Somatosensory Physiology

Question 1

What is adequate stimulus?

Answer 1

a stimulus to which a particular receptor responds

Question 2

List the different types of sensory receptors and their adequate stimulus

Answer 2

1. mechanoreceptors/proprioceptors: mechanical displacement
2. chemoreceptors: chemical stimulus
3. thermoreceptors: temperature
4. photoreceptors: light
5. nociceptors: intense, often harmful, stimuli that lead to pain sensation

Question 3

What are the 4 ways sensations are coded

Answer 3

1.modality: what type of sensation is this
2. location: where is this sensation coming from
3. intensity: how strong is this sensation
4. duration: how persistent is this sensation

Question 4

What are the 3 classes of mechanoreceptors

Answer 4

1. tactile
2. proprioceptors
3. baroreceptors

Question 5

What are the 4 tactile mechanoreceptors?

Answer 5

1. Merkel’s discs
2. Meissner’s corpuscles
3. Ruffini endings
4. Pacinian corpuscle

Question 6

Describe transduction mechanisms of cutaneous mechanoreceptors

Answer 6

mechanical displacement= generator potential
physical displacement makes the membrane more permeable to cations
depolarization can lead to an action potential

Question 7

Which mechanoreceptor is different in its action potential transduction? what is the pathway?

Answer 7

Merkel cells are modified epithelial cells which when activated by mechanical displacement release neurotransmitter which binds to receptors on the sensory afferent to initiate an action potential

Question 8

Whats the difference between rapid and slow adapting mechanorecpetors?

Answer 8

-Rapid adapting responses allow for detecting dynamic stimuli
-slow adapting responses allow detection of constant stimuli (pressure/touch)

Question 9

What is the purpose of lateral inhibition?

Answer 9

increases signal strength by quieting nearby “noise” (neurons)

Question 10

Describe thermoreceptors

Answer 10

-free nerve endings with high thermal sensitivity
-detect both warmth and cold although receptor dependent

Question 11

Describe thermoreceptor adaptation

Answer 11

-responsive to changes in temperature rather than constant temperature.
-allows the receptor to adapt to new steady state of firing when temperature is maintained

Question 12

Describe nociceptors

Answer 12

-free nerve endings that respond to intense stimulu
-transduction via transient receptor potential which are non selective cation channels

Question 13

What are the 3 subtypes of nociceptors and what stimuli do they respond to?

Answer 13

-mechanical: strong pressure, sharp objects
-thermal: burning heat, noxious cold
-chemical: pH extremes, environmental irritants, inflammatory mediators

Question 14

How is visceral pain different from somatic pain?

Answer 14

more diffuse, poorly localized and of a longer duration than somatic pain

Question 15

Which afferent fiber conducts signal from non noxious mechanoreceptors?

Answer 15

A beta

Question 16

Which afferent fiber conducts signal from a noxious mechanoreceptor for quick, intense pain?

Answer 16

A delta

Question 17

Which afferent fiber conducts signals from nociceptors?

Answer 17

C

Question 18

What are the 3 major somatosensory tracts? which afferent fibers pair which which tract?

Answer 18

1. Dorsal column medial lemniscal pathway: superficial sensation - A alpha A beta
2. Spinothalamic tract: pain, temp, pressure, and some proprioceptive pain- A delta and C fibers
3. Spinocerebellar tract: unconscious proprioception- A alpha fibers

Question 19

When is sensation considered conscious?

Answer 19

when the signal reaches the cortex

Question 20

Describe gate control theory of regulation

Answer 20

-ascending nociceptor signals can be surpressed by the activity of inhibitory interneurons, when these interneurons are active they prevent signal from reaching the thalamus. Activation of the c fiber causes not only the excitation of the second order neuron, but inhibitory to the interneuron that would normally prevent transmission of signal

Question 21

Why do you feel better when you rub your knee after you bump into somthing?

Answer 21

Because the pressure activation of A beta fibers reactivates the inhibitory interneuron which will decrease the nociceptor signal

Question 22

Describe descending pain modulation

Answer 22

Pain transmission at the level of the dorsal horn can be modulated by supraspinal inputs. The descending pathways originate in the brainstem/other cerebral structures. They release NT which leads to release of endogenous opioids which inhibit pain transmission at the dorsal horn level.