Touch Flashcards
Merkel disc density in fingertips:
70 per cmsq.
Meissner’s corpuscles density in fingertips:
150 per cmsq.
Pacinian corpuscles density in fingertips:
20-70 per cmsq.
Ruffini endings density in fingertips:
1-2 per cmsq.
Why are Pacinian corpuscles and Ruffini endings large?
Allows them to be more sensitive to mechanical displacement.
RA receptors detect:
Motion and vibration.
(RA) Meissner’s corpuscles tuned to what frequencies?
30-50Hz.
(RA) Pacinian corpuscles tuned to what frequency?
<500Hz.
Meissner’s corpuscle structure:
Stack of flattened, fluid-filled cells ecapsulating RA fibre.
Pacinian corpuscle structure:
Concentric, fluid-filled connective tissue lamellae encapsulating RA fibre.
How do Meissner’s and Pacinian corpuscles do rapid adaptation?
Fibres inside linked to stretch-sensitive ion channels which response to rapid deformation of capsule, firing at the start and end of stimulation.
SA receptors detect:
Object pressure and form.
Merkel’s discs sensitive to what? Makes them well suited for…
Sustained pressure and indentation. Picking up fine details such as reading braille.
Merkel discs’ structure:
specialised, disc-shaped cells closely associated w/ epidermal later of skin.
Ruffini endings sensitive to:
Stimulation that stretches skin along its long axis.
Ruffini endings structure:
Elongated fusiform structures that enclose collagen fibrils extending from the subcutaneous tissue to folds in the skin at joints.
What may Ruffini endings play a role in.
Stereognosis and proprioception.
Complementary roles, SA receptors provide … for … , while RA receptors alert the system to ….
A steady baseline for monitoring constant stimuli, alert the system to changes or new inputs.
In terms of complementary roles, when holding an object what are the roles for both RA and SA receptors?
SA: continuously assess grip strength and texture. RA: detect slippage or adjustments.
Why is parallel processing via diff. receptors useful?
Allows brain to process many object features at once, assisting speed of identification.
Describe the DCMLS.
Sensory neurons terminate in DC nuclei. 2nd order neurons cross the midline in the medulla synapsing in the gracile (lower) or cuneate (upper) nucleus, then ascend contralaterally to thalamus, terminate in ventral posterolateral.
Where is S1? What does it consist of?
Parietal lobe. Postcentral gyrus and posterior paracentral lobule.
Why is contralateral projection important>
Ensures tactile sensations processed in conjunction with movements controlled by the opposite hemisphere.
What shows that crude touch exists, and ascends via the anterolateral system?
Advanced syphillis, hardening of dorsal columns, touch but no localisation.
What shows that there is a role for touch in movement?
Somatosensory losses accompanied by motor deficits: clumsy movements, dystonia.
Mountcastle discovered vertical columns in S1 …. wide, spanning all 6 layers.
300-600um.
Neurons within columns receive inputs from same what?
Locality and class of touch receptor.
Columns provide what?
Anatomical structure that organisies sensory inputs to convey information about location and modality.
Columns within S1 arranged topographically such that…
Complete representation of each body part in each area of S1.
Cortical map corresponds roughly to…
Spinal dermatomes.
What body parts overrepresented in homunculus?
Hands, feet, tongue. (in humands and monkeys more cortical colmns devoted to fingers than entire trunk).
Cortical magnification. Closely correlated with …
The innervation density and thus the spatial acuity of the touch receptors in an area of skin.
How may S1 topographical organisation lead to phantom limbs?
Hand area is very close to the face.
Central pathways integrate responses of thousands of neurons through … , this allows what?
Neural convergence. This allows representation of global features of objects.
Example of increase in RF size up the pathway:
MD and MC RF tiny spots on fingertip, those of cortical neurons the entire fingertip.
Why are RFs progessively larger than in areas 3a and b in areas 1, 2 and 5?
3a and b input. Convergent inputs from neurons in area 3b that are stimulated together when hand is used. Area 5 has bilateral RFs, respond to touch at mirror image locations on both hands.
Neurons in areas 1 and 2 have Rfs that include …, and thus fire at higher rates when…, as such signalling the …, explaining why area 2 lesion leads to…
More than one finger, and thus fire at higher rates when several fingers touch simultaneously. As such signal the size and shape of objects held in hand. Explaining why area 2 lesion leads to deficit in size and shape differentiation.
Large RFs enable…
Integration of fragmented info from touch receptors.
What evidence highlights how active touch is different to passive?
Steinmetz et al (2000) used multiple electrode arrays to find that most neuron pairs in SII cortex fired synchronously and that the degree of synchrony was affected by the monkey’s attentional state.
Experimental ablation of S1 areas in monkeys. Lesions to 3b lead to…
Deficits in touch from particular body part.
Experimental ablation of S1 areas in monkeys. Lesions to 1 lead to…
Deficits in texture assesment.
Experimental ablation of S1 areas in monkeys. Lesions to 2 lead to…
Deficit in size and shape differentiation.
How do sensory receptors do lateral inhibition to increase resolution?
Neuron stimulated most sends out collaterals which contact GABAergic interneurons that inhibits neurons either side of it.
Somatotopic mapping: individual digits can be discriminated (X), and even along the phalanges of a single finger exhibit topographical distinction (X).
Kolasinski et al (2016).
Sanchez-Panchuelo et al (2012).
Lesions to S1 can cause:
Agraphesthesia (loss of vibration).
Hemisypesthesia (loss of fine touch).
What fibres may be involved in thermoception?
Adelta fibres.
