Motor systems - Persons

Question 1

Define a motor unit.

Answer 1

A motor unit is defined as an α motor neuron and the muscle fibers it innervates. A given muscle fiber is innervated by A SINGLE motor neuron.

A given motor neuron innervated many muscle fibers.

Question 2

Where is somatotopy seen in the spinal column, with regard to UMN and alpha motor neurons?

Answer 2

The anterior horn of the spinal cord is organized somatotopically, such that the shoulders are medial and the forearms/hands are lateral (a reflected mirror image about the midline).

Question 3

How are muscles recruited? Name and describe the three types of motor units discussed in lecture.

Answer 3

Slow: Small motor units, responsible for sustained activities (eg posture).

Fast Fatigue Resistant: Fast-twitch oxidative. Medium axon/fiber group size.

Fast Fatigable: Primarily glycolytic. Recruited at maximum effort (last). Large, myelinated axons innervate large groups of fibers.

Question 4

How does exercise change the proportion of slow, fast fatigue-resistant, and fast-fatigable motor units?

What determines the type of muscle fiber?

Answer 4

Exercise tends to replace FF with FFR, particularly endurance exercise.

Fiber type is determined by the motor neuron identity. For example, repeated stimulation of a neuron will tend to convert those fibers from FF to FFR or S, similar to the effect of exercise.

Question 5

What are the extrafusal muscle cells?

What is the innervation of the extrafusal cells?

Answer 5

Force generating fibers.

Innervated by the alpha motor neurons.

Question 6

What are the intrafusal cells?

What is the innervation of the intrafusal cells.

Answer 6

Spindle cells.

Have dual innervation.

1) include raw nerve endings with mechanosensors attached to the Ia sensory afferents (and some type II afferents)
2) Receive innervation (efferent) from gamma motor neurons

Question 7

When the patellar tendon is tapped, what circuit is activated? Describe the depolarization pattern of the neurons involved.

Answer 7

The stretch reflex.

Type Ia afferents within the reflex arc directly contact alpha motor neurons that contract the homonymous muscle – that is, the muscle that has been stretched (Quads). Ia afferents also contact inhibitory interneurons that reside within the spinal cord that then inhibit motor neurons controlling the opposing “antagonist” muscle (Hammies) . This means that during the stretch reflex, synergist and antagonist muscles are coordinated to contract and relax nearly simultaneously.

Question 8

What are the Golgi Tendon Organs (GTOs)? Where do they reside? What stimulus do the preferentially detect?

What is the afferent neron type?

Answer 8

Reside in the muscular/tendon junction. These are mechanosensors embedded in the collagen mesh of the tendon that preferentially detect muscle tension rather than passive stretch. They are in series, not in parallel (spindle cells are in parallel).

Ib afferent.

Mnemonic: finger trap. Admiral Ackbar.

Question 9

When you pick up something that is lighter than anticipated, what is the series of events that keeps you from flinging it into the air?

Answer 9

1) Extrafusal fibers contract more than expected
2) Intrafusal fibers go “slack”
3) Ia afferents leaving spindle fiber drop firing rate
4) alpha motor neuron reduces drive (decrease in tone)
5) less recruitment (muscle lengthens)

Heavier is the opposite. This is called rapid error correction or something equally vague.

Question 10

Muscle spindles maintain muscle ____; muscle GTOs maintain muscle ____.

Answer 10

length

force

Question 11

Explain why Ib GTO afferents inhibit the homonymous muscle and excite the antagonist muscle. Also explain how.

Answer 11

Ib afferents innervate both excitatory and inhibitory interneurons.

The antagonist muscle is excited, the homonymous muscle is inhibited. This tends to STABILIZE CONTRACTIONS.

Question 12

What is reciprocal innervation? (Basic sense)

Answer 12

Basically, it’s when a neuron innervates the homonymous and antagonist muscle. So the Ia afferents and the Ib afferents would both be examples, as both send connections to the homonymous and antagonist muscle groups.

Question 13

Lets learn more bizarre nomenclature that sounds like script from a B-list SciFi flick.

What is “Crossed Extensor Flexor Coupling” and “Extensor Flexor Coupling Circuits?”

Answer 13

The system that activates when you step on a tack.

Basically, this impulse causes relaxation of the ipsilateral extensor, contraction of the ipsilateral flexor, and contraction (stabilization) of the contralateral side (extensors).

Question 14

Send me back to the Central Patterning Generator, Scotty! I can’t walk!

Please explain the salient points of CPG so we can get this over with. (There are 5, believe it or not).

Answer 14

It is concluded that CPGs are responsible for coordinated extension-flexion alternation during locomotion.

1. CPG has a clock**
2. The ‘clock’ drives muscle contraction and relaxation.
3. CPGs interact via commissural fibers to coordinate between-limb use.
4. CPGs are modulated by descending pathways
5. Some of the same circuits involved in simple reflexes discussed earlier are flexibly engaged in locomotion.

Swing phase–> flexors active
Stance phase–> extensors active

**If the clock is Muslim, it will be arrested, resulting in paralysis.

No idea how this question will be asked on exam. Humans require more descending input than cats, loading on the knees and hips seems to trigger it more than moving the feet.

Question 15

Lateral white matter tracts carry ___ and ___ pathways controlling precise skilled movements involving distal parts of the limbs.

Answer 15

corticospinal; rubrospinal

Question 16

Brainstem projections within the medial ventral white matter primarily concern posture, balance, and orienting mechanisms. These pathways are (3):

Answer 16

Reticulospinal - anticipatory motor control (cut the RS tract, the cats will topple over), locomotor initiation

Colliculospinal (Tectospinal) - orientation of gaze and stance to a stimulus in space (explosions). Pineal tumor can prevent upward gaze. [Integration of auditory and visual maps]

Vestibulospinal - unconscious responses to vestibular stimulation. (Jamie Lannister)

Question 17

What are some differences between voluntary and reflexive movements?

Answer 17

Voluntary - can improve with practice, can be initiated internally.

Question 18

Differentiate between the primary motor cortex (M1), the premotor cortex, and the supplementary motor cortex.

Answer 18

M1 - CS tract, basic motor efferent

Premotor - Located laterally. Involved when movement is initiated by an external cue. For example, premotor cortex will be highly activated if a subject is asked to move his hand in response to a light stimulus. Also shows “mirrored movement”

Supplementary motor - Located medially. more complex, “self-cued” voluntary movement (eg mental rehearsal).

Question 19

Benefits of plasticity?

Answer 19

Injury recovery (eg stroke)
Perfect practice makes perfect
Brain-machine interfaces

Question 20

What differentiates the motor cortex from the sensory (in terms of cortical mapping)?

Answer 20

Motor is more plastic (no strict somatotopy), and less precise than the sensory. This is due to regulation of interneurons, as well as activation of muscle units, which may not be in the same location.

Think of the monkey bringing its hand to mouth regardless of initial position, based on stimulation of the same small region of cortex.

Question 21

Do single neurons in the motor cortex define directional motor movement?

Answer 21

No. Populations of neurons encode directional movement. Not single neurons. Monkey/joystick experiment. Direction is the average of all APs.

Question 22

Does degree of spasticity in UMN syndrome differ based on the latitude of the lesion?

What surgical intervention can help reduce spasticity? How might this work?

Answer 22

Yes. With spinal cord level damage, extensor muscles cannot support body even though they are rigid. With cortical or capsular damage they can sometimes. Called decerebrate rigidity.

Transection of dorsal roots can ameliorate spasticity possibly because it represents abnormal gain of the spinal cord stretch reflexes due to loss of descending inhibition.

Question 23

You are driving up to a yellow light in your car going about 45 mph. You quickly decide to run the red light because you don’t think you will be able to stop in time. Which part of your brain made this decision?

Answer 23

The lateral premotor cortex (aka ventral motor cortex) is responsible for learned movements in response to external stimuli. This area also plans movements that are to be executed with delay (i.e. conforming your hand to the shape you are going to grasp). Planning here begins up to a second before the activity begins.

Question 24

Your dog gets SO excited every time you reach for the leash that is hanging by the door. Every time you reach for it, he comes over and sits next to you wagging his tail. He does this because you have trained him to. Which region of his brain is responsible?

Answer 24

The lateral (ventral) premotor cortex does learned movement in response to a stimulus. Planning begins up to a second before the movement starts.

Question 25

You are about to run a 400 meter race. You have practiced every day for the past 3 months. You close your eyes and envision yourself starting the race, doing what you practiced and finishing perfectly, just as planned. Which part of your brain are you using?

Answer 25

The supplementary motor cortex is involved in the mental rehearsal of planned movements. Both the supplementary and premotor cortices are located ANTERIOR to the primary motor cortex.

Question 26

You write down your name on a piece of paper. Which part of the brain are you using?

Answer 26

Memorized tasks that are performed for a sufficiently long time shift from being controlled in the supplementary motor cortex and are eventually controlled by the primary motor cortex.