Physiology-Ascending Pathways

Question 1

Where is conscious sensation transmitted to?

Answer 1

Cerebral cortex

Question 2

Where is unconscious proprioception transmitted to?

Answer 2

Cerebellum

Question 3

All pathways for conscious sensation will have at least what 3 neurons?

Answer 3

Primary neuron: Cell body in DRG, Secondary neuron: Cell body in spinal cord or brainstem, Tertiary neuron: cell body is in the thalamus and projects into the primary somatosensory cortex

Question 4

The axon of which neuron is typically the one that will decussate to the opposite of the CNS in the spinal cord or brainstem?

Answer 4

Secondary neuron

Question 5

Where do axons conveying conscious sensation terminate at?

Answer 5

Thalamus

Question 6

What are the three types of somatic receptors?

Answer 6

Mechanoreceptors, nociceptors and thermoreceptors

Question 7

How do the different sensory neuron fibers differ in each of these categories: diameter, myelination, receptors, sensory modality and fiber tract?

Answer 7

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Question 8

What type of nerve root damage would be required to loose sensation in an entire dermatome?

Answer 8

Damage to 3 sequential nerve roots. This is because there is significant overlap between dermatomes.

Question 9

What sensory neuron fibers enter the zone of Lissauer and what fibers do not?

Answer 9

Large (A-alpha/Group 1a/1b) and Intermediate (A-beta/Group III) avoid the gray matter and travel up the white matter in the spinal cord. Small, thinly myelinated (A-delta/Group III) and unmyelinated (C/Group IV) enter the zone of Lissauer and synapse in the spinal cord gray matter.

Question 10

What is a fasciculus?

Answer 10

A bundle of axons with a common origin and termination in the CNS

Question 11

What are the four ascending somatosensory tracts?

Answer 11

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Question 12

Describe the characteristics of each category indicated below in the posterior column (lemniscal) ascending system:

Answer 12

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Question 13

Where do the primary afferent fibers travel from the lower thoracic, lumbar and sacral regions in regard to fasciculus gracilis?

Answer 13

The sacral axons enter first and are most medial. The lower thoracic axons (T7 and below) enter last and are most later. The lumbar axons are sandwiched between the two.

Question 14

What is the posterior funiculus include at T6 and above? Below T6?

Answer 14

T6 and above = Fasciculus gracilis + fasciculus cuneatus. Below T6 = Fasciculus gracilis

Question 15

Where do the primary afferent fibers travel from the upper thoracic, lower cervical and upper cervical regions in regard to fasciculus cuneatus?

Answer 15

Upper thoracics enter 1st and are most medial. Upper cervicals enter last and are most later. Lower cervicals are between the two.

Question 16

Where do the posterior column fibers running in fasciculus gracilis end up synapsing?

Answer 16

Nucleus gracilis in the medulla.

Question 17

Where do the posterior column fibers running in the fasciculus cuneus end up synapsing?

Answer 17

Nucleus cuneus in the medulla.

Question 18

You feel how soft your baby’s head is. The sensory touch stimulus travels up to the DRG, synapses, goes through the posterior column ascending system and into the medulla. At this point the fibers cross. What fibers cross and where do they land?

Answer 18

The primary axon synapses on the secondary neuron in the medulla. Secondary axons (internal arcuate fibers) which cross over and land in the medial lemniscus.

Question 19

Who is the headless man standing?

Answer 19

Medial lemniscus. Fibers from the nucleus gracilis and nucleus cuneus land here and transmit posterior column sensory information for all body parts except for the head.

Question 20

What happens to the orientation of the posterior column fibers in the medial lemniscus as you move from the medulla to the basilar pons?

Answer 20

The man “slips out on his feet” with the lower fibers lateral and upper fibers more medial.

Question 21

Where do the secondary axons of the posterior column ascending system synapse after they travel through the pons? Where do the tertiary axons travel to?

Answer 21

Thalamus (ventral posterolateral nucleus). The tertiary axons travel to the primary somatosensory cortex and form the homunculus.

Question 22

How is the homunculus organized in the primary somatosensory cortex?

Answer 22

Head, hands, arm, legs, genetilia.

Question 23

Describe the characteristics of each category indicated below in the ascending anterolateral spinothalamic system (ALSTS):

Answer 23

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Question 24

Where do the secondary axons cross over in the anterolateral spinothalamic system (ALSTS)?

Answer 24

They start in the Zone of Lissauer and cross over through the anterior white commissure in the spinal cord.

Question 25

Where do the secondary axons synapse in the anterolateral spinothalamic system (ALSTS)?

Answer 25

Ventral posterolateral nucleus (VPL) of the Thalamus

Question 26

How does the somatotopic arrangement compare in the anterolateral spinothalamic system (ALSTS) compare to that in the posterior column medial lemniscus?

Answer 26

ALSTS is much more crude in the anterolateral spinothalamic tract. Note that the most rostral regions are most medially placed.

Question 27

A patient comes to see you complaining of loss of light touch, pain and thermal sensation that started in his neck, moved to his arms and is now in his feet on the right side. CT scan reveals an inter medullary tumor. Is this tumor expanding medially or laterally?

Answer 27

Laterally. It started medial and is causing symptoms to manifest in the ALSTS somatotopic arrangement moving medial to lateral.

Question 28

Describe the characteristics of each category indicated below in the spinoreticular tract of the ascending anterolateral system:

Answer 28

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Question 29

How does the spinoreticular tract integrate with the anterolateral spinothalamic tract?

Answer 29

The ALSTT senses pain. The spinoreticular tract takes that pain sensation and peels off to the reticulothalamic tract (black line below). This tract goes to the inter laminar nucleus of the thalamus. The inter laminar nucleus of the thalamus sends diffuse signals to the cortex telling the brain that there is pain.

Question 30

Describe the characteristics of each category indicated below in the spinomesencephalic tract of the ascending anterolateral system:

Answer 30

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Question 31

Where do the fibers of the spinomesencephalic tract peel off to? What does this cause?

Answer 31

The peri-aqueductal gray matter in the mesencephalon. These neuron bodies have axons that go on to modulate and inhibit the pain response.

Question 32

Describe the characteristics of each category indicated below in the anterior spinocerebellar tract:

Answer 32

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Question 33

What system only has primary afferent neurons in the L1-S5 region?

Answer 33

Anterior spinocerebellar tract. This system deals with posture and lower extremity proprioception.

Question 34

Where do axons cross in the anterior spinocerebellar tract?

Answer 34

They avoid the Zone of Lissauer, but do end up synapsing in the gray matter and crossing over the anterior commissure in the spinal cord.

Question 35

Describe the characteristics of each category indicated below in the posterior spinocerebellar tract:

Answer 35

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Question 36

What nerve fibers run in the area indicated below?

Answer 36

This is the dorsal nucleus of Clarke. It begins at the L1-L2 region and axons of the posterior spinocerebellar tract synapse here. Note that because of its location the PSCT is not present below L2, even though it is composed of axons from L1-S2 levels.

Question 37

Where do future posterior spinocerebellar tract fibers travel if they come in before L2?

Answer 37

Posterior column (fasiculus gracilis).

Question 38

What ascending system is responsible for conveying proprioceptive information about each individual lower extremity muscle?

Answer 38

Posterior spinocerebellar tract.

Question 39

During surgery you create a unilateral lesion of L1-S5 dorsal roots (shown below). What will the clinical symptoms be?

Answer 39

You will lose all cutaneous sensation on the ipsilateral lower extremity.

Question 40

During surgery you create a unilateral lesion of L1 fasciculus gracilis (shown below). What will the clinical symptoms be?

Answer 40

Loss of tactile discrimination, vibration and conscious proprioception in the ipsilateral lower extremity below L1. This is due to loss of posterior column ascending fibers. Pain and temperature will remain in tact. Note that posterior column fibers do not cross, so you have ipsilateral symptoms.

Question 41

What clinical symptoms would present with a lesion in the medial lemniscus?

Answer 41

Loss of tactile discrimination and conscious proprioception on the CONTRALATERAL side of the body because the arcuate fibers cross over in the upper medulla, before the medial lemniscus.

Question 42

A patient presents with a lesion of the anterior spinothalamic tract at L1. What clinical symptoms will this patient manifest with?

Answer 42

Loss of pain and thermal sensation on the CONTRALATERAL extremity. Note that the patient will still have noticeable loss of touch because posterior column fibers remain in tact.

Question 43

A patient comes to see you with a lesion in the anterolateral spinothalamic tract in the medulla. What clinical symptoms will this patient present with?

Answer 43

Loss of pain and thermal sensation on the CONTRALATERAL side of the body. Note that the touch sensation will be maintained because posterior column fibers are preserved.

Question 44

Why do you see dissociated sensory loss in the medulla but not in the midbrain?

Answer 44

In the medulla, the aterolateral spinothalamic tract has a different blood supply and is distanced from the posterior column fibers in the medial lemniscus. In the midbrain, the medial lemniscus begins to lay flat and the fibers begin to intermingle with the ALSTT as they both approach the ventral posterolateral nucleus (VPL). Thus lesions in the midbrain would knock out both of these tracts presenting with CONTRALATERAL loss of touch, vibration, pain and thermal sense.