Descending Motor Systems Flashcards
What is a lower motor neuron?
Directly innervates striated muscle
Last neuron in chain
Only way movement can be initiated
What results from a lower motor neuron injury?
- Atonia: loss of muscle tone
- Loss of myotatic reflex
- Flaccid paralysis
- Spontaneous contractions
- Atrophy: loss of tissue
What is an upper motor neuron?
Axons descend from cortex and end on or near LMN
What results from an upper motor neuron lesion?
Hypertonia: tense while resting
Less atrophy then LMN lesion
How are motor neurons arranged in the anterior horn?
Flexors are posterior while extensors are anterior
Medial muscles are medial while distal muscles are distal
What is a motor unit?
Consist of one motor neuron and all the myofibers it innervates
What is the difference between a type 1 and type 2 muscle fiber?
Type 1: one slow fat (lipid-rich) red ox (oxidative, lots of mitochondria)
Type 2: fast, glycogen rich, few mitochondria
What is the influence of the basal ganglia, cerebellum, and association cortex on the motor system?
They affect the output of the upper motor neurons but have no direct impact on LMNs.
What effect do injuries to the basal ganglia, cerebellum, and association cortex have on muscles?
No weakness.
Involuntary movements and incoordination
What are the several origins of the corticospinal tract?
- Primary motor area (precentral gyrus)
- Somatic sensory area (postcentral gyrus)
- Premotor area (lateral surface)
- Supplementary motor area (medial surface)
- Superior parietal lobe
What is the function of the fibers in the corticospinal tract from the primary motor area? Result of lesion?
Contralateral voluntary movement and control of fine digital movement
Paralysis of contralateral muscles
What is the function of CST fibers from the premotor area? Lesion result?
- Plans movements in response to instructions
- controls proximal and axial musculature
- empathetic facial movements
Loss of ability to associate learned hand movements to verbal or visual cues
What is the function of CST fibers from the supplementary motor area?
Plans movement while thinking (ex. Play new music)
T/F: It is unlikely the CST has one simple function.
TRUE
Is it likely that cutting the corticospinal tract will result in total paralysis?
No. Not all movements are completely dependent on the CST
Describe the path of the corticospinal tract?
Descends thru cerebral peduncle -> basis Pontus -> medullary pyramid -> pyramidal decussation -> skeletal muscles
Do all fibers in the CST cross in the pyramidal decussation?
No. The Anterior CST fibers do not cross and are associated with axial muscle activity
What is the somatotopic organization of the CST?
Lateral: Leg
Medial: Arm
What is the function of the rubrospinal tract?
Control of shoulder and proximal arm
What is the function of the reticulospinal tract?
Control axial musculature for walking
What is the function of the vestibulospinal tract?
Control of axial musculature for balance
What is the function of the tectospinal tract?
Head turning reflexes in response to visual stimuli
T/F: Most neurons in the corticobulbar tract end directly on lower motor neurons.
FALSE
Most end on interneurons in the RF
What cranial nerves receive bilateral input from the corticobulbar tract?
CN V, CN VII, CN XII, nucleus ambiguus, and CN XI
Where does the corticobulbar tract originate?
In the face/mouth portion of the motor cortex
Describe how the corticobulbar pathway descends.
It descends with the CST and does not decussate
What is special about the way the corticobulbar pathway interacts with CN VII?
Upper facial muscles innervated bilaterally
Lower facial muscles only contralateral
Where is the corticospinal tract found throughout the medulla and pons? Where does it migrate to in the midbrain?
Anterior; anterolateral
Where is the spinothalmic tract throughout the medulla and pons? Where does it migrate to in the midbrain?
Lateral (to the ML); posterolateral (posterior to ML)
Where is the medial lemniscus throughout the brainstem?
Always found fairly medial
