Descending Motor Systems Flashcards
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
3) Hyperreflexia
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
run?
- 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)
Where is the primary motor area?
Area 4 on medial and lateral aspect
What is the function of the primary motor area?
Function:
1) Execution of contralateral voluntary movements
2) Control of fine digital movements
Where do projections from the Primary Motor Area go?
Projects to brainstem & spinal cord – some monosynaptic terminations on spinal cord motor neurons (hand)
What is the result of a lesion on the Primary Motor Area?
Paralysis of contralateral musculature
Where is the premotor area?
Area 6 on lateral aspect
What is the function of the premotor area?
- Plans movements in response to external cues (e.g., instructions)
- Control of proximal and axial musculature (trunk, shoulder, hip)
- May assemble empathetic facial movements
Where do projections from the Premotor Area go?
to primary motor area and reticular formation
Some fibers project to all spinal cord levels
What is the result of a lesion on the Premotor area?
Moderate weakness of contralateral proximal muscles
Loss of ability to associate learned hand movements to verbal or visual cues
Where is the supplementary motor area?
Area 6 on medial aspect
What is the function of the supplementary motor area?
Function:
- Plans movements while thinking (internally paced)
- Assembles (learns) new sequence (playing new music)
- Assembles previously learned sequence (music scale)
- “Imagines” movements
Where do projections from the supplementary motor area go?
Projects to premotor and primary motor areas
What areas does the Parietal Lobe take up?
Areas 3, 1, & 2 Somatic sensory areas; Areas 5 & 7 Superior parietal lobule
Where do projections from the Parietal Lobe go?
- Project to primary motor area - Direct motor patterns in response to sensory input
- Project to sensory areas of brainstem and spinal cord
Modulate sensory signals
Where do collateral projections from the Corticospinal tract go?
to basal ganglia, thalamus, reticular formation, various sensory nuclei (posterior column nuclei), posterior & intermediate horns of spinal cord
True or false: all movements are dependent on CST
False, if cut in monkeys, after a period of flaccid paralysis, they move again, fine finger movement lost permanently
Where does Corticospinal tract originate?
in cerebral cortex, precentral gyrus and nearby areas (Betz cells; layer V)
Where does Corticospinal Tract descends and decussate?
Descends thru cerebral peduncle, basis pontis, medullary pyramid, decussates at spinomedullary junction
Lateral CST
- 85% of fibers cross in decussation in medulla descend in lateral funiculus
- The CST is Somatotopically organized
Anterior CST
- 15% of fibers, uncrossed descend in anterior funiculus
* Axial muscle activity, some actually cross in anterior commissure prior to synapsing
Select the true response about the corticospinal tract (CST).
A) It has no somatotopic organization
B) It consists of small bundles in the midbrain
C) A left sided spinal cord lesion that involved the CST would cause ipsilateral weakness
D) Lesions are associated with flaccid paralysis
E) Decussates in the pons
D) Lesions are associated with flaccid paralysis
What does Rubrospinal tract
control?
Control of shoulder and proximal arm musculature
What does Reticulospinal tract control?
Control of axial musculature - walking
What does Vestibulospinal tract control?
Control of axial musculature - balance
What does Tectospinal tract control?
believed to be important in head turning reflexes in response to visual stimuli, unclear function in humans
Where is the origin of the Vestibulospinal tract?
Origin: vestibular nuclei in pons
Where does the Vestibulospinal tract receive input from?
Vestibular end organ and cerebellum (balance)
Lateral vestibulospinal tract
Projects via lateral funiculus to:
- Ipsilateral spinal cord, facilitates antigravity muscles
Medial vestibulospinal tract
Projects via anterior funiculus to:
Spinal cord cervical levels bilaterally, controls head movements in response to gravity
What is the function of the vestibulospinal tract?
Function:
- Mediates postural adjustments & head movements
- Antigravity reflexes
- Righting reflex (cats); righting reflex to head inversion
What is the origin of the Rubrospinal tract?
Origin: red nucleus of midbrain (so “rubro”)
Where does the Rubrospinal tract receive input from?
Receives input from:
Primary and premotor areas – shoulder and arm control
Cerebellum
What is the course of the Rubrospinal tract?
- Ventral tegmental decussation
- Lateral funiculus
Where do projections from the Rubrospinal tract area go?
Projects to contralateral spinal cord
What is the function of the rubrospinal tract?
Function: like that of vestibulospinal tract;
- Facilitates upper extremity flexor muscle tone
- Believed to be small in humans, some question significance
What is the origin of the Reticulospinal tract?
Origin: brainstem reticular formation (pons and medulla)
Where does the Reticulospinal tract receive input from?
Receives input from:
Motor and somatic sensory cortex
What is the course of the Reticulospinal tract?
- Brainstem - anterior to medial longitudinal fasciculus (MLF)
- Spinal cord – anterior funiculus
Where do projections from the Reticulospinal tract area go?
Spinal cord bilaterally (courses in anterior funiculus)
What is the function of the rubrospinal tract?
Supports rhythmic motor actions including walking
*May support recovery of function via projections to motor neurons controlling arm and hand
Where does the Tectospinal tract originate?
Originating here from superior colliculus (SC)
What is the course of the Corticobulbar pathway?
fibers leave the cerebral cortex and descend to the brainstem where they end directly on motor neurons (XII), but most end on interneurons in the reticular formation
Do III, IV, VI receive direct input from the Corticobulbar pathway?
No
What receives bilateral input from the Corticobulbar pathway?
V, VII, XII, nucleus ambiguus & XI
True or false: no corticobulbar decussation exists
True, it descends with CST to level of target nucleus, then splits off
Facial motor nucleus
- Exception to typical CBP pattern
- Motor neurons to lower facial muscles mainly innervated by contralateral cortex, but upper facial muscles innervated bilaterally
- Unilateral damage to CBP (e.g. in cerebral peduncle) result:
- Inability to smile or show teeth symmetrically; but ability to wrinkle forehead is unaffected
