2 - Spinal Cord and Motor Unit

Question 1

What are the three functions of skeletal muscles?

Answer 1

1. Execute voluntary movements, generate tension by contracting, pulling.
2. Maintain posture of body with respect to gravity
3. Product heat and metabolic energy (80% of muscle activity)

Question 2

Skeletal muscles are made of parallel bundles of ______, which are made of ______ _____ that contract.

Answer 2

Bundles of fascicles.

Fascicles are made of muscle fibers that contract.

Question 3

What are synergistic muscles? What are antagonistic muscles?

Answer 3

Synergistic: muscles that work across the joint together to produce similar actions (ie biceps and brachialis to cause flexion of arm.

Antagonistic: muscles that work in opposite directions against each other such as biceps and the triceps.

Question 4

What is the acton of flexors? Extensors?

Answer 4

Flexors: decrease the angle between bones or parts of the body

Extensors: increase the angle between bones or parts of the body.

Question 5

How does our nervous system directly control our muscles?

Answer 5

Lower motor neurons (efferents) are the final common pathway for movement.
-only way our nervous system can control movement of limbs and body both voluntary and involuntary.

Higher levels CNS (cortex, brainstem, basal ganglia, and cerebellum) influence lower motor neurons.

Question 6

What are muscles activated by? Describe these.

Answer 6

Muscles are activated by motor units.

A motor unit is one motor neuron and all of the muscle fibers it innervates. (all of the muscle fibers innervated by 1 motor neuron).

Question 7

What is a motor unit?

Answer 7

Smallest functional element of the motor system.

Smallest unit of muscle under neural control.

Smallest increment of tension that can be generated.

Question 8

What does the size of a motor unit vary with?

Answer 8

The precision of muscle movement for specific muscles.

Small muscle: small motor units generate small increments of force.

Large muscle: large motor units generate large increments of force.

Question 9

What are two things that influence the amount of force generated?

Answer 9

The number of muscle fibers in a motor unit.

The number of motor units activated.

Question 10

What is the organization of innervation of a normal motor unit?

Answer 10

1 motor neuron innervates muscle fibers in different fascicles within a muscle, and innervated muscle fibers are NOT adjacent to each other.

This is an efficient way to activate a muscle and produces smooth, graded increase in force so limb movement is smooth.

Question 11

How are low and high force movements generated?

Answer 11

Low force: few motor units across a muscle

High force: additional motor units recruited

Question 12

How does 1 motor neuron (MN) make muscles contract? How does this differ from when multiple motor units are causing contraction? Why is this the case?

Answer 12

One motor neuron contracts in synchrony.

CNS recruits different motor units asynchronously to help produce smooth, fluid movements.

Question 13

What is the goal of the CNS in recruiting motor units?

What does the CNS recognize when it comes to muscles?

Answer 13

To activate the fewest number of motor units to generate a smooth contraction in order to produce smooth movements that are energy-efficient.

CNS recognize motor units, not whole muscles.

Question 14

What is the organization of innervation of a motor unit in a pathological muscle?

Answer 14

1 motor neuron innervates many more muscle fibers all in one fascicle than normal.

Usually an indication that f deinnervation and reinnervation has occurred.

Question 15

What is the result of pathological muscle in which 1 MN innervates many muscle fibers all in one fascicle?

Answer 15

When increases forces are needed, the increments of tension generated are much larger, jerky, and not finely graded.

Produces movements too powerful for the force needed.

Fine, asynchronous, graded recruitment of motor units is lost.

Question 16

What is the appearance of a pathological muscle?

Answer 16

You lose the salt and pepper of light and dark muscle fibers types and instead have groupings where dark and light fibers cluster.

This suggests that motor neurons have died, been reinnervated, and induced a phenotype switch.

Question 17

What can be used to diagnose pathology in muscle or motor neurons?

Answer 17

Electromyography (EMG): needle electrodes are put in a muscle to record activity in motor units.

Question 18

What occurs with electromyography in a normal muscle at rest, during weak contraction, and during strong contraction?

Answer 18

At rest: no muscle activity

Weak contraction: few different motor units become activated

Strong contraction: all motor units are activated; full interference with many motor units contracting asynchronously.

Question 19

What occurs with electromyography in a pathogenic muscle at rest and during strong contraction? Why does this occur?

Answer 19

There’s a decrease in the # of MNs and the surviving motor neurons branch and innervate adjacent denervated muscle fibers.

At rest axons fire spontaneously: fasciculation (twitching/quivering muscles)

Giant unit: EMG is larger than normal because the few surviving MNs innervate many more fibers than normal. (large contraction)

Question 20

Reduced total number of motor units activated = reduced _______.

Answer 20

Interference.

This is because you’ve lost a lot of motor neurons that used to be innervating that muscle.

Question 21

What are lower motor neurons and what actions do they have in the spinal cord and brainstem nuclei?

Answer 21

Primary motor neurons that directly innervate skeletal muscle (alpha MNs)

Spinal cord (ventral horn, all levels): control muscles in neck, trunk, arms, legs

Brainstem nuclei (caudal medulla): control muscles in face, head, and neck.

Question 22

How are motor neurons organized in the spinal cord?

Answer 22

Into vertical columns called motor nuclei.

Motor neurons that innervate one muscle are usually clustered in the same motor nucleus (motor neuron pool) and can extend from 1-4 spinal segments.

Question 23

What is the function of medial motor nuclei? How are the neurons connected?

Answer 23

Have motor neurons that innervate axial muscles in neck and back.
-connected across many spinal segments by LONG propriospinal interneurons and can span the entire length of the cord to coordinate movements of neck, trunk, and pelvis.

Control balance and posture.

Question 24

What is the function of the lateral motor nuclei?

Answer 24

Motor neurons that innervate limb muscles and are connected across only a few spinal segments by SHORT propriospinal interneurons.
-more distal muscles have greater independence and a larger variety of movements.

Question 25

Describe the organization of the lateral motor nuclei?

Answer 25

Most medial motor neurons innervate proximal limb muscles.

Most lateral motor neurons innervate distal limb muscles (shortest connecting interneurons-flexibility)

Question 26

Flexors are more _____ within the lateral motor nuclei while extensors are more ____.

Answer 26

Flexors are more central

Extensors are more peripheral

Question 27

What are upper motor neurons? Where do they originate and travel to?

Answer 27

Neurons that originate in the brainstem and motor cortex.

Descend and synapse on lower MNs for descending control.

“premotor neurons”

Question 28

What are lower motor neurons? Where do they originate and travel to?

Answer 28

Primary MNs in spinal cord (ventral horn) and brainstem nuclei (ex. trigem motor nucleus, facial nucleus, oculomotor nucleus).

Directly innervate and control muscles.

Question 29

What effect does an upper motor neuron lesion have on reflexes and tone?

Answer 29

Causes hyperactive reflexes and increased muscle tone because you’ve lost the ability for inhibition of lower motor neurons.

Question 30

What are the ways in which motor units and muscle fibers differ?

Answer 30

Differ in:

• speed at which they’re recruited
• strength of contraction
• fatiguability

Question 31

All muscle fibers in one motor unit are _____?

Answer 31

Of a single type.

Question 32

What are the characteristics of slow-twitch motor units? What do they innervate? What type of metabolism do they do?

Answer 32

Innervate red, slow-twitch muscle fibers (type 1)

1. small muscle fibers, smaller MN cell bodies. Slow contraction.
2. Rely on oxidative catabolism that depends on glucose and O2 from blood to regernerate ATP (lots of mito and myoglobin).
3. Fatigue resistant: produce small tension for long duration

Question 33

What are slow-twitch motor units specialized for?

Answer 33

Long endurance: posture, standing, slow walking.

Question 34

What are characteristics of Fast fatigable motor units? What do they innervate? What type of metabolism do they do?

Answer 34

Innervate white, fast-twitch fibers (Type IIb).

1. Large fibers, usually large MN call bodies, large and fast contraction.
2. Anaerobic catabolism: depends on glycogen stores converted to lactic acid.
3. Fatigue easily because glycogen depleted.

Question 35

What are fast fatigable motor units specialized for?

Answer 35

Strength and speed: running, jumping, vigorous exercise, escape.

Question 36

What are characteristics of Fast fatigue-resistant motor units? What do they innervate? What type of metabolism do they do?

Answer 36

Innervate intermediate fibers; properties between slow and fast muscle fibers (type IIa)

Combine fast dynamics and contraction with enough aerobic capacity to resist fatigue for several minutes.

Question 37

What are fast fatigue-resistant motor units specialized for?

Answer 37

Endurance during exertion such as marathon running or endurance walking.

Question 38

Most muscles contain what type(s) of muscle fibers/motor units?

Answer 38

Most contain a mixture of all three types, but proportions differ depending on the main function of the muscle.

Question 39

What is a muscle twitch?

Answer 39

A muscle fiber’s contraction response to a single action potential.

Question 40

Compare the cell body size, diameter, and contraction force, and contraction time for slow twitch, fast fatigue-resistant, and fast fatiguable motor units? W

Answer 40

Cell body of slow-twitch is smallest, then fast-fatigue resistant is next in size, and fast fatiguable is largest.
-Same goes for muscle fiber diameter and size of the twitch force.

Contraction time the longest for slow-twitch, then fast-fatigue-resistant, then shortest contraction time for fast fatiguable.

Question 41

What ultimate force of a muscle depends on what three things?

Answer 41

1. The rate code: freq of AP firing in one MN (temporal summation)
2. Recruitment: # of motor units firing together (spatial summation).
3. Size principle: motor units are recruited in a fixed order of size for graded force. Smaller units (with fewer fibers) fire first, larger motor units (most fibers) fire last.

Question 42

What is the recruitment principle?

Answer 42

The more motor units activated, the stronger the contraction.

Question 43

What happens to cause force of contraction to increase?

Answer 43

Force increases as each motor unit fires more rapidly and more units are added.

Later units increase their force more rapidly.

Question 44

How are motor units recruited (ie what’s recruited first, second, and third?) How do they stop firing?

Answer 44

Slow twitch first, then fast fatigue-resistant, then fast fatiguable.

Motor units stop firing in reverse order.

Question 45

The size principle states that recruitment of motor units correlates with what?

Answer 45

1. Size of motor neuron cell body and axon

2. Size and strength of motor units

Question 46

Describe the properties of slow twitch neurons? When are they recruited?

Answer 46

Recruited first for small force tasks.

Small motor neuron cell body and axon, slower conduction velocity, innervate few muscle fibers.

1% of force generated by fast twitch fibers is from these.

Question 47

Describe the properties of fast fatiguable neurons? When are they recruited?

Answer 47

Recruited later for large force tasks.

Large motor neuron cell body and axon for fast conduction velocity, innervates many muscle fibers.

Produces large force rapidly.

Question 48

What are advantages of the size principle?

Answer 48

Allows smooth increase in force output by muscle. Movement is fluid, not jerky.

Minimizes fatigue and keeps power in reserve until needed.

Allows appropriate recruitment of muscle fibers for the task.

Question 49

What type of units are recruited for playing the piano? What about for high jumping?

Answer 49

Playing piano: small force by few slow-twitch units.

High jump: large force by many fast-twitch units.