Module E-03

Question 1

Start of the Rubrospinal Tract

Answer 1

Red nucleus in midbrain

Question 2

End of the Rubrospinal Tract

Answer 2

Alpha and gamma motor neurons in the anterior horn of the cervical spinal cord

Question 3

Route of the Rubrospinal Tract

Answer 3

Axons of red nuclear efferents decussate in the midbrain. Rubrospinal axons descend through the lateral brainstem and spinal cord, forming excitatory synapses on alpha and gamma motor neurons

Question 4

Function of the Rubrospinal Tract

Answer 4

Cells in the red nucleus receive excitatory connections from the ipsilateral motor cortex and from contralateral cerebellar nuclei. Most rubrospinal fibers end in cervical segments at synapses on lower motor neurons innervating arm muscles. Each red nucleus thus primarily facilitates
voluntary flexion of the muscles of the contralateral arm. Rubral influences on movements of the lower limbs are minimal

Question 5

How is voluntary movement modulated in the red nucleus?

Answer 5

The red nucleus receives cortical projections through the

corticorubral tract and hence facilitates voluntary activity

Question 6

Start of Medullary (Lateral) Reticulospinal Tract

Answer 6

Reticular nuclei in medulla

Question 7

End of Medullary (Lateral) Reticulospinal Tract

Answer 7

Interneurons that excite alpha and gamma motor neurons

Question 8

Route of Medullary (Lateral) Reticulospinal Tract

Answer 8

Many axons from the medullary reticular nucleus descend through the ipsilateral medulla and spinal cord. Some axons cross in the medulla and descend contralaterally in the cord. Both ipsilateral and contralateral tracts end mainly on INTERNEURONS that excite alpha and gamma motor neurons

Question 9

Function of Medullary (Lateral) Reticulospinal Tract

Answer 9

facilitate contraction of flexor muscles of the limbs

Question 10

What input does the Medullary (Lateral) Reticular nuclei receive?

Answer 10

receive bilateral cortical input from corticoreticular fibers and somatosensory projections through the spinoreticular tract (collaterals of the anterolateral system )

Question 11

Start of Pontine (Medial) Reticulospinal Tract

Answer 11

Reticular nuclei in the pons.

Question 12

End of Pontine (Medial) Reticulospinal Tract

Answer 12

Ipsilateral anterior horn of the spinal cord.

Synapses involve interneurons that excite mainly gamma motor neurons

Question 13

Route of Pontine (Medial) Reticulospinal Tract

Answer 13

Axons from the pontine reticular nuclei descend ipsilaterally through the medulla and ventral spinal cord

Question 14

Function of Pontine (Medial) Reticulospinal Tract

Answer 14

The excitatory actions of the pontine (medial) reticulospinal tract predominantly affect the gamma motor neurons of axial and limb extensor muscles. Most fibers in this pathway thus facilitate extension.

Question 15

What input does the Pontine (Medial) Reticular nuclei receive?

Answer 15

Receive bilateral cortical input through corticoreticular fibers. They also receive somatosensory projections
through the spinoreticular tract (collaterals of the anterolateral system

Question 16

Start of Spinoreticular Tract

Answer 16

Pain and temperature receptors

Question 17

End of Spinoreticular Tract

Answer 17

Reticular nuclei in pons and medulla

Question 18

Route f Spinoreticular Tract

Answer 18

This sensory pathway arises from collateral branches of

the anterolateral system

Question 19

Function of Spinoreticular Tract

Answer 19

relays sensory information (pain, temperature, and pressure) from one side of the body to the opposite to reticular nuclei. Its influence on the pontine reticular nuclei may facilitate contraction of the extensor muscles of the ipsilateral limb and trunk. Associated postural adjustment may place weight on the foot not subject to potentially threatening stimuli (e.g., a sharp piece of glass under the opposite foot)

Question 20

Start of Lateral Vestibulospinal Tract

Answer 20

Lateral vestibular nucleus

Question 21

End of Lateral Vestibulospinal Tract

Answer 21

The axons of the lateral vestibular nucleus synapse in the anterior horn of the spinal cord on excitatory interneurons that, in turn, excite alpha motor neurons.

Question 22

Route of Lateral Vestibulospinal Tract

Answer 22

Axons of lateral vestibular nuclear cells descend in the ipsilateral medulla and spinal cord

Question 23

Function of Lateral Vestibulospinal Tract

Answer 23

The lateral vestibulospinal tract predominantly excites the alpha motor neurons of the axial and limb extensor (anti-gravity muscles). This tract thus regulates muscular responses necessary for balance.

Question 24

What input does the Lateral Vestibular nuclei receive?

Answer 24

Receives input from vestibular organs (inner ear) and the
cerebellum.
vestibular organ—->excitatory
cerebellum—–> inhibitory

Question 25

Start of Medial Vestibulospinal Tract

Answer 25

Medial vestibular nucleus

Question 26

End of Medial Vestibulospinal Tract

Answer 26

This tract ends with synapses on alpha motor neurons at the cervical and upper thoracic levels.

Question 27

Route of Medial Vestibulospinal Tract

Answer 27

Axons of medial vestibular nuclear efferents descend

mainly ipsilaterally into the spinal cord and run as the caudal extension of the medial longitudinal fasciculus (MLF).

Question 28

Function of Medial Vestibulospinal Tract

Answer 28

The medial vestibulospinal tract plays important and complicated roles in muscular contraction of the neck, often in close coordination with movements of the eyes.many of the axons forming the pathway release glycine, an inhibitory amino acid neurotransmitter

Question 29

What input does the Medial Vestibular nuclei receive?

Answer 29

Receives input from vestibular organs in the inner ear and the cerebellum
inhibition of alpha (glycine)
vestibular organ—->excitatory
cerebellum—–>inhibitory

Question 30

What are muscle spindles ?

Answer 30

Specialized aggregations of intrafusal fibers form muscle spindles. They are proprioceptors tied to sensory fibers that encode stretching of the muscle.

Question 31

How do muscle spindles detect extrafusal muscle contraction?

Answer 31

Gamma motor neurons and alpha motor neurons are simultaneously active, leading to coordinated contraction of intrafusal and extrafusal muscle fibers. This process
thus “tunes” intrafusal muscle fibers, ensuring that the muscle spindle is sensitive to muscular stretch, irrespective of state of the extrafusal fibers

Question 32

Which nerves make up the afferent limb of the myotatic reflex?

Answer 32

Ia afferent excites

the alpha motor neurons serving the same muscle

Question 33

Fibers of which descending tracts activate voluntary contraction?

Answer 33

Descending fibers of the lateral corticospinal, rubrospinal, and reticulospinal tracts coactivate lower motor neurons during voluntary movement.
The pontine reticulospinal tract has a major excitatory influence on the gamma motor neurons and, therefore, can activate muscle contraction by the so-called “gamma loop

Question 34

What is the gamma loop?

Answer 34

Gamma motor neurons stimulate the muscle spindles and thereby indirectly stimulate alpha motor neurons (and hence extrafusal muscle fibers) via the Ia fibers forming the afferent limb of the myotatic reflex arc

Question 35

What is the Purpose of co-activation of alpha and gamma motor neurons?

Answer 35

To ensure that contraction of the intrafusal fibers is tuned to allow the neuromuscular spindle to measure stretching of the muscle.

Question 36

What is decorticate posturing?

Answer 36

following a painful stimulus or spontaneously, the affected

patient will flex the upper limbs and extend the lower limbs

Question 37

Where is the lesion if decorticate positioning is observed?

Answer 37

A lesion rostral to the red nucleus that impairs corticobulbar and corticospinal fibers

Question 38

What does decorticate positioning indicate in the following tracts:

a) corticospinal tract
b) corticobulbar tract
c) rubrospinal tract
d) Medullary reticulospinal tract
e) Pontine reticulospinal tract
f) vestibulospinal tract

Answer 38

a) interrupted – mainly flexion impaired.
b) interrupted – paralysis of motor cranial nerves
c) intact – flexion of the arms
d) intact – flexion of extremities
e) intact – extension of extremities
f) intact – extension of extremities (particularly the lateral vestibulospinal tract)

Question 39

Why are the arms flexed in decorticate positioning?

Answer 39

The flexor input of the rubrospinal tract is still present. Hence, there is strong flexor innervation, which is much greater than the extensor input of pontine reticulospinal and the vestibulospinal tracts

Question 40

What is decerebrate posturing?

Answer 40

After a noxious stimulus or spontaneously, the patient extends the upper and lower limbs

Question 41

Where is the lesion if decerebrate positioning is observed?

Answer 41

A lesion at or caudal to the red nucleus that impairs the

corticospinal, corticobulbar and rubrospinal fibers

Question 42

What does Decerebrate positioning indicate in the following tracts:

a) corticospinal tract
b) corticobulbar tract
c) rubrospinal tract
d) Medullary reticulospinal tract
e) Pontine reticulospinal tract
f) vestibulospinal tract

Answer 42

a) interrupted – mainly flexion impaired.
b) interrupted – paralysis of motor cranial nerves
c) interrupted – no flexion of arms
d) intact – flexion of extremities
e) intact – extension of extremities
f) intact – extension of extremities (particularly the lateral vestibulospinal tract)

Question 43

Why is there extension of upper and lower limbs in decerebrate positioning?

Answer 43

only the medullary reticulospinal tract promotes flexion, which is overcome by the extensor innervation of the pontine reticulospinal and vestibulospinal tracts