Neuro: Lower Motor Neuron Circuits And Motor Control

Front 1

All commands for movement, whether reflexive or voluntary, are ultimately conveyed to the muscles by the activity of the ____________ ; thus these neurons comprise what is called the “final common path” for initiating movement.

A. Basal ganglia
B. Cerebellum
C. Lower motor neurons
D. Upper motor neurons

Back 1

C. Lower motor neurons

Front 2

Another motor subsystem is the ____________, whose cell bodies lie in the brainstem or cerebral cortex and whose axons descend to synapse with local circuit neurons or with lower motor neurons directly.

A. Basal ganglia
B. Cerebellum
C. Lower motor neurons
D. Upper motor neurons

Back 2

D. Upper motor neurons

Front 3

_____________ acts via efferent pathways to the upper motor neurons detecting and attenuating the difference or “motor error” between an intended movement and the movement actually performed. Thus, real-time adjustment and long-term reductions in these inevitable motor errors (a form of motor learning) occur.

A. Basal ganglia
B. Cerebellum
C. Lower motor neurons
D. Upper motor neurons

Back 3

B. Cerebellum

Front 4

________ suppresses unwanted movements and prepare upper motor neuron circuits for the initiation of movements.

A. Basal ganglia
B. Cerebellum
C. Lower motor neurons
D. Upper motor neurons

Back 4

A. Basal ganglia

Front 5

Neurons that innervate the axial musculature (like postural muscles of the trunk) are located [ laterally / medially] in the spinal cord.

Back 5

medially

Front 6

Compare and contrast medial local circuit neurons of the lower motor neurons to that of the lateral local circuit neurons of the lower motor neurons with regards to:
A. what horn they are located in
B. difference in how many projections to spinal cord segments
C. ipsilateral versus bilateral

Back 6

Medial local circuit neurons are:
A. in medial ventral horn
B. have axons that project to many spinal cord segments
C. including some that crosst he midline (bilateral)

In contrast, the lateral local circuit neurons
A. are of the intermediate zone
B. have shorter axons that extend to fewer than five segments
C. are predominantely ipsilateral

Front 7

Two types of lower motor neurons exist in motor neuronal pools of the ventral horn. [ alpha / gamma ] motor neurons innervate specialized muscle fibers that, in combo with the nerve fibers that innervate them, are actually sensory receptors called muscle spindles called intrafusal muscle fibers.

Back 7

gamma motor neurons

Front 8

Two types of lower motor neurons exist in motor neuronal pools of the ventral horn. [ alpha / gamma ] motor neurons function to regulate the sensory input by setting the intrafusal muscle fibers to an appropriate length.

Back 8

gamma motor neurons

Front 9

Two types of lower motor neurons exist in motor neuronal pools of the ventral horn. [ alpha / gamma ] motor neurons innervate extrafusal muscle fibers which are striated muscle fibers that actually generate the forces needed for posture and movement

Back 9

alpha motor neurons

Front 10

True or False:

• A motor unit is comprised of a single alpha motor neuron and its associated muscle fibers. They can constitute the smallest unit of force to produce a movement.

Back 10

True

Front 11

Motor units differ in the types of muscle fibers they innervate. Small motor units are also called [ S / FF / FR ].

Back 11

S or slow motor units

Front 12

Which type of muscle fiber does this describe?
• Comprised of small alpha motor neurons
• Generate small forces
• Are resistant to fatigue
• Important for activities that require sustained muscular contraction
• Have lowest thresholds for activation
• Are tonically active during motor acts

Back 12

S or slow motor units

Front 13

Which type of muscle fiber does this describe?
• Important for brief exertions that require large forces
• Fastest
• Generate the largest forces
• Fatiguable

Back 13

FF or fast fatiguable motor units

Front 14

Which type of muscle fiber does this describe?
• Are of intermediate size
• Generate 2x force of S, but not as much as FF.
• Resistant to fatigue

Back 14

FR of fast fatigue-resistant motor units

Front 15

What is the recruitment order of the various motor units?

(FF, FR, S)

Back 15

S → FR → FF (but can’t count on FF for a long time)

Front 16

[ Nuclear bag fibers / Nuclear chain fibers ] relates velocity of stretch; if stretched/jerked, responds quickly.

Back 16

Nuclear bag fibers

Front 17

[ Nuclear bag fibers / Nuclear chain fibers ] monitors stretch and tells us where we’re stretching ; doesn’t respond as quickly.

Back 17

Nuclear chain fibers

Front 18

True or False:
Both nuclear bag fibers and nuclear chain fibers have Group Ia afferents coiled around the central part of each class of intrafusal fiber but only nuclear chain fibers have group II afferents.

Back 18

Front 19

Detail the stretch reflex.
- afferent fibers?
- mono/di/polysynaptic?
- output?
- other innervation?

Back 19

Hint 19

• Input: group Ia afferent senses increased load
• Monosynaptic into CNS
• Output: homonymous alpha motor neuron activated
• Note: reciprocal innervation is also induced (synergistic muscles + , antagonistic muscles - )

Front 20

Golgi tendon organs are encapsulated afferent nerve endings located at the junction of a muscle and a tendon. Each tendon is inn. by a single group of ___________.

Back 20

Type Ib neurons

Front 21

True or False:
Golgi tendons are very sensitive to increases in muscle tension due to muscle contraction but, unlike spindles, are relatively insensitive to passive stretch.

Back 21

True

The muscle spindle system is a feedback system that monitors and maintains muscle length, and Golgi tendon system is a feedback system that monitors and maintains muscle force.

Front 22

Detail the Golgi Tendon reflex.
- afferent fiber input
- mono/di/polysynaptic?
- output?

Back 22

Hint 22

• Increase in muscle force is sensed by GTO. The Ib afferent comes into the spinal cord and activates an inhibitory interneuron that inhibits the alpha motor neuron of the homonymous muscle. (The response to contraction is to come back and inhibit).
o Note that this is thus a disynaptic inhibition
o Reciprocal innervation occuring: synergists - , antagonists +

Front 23

Detail the flexion reflex pathway, where stimulation of nociceptive sensory fibers (Groups II, III’s alpha-delta, IV’s C fibers) leads to withdrawal of the limb from the source of pain. How?

Back 23

Hint 23

o Excitation of ipsilateral flexor muscles and reciprocal inhibition of ipsilateral extensor muscles. Flexion of stimulated limb is also accompanied by an opposite reaction in the contralateral alimb (ie. Contralateral extensor muscles are excited while flexor muscles are inhibited).
• Ipsilateral flexion + (withdrawl)
• Ipsilateral extension – (inhibited)
o This crossed extension reflex provides postural support during withdrawal of the affected limb from painful stimulus.
• Contralateral flexion – (inhibited)
• Contralateral extension + (withdrawl)

Front 24

• Complex signs and symptoms that can occur from damage to the lower motor neurons of the brainstem and spinal cord is referred to as the “lower motor neuron syndrome” and include what 4 + 1 later = 5 symptoms?

Back 24

- Paralysis (loss of movement) or paresis (weakness) of affected muscles, depending on damage
- loss of reflexes (areflexia)
- loss of muscletone
- a later effect of atrophy of the affected muscles due to disuse and denervation
-fasciculations

Front 25

What disease is caused by slow degeneration of alpha-motor neurons in ventral horn of the spinal cord and brainstem (lower motor neurons) and of neurons in the motor cortex (upper motor neurons)?

Back 25

ALS (Amyotrophic lateral sclerosis)