Lectures 29-30: Motor Systems Flashcards
Postural base
Positions that we take against force of gravity
The brainstem nuclei are critical for what aspect of motor control?
Postural base
What are the parts of the pyramidal motor system
Motor cortex, brainstem, spinal cord (and reciprocal connections)
Force-generating muscle fiber types, metabolism, function (2)
Fast-fatigable (glycolysis, power) and fatigue-resistant/slow (oxidative metabolism, posture-fine-scale movement)
Which muscle fiber type is able to generate a lot of force quickly?
Fast-fatigable
Intrafusal fibers are a part of the…function? Do they play a role in force generation?
Muscle spindle; adjust sensitivity of muscle spindle; NO
What motorneurons innervates extrafusal fibers?
Alpha-motorneurons
What motorneurons innervates intrafusal fibers?
Gamma-motorneurons
What is the significance of lower motorneuron pools?
All motorneurons in a pool/column innervate the same muscle
Final common pathway
Motorneurons, the route by which the CNS initiates and controls movement (includes alpha and gamma muscle fibers)
Motor unit
One alpha-motorneuron and all the extrafusal muscle fibers that its axon innervates
What is the relationship between alpha-MN size, motor unit size, and muscle fiber type?
Small alpha-MN innervates small motor units of “fatigue-resistent” muscle fibers; large alpha-MN innervates large motor units of “fast-fatigable” muscle fibers
Which fires first: small or large MNs? What is this called? Why?
Small MN fire first; size principle
Gamma motorneurons control what aspect of the motor capsule? What happens next?
Contractility of motor ends; feedback from non-contractile middle via sensory fibers
What sensory fibers carry back sensory feedback from the muscle capsule?
Group IA and Group II
Ways to stimulate muscle spindle (2)
- Stretch muscle overall; 2. Contract intrafusal fibers via gamma-MNs
When does a 1a fiber discharge (in what part of muscle)? When would 1a fiber be silent and how do we prevent this?
During stretch of muscle spindle; the fiber would be silent during muscle contraction but gamma-MN pulls polar ends of muscle spindle, maintaining load on spindle
The myotatic reflex requires how many neurons? What types? Describe this pathway.
Two; alpha-MNs and group Ia/II fibers; passive stretch –> Group Ia/II fibers carry information into dorsal horn –> synapse on alpha-MN –> excited agonist/synergist muscles (contraction)
How do antagonist muscles get involved in the myotatic reflex?
The same Group Ia/II sensory neuron synapse on an inhibitory interneuron, inhibiting motor activity of the antagonist muscle
What cell type shuts off the myotatic reflex?
Renshaw cells
Functions of deep tendon reflexes? (2)
Static posture and anticipatory activity (anticipation of future loads via gamma-MNs)
How is anticipatory activity possible?
Cortical input –> Gamma-MNs stretch polar ends –> Group Ia/II perception of “stretch” –> feedback to alpha-MNs –> contraction/relaxation of muscle pairs
Symptoms of LMN syndromes (3)
Flaccid paralysis (weakness, hypotonia, atrophy), hyporeflexia or areflexia (if LMNs have died), abnormal firing of LMNs (fasiculations)
Golgi Tendon Organs connect…and sense what using what types of fibers (2)?
Muscle to tendon; tension via contraction of collagen fibers on Group Ib axons
Golgi Tendon Organs are involved in what reflex? What does this prevent?
Inverse myotatic reflex; over-contraction
Describe somatotopy of ventral spinal cord motorneuron pools
(around a clock) flexor –> proximal –> extensor –> distal
Lateral spinal tracts and what they control largely (through access to what?)
Lateral coticospinal tract and rubropsinal tract; fine movements via access to distal flexors, extensors
Medial spinal tracts and what they control (through access to what?)
Tectospinal tract, vestibulospinal tracts (lateral and medial), reticulospinal tracts (lateral and medial); postural control via access to proximal flexors, extensors
Describe pathway of CST
Cortex –> cerebral peduncle –> pyramindal tract –> pyramidal decussation –> LCST (90%) and vCST (10%)
Where does the vCST travel?
Only to lower cervical
Lateral component UMNs mostly terminate on…
Interneurons
Describe pathway of RST
Cortex –> red nucleus –> decussation –> RST
RST facilitates what type of muscle? Inhibits what type of muscles?
Facilitates flexors, inhibits extensors
T/F: There is somatopy in the descending cortical motor projections?
True!
Tectospinal tract function. Where?
Facilitates head, neck rotation; high cervical only
Reticulospinal traction function.
Balanced control of axial/anti-gravity muscles
Describe pathway of RST
Cortex –> pontine/medial (excitatory, ipsilateral) and medullary/lateral (inhibitory, contralateral) reticular nuclei –> RST. FILL IN.
Vestibulospinal tract function. What controls it?
Medial: neck; Lateral: excitatory influence on axial/anti-gravity muscles; cerebellum
Describe pathway of LVST
Cerebellum (inhibitory) –> lateral vestibular nucleus –> LVST
The anti-gravity muscles favor excitatory/inhibitory inputs?
Excitatory (two excitatory pathways: LVST and Pontine RST)
If you make a midbrain cut what happens to anti-gravity extensor muscles?
Cut off motor cortex control and thus MRN inhibitory input –> hypertonus of anti-gravity muscles
If you cut the DRG of an animal with a midbrain cut, what happens? Therefore?
The hypertonus will relax. Descending motor system actually acts on gamma-MN and NOT alpha-MN
Spasticity and rigidity are both…
Forms of hypertonia (excessive muscle tone)
What is spasticity. Upper or lower?
Hyperactive stretch reflexes, hypertonus more evident in axial/extensor muscle groups; damage to UMNs, internal capsule, cord
What is rigidity? Disease of what structure?
Hypoactive stretch reflexes, hypertonus equal between extensors/flexors; basal ganglia disease (Parkinson’s)
Cortical areas that control movement (4)
- Overall plan areas (pre-motor areas); 2. Primary motor cortex (M1); 3. Areas responsible for concept of extrapersonal space and motor goals (parietal); 4. Sensory-feedback areas
Defining features of motor cortical areas (3)
- All “fire” before movement onset; 2. All have direction connections with spinal cord; 3 Unique histological features (output layers V and VI are dominant)
What percentage of corticispinal neurons are small? What about large (and name)?
90% = small; 10% = Betz
Describe the function of small corticospinal neurons
Slow, fine movements: firing proportional to force, selective to direction, sensitive to sensory feedback
Describe the function of Betz cells
Ballistic, powerful movements: all-or-none firing, not directionally selective or sensitive to sensory feedback
Pre-motor cortex is involved in what? What side?
Preparing upcoming motor acts based on sensory-related cues; contralateral
Supplementary motor cortex is involved in what? What side?
Internally generated motor planning (rehearsal); bilateral
UMN syndromes…EMG elicits?
Weakness, hypertonia, hyper-reflexia, spasticity, mild atrophy; EMG = decreased muscle recruitment
LMN syndromes…EMG elicits?
Weakness, hypotonia, hypo-areflexia, significant atrophy; EMG = abnormal firing of LMNs, fasciculations
Motor exam: observation (5)
Observe: gait, symmetry, bulk, atrophy, fasciculations
MRC scale uses what numbers? Describe the key middle number
0 - 5; 3 = movement against gravity
UMN deficit: weakness distributing (adductors vs abductors).
Adductors are stronger because abductors are affected MORE
UMN deficit: In upper extremities, flexors or extensors affected?
Extensors affected (arm flexed in)
UMN deficit: In lower extremities, flexors or extensors
Flexors affected (can’t flex foot = foot drop) –> anti-gravity muscles
Reflex scale, what is normal?
0 - +4, +2 = normal
Babinski equivalent in upper body
Hoffman sign (finger flicking)
Describe hemiplegic gait
Arm in flexion with extensor weakness, hand and finger tightly clenched, outward swinging of leg
Spastic gait
Both legs are stiff, walking slow and difficult, hyperreflexia, clonus
Frontal gait
Seen in normal pressure hydrocephalus; looks like feet are stuck to ground, small steps (magnetic gait)
ALS involves both…must include some symptoms above
UMN and LMN; foramen magnum (because of cervical spondalosus on differential)
Describe ALS. What will the history be like?
Progressive diffuse LMN w/ UMN dysfunction, pelvic floor muscles affected; history = fasciculations with weakness over weeks to months, clumsiness, difficulty swallowing
UMN in ALS
Stiffness, spasticity, clumsiness, hyperactive reflexes, Babinski
LMN in ALS
Weakness, wasting/fasciculations, hypo/areflexia
What is spared in ALS?
EOMs, intact bowel/bladder, no cognitive or sensory changes
What must be excluded in ALS?
Cervical sondylosis (so look at jaw jerk reflex), tumor, syrinx
Primary lateral sclerosis (compare to ALS)
ALS minus LMN involvement (only UMN)
Progressive spinal muscular atrophy (LMN/UMN and cause)
Only LMN involvement; congenital
Treatment and prognosis of ALS
Treatment: Riluzole; Prognosis: months to 3-4 years after Dx
How do you get UMN and LMN in cervical spondylosis?
UMN = spinal cord compression; LMN = nerve roots but NO INVOLVEMENT ABOVE FORAMEN MAGNUM
Guillain-Barre (description and causes)
Acute autoimmune polyneropathy: rapidly progressing paralysis, ascending w/ dysautonomia; often follows infection (campylobacter jejuni)
When you’re _____ with G-B, what is lost?
Weak; reflexes
How do you Dx G-B? Treatment?
Increased CSF protein w/out increased cells (albuminocytological dissociation); plasmapheresis
Miller Fisher sydrome
Like G-B, but w/ ophthalmoplegia and ataxia
Steppage gait is characteristic of what disorder?
LMN neuropathy; patient picks up foot with conscious effort to avoid tripping over it by lifting up proximal leg
What is the most common disorder of NMJ? Describe. Important clinical symptom.
Myasthenia gravis; antibodies targeted against AChR; reduction of numbers of AChR and damage to post-synaptic membrane; fatigable
Differential diagnosis for Myasthenia gravis
Eaton Lambert, botulism, myopathy
Myasthenia gravis treatment
AChE inhibitors, thymectomy, corticosteroids, IVIG/plasmaphoresis (can be quite effective)
Myopathy (muscle problem) symptoms
Weakness is proximal (not distal), normal sensation, muscle pain/cramps, DTRs preserved relatively
How to diagnose a myopathy
Look for elevated CK, EMG, muscle biopsy, genetic testing
One important myopathy to know…
Statin-induced myopathy (tested w/ elevated CK)
Classification of myopathies (general, 2)
Hereditary and acquired
Myotonia is a disorder of the…symptom?
Muscle membrane; delayed relaxation after contraction
Botulism and infant botulism cause and presentation
Toxin prevents ACh release, descending paralysis, respiratory failure; acute floppy baby (due to spore ingestion)
You can get paralysis by these two animals. These both look like what?
Ticks, fish (from consumption); G-B syndrome
What area of the spinal cord is affected in polio? UMN or LMN? What kind of paralysis?
Anterior horn cells; LMN lesion only; flaccid paralysis
T/F: ALS sometimes has sensory involvement
False! ALS is motor neuron only
