What determines the release of neurotransmitter on a sensory neurone?
The pattern of action potentials arriving at the axon terminal
What determines sensory acuity?
Density of innervation and receptive field
What are the three types of fibres that are responsible for cutaneous innervation?
Describe A beta fibres
Large myelinated (30-70m/s) used for touch pressure and vibration
Describe A delta fibres
Small myelinated (5-30 m/s) cold, fast pain, pressure
Describe C type fibres
Unmyelinated fibres (0.5 - 2 m/s) warmth, slow pain
What type of fibres mediate proprioception?
2 types of primary afferent fibres: –Aα & Aβ eg muscle spindles, golgi tendon organs etc
How do the cutaneous nerves enter the spinal cord?
Via the dorsal route ganglia (or cranial nerve ganglia for head)
What is the travel of mechanoreceptive fibres (A alpha and A beta)?
–project straight up through ipsilateral dorsal columns
–synapse in cuneate & gracile nuclei
–the 2nd order fibres cross over midline (decussate) in the brain stem & project to reticular formation, thalamus and cortex
What is the travel of thermoreceptive and nociceptive fibres (A delta and C)?
–synapse in the dorsal horn
–the 2nd order fibres cross over the midline in the spinal cord
–project up through the contralateral spinothalamic (anterolateral) tract to reticular formation, thalamus and cortex
What is the effect of damage to dorsal columns?
–causes loss of touch, vibration, proprioception below lesion on ipsilateral side
What is the effect of damage to anterolateral quadrant?
causes loss of nociceptive & temperature sensation below lesion on contralateral side
Where is the ultimate termination of transmission of sensory transmission?
Somatosensory cortex (S1) of the post central gyrus - endings are grouped according to the location of the receptors
What determines the representation of nerve termination in the somatosensory cortex?
Related to the density of receptors in each location. Produces a sensory homunculus
What is meant by a a rapidly and a slowly adapting sensory receptor?
What is the effect of convergence?
Saves on neurones
May underlie referred pain
What are specific and non-specific pathways?
What is a lateral inhibitor?
–activation of one sensory input causes synaptic inhibition of its neighbours
What is the effect of lateral inhibition?
Gives better definition of boundaries
Cleans up sensory inhibition
What activates nociceptors?
Low pH, heat (ASIC, TRPV1)
Local chemical mediators (bradykinin, histamine, prostaglandins)
What is the effect of A alpha and beta fibres on A delta and C fibres?
Aα/β fibres activate inhibitory interneurones which release opioid peptides that inhibit transmitter release from A delta and C fibres - ‘closing the gate’
How can the inhibitory interneurones also be activated?
Descending pathways from PAG and NRM can also activate the inhibitory interneurones
How do NSAIDS work?
Prostaglandins sensitise nociceptors to bradykinin bradykinin is released in response to cell damage. Cyclo-oxygenase converts arachidonic acid to prostaglandins. NSAIDs inhibit this enzyme so reduce the nociceptor sensitivity to bradykinin.
What type of pain do NSAIDs work well for?
Pain associated with inflammation
NSAIDs are analgesic, antipyretic and anti-inflammatory
How do local anaesthetics work?
–block Na+ action potential and therefore all axonal transmission
•Trans cutaneous electric nerve stimulation (TENS) ?
Think it may also 'close the gate'?
What are the three ways opiates work?
–reduce sensitivity of nociceptors
–block transmitter release in dorsal horn (hence epidural administration)
–activate descending inhibitory pathways