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Neurophysiology & Human Behaviour > Somatosensation > Flashcards

Somatosensation Flashcards

1
Q

What are the types of sensory nerves?

A
  • Slowly adapting type I (SAI)
  • Slowly adapting type II (SAII)
  • Rapidly adapting type I (RAI)
  • Rapidly adapting type II (RAII)
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2
Q

What are the types of sensory nerve endings and the fibre types that possess them?

A
  • Naked ending: Fibres terminate without any accessory structures. Aδ, C fibres
  • Encapsulated endings: Fibres terminate within accessory structures that aid in sensation. Aβ fibres
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3
Q

What type of accessory structure is associated with SAI fibres?

A
  • Merkel cells
  • Incomplete adaptation
  • Contact (texture, edges, points)
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4
Q

What type of accessory structure is associated with SAII fibres?

A
  • Ruffini endings
  • Incomplete adaptation
  • Tension, folding, joint movements
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5
Q

What type of accessory structure is associated with RAI fibres?

A
  • Meissner’s corpuscules
  • Complete adaptation
  • Shearing
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6
Q

What types of accessory structure is associated with RAII fibres?

A
  • Pacinian corpuscules
  • Complete adaptation
  • Deep pressure
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7
Q

What are accessory structures?

A

Accessory structures are structural components of sense organs which may play an importanty role in protection, conduction, concentration, analysis, sensitisation or inhibition; but are not directly involved in transduction.

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8
Q

What features makes Meissner’s corpuscules good at detecting shearing?

A

Mechanical coupling to papillary ridges in skin

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9
Q

Where are pacinian corpuscules found?

A
  • Dermis of glabrous skin
  • Connective tissues of muscles
  • Periosteum of bone
  • Mensentery of abdomen
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10
Q

What is the maximum sensitivity frequency of Pacinican corpuscules?

A

~200 - 250Hz

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11
Q

What is the maximum sensitivity frequency of Meissner’s corpuscules?

A

~10-40Hz

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12
Q

What is the sensitivity range of Merkel cells?

A

200μm - 1500μm skin deformation

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13
Q

Where are Ruffini endings found?

A
  • Wrist
  • Finger joints
  • Skin folds of palm
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14
Q

What types of receptive fields possessed by each type of fibre?

A
  • Type I: Small localised areas of high sensitivity
  • Type II: Small area of high sensitivity surrounded by large area of low sensitivity
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15
Q

Which types of fibres are involved in high acuity tactility and why?

A
  • SAI (Merkel cells) and RAI (Meissner’s corpuscules)
  • These have the smallest receptive fields
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16
Q

What are the functions of slow/rapidly adapting fibres?

A
  • Slow adapting fibres: Detection of details in stationsary objects such as edges, corners, curvature, points. Gives detail about shape, texture and size of object.
  • Rapidly adapting fibres: Detectin of quickly changing stimuli such as vibrations and movement.
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17
Q

How is stimulus intensity encoded by different types of tactile fibres?

A
  • Rapidly adapting: Recruitment, due to phase-locking not allowing frequency code.
  • Slowly adapting: Frequency code.
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18
Q

Which fibres are concerned with temperature sensation?

A

Aδ, C fibres

19
Q

How many colds and warm spots are there?

A
  • ~700 cold
  • ~24 warm
20
Q

What types of receptor molecules are involved in temperature detection?

A
  • Transient receptor potential (TRP) channels
  • Warm (e.g. TRPV1)
  • Cold (e.g. TRPM8)
21
Q

What causes paradoxical cold?

A

>45oC, cold receptors are stimulated to give cold sensation.

22
Q

What is the principle behind “labelled lines”?

A

Sensory experience is determined by the central connection of a particular sensory neurone, not the stimulus that stimulates it.

23
Q

What are the types of axons found in nerve bundle and their associated conduction velocities?

A
  • Aα: Myelinated motor fibres (80-120 m/s)
  • Aβ: Myelinated somatic sensory fibres (40-80 m/s)
  • Aδ: Thinly myelinated temperature and pain fibres (5-30 m/s)
  • C: Unmyelinated temperature and pain fibres (0.5-2 m/s)
24
Q

Which class of axons are affected by anoxia?

A

Aβ fibres (somatosensation)

25
Q

Which class of axons are affected by local anaesthetics?

A

Aδ/C fibres (pain/temperature)

26
Q

What are the functions of the spinal cord?

A
  • Motor innervation to skeletal muscle (ventral root)
  • Somatosensory innervation to skin (dorsal root)
  • Automatic innervation to viscera
27
Q

How many pairs of spinal nerves are there?

A

31

28
Q

What is the nature of termination in the spinal cord for the somatosensation fibres?

A
  • Both Aβ and Aδ/C fibres terminate in the dorsal horn of the grey matter.
  • Aδ/C fibres terminate directly in layers I and II of the dorsal horn.
  • Aβ fibres wind around dorsally and enter layers III-V of the dorsal horn.
29
Q

What is the function of the dorsal column-medial lemniscus (DC-ML) system?

A

Central pathway through which touch and proprioceptive information from Aβ fibres are relayed.

30
Q

What is the structure of the DC-ML system?

A
  1. As Aβ fibre enters the spinal cord, it bifurcates into a short branch entering the dorsal horn and a long branch ascending in the dorsal column.
  2. Below the mid-thoracic level (T7), nerve fibres enter the gracile fascicle on the medial aspects of the dorsal column.
  3. Above the mid-thoracic level, the dorsal column is divided by a sulcus and subsequent spinal nerve entering the column enters a more lateral cuneate fascicle.
  4. Nerve fibres in the cuneate/gracile fascicles synapse in the cuneate/gracile nuclei of the medulla oblongata respectively, together forming the dorsal column nuclei.
  5. Axons from the dorsal column nuclei cross the midline to ascend as the medial lemniscus contralaterally, being joined by fibres from the trigeminal nuclei (receiving sensory afferents from head).
  6. Fibres synapse in the ventroposterior medial/lateral nuclei of the thalamus before projecting into the primary/secondary somatosensory cortex.
31
Q

What is the topographic arrangement of fibres in the DC-ML system?

A
  • Medial-most fibres from the legs and sacral region.
  • Lateral-most fibres from the arm.
32
Q

What is the function of the spinothalamic (ST) system?

A

Primary pathway through which temperature and nociceptive information from Aδ and C fibres are relayed to brain.

33
Q

What is the structure of the ST system?

A
  1. Nociceptive fibres entering spinal cord give both ascending and descending branches into Lissauer’s tract.
  2. Branches from Lissauer’s tract enter the grey matter and synapse with second order cells principally in substantia gelatinosa. This synapse allows for gating mechanisms (e.g. gate-control of pain).
  3. Fibres originating from lamina I ascend in the lateral spinothalamic tract while fibres originating from lamina V ascend in the ventral spinothalamic tract; both crossing over at the midline to ascend on the contralateral side. Together, these 2 tracts form the anterolateral system.
  4. At the medial lemniscus, fibres from the trigeminal nucleus cross the midline to join the tract.
  5. From the medial lemniscus, fibres in the spinothalamic tract synapse in the ventral posterior lateral/medial nuclei of the thalamus before projecting various areas of the brain.
34
Q

Which areas of the brain do fibres in the ST system project?

A
  • Primary somatosensory nucleus
  • Insula
  • Anterior cingulate cortex (ACC)
35
Q

What additional tracts are given off by the anterolateal tract of the ST system?

A
  1. Spinoreticular tract: Originates from laminae VII and VIII and terminates in the reticular formation and the thalamus.
  2. Spinomesencephalic tract: Originates from laminae I and V and terminates in the mesencephalic reticular formation and the periaqueductal grey of the midbrain.
36
Q

What are the origins of sensory inputs into various parts of the thalamus from the DC-ML and ST systems?

A

Ventroposteromedial nucleus: Head

Ventroposterolateral nucleus: Body

37
Q

What types of clinical conditions are associated with spinal cord transections?

A
  • Spinal cord hemisection (Brown-Sequard syndrome):
    1. Ipsilateral loss of tactility and proprioception.
    2. Contralateral loss of pain and temperature.
  • Posterior column syndrome (e.g. tabes dorsalis):
    1. Pain and temperature unaffected.
    2. Bilateral loss of tactility and proprioception.
  • Syringomyelia (fluid-filled cavity in spinal cord):
    1. Bilateral loss of pain and temperature.
    2. Tactility and proprioception unaffected.
  • Total transection:

Bilateral loss of all senses.

38
Q

What are the principles behind referred pain?

A

Neurones in lamina V of spinal cord synapse with both Aδ/C fibres from skin and visceral afferents.

39
Q

What is the location of the primary somatosensory cortex?

A

Posterior to the central sulcus in post-central gyrus.

40
Q

What are the Brodmann’s areas associated with primary somatosensory cortex (S1)?

A

3a, 3b, 1, 2

41
Q

What is the nature of connectivity in S1?

A
  1. Layers I-III project to higher cortical areas via pyramidal cells and are involved with association.

Layer IV receive afferent inputs from the thalamus.

Layer V sends efferent projections to the thalamus.

Layer VI sends efferent projections to other subcortical regions.

42
Q

What is referred pain?

A

Pain felt in a part of the body away from the actual source of pain.

43
Q

What is the astereognosis and what is it caused by?

A
  • Patient is able to feel objects, but are unable to use tactile information to describe the shape of object.
  • Lesion in posterior parietal region of somatosensory cortex.

Neurophysiology & Human Behaviour (20 decks)

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