Sensory systems Flashcards
What are the different sensory receptors?
each type of sensory information is associated with a specific receptor type responding to a specific sensory modality - they may have free nerve endings eg nociceptors, cold receptors or a complex structure eg Pacinian corpuscle
what is the difference between physiological and pharmacological receptors?
physiological (sensory) receptors have terminals in the periphery and respond to specific stimuli and pharmacological (protein) receptors are generally a protein in the postsynaptic membrane
what is signal transduction?
all sensory receptors transduce their adequate stimulus into a depolarisation producing the receptor (generator) potential - the size of this potential encodes the intensity of the stimulus and evokes the firing of the action potentials for long distance transmission - the frequency if the action potentials encodes the intensity of the stimulus and the receptive field encodes the location of the stimulus
explain acuity.
can be tested by the 2-point acuity test- a high density of innovation causes high acuity but does not affect the sensitivity
what are the 3 types of primary afferent fibres that mediate cutaneous sensation?
A beta= large myelinated (30-70m/s)- touch, pressure and vibration
A delta= small myelinated (5-30m/s)- cold, “fast” pain and pressure
C= unmyelinated (0.5-2m/s)- warmth and “slow” pain
which of these primary afferent fibres mediate proprioception?
A alpha and A beta
where do all primary afferent fibres enter the spinal cord?
dorsal root ganglia
describe the transmission of mechanoreceptive sensory information.
mechanoreceptive fibres (A alpha and A beta) fibres come through the dorsal horn of the spinal cord and project straight up through the ipsilateral dorsal columns and synapse a the cuneate and gracile nuclei (at the top of the spinal cord) - 2nd order fibres decussate in the brain stem and project to the reticular formation, thalamus and the cortex on the contralateral side
describe the transmission of the thermoreceptive and nociceptive sensory information.
the thermoreceptive and nociceptive (A delta and C) fibres come through the dorsal horn and almost immediately make a synapse - the 2nd order fibres decussate in the spinal cord and project up through the contralateral spinothalamic (anterolateral) tract to the reticulate formation, thalamus and cortex
what would damage to the dorsal column cause?
loss of touch, vibration and proprioception below lesion on the ipsilateral side
what would damage to the anterolateral quadrant cause?
loss of nociceptive and temperature sensation below lesion on the contralateral side
how is the sensory homunculus produced?
transmission of sensory information ends in the somatosensory cortex in the post central gyrus and the endings are grouped according to the location of the receptors and the extent of representation is related to the density of receptors in each location
explain adaptation in the processing of sensory pathways.
can have rapidly adapting receptors which fire action potentials initially in response to a stimulus but then quickly adapts to the response and stops firing APs and continues until more AP signal to stop or slowly adapting receptors which show initial burst of APs when stimulus is put on but keeps firing throughout the stimulus
explain convergence in the processing of sensory pathways.
2 neurones converging onto the second neurone which saves the number of axons travelling up and down the spinal cord or thalamus to the cortex but reduces the acuity= can’t tell which initial neurone it has originated from and can cause referred pain
explain lateral inhibition in the processing of sensory pathways.
activation of one sensory input causes synaptic inhibition of its neighbours which gives better definition of its boundaries
explain perception in the processing of sensory pathways.
sensory information can come in but it may or may not make a difference to you ie looking at a picture of a scary dog is not going to make you run away but if in real life you would do
why does not all information reach the brain?
as most of it you don’t pay attention to and a massive amount of filtering goes on so that you only focus on the important things and not the things which don’t matter
explain the concept of pain.
it is entirely subjective and only exists inside your head - can be sharp, stabbing pain or diffuse, throbbing pain (initially localised and then spreads due to delay between A delta and C fibres) - can be acute or chronic - can be visceral (pain from inflation of organs) - an be referred (convergence of nociceptors from 2 different regions) - can be phantom limb pain (sensation from amputated limb)
describe signal transduction in nociceptors.
nociceptor fibres are free nerve endings and detect noxious/damaging stimuli in 2 ways and are activated by low pH, heat etc which causes depolarisation and activation of voltage gated Na channels which triggers APs or the other route is when tissue gets damaged local chemical mediators are released like bradykinin, histamine and prostaglandins when bind to G protein couples receptors which causes depolarisation and firing of APs
what then happens to the A delta and C fibres?
come through the dorsal root ganglion and make a synapse in the dorsal horn - the 2nd order fibres decussates and travels up the anterolateral spinothalamic tract on the contralateral side to the thalamus and the cortex
explain the gate control theory of pain.
activity of A beta fibres activates inhibitory interneurons which release opioid peptides (endorphins) that inhibit transmitter release from A delta and C fibres hence closing the gate
how do local anaesthetics work?
block sodium action potential which will therefore block all axonal transmission
how does TENS (trans cutaneous electric nerve stimulation) work?
electric pads on the skin and stimulate electrically - it is easier to stimulate large diameter fibres than smaller diameter fibres and so you can turn up the strength to the point where you are activating A beta fibres but not A delta and C fibres so the inhibitory interneurons close the gate
how do opiates like morphine work?
opioid receptors tend to hyper polarise the terminals keeping them further away from reaching the threshold so more depolarisation is needed before they start firing and so
how do opiates like morphine work?
opioid receptors tend to hyper polarise the terminals keeping them further away from reaching the threshold so more depolarisation is needed before they start firing= reduces the sensitivity of nociceptors and they also block transmitter release in the dorsal horn (across the gate)
how can descending controls coming from the Brian also activate inhibitory interneurons?
they can come from 2 regions= periaqueductal grey matter (PAG) which surrounds the central aqueduct and protest to the nucleus raphe magnus (NRM) which project down the spinal cord and release an excitatory transmitter which activates the inhibitory interneurons which will release the endogenous opioids which inhibit transmission
