Chapter 3: Motor Paralysis

Question 1

What comprises the motor unit?

Answer 1

nerve cell + its axons + muscle fibers it subserves

Question 2

What is the Sherrington Law?

Answer 2

“aka Law of Reciprocal innervation

the extensor muscles must relax at the same rate as the flexors contract “

Question 3

What is an example of isotonic contraction?

Answer 3

motor activities that alter the position of a limb or other parts of the body

Question 4

What is an example of isometric contraction?

Answer 4

motor activites that stablize the posture

Question 5

What to you call movements that are performed slowly?

Answer 5

Ramp movements

Question 6

What do you call very rapid movements which are too fast for sensory control?

Answer 6

Ballistic or phasic

Question 7

Describe the triphasic movement of large motor units.

Answer 7

“1. Initial burst of activity in agonist muscles

2. burst in the antagonists
3. smaller burst in the agonist

Large agonist burst -> antagonist burst -> small agonist burst”

Question 8

What determines the speed and distance of the movement?

Answer 8

Strength of the initial agonist burst

Question 9

What sets the pattern and timing of the muscle action in any projected motor performance?

Answer 9

Basal ganglia and cerebellum

Question 10

What mediates the suprasegmental control of the axial and proximal limb musculature (antigravity postural mechanisms)?

Answer 10

Reticulospinal Tract

Vestibulospinal Tract

Question 11

The muscle stretch (Tendon) Reflex and Muscle Tone is dependent on what?

Answer 11

1. alpha motor neurons (large anterior horn cells)
2. muscle spindles and their afferent fibers
3. gamma neurons (small anterior horn cells)
   …whose axons terminate on the specialized intrafusal muscle fibers (nuclear chain fibers) within the spindles

Question 12

Describe the process of the monophasic (myotatic) stretch reflex?

Answer 12

“aka. Tendon reflex/Tendon Jerk

tap on a tendon stretches -> vibratory wave of the spindle and activates its nuclear bag fibers -> afferent projections -> synapse directly with alpha motor neurons in the same and adjacent spinal segments -> impulses to skeletal muscle fibers

within 25ms”

Question 13

What happens with the alpa motor neurons of the antagonist muscles during the tendon jerk?

Answer 13

the alpha neurons of antagonist muscles are simultaneously inhibited but through disynaptic rather than monosynaptic connections

accomplished by:

1. Reciprocal inhibition (inhibitory interneurons) which also receive input from descending pathways.
2. Recurrent inhibition - Renshaw cells also participate by providing negative feedback through inhibitory synapses of alpha motor neurons

Question 14

What determines the level of activity of the tendon reflexes and muscle tone (the responsiveness of muscle to stretch)?

Answer 14

Tension in the spindle fibers and the state of excitability of the alpha and gamma neurons (influenced greatly by descending fiber system)

Question 15

Where is the alpha motor neurons located?

Answer 15

anterior gray matter (anterior horn) of the spinal cord

Question 16

Which part of the anterior horn supply the trunk or axial muscles?

Answer 16

Medial part of the anterior horn

Question 17

Which part of the anterior horn supply the appendicular muscles?

Answer 17

Lateral parts of the anterior horn

Question 18

What is the neurotransmitter used at the NMJ by the large neurons of the anterior horns of the spinal cord?

Answer 18

Acetylcholine

Question 19

What is the main NT of the descending corticospinal tract?

Answer 19

Aspartate and Glutamate

Question 20

What is the NT released by Renshaw cells (responsible for recurrent inhibition), as well as the NT released by interneurons that mediate reciprocal inhibition during reflex action?

Answer 20

Glycine

Question 21

What is the inhibitory NT of interneurons in the posterior horn?

Answer 21

GABA

Question 22

What are typical findings of lower motor neuron lesion?

Answer 22

flaccidity
hypotonia/atonia
areflexia

Question 23

What is the % of muscle atrophy in denervation atrophy?

Answer 23

extreme atrophy
reduced 20-30% of its original bulk within 3 to 4 months

*worse than disuse atrophy

Question 24

What is the probable cause of spinal shock?

Answer 24

interruption of the descending tonic excitatory impulses (which normally maintain a sufficient level of excitation in spinal motor neurons to permit the peripheral activation of segemental reflexes

Question 25

What medication can attenuate spinal shock?

Answer 25

“Naloxone (opiate antagonist)

• **suggest that spinal shock is mediated by the release of previously stored endogenous opiates from the distal terminals of neurons in the periaqueductal gray matter
• > once depleted the presynaptic inhibition of motor neuron ceases -> end of spinal shock and beginning of the period of spasticity”