Motor Unit

Question 1

Motor systems: translate neural signals in the
_____ & _____ _____.

Answer 1

• brain & spinal cord
• ⇒ contractile force in muscles
• ⇒ move limbs, body

Question 2

Functions of Skeletal Muscle:

Answer 2

to move bones, joints, skin

1. execute voluntary movements
   
   • ex: rapid escape response
2. maintain posture with respect to gravity
3. produce tremendous heat and metabolic energy

Question 3

How are skeletal muscles the effectors of movements?

Answer 3

• Muscles are specialized to generate tension by contraction:
  
  1. shortening contractions
  2. isometric (equal length) contraction
  3. lengthening contraction
• Skeletal muscle made up of:
  
  1. Parallel bundles of fascicles
  2. Fascicles made of muscle fibers
• Muscles that work across the joint together (produce similar action) = synergistic
• Muscles that work in opposite direction (work against each other) = antagonistic

Question 4

How do muscles generate tension?

Answer 4

• By pulling
• Each joint require at least two muscles pulling in opposite directions:
  
  • antagonist muscles: Extensors and Flexors.
  • Flexor moves limb toward body; Extensor moves limb away from body
  • Biceps = flexor; Triceps = extensor

Question 5

Muscles are activated by _____ _____.

Answer 5

motor units

Question 6

What makes up a motor unit? What are its properties?

Answer 6

• One motor neuron (MN) and all of the muscle fibers it innervates
• Smallest functional element of the motor system.
  
  • Smallest unit of muscle under neural control
  • Provides the smallest increment of tension that can be generated

Question 7

How does the size of the motor unit vary?

Answer 7

• Size of motor unit varies with precision of neural control over muscle movement:
  
  • Stapedius: 1 MN: 2-3 muscle fibers
  • Extraocular muscle: 1 MN: 10 muscle fibers
  • Digits: 1 MN: 5-10 muscle fibers; gives precise control
  • Gastrocnemius: 1 MN: 1000 muscle fibers; provides support, gives power

Question 8

Motor units in normal muscle:

Answer 8

• 1 MN innervates many muscle fibers in different fascicles within one muscle (fibers are not adjacent to each other)
• Provides an efficient way to activate a muscle:

Question 9

How is movement coordinated in a normal motor unit? How is smooth limb movement achieved?

Answer 9

1. During low force movement, a few motor units are recruited across the muscle
2. Force needed to generate a larger movement increases ⇒ additional motor units across the muscle(s) are recruited
   * Produces a smooth, graded increase in force so that limb movement is smooth (not jerky)
   * Also, the different motor units are activated asynchronously
   
   • helps produce smooth, fluid movements

Question 10

What is the CNS role in a normal motor unit?

Answer 10

• recruits motor units in muscles crossing a joint
• Goal: to activate the fewest number of motor units to accomplish the movement task in a smooth, efficient manner
• does not “recognize” whole muscles
  
  • recognizes and uses motor units as increments of force generation in order to produce smooth, fluid, energy-efficient movement

Question 11

Motor units in pathological muscle:

Answer 11

• motor unit innervates many muscle fibers that are all in one fascicle
  
  • often deinnervation-reinnervation has occurred
  • muscle unit is overall much larger
• When greater forces are needed:
  
  • increments of tension generated are larger, jerky and not finely graded
  • jerky movements that are too powerful for the force needed
  • fine, asynchronous, graded recruitment is lost

Question 12

How is motor unit pathology diagnosed?

Answer 12

• Complex action potentials in muscle can be recorded as an Electromyogram (EMG)
• A large single motor unit potential can mean denervation or reinnervation by sprouting
  
  • enlarges the motor unit

Question 13

1. Spinal cord has motor neurons that control muscles in ____, _____, _____.
2. Brainstem has motor neurons that control muscles in ____, ____.

Answer 13

1. Spinal cord: neck, trunk, limbs
2. Brainstem: face, head

Question 14

How are motor neurons in the spinal cord organized?

Answer 14

• **motor nucleus: **vertical column (extending 1-4 segments)
• **motor neuron pool: **Motor neurons that innervate the same muscle are found in same motor nucleus

Question 15

Somatotopic organization of motor nuclei in ventral horn of spinal cord:

Answer 15

• Medial motor nuclei: motor neurons that innervate axial muscles of neck, back.
  
  • Connected across many segments by propriospinal neurons (long axons, branch extensively)
• Lateral motor nuclei: innervate limb muscles
  
  • Most medial innervate proximal limb muscles
  • Most lateral innervate distal limb muscles
  • Connected across few segments by propriospinal neurons (short axons, branch less)

Question 16

1. Flexors are nearer to the _____ of the spinal cord.
2. Extensors are more __________.

Answer 16

1. Flexors: center
2. Extensors: peripheral

Question 17

How do the axons of motor neuron exit?

Answer 17

Through ventral root and terminate in the muscle

Question 18

How does the size of the muscle affect the motor unit and amount of force generated?

Answer 18

• Small muscle: small motor units: small increment of force
• Large muscle: large motor units: large increment of force

Question 19

Lower motor neurons:

Answer 19

• Primary motor neurons in spinal cord (ventral horn) and brainstem nuclei
  
  • ex: trigeminal motor nucleus, occulomotor nucleus, facial nucleus, etc.
• directly innervate and control muscles

Question 20

Upper motor neurons:

Answer 20

• Neurons that originate in brainstem and motor cortex
  
  • ex: corticospinal tract
• Descend and synapse on lower motor neurons to convey descending control of movement
  
  • Often called “premotor” neurons

Question 21

What are the clinical signs of upper motor neuron and lower motor neuron lesions?

Answer 21

Question 22

Types of Motor units and Muscle Fibers:

Answer 22

• Differ in speed at which they are recruited to fire, strength of contraction
• Fatigability
• Types:
  
  1. Slow-twitch
  2. Fast Fatigable motor units
  3. Fast Fatigue-resistant

Question 23

Slow-twitch motor units:

Answer 23

• innervate red, slow-twitch muscle fibers (type 1)
  
  • smaller fibers; small MN; slow contraction, smaller tension output
• oxidative catabolism: depend on glucose and oxygen ⇒ regenerate ATP; large numbers of mitochondria; myoglobin
• fatigue resistant
  
  • produce small tension for long duration without fatigue
  • specialized for endurance:
    
    • posture, standing and walking

Question 24

Fast Fatigable motor units:

Answer 24

• innervate white, fast-twitch fibers (type II)
  
  • larger fibers; large MN; fast, large contraction
• anaerobic catabolism: depend on glycogen stores that are converted into lactic acid
• fatigue because glycogen is rapidly depleted
  
  • produce brief burst of force, take hours to recover fully
  • strength and speed:
    
    • running, jumping, vigorous exercise, escape

Question 25

Fast Fatigue-resistant:

Answer 25

• innervate intermediate fibers
  
  • properties between red & white muscle fibers
• combines fast twitch dynamics and contraction with enough aerobic capacity to resist fatigue for several minutes
• exercise with endurance:
  
  • ex: marathon running

Question 26

What does the maximum force exerted depends on?

Answer 26

Rate of firing and recruitment:

1. Rate code: frequency of action potential firing in motor neuron
2. Recruitment: number of motor units firing
3. Size principle: smallest motor units fire first; largest fire last

Question 27

Size Principle:

Answer 27

1. ​​Slow: small motor neurons: low threshold for firing
2. Fast: larger motor neurons: higher threshold for firing
   * Recruitment of motor units correlated with:
   
   1. size of motor neuron cell body and axon
   2. size and strength of motor units

Question 28

Describe slow twitch and fast twitch fibers in relation to the size principle:

Answer 28

1. Slow twitch:
   
   • small motor neuron cell body, small axons, slower conduction velocity, innervate fewer fibers
   • Small cell body is brought to action potential threshold easily
   • ⇒ produce smaller force
2. ​Fast twitch:
   
   • large motor neuron cell body, large axons, high conduction velocity, innervate many fibers
   • Larger cell body takes more input (more EPSPs) to bring to threshold for an action potential
   • ⇒ produce large force rapidly

Question 29

What is the order in which motor neurons are recruited in the ventral horn?

Answer 29

Smallest ⇒ Largest

• Small force required ⇒ slow-twitch fibers
• More force ⇒ fast fatigue-resistant
• Maximum force ⇒ fast fatigable units
• Fibers stop firing in reverse order

Question 30

Why is the size principle advantageous?

Answer 30

• Allows a smooth increase in force output by muscle
  
  • Movement is fluid, not jerky
• Minimizes fatigue
  
  • Keeps power in reserve until needed
• Allows appropriate recruitment of fibers for job:
  
  • playing piano ⇒ small force ⇒ few slow-twitch units
  • high jump ⇒ large force ⇒ many fast-twitch units