HNS13 Somaesthetic Pathway

Question 1

Sensation vs Perception

Answer 1

Perception: Conscious interpretation of stimuli from environment

Sensation may not turn into perception
—> may not process in cerebral cortex

Question 2

Sensations

Answer 2

Conscious (Cerebral cortex)
—> Exteroception: sense of direct interaction with external world as it impacts on the body (e.g. touch, pressure, temperature)
—> Proprioception: sense of oneself posture and movement (by muscles, tendons, joints)

Unconscious (e.g. Cerebellum)
—> Proprioception: sense of oneself posture and movement
—> Interoception: sense of function of major organ systems of body and internal state (e.g. pH, BP)

Question 3

Organisation of somatosensory system

Answer 3

1. Receptor level: sensory receptors
2. Circuit level: processing in ascending pathways
3. Perceptual level: processing in cortical sensory areas

Question 4

Types of sensory receptor (by function)

Answer 4

1. General sensory:
   - receptors responsive to general stimuli such as touch, temperature, pain, vibration
2. Special sensory:
   - receptors responsive to special stimuli such as light, odour, taste, balance, sound

Question 5

Sensory neurons

Answer 5

• Pseudounipolar neuron
• 2 functionally distinct segments —> both function as axons
  —> one extend to peripheral skin / muscle, synapse with receptor cells
  —> other to central spinal cord
• cell body resides in ganglion
  —> dorsal root ganglion OR
  —> cranial ganglion

Question 6

Types of axons

Answer 6

Type Aα:

• Proprioceptors of skeletal muscle
• highly myelinated
• largest diameter

Type Aβ:
- Mechanoreceptors of skin (Touch)

Type Aδ:
- Pain, temperature

Type C:

• Temperature, pain, itch
• unmyelinated (much slower conduction speed)
• smallest diameter

Question 7

3 Major ascending pathways to the brain

Answer 7

1. Spinothalamic (anterolateral) pathway
   - pain
   - temperature
   - crude touch
2. Dorsal column-medial lemniscus (posterior column) pathway
   - arm and upper body
   - leg and lower body
3. Spinocerebellar pathway
   - anterior
   - posterior

ALL pathways conduct sensory impulses upward, through chains of 2/3 successive neurons to various areas of brain

Question 8

Spinothalamic, DC, Spinocerebellar pathway

Answer 8

Spinothalamic pathway:

• Transmit impulses via Thalamus to Sensory cortex for conscious interpretation
• 1st order neuron in dorsal root ganglion
• ***2nd order neuron in spinal cord + cross at spinal cord level

DC pathway:

• Transmit impulses via Thalamus to Sensory cortex for conscious interpretation
• 1st order neuron in dorsal root ganglion
• ***2nd order neuron in medulla oblongata (Cuneate nucleus + Gracile nucleus) + cross at medulla oblongata level

Spinocerebellar pathway:
- Terminates in Cerebellum, does NOT contribute to sensory perception

Question 9

First-order neuron

Answer 9

• Cell bodies reside in a ***ganglion (Dorsal root / Cranial)
• Conduct impulses from **Cutaneous receptors / Proprioceptors to **Spinal cord / Brainstem —> then synapse with second-order neuron
• some synapses with interneurons in CNS (contribute to reflexes)

Question 10

Second-order neuron

Answer 10

• Resides in ***Dorsal horn of Spinal cord / Brainstem
• Conduct impulses to ***Thalamus / Cerebellum —> then synapse
• Axons cross to ***opposite sides of body (decussation)

Question 11

Third-order neuron

Answer 11

• Locates in ***Thalamus
• Conduct impulses to ***Cerebral cortex
• Synapse with neurons of primary sensory cortex found in Spinothalamic and DC pathway

N.B.: within CNS (Spinal cord, Thalamus), there may be several short interneurons between input neuron and output neuron (i.e. between 1st / 2nd order neuron, between 2nd / 3rd order neuron)

Question 12

Dorsal column-medial lemniscus pathway

Answer 12

Pathways formed by:
1. Paired tracts of **dorsal **white column of spinal cord (Cuneate fasciculus + Gracile fasciculus) (ipsilateral)
—> Cross in medulla oblongata —>
2. Medial lemniscus (contralateral side)

• **Cuneate fasciculus: ***upper body, more lateral of spinal cord (記: CU)
• **Gracile fasciculus: ***lower body, more medial of spinal cord

(When enter posterolateral side of spinal cord —> bifurcate —> major branch ascend (Gracile fasciculus, Cuneate fasciculus), minor branch travel down —> terminate in some neuron which contribute to spinal reflexes)

Fibre:

• Larger diameter myelinated
• Aα, Aβ, Aδ

Function:

• ***Discriminative touch
• ***Pressure
• ***Vibratory sense
• Limb ***proprioception
• Conscious muscle joint sense

Receptor:

• Merkel cell (Mechanoreceptors)
• Pacinian corpuscle (Mechanoreceptors)
• Muscle spindle (Proprioceptors)
• Tendon organ (Proprioceptors)

1st order neuron:
- Dorsal root ganglion

2nd order neuron:
- Gracile nucleus + Cuneate nucleus (in **medulla) —> then **cross in medulla along ***medial lemniscus

3rd order neuron:
- Ventral posterolateral (VPL) nucleus of Thalamus

Destination:
- Primary somatosensory cortex

Question 13

Spinothalamic pathway

Answer 13

Pathways:
1. Anterior spinothalamic tract (light touch)
2. Lateral spinothalamic tract (pain, temperature)
—> Cross in spinal cord at Anterior white commissure

(When enter posterolateral side of spinal cord —> branch out —> some branches travel up / down (rostral-caudally) for 3-5 spinal levels)

Fibre:

• Smaller diameter myelinated
• Unmyelinated fibres terminate in dorsal horn
• Aδ, C

Function:

• ***Pain
• ***Temperature
• Light/crude (non-discriminative) touch
• Itch

Receptor:
- Free nerve endings

1st order neuron:
- Dorsal root ganglion

2nd order neuron:
- Dorsal horn of **grey matter of spinal cord —> then **cross in spinal cord at anterior white commissure

3rd order neuron:
- Ventral posterolateral (VPL) nucleus of Thalamus

Destination:
- Primary somatosensory cortex

Question 14

Spinocerebellar pathway

Answer 14

Pathway:

• ONLY 2 neurons
• terminate in ***ipsilateral cerebellum

1. Dorsal spinocerebellar tract (NOT cross)
   - **Lower trunks + limbs **proprioceptive afferents
   - synapse with Clarke’s column cells / Nucleus dorsalis
2. Ventral spinocerebellar tract (cross at spinal cord —> travel up to Superior cerebellar peduncle (pons) —> cross back to cerebellum) (cross TWICE / 即係無cross過)
   - **Lower limbs spinal **motor neurons + **interneurons afferents
   - Originate from **ventral horn of spinal cord
3. Cuneocerebellar tract (neurons originate from external cuneate nucleus) (medulla) (NOT cross)
   - **Upper limbs + Neck **proprioceptive afferents

Function:

1. Unconscious proprioception
2. Convey information from muscle / tendon stretch to Cerebellum —> coordinate skeletal muscle activity
3. NOT contribute to conscious sensation

Question 15

Cranial nerves with somatosensory function

Answer 15

1. CN5 Trigeminal
2. CN7 Facial
3. CN9 Glossopharyngeal
4. CN10 Vagus

Question 16

Trigeminothalamic tract

Answer 16

Function:
- Somatosensation from head and face

1st order neuron / primary sensory neuron:

• Trigeminal ganglion (Ipsilateral) —> ***Spinal trigeminal tract —> Principal / Spinal trigeminal nucleus
• Ganglia of CN7, 9, 10 (Ipsilateral) —> ***Spinal trigeminal tract —> Spinal trigeminal nucleus

2nd order neuron:

1. **Discriminative touch of CN5:
   - Principal trigeminal nucleus (Pons)
   - Some cross in pons —> Anterior **trigeminothalamic tract —> VPM
   - Some do not cross —> Posterior ***trigeminothalamic tract —> VPM
2. **Pain, Temperature of CN5 and CN7, 9, 10:
   - Spinal trigeminal nucleus (Medulla / Spinal cord)
   - cross in medulla / spinal cord —> Anterior **trigeminothalamic tract —> VPM

3rd order neuron:
- ***VPM nucleus of Thalamus

Destination:
- Primary somatosensory cortex

N.B.: Spinal trigeminal tract =/ Trigeminothalamic tract

Question 17

Blood supply of medulla: Posterior inferior cerebellar artery (PICA)

Answer 17

Occlusion of PICA —> Wallenberg syndrome / PICA syndrome / Lateral medullary syndrome:

1. Contralateral loss of pain + temperature sensation from body (Spinothalamic tract)
   (∵已經cross左)
2. Ipsilateral loss of pain + temperature sensation from face (Spinal trigeminal tract and nucleus)
   (∵未cross)
3. Vertigo + Nystagmus (Vestibular nuclei)
4. Loss of taste from ipsilateral half of tongue (Solitary tract and nucleus)
5. ***Hoarseness + Dysphagia (Nucleus ambiguus / roots of CN9, 10) (distinguished from AICA syndrome)

Question 18

Blood supply of medulla: Anterior inferior cerebellar artery (AICA)

Answer 18

AICA syndrome / Lateral pontine syndrome:

Distinct from PICA syndrome:

1. Ipsilateral hearing impairment (Cochlear nucleus)
2. Ipsilateral facial paralysis, ↓ taste from anterior 2/3 tongue, ↓ lacrimation and salivation (Facial nerve)

Question 19

Dermatome

Answer 19

• Area of skin supplied by right and left dorsal roots of a single spinal segment
• dermatomes formed by peripheral processes of adjacent spinal nerves ***overlap on body surface

Question 20

Sensory information processing

Answer 20

• Most somatic sensory information relayed to Thalamus for processing
• Adaptation reduces amount of information reaching cerebral cortex
• Small fraction (~1%) of sensory information coming in projects to cerebral cortex and reaches our awareness
• Damage / disease of Primary sensory neurons (peripheral nerve injury) —> absence of sensation from region of body supplied by those nerves
• Damage to Thalamic projection neurons in Spinal cord / Brainstem / 3rd order neurons in cerebral hemisphere —> loss of sensation from body below level of lesion

Question 21

Case study: Brown-Sequard syndrome

Answer 21

Hemisection / damage to half of spinal cord (係某一層壞左)

1. Ipsilateral loss of proprioceptive sensation, 2-point discrimination below level of lesion (due to damage to ascending dorsal columns)
   (∵已經未去到medulla cross已經壞左)
2. Contralateral loss of pain and temperature sensation ***a few levels below lesion (∵ at level of / a few levels below the lesion some 2nd order neurons do not cross until climb up a few levels)
   (∵爬上幾層先cross)
3. Ipsilateral upper motor neuron paralysis (Spastic paralysis) below level of lesion (due to damage to descending lateral corticospinal tract)
   (∵落黎個陣已經係medulla cross左)
4. Ipsilateral loss of lower motor neuron function (Flaccid paralysis) ***at the level of lesion due to direct damage to ventral and dorsal grey matter
5. Ipsilateral loss of sensations ***at the level of lesion due to direct damage to ventral and dorsal grey matter