06_Ascending Sensory Pathways of the Spinal Cord and Brainstem

Question 1

What are the eight ascending sensory pathways of the spinal cord and brainstem?

Answer 1

1. DCML - Dorsal column medial lemniscal pathway
2. Trigeminal lemniscal pathway
3. (Lateral) Spinothalamic tract
4. Trigeminothalamic tract
5. DSCT - Dorsal Spinocerebellar tract
6. SCCT - Spinocuneocerebellar tract
7. VSCT - Ventral spinocerebellar tract
8. RSCT - Rostral spinocerebellar tract

Question 2

What sensations do DCML and Trigeminal lemniscal carry?

Answer 2

DCML - Discriminative tactile sensations from the body

Trigem Lemniscal - Discriminative tactile sensations from the face

Question 3

What are the five discriminative tactile sensations?

DCML and Trigeminal lemniscal pathways

Answer 3

1. Discriminative touch
2. Two-point discrimination
3. Vibration
4. Kinesthesis (aware of movements of legs/arms)
5. Sterognosis (recognition via texture, size, spatial, and temp)

Question 4

DCML

- Neuronal pathway, including decussation

Answer 4

1st Order: Dorsal root ganglia

2nd Order: Nucleus gracilis (LE) and nucleus cuneatus (UE) - ascend as Medial lemniscus

Decussation - internal arcuate fibers (medial lemniscus) at medulla midline

3rd Order: VPL in thalamus

Destination: Primary somatosensory tract via corona radiata

Question 5

DCML and Trigeminal leminscal

- Afferent input

Answer 5

A-beta afferents

Question 6

Trigeminal leminscal

- Neuronal pathway, including decussation

Answer 6

1st Order: Trigeminal ganglion in Pons

2nd Order: Main sensory nucleus (Trigeminal leminscus)

Decussation - Main sensory nucleus neurons at Pons

3rd Order: VPM in thalamus

Destination: Primary somatosensory tract via corona radiata

Question 7

What sensations do (lateral) Spinothalamic tract and Trigeminothalamic tract carry?

Answer 7

(lateral) Spinothalamic tract - Pain, temp, and crude touch from body

Trigeminalthalamic tract - Pain, temp, and crude touch from face

Question 8

(lateral) Spinothalamic tract

- Neuronal pathway, including decussation

Answer 8

1st Order: Dorsal root ganglia (A-delta and C afferents) via lateral regions of Lissauer’s tract

2nd Order: Substantia gelatinosa of Rexed’s lamina II

Decussation: Spinothalamic tract ascends 1-2 levels and crosses @ anterior white commissure

3rd Order: VPL in thalamus

Destination: Primary somatosensory tract via corona radiata

Question 9

Some fibers of the spinothalamic tract terminate in the “brainstem reticular formation”. What are these fibers named?

Answer 9

Spinoreticular tract fibers

Question 10

Trigeminothalamic tract

- Neuronal pathway, including decussation and destination

Answer 10

1st Order: Trigeminal ganglion (A-delta and C afferents)

Descend ipsilaterally

2nd Order: Spinal nucleus of trigeminal nerve (trigeminal leminscus)

Decussation: Medulla

3rd Order: VPM in thalamus

Destionation: Primary somatosensory tract via corona radiata

Question 11

DSCT (Dorsal spinocerebellar tract)
SCCT (Spinocuneocerebellar tract)
- Sensations carried

Answer 11

DSCT - Subconscious proprioception of LE and trunk

SCCT - Subconscious proprioception of UE

(Fine coordination of posture and movement)

Question 12

DSCT

- Neuronal pathway, including decussation and destination

Answer 12

1st Order: DRG (Group Ia, II and Ib)

2nd Order: Clark’s column in dorsal horn T6 and below - also alpha motor neurons directly

Destination: Cerebellum (via Inferior cerebellar peduncle)

No decussation, ipsilateral

Question 13

SCCT

- Neuronal pathway, including decussation and destination

Answer 13

1st Order: DRG (Group Ia, II, and Ib)

2nd Order: Cuneate nucleus

Destination: Cerebellum (via Inferior cerebellar peduncle)

Question 14

VSCT (Ventral spinocerebellar tract)
RSCT (Rostral spinocerebellar tract)
- Sensations carried

Answer 14

VSCT - Info about descending motor signals to thorax and lumbar SC to cerebellum

RSCT - Info about descending motor signals to cervical SC to cerebellum

Question 15

VSCT

- Neuronal pathway, including decussation and destination

Answer 15

1st order: diffuse neurons in Lamina V, VI, and VII of thorax and lumbar SC

Decussation: Anterior white commissure

Destination: Superior cerebellar peduncle (bilateral)

Question 16

RSCT

- Neuronal pathway, including decussation and destination

Answer 16

1st Order: diffuse neurons in Lamina V, VI, and VII of cervical SC

No decussation

Destination: Ipsilateral cerebellum via Superior and Inferior cerebellar peduncles

Question 17

What is a reflex?

Answer 17

Involuntary, stereotyped response to a sensory input

Question 18

Most reflexes involve what structures?

Answer 18

1. A receptor
2. Afferent neuron
3. Efferent neuron

Question 19

Why does the stretch reflex represent the simplest segmental reflex?

Answer 19

Only two neurons (afferent and efferent)

Most neurons involve one or more interneurons, whereas stretch reflect is monosynaptic

Question 20

What are other names for the stretch reflex?

Answer 20

A. Knee jerk reflex (patellar tendon)
B. Deep tendon reflex (DTR)
C. Myotatic reflex
D. Ia monosynaptic reflex

Question 21

Stretch reflex

• Mechanism
  
  • What afferent neurons?
  • What gets excited?
  • What contracts?

Answer 21

A muscle stretch activates Ia (and Group II) afferents of the muscle spindle.

This activation monosynaptically excites alpha motor neurons, causing contraction of the muscle in which the muscle spindle lies (i.e. homonymous muscle).

Question 22

What proprioceptor is notably not involved in the stretch reflex?

Answer 22

Golgi tendon organs are not involved