7. Sensory Pathways Flashcards
Define sensory modalities.
A modality is a type of stimulus (e.g. hot, cold, touch…)
Modalities have specialised receptors, which will transmit information through specific anatomical pathways to the brain.
List the major sensory modalities.
- Touch, Pressure, Vibration –> mechanoreceptors
- Proprioreception (joint position, muscle length, muscle tension)
- Temperature –> thermoreceptor
- Nociception –> nociceptors
Describe the types of sensory fibres.
- Falls into three categories
- A alpha
-
A beta
- Very fast, transducting
- Large
- Myelinated
- Innocuous mechanical stimulation
-
A delta
- Myelinated
- Not as large but still quick
- Pain and temperature signals
-
C
- No myelination
- Slow
- Slow pain
Together they form a peripheral nerve.
Define receptor.
‘’sensory receptors are transducers that convert energy from the environment into neuronal action potentials’’
Describe thermoreceptors.
- A-delta and C fibres
- Free nerve endings
- Transient Receptor Potential (TRP) ion channels
- 4 heat activated:
- TRPV1-TRPV4
- 2 cold activated:
- TRPM8
- TRPA1
Describe mechanoreceptors
- Meissner’s Corpuscle
- Fine discriminative touch, low frequency vibration
- Merkel Cells
- Light touch and superficial pressure
- Pacinian Corpuscle
- Detects deep pressure, high frequency vibration and tickling
- Ruffini endings
- Continuous pressure or touch and stretch
Define stimulus threshold
‘’A threshold is the point of intensity at which the person can just detect the presence of a stimulus 50% of the time (absolute threshold)’’
What does stimulus intensity depend on?
Increased stimulus strength and duration = increased neurotransmitter release = greater intensity
Explain the adaptation: tonic receptor.
- These receptors detect continuous stimulus strength
- Continue to transmit impulses to the brain as long as the stimulus is present
- Keeps the brain constantly informed of the status of the body
- e.g. Merkel cells
- Slowly adapt allowing for superficial pressure and fine touch to be perceived
Explain the adaptation: phasic receptor.
- Detect a change in stimulus strength
- Transmit an impulse at the start and the end of the stimulus
- e.g. when a change is taking place
-
The pacinian receptor
- Sudden pressure excites receptor
- Transmits a signal again when pressure is released
Define receptive fields.
The receptive field is the region on the skin which causes activation of a single sensory neuron when activated
What is two point discrimination?
- Minimum distance at which 2 points are perceived as separate
- Related to the size of the receptive field
How is the dorsal horn neurone divided?
- Those with axons that project to the brain (projection neurons)
- Those with axons that remain in the spinal cord (interneurons)
Define Lateral inhibition.
Lateral inhibition enhances the difference between adjacent inputs.
- A receptive field can overlap with another receptive field
- Difficult to disntiguish between 2 stimulus locations
- Lateral inhibition prevents the overlap of receptive fields
- Facilitates pinpoint accuracy in localisation of the stimulus
- Mediated by inhibitory interneurones within the dorsal horn of spinal cord
- Facilitates enhanced sensory perception (discrimination)
Describe the ascending pathway of touch and proprioception
The dorsal column system
- Innocuous mechanical stimuli
- Fine discriminative touch
- Vibration
- A-beta fibres enter via the dorsal horn and enter the ascending dorsal column pathways
- Information conveyed from lower limbs and body (below T6) travel ipsilaterally along the gracile/cuneate tract
1st order neurones terminate in the medulla
- Fibers in the Gracile tract have their first synapse in the gracile nucleus
- Fibres in the cuneate tract have their first synase in the cuneate nucleus
2nd order neurones cross in the medulla
- Second order axons ducussate (cross the midline) on the caudal medulla
- Forms the contralateral medial lemniscus tract
- The axons of the second order neurones terminate in the ventral posterior lateral nucleus of the thalamus (VPL)
3rd order neurons terminate in the somatosensory cortex
- 3rd order neurons terminate in the somatosensory cortex
- Size of somatotopic areas is proportional to density of sensory receptors in that body region (somatosensory homunculus)
- Pain and temperature localisation not as precise
Describe the ascending pathway of pain, temperature and crude touch
This is the spinothalamic (anterolateral pathway)
‘’Crude touch is a sensory modality that allows the subject to sense that something has touched them, without being able to localize where they were touched (contrasting fine touch)’’
Crude touch is mediated by Adelta fibres (Free nerve ending). Fine touch is mediated by Abeta fibres (Meissner’s corpuscles)
1st order neurons terminate in the dorsal horn
- Primary afferent axons terminate upon entering the spinal cord
- Second order neurons decussate immediately in the spinal cord and form the spinothalamic tract
2nd order neurons terminate in the thalamus
- 2nd order neurons terminate in the ventral posterior lateral (VPL) nucleus of the thalamus
- 3rd order neurons from the VPL project to the somatosensory cortex
Descrive anterior spinal cord lesion.
- Blocked anterior spinal artery causes ischemic damage to the anterior part of the spinal cord
- Spinothalamic tract damage causes pain and temperature loss below the level of the lesion
- Retained light touch, vibration and 2-point discrimination due to intact dorsal columns
Describe nociceptors.
- A-delta fibres mediate sharp, intense or first pain
- Type 1: noxious mechanical
- Type 2: noxious heat
- C fibres mediate dull, aching or second pain
- Noxious thermal, mechanical and chemical stimuli (polymodal)
Describe the first synpase in the pain pathway.
From A-delta/C fibre to Dorsal horn neuron
Describe the pain pathway in terms of sensory and emotional
Describe the pain matrix.
- Cortex
- SI
- SII
- Insula cortex
- Anterior cingulate cortex
- Prefrontal cortex
- Amygdala
- Cerebellum
- Brainstem
How can nociceptive input be gated?
- Inhibition of primary afferent inputs before they are transmitted to the brain through ascending pathways
-
Descending control pathways: Facilitation and inhibition of nociceptive processing in the dorsal horn by:
- Serotonin
- Noradrenaline
Summarise the mechanisms of somatosensory disruption
-
Central Sensitisation
- Adjacent A-delta fibre sensitisation in the dorsal horn- Decreases threshold to peripheral stimuli at an adjacent site to the injury
- Expansion of receptive field
- Spontaneous pain
-
Peripheral Sensitation
- C-fibre sensitisation in periphery - decreases threshold to peripheral stimuli at the site of injury
Define Hyperalgesia.
Increased pain from a stimulus that normally provokes pain.
Allodynia
pain due to a stimulus that does not normally provoke pain
