Descending spinal cord tracts

Question 1

What are they main motor outputs of the corticospinal tract?

Answer 1

1. Primary motor cortex (precentral gyrus)
2. supplementary and premotor areas
3. sensory cortex (minor)

Question 2

The corticospinal tract is divided into the _________ and ________corticospinal tracts.

Answer 2

Ventral (anterior);

lateral

Question 3

About 80% of the fibres originate from the _______________ . These fibres send axons to form the corticospinal tract. Joined by fibres from the supplementary and premotor areas, as well as some minor fibres from the sensory cortex and superior parietal lobule, project to dorsal column nuclei and dorsal horn of spinal cord to modulate pain transmission.

Answer 3

primary motor cortex (precentral gyrus, where big motor neurons can
be found)

Question 4

What kind of fibers do the corticospinal tract contains?

Answer 4

Glutaminergic and excitatory fibers.

Question 5

The pathways divided into upper motor neurons and lower motor neurons. State the definitions of these 2 types of neurons.

Answer 5

1. Upper motor neurons: corticospinal or corticonuclear neurons
2. Lower motor neurons: motor neurons in the brainstem and ventral horn of spinal cord

Question 6

What are the functions of the corticospinal tract?

Answer 6

1. Coordinate muscle movements through the inhibitory interneurons (suppress the excitability of postsynaptic neurons) through GABA (NT)
2. Contraction of flexor stimulates the muscle spindles which send sensory signals via the dorsal root into the spinal
   cord.
   The proprioceptive signals synapse with the motor neuron that connect to the flexor, forming a reflex loop
   through Ia afferents motor neurons > regulate the muscle tone

Question 7

The reflex loop that control contraction and relaxation (muscle tone) is under which 2 controls?

Answer 7

1. Tonic activation

2, Influence of the corticospinal tract

Question 8

State the course of the corticospinal tract.

Answer 8

1. Descends from the cortex through the internal capsule and down the brainstem through the basal pons and medulla > form basis pedunculli in midbrain and pyramid in ventral region of medulla
2. 80% fibers decussate at the junction of medulla and spinal cord (pyramidal decussation), changing from ventral to dorsal-lateral position in spinal cord and descend to form the lateral corticospinal tract
3. synapse with motor neurons directly/ with interneurons in spinal cord before affecting activity of motor neurons in ventral horn of contralateral spinal cord

Question 9

After corticospinal fibers decussate at the junction of medulla and spinal cord, what is the change of position in the spinal cord?

Answer 9

From ventral to dorsolateral

Question 10

What is the function and targets of the corticonuclear/corticobulbar tracts?

Answer 10

Control the motor nuclei in brainstem.
They target the motor nuclei that contribute to the cranial nerves (e.g. Trigeminal motor nuclei in pons/ facial motor nuclei in pons/ nucleus ambiguus in medulla/ hypoglossal nucleus)

Question 11

In stoke, why patient is unable to bend the limb ?

Answer 11

The descending motor influence is lost, reflex loop is constantly activated without inhibition on its own. All the muscles contract at the same time, generating spasm and paralysis.

Question 12

What is spastic gait?

Answer 12

with flexed upper limb but extended lower limb

the hips and knees are not flexed enough for the foot to clear the ground

In case of a stroke, the descending motor signals are lost. Without the descending influence, the reflex
loop is constantly activated without inhibition on its own. The patient will be unable to bend the limb (or
require a great force to flex it) as all the muscles contract at the same time all the time, generating spasm and paralysis.

Question 13

What is clonus hyper reflexia?

Answer 13

Upper motor neuron lesion, in which when we bend the ankle joint with force, in absence of corticospinal tract, the foot will rebound (try to plantar flex) due to exaggerated reflex.

• Positive Babinski’s reflex: dorsiflexed big toe when touch on plantar surface

Question 14

Which of the following examples will cause upper motor neuron lesion?
A. Tumour compression in the area 4 of cerebral cortex
B. Hemorrhage of the internal capsule
C. Damage to brainstem that causes degeneration of neurons
D. Lesions in spinal cord (ventral horn)
E. Lesions along peripheral nerve

Answer 14

A,B,C

Question 15

State the signs of upper motor neuron lesion.

Answer 15

1. Exaggerate reflex from muscle spindle (spastic paralysis)
2. Increased deep tendon reflex (Babinski’s reflex)
3. No muscle atrophy as peripheral nerves are still intact
4. No fasciculations and fibrillations

Question 16

What kind of lesion does ALS belong to? Why?

Answer 16

Lower motor neuron lesion, due to degeneration of ventral horn motor neurons

Question 17

State the signs of lower motor neuron lesion.

Answer 17

1. With the neurons, muscles cannot contract, thus flaccid, complete paralysis
2. Damaged reflex arc > no reflex
3. Lack of trophic factors in nerves > muscle atrophy
4. Spontaneous activities (fibrillations and fasciculations) due to release of Ca2+ from degenerating muscles

Question 18

Name the 4 descending tracts other than the corticospinal tract.

Answer 18

Reticulospinal tract;
Vestibulospinal tract;
Tectospinal tract;
Raphespinal tract

Question 19

Where is the motor supply originated from in reticulospinal tract? (be specific)

Answer 19

Reticular formation in pons and medulla

Question 20

State the target neurons of reticulospinal tract.

Answer 20

Interneurons adjacent to motor neurons in the ventral horn which supply trunk or proximal limb muscles
(do not usually interact directly with motor neurons)

Question 21

What is the function of the reticulospinal tract?

Answer 21

Through the locomotor center to generate controls of coutine locomotion (walking, running, postural controls such as standing still)

Question 22

Which of the following is true about reticulospinal tract?

A. It relies on cerebral cortex rather than spinal circuitry
B. It is under the influences of premotor cortex of cerebral region (higher brain control)
C. When cerebral cortex is removed, animal cannot generate walking movement due to no cortical control to the brainstem and spinal cord

Answer 22

B only

A: Rely on a spinal circuitry instead of cerebral cortex (autonomic system built in spinal cord)

C: Even when cerebral cortex is removed, leaving no cortical control to the brainstem and spinal cord, the animal can still generate walking movement in the proximal limb due to existence of the reticulospinal tract and the control within the spinal cord by locomotor centre.

Question 23

What are the functions of vestibulospinal tract?

Answer 23

1. Postural reflexes

2. Control of eye movement

Question 24

State the course of the vestibulospinal tract.

Answer 24

1. Ascends via the medial longitudinal fasciculus for the control of eye movement (innervating CN3,4,6)
2. Descends along descending component of MLF (medial longitudinal fasciculus)
3. Vestibulospinal tract synapse at cervical level descend towards motor neurons of spinal cord that control antigravity muscles
4. as it descends, it runs close and intermingles with tectospinal tract

Question 25

The 2 descending tracts (medial longitudinal fasciculus and vestibulospinal) target the neurons in _______________ that innervates the ___________muscles.

Answer 25

Anterior horn (both interneurons and motor neurons) ;
extensor

Question 26

What is the motor function of tectospinal tract?

Answer 26

Reflexes to orient head towards sources of visual/ auditory stimulations (turn the head in response to sudden visual/ auditory stimuli)

Question 27

The tectospinal tract originates from the ________________ in the roof of midbrain.

Answer 27

Superior colliculus

Question 28

State the course of the tectospinal tract.

Answer 28

1. Tectum of midbrain receives optic output which is then projected to the superior colliculus
2. Fibers then crosses the midline immediately and courses in the contralateral ventral white column to descend to ventral gray horn at cervical and upper thoracic levels > contraction of neck muscles

Question 29

What is the function of the raphespinal tract?

Answer 29

to modulate pain signals that go into the spinothalamic tract > analgesic effect

Question 30

Which of the followings about raphespinal tract is false?

A. It originates from the other part of the RF , midline of pons and medulla
B. Raphe nuclei sends fibers down the spinal cord which synapse with ventral horn neurons
C. It is under the influence of the periaqueductal gray matter
D. It produces analgesic effect by largely stopping the pain transmission from dorsal horn

Answer 30

B : Raphe nuclei which sends nerve fibres down spinal cord which then synapse with dorsal horn neurons (especially
gelatinosa cells)

Question 31

What are the functions of central autonomic descending pathways?

Answer 31

modulate autonomic functions like blood pressure, pulse and respiratory rates and sweating

Question 32

What do the central autonomic pathways originate and terminate?

Answer 32

Originate from hypothalamus and brainstem;
Terminate in the preganglionic sympathetic neurons (lateral horn at thoracic segments) and parasympathetic neurons (sacral levels)