Week 9 - Spinal Cord, Major Tracts & Brain Stem

Question 1

What is Corticospinal tract for?

Answer 1

Efferent Motor pathway

Question 2

What are the two pathways to the motor neurons in the spinal cord?

Answer 2

Anterior and Lateral Corticospinal Tract

Question 3

Where are Upper Motor Neurons found?

Answer 3

Sit in the motor cortical areas (primary, supplementary & pre-motor)

Question 4

Describe the whole Corticospinal Tract Pathway

Answer 4

1. UMN
2. Fibers descend through the internal capsule and thalamus where they give off extensive co-laterals
3. Fibers pass through the midbrain in the cerebral peduncles
4. Fibers pass through the pons, where many terminate on pontine relay nuclei (relay back to cerebellum contralaterally so some of pathway is gone at this point)
5. Inferior to Pons, 75% of original no of axons is lost -> housed in Medullary pyramids
6. In the caudal medulla, most fibers cross to contralateral side while some still on ipsilateral side
7. Fibers in lateral CS tract = contralateral innervation while fibers in anterior CS tract = mostly ipsilateral

Question 5

What is the brainstem consisted of?

Answer 5

Midbrain
Pons
Medulla

Question 6

What do you notice when you split the ventral horn of the spinal cord in half horizontally?

Answer 6

Extensors = more ventral
Flexors = more dorsal

Question 7

What are the Cerebral Peduncles?

Answer 7

Structures on the midbrain that transmit the corticospinal tract signals down to the pons

Question 8

What is the Dorsal Column Medial Lemniscus (DCML) pathway for?

Answer 8

Afferent touch sensation and proprioception

Question 9

Describe the whole DCML pathway

Answer 9

1. Sensory neurons in DRG extend axons superiorly through respective dorsal column giving off extensive collaterals (branches) as they ascend
2. The more superior in the body, the more lateral in the dorsal column // The more inferior, the more medial in dorsal column
   Upper Limb = Cuneate |Lower Limb = Gracile
3. The postsynaptic cells cross the brain, travel to medial lemniscus and synapse in the thalamus (Ventral Posterolateral Nucleus (VPL))
4. Sensory fibres innervating tissues derived from pharyngeal arches synapse in trigeminal nuclei, cross in brain stem and project to Thalamus Ventral Posteromedial Nucleus (VPM)
5. Projections from thalamus -> somatosensory cortex

Question 10

What happens in the Brainstem in the DCML?

Answer 10

Cross in medial lemniscus

Question 11

What is the Spinothalamic Tract for?

Answer 11

Convey pain and temp

Question 12

Simple description of Spinothalamic tract

Answer 12

Afferents (sensory) -> DRG -> Dorsal horn -> Synapse at Dorsal Horn -> Synaptic connections land on projection neurons -> Projection neurons crossover to contralateral side at anterolateral region -> then go to Thalamus

Question 13

Describe whole Spinothalamic Tract

Answer 13

1. Sens neurons in DRG -> Synapse onto relay neurons in dorsal horn -> neurons cross to contralateral side and ascend in anterolateral white matter of spinal cord to thalamus VPL
2. Sens fibers innervating tissues in branchial structures -> cell bodies in trigeminal ganglion -> travel in spinal trigeminal tract -> synapse in trigeminal nuclei -> cross brain stem and project to thalamus (VPM)

Question 14

Where are LMNs situated? and where are their axons extended?

Answer 14

Sit in Ventral horn of spinal cord and extend axon down ventral root to mixed spinal nerve then to the muscle

Question 15

Where are UMNs situated? and where does it go?

Answer 15

In the Precentral Gyrus of the Primary Motor Cortex
Goes down the brain via the Corticospinal tract
(in blue on pic)

Question 16

What is the green line in this image and why is it required?

Answer 16

Sensory fibre (stretch receptor) -> sends afferent info back -> DRG -> Propiroceptor -> Dorsal column in cuneate part -> bottom of brainstem -> cross contralaterally -> synapse in thalamus -> Post central gyrus

This sensory info is required to understand the movement and position

Question 17

What is special about the sensory branches that go back to the brain?

Answer 17

They give off collateral branches

Question 18

What is important about the collateral branches of the sensory neurons?

Answer 18

Give direct info to the LMN about the stretch of the muscle -> saves time by not having to go all the way to the brain

Also can inhibit the antagonist -> eg if you want to contract the bicep in a quick movement, the antagonist, the tricep, must be inhibited -> does this w GABA

Question 19

What are the key components of a Reflex Arc?

Answer 19

1. Afferent (sensory fibre)
2. CNS relay
3. Efferent (motor fibre)

Question 20

What is important about interneurons in the motor response?

Answer 20

They receive most of the motor output from the brain -> modulate activity in motor neurons

Question 21

How many neurons in the in sensory and motor pathways?

Answer 21

sensory -> 3 neurons
motor -> 2 neurons

Question 22

E.g In the Biceps reflex arc at around C6, what happens if theres damage been done at C4?

Answer 22

The interneurons now unable to get to C6 therefore cannot modulate the motor neurons and cannot have its inhibitory effect on them anymore.
–> in the reflex arc, motor neurons receiving much more sensory information -> will have an exaggerated reaction compared to normal