Sensory systems

Question 1

What are the different sensory receptors?

Answer 1

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

Question 2

what is the difference between physiological and pharmacological receptors?

Answer 2

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

Question 3

what is signal transduction?

Answer 3

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

Question 4

explain acuity.

Answer 4

can be tested by the 2-point acuity test- a high density of innovation causes high acuity but does not affect the sensitivity

Question 5

what are the 3 types of primary afferent fibres that mediate cutaneous sensation?

Answer 5

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

Question 6

which of these primary afferent fibres mediate proprioception?

Answer 6

A alpha and A beta

Question 7

where do all primary afferent fibres enter the spinal cord?

Answer 7

dorsal root ganglia

Question 8

describe the transmission of mechanoreceptive sensory information.

Answer 8

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

Question 9

describe the transmission of the thermoreceptive and nociceptive sensory information.

Answer 9

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

Question 10

what would damage to the dorsal column cause?

Answer 10

loss of touch, vibration and proprioception below lesion on the ipsilateral side

Question 11

what would damage to the anterolateral quadrant cause?

Answer 11

loss of nociceptive and temperature sensation below lesion on the contralateral side

Question 12

how is the sensory homunculus produced?

Answer 12

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

Question 13

explain adaptation in the processing of sensory pathways.

Answer 13

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

Question 14

explain convergence in the processing of sensory pathways.

Answer 14

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

Question 15

explain lateral inhibition in the processing of sensory pathways.

Answer 15

activation of one sensory input causes synaptic inhibition of its neighbours which gives better definition of its boundaries

Question 16

explain perception in the processing of sensory pathways.

Answer 16

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

Question 17

why does not all information reach the brain?

Answer 17

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

Question 18

explain the concept of pain.

Answer 18

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)

Question 19

describe signal transduction in nociceptors.

Answer 19

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

Question 20

what then happens to the A delta and C fibres?

Answer 20

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

Question 21

explain the gate control theory of pain.

Answer 21

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

Question 22

how do local anaesthetics work?

Answer 22

block sodium action potential which will therefore block all axonal transmission

Question 23

how does TENS (trans cutaneous electric nerve stimulation) work?

Answer 23

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

Question 24

how do opiates like morphine work?

Answer 24

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

Question 25

how do opiates like morphine work?

Answer 25

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)

Question 26

how can descending controls coming from the Brian also activate inhibitory interneurons?

Answer 26

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