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LCRS - neuro > The somatosensory system > Flashcards

The somatosensory system Flashcards

1
Q

What is the function of the somatosensory system?

A

interpretation of bodily sensations through sensory receptors linked to brain centres that process information by nerve tracts.

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2
Q

Name the modalities of the somatosensory system.

A

mechanical (touch)

thermal (heat)

proprioception (mechanical displacement of joints and muscles)

nociception (noxious)

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3
Q

Briefly describe how sensory nerves are adapted for their function.

A

Individual axons have modified terminals - free nerve endings for thermo/nociceptors, enclosed for mechanoreceptors.

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4
Q

Compare A-alpha, B-beta, A-delta and C fibres.

A

A-alpha - very fast. skeletal muscle propriocepton. Large diameter, myelinated.

A-beta - quite fast. Innocuous mechanical. Medium diameter. Myelinated.

A-delta - fast. noxious mechanical + thermal + chemical stimulation. Small diameter. Myelinated.

C - slow. Noxious mechanical + thermal + chemical stimulation. Small diameter. Unmyelinated.

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5
Q

Define receptor.

A

Transducer that converts environmental stimulus –> action potential.

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6
Q

What is the role of transient receptor potential (TRP) ion channcels in thermoreceptors?

A

Perception of small changes in temperature.

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7
Q

What receptor types are responsible for cold and hot sensory information?

A

Cold - A - delta. TRPM8/A1.

Hot - C. TRPV1-4.

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8
Q

Name 4 types of mechanoreceptor.

A

Meissner’s Corpuscle.

Merkel cells

Pacinian Corpuscle

Ruffini endings.

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9
Q

What sensory information are Meissner’s Corpuscles responsible for?

A

fine touch discrimination, low frequency vibration.

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10
Q

What sensory information are Merkel cells responsible for?

A

light touch and superficial pressure

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11
Q

What sensory information are Pacinian Corpuscles responsible for?

A

deep pressure and high frequency vibration

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12
Q

What sensory information are Ruffini endings responsible for?

A

continuous pressure/touch and stretch

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13
Q

Define stimulus threshold.

A

point of intensity at which a person can just detect the presence of a stimulus 50% of the time

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14
Q

Define stimulus intensity.

A

ncreased strength and duration of the stimulus leads to greater NT release and greater intensity - encoded by frequency of neurone firing

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15
Q

Define adaptation.

A

ability of a receptor to stop firing while a stimulus is in place

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16
Q

What is the difference between tonic and phasic receptors?

A

Tonic - adapt slowly. Detect continuous stimulus strength. Allow constant perception of superficial pressure (e.g. Merkel cells).

Phasic - adapt rapidly. Detect change in stimulus strength. Imulse transmitted at start and end of stimuli (e.g. Pacinian receptors).

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17
Q

Define receptive field.

A

region of skin causing the activation of a single sensory neurone when activated

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18
Q

Where can small and large receptive fields be found?

A

Small - limbs. For fine detail detection and precise perception (e.g. fingers).

Large - back. Allow changes detected over large area. Less precise.

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19
Q

Define two point discrimination.

A

minimum ditance at which two points are perceived as separate.

20
Q

What is the role of lateral inhibition? How is it mediated?

A

prevent overlap of receptive fields to facilitate pinpoint accuracy, mediated by inhibitory interneurons within the dorsal horn of the cord

21
Q

Explain the mechanism of lateral inhibition.

A

most active neuron sends impulses down interneurons to inhibit adjacent neurones and pinpoint stimulus

22
Q

Where are the cell bodies of inhibitory neurons involved in lateral inhibition located?

A

DRG in body, trigeminal ganglia in face.

23
Q

Where is the primary somatosensory cortex located?

A

located in the postcentral gyrus

24
Q

What is the role of the primary somatosensor cortex?

A

receive somatosensory input

25
Q

Where is the parietal association cortex located?

A

posterior to the primary cortex

26
Q

What is the role of the parietal association cortex?

A

integrate sensory information to yield understanding of object/environment

27
Q

Explain somatotopic organisation.

A

Each cortex contains representation of the contralateral side of the body (toes (superior) –> mouth (inferior).

28
Q

Give the 6 steps of the doral column system sensory pathway for innocuous mechanical stimuli.

A
  1. A-beta fibres enter via dorsal horn
  2. 1st Order fibres travel up ascending dorsal (posterior) column pathways ipsilaterally
    - Lower limb fibres –> gracile tract (more medial)
    - Upper limb fibres –> cuneate tract (more lateral)
  3. 1st Order neurones terminate in the medulla
    - Lower limb / gracile fibres synapse in gracile nucleus (more medial)
    - Upper limb / cuneate fibres synapse in cuneate nucleus (more lateral)
  4. 2nd Order neurones cross in the caudal medulla, forming the contralateral medial lemniscus tract
  5. 2nd Order neurones terminate in the ventral posterior lateral nucleus of the thalamus
  6. 3rd Order neurones project to, and terminate in the somatosensory cortex, with the size of somatotopic areas proportional to the density of sensory receptors in that body region
29
Q

Give the 4 steps of the spinothalamic (anterolateral) somatosensory pathway.

A
  1. 1st Order neurones terminate in the dorsal horn
  2. 2nd Order neurones decussate immediately and form the spinothalamic tracts contralaterally
  3. 2nd Order neurones terminate in the ventral posterior lateral nucleus of the thalamus
  4. 3rd Order neurones project to, and terminate in the somatosensory cortex, with the size of somatotopic areas proportional to the density of sensory receptors in that body region
30
Q

What sensations are conveyed through the spinothalamic pathway?

A

Pain and temperature (lateral tract)

crude touch (anterior tract).

31
Q

Give the key differences between the spinothalamic and dorsal column pathways.

A

Dorsal - info –> medulla before decussation. Spinothalamic = decussation in cord before entering medulla.

32
Q

What is peripheral sensitisation?

A

Reduce theshold to peripheral stimuli at injury site. Tissue damage –> mediator release –> increased sensitivity of nociceptors and healing.

33
Q

What causes nociceptive pain?

A

noxious stimulation of receptors.

34
Q

What causes neuropathic pain?

A

lesion/disease of somatosensory system. e.g. diabetes.

35
Q

What is central sensitisation?

A

reduced theshold for peripheral stimuli at side adjacent to injury.

36
Q

Define allodynia.

A

pain due to stimulus that does not normally provoke pain

37
Q

What is hyperalgesia?

A

increased pain from stimulus that normally provokes pain

38
Q

What is the difference between A-delta and C fibres in the communication of pain sensation?

A

A-delta - mediate sharp, intense pain. T1 = noxious mechanical. T2 = noxious heat.

C-fibres - mediate dull,aching pain. Noxious thermal, chemical, mechanical stimuli.

39
Q

What NT is largely responsible for pain signalling.

A

Glutamate. Excitatory.

40
Q

What tracts convey the sensory and emotional components of pain?

A

emotional - spinoreticular tract.

sensory - lateral spinothalamic tract.

41
Q

Where is nociceptive processing facilitated/inhibited in the descending control pathway?

A

periaqueductal grey matter.

(descending inhibition - attenuation of dorsal horn responses to peripheral stimuli with NA,

descending facilitation - increase of dorsal horn responses to perhiperal stimuli with seratonin).

42
Q

Explain gate control theory.

A

Pain sensation cab be blocked by inhibiting primary afferent inputs before transmission reaches brain.

43
Q

Explain the mechanism of gate control theory.

A

Activation of A-beta fibres activates inhibitory neuron that blocks activated C-fibres. Hence rubbing alleviates pain.

44
Q

Why are vibration and 2 point discrimination sensations retained in damage to the anterior spinal cord?

A

Only spinothalamic tract damaged, not the dorsal column pathway.

45
Q
A

LCRS - neuro (16 decks)

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