Somatosensation

Question 1

What are the types of sensory nerves?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

What type of accessory structure is associated with SAI fibres?

Answer 3

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

Question 4

What type of accessory structure is associated with SAII fibres?

Answer 4

• Ruffini endings
• Incomplete adaptation
• Tension, folding, joint movements

Question 5

What type of accessory structure is associated with RAI fibres?

Answer 5

• Meissner’s corpuscules
• Complete adaptation
• Shearing

Question 6

What types of accessory structure is associated with RAII fibres?

Answer 6

• Pacinian corpuscules
• Complete adaptation
• Deep pressure

Question 7

What are accessory structures?

Answer 7

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.

Question 8

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

Answer 8

Mechanical coupling to papillary ridges in skin

Question 9

Where are pacinian corpuscules found?

Answer 9

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

Question 10

What is the maximum sensitivity frequency of Pacinican corpuscules?

Answer 10

~200 - 250Hz

Question 11

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

Answer 11

~10-40Hz

Question 12

What is the sensitivity range of Merkel cells?

Answer 12

200μm - 1500μm skin deformation

Question 13

Where are Ruffini endings found?

Answer 13

• Wrist
• Finger joints
• Skin folds of palm

Question 14

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

Answer 14

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

Question 15

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

Answer 15

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

Question 16

What are the functions of slow/rapidly adapting fibres?

Answer 16

• 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.

Question 17

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

Answer 17

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

Question 18

Which fibres are concerned with temperature sensation?

Answer 18

Aδ, C fibres

Question 19

How many colds and warm spots are there?

Answer 19

• ~700 cold
• ~24 warm

Question 20

What types of receptor molecules are involved in temperature detection?

Answer 20

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

Question 21

What causes paradoxical cold?

Answer 21

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

Question 22

What is the principle behind “labelled lines”?

Answer 22

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

Question 23

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

Answer 23

• 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)

Question 24

Which class of axons are affected by anoxia?

Answer 24

Aβ fibres (somatosensation)

Question 25

Which class of axons are affected by local anaesthetics?

Answer 25

Aδ/C fibres (pain/temperature)

Question 26

What are the functions of the spinal cord?

Answer 26

- Motor innervation to skeletal muscle (ventral root) - Somatosensory innervation to skin (dorsal root) - Automatic innervation to viscera

Question 27

How many pairs of spinal nerves are there?

Answer 27

31

Question 28

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

Answer 28

- 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.

Question 29

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

Answer 29

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

Question 30

What is the structure of the DC-ML system?

Answer 30

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.

Question 31

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

Answer 31

- Medial-most fibres from the legs and sacral region. - Lateral-most fibres from the arm.

Question 32

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

Answer 32

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

Question 33

What is the structure of the ST system?

Answer 33

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.

Question 34

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

Answer 34

- Primary somatosensory nucleus - Insula - Anterior cingulate cortex (ACC)

Question 35

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

Answer 35

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.

Question 36

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

Answer 36

Ventroposteromedial nucleus: Head Ventroposterolateral nucleus: Body

Question 37

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

Answer 37

- 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.

Question 38

What are the principles behind referred pain?

Answer 38

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

Question 39

What is the location of the primary somatosensory cortex?

Answer 39

Posterior to the central sulcus in post-central gyrus.

Question 40

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

Answer 40

3a, 3b, 1, 2

Question 41

What is the nature of connectivity in S1?

Answer 41

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.

Question 42

What is referred pain?

Answer 42

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

Question 43

What is the astereognosis and what is it caused by?

Answer 43

- 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.