The sodium channels in

the neurone membrane

are pressure sensitive

An increase in

pressure causessodium channels to

open

pressure


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Pressure on the skin is transmitted toto the corpuscle in the dermis The shape of the corpuscle is changed

Causing sodium ion channels in theneurone membrane to open

Sodium ions diffuse into the neurone

down the concentration gradient

Depolarising the membrane

Called a generator potential

The greater the pressure the more

sodium channels open causing a bigger

generator potential

If the threshold of that neurone isreached

An action potential develops and is

transmitted along the sensory neurone

pressure


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Pressure causes sodium channels inthe neurone membrane to openNa+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Neurone ending

in corpuscle

pressure

pressure

Pressurecauses sodium channels to open

Sodium ions diffuseinto the neurone

Down the concentration gradient

Creating a generator potential


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The greater the pressure the moresodium channels open

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Neurone ending

in corpuscle

pressure

pressure

pressure

More sodium ions diffuseindown theconcentration gradient

A larger generator potentialis created

If the threshold is reached an action potentialdevelops

Action potentials will continue to be developedwhile the generator potential is at or above thethreshold

The greater the pressure the greater the

frequency of nerve impulsesalong the neurone

The maximum frequency is limited by

the refractory period


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Pacinian Corpuscles Show AGraded Response

time

Potentialdifferenceacrossmembrane

threshold

restingpotential

Generatorpotential

Action potentialsat low frequency

pressure


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Greater pressure results in a higherfrequency of impulses along the neurone

time

Potentialdifferenceacrossmembrane

threshold

restingpotential

Generatorpotential

Action potentials at highfrequency and for alonger time

Greaterpressure


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Laws of Sensory Perception:

Weber-Fechners Law of Perception: is a basic psy-

chophysical Law. The bigger is the intensity of sti-mulation, the higher is the magnitude of sensation.Magnitude of sensation E = log S, ( S - intensity ofstimulation )or in a modified form :

Stevensons Law: FAP= k . Sn

(FAP is rate of APs froma receptor, k- constant, n=1 is valid formechanoreceptors, n1, for fotoreceptors, n 1, forpain receptors.

The Law of Projection:Each sense occupies the uniq-ue and separated site within the brain cortex.Therefore, we are able to distinguish the individualstimuli - like touch, pressure, pain sensations, lightor sound.

The Law of Adaptation


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The Law of Adaptation Adaptation-is an internal electric property(caused by

membrane properties of the receptor) to respond whenthe long-term stimulus of a constant intensity is applied.Actualy, it is a drop of the receptor excitability to giverise the GP and then the APs

Receptors with Rapid Adaptationof theirBurst Activity - their fire just for a short time, during

the constant (maintained ) stimulation ( as typically seenin touch, pressure, taste and smell receptors.)

Receptors with Low Adaptation of theirBurst Activity-they fire for a long time with only a lowdrop of their firing activity (as seen in the pain, cold, heat

receptors, baroreceptors, in pulmonary stretch recepors,the chemoreceptors, carotid baroreceptors or in thepulmonary stretch receptors).

The receptors with Low Adaptation areinvolved in a control of blood pressure, in control of

breathing , in responses of body to the pain, etc.


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Receptor Adaptation


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Sensory perception

- it is an ability to distinguish, detect, utilize

some feelings , and answer to many informa-tion that come to the brain through the reflexarc.

- The reflex arcconsists of receptor, afferent

nerve pathway, the central nervous sys tem(brain and spine),efferent pathwayand effector(muscle...)

- Information from an environment come to our

body and are processed by our senses (Touch,Taste, Smell, Vision, Hearing).Reaching thebody the information are coded in 2 forms: asa local electric response (local potential), andthe general action potential (AP)


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Reflex arc

l ( R) d A i


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Local response (LR) and Action

potential (AP)

Local electric response-takes local place, it does notspread to the vicinity, when its magnitude reachesmore than 10 mV then, in turn AP is produced. Thistype of coding is so called AMPLITUDE. (i.e. the

stronger is stimulus, the higher is amplitude ofresponse (examples: receptor potential (generatorpotential), EPSP and IPSP or end-plate potential).

Action potentialis a generally spreading electricity,

being under the Law All or NoneThis type of coding isnamed FREQUENCY. I.e.the stronger is stimulus thehigher is a rate of APs from the receptors. The brainknows that a higher frequency of action potentialsmeans a stronger stimulus (and vice versa)


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Adaptation When pressure is first applied impulses are transmitted

along the neurone

With continuous pressure the frequency of the action

potentials decreases, and then stop This is known as adaptation (it occurs in most sensory

receptors)

It prevents the nervous system being overloaded with

insignificant information e.g. the pressure from clothing


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Questions1. Where are Pacinian corpuscles found in the body?

(1mark)

2. Explain how pressure on the skin is perceived.(5marks)

3. Explain why slight pressure on part of the skin may go

unnoticed.

(2marks)

4. Explain how differences in pressure applied to the same

part of the skin are detected.

(2marks)

5. What is meant by adaptation and why is it useful?(2marks)

Click on the marks to checkyour answers

Click here to finish


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Answer Q1 In the dermis of the skin, tendons and

jointsBack to question


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Answer Q2 Pressure deforms the Pacinian corpuscle Causing sodium channels to open in the neurone

membrane

Sodium ions diffuse into the neurone

Down a concentration gradient

Creating a generator potential/depolarising the

membrane

If the generator potential is above the threshold

Impulses /action potentials are conducted along the

sensory neurone to the brain

Any 5 from the above

Back to question


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Answer Q3 Slight pressure will cause fewer sodium

ion channels to open

Fewer sodium ions diffuse in creating alower generator potential

If this is lower than the threshold there

will be no action potentials/no impulses tothe brain

Any 2 from the above

Back to question


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Answer Q4 The greater the pressure applied the

greater the number of sodium ion

channels opened The greater the generator potential

created

Larger generator potentials cause anincrease in the frequency of impulses

sent along the neurone

Back to question


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Answer Q5 Continuousstimulation of a sensory

neurone can result in a decreased

frequency of impulses and even stopthem, this is known as adaptation.

It is useful as it prevents overloading the

nervous system with insignificantinformation e.g. the pressure from

clothing

Back to question


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What synoptic links can you think of? Relationship between structure, shape and

function of protein

Facilitated diffusion and active transport

Respirationenergy required fromrespiration to restore receptors after

transduction / presence of mitochondria

C 12 Mechano Reception

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