Sensory system: The somatic sensory system
S. Chung (5/13/09)
http://www.physpharm.fmd.uwo.ca/undergrad/sensesweb/index.htm
1. What is the modality in sensory system?
2. Different type of receptors for different sensory mode (exteroceptors, interoceptors)
Somatosensory signaling
1. Receptor (generator) potential - graded potential
* Threshold stimulation - http://itc.gsw.edu/faculty/gfisk/anim/threshold.swf
2. Information coding – frequency & population code of stimulus intensity
3. Physiological consequence of (slow and fast) adaptation of sensory receptors
4. Receptive field

Testing the receptive field: two-point discrimination: density of the receptors
5. Distinguishing each stimuli - labeled-line code of stimulus quality
Somatosensory pathway
1. Dermatome (30 spinal segment): area of skin innervated by each dorsal root
* Overlapping of somatic sensation with adjacent dorsal roots
2. Somatic sensations carried by Dorsal column-medial lemniscus pathway
* Trigeminal nerves
3. Somatotopic organization of somatic sensory information
4. Neural connections: transformation of information
* Lateral (surround) inhibition – contrast enhancement
5. Somatosensory Cortex: each column of area 3 receives input from one afferent type
6. Cortical map plasticity: Phantom limbs
Pain
1. Pain stimuli – mechanical stimulation, temperature, chemicals
2. Hyperalgesia – bradykinin, prostaglandin, substance P
3. Double pain – A fiber, C fiber
4. Referred pain: Convergence
http://www.med.umich.edu/lrc/coursepages/M1/anatomy/html/surface/abdomen/referred.html
5. Spinothalamic pathway
6. Regulation of pain
* gate theory of pain
** descending regulation – periaqueductal gray matter (PAG)
*** endogenous opiates
7. Temperature - thermoreceptors
Modality in Sensory system
Transduction of stimulation energy
Adaptation of sensory receptors
Overlapping of innervation area
Descending regulation of sensory pathway
Problems for “Somatic sensory system”
1. Which is incorrect combination for modality - receptor type?
a) vision – photoreceptor
b) somatic – mechanoreceptor
c) audition – chemoreceptor
d) taste – chemoreceptor
2. The information for the stimulus strength is coded by (
).
a) amplitude of action potential
b) width of action potential
c) frequency of action potential
d) shape of action potential
3. Even though one sensory nerve is disconnected, sensation from the receptive field is partially preserved. How?
a) because the receptive field is wide.
b) because the receptive field is overlapped.
c) because the information from the disconnected neuron is relayed through different pathway other than axon.
d) because of the rapid regeneration of the axon.
4. Which fiber has the slowest conduction velocity?
a) A
b) A
c) A
d) C
5. Rapid decrease of action potential frequency even in the continuous presence of stimulation is called (
a) adaptation
b) convergence
c) lateral inhibition d) divergence
6. Which sense is not using anterolateral pathway?
7.
a) pain
b) mechanical sense with low threshold
c) mechanical sense with high threshold
d) temperature
The generator (receptor) potential in sensory endings is due to
A.
B.
C.
D.
E.
8.
increased rate of active transport by the Na-K-ATPase pump
release of stored Ca2+ from intracellular stores
opening of voltage-gated ion channels
opening of non-selective ion channels
closing of K+ selective ion channels
When a noxious stimulus is applied to a nociceptor, the receptor will discharge
A.
B.
C.
D.
only when the stimulus is first applied
when the stimulus is first applied and again when the stimulus is withdrawn
only while the noxious stimulus is increasing but not when it remains constant
as long as the noxious stimulus is present
).
E. any of the above, depending upon the specific nociceptive ending
9. The relation between stimulus strength and conscious sensation of stimulus intensity can be characterized as (note:
jnd = "just noticeable difference", the smallest increment in stimulus intensity that can be recognized)
A.
B.
C.
D.
E.
10.
jnd increases as stimulus strength increases
jnd decreases as stimulus strength increases
jnd is constant is as long as the stimulus strength is above threshold and below receptor saturation
jnd is variable, as sensation intensity is subjective and depends on past experience
jnd is variable, depending on the sensory modality and submodality
"Lateral inhibition" is a mechanism for
A.
B.
C.
D.
E.
preventing excess stimulation of sensory endings
increasing the precision of stimulus localization
selectively reducing the intensity of pain sensation
increasing sensitivity to low intensity stimuli
responding to complex stimuli, such as "wetness"
11. Loss of pain and temperature sensation from the left side of the face with other facial sensation remaining normal
could be due to a lesion in the
A.
B.
C.
D.
E.
12.
left trigeminal lemniscus
right trigeminal lemniscus
left trigeminal caudalis subnucleus
right trigeminal caudalis subnucleus
left trigeminal mesencephalic nucleus
"Phantom limb" is a example of
A.
B.
C.
D.
E.
sensitization of cutaneous pain endings due to injury
sensory projection
referred pain
pain modulation
psychological hallucination
13. Loss of touch-pressure sensation from dermatomes T5-T6-T7 on the right side with normal sensation otherwise
could be due to
A.
B.
C.
D.
E.
destruction of the right dorsal root ganglia at T5-T6-T7
a tumor pressing on the medial aspect of nerves entering the spinal dorsal horn at T5-T6-T7 on the right side
a lesion of the right dorsal column just above T5
a lesion of the left dorsal column just above T5
a lesion in the ventral white commissure at T5-T6-T7
14. Which of the following would not be expected to reduce the intensity of pain sensation?
A.
B.
C.
D.
E.
electrical stimulation of the dorsal columns
electrical stimulation of the spinothalamic and spinoreticular tracts
electrical stimulation of the periaqueductal gray (PAG)
electrical stimulation of the raphe nucleus
none of the above, as all of the above would reduce pain sensation
15. Opioid peptides are released from
A. nociceptive afferent endings
B. first order nociceptive terminal in the dorsal horn
C. first order mechanosensitive terminals in the dorsal horn
D. terminals of descending fibers from the raphe nucleus in the dorsal horn
E. both C and D above
16.
Aspirin, an agent that inhibits prostaglandin synthesis, can reduce the pain of headache by
A.
B.
C.
D.
E.
17.
reducing the sensitivity of cranial nociceptors
reducing synaptic excitability in the spinal trigeminal nucleus
releasing enkephalin in the spinal trigeminal nucleus
depressing the thalamic intralaminar nuclei
preventing action potential transmission in C-fibers
The nerve endings in dentinal tubules are
A.
B.
C.
D.
E.
motion sensitive encapsulated endings mediating the sense of tooth movement
unencapsulated mechcanoreceptors mediating the sensation of crude touch
nociceptors responsible for the pain of pulpitis (tooth pulp inflammation)
nociceptors contributing to dentinal sensitivity
none of the above; this is a trick question (as usual) as there are no nerve endings in dentine