Somatosensory Pathways Flashcards
What are the classifications of sensation?
- General sensation > somatic + visercal
- Special sensation
What are the types of general sensation?
Visceral - unconscious
Somatic - conscious
What are the special senses?
Vision
Hearing
Balance
Taste
Smell
What are modalities?
Units of sensation relying on a distinct receptor type
Part of somatic sensation
What are the types of modalities?
-
Spinothalamic system:
-temperature
-pain
-pressure/crude touch
. -
Dorsal column-medial lemniscus system:
-vibrations
-proprioception
-fine touch
-two point discrimination
What is two point discrimination?
The ability to distinguish between two nearby objects touching the skin as two distinct points, not one
What modalities are part of the spinothalamic system?
What are the associated receptors?
Temperature thermoreceptors
Pain nociceptors
Pressure/crude touch mechanoreceptors
What modalities are part of the dorsal column-medial lemniscus system?
What are the associated receptors?
- Vibration mechanoreceptors
- Proprioception muscle spindles + Golgi tendon organs
- Fine touch mechanoreceptors
- Two point discrimination mechanoreceptors
Location of the cell body of first order neurones
Dorsal root ganglion
Location of the cell body of second order neurones
Dorsal horn - spinothalamic pathway
Medulla - dorsal column
Location of the cell body of third order neurones
Thalamus
Difference in strong and weak receptor activation
- Strong receptor activation > high frequency AP
- Weak receptor activation > low frequency AP
Difference between rapidly + slowly adapting receptors
Examples
- rapidly adapting receptors: frequency of firing diminishes rapidly e.g. mechanoreceptors
- slowly adapting receptors: frequency of firing changes very little e.g. nociceptors
What difference between large and small receptive fields?
Examples of a location for each
- large receptive field: low sensory acuity/poor 2 point discrimination e.g. skin on back
- small receptive field: high sensory acuity/good 2 point discrimination e.g. skin on fingertips
What is the function of somatotropy representation?
- This way of organisation minimises the amount of ‘wiring’ required to transmit sensory information
- Dermatome modalities converge on the way to the homunculus
Describe the layout of the dorsal column-medial lemniscus system
-
1st order neurones ascend ipsilaterally through dorsal columns of spinal cord:
-lower body ascend through gracile fasciculus to gracile nucleus in medulla
-upper body ascend through cuneate fasciculus to cuneate nucleus in medulla
-lower body is more medial than upper body - 2nd order neurones decussate to thalamus in medial lemniscus
-
3rd order neurones > primary sensory cortex:
-lower body to medial portion
-upper body to lateral portion
Topographical organisation of the dorsal columns
Lower body - medial
Upper body - lateral
What effect could a central lesion of the spinal cord do in relation to dorsal column medial lemniscus system?
Affects dorsal column modalities radiating up the body:
-fine touch
-two point discrimination
-proprioception
-vibration
What is syringomelia?
Disorder characterised by development of syrinx in central canal > effects sphinothalamic modalities decussated in white ventral commissure
What spinal levels correspond to the upper vs lower body?
upper - T6 + above
lower - T7 + below
Describe the first order neurones ascent in the dorsal column
Ascend ipsilaterally to medulla
- lower body: up through gracile fasciculus to gracile nucleus
- upper body: up through cuneate fasciculus to cuneate nucleus
Describe the connection of 3rd order neurones of the upper vs lower body to the primary sensory cortex
upper body to lateral portion
lower body to medial portion
Describe the layout of the spinothalamic pathway
- 1st order neurones project to ipsilateral side of cord
- 2nd order cell bodies are in dorsal horn
- 2nd order cells decussate at ventral white commissure > form spinothalamic tract > to thalamus
-
3rd order neurones > primary sensory cortex:
-lower body to lateral portion
-upper body to medial portion
Outline Brown-Sequard syndrome
Lesion to one half of the spinal cord > unilaterally completely destroying:
- dorsal + ventral horn
- dorsal + ventral roots
- all other cord grey matter
- all white matter pathways
.
This causes:
- ipsilateral complete anaesthesia affecting single dermatome (destruction of dorsal structure)
- ipsilateral loss of dorsal column modalities below
- contralateral loss of spinothalamic modalities below
What does brown sequard syndrome cause?
- ipsilateral complete anaesthesia affecting single dermatome (destruction of dorsal structure)
- ipsilateral loss of dorsal column modalities below
- contralateral loss of spinothalamic modalities below
Outline Lissauer’s tract
1st order neurones can ascend a couple spinal cord levels in Lissauer’s trac before synapsing to 2nd order neurones in the dorsal horn
Why does rubbing a sore area relieve pain?
Rubbing a sore area activates encephalinergic interneurones > encephalins release > inhibits Nociceptors > reduced pain
What effect could a central lesion of the spinal cord do in relation to spinothalamic system?
Affects the spinothalamic modailites radiating down the body:
-pain
-pressure/crude touch
-temperature
What is a syrinx?
Fusiform cyst
What is the most common location affected by syringomelia?
Cervical region of central canal
What are the initial symptoms of syringomelia?
- damage to Spinothalamic pathway
- loss of sensitivity to pain + thermal stimuli
- preserved sensitivity to touch + proprioception
