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Med Neuro Block 3 > Spinal Cord and Motor Unit 2: Motor Unit > Flashcards

Spinal Cord and Motor Unit 2: Motor Unit Flashcards

1
Q

What are the functions of skeletal muscle

A
  • move bones joints and skin
    1. They execute voluntary movements ( rapid escape, goal directed movements)
  1. They maintain posture of the body with respect to gravity
  2. they produce heat and metabolic energy (80% of muscle activity)
    - skeletal muscle are the effectors of movement they generate tension by contracting and pulling
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2
Q

Describe the anatomy/structure of a skeletal muscle

A

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

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3
Q

What are synergistic muscles

A

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

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4
Q

What are antagonistic muscles

A

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

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5
Q

How many antagonistic muscles does each joint require

A

each joint requires at least two antagonistic muscles

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6
Q

What do flexors do

A

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

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7
Q

what do extensors do

A

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

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8
Q

What is the Final common pathway

for movement

A

the LOWER motor neurons

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9
Q

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

A

the LOWER motor neurons

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10
Q

How does our nervous system directly control our muscles?

A

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)

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11
Q

Muscles are activated by________

what is the general definition of __________

A

motor units

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

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12
Q

What is the 3 point definition of a motor unit

A
    • Smallest functional element of the motor system
    • Smallest unit of muscle under neural control
    • Smallest increment of tension that can be generated
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13
Q

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

A
  • *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

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14
Q

Force in a muscle is determined by what two things

A

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

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15
Q

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

A

more!

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16
Q

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

A
  • 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

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17
Q

Muscle fibers innervated by 1 MN contract in

A

synchrony

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18
Q

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

A

asynchronously

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19
Q

How does the CNS help ensure smooth movements

A

-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.

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20
Q

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

A

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

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21
Q

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

A
  • 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.
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22
Q

How is muscle fiber type determined

A

by the motor neuron that innervates it

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23
Q

Light=

A

Slow fibers (type I)

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24
Q

Dark=

A

Fast fibers (type II)

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25
Q

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

A

evenly distributed

26
Q

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

A
  • 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.
27
Q

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

A

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

28
Q

EMG normal muscle

A

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.

29
Q

EMG pathological muscle

A 
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
30
Q

Lower motor neurons:

A

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
31
Q

Motor nucleus

A

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

32
Q

Medial motor nuclei

A

-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

33
Q

Lateral motor nuclei

A

-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.

34
Q

Among the lateral motor nuclei:

Most medial innervate

A

proximal limb muscles

35
Q

Among the lateral motor nuclei:

Most lateral innervate

A

distal limb muscles

36
Q

Are flexors more central or more peripheral

A

more central

37
Q

are extensors more central or more peripheral

A

more peripheral

38
Q

Lower motor neurons:

A

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

39
Q

Upper motor neurons:

A

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

40
Q

Why does nervous system need different types of motor units?

A

To perform different activities!

41
Q

Motor units and muscle fibers differ in:

A
  • speed at which they are recruited
  • strength of contraction
  • fatigability
42
Q

Types of motor units

A
  1. slow twitch motor units
    2 fast fatigable
  2. fast-fatigue resistant
43
Q

What do slow twitch motor units innervate

A

red, slow twitch muscle fibers (type I)

44
Q

slow twitch motor units

A

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

45
Q

What are slow twitch motor units for

A

** specialized for long endurance: posture, standing,

slow walking

46
Q

What do fast fatiguable motor units innervate

A

white, fast-twitch fibers (type IIb)

47
Q

Fast Fatigable

A

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

48
Q

What are fast fatiguable motor units for

A

** specialized for strength and speed:

running, jumping, vigorous exercise, escape

49
Q

What do Fast fatigue-Resistant motor units innervate

A

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

50
Q

Fast fatigue resistance

A

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
51
Q

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

A

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

52
Q

Ultimate force depends on

A

rate of firing and recruitment

53
Q

Rate code

A

frequency of action potential firing in one MN

ex. of Temporal summation

54
Q

Recruitment

A

number of motor units firing together

ex. of Spatial summation

55
Q

Recruitment principle:

A

The more motor units activated, the stronger the contraction

56
Q

Force of a contraction increases as

A
  • each motor unit fires more rapidly
  • more units are added
  • later units ⇑ force rapidly
57
Q

Size principle:

A

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

58
Q

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

A

Size of motor neuron cell body and axon

Size and strength of motor units

59
Q

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

A

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

60
Q

What motor neurons are recruited second

A 
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

61
Q

Advantages of size principle:

A

– 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

Med Neuro Block 3 (10 decks)

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