Sensory Systems (and Physiology of Pain)

Question 1

Describe the range of sensory receptors

Answer 1

Each type of sensory information is associated with a specific receptor type responding to a specific sensory modality

i.e. mechanoreceptors, chemoreceptors, thermoreceptors, nociceptors, proprioceptors

Question 2

How do sensory receptors initiated?

Answer 2

Respond to stimulus over a specific area, called the receptive field

Question 3

What is signal transduction?

Answer 3

All sensory receptors transduce their adequate stimulus into a depolarisation, the receptor (generator) potential.

Question 4

What does the size of the receptor potential reflect?

Answer 4

Intensity of stimulus

Question 5

What is the function of the receptor (generator) potential?

Answer 5

Evokes firing of action potentials for long distance transmission

• The frequency of AP encodes intensity of stimulus
• Receptive field encodes location of stimulus

Question 6

What do receptors give information on?

Answer 6

Modality, intensity and location of the stimulus

Question 7

What determines acuity?

Answer 7

Density of innervation and size of receptive fields

Question 8

What are the three different types of primary afferent fibres which mediate cutaneous sensation?

Answer 8

• Aβ
• Aδ
• C

Question 9

Describe Aβ primary afferent fibres

Answer 9

Large myelinated (30-70m/s) touch, pressure, vibration

Question 10

Describe Aδ primary afferent fibres

Answer 10

Small myelinated (5-30m/s) cold, ‘fast’ pain, pressure

Question 11

Describe C primary afferent fibres

Answer 11

Unmyelinated fibres (0.5-2m/s) warmth, ‘slow’ pain

Question 12

What are the two primary afferent fibres responsible for proprioception?

Answer 12

Aβ and Aα (i.e. muscle spindles, golgi tendon organs)

Question 13

Describe the pathway of mechanoreceptive (Aβ and Aα) fibres

Answer 13

1. Project straight up through ipsilateral dorsal columns
2. Synapse in cuneate and gracile nuclei of the medulla
3. The 2nd order fibres cross over at the decussation in the brainstem and project to reticular formation, thalamus and cortex

Question 14

Describe the pathway of thermoreceptive and nociceptive (Aδ & C) fibres

Answer 14

1. Synapse in the dorsal horn
2. 2nd order fibres cross over the midline in the spinal cord
3. Project up through the contralateral spinothalamic (anterolateral) tract to reticular formation, thalamus and cortex

Question 15

What are the effects of damage to dorsal columns?

Answer 15

Causes loss of touch, vibration, proprioception below lesion on ipsilateral side

Question 16

What are the effects of damage to anterior columns?

Answer 16

Causes loss of nociceptive and temperature sensation below lesion of contralateral side

Question 17

Where do the ascending tracts terminates?

Answer 17

In the somatosensory cortex (S1) of the post central gyrus

Question 18

What produces the sensory homunculus?

Answer 18

Endings are grouped according to the location of the receptors

Extent of representation is related to the density of receptors in each location

Question 19

What are the three different types of processing in sensory pathways

Answer 19

Adaptive, convergence and lateral inhibition

Question 20

Describe adaptive processing in sensory pathways

Answer 20

Change in sensitivity of your perception of a stimulus – change over time in the responsiveness of the sensory receptor to a constant stimulus.

Question 21

Describe convergence processing in sensory pathways

Answer 21

Saves on neurones, but reduces acuity and may underlie referred pain

Question 22

Describe lateral inhibition processing in sensory pathways

Answer 22

• Activation of one sensory input causes synaptic inhibition of its neighbours
• Gives better definition of boundaries
• Cleans up sensory information

Question 23

List seven types of pain

Answer 23

• Sharp stabbing vs diffuse, throbbing pain
• Fast (initial) pain vs slow (delayed) pain
• Acute vs chronic pain
• Visceral pain
• Referred pain
• Phantom limb pain

Question 24

How in signal transduction in nociceptors are activated?

Answer 24

• Low pH, heat (via ASIC, TRPV1 etc)

* Local chemical mediators (i.e. bradykinin, histamine, prostaglandins)

Question 25

How do reflexes begin?

Answer 25

Activation of neurone free nerve endings. Ion channels responding to a variety of chemical, mechanical and thermal stimuli creates a graded potential that trigger AP if the stimulus is strong enough

Question 26

Describe how reflexes continue after AP is initiated?

Answer 26

The primary sensory neurones from nociceptors (Aδ & C) terminate in the dorsal horn of the spinal cord. Ascending pathways enter the thalamus and sensory areas of the cortex.

Question 27

What is Gate Control Theory?

Answer 27

Aβ fibres carrying sensory info abut mechanical stimuli help block pain transmission. They synapse on inhibitory interneurone and enhance the interneuron’s inhibitory activity. If simultaneous stimuli reach inhib. Interneurons from the Aβ and C fibres, the integrated response is partial inhibition of the ascending pain pathway so that pain perceived by the brain is lessened. The inhib. Interneuron release opiod peptide (endorphins) which inhibit transmitter release from Aδ & C fibres

Question 28

What are four analgesics?

Answer 28

• NSAIDs
• Local anaesthetics
• Trans-cutaneous electric nerve stimulation (TENS)
• Opiates (i.e. morphine)

Question 29

Why are NSAIDs analgesics?

Answer 29

• Prostaglandin sensitise nociceptors to bradykinin

* They inhibit cyclo-oxygenase which converts arachidonic acid to prostaglandins

Question 30

What are the actions of local anaesthetics?

Answer 30

Block Na action potential and therefore all axonal transmission

Question 31

What are the actions of opiates?

Answer 31

• Reduce sensitivity of nociceptos
• Blocks transmitter release in dorsal horn (hence epidural administration)
• Activate descending inhibitory pathways