The Somatosensory System: Mechanoreceptors

Question 1

How can a physical or chemical stimulus cause excitation of primary afferent fibres

Answer 1

• open cation-sensitivev ion channels
• influx of +ve ions (mainly Na+)
• depolarisation of nerve terminal (receptor potential)
• AP firing

Question 2

Describe the relationship between the strength of the stimulus and the amplitude of receptor potential

Answer 2

• the amount of depolarisation determines the firing frequency at neuron
• linear

Question 3

Slow adapting tonic/static response

Answer 3

• continuous information to CNS while terminal deformed
• provides information about position, degree of stretch, or force
• e.g. stretch receptors

Question 4

Rapidly adapting phasic/dynamic response

Answer 4

• detects changes in stimulus strength (e.g. rate of movement)
• number of impulses proportional to rate of change of stimulus
• e.g. some muscle spindle afferents, hair follicle afferents

Question 5

Vary rapidly acting very phasic/dynamic response

Answer 5

• responds only to very fast movement, such as rapid vibration
• e.g. Pacinian corpuscle

Question 6

Modality

Answer 6

• principle type of adequate stimulus
• transduced into electrical signal by primary afferent neuron

Question 7

Threshold

Answer 7

• stimulus that is necessary for a response evoke a neuronal response

Question 8

Threshold of somatosensory receptors innervating skin

Answer 8

• low / high-intensity units
• low-threshold units respond to non-damaging and non-painful stimuli
• high-threshold units respond to potentially damaging, not low-intensity stimuli (can be perceived as painful)

Question 9

Response of chemical nociceptors

Answer 9

respond to substances in tissue (inflammation) e.g. prostaglandins, bradykinin, serotonin, histamine, etc.

Question 10

Adaptation rate

Answer 10

• how much the neuron carries on firing in the presence of a continuous and sustained stimulus
• influenced by specialised structures that surround terminals of some sensory neurons
• tonic and phasic components

Question 11

Conduction velocity

Answer 11

• speed at which an AP travels along axon of neuron
• travels faster down Group IV neurons (temp, pain, itch) at 0.5-2.0m per s
• travels slower down Group I neurons (proprioceptors of skeletal muscle) at 80-120 m per s

Question 12

Factors determining receptor field

Answer 12

• site and extent of peripheral termination
• pattern and etent of neurons peripheral terminals in skin
• can be measured using 2-point discrimination