Flashcards in Descending Motor Systems Deck (74)
What does the Lower Motor Neuron innervate?
striated muscles, it directly signals the muscle to contract which is the only way movement can be initiated
True or false: The Lower Motor Neuron is the first in a chain of neurons.
False, it is the last.
What two parts are included in the Lower Motor Neuron?
1) Alpha motor neuron - extrafusal muscle fibers
2) Gamma motor neuron - intrafusal muscle fibers
What are the symptoms of a Lower Motor Neuron lesion?
1) Atonia - loss of muscle tone
2) Areflexia - loss of myotatic (knee jerk) reflex
3) Flaccid paralysis
4) Fasciculations - spontaneous muscle contractions
5) Atrophy - loss of muscle tissue
What is an Upper Motor Neuron?
Axons that descend from the cortex, and end on or near LMN
What are the symptoms of an Upper Motor Neuron lesion?
1) Spastic paralysis (paresis)
2) Hypertonia (increased resting tension) - Arm flexors, leg extensors
4) Pathologic reflexes, e.g. negative plantar reflex or Babinski sign
5) Big toe dorsoflexion with fanning of other toes when side of heal is stroked
What are fascinations?
spontaneous contractions of small groups of muscle fibers that can be visible at the skin surface.
What are fibrillations?
are contractions of individual muscle fibers that can not be seen visually but are detected using electrical monitoring.
What are clonus?
a rapid series of alternating muscle contractions that occur in response to the sudden stretch of a muscle.
During a routine exam you note that the patient has tongue fasciculations. This raises concern for:
A) Corticospinal tract injury
B) Primary myopathy
C) Lower motor neuron injury
D) Neuromuscular junction disorder
E) Hypoglossal nuclear injury
C) Lower motor neuron injury, decreased muscles tone and stretch reflexes or severe atrophy would also indicate this.
UMN injury would have increased muscle tone, stretch reflexes and mild atrophy, and clonus/pathologic reflexes like Babinski sign etc.
*Both show decreased muscle strength
Where are lower motor neuron cell bodies?
in anterior horn
LMN axons in ventral root divide into ...
terminal branches widely distributed in target muscle, each branch ends at one neuromuscular junction
What is the systematic arrangement of motor neurons?
1) Neurons controlling axial muscles are medial to those controlling distal muscles
2) Neurons controlling flexors are located posterior to the extensor groups
What is a motor unit?
1 motor neuron + all myofibers it innervates = motor unit
They vary in size, related to control we have over the muscle
How many muscle fibers are in an extraocular muscle motor unit?
10 myofibers/motor unit
*Large antigravity muscle like gastrocneumius 100s up to 1000myofibers/ motor unit
What are the three kinds of muscle fibers?
1) Standing: contract weakly for long periods
2) Running: contract strongly for short/long periods
3) Jump: contract very strongly for very short periods
*Each muscle fiber type populates one motor unit, no mixing
What is the difference between Type 1 and Type 2 muscle fibers?
1) “one (type 1) slow (twitch) fat (lipid-rich) red (appearance) ox (oxidative, mitochondria-rich), (duck breast muscle)
2) Type 2 - fast twitch, scant lipids, abundant glycogen, few mitochondria, (turkey breast muscle)
How does motor control work?
1) Basal ganglia and cerebellum influence cerebral cortical output to cord and brainstem
2) Basal ganglia, cerebellum and association cortex are vital in design, choice and monitoring of movement, but note they have no direct effect on LMN
3) Damage to these areas does not cause weakness, may have involuntary movements, incoordination, difficulty initiating movement
Hows does motor control work in higher centers?
- Hierarchical in that cortex “decides” what movement should occur - Premotor cortex plans and tells motor cortex and then the LMN what to do
- Parallel arrangement as premotor cortex can directly “talk to” LMN
- Basal ganglia and cerebellum involved in planning and monitoring movements, have no (few) outputs to spinal cord instead they effect motor and premotor cortex
Where do descending motor pathways terminate or synapse?
Dorsal horn neurons & Interneurons
Where does the Corticospinal tract (pyramidal tract) run?
- Cortex to spinal cord, classic upper motor neuron
- Some fibers to hand region of cord terminate on primary motor neurons in anterior horn
Where does the Corticobulbar (corticonuclear) tract run?
- Cortex to brainstem
Where does the Corticopontine tract
- Cortex to basilar pons
What are other descending motor pathways?
1) Rubrospinal tract
2) Reticulospinal tract
3) Vestibulospinal tract
What percentage of the corticospinal tract fibers is from the precentral gyrus?
40% from primary motor area (Area 4)
What percentage of the corticospinal tract fibers is from the post central gyrus?
~ 25% from Somatic sensory area (Areas 3, 1, & 2)
What percentage of the corticospinal tract fibers is from the Premotor area?
~ 20% Area 6, lateral surface
What percentage of the corticospinal tract fibers is from the Supplementary motor area?
~ 10%, Area 6, medial surface
What percentage of the corticospinal tract fibers is from the Superior parietal lobule?
~ 5% (Areas 5 & 7)