Lecture 9: Somatosensory system: Discriminative (fine) touch Flashcards
What are meissner’s corpuscles?
Superficial mechanoreceptors found only at the epidermal-dermal boarder in glabrous skin. A capsule full of Schwann cells with nerve ending between fibres. It detects stroking, changes in texture (around 50Hz) They have a SMALL receptive field with RAPID adaption
What are merkel’s disk cells?
Superficial mechanoreceptors found in both hairy and glabrous skin. It detects sustained light touch and pressure. The have a SMALL receptive field and are SLOWLY adapting. consist of a separate cell for mechanosensation with afferent fibre.
What are pacinian corpuscles?
Deep mechanoreceptors found in both hairy and glabrous skin, detecting vibration (200-300 Hz). They have a LARGE receptive field and are RAPIDLY adapting.
What are ruffini’s endings?
Deep mechanoreceptors found in hairy and glabrous skin that detect pressure and stretch deep within the skin. They have a LARGE receptive field and are SLOWLY adapting.
What are hair follicle receptors?
Wraps around hair follice
What are free nerve endings?
superficial mechanoreceptors found only in hairy skin that respond to touch, pain, stretch, itch, pressure and temperature.
How is the dorsal horn of the spinal cord organised? Which fibres synapse where?
A-delta fibres synapse in laye I of the dorsal horn, C-fibres synapse in layer II and A-beta fibres (mechanoreceptors) synapses in layers III - V
Describe the structure and function of the 5 different types of mechanoreceptors.
Pacinian corpusle: It is oval shaped, and encapsulated. It consists of 20-60 concentric layers of epithelial cells, with fluid spaces between layers. In the centre of the corpuscle is the inner core, a fluid-filled cavity with a single unmyelinated nerve fibre. Applied forces are transmitted through the interior of the capsule reaching the nerve fibre, but are not maintained due to the elastic properties of the capsular layers. Thus they are “rapidly adapting”, and respond to changing stimuli, particularly high frequency stimulation of 300Hz. They are also extremely sensitive being able to respond to skin indentations of as little as of 1μm. Activation of Pacinian corpuscle’s are perceived by us as vibrations, such as that caused by a loud speaker or pneumatic drill.Ruffini’s ending:It is a thin cigar shaped corpuscle traversed by longitudinally strands of connective tissue. A sensory fibre enters and branches, so that many small processes are interspersed along the collagenous strands. Since collagen is not very elastic, deformation is maintained as long as applied force is maintained. Thus the Ruffini’s ending is “slowly adapting”, and responds to ongoing pressure and stretch, deep in the skin, such as when gripping a tennis racket tightly.Meissner’s corpuscle: It is oval shaped with a connective tissue capsule, with stacked layer of Schwann cells inside. Myelinated nerve fibres enter at the base, and lose their myelin. The nerve fibres branch further, with single fibres between the Schwann cells stacks. Applied forces are transmitted through the interior of the capsule reaching the nerve fibres between the Schwann cells. The insulating properties of the Schwann cells mean that activation of the mechanosensitive ion channels in the nerve fibres are not maintained if the applied force is ongoing. Thus it is “rapidly adapting”, responding to changing stimuli around 50Hz and being sensitive to skin indentations as small as 50 microns. Activation of Meissner’s corpuscles are perceived by us as stroking, changes in texture, and flutter.Merkel ending/disc: It is non-encapsulated and consists of a nerve terminal and a flattened non-neural epithelial cell (Merkel cell). The Merkel cell is the mechanically sensitive part, making a synapse like junction with the nerve terminal. Applied pressure leads to ongoing activation of the Merkel cell, due to the lack of encapsulation, thus it is “slowly adapting”, and is perceived by us as sustained light touch and pressure. One example of this is during braille reading, where the Merkel endings are responsible for encoding the braille dots with high fidelityFree nerve ending: They are non-encapsulated bare nerve terminals, resembling plant roots. Free nerve endings contain different complements of ion channels and thus are sensitive to different stimuli, such as touch if mechanosensitive ion channels are present or pain/temperature if nociceptive or thermoceptive ion channels are present. Mechanosensitive and nociceptive free nerve endings are constantly activated by ongoing stimuli and are “slowly adapting”, whilst thermoceptors are “rapidly adapting”.Hair-follicle receptors: Free nerve endings that wrap around the base of a each hair. They detect movement of the hair, and are “rapidly adapting”, being activated only by stimulus onset/offset
Describe the different types of primary afferent fibres and where they synapse in the dorsal horn
A-alpha fibres
* Proprioception synapse on second order neurons in layer 3-5
A-beta fibres
* Mechanoreceptors of the skin synapse on second order neurons in layer 3-5
A-delta fibres
* Pain and temperature synapse on second order neurons in layer 1-5
C fibres
* Temperature, pain and itch synapse on second order neurons in layer 1-2
Describe the dorsal column - medial lemniscus pathway
