Module E-03 Flashcards Preview

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Flashcards in Module E-03 Deck (43):
1

Start of the Rubrospinal Tract

Red nucleus in midbrain

2

End of the Rubrospinal Tract

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

3

Route of the Rubrospinal Tract

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

4

Function of the Rubrospinal Tract

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

5

How is voluntary movement modulated in the red nucleus?

The red nucleus receives cortical projections through the
corticorubral tract and hence facilitates voluntary activity

6

Start of Medullary (Lateral) Reticulospinal Tract

Reticular nuclei in medulla

7

End of Medullary (Lateral) Reticulospinal Tract

Interneurons that excite alpha and gamma motor neurons

8

Route of Medullary (Lateral) Reticulospinal Tract

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

9

Function of Medullary (Lateral) Reticulospinal Tract

facilitate contraction of flexor muscles of the limbs

10

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

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

11

Start of Pontine (Medial) Reticulospinal Tract

Reticular nuclei in the pons.

12

End of Pontine (Medial) Reticulospinal Tract

Ipsilateral anterior horn of the spinal cord.
Synapses involve interneurons that excite mainly gamma motor neurons

13

Route of Pontine (Medial) Reticulospinal Tract

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

14

Function of Pontine (Medial) Reticulospinal Tract

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.

15

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

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

16

Start of Spinoreticular Tract

Pain and temperature receptors

17

End of Spinoreticular Tract

Reticular nuclei in pons and medulla

18

Route f Spinoreticular Tract

This sensory pathway arises from collateral branches of
the anterolateral system

19

Function of Spinoreticular Tract

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)

20

Start of Lateral Vestibulospinal Tract

Lateral vestibular nucleus

21

End of Lateral Vestibulospinal Tract

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.

22

Route of Lateral Vestibulospinal Tract

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

23

Function of Lateral Vestibulospinal Tract

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.

24

What input does the Lateral Vestibular nuclei receive?

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

25

Start of Medial Vestibulospinal Tract

Medial vestibular nucleus

26

End of Medial Vestibulospinal Tract

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

27

Route of Medial Vestibulospinal Tract

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).

28

Function of Medial Vestibulospinal Tract

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

29

What input does the Medial Vestibular nuclei receive?

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

30

What are muscle spindles ?

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

31

How do muscle spindles detect extrafusal muscle contraction?

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

32

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

Ia afferent excites
the alpha motor neurons serving the same muscle

33

Fibers of which descending tracts activate voluntary contraction?

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

34

What is the gamma loop?

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

35

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

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

36

What is decorticate posturing?

following a painful stimulus or spontaneously, the affected
patient will flex the upper limbs and extend the lower limbs

37

Where is the lesion if decorticate positioning is observed?

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

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

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)

39

Why are the arms flexed in decorticate positioning?

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

40

What is decerebrate posturing?

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

41

Where is the lesion if decerebrate positioning is observed?

A lesion at or caudal to the red nucleus that impairs the
corticospinal, corticobulbar and rubrospinal fibers

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

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)

43

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

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