Somatosensory system Flashcards by Joanna Mitchell

What does the somatosensory system mediate?

Fine discriminatory touch (light touch, pressure, vibration, flutter) 
Stretch (mechanosensation) 
Proprioception 
Thermosensation 
Nociception (pain) 
Pruriception

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What are the 3 broad categories of the somatosensory system?

Exteroceptive division
Proprioceptive
Enteroceptive

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What is the exteroceptive division?

Registers information from the surface of the body by numerous receptor types

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What is the propceptive division?

Monitors posture and movement with sensors in the muscles, tendons and joints

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What is the enteroceptive division?

Reports upon the internal state of the body and is closely related to autonomic function

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What are the neurones involved in the somatosensory pathway?

1st order (PNS): cell body location in the dorsal root ganglion or cranial ganglion 
2nd order (CNS): cell body location in dorsal horn of spinal cord or brainstem nuclei 
3rd order (CNS): cell body in thalamic nuclei

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What will stimulus (mechanical, thermal or chemical) do to ion channels?

Opens cation selective ion channels in the peripheral terminal of primary sensory afferents eliciting a depolarizing receptor potential

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What determines the amplitude of the receptor potential

Stimulus intensity

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What are important properties of sensory units?

Modality - what type of stimulus excites the sensory receptor
Threshold - what intensity of the stimulus is required for excitation of the sensory receptor
Adaptation rate - does the sensory unit discharge action potentials continuously during the stimulus or does it respond preferentially to a changing stimulus
Conduction velocity - how rapidly does the sensory unit conduct action potentials along its axon
Site and extent of peripheral termination

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What is the sensory unit for touch, pressure and vibration modality?

Skin mechanoreceptors

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What is the sensory unit for proprioception modality?

Joint and muscle mechanoreceptord

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What is the sensory unit for temperature modality?

Cold and warm thermoreceptors

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What is the sensory unit for pain modality?

Mechanical, thermal and polymodal nociceptors

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What is the sensory unit for itch modality?

Itch receptors

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What does threshold relate to?

The intensity of a stimulus required to excite a sensory unit

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What is a low threshold unir?

Responds to low intensity (non-damaging) stimuli

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What is a low threshold mechanoreceptor?

Mediates fine discriminatory touch

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What is a low threshold thermoreceptor?

Mediates cold through to hot

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What is an example of a high threshold unit?

Nociceptors; responds to high (noxious) but not low intensity stimuli

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What is a high threshold mechanoreceptor?

Mechanical nociceptors that respond to high intensity mechanical stimuli

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What is a thermal nociceptor?

Responds to extreme degrees of heat ; >45 or <10

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What do chemical nociceptors respond to?

Substances in tissue; 
Inflammation 
Prostaglandins
Bradykinin
Serotonin
Histamine
K+ 
H+ ATP

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What will polymodal nociceptors respond to?

At least 2 of:
High threshold mechanoreceptors
Thermal nociceptors
Chemical nocicpeotrs

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What is adaptation in the somatosensory context?

Feature of sensory units that determines whether they change their firing rate only in response to a stimulus of changing intensity, or fire continuously throughout a constant stimulus

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What does a slowly adapting/ tonic response convey?

Continuous information to CNS while terminal deformed | Provides information about position, degree of stretch or force e.g. stretch receptors

What does a fast adapting/ dynamic response convey?

Detects changes in stimulus strength (rate of movement) Number of impulses is proportional to the rate change of stimulus Includes muscle spindles, hair follicle afferents

What does a very fast adapting/ very dynamic response convey?

Responds only to very fast movement, such as rapid vibration E.g. pacinian corpuscle

What differentiates the different types of primary sensory afferent fibres?

Axon diameter Extent of myelination Conduction velocity Assoc sensory receptor

What are the different types of primary sensory afferent fibres?

Alpha - group 1 Beta - group 2 Delta - group 3 C - group 4

Describe an alpha fibre?

Thickest diameter Thick myelination Fastest conduction velocity at 80-120 m/s Sensory receptor for proprioceptors of skeletal muscle

Describe a beta fibre

Medium diameter Moderate myelination Conduction velocity of 35-75 m/s Sensory receptor for mechanoreceptors of skin

Describe a delta fibre

Small diameter Thin myelination Conduction velocity of 5-20 m/s Sensory receptor for pain and temp

Describe a C fibre

Smallest diameter No myelination Conduction velocity of 0/5-2.0 Sensory receptor for temp, pain and itch

What is the receptive field?

Target territory from which a sensory unit can be excited

What is sensory acuity?

Fineness of discrimination

How does sensory acuity relate with RF size?

Correlates inversely

What is two point discrimination ?

An important measure of somatosensory function

How is two point discrimination clinically tested?

Applying simultaneously two sharp point stimuli, separated by a variable distance at different sites on the body surface

What is the relationship between 2 point threshold and the diameter of corresponding RF?

Regions with the highest discriminative capacity have the smallest RFs

What are the different types of cutaneous receptors?

``` Free nerve endings Meissner's corpuscle Merkel's disc Hair end organs Krause end bulbs Ruffini endings Pacinian corpuscles ```

Where can free nerve endings be found and what is the function?

Ubiquitous distribution | Pain, heat, cold

Where can meissner's corpuscles be found and what is the function?

Abundant in skin locations where two point discrimination is highest, not present in hairy skin Touch

Where can merkel's discs be found and what is the function?

Same in meissner's corpuscle but present in moderate numbers in hairy skin Touch

Where can krause end bulbs be found and what is the function?

At border of dry skin and mucous membranes | Touch

Where can ruffini endings be found and what is the function?

Within dermis and joint capsules | Pressure

Where can pacinian corpuscles be found and what is the function?

Within dermins and fascia | Pressure

What are the subdivisions of skin LTM?

Rate of adaptation: Fast or slow | Size of receptive field: small (type 1) or wide (type 2)

What are the receptors and parent fibres of free nerve endings?

Parent nerve fibre: delta of C | No receptor

What are the receptors and parent fibres of merkel discs?

Parent fibre: beta | Slow adapting type 1 unit receptor

What are the receptors and the parent fibre type of meissner's corpuscles?

Parent fibre: beta | Fast adapting type 1 unit

What are the receptors and the parent fibre type of ruffini endings?

Parent fibre: beta | Slow adapting type 2 unit

What are the receptors and the parent fibre type of pacinian corpuscles?

Parent fibre: betea | Fast adapting type 2 unit

What is the maximal vibration of human detection?

150 Hz; this is why a 128 tuning fork is utilised

What fibre class and termination in the spinal cord does nociceptors utilise?

Fibre: delta/C Termination: laminae 1 and 2

What fibre class and termination in the spinal cord does LTMs utilise?

Fibre: beta Termination: laminae 3-6

What fibre class and termination in the spinal cord does proprioceptors utilise?

Fibre: alpha Termination: laminae 7-9

What is the laminae of rexed?

Subdivisions of grey matter

What sensory information does the dorsal column medial lemniscal pathway process?

Discriminatory touch Pressure Vibration Proprioception

Where will the dorsal column fibres decussate?

Medulla at the great sensory decussation to then ascend in the medial lemniscus to the ventral posterior lateral nucleus of the thalamus

What sensory information does the spinothalamic tract process?

Pain, thermosensation, crude touch, itch, tickle

What is the difference between the medial gracile tract and the lateral cuneate tract of the dorsal column?

Gracile nuclei: lower limbs and lower trunk - below T6 | Cuneate tract: upper limbs and upper trunk - above T6

How will the neurones of the DCML pathway reach the primary somatosensory cortex of the post central gyrus from the thalamus?

Posterior internal capsule

What is stereognosis?

Ability to recognise an object by feeling it

What is contrast enhancement?

As information is conveyed from one neurone to the next in a sensory pathway, differences in the activity of adjacent neurones are amplified producing a contrast enhancement

What is lateral inhibition?

When one neurone is active, it will inhibit the activity of its neighbours via inhibitory interneurons

What will the trigeminothalamic pathway process?

General somatic information from the anterior head, oral and nasal cavities, sinuses, intracranial dura and cerebral arteries

Where are the soma of sensory neurones of the trigeminal nerve found?

Trigeminal sensory ganglion

What will synapse in the spinal nucleus?

Pain and temp from face

How will information from the trigeminal nuclei reach the thalamus?

Via trigeminal lemniscus to the ventroposteriomedial nucleus

How will information from the VPM nucleus of the thalamus reach the cortex?

Via thalamocortical neurones

Which brodmann areas is the somatosensory cortex in the parietal lobe associated with?

1,2,3a and 3b

Describe the sensory homunculus

Toes at top Tongue at bottom Hand separates the head from the face

How many layers are there to the somatosensory cortex?

6 cell layers

Which layer of the somatosensory cortex receives the majority of neurones from the thalamus?

What are the layers of the somatosensory cortex?

``` 1 = molecular 2 = external granular 3 = external pyramidal 4 = internal granular 5 = internal pyramidal 6 = multiform ```

What does the posterior parietal cortex receive?

Information from the somatosensory cortex and other cortical areas such as visual and auditory and subcortical areas such as the thalamus

What does the posterior parietal cortex do?

Deciphers the deeper meaning of information from the somatosensory cortex

What can damage to the posterior parietal cortex result in?

Bizarre neurological disorders; agnosia, astereognosis, hemispatial neglect syndrome

What is hemispatial neglect syndrome?

Damage to right parietal cortex | Patients believe that the left side of the world does not exist

Somatosensory system Flashcards

(79 cards)