Sensory Physiology (Pierce)

Question 1

What two schemes are peripheral nerves classified as ?

Answer 1

Their contribution to the compound action potential

Fiber diameter, myelin thickness and conduction velocity (I, II, III and IV)

Question 2

What determines a fibers contribution to the compound action potential?

Answer 2

Conduction velocity (often measured in diagnostic testing along with compound action potential)

Question 3

What are some important properties of sensory receptors?

Answer 3

They are specific for a narrow range of input, perform a common functions in unique ways, convert the signal they receive into change of membrane voltage

Question 4

What is a generator potential?

Answer 4

Stimulus received by the somatosensory receptor produces this potential. When it reaches a threshold it generates the spike potential and carried over to the CNS

Question 5

Role of the thalamus in somatosensory processing

Answer 5

All sensory systems (except for olfaction) relay info to the nuclei of the thalamus before they go to specific places in the cortex. Usually two nuclei involved (one for body and one for face)

Question 6

How are the neurons in the thalamic nuclei arranged?

Answer 6

Neurons performing a certain function are segregated together

Question 7

What is the relationship between receptor density and stimulus strength?

Answer 7

The stronger the stimulus, the more receptors involved.

Question 8

How do somatosensory receptors work?

Answer 8

Encode the intensity of the stimulus as receptor potential > digital pulse code

Question 9

What is receptor adaptation?

Answer 9

Stimulus is unchanged (no change in position or amplitude) for several minutes - neuron response is lost (sensory adaptation)

Question 10

Slowly adapting receptors vs. rapidly adapting receptors

Answer 10

• respond to prolonged and constant stimulation

- respond only at the beginning or end of a stimulus, only active when the stimulus intensity changes

Question 11

What are receptive fields?

Answer 11

Made up of mechanoreceptor fibers that innervate an area of the skin (the receptive field)

Question 12

Describe the significance of 2 point discrimination

Answer 12

It is basically the ability to distinguish that two different objects touching the skin are separate and not one.

Question 13

Tactile acuity (keenness of touch) is highest in ()
Lowes in ()

Answer 13

Fingertips and lips (smallest receptive fields)

Calf, back and thigh (largest receptive fields)

Question 14

How does the size of receptive fields affect 2 point discrimination?

Answer 14

Primary neurons converge on the same field (due to the size), so ends up being perceived as a single point. Not the case for smaller receptive fields.

Question 15

What is presynaptic inhibition? What does this do?

Answer 15

Powerful form of inhibitory control of primary afferents. It is basically a diminished excitatory signal. End effect is that the brain’s ability to localize a signal is improved

Question 16

How does presynaptic inhibition diminish the excitatory signal?

Answer 16

GABA > Cl- influx into axon > hyperpolarizaton > less Ca2+ enters the cell > less NT release
Affects the neighbors of the neurons receiving the stimulus. Signal is narrowed by much by the time it reaches the cortex

Question 17

What is the Somatosensory Area I (SI)?

Answer 17

Primary somatosensory cortex. Takes care of position sense, size and shape discrimination info.

Question 18

What is somatosensory area II (SII)

Answer 18

Secondary somatosensory cortex. Takes care of comparisons/differences between sensation and remembering sensation. Receives input from S1

Question 19

What is the parieto-temporal-occipital association area (PTO)?

Answer 19

Takes care of high level interpretation of sensory inputs. Things like spatial perception, identification etc. Receives input from multiple sensory areas.

Question 20

Describe the doctrine of specific nerve energies and how does this relate to the Law of Projections?

Answer 20

What you “sense” is determined by the receptors connected to the somatosensory pathway. So you can stimulate any part of the afferent pathway and you will perceive a similar sensation.

Question 21

Pain vs nociception

Answer 21

Pain is the sensation. Nociception is the neural process of encoding the noxious stimuli

Question 22

Hyperaesthesia
Hyperalgesia
Allydonia

Answer 22

Increased sensitivity to stimulation
Increased pain from stimulus that normally provokes pain
Pain due to stimulus that does not normally provoke pain

Question 23

Fast vs slow pain

Answer 23

• Impulses travel through thinly myelinated A- fibers. Pain goes away quickly
• Impulses travel through non-myelinated C fibers. Pain persists longer

Question 24

Nociceptors and modalities:

Also what are silent nociceptors?

Answer 24

• Could be thermal, mechanical, chemical or polymodal (can perceive different types of pain sensations).
• Could be related to phenotype switching

Question 25

What are free nerve endings?

Answer 25

Terminal ends of the A-gamma or C pain fibers. They do NOT have specialized receptor cells

Question 26

Peptidergic free nerve endings:

Answer 26

Expresses neuropeptides (Substance P, CGRP) Respond to NGF Includes most visceral and half of cutaneous afferents (chronic inflammation and visceral pain)

Question 27

Non peptidergic free nerve endings:

Answer 27

(basically the opposite of peptidergic) Does not express neuropeptides, responds to GDNF, half of cutaneous afferents and few visceral afferents (diabetic neuropathy)

Question 28

TRP receptors:

Answer 28

Nociceptors on the ends of the nerve endings. Ligand gated and lets Ca/Na/K in. (CANaK) - aka not specialized 3 types: V1, A1, M8

Question 29

Na v I.7 channel What is it? Mutations result in?

Answer 29

Another nociceptor. Permeable to sodium. I type mutation = pain sensitive areas Other mutations result in extreme pain disorder, when channel does not inactivate properly

Question 30

Other pain receptors:

Answer 30

P2X - ATP activated ASIC - H+ activated SP&CGRP, Histamine and kinins

Question 31

What modulates nociceptors and pain processing?

Answer 31

Interneurons and descending inhibition (input is reduced as it makes it way down to the cortex)

Question 32

Describe the gate control theory of pain

Answer 32

No input from free nerve endings so interneuron suppresses the pain pathway and blocks it (inhibitory interneuron). No further signal to the brain = no pain.

Question 33

How is gate control of pain induced (why does massaging the pain make it feel better)?

Answer 33

normal sensory stimulus stimulates the A-beta fiber > Central process synapses to the dorsal horn on the inhibitory interneuron, releases EAA > Activated interneuron releases glycine and inhibits the secondary sensory neuron > blockage of the nociceptive pathway.

Question 34

Describe the pathway of descending inhibition

Answer 34

Opiates, EAA And cannaboids activate the periaqueductal gray (PAG) > Locus coeruleus/raphe > release NE and serotonin on dorsal horn > activation of inhibitory interneurons > which releases opiates > which activate mu receptors on c-fiber terminals > reduction of stimulus from the C-fiber = reduced pain sensation

Question 35

Central vs peripheral sensitization

Answer 35

- Activity dependent synaptic plasticity in the spinal cord which increases hypersensitivity to pain post injury. - change in structures etc. of the PNS resulting in increased pain sensation

Question 36

How does central sensitization happen?

Answer 36

Reduced threshold of the dorsal axons to pain stimulus, alterations to ion channels, expansion of the receptive field Persistent stimulation of various intracellular mechanisms = end result is to be hypersensitive to pain

Question 37

How does peripheral sensitization happen?

Answer 37

Neuroimmune activation (Prostaglandin e.g. bradykinin release from nearby cells after an infection) sensitizes the peripheral nociceptors = hypersensitive to pain

Question 38

How is pain processed in the cortex?

Answer 38

S1 and S2 receive input from nociceptors and overall localizes the pain > insular cortex > interpretation of pain

Question 39

What is the insular cortex?

Answer 39

Located in the insula. Integrates pain related signals and important in pain interpretation. Damage results in asymbolia - failure to interpret symbols

Question 40

Emotional component of pain is thanks to the …

Answer 40

Amygdala

Question 41

How does visceral pain get processed?

Answer 41

Visceral input travels via the autonomics > hypothalamus and medulla (integrates physiological changes associated with visceral pain)

Question 42

General concepts about the different stimuli that activate nociceptors in different organs

Answer 42

Stimuli is not the same to activate nociceptors in the skin vs joints vs. muscles vs. viscera

Question 43

What is referred pain?

Answer 43

Brain needs experience to localize pain. Early visceral pain is not localized and could travel to other areas. This is due to convergence of the afferents on the dorsal horn and antidromic signaling (e.g why you have diffuse flank pain when you have kidney stone)