HNS13 Somaesthetic Pathway Flashcards
Sensation vs Perception
Perception: Conscious interpretation of stimuli from environment
Sensation may not turn into perception
—> may not process in cerebral cortex
Sensations
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)
Organisation of somatosensory system
- Receptor level: sensory receptors
- Circuit level: processing in ascending pathways
- Perceptual level: processing in cortical sensory areas
Types of sensory receptor (by function)
- General sensory:
- receptors responsive to general stimuli such as touch, temperature, pain, vibration - Special sensory:
- receptors responsive to special stimuli such as light, odour, taste, balance, sound
Sensory neurons
- 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
Types of axons
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
3 Major ascending pathways to the brain
- Spinothalamic (anterolateral) pathway
- pain
- temperature
- crude touch - Dorsal column-medial lemniscus (posterior column) pathway
- arm and upper body
- leg and lower body - Spinocerebellar pathway
- anterior
- posterior
ALL pathways conduct sensory impulses upward, through chains of 2/3 successive neurons to various areas of brain
Spinothalamic, DC, Spinocerebellar pathway
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
First-order neuron
- 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)
Second-order neuron
- Resides in ***Dorsal horn of Spinal cord / Brainstem
- Conduct impulses to ***Thalamus / Cerebellum —> then synapse
- Axons cross to ***opposite sides of body (decussation)
Third-order neuron
- 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)
Dorsal column-medial lemniscus pathway
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
Spinothalamic pathway
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
Spinocerebellar pathway
Pathway:
- ONLY 2 neurons
- terminate in ***ipsilateral cerebellum
- Dorsal spinocerebellar tract (NOT cross)
- **Lower trunks + limbs **proprioceptive afferents
- synapse with Clarke’s column cells / Nucleus dorsalis - 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 - Cuneocerebellar tract (neurons originate from external cuneate nucleus) (medulla) (NOT cross)
- **Upper limbs + Neck **proprioceptive afferents
Function:
- Unconscious proprioception
- Convey information from muscle / tendon stretch to Cerebellum —> coordinate skeletal muscle activity
- NOT contribute to conscious sensation
Cranial nerves with somatosensory function
- CN5 Trigeminal
- CN7 Facial
- CN9 Glossopharyngeal
- CN10 Vagus
Trigeminothalamic tract
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:
-
**Discriminative touch of CN5:
- Principal trigeminal nucleus (Pons)
- Some cross in pons —> Anterior **trigeminothalamic tract —> VPM
- Some do not cross —> Posterior ***trigeminothalamic tract —> VPM -
**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
Blood supply of medulla: Posterior inferior cerebellar artery (PICA)
Occlusion of PICA —> Wallenberg syndrome / PICA syndrome / Lateral medullary syndrome:
- Contralateral loss of pain + temperature sensation from body (Spinothalamic tract)
(∵已經cross左) - Ipsilateral loss of pain + temperature sensation from face (Spinal trigeminal tract and nucleus)
(∵未cross) - Vertigo + Nystagmus (Vestibular nuclei)
- Loss of taste from ipsilateral half of tongue (Solitary tract and nucleus)
- ***Hoarseness + Dysphagia (Nucleus ambiguus / roots of CN9, 10) (distinguished from AICA syndrome)
Blood supply of medulla: Anterior inferior cerebellar artery (AICA)
AICA syndrome / Lateral pontine syndrome:
Distinct from PICA syndrome:
- Ipsilateral hearing impairment (Cochlear nucleus)
- Ipsilateral facial paralysis, ↓ taste from anterior 2/3 tongue, ↓ lacrimation and salivation (Facial nerve)
Dermatome
- 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
Sensory information processing
- 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
Case study: Brown-Sequard syndrome
Hemisection / damage to half of spinal cord (係某一層壞左)
- 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) - 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) - 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) - Ipsilateral loss of lower motor neuron function (Flaccid paralysis) ***at the level of lesion due to direct damage to ventral and dorsal grey matter
- Ipsilateral loss of sensations ***at the level of lesion due to direct damage to ventral and dorsal grey matter
