1 Motor Systems Weyand Narrative

Question 1

What does feedback tell us?

Answer 1

whether what we wanted to do was actually done

Question 2

What does threshold do?

Answer 2

facilitate to do or not to do an act

Question 3

What part of the brain does feedback and corrections run through?

Answer 3

cerebellum

Question 4

Thresholding and behavior selection occurs where?

Answer 4

basal ganglia

Question 5

what does alpha motor neuron innervate?

Answer 5

extrafusal fibers

Question 6

what does gamma motor neuron innervate

Answer 6

intrafusal fibers of the muscle spindle organ

Question 7

where are lower motor neurons located?

Answer 7

Rexed’s Lamina IX (ventral horn of spinal cord) + motor nuclei of cranial nerves

Question 8

What is the neurotransmitter of LMNs? What receptor do they bind to?

Answer 8

Acetylcholine; Nicotinic Cholinergic Receptors (nAch)

Question 9

What is Polio? What does it do?

Answer 9

Polio is a virus that attacks LMNs

Question 10

What is Amyotropic Lateral Sclerosis (ALS)? What does it do?

Answer 10

ALS attacks and wastes both upper and lower motor neurons. Sometimes only LMNs.

Question 11

What are the 3 types of muscle fibers?

Answer 11

slow twitch (least force, high endurance; lots of mitochondria), fast twitch fatigue resistant (fewer mitochondria), fast twitch fatigueable (twice as powerful, little stamina)

Question 12

How do muscles increase power?

Answer 12

increase frequency of output & if not enough, increase number of motor units (recruitment)

Question 13

What is the size principle? How does it apply?

Answer 13

LMNs doing the most work have small cell bodies & innervate slow-twitch fibers. When there is a demand, they increase frequency of action potentials. If help is needed, they recruit in order from small cell bodies to large cell bodies (slow twitch fibers, fast twitch fatigue-resistant fibers, and lastly fast-twitch fatiguable fibers - which have the largest cell bodies and power)

Question 14

What are the components of the myotactic reflex? How does it work?

Answer 14

Sensory (Ia fibers attached to muscle spindle receptor) & Motor (alpha motor neuron). Stretch of the muscle (via tapping tendon) also stretching muscle spindle organ, which sets off Ia fiber to produce several action potentials. “contract! contract! you’re being stretched!” to the alpha motor neuron wired to the stretched muscle. Ia fibers also has a collateral that makes synaptic contact with an inhibitory interneuron (Ia inhibitory interneuron) that inhibits the alpha motor neuron going to the antagonistic muscle. (called Reciprocal Inhibition). Ia fibers, thirdly, send a ‘cc’ of what’s going on to the brain via ascending lemniscal fibers & ascending spinocerebellar fibers, which finds its way back up to the motor cortex.

Question 15

What does the Spinocerebellar Tract convey?

Answer 15

proprioception via muscle spindle fibers & Golgi tendon organs. cell bodies in DRG, synapse in lamina VII (Dorsal Nucleus of Clark)

Question 16

What is the purpose of the gamma motor neuron?

Answer 16

ensures the muscle spindle organ is always “loaded” (under tension) - attaching to the two poles of the muscle spindle & stretching the bag. This allows Ia fibers to provide continuous readings of muscle length via muscle spindle organ receptor.

Question 17

What is the life purpose of the Ia fibers?

Answer 17

provide information on muscle length – tells us about current posture,& position in space (proprioception)

Question 18

What does the Romberg Test do?

Answer 18

testing equilibrium of Ia fibers (close eyes and remain balanced); any body sway is a positive test

Question 19

What are the components of the Inverse Myotactic Reflex? How does it work?

Answer 19

Afferent limb (Ib fibers innervating Golgi Tendon Organ). Golgi Tendon Organ is in series with extrafusal muscle at the end of the muscle with the tendons. During muscle contraction, capsule stretches & Ib fibers are squeezed by collagen fibers (opens Na+ channels) & action potentials fire (proportional to contractile force). Ib fibers monosynaptic contact with inhibitory interneurons, which contact with alpha motor neurons innervating contracting muscle. (disynaptic reflex) “I’m being squeezed! stop! stop!”. Ib fibers also excites (excitatory interneuron) antagonistic muscle to contract (Reciprocal Inhibition). Ib fibers also send a ‘cc’ of its output up the dorsal columns to the cerebellum (via spinocerebellar fibers), which finds its way to the motor cortex

Question 20

How does Decerebrate Rigidity present?

Answer 20

extension of all 4 limbs

Question 21

What is the cause of Decerebrate Rigidity?

Answer 21

loss of both corticospinal tract & rubrospinal tract, leaving the lateral vestibulospinal and medial reticulospinal tracts; the lateral vestibulospinal and medial reticulospinal tracts have a bias for extensors. They cause gamma motor neurons to fire, which causes Ia fibers to fire, which causes a continuous myotactic reflex.

Question 22

what do the lateral vestibulospinal and reticulo spinal tracts have a bias for?

Answer 22

extensors & gamma motor neurons.

Question 23

What is posture?

Answer 23

the relative position of all the limbs

Question 24

How does the myotactic reflex regulate postural control?

Answer 24

By setting muscle length, one is setting posture.

Question 25

How does the myotactic reflex regulate tone?

Answer 25

tone can be set high or low, reflecting the relative activation of flexors and extensors. (high tone is resistant to passive movement) Tone is asserted by corticospinal & vestibulospinal tracts.

Question 26

What do UMNs do relative to LMNs?

Answer 26

UMNs tell LMNs WHAT to do. NOT HOW.

Question 27

how does LMN damage present?

Answer 27

paralysis, muscle atrophy, fasciculations (twitches)

Question 28

how does UMN damage present?

Answer 28

paresis, apraxia, odd reflex responses (Babinski Sign, Hoffman’s Sign), resistance to passive movement (spasticity)

Question 29

What is the Babinski Sign? How is it performed?

Answer 29

Stroke bottom of foot, positive sign shows hyperreflexia dorsiflexion of big toe as other toes fan out; (normal in infants)

Question 30

What is Hoffman’s Sign? How is it performed?

Answer 30

Tapping or flicking of nail or distal phalynx of middle or ring finger, positive sign shows hyperreflexia flexion of thumb

Question 31

What neurons (of which tracks) are considered upper motor neurons?

Answer 31

neurons whose axons lie within the corticospinal tract (IMPORTANT), rubrospinal tract, vestibulospinal tract, reticulospinal tract, & corticonuclear tract