Sensory Systems (Including Physiology of Pain) Flashcards
sensory receptors respond to stimulus over a specific area, what is the area called?
receptive field
what do sensory receptors transduce their adequate stimulus into?
a depolarisation, the receptor (generator) potential
what encodes the intensity of the stimulus
size of receptor potential
then frequency of action potentials
what does the receptor potential evoke?
firing of action potentials for long distance transmission
what encodes the location of the stimulus?
receptive field
what is proportional to duration of a series of action potentials?
stimulus duration
what determines acuity?
Density of innervation, and size of receptive fields
what are 3 types of primary afferent fibres that mediate cutaneous sensation?
Aβ
Aδ
C
describe Aβ fibres
large myelinated (30-70m/s) touch, pressure, vibration
describe Aδ fibres
small myelinated (5-30m/s) cold, “fast” pain, pressure
describe C fibres
unmyelinated fibres (0.5-2m/s) warmth, “slow” pain
what are 2 types of primary afferent fibres that mediate proprioception & mechanoreception?
Aα & Aβ
what are 2 types of primary afferent fibres that mediate thermoreception & nociception ?
Aδ & C
describe the pathway of mechanoreceptive fibres
project straight up through ipsilateral dorsal columns
synapse in cuneate & gracile nuclei
the 2nd order fibres cross over midline in the brain stem
describe the pathway of thermoreceptive & nociceptive fibres
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
where do the Primary afferent fibres project in the brain?
reticular formation, thalamus and cortex
what is affected when there is damage to dorsal columns?
causes loss of touch, vibration, proprioception below lesion on ipsilateral side
what is affected when there is damage to anterolateral quadrant?
causes loss of nociceptive & temperature sensation below lesion on contralateral side
where is the ultimate termination of sensory information?
sensory homunculus
in the somatosensory cortex (S1) of the postcentral gyrus
what is neural/sensory adaption?
change over time in the responsiveness of the sensory system to a constant stimulus
ie action potentials stop firing after putting on clothes
what is the difference between fast and slow adaptation?
Fast adaptation occurs immediately after stimulus presentation i.e., within 100s of milliseconds.
Slow adaptive processes that take minutes-days
what is neural convergence?
One postsynaptic cell receives convergent input from a number of different presynaptic cells (saves on neurones but reduces acuity, may underlie referred pain)
describe lateral inhibition
activation of one sensory fibre causes synaptic inhibition of its neighbours
gives better definition of boundaries
cleans up sensory information
what is chronic and phantom limb pain due to?
neural plasticity
what are nociceptors activated by?
low pH, heat (via ASIC, TRPV1 etc)
local chemical mediators (eg bradykinin etc)
what are the two ways the body inhibits pain?
Segmental controls
Descending controls
what activates the inhibitory interneurones in segmental controls?
activity in Aα/β fibres
describe how Inhibitory interneurones inhibit pain
release opioid peptides (endorphins) that inhibit transmitter release from Aδ/C fibres, hence “closing the gate”
what activates the Inhibitory interneurones in decending controls?
descending pathways from brain:
Peri-aqueductal grey matter (PAG) Nucleus raphe magnus (NRM)
how do Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit pain?
prostaglandins sensitise nociceptors to bradykinin
NSAIDS block prostaglandins so bradykinin signal stays small
how are NSAIDS anti-inflammatory?
they inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins
how do Local anaesthetics inhibit pain?
block Na+ action potential and therefore all axonal transmission
(why area affected is numb)
how do Trans cutaneous electric nerve stimulation (TENS) inhibit pain?
increases activity in Aα/β fibres (segmental control activated)
how do Opiates (eg morphine)
inhibit pain?
reduce sensitivity of nociceptors
block transmitter release in dorsal horn (hence epidural administration)
activate descending inhibitory pathways
