The somatosensory system

Taste

Smell

Practical tasks

1. Examination of cutaneous sensitivity

2. Skin localisation

3. Adaptation of skin mechanoreceptors

4. Detection of olfactory sensations

5. Detection of taste sensations

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Senses 2

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The somatosensory system

- allows for sensation of the body and movements of its parts

Sensory receptors of the somatosensory system

1. Exteroceptors – cutaneous sensitivity (skin receptors)

- touch, pressure, vibrations, itching, tickling (mechanoreceptors)

- cold, warmth (thermoreceptors)

- pain (nociceptors) 2. Proprioceptors (Muscle spindle, Goldi tendon organ)

- position of the body - position of individual parts of the body

- muscle tone - movement of parts of the body

3. Interoceptors – perceptions from the internal organs

Cutaneous mechanoreceptors

1/ free nerve endings

2/ specialised receptors

– round corpuscles in the skin

– layers of cells surrounded by connective tissue

– branches of afferent nerves among cells

• various types of receptors exist - differ in:

– location in the skin (superficial, deep)

– sensitivity to different qualities of the stimulus:

- intensity

- direction

- speed

- adaptability – ability to detect duration of the

stimulus

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Adequate stimulus – mechanic energy

• touch – stimulus of a low intensity

• pressure – stimulus of a higher intensity

• vibration – series of regular repeated stimuli

Main types of touch receptors

Receptor

type/adaptability

Sensation Location

Merkel’s discs (slow)

Sustained touch and pressure Movement of a stimulus

Base of epidermis

Ruffini

corpuscles (slow)

Sustained deep pressure Deep in dermis and

hypodermis

Meissner’s

corpuscles

(rapid)

Texture

Slow vibrations

2 point discrimination

Upper dermis

Pacinian corpus-

cles (very rapid)

Deep pressure

Fast vibrations Deep in dermis

Hair end organ Light touch Around hair follicles

Free nerve endings

Touch, pressure

(+ heat, cold, nociception) Throughout the skin

Adaptability of a receptor

- the stimulation continues, but a decline in generator potential and thus frequency of

AP occurs, or disappears

- reduced input to CNS – less intense sensation / no sensation (e.g. clothes - skin)

receptors

- rapidly adapting

- when the stimulation starts – AP is generated

- although the stimulus is still acting, receptors stop to fire AP after some time

- when the stimulation is ceased, the change causes AP

- slowly adapting – fire AP in as long as the stimulation persists, but

- at the beginning a strong response, then it becomes weaker

- non -adapting – fire constant AP in as long as the stimulation persists

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Most sensitive areas of the body

• tip of the tongue, lips

• fingertips - in proximal direction of hands the sensitivity decreases

sensed 1 point sensed 2 points

Sensitivity of the skin depends on

• receptor count per area (density of receptors)

– higher count = better sensitivity

• one receptor „collects“ stimuli from a smaller

surface – better discrimination The receptive field of a cutaneous receptor

– portion of the skin which, when stimulated,

affects the activity of receptor

• large receptive fields – lower sensitivity

• mall receptive fields - higher sensitivity

• count of nerve fibres – 2 stimuli are perceived

as 2 if 2 receptors are stimulated, each of them

having own sensitive fibre (nerve)

thickness of the skin skin temperature presence of sweat age

large receptive

fields

small receptive

fields

A. Specific (free nerve endings)

1. Cold receptors

– sensitive to lower than body temperatures

2. Warmth receptors

– sensitive to normal temperatures and higher than body temperature up to 48 ° C

B. Non specific

• nociceptors

– 1. type: sensitive to temperatures higher than 40 ° C

– 2. type: respond to temperatures less than 10 ° C

Adequate stimulus: change in temperature

Non – adequate stimuli: menthol, CO2 (perception of cold)

Cutaneous thermoreceptors

Receptor count (per unit of skin surface area)

pain > touch > cold > heat

Adaptability of thermoreceptors

- fast - after a few moments we stop to feel the change in temperature in full

extent (e.g. cold is perceived as less cold)

Pain

Somatic pain

- superficial (skin receptors)

- deep (receptors in muscles, joints, connective tissue)

Visceral pain (receptors in viscera)

Fast pain

- begins 0,1 s after stimulation

- sharp, acute, electric

Slow pain

- begins 1 s after stimulation, increases slowly

- burning, nauseous, chronic pain

Pain receptors – nociceptors

sensitive to high intensity of stimuli

sensitive to chemical, mechnical, thermic stimuli

non-adaptable

Phantom pain

= pain that feels like it's coming from a body part that was amputated

Explanation

• the nerve endings at the site of the amputation continue to send

pain signals to the brain that make the sensation as thouth the limb

was still there

Referred pain

e.g. patients with heart attack often feel pain of the left arm

= activation of nociceptors in the viscera results in a perception of pain on the body surface

Explanation

- information from nociceptors (both superficial visceral) converges to the same synapse in the spinal cord (spinothalamic tract neurons)

- the brain interprets the information coming from visceral receptors as having arisen from receptors on the body surface - since this is typically where nociceptive stimuli originate

http://www.d.umn.edu/~jfitzake/Lectures/UndergradPharmacy/Sensory

Physiology/Somatosensation/Figures/ReferredPain.jpg

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General somatosensory pathway

3 neurons

1. periphery spinal cord

- cell bodies in

- dorsal root ganglia

- ganglia of sensory cranial nerves

2. spinal cord (brainstem) thalamus

- all secondary neurons cross the midline

- shortly after entering the spinal cord (pain,

temperature, coarse touch)

- in medulla oblongata (fine touch, vibration,

proprioception)

- the sensations are processed in

contralateral hemisphere

- projections to reticular system, cerebellum

(balance, movement, activation of CNS)

3. thalamus postcentral gyrus (parietal

lobe)

Types of afferent nerves transmitting sensory information

• Ab sensory nerves: 30 – 70 m/s (all types of receptors except free nerve endings)

• tactile sensitivity

• Ad: 5 – 30 m/s (free nerve endings)

• pain

• temperature

• C: less than 2m/s (free nerve endings)

• pain

• temperature

slow pathways fast pathways

– rough, less precise sensations – fine, quick, exact sensations

Dermatomes

• A dermatome is an area of skin

that is mainly supplied by a

single spinal nerve

• (partial overlapping with

neighbouring areas)

• Each of these nerves relays

sensation (including pain) from

a particular region of skin to the

brain

• Dermatomes are useful in

neurology for finding the site of

damage to the spine

The sensory homunculus

Examination of cutaneous sensitivity

Tactile sensitivity

Procedure:

• stamp palmar side of 3. finger (fingertip) – use stamp size 1x1cm

• stamp forearm – use stamp size 1x1cm

• the examined person must not look at the examination

• touch subsequently each of 100 squares in stamped area of the skin with Frey hair,

if the examined person feels a touch, he informs the examiner („yes“)

• for each „yes“ the examiner writes a „+“ into the respective field in the manual

Result

• number of fields marked by „+“ = count

of receptors per 1 cm2 on the fingertip

and the forearm

Conclusion

explain: is the observation normal/expected?

+ +

+ + +

+ +

+

Thermal sensitivity – examination of cold receptor count

Procedure:

•stamp palmar side of 3. finger – use stamp size 1,5x1,5 cm (2,25 cm2)

•stamp forearm – use stamp size 1,5x1,5 cm

•touch subsequently each of 100 squares of the stamped area of skin with cold

metal stick, the examined person must not look at the examination

•after each stimulus dip the stick into ice in order to maintain it cold

•if the examined person feels cold (not a touch), he informs the examiner

• for each „yes“ the examiner writes a „+“ into the respective field in the manual

Result

•count of receptors per 1 cm2

(result divided by 2,25) on fingertip

and forearm

Conclusion:

explain: is the observation normal/expected?

+ +

+ + +

+ +

+

Thermal sensitivity – examination of warmth receptor count

Procedure:

• stamp the palmar side of 3. finger – use stamp size 1,5x1,5 cm (2,25 cm2)

• stamp forearm – use stamp size 1,5x1,5 cm

• touch each of 100 squares in stamped area with a warm metal stick

(the examined person must not look at the examination)

• after each stimulus dip the stick into warm water in order to maintain it cold

• if the examined person feels warmth (not a touch), he informs the examiner

• for each „yes“ the examiner writes a „+“ into the respective field in the note book

Result

•count of receptors per 1 cm2

(result divided by 2,25)

on fingertip and forearm

Conclusion:

• explain: is the observation normal/expected

+ +

+ + +

+ +

+

Examination of discrimination sensitivity (2 point discrimination)

(i.e. what is the smallest distance in which 2 tactile stimuli are

recognized as 2 and not as 1 ?)

Procedure:

• touch the skin with both spikes of a compass at one time

• start with small distance of spikes, increase the distance gradually

• measure and record the minimum distance when the examined person can

recognize 2 stimuli

• make this examination on

• fingertip (3rd finger)

• dorsum manus

• palm of the hand

• forearm

• upper arm

Result: distance in mm (cm)

Conclusion: is the observation normal/expected?

Weber's experiment

Procedure:

• prepare 3 containers

• fill them with water – temperature 15,25, 35°C

• put one hand into the container with 15 °C, the other one into

the container with 35 °C for approx 30 s

• put both hands into the middle container with 25 °C water

Result: describe your observation

Conclusion: explain the observation (adaptability...)

35°C 25°C 15°C

1 1 2 2

Procedure

• mark 3 points with a marker

- fingertip

- dorsum manus

- forearm

• patient closes his eyes

• the examiner touches a dot and asks the patient to touch the same place (for all dots)

Result:

- did the patient indicate exactly the same place?

- if not, what was the distance between the marked point and the point shown by the examinee?

Conclusion:

- explain the observation 2

Accuracy of tactile localization

Procedure:

- put a cork on the forearm of the examined person and leave it there

(the patient must not look at the arm)

after putting the cork on the arm the volunteer feels touch of the cork

in a few seconds adaptation occurs - the person does not „feel“ the

presence of the cork any more

- remove the cork quickly

the volunteer is supposed to feel the change – that the cork was removed

- repeat the experiment with a metal object

Result:

- describe your observation

Conclusion

- explain your observation (adaptation .....)

Adaptation of skin mechanoreceptors

Sense of taste

Elementary (primary) tastes /stimuli 1. sour - acids (H+ concentration) 2. salty - ionized salts, mainly Na+

3. sweet - sugars, aldehydes, glycols, some amino acids, etc.

4. bitter - long chain organic substances, alkaloids (e.g. quinine, caffeine, nicotine)

- causes rejection of food – many deadly toxins are bitter

5. umami – taste of foods rich in monosodium glutamate („meaty“ taste, chinese food taste)

6. kokumi

Sense of taste

taste receptors – sensitive to different chemical stimuli

adequate stimulus – chemical substances dissolved in water

hundreds of different tastes can be perceived – all are combinations of

elementary taste sensations

individual receptors generate receptor potential/action potential of varying

intensity – the sum of potentials results in a particular taste senastion

Taste buds

Location

• mainly - in papillae of the tongue

• palate, pharynx, larynx

Composition

• supporting cells

• sensory cells

• their tips are arranged around a taste pore

• microvilli of the tips protrude into the taste

pore = receptor surface

Specificity of the taste buds

- individual taste buds respond mostly

to one of the primary tastes

The complec taste experience depends on stimulation of

- taste receptors

- pain receptors (e.g. pepper)

- touch receptors (texture of the food)

(+ smell of the food)

• taste receptors are partially (slowly) adaptable

– when stimulated a strong immediate signal is transmitted by the taste nerve

– later a weaker continuous signal is transmitted

Threshold for taste

- bitter taste – the most sensitive

- detectable concentrations

Bitter (quinin) 0,000008 mole

Sour (HCl) 0,0009 mole

Salty (NaCl) 0,01 mole

Sweet (sucrose) 0,01 mole

http://www-psych.stanford.edu/~lera/psych115s/notes/lecture11/images/tstoverview.jpg

Hippocampus

- memory of taste

Amygdala, hypothalamus

– emotional quality of taste

Somatosensory cortex, frontal cortex –

conscious perception of tase

Taste buds

Thalamus

Med.oblongata

Gustatory pathway

Detection of gustatory sensation

Procedure A

• pour a sample of sweet, sour, salty and bitter solution

into separate glass containers

use a set of containers with solutions of different

concentration of

a/ saccharose

b/ citric acid

- dip a stick into the container and touch the tongue

of the examinee

- find the threshold concentration (the examinee

can detect the taste) – for both tastes

(record the number of the container)

Result and conclusion

compare the sensitivity of your examinee to both tastes

Procedure B

• test the threshold

• dip a stick with cotton swab in one solution (the patient

must not know the taste !!)

• touch different regions of tongue with the stick

• at the end of examination the patient has to determine

– what taste he felt (sweet, salty.....)

– in which part of tongue he felt the taste maximally

• repeat the procedure for all elementary tastes

(use the bitter taste at the end)

• after testing of each taste, the examinee has to wash

his mouth with water

Result and conclusion

describe your observations

Sense of olfaction

Sense of olfaction (smell)

• adequate stimulus – chemical substances present in air

• human – microsmate – less sensitive sense of olfaction

• receptors in superior part of each nostril (2,4 cm2) – regio olfactoria

• sensory cells (olfactory cells) = bipolar nerve cells

• project olfactory cilia into mucus that covers mucosa of the nasal cavity (secreted

by Bowmans glands)

• cilia capture the chemical

stimuli (odours) – the substance

must be dissolved in mucus

• Henning´s theory - six elementary odours :

– flowers

– fruit

– resin

– spices

– putrid

– burns

• i.e. smell receptors are specialized to indiviual odours

• all other odours are a combination of elementary odours

• there are also othes classifications : – camphoraceous, musky, floral, pepperminty, ethereal, pungent, putrid

– 50 or even 100 elementary odours

• threshold concentrations – very low

• poor discrimination of intensites of stimuli (gradations)

• adaptation of receptors: very fast in 1st second, then proceeds only slowly

• affective component od odours (pleasant, unpleasant)

Olfactory pathways

Detection of olfactory sensations

Principle

• by sniffing the solutions of olfactory substances we examine the

ability to distinguish primary odours and determine their

thresholds

Procedure

A. by sniffing get familiar with the 4 available primary odours – flowers,

fruit, resin, spices (use bottles 1)

B. test the threshold for odours

• from a set of 10 bottles with increasing concentration select the least concentrated

solution (bottle 10)

• remove the stopper from one of the bottles and hold it about 4 cm under your partner's

nose about 2 seconds

• ask your partner to identify the odor, and then replace the stoppe

• record the number of bottle in which the examinee can recognize

the odour = threshold

Result: record the threshold values

Conclusion: compare the sensitivity to different odours

C. Test the memory for odours

• let the examinee sniff to an „unknown“ odour

• the examinee has to determine the odour

Result and conclusion: describe your observation