Spinal Cord and Motor Unit 2: Motor Unit

Question 1

What are the functions of skeletal muscle

Answer 1

• move bones joints and skin
  1. They execute voluntary movements ( rapid escape, goal directed movements)

2. They maintain posture of the body with respect to gravity
3. they produce heat and metabolic energy (80% of muscle activity)
   - skeletal muscle are the effectors of movement they generate tension by contracting and pulling

Question 2

Describe the anatomy/structure of a skeletal muscle

Answer 2

the skeletal muscle is made up of parallel bundles of fascicles which are made up of muscle fibers which contract.

Question 3

What are synergistic muscles

Answer 3

muscles that work across the joint together (for the same action) they produce similar actions

ex: Biceps muscle and brachialis work together to produce flexion

Question 4

What are antagonistic muscles

Answer 4

muscles that work in opposite directions, aka work against each other.

Biceps work for flexion (with the brachialis in synergy)
VS.
The triceps work for extension.

So the biceps/brachilais and the triceps are antagonisitic

Question 5

How many antagonistic muscles does each joint require

Answer 5

each joint requires at least two antagonistic muscles

Question 6

What do flexors do

Answer 6

they decrease the angle between bones or the angle between parts of the body

Question 7

what do extensors do

Answer 7

they increase the angle between bones or between parts of the body

Question 8

What is the Final common pathway

for movement

Answer 8

the LOWER motor neurons

Question 9

What is Only way our nervous system can control movement of limbs and body; voluntary and involuntary

Answer 9

the LOWER motor neurons

Question 10

How does our nervous system directly control our muscles?

Answer 10

Combination of the lower motor neurons that directly stimulate muscle contraction and the higher levels of CNS that stimulate the lower motor neurons (cortex, brainstem, basal ganglia, cerebellum)

Question 11

Muscles are activated by________

what is the general definition of __________

Answer 11

motor units

A motor unit is one motor neuron and the muscle fibers it innervates

Question 12

What is the 3 point definition of a motor unit

Answer 12

• • Smallest functional element of the motor system
• • Smallest unit of muscle under neural control
• • Smallest increment of tension that can be generated

Question 13

describe the relationship between the size of the muscle/amount of motor units that innervate it and the level of force and precision in control of the muscle

Answer 13

• *Small muscle: small motor units ⇒ generate small increments of force
• *Large muscle: large motor units ⇒ generate large increments of force

Force generated is determined by number of muscle fibers in a motor unit & Number of motor units activated

Question 14

Force in a muscle is determined by what two things

Answer 14

the number of muscle fibers in one motor unit (meaning under the control of one motor neuron and therefore one action potential)

AND

the number of motor units that are activated

Question 15

For a higher force movement are more or less motor units recruited

Answer 15

more!

Question 16

How do we ensure a smooth graded increase in force of contraction in a muscle so that movements are smooth

Answer 16

• Normal muscle: 1 MN innervates muscle fibers in DIFFERENTt fascicles within a muscle (innervated muscle fibers not adjacent)
• as more force is needed more motor units are recruited

We are stimulating fibers in lots of different fascicles

Question 17

Muscle fibers innervated by 1 MN contract in

Answer 17

synchrony

Question 18

CNS recruits different motor units ____________ which Helps produce smooth, fluid movements.

Answer 18

asynchronously

Question 19

How does the CNS help ensure smooth movements

Answer 19

-recruits different motor units (that induce fibers from different fascicles) asynchronously
-the goal is to activate the fewest number of motor units to generate the smooth contraction
CNS doesn’t recognize whole muscles; rather uses motor units to produce smooth, energy-efficient contraction, smooth movement.

Question 20

Describe the difference between a motor unit in normal muscel vs pathological muscle

Answer 20

normal muscle is when one motor unit innervates muscle fibers that are within DIFFERENT fascicles

in a pathological muscle one motor unit innervates several fibers that are all within the SAME fascicle. this usually indicates that deinnervation and then reinnervation has occurred

Question 21

What are the results of pathological muscle (the fact that one motor neuron innervates fibers within the same fascicle)

Answer 21

• When increasing forces are needed, the increments of tension generated are much larger, jerky and not finely graded.
• This produces jerky movements that are too powerful for the force needed.
• The fine, asynchronous, graded recruitment of motor units is lost.

Question 22

How is muscle fiber type determined

Answer 22

by the motor neuron that innervates it

Question 23

Light=

Answer 23

Slow fibers (type I)

Question 24

Dark=

Answer 24

Fast fibers (type II)

Question 25

Describe the distribution of light and dark fibers in a normal msucle

Answer 25

evenly distributed

Question 26

describe the distribution of light and dark fibers in a pathological muscle

Answer 26

- Fiber type “grouping” Dark fibers cluster together Light fibers cluster together - Some motor units degenerate, muscle becomes denervated - Surviving motor units sprout axons that have reinnervated the adjacent denervated fibers and induced a phenotype switch in the muscle fibers.

Question 27

What is used to diagnose pathology in muscle or motor neurons.

Answer 27

Electromyography (EMG) used to diagnose pathology in muscle or motor neurons. Needle electrodes are put in muscle to record activity in motor units.

Question 28

EMG normal muscle

Answer 28

Normal muscle: -- Muscle fibers innervated by a single motor neuron are not usually adjacent to each other -- At rest: no activity in muscle -- Weak contraction: a few different motor units become activated -- Strong contraction: all motor units are activated. Full interference with many motor units contracting asynchronously.

Question 29

EMG pathological muscle

Answer 29

``` Neurogenic disease ⇓Number of motor neurons -- Some motor neurons degenerate (muscle fibers become denervated). -- Surviving motor neurons branch and innervate adjacent denervated muscle fibers. -- Rest: axons fire spontaneously “Fasciculations” spontaneous action potentials; twitching, quivering muscles ⇒ Lower MN disease ```

Question 30

Lower motor neurons:

Answer 30

primary motor neurons that directly innervate skeletal muscles (alpha MNs) Lower motor neurons: primary motor neurons that directly innervate skeletal muscles (alpha MNs) Spinal cord (ventral horn, all spinal levels):⇒ control muscles in neck, trunk, arms, legs ``` Brainstem nuclei (caudal medulla): ⇒ control muscles in face, head, neck ```

Question 31

Motor nucleus

Answer 31

Motor neurons are organized into vertical columns in cord: called the motor nucleus -motor neurons that innervtae the same muscle usually clustered within the same motor nucleus

Question 32

Medial motor nuclei

Answer 32

-motor neurons that innervate axial muscles in neck, back -Are connected across many spinal segments by long propriospinal interneurons --sometimes span entire length of spinal cord to coordinate movements of neck, trunk, pelvis -responisble for balance and posture

Question 33

Lateral motor nuclei

Answer 33

-motor neurons that innervate limb muscles -connected across only a few spinal segments by short propriospinal interneurons. -Thus, more distal muscles have greater independence, larger variety of muscle movements.

Question 34

Among the lateral motor nuclei: | Most medial innervate

Answer 34

proximal limb muscles

Question 35

Among the lateral motor nuclei: | Most lateral innervate

Answer 35

distal limb muscles

Question 36

Are flexors more central or more peripheral

Answer 36

more central

Question 37

are extensors more central or more peripheral

Answer 37

more peripheral

Question 38

Lower motor neurons:

Answer 38

Primary MNs in spinal cord (ventral horn) and brainstem nuclei and Directly innervate, control muscles (ex: trigeminal motor nucleus, facial nucleus, occulomotor nucleus)

Question 39

Upper motor neurons:

Answer 39

Neurons that originate in brainstem, motor cortex (ex: corticospinal tract). Descend, synapse on lower MNs; Descending control “premotor neurons”

Question 40

Why does nervous system need different types of motor units?

Answer 40

To perform different activities!

Question 41

Motor units and muscle fibers differ in:

Answer 41

- speed at which they are recruited - strength of contraction - fatigability

Question 42

Types of motor units

Answer 42

1. slow twitch motor units 2 fast fatigable 3. fast-fatigue resistant

Question 43

What do slow twitch motor units innervate

Answer 43

red, slow twitch muscle fibers (type I)

Question 44

slow twitch motor units

Answer 44

a) Smaller muscle fibers; have smaller MN cell bodies ⇒ produce slow contraction, smaller tension output b) Rely on oxidative catabolism: depend on glucose and oxygen from blood to regenerate ATP - large numbers of mitochondria - lots of myoglobin c) Fatigue resistant ⇒ produce small tension for long duration; no fatigue

Question 45

What are slow twitch motor units for

Answer 45

** specialized for long endurance: posture, standing, | slow walking

Question 46

What do fast fatiguable motor units innervate

Answer 46

white, fast-twitch fibers (type IIb)

Question 47

Fast Fatigable

Answer 47

a) Larger muscle fibers, usually larger MN cell bodies ⇒ large and fast contraction b) Rely on anaerobic catabolism: depend on glycogen stores that are converted into lactic acid c) Fatigue easily because glycogen is rapidly depleted ⇒ produce brief burst of intense force; takes hours to recover fully

Question 48

What are fast fatiguable motor units for

Answer 48

** specialized for strength and speed: | running, jumping, vigorous exercise, escape

Question 49

What do Fast fatigue-Resistant motor units innervate

Answer 49

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

Question 50

Fast fatigue resistance

Answer 50

Combine fast dynamics and contraction with enough aerobic capacity to resist fatigue for several minutes * * specialized for endurance during exertion example: Marathon running, endurance walking

Question 51

Describe the comparison of the grams of force produced per action potential for each type of motor unit

Answer 51

slow twitch- moderate force for long period of time fast fatiguable-HUGE force for short period of time fast-fatigue resistant-moderate forcer for moderate period of time

Question 52

Ultimate force depends on

Answer 52

rate of firing and recruitment

Question 53

Rate code

Answer 53

frequency of action potential firing in one MN | ex. of Temporal summation

Question 54

Recruitment

Answer 54

number of motor units firing together | ex. of Spatial summation

Question 55

Recruitment principle:

Answer 55

The more motor units activated, the stronger the contraction

Question 56

Force of a contraction increases as

Answer 56

- each motor unit fires more rapidly - more units are added - later units ⇑ force rapidly

Question 57

Size principle:

Answer 57

Motor units are recruited in fixed order of size; graded force Smallest motor units (lower threshold), fewest fibers, fire first Largest motor units (higher threshold), most fibers, fire last

Question 58

According to the size principle recruitment of motor units correlated with what two things

Answer 58

Size of motor neuron cell body and axon | Size and strength of motor units

Question 59

What motor neurons are recruited first and what are they recruited for

Answer 59

slow twitch are recruited first bc -small motor neuron cell body and axon -slower conduction velocity Produce smaller force; 1% of force generated by fast twitch fibers Recruited first for small force tasks

Question 60

What motor neurons are recruited second

Answer 60

``` Fast Fatigable -- large motor neuron cell body and axon -- fast conduction velocity -- innervate many muscle fibers ⇒ Produce large force rapidly ``` Recruited later for large force tasks

Question 61

Advantages of size principle:

Answer 61

-- Allows a smooth increase in force output by muscle Movement is fluid, not jerky -- Minimizes fatigue -- Keeps power in reserve until needed (to escape danger; run, jump) -- Allows appropriate recruitment of muscle fibers for the task