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Sensation of touch - Coggle Diagram
Sensation of touch
Receptor types
Meissner's corpuscle
- superficial - collagen links to BM of overlying epidermis
- rapid adaptation, light touch mechano-R
- highly sensitive, low threshold: 10-25 corpuscles/DRG; high density in fingertips
Merkel's disc
- superifical - attachment to basal epidermis
- skin deformation and contact(form/texture) detection
- slow adapting; 10-25/DRG
EE: Maririch et al, 2009 - Atoh KO mice
- Atoh = TF: KO leads to absence of merkel's disc
- Skin/nerve preparations - loss of light-touch response
Pacinian corpuscle
- LARGE receptive field - within dermis; 1/DRG
- rapid adaptation, vibration and pressure sensitive
- concentric lamellae of Schwann cells around single corpuscle - increased sensitivity
EE: Lowenstein et al, 1959 - Lamellae importance
- lamellae removal doesnt affect AP generation
- partial degeneration of nerve endings prevent pd generation
EE: Lowenstein et al, 1965 - cont.
- Corpuscle lacking lamellae slower adapting
- Dissipates applied pressure: key for reset and rapid apadtation
Ruffini endings
- located within dermis; 1/DRG cell
- respond to tissue folding and tension
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Transmission
Myelinated A beta fibres
- conduction v: 35-75m/s
Unmyelinated C
- conduction v: 0.5-2m/s; convey dull, burning pain
Pathway from dermatomes to spinal nerve roots
- 1' sensory neuron enters via dorsal root
- each root has specific dermatome; often overlap with neighbouring dermatomes
CC: Shingles
- illustrates existence of specific dermatomes
- Varicella zoster virus infects ALL nerves of one spinal root
- corresponding skin = inflamed; map dermatomes
A beta fibre within spinal cord/brainstem
- some(mainly proprio) remain and synapse with interneurons - motor neuron interaction - adjust muscle tone
- rest(proprio and sensation) - no synapse - project directly up; feed to brain for fast transduction
Either med. gracile fascicle or lat. cuneate fascicle in post. white matter
- MGF: T7-S5 - mostly lower limbs
- LCF: C1-T6 - upper limbs
- axon termination in respective nuclei @ junction of spinal cord and medulla --> decussation
CC: Brown Sequard syndrome
- Hemisection of spinal cord; lesion of BOTH somatosensory systems
- one/both dorsal column: ipsilateral loss of vibration, proprio and fine touch
- spinothalamic tract: pain and temp lost from contralateral side 1-2 segments below lesion --> fibres ducussate @ level of entry
proprio afferents from leg ascend to level T
- synapse @ Clarke's column
- relay onto ipsilateral cerebellum via spinocerebellar tract
Thalamus: modulation of sensory information
- White matter tract called Medial lemniscus
- passes through pons, medulla and midbrain
- terminates in lat. ventroposterior nucleus of thalamus
Information relayed and modulated
- lat. inhibition in dorsal column and thalamic nuclei --> contrat enhancement
Desc. control pathways modify signal
- 50% fibers project downwards from somatosensory cortex
Information from head/neck via trigeminal nerve
- enters medial ventroposterior nucleus
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