Somatosensation and Pain Flashcards
Detail the different types of mechanoreceptors:
Meissner corpuscle: RAI, surface, most sensitive to 10-50Hz
Merkel cell: SAI, surface, sharpest spatial resolution using piezo2 channel
Pacinian corpuscle: RAII, deep
Ruffini endings: SAII, deep, best for skin stretching
Free nerve endings: general sensation
What is the Nyquist theory?
Suggest >2 receptors needed to distinguish two points
Highest density of receptors at fingertips and lips.
Describe the different thermoreceptors and their positioning.
Found in dermis, skeletal muscle, liver and hypothalamus.
Trpv1 channel: heat >43C and capsacin causes Ca2+/Na+ entry
Trpm8 channel: cold <28C and menthol
Explain how labelled line coding causes paradoxical cold:
Activity of a cold fibre is experienced as cold irrespective of physical nature of stimulus
Cold receptors can express Trpv1 as well meaning cold experienced when hot if over large area:
- If large area is exposed to heat – sensed as painful/hot
- If small area exposed to heat – cold sensed
Describe the range of nociceptors in the dermis:
- Trpv1/2/3 = extreme heat
- Trpm8 = extreme cold
- ASIC = acid
- DRASIC = acid or high pressure (injection)
- Polymodal receptors respond to a variety of noxious stimuli (chemical irritants which indicate cell damage)
Firing rate increases with destructiveness of stimulus.
Which types of axons relay information to the thalamus for somatosensation?
Myelinated Axons (Largest)
- Aα – proprioception
- Aβ – touch sensation
- Aδ – nociception (smaller fibre)
Unmyelinated axons = C-fibres
Pain felt by bi-stimulation of Aδ and C fibres
What is the difference between the dorsal horn and column?
Horn = grey matter
- Nerve cell bodies
Column = white matter
- Nerve axons
Describe the DC-ML pathway organisation:
Aα and Aβ (large) fibres bifurcate on entry to spinal cord:
- Long branch to dorsal column
- Short branch to dorsal horn
Somatotopic organisation:
- Legs medially, arms laterally (since added on higher up)
Axons leave dorsal column, cross sides in medulla and enter thalamus at the medial lemniscus.
Which parts of the dorsal horn receive nociceptive stimulation?
Spinothalamic tract:
- Stimulation of lamina I from Aδ and C fibres
- Aβ gives position information in parallel - innervated lamina IV
- Cross over at spinal cord level to contralateral side
What is Brown-Sequard syndrome and what does it show?
Hemisection of spinal cord:
- Loss of temperature and pain sensation contralateral
- Loss of mechanosensation on ipsilateral
Describe organisation in the somatosensory cortex
Columns: preserve the area of the body relevant
- All neurons in a column receive information from same skin area
- Individual neurons respond to a single modality (e.g. RAI, hot pain, vibration…)
Layers: give inhibition or excitation information
- Spiny stellate cells = excitatory
- Smooth stellate cells = inhibitory
Whole representation is homunculus (Brodmann’s map)
What is the role of the Thalamus in somatosensory organisation?
Assimilates information (from DC-ML and nociceptive pathways) using summation and relays to the somatosensory cortex
- Input into layer IV
- Output from layer I
What are the stages nociceptive signalling?
- Transduction: (primary neuron such as Aδ stimulation)
- Conduction: secondary neuron activated in dorsal horn
- Transmission: tertiary neuron assimilates information in horn and crosses contralaterally. Relays through 3 spino pathways
- Modulation: response of secondary neurons can be supressed/facilitated by descending modulation from the periaqueductal gray matter PAG)
- Perception: information decoded in the cingulate cortex
Describe the three parallel transmission pathways for nociception in the dorsal horn:
Spinothalamic tract (STT) (nociceptive input)— laminae I, V , VI, VII
Spinomesenphalic tract (SMT) (control and inhibition of pain) – laminae I, III-V, VII, X
Spinohypothalamic tract (SHT) (modulation, movement and emotional ties to pain) – laminae VII, VIII
Relays using NK1 receptors on post-synaptic side.
Describe descending modulation of nociception:
Response of 2nd ary neurons can be supressed or facilitated:
- Inhibiting sensitisation of nociceptors is how ibuprofen, aspirin and cyclooxygenase work
- Descending modulation system headed by periaqueductal gray (PAG) matter (mid-brain) and raphe nucleus – electrical stimulation can produce enough analgesia to perform surgery
