13_Somatosensory_Q and A_Jonathan

Question 1

What is the schema of most somatosensory pathways?

Answer 1

first-order neuron located in the periphery that has a specialized receptor end
second-order neuron whose cell body is located in the spinal cord or brainstem
third-order neuron whose cell body is located in the thalamus.

Question 2

How do pathways that carry unconscious information differ from the schema of somatosensory pathways?

Answer 2

Pathways that carry unconscious information generally have two neurons instead of three.

Question 3

Through what type of nerves does most sensation for the skin travel in?
What is the exception?

Answer 3

skin of the body&raquo_space; spinal nerves.
Skin of the face anterior scalp&raquo_space; CN V, the trigeminal nerve.
For this reason, sensory information from the face travels on a separate pathway than the rest of the body.

Question 4

What is the Dorsal Column-Medial Lemniscus system (DC-ML)?

Answer 4

carries fine tactile and conscious proprioceptive information from the body.

Question 5

What is the Anterolateral System (ALS) aka spinalthalamic system?

Answer 5

carries pain, temperature and crude tactile information.

Question 6

What is the trigeminal mechanosensory system?

What is the spinal trigeminal system?

Answer 6

Information from the face travels via the trigeminal mechanosensory system (fine tactile and conscious proprioception)
spinal trigeminal system (pain, temperature and crude tactile information).

Question 7

What somatosensory pathways transmit fine tactile and proprioceptive information?

Answer 7

Dorsal Column Medial Lemniscus System (DC-ML) (Body)

Trigeminal Mechanosensory System (Face)

Question 8

What somatosensory pathways transmit pain, temperature, and crude touch information?

Answer 8

Anterolateral system (ALS) aka Spinothalamic system (Body)
Spinal Trigeminal System (Face)

Question 9

What transmits unconscious proprioceptive information?

Answer 9

dorsal spinocerebellar

cuneocerebellar tracts.

Question 10

It is important to realize that there are descending modulatory pathways in the somatosensory system that modify the ascending information.

Answer 10

It is important to realize that there are descending modulatory pathways in the somatosensory system that modify the ascending information.

Question 11

What type of touch…
detect fine touch, motion detection (when objects are moved across the skin)?
detect vibrations, especially from objects such as tools or a pencil?
detect stretching of the skin?

Answer 11

Meissner’s corpuscle – detect fine touch, motion detection (when objects are moved across the skin)
Pacinian corpuscle – detect vibrations, especially from objects such as tools or a pencil
Ruffini’s corpuscles – detect stretching of the skin

Question 12

What receptors detect…
detect touch and pressure, especially from edges, points, corners, curves; useful for resolving form and texture of an object?
detect movement of hair?

Answer 12

Merkel’s disks – detect touch and pressure, especially from edges, points, corners, curves; useful for resolving form and texture of an object
Hair follicle receptor – detect movement of hair

Question 13

What are the three types of proprioceptive receptors?

Where are each found?

Answer 13

Muscle spindles – found in skeletal muscles; detect stretching of a muscle, thus convey information about muscle length
Golgi tendon organs – found in tendons; detect tension in a muscle
Joint receptors – found in the ligaments and capsules of joints; detect joint position

Question 14

Explain the nerve path of DC-ML proprioceptive nerves. Individual questions follow.

Answer 14

Peripheral axons to spinal nerves (neuronal cell bodies in dorsal root ganglia).
dorsal root to ascend through the ipsilateral dorsal column (dorsal funiculus) to the caudal medulla.
Ascending branches inferior to T6 ascend in the medial portion of the dorsal column aka fasciculus gracilis (gracile tract).
Ascending branches superior to T6 ascend in the lateral portion of the dorsal column aka fasciculus cuneatus (cuneate tract).
At the spino-medullary junction, neurons in the fasciculus gracilis and cuneatus project into nucleus gracilis and nucleus cuneatus to synapse with cell bodies of second order neurons.
The second order axons decussate to form internal arcuate tract.
Nerve fibers are now known as the medial lemniscus (a lemniscus is a tract that has a flat shape like a ribbon).
The second order fibers terminate in the ventral posterior lateral (VPL) nucleus of the thalamus, where they synapse with cell bodies of third order neurons.
third order neurons pass through the posterior limb of the internal capsule and project ipsilateral to the postcentral gyrus and posterior part of the paracentral lobule (the paracentral lobule is the medial surface of the pre and postcentral gyri).
The cortex of these gyri is known as the primary somatosensory cortex (SI).

Note: nucleus gracilis and nucleus cuneatus (collectively the dorsal column nuclei)

(some axons enter the gray matter to participate in reflexes, others may modulate ascending information).

Question 15

Ascending branches inferior to T6 ascend in the medial portion of the dorsal column called?
Ascending branches superior to T6 ascend in the lateral portion of the dorsal column called?

Answer 15

Below T6: fasciculus gracilis (gracile tract).

Above T6: fasciculus cuneatus (cuneate tract).

Question 16

Where does the decussation occur for proprioceptive nerves?

Answer 16

in the caudal portion of the medulla, just superior to the spino-medullary junction.

Question 17

What is the decussation tract called for proprioceptive nerves?

Answer 17

internal arcuate tract.

Question 18

Above decussation what are the proprioceptive tracts called?

Answer 18

Nerve fibers are now known as the medial lemniscus (a lemniscus is a tract that has a flat shape like a ribbon).

Question 19

Where do 2nd order proprioceptive nerve fibers terminate?

Answer 19

ventral posterior lateral (VPL) nucleus of the thalamus, where they synapse with cell bodies of third order neurons.

Question 20

What is the course of the 3rd order proprioceptive nerves?

Answer 20

third order neurons pass through the posterior limb of the internal capsule and project ipsilateral to the postcentral gyrus and posterior part of the paracentral lobule

Question 21

What is the paracentral lobule?

Answer 21

the medial surface of the pre and postcentral gyri

Question 22

What is the primary somatosenory cortex?

Answer 22

The cortex of the pre and postcentral gyris.

Question 23

What is Somatotopy?

Answer 23

organization by body region.

Question 24

What is the primary somatosensory cortex (SI)?

Answer 24

the postcentral gyrus and the posterior part of the paracentral lobule.

Question 25

Explain the steps of the trigeminal mechanosensory pathway.

Answer 25

1st order neurons cell bodies in trigeminal ganglion synapse onto…
Priniciple (chief) nucleus of the trigeminal complex in the mid-pons
2nd order neuron desucates in the mid pons
Travels ups the medial lemniscus
The trigeminal fibers within the medial lemniscus form the trigeminothalamic tract, which is also known as the trigeminal lemniscus.
Synapses onto the ventral posterior medial nucleus of the thalamus
3rd order neuron terminates onto to the somatosensory cortex (specifically the face area).

Question 26

What are the three subdivisions of the trigeminal brainstem complex?
What information does each subdivision process?

Answer 26

principal nucleus for mechanosensory information
spinal nucleus for pain and temperature information
mesencephalic nucleus for proprioceptive information from muscles innervated by CN V.

Question 27

Are ALS receptors encapsulated or unencapsulated? Why?

Answer 27

Encapsulated receptors are quick adaptors. We want pain receptors to be slow adaptors: unencapsulated.
Thermoreceptors, nociceptors, and mechanoreceptors for crude touch are unencapsulated receptors (bare nerve endings) that are part of type III and IV neurons.

Question 28

What is crude touch?

Answer 28

Crude touch is a light type of touch that yields little information about an object, other than the sensation of contact.

Question 29

At what temperatures are heat receptors activated?

At what temperatures are cold receptors activated?

Answer 29

29 to 45 degrees C

5 to 40 degrees C

Question 30

What family of receptors are thermoreceptors?

Answer 30

ion channels belonging to the transient receptor potential (TRP) family.

Question 31

What stimuli do nociceptors detect?

Answer 31

Unique because detect, Mechanical, Thermal, and Chemical
Activated only when “intense” stimuli is presented: high receptor thresholds.
Although there are three types of stimuli, pain is pain

Question 32

What are the categories of pain perceptions axons?

Answer 32

A (type III) myelinated axons conduct “fast pain” sensations that are sharp and well-localized.
C (type IV) unmyelinated axons conduct “slow pain” sensations, i.e. poorly localized sensations that are burning and long-lasting.

Example: when you cut your finger – first you feel a sharp pain where you were cut; afterwards your entire finger hurts (aching or throbbing pain)

Question 33

What do A (type III) myelinated axons conduct?

What do C (type IV) unmyelinated axons conduct?

Answer 33

A (type III) myelinated axons conduct “fast pain” sensations that are sharp and well-localized.
C (type IV) unmyelinated axons conduct “slow pain” sensations, i.e. poorly localized sensations that are burning and long-lasting.

Question 34

What is TRPV1?

Answer 34

TRPV1 (VR1) receptor which is activated by heat, acid, and capsaicin (a noxious chemical found in chili peppers).

Additional channels have been discovered in the TRP family as well as the degenerin family of proteins (DEG/ENaC).

Question 35

What is Allodynia?

Answer 35

Nociceptors are sensitized (i.e. their thresholds are lowered) by chemicals released during inflammation.
innocuous stimulation of the damaged area is perceived as painful ex: sunburn and a back scratch

Question 36

What is hyperalgesia?

Answer 36

painful stimuli in the damaged area are perceived as more painful than normal

Question 37

What inflammatory mediators are involved in allodynia and hyperalgesia?

Answer 37

prostaglandins and bradykinin
Hystamine from mast cells
Substance P from nociceptors themselves

5-HT = serotonin; CGRP = calcitonin gene-related peptide

Question 38

Explain the course of the ALS pain fibers.

Answer 38

Primary neuron cell bodies in Dorsal root ganglion
At spinal nerve level bifurcates: some synapse to 2nd neurons. Some travel up or down the spine in the dorsolateral tract of Lissauer (a small tract adjacent to the dorsal horn), and then enter the dorsal horn.
and synapse with other 2nd neurons at other spinal levels
2nd neurons decussate immediately through the anterior white commisure
Then travel anterior laterally up the Spinalthalamic Tract (STT)
third order neurons cell bodies are in the VPL nucleus and project through the posterior limb of the internal capsule to the primary somatosensory cortex.
Other axons that are concerned with emotional aspects of pain and pain modulation project to a variety of locations, such as the reticular formation of the medulla and the periaqueductal gray matter of the midbrain.

Question 39

What is the dorsolateral tract of Lissauer?

Answer 39

Pain nerves in the ALS system At spinal nerve level bifurcates: some synapse to 2nd neurons. Some travel up or down the spine in the dorsolateral tract of Lissauer (a small tract adjacent to the dorsal horn), and then enter the dorsal horn.

Question 40

Where do the emotional aspects of pain 2nd order neurons synapse?

Answer 40

the reticular formation of the medulla and the periaqueductal gray matter of the midbrain.

Question 41

What are the Laminae of the Spinal Gray Matter?

Answer 41

The gray matter of the spinal cord is divided into horns, they further divided into layers (laminae).

Question 42

What layers do 1st order neurons synapse with 2nd order neurons? (ie, the 2nd order neuron cell bodies are found within these layers)

Answer 42

1st order neurons synapse with 2nd order neurons at specific layers I, II, and V
I is the marginal zone
II is the Substantia Gelatinosa
V is ?

Question 43

What are the names of the layers I through VI?

Answer 43

I marginal zone
II Substantia Gelatinosa
III and IV Nucleus Proprius
V?
VI Base of the Dorsal Horn

Question 44

Explain the course of the Spinal Trigeminal System.

Pain, Temperature, and Crude Touch from the face

Answer 44

1st order cell bodies in trigeminal ganglion
Enter the pons and descend in the Spinal Trigeminal Tract to the caudal medulla and upper cervical area
Synapse with 2nd order neurons in the Spinal Nucleus of the Trigeminal Brainstem (one of three sections of the trigeminal brainstem)
Decussate and travel up the Trigeminothalamic Tract
Synapse with 3rd order neurons at the Ventral Posterior Medial Nucleus
The third order neurons of the trigeminal system project to the face area of the somatosensory cortex through the posterior limb of the internal capsule.

Note: the Spinal Nucleus of the Trigeminal Brainstem extends from the midpons to the cervical spinal cord.

Question 45

What is the periaqueductal gray matter of the midbrain?

Answer 45

Descending pathways for pain modulation
These pain-modulating pathways project primarily to neurons (nociceptive afferents, interneurons, projection neurons) in the dorsal horn, which in turn regulate the activity of ascending pain pathways (e.g. to reduce the ascending information so pain is diminished).
There are similar pathways that project to the spinal trigeminal nucleus for facial pain.

Question 46

How do pain modulating pathways work?

Answer 46

use endogenous opiate peptides (enkephalins, endorphins, and dynorphins) as neurotransmitters.

Question 47

Explain the Gate Theory of Pain.

Answer 47

Example:
stub your toe: smaller pain fibers (A and C) fibers stimulated
Rub the toe to make it feel better: large diameter mechanosensory fibers (A and A) stimulate an inhibitory interneuron to diminish the pain

Details
Figure: Purves 10.8. Another group of neurons that modulates pain are the large-diameter first order mechanosensory fibers (A and A) traveling with the smaller pain fibers (A and C) in the dorsal roots. When the large fibers are stimulated, they stimulate inhibitory interneurons in the dorsal horn that suppress pain fibers. This theory is called the gate theory of pain, because mechanosensory fibers act as a gate to allow or suppress pain information from traveling to higher centers. One example that provides support for this theory is why you instinctively rub a stubbed toe or other injured body part. In the clinical setting, transcutaneous electrical nerve stimulation (TENS) devices are used to reduce chronic pain by
stimulating A and A fibers.

Question 48

In the Spinocerebellar pathway.
What are two important tracts?
How many neurons are in relay?
Ipsilateral or contralateral?
Conscious or unconsious?

Answer 48

dorsal spinocerebellar tract and the cuneocerebellar tract.
convey proprioceptive information directly to the cerebellum via a two-neuron chain (unlike most three-neuron sensory pathways).
not consciously perceived since it does not reach the cerebral cortex.
Ipsilateral

Note: Similar to the DC-ML, information in the Spinocerebellar pathways from the lower part of the body travels on a different tract than that from the upper part of the body. Note that both of the above-mentioned tracts are ipsilateral (fibers do not cross).

Question 49

Explain the course of the Spinalcerebellar pathway.
Where do 1st order fibers below the neck synapse?
Through what tract do 2nd order fibers of the (below the neck) spinal cerebellar pathway ascend in?

Answer 49

1st order fibers below the neck synapse with second order neurons in the dorsal nucleus of Clarke (a nuclear column found in the intermediate zone of spinal gray matter between C8-L3).
Fibers below L3 ascend in the ipsilateral gracile tract before synapsing.
2nd order fibers ascend to the cerebellum in the dorsolateral funiculus as the dorsal spinocerebellar tract (DSCT).

Question 50

Explain the course of the Spinalcerebellar pathway.
Where do 1st order fibers from the cervical region ascend in? synapse with?
Through what tract do 2nd order fibers of the (from the cervical region) spinal cerebellar pathway ascend in?

Answer 50

1st order fibers from the cervical region ascend to the medulla in the cuneate tract and synapse with second order fibers in the external (lateral) cuneate nucleus.
2nd order fibers travel with the DSCT to the cerebellum; these fibers form the cuneocerebellar tract.