Somatosensory pathways

Question 1

What is somatosensory system responsible for mediating?

Answer 1

Touch and proprioception

Question 2

What are the mechanosensitive channels thought to be?

Answer 2

TRPs probs

Role for piezo2 channels

Question 3

What are the 4 types of mechanoreceptor in glabrous skin?

Answer 3

Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, and Ruffini’s endings

Question 4

Which receptors have very small receptive fields? Which are large?

Answer 4

Found that Meissner’s corpuscles and Merkel’s discs had very small receptive fields while Ruffini’s endings and Pacinian capsules had very large

Question 5

Describe rapidly adaptive mechanoreceptors vs slowly adapting

Answer 5

Rapidly adapting mechanoreceptors respond quickly at first but then stop firing as the stimulus continues
Slowly adapting mechanoreceptors produce a more sustained response.

Question 6

Draw the table summarising the receptors

Answer 6

OneNote

Question 7

Describe Pacinian corpuscles

Answer 7

Myelinate Abeta enclosed by concentric layers of connective tissue

Question 8

What are the three types of follicle receptor in non-glabrous skin?

Answer 8

Hair guard
Hair tylotrich
Hair down

Question 9

Which nerves are involved?

Answer 9

Abeta subgroup

C fibers → normally involved in pain but thought they could be involved in pleasant touch

Question 10

Where do the nerve endings go?

Answer 10

dorsal horn of spinal cord

Question 11

Describe pathway to the cortex

Answer 11

dorsal horn (where they can synapse with second order neurons to form a reflex arc) → travel up the ipsilateral dorsal column → terminate in the medulla in the dorsal column nuclei → decussate before continuing to ascend on the contralateral side of the medial lemniscus → VPL of the thalamus → internal capsule → ipsilateral postcentral gyrus of cerebral cortex (S1)

Question 12

How are the dorsal columns within the spinal cord organised?

Answer 12

• fibres conveying info about lower limbs = medially and travel upwards as gracile tract
• fibres about upper limbs = more lateral cuneate tract
• these terminate in the gracile nucleus and the cuneate nucleus of the dorsal column nuclei respectively

Question 13

How does information from the face reach S1?

Answer 13

It is, however, the trigeminothalamic system that is responsible for conveying tactile information about the face
Primary sensory neurons in trigeminal nerve ganglion
Enter at the level of the pons
Terminate on neurons in the trigeminal brainstem complex
Decussate and enter the ventral posterior medial (VPM) nucleus of the thalamus via the trigeminal meniscus
Delivered to the ipsilateral areas of the primary and secondary somatosensory cortices

Question 14

Describe anterior choroidal syndrome

Answer 14

Anterior choroidal syndrome
Occlusion of the anterior choroidal artery supplies the posterior limb of the internal capsule
Causes contralateral hemihypesthesia (reduction in sensation on one part of the body)
Other clues: contralateral hemiplegia (paralysis on one side) and homonymous hemianopia (blindness)

Question 15

Draw somatosensory pathway

Answer 15

OneNote

Question 16

Describe wallenburg syndrome

Answer 16

Injury to the lateral medulla

• loss of pain and temperature contralaterally while touch is spared
• PICA is thrombosed, leads to ischemia and contralateral loss of pain and temperature sensation, touch is not lost because it has a different blood supply and doesn’t travel in the same channel → dorsal column supplied by the small arteries of the vertebral basilar circulation

Question 17

Where is touch information altered on the pathway?

Answer 17

In both the dorsal column and the thalamic nucleus

• common alteration is contrast enhancement which serves to amplify differences in activity of neighbouring neurons
• one mechanisms achieving this is lateral inhibition (neighbouring neurons inhibit one another, thus enhancing spatial contrast and allowing fine tactile discrimination)

Question 18

Where is the somatosensory cortex located?

Answer 18

In the postcentral gyrus of the parietal lobe

Question 19

What is S1 formed from?

Answer 19

Brodmann’s areas 1, 2, 3a and 3b

Question 20

What order do the Brodmann’s areas come in in S1?

Answer 20

3a, 3b, 1, 2 in that order, anterior → posterior

Draw the diagram from OneNote

Question 21

Which part of S1 do most afferents terminate in?

Answer 21

3a and 3b which in turn project to areas 1, 2 and S2

Question 22

Where is S2 located?

Answer 22

Posterior parietal lobe

Question 23

What are the crude roles of these areas?

Answer 23

Area 3 - shape and texture discrimination
Area 2 - shape
Area 1 - texture

Question 24

Describe the somatotopic organisation of S1

Answer 24

Feet, legs, arms, trunk, face → medial to lateral

Question 25

What does the somatosensory association get input from?

Answer 25

Somatosensory, visual and auditory systems

Question 26

What does slowly adapting mean

Answer 26

means surface stimulation causes continuous APs until removed (firing frequency
directly proportional to extent of stimulus)

Question 27

What does rapidly adapting mean?

Answer 27

same stimulus causes a

transient AP then silence followed by a small AP when removed

Question 28

What sort of fibres do cold receptors have?

Answer 28

A delta

Question 29

What sort of fibres do warm receptors have?

Answer 29

C fibres

Question 30

Where does lateral inhibition occur?

Answer 30

Dorsal column nuclei

Question 31

What do lesions in S1 impair?

Answer 31

Somatosensation (dysfunction in texture, size and shape discrimination)

Question 32

What indicates that the spinal cord is only partially damaged?

Answer 32

there must be some preservation of sensation or motion in the areas
innervated by S4 to S5 (eg. voluntaryexternal anal sphinctercontraction) – the nerves in this area are
connected to the very lowest region of the spinal cord, and retaining sensation and function in these
parts of the body indicates that the spinal cord is only partially damaged.

Question 33

What do lesions at the lumbrosacral level cause?

Answer 33

result in decreased control of legs and hips, genitourinary system, anus

Question 34

What do lesions of thoracic area cause?

Answer 34

in addition to problems found in lower-level injuries, thoracic spinal lesions can affect
muscles in the trunk (the lower the level of injury, the less extensive its effects):

Question 35

What do injuries from T1-T8 cause?

Answer 35

inability to control abdominal muscles, may affect trunk stability

Question 36

What do injuries from T9-T12 cause?

Answer 36

partial loss of trunk and abdominal muscle control

Question 37

What do injuries at the cervial level result in?

Answer 37

full or partialtetraplegia(aka. quadriplegia);
depending on specific location and severity of trauma, limited function may be retained:
 C1-C4 – full paralysis of the limbs; cannot breathe without mechanical ventilation
 C5 – paralysis of wrists, hands and triceps; difficulty coughing and clearing secretions
 C6 – paraysis of wrist flexors, triceps and hands; respiratory function as above

Question 38

What does complete transection cause?

Answer 38

Complete transection is a complete tear in the spinal cord; it results in absence of movement,
sensation, and body organ function below the level of the injury. Commonly thoracic or cervical
- Complete injury (all function lost) may also occur even if spinal cord is not severed

Question 39

Describe Brown-Sequard syndrome

Answer 39

This syndromeoccurs when the spinal cord is injured on one side much more than the other
- It is rare for the spinal cord to be truly hemisected (severed on one side), but partial lesions due to
penetrating wounds (such as gunshot or knife wounds) or fractured vertebrae or tumors are common
- On the ipsilateral side of the injury (same side), the body loses motor function,proprioception, and
senses of vibration and touch
- On the contralateral (opposite side) of the injury, there is a loss of pain and temperature sensations

Question 40

Describe syringomyelia

Answer 40

• Syringomyelia is a generic term referring to a disorder in which acystor cavity forms within thespinal
  cord – this cyst, called asyrinx, can expand and elongate over time, destroying the spinal cord
• The damage may result in pain,paralysis, weakness; stiffness in the back, shoulders, and extremities
• It may also cause a loss of the ability to feel extremes of hot or cold, especially in the hands
• The disorder generally leads to a cape-like loss of pain and temperature sensation along back and arms
• Each patient experiences a different combination of symptoms – these symptoms typically vary
  depending on the extent and, often more critically, to the location of the syrinx within the spinal cord
• A syrinx may also cause disruptions in theparasympatheticandsympathetic nervous systems, leading
  to abnormal body temperature or sweating, bowel control issues, or other problems.
• If the syrinx is higher up in the spinal cord or affecting the brainstem as in syringobulbia,vocal cord
  paralysis, ipsilateral tongue wasting,trigeminal nervesensory loss, and other signs may occur.
• Classically, syringomyelia spares thedorsal column/medial lemniscusof the spinal cord, leaving
  pressure, vibration, touch andproprioceptionintact in the upper extremities.

Question 41

What are the spinal levels of the dermatomes and basic reflexes

Answer 41

Biceps - C5
Triceps - C7
Knee - L3/4
Ankle - S1