The Somatosensory System Flashcards
What are sensory receptors
Transducers that converts energy from the environment into neuronal action potentials
What are the types of sensory neurones
Aβ-fibres - innocuous mechanical stimualtion. Myelinated and large, very fast
Aδ-fibres - noxious mechanical and thermal stimulation. Myelinated and thinner than Ab, fast
C-fibres - noxious mechanical, thermal and chemical stimulation, non-myelinated, slow
What are the somatosensory modalities
Touch – detection of light mechanical stimuli
Thermal – detection of temperature
Nociceptor – noxious and pain
Proprioception – mechanical displacement of muscles and joints
What kind of nerve endings do receptors have
Thermo receptors - free nerve
nociceptors - free nerve
Mechanoreceptors - enclosed nerve endings
Describe thermoreceptors
Aδ- and C-fibres Free nerve ending Transient receptor potential (TRP) ion channels 4 are heat activated: TRPV1 - 4 2 are cold activated: TRPM8, TRPA1
Describe the mechanoreceptors
Meissner’s corpuscle - fine discriminative touch, low frequency vibration
Merkel cells - light touch and superficial pressure
Pacinian corpuscle - Detects depp pressure, high frequency vibration and tickling
Ruffini endings - continuous pressure or touch and stretch
What is a threshold
Point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)
What are tonic receptors
Detects continuous stimulus strength
Continues to transmit impulses to the brain as long as the stimulus is present
Keeps the brain constantly informed of the status of the body
Give an example of tonic receptors
Merkel cells
Slowly adapt allowing for superficial pressure and fine touch to be percieved
What are phasic receptors
Detects changes in stimulus strength
Transmits an impulse at the start and the end of the stimulus (when a change is taking place)
Give and example of phasic receptors
Pacinian receptors
Sudden pressure excites receptor
Transmits a signal again when pressure is released
What is a receptive field
Region on the skin which causes activation of a single sensory neurones when activated
Compare small to large receptive fields
small - allow for detection of fine detail over small areas, precise perception e.g. fingers have many densely packed mechanoreceptors with small receptive fields
Large - allows the cells to detect changes one a wider area, less precise
What is two point discrimination
Minimum distance at which two points are perceived as separate
Related to the size of the receptive field
Where are the cell bodies found for the body and the face
Dorsal root ganglia (body) Trigeminal ganglia (face)
What are the two types of dorsal horn neurons
Neurones with axons that project into the brain (projection neurons)
Neurons with axons that remain in the spinal cord (interneurons)
What is lateral inhibition
prevents the overlap between two receptive fields
Facilitates pinpoint accuracy in localisation of the stimulus
Mediated by inhibitory interneurons within the dorsal horn of the spinal cord
Facilitates enhances sensory perception
What is the somatosensory pathway for touch and proprioception for the lower limbs (which vertebral level)
Below T6
- Aβ fibres enter the dorsal horn
- Aβ fibres enter the ipsilateral ascending dorsal column pathway
- Information travels ipsilaterally along the gracile tract (inner)
- Synapse in the gracile nucleus
- Crosses/ decussate in the caudal medulla (second order) to form the contralateral medial lemniscus tract
- Projection to the ventral posterior lateral nucleus of the thalamus
- Projection to the somatosensory cortex (third order neuron)
What is the somatosensory pathway for touch and proprioception for the upper limbs (which vertebral level)
Above T6
- Aβ fibres enter the dorsal horn
- Aβ fibres enter the ipsilateral ascending dorsal column pathway
- Information travels ipsilaterally along the cuneate tract (outer)
- Synapse in the cuneate nucleus
- Crosses/ decussate in the caudal medulla (second order) to form the contralateral medial lemniscus tract
- Projection to the ventral posterior lateral nucleus of the thalamus
- Projection to the somatosensory cortex (third order neuron)
What is size of somatotopic areas proportional to
Density of sensory receptors in that body region (somatosensory homunculus)
Pain and temp. localisation is not as precise)
Describe the somatosensory pathway for pain and temperature
- Sensations ascend within the lateral spinothalamic tract (crude touch in the anterior spinothalamic tract)
- Synapse in the spinal cord
- Decussate immediately in the spinal cord to form the spinothalamic tract
- 2nd order terminates in the ventral posterior lateral nucleus of the thalamus
What are the key differences between dorsal column and spinothalamic tracts
Spinothalamic - Pain, temp., coarse touch
Dorsal - light touch, vibration, 2 -point discrimination
What is crude touch and fine touch mediated by
crude - Aδ fibres (free nerve ending)
fine touch - Aβ fibres (meissner’s corpuscles)
What is the clinical importance of the 2 point discrimination to assess posterior column potency
Psychophysical assessment
Evaluate loss and gain to thermal and mechanical stimuli Normative data for face, hand foot and back
Describe an anterior spinal cord lesion
Blocked anterior spinal artery causes ischaemic damage to the anterior part of the spinal cord
Spinothalamic tract damage causes pain and temp. loss below the level of the lesion
Retained light touch, vibration and 2-point discrimination (dorsal column intact)
What is pain
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage
What are the nociceptors
Aδ fibers - mediate sharp, intense or first pain
C-fibres - mediate dull, aching or second pain. Noxious thermal, mechanical and chemical stimuli (polymodal)
What are the types of Aδ fibers
Type 1 - noxious mechanical
Type 2 - noxious heat
Which NT is involved in spinal cord nociceptive processing (first synapse)
Glutamate released from sensory afferents in response to acute or persistent noxious stimuli
Where are the sensory and components of the pain pathways carried
Sensory - lateral spinothalamic tract -> thalamus -> cortex
Emotional - spinoreticular tract -> parabrachial area -> hypothalamus/amygdala
What is involved in the pain matrix
Cortex: SI, SII, insula cortex, anterior cingulate cortex, prefrontal cortex
Amygdala
Cerebellum
Brainstem
What is gate control theory
Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways
Describe the descending control pathway
Periaqueductal grey (PAG) Facilitation and inhibition of nociceptive processing in the dorsal horn Monoamines (seratonin, NA)
What can nociceptive pain be associated with and what is it
Skin Muscles Ligaments Joints Bones Viscera
Noxious stimulation go a nociceptor (somatic or viscera)
What can neuropathic pain be associated with and what is it
Sciatica Diabetes Trauma Chemotherapy Post-surgical
Lesion of disease of the somatosensory system
Give examples of mixed nociceptive and neuropathic pain
Osteoarthritis
Low back pain
Compare peripheral sensitisation to central sensitisation
peripheral - Reduced thresholds to peripheral stimuli at the site of injury
Central - Reduced thresholds to peripheral stimuli at an adjacent site to the injury
Expansion of receptive field
Spontaneous pain
What is allodynia
pain due to a stimulus that does not normally provoke pain
What is hyperalgesia
Increased pain from a stimulus that normally provokes pain
Primary - site of injury/pain
Secondary - area around it
