Somatosensory physiology Flashcards
What are the 4 main classes of receptors in the somatosensory system?
1) Tactile (innocuous - not painful)
2) Nociceptive
3) Proprioceptive
4) Thermal
What are the 4 types of innocuous tactile receptors?
1) Merkel
2) Ruffini
3) Meissener
4) Pacinian
What are the 3 types of proprioceptive receptors?
1) Muscle (spindle) - in muscles
2) Golgi tendon receptors - in joints
3) Some inputs from cutaneous mechanoreceptors
How are thermal sensations mediated?
Mediated by thermoreceptors localized to discrete zones that exhibit hot and cold sensitivity
By what 3 receptors are nociceptive (painful) sensations mediated?
1) Mechanical nociceptors
2) Thermal nociceptors
3) Polymodal nociceptors
What are the 5 types of sensory receptors in the skin, where is each located?
1) Meissener’s corpuscle - Just beneath epidermis (between papillae)
2) Merkel’s discs - Just beneath the epidermis (aligned with papillae)
3) Pacinian corpuscles - in the subcutaneous tissue
4) Ruffini’s corpuscles - deep in the dermis
What is the morphology of Meissener’s corpuscles?
Looping axon terminals that intertwine supporting cells
What is the morphology of Merkel discs?
Dome structure atop axon terminals
What is the morphology of pacinian corpuscles?
Sensory axon surrounding by fluid filled capsule, onion shaped appearance
What is the morphology of Ruffini endings?
Nerve terminals intertwined with collagen fibrils
What is the morphology of nociceptors?
Free nerve endings that penetrate epithelial cells, no morphological specialization
Which type of cutaneous sensory receptor has the smallest receptive field?
Merkel discs
Which type of sensory receptor has the largest receptive field?
Pacinian corpuscles
Which cutaneous sensory receptors are rapidly adapting?
Meissener’s corpuscles and pacinian corpuscles
Which cutaneous sensory receptors are slow adapting?
Merkel’s discs and Ruffini endings
Which 2 types of cutaneous sensory receptors have a small receptive field?
Meisseners and merkels discs
Which 2 types of cutaneous sensory receptors have a large receptive field?
Pacinian corpuscles and Ruffini endings
What is the role of Merkel cells in perception?
Detect pressure, form and texture
What is the role of Meissener’s corpuscles in perception?
Detect flutter and motion
What is the role of Ruffini corpuscles in perception?
Detect skin stretch
What is the role of Pacinian corpuscles in perception?
Detect vibration (250-350 Hz)
Which type of cutaneous sensory receptor is involved in reading braille?
Merkel afferents
In terms of 2 point discriminatory thresholds across the body, what is sensitivity correlated with?
Density of sensory innervation
What are the 2 areas of high sensitivity (small 2 point discriminatory threshold)?
1) Fingertips
2) face
What are the 2 areas of low sensitivity (large 2 point discriminatory threshold)?
1) Torso
2) limbs
What are the 4 ‘types’ of temperatures that different thermoreceptors respond to?
1) Cold pain
2) Cold
3) Warm
4) Hot pain
Temperatures ranging from noxious heat to noxious cold activate several members of which family of receptors?
The Trp family of receptors
Which receptor does menthol activate?
Trpm8
Which receptor does capsaicin activate?
Trpv1
How does capsaicin produce the pungent burning sensation in the mouth?
Belongs to a group of chemicals known as vanilloids Acts on thermal receptors (class of nociceptor) VR1 is expressed in cultured cells Capsaicin causes an inward Ca2+ current which is dose dependent according to 'hotness of the pepper' - this receptor is also activated by heat
What proprioceptor is found in muscles, what does it do?
Muscle spindles
Provide sensory feedback from muscle fibres on body position and movement
What proprioceptor is found in joints?
Golgi tendon organ
Regulate muscle tension or force of contracting and prevent muscle overloading
Conduction velocity of a nerve fibre is positively correlated with what?
Axon diameter
Large diameter, rapidly conducting afferents are associated with which type of somatosensory receptor?
Low threshold mechanoreceptors
Small diameter, slow conducting afferents are associated with what 2 types of somatosensory receptor?
1) Nociceptors
2) Thermoreceptors
How can the Herpes Zoster Virus enable us to visualise dermatomes?
Virus lurks in primary sensory neurones then can reactivate as shingles - get inflammation and blistering on skin of area of affected dorsal root/sensory neuron
Innocuous sensitivities are carried through the CNS by which pathway?
Dorsal column pathway
Noxious and thermal sensitivities are carried through the CNS by what pathway?
Spinothalamic pathway
What are Brodmann areas of the brain?
47 areas within the cerebral cortex which subserve different physiological functions
Where does the secondary somatosensory cortex lie?
Adjacent to the primary somatosensory cortex (S1) along the lateral sulcus
What is the function of the secondary somatosensory cortex?
Plays a key role in sensory and motor integration
Receives corpus callosum inputs to form a ‘joined-up’ body image
Build info from multiple body areas and form a body image
Where are the association areas localized to, what is there function?
Localised to the posterior parietal cortex
Play a role in integration of multimodal senses
Damage to the posterior parietal cortex (ie the association areas) leads to what 2 unusual neurological disorders?
1) Astereoagnosia - the inability to identify objects on basis of touch alone - would only be able to confirm what it was by using other sense eg. vision
2) Neglect syndrome - body part or visual field is disregarded
After passing through the ventrobasal complex in the thalamus and to the post central gyrus what areas does somatosensory info then pass to?
Passes to the posterior parietal (secondary association area) then from there to:
1) Motor system
2) Polysensory association areas
3) Limbic system (learning and memory)
