Motor System I, II, and III

Question 1

Interestingly, there is a tunicate (a small, waterborne organism) that ________ its brain after implanting into a stationary location.

Answer 1

dissolves

Question 2

The local circuit neurons do what?

Answer 2

They coordinate lower motor neurons (so that similar groups function together)

Question 3

The motor unit is defined as _______________.

Answer 3

the motor neuron and the muscle fibers that it innervates

Question 4

Another name for lower motor neuron is ____________.

Answer 4

alpha motor neuron

Question 5

Describe the three size categories of motor units.

Answer 5

Slow: recruited first; small number of muscle fibers; generate small forces; fatigue slowly; high input resistance

Fast, fatigue-resistant: recruited second; medium number of muscle fibers; generate large forces; fatigue slowly

Fast, fatigable: recruited last; large number of muscle fibers; generate large forces; fatigue fast

Question 6

True or false: small motor neurons have higher resistance.

Answer 6

True! (Fewer channels)

Question 7

What is the consequence of increased resistance in smaller neurons?

Answer 7

Smaller neurons will have a higher voltage EPP with the same current input compared to large neurons.

Question 8

Weirdly, the muscle fiber type (fast-twitch or slow-twitch) is determined by ____________.

Answer 8

the type of motor neuron that stimulates it; switching neuron types will induce the muscle fiber to switch

Question 9

Just anterior to the lateral corticospinal tract in the spine is the _____________. Where does this come from?

Answer 9

Rubrospinal tract; midbrain

Question 10

The stretch reflex is also called the __________.

Answer 10

myotatic reflex

Question 11

A alpha sensory fibers connect to what subtype of muscle fibers?

Answer 11

Intrafusal – a type of muscle spindle that senses stretching; it is embedded within extrafusal muscle (the standard kind of muscle that responds to lower motor neuron stimulation

Question 12

How many lower motor neurons do A alpha neurons innervate?

Answer 12

Many; they innervate multiple neurons that work together as well as inhibitory neurons that synapse on opposite muscles to relax opposite to direction of the reflex

Question 13

What is the basis of the stretch reflex (that is, what is the reason we have it)?

Answer 13

It is a feedback mechanism for when things are heavier or lighter than we expect. For instance, if you expect something to be heavy, your extrafusal muscle fibers will contract a lot in anticipation; if it is lighter, then you will suddenly over contract and the intrafusal fiber will become slackened, producing a decrease in basal firing tone and causing the extrafusal muscles to stop contracting as much. The opposite also happens.

Question 14

Golgi tendon organs lie between _____________.

Answer 14

muscles and tendons in a cartilaginous capsule

Question 15

Overstretching muscles leads to inhibition via the _____________.

Answer 15

A beta neurons leading off the Golgi tendon organ

Question 16

True or false: Golgi tendon neurons mediate the passive stretch reflex.

Answer 16

False! Golgi tendon organs balance active stretching, while the A alpha neurons from the intrafusal fibers mediate the passive reflex

Question 17

Golgi tendon organs are not ___________.

Answer 17

contractile (they just sense stretching and inhibit excessive motor neuron stimulation)

Question 18

Pain fibers synapse on ___________ in the spinal cord to mediate the pain reflex. The coupling is opposite, so if you step on a nail, then the injured leg will flex while the opposite leg will extend.

Answer 18

excitatory interneurons (a network of neurons that coordinates a retracting movement)

Question 19

What is central pattern generation?

Answer 19

A process in which rhythmic motor actions occur in the absence of sensory or cerebral involvement

Question 20

What is the evidence for central pattern generation being generated in the spinal cord?

Answer 20

Cats with thoracic spinal cord transections can still generate left-right walking with the assistance of a treadmill; further, severing the dorsal roots eliminates this ability, but adding L-DOPA regenerates the CPG!

Question 21

What part of the brain controls the axial muscles?

Answer 21

The brainstem

Question 22

What is channelrhodopsin?

Answer 22

An experimentally created sodium channel that opens in response to blue light; other channels exist for inhibitory channels.

Question 23

Central pattern generation seems to begin in the _____________.

Answer 23

mesencephalon

Question 24

What tract mediates the unexpected fall reflex?

Answer 24

The vestibulospinal tract

Question 25

The cortex is not set up to innervate just one muscle. How does this happen?

Answer 25

Upper motor neurons synapse on many lower motor neurons.

Question 26

What research indicates that the motor cortex operates at a higher level (that is, not simple muscle activation)?

Answer 26

Stimulating the brains of macaque monkeys elicits certain complex actions that seem purpose-driven; for instance, stimulating one area caused the monkeys to bring their hands to their mouths regardless of where the hand started out.

Question 27

What is the big problem in neural bypass technology?

Answer 27

The electrodes “die.” A few months after implanting them, the electrodes stop working.

Question 28

What is the basic purpose of gamma motor neurons?

Answer 28

They stimulate the intrafusal muscle spindles to contract when the extrafusal muscles contract, thus making sure that muscle spindles remain sensitive to stretch across a range of lengths.

Question 29

Intrafusal spindles rely on ___________ neurons, and Golgi tendon organs rely on ___________.

Answer 29

A-alpha; A-beta

Question 30

I-a neurons also synapse on _______________.

Answer 30

inhibitory interneurons for the muscles that go counter to direction of the reflex (I-b neurons do the opposite)

Question 31

Anticipatory control, such as when the calf muscles flex in anticipation of flexing the biceps, is mediated by _______________.

Answer 31

the reticular formation