Somatosensory Flashcards
Why is somatosensory system important?
The somatosensory system provides a crucial line of communication between us & outside world
What conditions occur when you damage peripheral or central components of somatosensory system?
Damage to peripheral or central components results in conditions such as neuropathy, neuralgia, phantom pain
What are the 4 main classess of somatosensory receptors?
Tactile
Proprioception
Thermal sensations
Nociceptive (painful) sensations
What receptors are involved in Tactile sensation?
mediated by low threshold Mechanoreceptors, Merkel, Ruffini, Meissner & Pacinian types
What is Tactile sensation?
Not harm ful sensation to light touch
What mediates proprioception?
Mediated by muscle (spindle) & joint (Golgi tendon) receptors, some inputs from cutaneous mechanoreceptors
Thermal sensation is mediated by what?
Mediated by thermoreceptors localized to discrete zones that exhibit hot & cold sensitivity
What mediates Mociceptive (painful) sensation?
Mediated by mechanical, thermal & polymodal nociceptors
What are the different sensory receptors of the skin?
Free nerve endings Meckels disks Meissners corpuscle Pacinian corpuscle Ruffini's corpuscles
What are the 3 layers of the skin?
Epidermis
Dermis
Subcutis/hypodermis
Where are each of the sensory receptors of the skin found?
Meissners and Merkel are found just beneath the epidermis. Meissners sit between the dermal pupillae and Merkels are aligned with the pupillae.
Ruffini’s corpuscles located deep in dermis, long axis of the corpuscle is orientated parallel to the skin.
Pacinian corpuscles are large encapsulated endings located in the subcutaneous tissue.
What is the morphology of cutaneous skin receptors?
A. Meissners Corpuscles - looping axonal terminals that inter-twine supporting cells
B. Merkel’s - dome structure atop axon terminals
C. Pacinian Corpuscles - sensory axon surrounded by fluid filled capsule, onion-shaped appearance
D. Ruffini endings - nerve terminals intertwined with collagen fibrils
E. Nociceptors - free nerve endings that penetrate epithelial cells, no morphological specialization
What is the area, function and speed of adapatibility of merkel cells?
Dermo-epidermal boundary
Slow adapting
Pressure form, texture
Small receptive field size
What is the area, function and speed of adapatibility of meissner corpuscle?
Papillary dermis
Rapidly adapting
Flutter, motion
Small receptive field size
What is the area, function and speed of adapatibility of Pacinian corpuscle?
Deep in subcutis
Rapidly adapting
Vibration –> mechanical distortion
Large receptive field size
What is the area, function and speed of adapatibility of Ruffini corupuscle?
Dermis
Slow adapting
Stretching and shearing of skin
Large receptive field size
What is the fiber type of all the skin reecptors?
Ab
How do you test the receptive field of cutaneous sensory receptors?
Testing the receptive field of sensory receptors using a stimulus probe on the hand, and recording action potentials from a single median nerve axon.
Results for main types of cutaneous sensory receptors.
Is two point discrimiation same across the whole body?
Two point discrimination thresholds vary across body surface
Sensitivity is correlated with density of sensory innervation
What areas have high and low two point discrimination?
Areas of high sensitivity – fingertips , face
Areas of low sensitivity – torso, limbs
What is the range of temperatures sensory fibres can response to?
cold pain
cold
warm
hot pain
What family of receptors are activated by temperature?
Trp family of receptors
What is the methanol receptor?
Trpm8 and is a cold receptor
What is the Capsaicin receptor ?
TRPV1/VR1 –> heat receptor
Where is capsaicin found?
Extract of chilli peppers
What chemical group does capsaicin belong? to?
Vanilloids
What receptor does Capsaicin act on and what is the consequence?
Act on thermal receptor that produces an inward Ca current.
What is the function of muscle spindles?
Muscle spindles provide sensory feedback from muscle fibres on body position and movement.
What is the function of golgi tendons?
Golgi tendon organs regulate muscle tension or force of contraction, and prevent muscle overloading.
What is the general principle for large diameter receptors?
Rapidly conducting afferents (I/II) associated with low threshold mechanoreceptors.
What is the general principle for small diameter receptors?
Slow conducting afferents (III/IV) associated with nociceptors and thermoreceptors.
What is the conduction velocity positively correlated with?
Conduction velocity is positively correlated with axon diameter
What does dermatome represent?
Each dermatome represents that area of skin innervated by the left and right dorsal roots of a single spinal segment
What do dorsal roots contain?
Dorsal roots contain sensory afferents innervating receptors of peripheral targets e.g. in skin & muscle
Are dermatome boundaries absolute?
Boundaries are not absolute
Overlap between adjacent dermatomes
What is the process of developing shingles?
Herpes Zoster Virus (Chickenpox)
Virus lurks in primary sensory neurons
Can re-activate as Shingles
Inflammation & blistering on skin area of affected dorsal root/sensory neuron (e.g. L4)
Via what pathway does innocous snesitivies ascend?
Via the dorsal column pathway
Via what pathway does Noxious and thermal sensitivity ascend?
Spinothalamic pathway
Where is primary somatosensroy cortex (S1) found? What is its function?
Localized to postcentral gyrus and flanked by the central sulcus and the postcentral sulcus
Receives strong somatosensory inputs from thalamus
Where is secondary somatosensroy cortex (S2) found? What is its function?
Lies adjacent to S1 along the lateral sulcus
Plays a key role in sensory and motor integration
Receives corpus callosum inputs to form ‘joined -up’ body image
Build info from multiple body areas, body image
What is the function of sensory assoication areas?
Localized to posterior parietal cortex
Plays role in integration of multimodal senses
Who produced the idea of sensory and motor homunculus?
Dr Wilder Penfield
What neurological disorders can occur due to sensory asosication area ( posteirorparietal cortex)?
Astereoagnosia
Neglect
Agnosia
What is asteroeangosia?
The inability to identify objects on basis of touch alone but can recognize object by sight or sound
What is neglect syndrome?
Body part or visual field is disregarded.
Damage to what hemisphere usually causes neglect syndrome and what is the prognosis?
Most common after damage to the right hemispere, and usually improve or disappear with time
What is agnosia?
Inability to recognise objects despite normal sensory functioning
What are the common symptoms of lateral medullary syndrome?
Loss of pain and temperature on the ipsilateral side of the face but contralateral side of the body
What causes lateral medullary syndrome?
little stroke in the medulla
Why do you also get Rapidly developed vertigo, unsteadiness, hoarse voice and difficulty swallowing ?
The lower cranial nerves and sympathetic nerves come out of the medulla so these get damage too
In what part of the circle of willis does a stroke occur to cause lateral medulalry syndrome?
Posterior inferior cerebellar artery
What caues Anterior spinal syndrome?
The anterior spinal artery is occluded so lack of blood supply to the motor area and spinothalamic area of the spinal cord
What surgical procedure can cause Anterior spinal syndrome?
Clamping the aorta
What is the outcome of Anterior spinal syndrome?
Loss of both motor innervation and pain and temperature sensation.
Vibration and proprioception is preserved
What would be the outcome of a lacunar stroke of the thalamus?
Can get sudden onset loss of sensation of the single side of the whole body including the face.
Loss of sensation to all modalities
What is lacunar stroke associated with?
Usually associated with smoking, high blood pressure or diabetes
What are the symtpoms of Syringomyelia?
Gradually develop loss of pain and temperature sensation in both hands and across her body in a cape like distribution.
Also get wasting in small muscles of the hands and weakness in the legs.
What is Syringomyelia?
Rare expansion of spinal canal
What does Syringomyelia effect?
Affects crossing fibres first
Pain and temperature sensation
Later can cause paraparesis and root lesions (wasted hands)
How is Syringomyelia treated?
Surgically
What is peripheral neuropathy?
all the nerve endings are damaged of the peripheral nerves
What are the causes of peripheral neuropathy?
Diabetes mellitus Autoimmune (Guillain-Barre, CIDP, vasculitis) Toxic (including drugs) Vitamin deficiency Paraneoplastic Chronic kidney, liver disease Inherited (Charcot-marie-Tooth disease)
What is the cause of Subacute combined degeneration?
Caused by B12 deficiency
What degeneration do you get in Subacute combined degeneration?
Degeneration of dorsal column but the rest of the cord is still intact so still feel things
Why is dorsal column pathway effected but not spinothalamic pathway in subacute combined degeneration?
B12 is used to make myelin so axons that don’t have myelin such as spinothalamic are actually protected by this
What antibody is subacute combined degeneration assoicated with and what is the consequence?
Usually associated with Anti-parietal cell antibodies–> these destroy the parietal cells that absorb B12 even though they are taking in B12
What is complications of subacute combined degeneration ?
Cause also a peripheral neuropathy or dementia
Why is it important to treat subacute combined degeneration qucikly?
The treatment will stop the degeneration but won’t reverse it so the effects are done.
What is the treatment of subactute combined degeneration?
You cannot take B12 tablets because you can’t absorb it but you can use B12 injection
