Ascending and Descending Pathways

Question 1

What are the 3 important tracts of the CNS? What is each tract for?

Answer 1

• Dorsal Column Pathway; Precise touch, Joint position, Sense, Proprioception
• Spinothalamic Tract; Pain, Temperature, Poorly localised tactile sensation
• Pyramidal Tract (Motor pathway); Corticospinal (limbs/trunk), Corticobulbar (cranial nerve muscle nuclei)

Question 2

Where is the Primary Motor Cortex (Motor Strip/M1)?

What does it control?

What does damage to it cause?

Where is the Primary Somatosensory Cortex (Sensory Strip/S1)?

Which projections does it receive?

What is it concerned with?

What is Somatotopy?

How does the body correspond to the cortex?

Answer 2

• Precentral gyrus of frontal lobe
• Voluntary movement of OPPOSITE half of body
• Weakness, Paralysis

• Postcentral gyrus of parietal lobe
• Receives ascending (sensory) projections for all sensory modalities; light touch, joint position sense, pain, temperature
• Concerned with sensations from OPPOSITE half of body
• Point-for-point correspondence of an area of the body to a specific point in the cortex
• Lower body occupies the medial surface of the hemisphere, Upper limbs occupy the superior curved part of the hemisphere, Face/tongue occupy the inferior curved part of the hemisphere

Question 3

Primary Motor Pathway:
What are the 2 components of it?

What are the projections from the Corticospinal Tract?

What does this tract control?

What is the role of the Corticobulbar Tract? What does it control?

Answer 3

• Corticospinal Tract, Corticobulbar Tract
• From motor and premotor areas to all levels of the spinal cord
• Voluntary movements of contralateral limbs/torso
• Voluntary motor supply to the brain stem (motor cranial nerve nuclei); Controls movements of the jaw, face, tongue, larynx, pharynx

Question 4

Corticospinal Tract:
Where do 2/3 of its fibres come from? Where do they go?

Where do 1/3 of its fibres come from? Where do they go? What is the function of these fibres?

1. What is the initial path these fibres take to enter the brainstem?

Why can the smallest damage to the internal capsule lead to complete contralateral paralysis?

2. What is the next path these fibres take to enter the spinal cord?

What do most fibres do directly before they enter the spinal cord (lowest border of medulla)?

3. What’s the path 90% of the fibres then take in the spinal cord? What are they important in controlling?

What’s the path 10% of the fibres then take in the spinal cord? What are the concerned with?

Answer 4

• From motor and premotor areas; axons project to the anterior horn of spinal cord, where they synapse onto spinal motor neurons via interneurons
• From parietal lobe; axons project to the dorsal horn of spinal cord - helps “filter out” sensations generated by movement

• Fibres pass through the fan-like corona radiata, then enter the posterior limb of the internal capsule
• Internal capsule is like a “bottle-neck” for all the fibres to pass through

• Fibres descend through the anterior part of the brain stem, passing in turn through:
  1. Crus Cerebri (Peduncle); most anterior midbrain
  2. Basilar Pons; breaks up into fascicles here
  3. Pyramids of medulla
• Decussate

• Pass posteriorly and laterally to enter the lateral column of the spinal cord as the Lateral (crossed) Corticospinal tract; important for control of distal limb flexors, and therefore required for dexterity
• Continue in anterior part of the cord as the Anterior (uncrossed) Corticospinal tract; concerned with proximal/axial musculature

Question 5

What are UMNs?

What are LMNs?

What the difference between damage being in the brain/brainstem compared to being in the spinal cord (after decussation)?

Answer 5

• Has its cell body in the motor/premotor cortex and an axon in the corticospinal tract; synapses onto LMNs
• Has its cell body in the anterior horn of the spinal cord and axon leaves as peripheral nerve to reach their target muscle
• • Damage is in the brain/brain stem = Contralateral weakness
  • Damage in spinal cord = Ipsilateral weakness

Question 6

Somatic Sensory Pathway:
What are the 2 components of it?

What is the similarity in their arrangements?

Where are these 3 neurons found in the pathway?

What is the main difference between their arrangements?

Answer 6

• Dorsal Column Pathway, Spinothalamic Tract
• Both have a First, Second, and Third Order Neuron between the periphery and sensory cortex
• • First neuron in the dorsal root ganglion
  • Second neuron crosses the midline and ascends to the thalamus
  • Third neuron in Ventral Posterior Nucleus (VPN) of thalamus and projects to the primary SS cortex
• The position of the second neuron, and where they decussate

Question 7

Dorsal Column Pathway:
What is it concerned with?

How is the vibration sense tested?

Where does this pathway originate from?
→ What types of fibres are used for transmission?

1. What path do the First-order neurons take? Where do they synapse with Second-order neurons?

Where do the fibres from the lower and upper half of the body travel in the spinal cord?

2. What path do the Second-order neurons take? Where do they synapse with Third-order neurons?

What’s it called where all the sensory fibres pass the midline at the same place?

What do they ascend through after they decussate?

3. What path do the Third-order neurons take? Where do they end?

Answer 7

• FINE TOUCH, PROPRIOCEPTION, VIBRATION, Joint position sense
• High-amplitude, Low-frequency (128Hz) tuning fork
• Originates from low-threshold MECHANORECEPTORS → Large, thick, myelinated Aα/β fibres (HIGH VELOCITY)

• Has its cell body in the dorsal root ganglion, and it enters the spinal cord via the dorsal root, joining the dorsal column; synapses with Second-order neuron in Dorsal Column Nuclei
• • Upper half (↑T6); ascend in the lateral part of the dorsal column = CUNEATE FASCICULUS
  • Lower half (↓T6); ascend in the medial part of the dorsal column = GRACILE FASCICULUS

• Has its cell body in the dorsal column nucleus, and its axon crosses the midline in the medulla; synapses with Third-order neuron in the Ventral Posterior Nucleus (VPN) of the thalamus
• Great Sensory Decussation
• The Medial Lemniscus

• Ascends through posterior limb of internal capsule (behind descending corticospinal fibres); ends in the primary sensory cortex (post-central gyrus)

Question 8

Spinothalamic Tract:
What is it concerned with?

How are both of these tested?

Where does this pathway originate from?
→ What types of fibres are used for transmission?

1. What path do the First-order neurons take? Where do they synapse with Second-order neurons?
2. What path do the Second-order neurons take? Where do they synapse with Third-order neurons?

What is the Paleo-spinothalamic tract? What is the pain like here?

3. What path do the Third-order neurons take? Where do they end?

Answer 8

• PAIN AND TEMPERATURE
• Neurological pins and cold Volatile spray
• Originates from NOCICPETORS and THERMORECEPTORS
  → Thin, myelinated Aδ and unmyelinated C fibres; (SLOW VELOCITY)

• Has its cell body in the dorsal root ganglion; enters dorsal root to synapse with the Second-order neuron STRAGHT AWAY

• Crosses midline to travel in front of anterior horn and between the lateral and anterior spinal columns, then travels behind the olive, enters the pons, and ends in the lateral VPN; synapses with Third-order neuron in lateral VPN
• Some of the fibres go to the brain stem before travelling to the thalamus; Indirect, Slower pain

• Travel through the posterior limb of internal capsule to end in the primary sensory cortex