somatosensory system Flashcards
somatosensory system overview
provides crucial line of communication between us and the outside world, unusual sensory system as receptors are distributed throughout the body including skin, musculoskeletal system and internal organs, provides wide variety of information, main receptors; tactile (innocuous), nociceptive, proprioceptive, thermal, damage to peripheral or central components results in conditions such as neuropathy, neuralgia, phantom pain
mechanotransduction
pressure sensitive sodium channels - stimulus=channel opening, depolarisation if large enough stimulus, transduction mechanisms largely unknown - stretch activated; forces in the lipid bilayer change, alterations in the tension or curvature, tethered; channels are linked to ECM or cytoskeletal proteins, tension in linkages controls gating, direct or indirect channel domains, indirect gated; channels are coupled to mechanically sensitive signalling intermediates, intrinsically slower, examples of candidate channel types; DEG/ENaC, TRPN and TRPA, TRPV
peripheral innervation
skin and deeper structures (viscera or muscles) innervated by vast network of peripheral nerves, axons carrying information from the somatic receptors are referred to as primary/sensory afferents, primary afferents enter the spinal cord via a spinal nerve and the Doral root, axons carrying information from spinal cord to periphery referred to as motor efferents, efferents leave the spinal cord via a ventral root and spinal nerve
dermatomal map of human body
dermatome - area of skin mainly supplied by afferent nerve fibres from a single dorsal root of the spinal nerve, direct correspondence between skin dermatome (numbering) and cervical, lumbar, thoracic and sacral segments of spinal cord (dermatome not the same as receptive field of sensory receptor
dermatomes
boundaries not absolute, overlapping between adjacent dermatomes
receptive fields
area of skin where stimulus evokes sensory receptor activation, in order to be effective the stimulus needs to lie within area of skin innervated by the neurone
two point discrimination thresholds
varies across body surface, sensitivity is correlated with density of sensory innervation, areas of high sensitivity = fingertips and face, areas of low sensitivity = torso and limbs
classes of somatosensory receptors
tactile (innocuous touch sensations - mediated by low threshold mechanoreceptors, Merkel, Ruffini, Meissner, and Pacinian types, proprioception - mediated by muscle (spindle) and joint (Golgi tendon) receptors, some inputs from cutaneous mechanoreceptors, thermal receptors - mediated by thermoreceptors localised to discrete zones that exhibit hot and cold sensitivity, free nerve endings, nociceptive sensations - mediated by mechanical, thermal and polymodal nociceptors, free nerve endings
morphology of cutaneous receptors
Meissner’s corpuscles - comprised of looping axonal terminals and intertwined supporting cells. low frequency vibrations, Merkel’s discs - dome structure comprised of axon terminals and Merkel cells, small forms and shapes, Pacinian corpuscles - sensory axon surrounded by fluid filled capsule, gives it an onion-shaped appearance, high frequency vibrations, Ruffini’s corpuscles - nerve terminals intertwined with collagen fibrils (20%), pressure, free nerve endings - penetrate into epithelial cells, no apparent morphological specialisation, noxious (harmful)
human hand receptive field map
variable in size, discrete zones characteristic of Meissner’s (RA) and Merkel’s (SA1), broad regions more characteristic of Pacinian (RA) and Ruffini (SA2)
cutaneous mechanoreceptors
selective activation of Merkel cell and Ruffini endings causes sensation of steady pressure, selective activation of Meissner and Pacinian corpuscles causes tingling and vibratory sensation, Meissner and Pacinian corpuscles adapt rapidly (respond quickly and adapt - stop firing)
rapidly adapting cutaneous mechanoreceptors
Pacinian - localised to deep dermis, detects high frequency vibrations of 250-350 Hz, tuned to fine textures and movement, Meissner - found just below epidermis, detects low frequency 30-50 Hz flutter, rough textures and movement, rapid adaptors important for sensory feedback during motor tasks e.g. grip
slowly adapting cutaneous mechanoreceptors
Ruffini’s - found in deep dermis, detects stretch, indentation or lateral movement of skin e.g. on limb movement, Merkel’s - found at epidermal/dermal boarder, detects form, shapes, rough edges, boarders, texture (most important for brail reading
hair follicles
class of mechanoreceptors, can be slow or fast adapting, located in dermis and epidermis, detect movement across skin or brushing
somatosensory resolution
high spatial resolution - <0.5 mm and high temporal resolution - 100 characters par minute
thermoreceptors
different sensory fibres respond to different temperatures (cold pain, cold, hot, hot pain), temperature ranging from noxious heat to noxious cold activate several members of the Top family of receptors, menthol activates Trpm8 and capsaicin activates Trpv1
capsaicin and the VR1 receptor
capsaicin is an extract of chilli peppers, belongs to group of chemicals know as vanilloids, causes pungent burning sensation in the mouth, act on thermal receptors (class of noiceceptors), recently cloned the vanilloid receptor VR1, VR1 expressed in cultured cells, capsaicin produces inward calcium ion current, dose dependent relating to perceived hotness of pepper, also activated by heat
afferent termination patterns
thick afferents terminate in deeper laminae, thin and unmyelinated afferent terminate in superficial laminae, every afferent generates a glutaminergic output so all the afferents terminating in the spinal cord are excitatory, inhibition comes from interneurones in spinal cord,
afferents and the laminae they terminate
A alpha fibres - laminae VI-IX (muscle afferents), A beta fibres - laminae III-VI (skin mechanoreceptors), A delta fibres - laminae I & II (pain and temperature), C fibres - laminae I & II (pain, itch and temperature)
dorsal column-medial lemniscal pathway
processes innocuous inputs from receptors in skin, muscles and joints, two components - fasciculus gracilis; lower body and legs, everything caudal to T6, fasciculus cuneatus; upper body and arms, everything rostral to T6, it is an ipsilateral pathway - first order afferents project to medulla (n. cuneatus and n. gracilis), second order neurones decussate in caudal medulla
clinical sensory loss - peripheral neuropathy
nerve or axon lesion; diabetes, vitamin deficiencies, lead poisoning, acute effects; loss of reflex, distal weakness, secondary complications like ulcers and deformed joints,
clinical sensory loss - central sensitisation
drop in threshold for pain; often seen in postoperative pain
clinical sensory loss - allodynia
pain in response to non-noxious stimulus
clinical sensory loss - synaesthesia
confusion of sensory modality, e.g. hearing number evokes a flavour sensation
clinical sensory loss - spinal cord injuries
causes disruption of transmission of signal, hemisection of spinal cord that interrupts all ascending fibres on one side will result in loss of deep pressure and vibration sense below the level of the section on the same side and loss of pain, temperature and light touch on the other side (Brown-Séquard syndrome), central cord syndrome - arises when central, fluid filled spinal cavity becomes distended as a result of congenital abnormalities of cerebrospinal fluid flow, trauma or inflammation (termed syringomyelia),
