Sensory Systems Flashcards
Physiological receptors are
sensory receptors
Pharmacological receptors are
protein receptors
All sensory receptors transduce their adequate stimulus into
a depolarisation signal which produces the receptor (generator) potential
What type of potential is the receptor potential?
Graded local detrimental potential
The size of the receptor potential is related to
the intensity of the stimulus
The receptor potential triggers
the firing of action potentials
Action potentials are always the same size, so intensity of the stimulus is determined by
the frequency of action potentials rather than their size
The receptive field encodes
the location of the stimulus, giving information on the modality, intensity and location of the stimulus
Thermoreceptive and nociceptive information travels via what fibres?
A-delta and C fibres
Where do A-delta and C fibres synapse?
In the dorsal horn
A-delta and C fibres come through the dorsal horn and almost immediately synapse onto
the second order fibre
The second order fibres of A-delta and C fibres cross over
in the midline of the spinal cord
Once the second order fibres of A-delta and C fibres have crossed over, they
project up through the contralateral spinothalamic (anterolateral) tract to the reticular formation, thalamus and cortex
Damage to the spinothalamic (anterolateral) quadrant causes
loss of nociceptive and temperature sensation below the lesion on the contralateral side
Mechanoreceptive information travels via what fibres?
A-alpha and A-beta fibres
A-alpha and A-beta fibres project
straight up through the ipsilateral dorsal columns of the spinal cord
A-alpha and A-beta fibres synapse
on the same side, in the cuneate and gracile nuclei
The second order fibres of A-alpha and A-beta fibres cross over
in the midline of the spinal cord and synapse in the brain stem
Once the second order fibres of A-alpha and A-beta fibres have synapsed in the brain stem, the information goes to
the reticular formation, thalamus and cortex
Signal transduction in nociceptors is activated by
Low pH
Heat
Local chemical mediators
When are local chemical mediators released?
When the skin is crushed, leading to depolarisation, action potential firing and excitation of the nociceptors
Bradykinin, histamine and prostaglandins are examples of
local chemical mediators
How is low pH detected and what does this cause?
an acid sensing ion channel (ASIC) which opens its integral ion channel to cause depolarisation, action potential firing and excitation
Noxious heat causes what ion channel to open?
VR1 ion channel
Why might pain originating from the viscera cause referred pain to a somatic structure from the same dermatome?
Visceral pain is generally poorly localised
Convergence of sensory fibres - fewer neurones reduces the acuity
According to the gate control theory, pain signals
are not free to reach the brain as soon as they are generated at the injured tissue/site
Pain signals need to encounter
neurological gates at the spinal cord level which determine whether the pain signals should reach the brain or not
Pain is perceived when
neurological gates give way to the pain signals
Pain is less intense or not perceived when
the neurological gate closes
Peri-aqueductal grey matter cell bodies transmit to the
nucleus raphe magnus
Fibres travel from the nucleus raphe magus down what?
The spinal cord and descending tracts
Function of peri-aqueductal grey matter cell bodies that transmit to the NRM
Activate inhibitory interneurons that release opioid peptides and stop the transmitter release, closing the neurological gates
Function of prostaglandins
release G proteins when stimulated which sensitise the nociceptors to bradykinin
Why do NSAIDs work well against pain associated with inflammation?
They are anti-pyretic and anti-inflammatory
They inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins (prostaglandins sensitise nociceptors)
Local anaesthetics block
the voltage gated sodium channels from transmitting the action potential and therefore block axonal transmission
How does transcutaneous electric nerve stimulation (TENS) work?
Electrically stimulates the nerve fibres to close the gate
How do opiates e.g. morphine work?
Reduce sensitivity of nociceptors
Block transmitter release in the dorsal horn
Activate descending inhibitory pathways
What route of opiate administration is most effective and why?
Epidural
Allows them to be delivered around the spinal cord where they have best access to the synapses at the top levels of the spinal cord and can block transmission
