Sensory Systems Flashcards
what is the receptive field?
the specific area that a receptor responds to
what do all sensory receptors transduce the adequate stimulus into?
a depolarisation, the receptor generator potential
what does the size of the receptor potential encode?
the intensity of the stimulus
what does the receptor potential then evoke?
firing of action potential for long distance transmission
what does the frequency of the action potential encode?
the size of the stimulus
what determines acuity?
the density of innervation and size of receptive fields
what are A beta afferent fibres?
they are large myelinated fibres sensing touch, pressure and vibration
what are A omega afferent fibres?
small myelinated fibres sensing cold, fast pain and pressure
what are C afferent fibres?
unmyelinated fibres sensing warmth, slow pain
what primary afferent fibres mediate proprioception?
a alpha and A beta fibres
what type of fibres are mechanoreceptive fibres?
> A alpha
> A beta
where do mechanoreceptive fibres synapse in?
the cuneate and gracile nuclei
where do the 2nd order mechanoreceptive fibres project to?
> reticular formation
thalamus
cortex
what fibres are the thermoreceptive and nociceptive fibres?
A omega and C fibres
where do thermoreceptive and nociceptive fibres synapse?
in the dorsal horn
where do second order mechanoreceptor fibres cross over?
in the brainstem
where do thermoreceptive and nociceptive fibres cross the midline?
in the spinal cord
where do the thermoreceptive and nociceptive fibres project to?
reticular formation
thalamus
cortex
how will damage to the dorsal column affect sensory information?
there will be loss of touch, vibration, proprioception below lesion on the ipsilateral side
how will damage to the anterolateral quadrant affect transmission of sensory information?
there will be loss of nociceptive and temperature sensation below the lesion on contralateral side
where is the ultimate termination of sensory information?
the somatosensory cortex of the postcentral gyrus
how are the endings of sensory fibres grouped together?
they are grouped according to the location of the receptors
what produces a sensory homunculus?
grouping of ending of sensory nerve fibres according to the location of their receptors. the extent of the representation is related to the density of receptors in each location.
describe the action potential firing rate in a rapidly adapting neuron when there is a prolonged stimulus
action potential are fired at the beginning of the stimulus then after a brief period of time they stop firing. they fire again when the stimulus stops.
what does convergence reduce?
acuity
what is lateral inhibition?
activation of one sensory input causes synaptic inhibition of its neighbours giving a better definition of boundaries and cleaning up sensory information
describe how opioid peptides (endorphins) “close the gate” and inhibit A omega and C neurons
inhibitory interneurons release opioid peptides that inhibit transmitter release from A omega and C fibres
describe gate closing in segmental control
activity in A alpha and beta fibres activates inhibitory interneurons that inhibit neurotransmitter release from A omega and C fibres
what do prostaglandins sensitise?
nociceptors to bradykinin
how do NSAIDs work as an analgesic?
they block cyclo-oxygenase that generates prostaglandins
how do local anaesthetic work?
they block Na+ action potential and therefore all axonal transmissions
how do opiates work?
> reduce the sensitivity of nociceptors
block transmitter release in the dorsal horn
activate descending inhibitory pathways
how does trans cutaneous electrical nerve stimulation?
> activation of large diameter mechanoreceptor fibres in the same segment as the stimulus
this stimulates the release of endogenous opioid peptides from interneurons in the dorsal horn
this inhibits synaptic transmission between primary afferent fibres and other fibres.
