Motor functions of the Brainstem

Question 1

What are the functions of the brain stem?

Answer 1

• The brainstem is responsible for:
1. Controlling respiration
2. Controlling the cardiovascular system
3. Controlling gastrointestinal function
4. Controlling many stereotyped movements of the body
5. Controlling equilibrium
6. Controlling eye movement

Question 2

What is the role of the reticular and vestibular nuclei?

Answer 2

Support of the body against gravity

Question 3

Describe the 2 groups of reticular nuclei?

Answer 3

1. The pontine reticular nuclei, located in the pons but extending into the mesencephalon
2. The medullary reticular nuclei, which extend the entire extent of the medulla
   - The two nuclei function mainly antagonistically to each other
   > The pontine excite the antigravity muscles and the medullary inhibit them

Question 4

Describe the pontine reticular nuclei?

Answer 4

• The pontine reticular nuclei transmit excitatory signals downwards into the cord through pontine (or medial)
  reticulospinal tract
• The fibers of this pathway terminate on the medial anterior motor neurons that excite the muscles that support the body against gravity
• The pontine reticular nuclei have high degree of natural excitability
• They also receive excitatory signals from local circuits within the brain stem and from the vestibular nuclei

Question 5

Describe the medullary reticular nuclei?

Answer 5

• The medullary nuclei transmit inhibitory signals to the same antigravity anterior motor neurons by way of the medullary (or lateral) reticulospinal tract
• It receives strong input collaterals from: i. the corticospinal tract
  ii. rubrospinal tract and other motor pathways
• These normally activate the medullary reticular inhibitory system to counterbalance the excitatory signals from the pontine reticular system

Question 6

Name the 4 vestibular nuclei on each side of the brain stem?

Answer 6

• Superior
• Medial
• lateral
• inferior

Question 7

Describe the role of the vestibular nuclei in exciting antigravity muscles?

Answer 7

• Vestibular nuclei receive input from the cranial nerve VIll and other parts of the brain, including the cerebellum, and the visual and somatic sensory systems
• This combines incoming vestibular information with da about the motor system and other sensory modalities
• The lateral vestibular nuclei transmit strong excitatory signals by way of both the lateral and medial vestibulospinal tracts to spinal motor neurons controlling muscles in the legs that help maintain posture
• Without the support of the vestibular nuclei, the pontine reticular system loses much of its force

Question 8

Specific role of the vestibular nuclei?

Answer 8

The specific role of the vestibular nuclei is to control selectively the excitatory signals to the different antigravity muscles to maintain equilibrium in response to signals from the vestibular apparatus

Question 9

Decerebrate animal?

Answer 9

• Sectioned brain stem between the pons and the mesencephalon, leaving both the pontine and medullary reticular system and vestibular system lead to decerebrate rigidity
• The rigidity is not in all muscles but only in antigravity muscles: muscles of the neck, trunk and the extensors of the leg

Question 10

Cause of Decerebrate rigidity?

Answer 10

The cause of decerebrate rigidity is blockage of the normally strong excitatory input to the medullary reticular nuclei from cerebral cortex, red nuclei and basal ganglia

Question 11

What organs detect sensations of equilibrium?

Answer 11

The vestibular apparatus located in the inner ear is the organ that detects sensations of equilibrium
• The utricle, the semicircular ducts and the saccule are integral parts of the equilibrium mechanism

Question 12

Vestibular labyrinth?

Answer 12

The vestibular labyrinth includes two types of structures with different functions:
1. the otolith organs (utricle & saccule)
which detect the force of gravity and tilts of the head (static equilibrium)
2. the semicircular canals which are sensitive to head rotation (dynamic equilibrium

Question 13

Macula?

Answer 13

Located inside the utricle and saccule is a small sensory epithelium called macula which detects the normal orientation of the head

Question 14

Semicircular canals?

Answer 14

• The three semicircular canals are arranged at right angles to each other so that they represent all three planes in space
• The ducts are filled with a fluid called endolymph which excites the sensory organ of the ampulla when flowing

Question 15

Vestibular postural reflexes?

Answer 15

• Sudden changes in the orientation of an animal in space elicit reflexes that help to maintain equilibrium and posture
• If an animal is suddenly pushed to the right, before it fall more than a few degrees, its right leg extends instantaneously
• This mechanism anticipates that the animal will be off balance in a few seconds and makes appropriate adjustments to prevent this

Question 16

Neck proprioceptors and equilibrium?

Answer 16

The neck proprioceptors
.
The vestibular apparatus detects the orientation and movements only of the head
Since vestibular signals cannot distinguish whether the head or the whole body is moving when the head moves on a stationary trunk, the neck-proprioceptive input provides the necessary information about head movements relative to the trunk
• The proprioceptors of the neck and the body transmit information into the vestibular and reticular nuclei of the brain stem
• Accordingly, neck muscles are richly endowed with spindles whose density is much higher than in thigh and shoulder muscles

Question 17

Proprioceotion and exteroceptive information from other parts of the body and equilibrium?

Answer 17

• Proprioceptive information from other parts of the body is important in the maintenance of equilibrium
• For instance, pressure sensations from the footpads can tell one
- whether weight is distributed equally between the two feet
- whether weight is more forward or backward on the feet

Question 18

Visual information and equilibrium?

Answer 18

People with complete destruction of vestibular apparatus have almost normal equilibrium as long as their eyes are open and they perform all motions slowly

Question 19

Lateral medullary syndrome?

Answer 19

Aka Wallenberg syndrome/posterior inferior cerebellar artery (PICA)
syndrome
- due to obstruction in vessels supplying the medulla (vertebral artery, PICA), resulting in brainstem ischemia or infarction.
- causes abnormalities in the vestibulocerebellar, sensory, bulbar, respiratory, and autonomic systems
- vertigo, ipsilateral paralysis of the soft palate, larynx, and pharynx, dysphonia, dysphagia, ipsilateral facial numbness and loss of corneal reflexes, ipsilateral cerebellar asynergy, and hypotonia