Somatosensory system Flashcards
What does the somatosensory system mediate?
Fine discriminatory touch (light touch, pressure, vibration, flutter) Stretch (mechanosensation) Proprioception Thermosensation Nociception (pain) Pruriception
What are the 3 broad categories of the somatosensory system?
Exteroceptive division
Proprioceptive
Enteroceptive
What is the exteroceptive division?
Registers information from the surface of the body by numerous receptor types
What is the propceptive division?
Monitors posture and movement with sensors in the muscles, tendons and joints
What is the enteroceptive division?
Reports upon the internal state of the body and is closely related to autonomic function
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
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
What determines the amplitude of the receptor potential
Stimulus intensity
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
What is the sensory unit for touch, pressure and vibration modality?
Skin mechanoreceptors
What is the sensory unit for proprioception modality?
Joint and muscle mechanoreceptord
What is the sensory unit for temperature modality?
Cold and warm thermoreceptors
What is the sensory unit for pain modality?
Mechanical, thermal and polymodal nociceptors
What is the sensory unit for itch modality?
Itch receptors
What does threshold relate to?
The intensity of a stimulus required to excite a sensory unit
What is a low threshold unir?
Responds to low intensity (non-damaging) stimuli
What is a low threshold mechanoreceptor?
Mediates fine discriminatory touch
What is a low threshold thermoreceptor?
Mediates cold through to hot
What is an example of a high threshold unit?
Nociceptors; responds to high (noxious) but not low intensity stimuli
What is a high threshold mechanoreceptor?
Mechanical nociceptors that respond to high intensity mechanical stimuli
What is a thermal nociceptor?
Responds to extreme degrees of heat ; >45 or <10
What do chemical nociceptors respond to?
Substances in tissue; Inflammation Prostaglandins Bradykinin Serotonin Histamine K+ H+ ATP
What will polymodal nociceptors respond to?
At least 2 of:
High threshold mechanoreceptors
Thermal nociceptors
Chemical nocicpeotrs
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
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?
4
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
