Control of Movement: Motoneurons & motor units

Question 1

Muscle fibres are _____ cells activated by _____ in the ______innnervating the ______

Answer 1

Muscle fibres are EXCITABLE cells that are activated by ACTION POTENTIALS in the A-MOTOR UNIT innervating the MOTOR UNIT.

Question 2

How can Muscle AP be recorded

Answer 2

Muscle APs can be recorded percutaneously by placement of recording electrodes on the skin above the muscle.

Question 3

EMG: electromyograph:
What needs to happen in order for EMG activity to be recorded
results can reveal..

Answer 3

Voluntary contraction results in electromyograph (EMG) activity.

Results can reveal nerve dysfunction, muscle dysfunction or problems with neuromuscular transmission.

Question 4

Disorders of movement

Answer 4

Epilepsy
* Myasthenia gravis
* Poliomyelitis (Infantile paralysis)
* Stroke
* Parkinsonism (paralysis agitans)
* Huntingtons chorea
* Multiple sclerosis
* Cerebral palsy
* Spinal cord injury
* Brain tumors
-Neuropathies
* Toxins (eg. Alcohol)
* Infections (eg. AIDS, encephalitis)
* Syringomyelia (spinal cord cyst)
* Amyotrophic lateral sclerosis (ALS)

Question 5

Movement by skeletal muscle is initiated by ?

Answer 5

Brain.
APs sent to the upper motor neurons of the Primary Motor Cortex in the precentral gyrus initiate movement.

Question 6

Label the areas of the brain

Answer 6

1. Frontal
2. Frontal eye field
3. Premotor/Supplementary motor cortex
4. Primary motor cortex
5. Central Sulcus
6. Prefrontal cortex

Question 7

What are the roles of the different areas of the brain

Answer 7

1. Primary Motor cortex:
   - Voluntary muscle movement
2. Premotor/Supplementary motor cortex
   - Planning/co ordination of movement
3. Frontal eye field
   - Voluntary rapid eye movement
4. Prefrontal cortex:
   - Executive functions, behaviour and personality

Question 8

What are the three classes of movement + description and example

Answer 8

VOLUNTARY: purposeful, goal-directed movements initiated from within CNS.

REFLEXES: (somatic) automatic movements triggered in response to a specific sensory stimulus.
e.g. vestibular, stretch, & withdrawal reflexes

RHYTHMIC MOTOR PATTERNS: initiation and termination is voluntary, but the actual movement is more
stereotyped.
* e.g. breathing, locomotion, chewing

Question 9

Initiation and co-ordination of movement
What elements are there in order to create this

Answer 9

Movement of groups of muscles, and the activation patterns of individual muscles, must be coordinated

The CNS must also simultaneously coordinate the actions of agonist and antagonist groups of muscle

The starting position of the individual muscle fibres and body position must be taken into account (i.e., via input from proprioceptors and muscle receptors)

During muscle activity there must be stability provided across joints that are often far removed from the muscle in question

Question 10

What are the Components of the motor system

Answer 10

1. Forebrain:
   Voluntary movements.
2. Spinal cord & brainstem:
   - Reflex movements & rhythmic motor patterns.

• Thalamus is the ‘relay station’ for incoming motor & sensory signals
• Cerebellum detects & corrects “motor error”
• Basal ganglia “approve or reject” movement signals

Question 11

Draw a diagram showing the components of the motor system

Answer 11

Lecture Slide

Question 12

Motoneurons: Draw one labelled with features

Answer 12

Lecture Slide

Question 13

Motoneurons:
Location
Types

Answer 13

Location:
* Spinal cord
* Brain stem

Two types:
* α (alpha) motoneurons
* γ (gamma) motoneurons

Question 14

Describe the two types of motoneurons

Answer 14

1. Alpha motor neurons carry movement instructions from the brain & spinal cord to the muscles, synapsing on extrafusal muscle fibres.
   * Their axons are the longest in the body — a single axon can stretch from the base of the spinal cord to the toes.
2. Gamma motor neurons located in the same motor nucleus as alpha motor neurons supplying a muscle, synapsing on the intrafusal muscle fibres within muscle spindles

Question 15

Draw an ALPHA Motorneuron

Answer 15

Anterior horn cells (“motoneurons) located in anterior grey matter of spinal cord at every level.

Concentrated in cervical (neck) and lumbosacral (lower back) enlargements.

Question 16

Outputs of the somatic and autonomic NS: Diagram

Answer 16

Lecture Slide

Question 17

Somatic motor pathway:
What does it control
Where are the cells in pathway located
Where do the cells lie in terms of muscle

Answer 17

Somatic motor pathway controls movement.

Cell bodies of somatic motor neurons are contained either in the ventral (anterior) horn of the spinal cord or in the brain.

Somatic motor neurons branch close to their target muscle with clusters of axon terminals lying above the motor endplate of each muscle fibre innervated.

Question 18

How can a muscle fibre have multiple motoneurons?

Answer 18

Each muscle fiber is innervated by only one motoneuron at a single neuromuscular junction (‘end-plate’); but each motoneuron innervates many muscle fibers

Question 19

Whats the innervation ratio
What does the ratio depend on

Answer 19

number of muscle fibers innervated by an individual motoneuron, ranges from 5 to 2000 depending on the function of the muscle.

Question 20

How many Motor neurons innervate muscles for FINE motor control

Answer 20

10-100

Question 21

What must ALL muscle fibres do to have synchrony

How do muscles get more force?

Answer 21

All muscle fibres within a motor unit contract in synchrony.

Can increase force of contraction by
1. recruitment of motor units.
2. Increased frequency of APs in individual α-motoneurons.

Question 22

What is the Classification of motor units
What are they based on?
What are they determined by?

Answer 22

S type (slow) or Type I

FF type (fast, fatigable) or Type II B (or Type II X)

Also a rare intermediate type (Type II A)

Based on anatomical, physiological & biochemical properties:
Small oxidative units (lower threshold, fire first) THEN Large glycolytic units

DETERMINED BY MyHC isoforms

Question 23

Skeletal muscle fibre types and what determines the type made

Answer 23

3 myosin heavy chain (MyHC) proteins expressed in human skeletal muscle determine fiber type:
MyHC I
MyHC IIA
MyHC IIX.z

Each type has distinct contractile characteristics & metabolic profiles.

Question 24

Muscle fibre type composition

Answer 24

Muscle fiber type composition can exist along a continuum: expressing either a single MyHC isoform, or co-expressing multiple MyHC isoforms (MyHC I/IIA, IIA/IIX, and I/IIA/IIX) to give 6 different MyHC fiber isotypes.

Question 25

Fibre type characteristics
Slow (type 1) vs Fast (Type IIB)
Number of motor units
Axon diameter
Number of terminals
Tetanic tension
Speed of contraction
Fatigue
Metabolism
Myoglobin (binds O2) Glycogen (glucose store) Mitochondria
Capillaries
Muscle fibres
Recruitment
(NANTSFMMGMCMR)

Answer 25

Slow:
Number of motor units: Many
Axon diameter: Small
Number of terminals: Few
Tetanic tension: Small
Speed of contraction: Slow
Fatigue: Little/None
Metabolism: Aerobic
Myoglobin (binds O2): Plentiful
Glycogen (glucose store): Little
Mitochondria: High density
Capillaries: Rich Supply
Muscle fibres: Small, red
Recruitment: Early

FAST:
Number of motor units: Few
Axon diameter: Large
Number of terminals: Many
Tetanic tension: Large
Speed of contraction: Fast
Fatigue: Rapid
Metabolism: Anaerobic
Myoglobin (binds O2): Scanty Glycogen (glucose store): Much
Mitochondria: Low density
Capillaries: Few
Muscle fibres: Large, pale
Recruitment: Late

Question 26

What creates a twitch in muscle

Answer 26

A single action potential in a MN causes a “twitch” (short-lasting contraction of all muscle fibres belonging to the motor unit)

Question 27

Causes of summation of twitches

Answer 27

Increased frequency of action potentials in MN leads to sustained contraction through summation of twitches.

Question 28

Describe the size principle in terms of recruitment of motor units

Answer 28

Small motor neurons are more excitable, conduct APs more slowly, and excite fewer fibers that tend to be slow twitch (type I)

Large motor neurons are less excitable, conduct action potentials more rapidly, and excite many fibers that tend to be fast twitch (type II).

(Rm, membrane resistance.)

SO
‘S’ or ‘Type I’ are recruited first (i.e., they are active during relatively weak contractions)

‘FF’ or ‘Type II’ are recruited only at relatively high levels of muscle force

Question 29

Physiological consequences of the ‘size principle’:

Answer 29

• ‘S’ type motor units ‘fire’ almost always.
• ‘S type motor units are best suited for carrying sustained but small loads.
• Weak contractions can be graded with greater precision than strong contractions.
• Necessity to exercise (hard!) to prevent atrophy of ‘FF’ units.