Function of Neural Control of Human Movement: Chapter 19

Define a motor unit

Explain a neuromuscular junction

Neuromuscular junction

- Intersurface between the end of a myelinated motor neuron and muscle fiber

- Transmits nerve impulse to initiate muscle action

NMJ has 5 common features

1. Presence of Schwann cells (form myelin sheaths which greatly increases nerve conduction velocity)

2. Terminal section has acetylcholine

3. Basement membrane that lines the synaptic space (extracellular matrix that contributes to the adhesion of the nerve to muscle receptors)

4. Membrane across cleft that has ACh receptors

5. Connector microtubules at postsynaptic membrane that that transmits electrical signals deep within muscle fiber

Describe the concept of neural facilitation

Facilitation

- Occurs at NMJ when ACh release causes sub-treshold excitation that lowers resting membrane potential of connective neuron

- Results in increased "tendency" to fire

Neural facilitation (disinhibition)

Affectes neurons in the CNS and occurs when:

- Motor neurons become less sensitive to inhibitory neurotransmitters and/or

- Reduced quantity of inhibitory neurotransmitter substance transported to the motor neuron

Neural facilitation allows for

- Full muscle activation during all-out effort

- Largely account for rapid strength gains in early stage of resistance training

How many motor neurons typical individual posses?

420,000

Motor unit

- Functional unit of muscle

- Consists of the alpha motor neuron and the specific muscle fiber it supplies

- Each single fiber receives input from one motor unit

- A single motor unit can supply many fibers

# of muscle fibers per motor neuron relates to

muscle function

- Large muscles = motor neuron may control 2000-3000 muscle fibers for less complex movements

Individual and combined motor units actions produce

specific muscle actions

Motor Unit Functional Characteristics

- Contains only one specific muscle fiber type (type I or II) or a subdivision of the type II with same metabolic profile

Motor units can be categorized based on 3 properties of the muscles they supply which include...

1. Twitch characteristics

2. Tension characteristics

3. Fatigue resistance

Motor unit- Twitch characteristics

- Motor units generate high, intermediate or low force in response to single electrical stimulus

- MU that produce low force also exhibit a slow shortening time (reduced rate of force development) and fatigue resistance

- MU that generate high force also shorten at a fast rate, but fatigue earlier

There are 3 common motor unit categories

1. fast fatiguable ( FF- type Iix)

2. Fast fatiguable resistant (FR-type IIa)

3. Slow (s-type I)

All or non principle

Stimulus strong enough to trigger an action potential in a motor neuron activates all of the accompanying fibers (in the motor unit) to contract synchronously

- Single motor unit cannot exert a force gradation

Understand and explain tension characteristic, gradation of forces control of motor unit function, motor unit activity, fatigue resistance

How does the entire muscle exert a graded force?

1. Increasing the number of motor units recruited

- Muscle force generated is greatest when activated by all of its motor units

2. Increasing the frequency of motor unit discharge

- repetitive stimuli that reach muscle before it relaxes increase total tension versus a single stimuli

Size principle of motor

- A muscle force requirements increase progressively larger axons recruited the required motor neurons

- All motor units within a muscle do not fire at the same time (ex: picking up a pencil vs DL)

- Blending recruitment of motor units and modification of their firing rate permits optimal patterns of neural discharge

- Differential control of motor unit firing patterns represents major factor distinguishing skilled from unskilled performance (ex: difference between novice and expert guitar players)

Fatigue resistance

- Represents decline in muscle tension or force capacity with repeated stimulation or during a given time period

- Motor unit fatigue occurs as a result of many complex factors

-During prolonged submaximal activity, increased motor unit recruitment maintains the necessary force output

- During all-out exercise (that indicated all motor units) decreased neural activity accompanies fatigue as measured by EMG

Explain motor unit functional characteristics

Efficient force gradation is critical for

performing many common functional tasks