Neuromuscular system

Summarise the neuromuscular system

The neuro-muscular system includes all the muscles in the body and the nerves serving them

It is a complex link between the muscular system and the nervous system

Every movement the body makes requires communication between the brain and the muscles

How does an impulse travel

Nerve impulses sent from the cerebellum in the brain cause skeletal muscle to contract

These electrical impulses are sent down a specialised nerve called a motor neurone

At the muscle fibre end of the motor neurone, this nerve impulse terminates at a synaptic end bulb which is found at the junction to the muscle fibre

The muscle fibre side of this junction is called a neuro-muscular junction or motor end plate

A signal is transmitted between the motor end plate and a muscle fibre causing the muscle fibre to contract and exert force

Cell Body

Receives stimuli from other neurones, contains nucleus to control the functions of the cell

Mitochondrion

Generate chemical energy needed to power the cell

Nucleus

Brain of the nerve cell

Dendrites

Branch-like extensions act like antennae. They collect/receive information from other neurones, and pass it to the cell body

Axon

Carries the electrical signals away from the cell body to the axon

Myelin sheath

Surrounds the axon, which electrically insulates it

Schwann cell

Wraps around the axon, produces the myelin sheath

Nodes of Ranvier

Gaps in the myelin sheath, where the ‘action potential’ jumps from node to node

Axon terminal

The end of the axon. Makes the synapse with a muscle cell (the motor unit)

Neuromuscular junction

Transmits the nerve impulse into the muscle fibres causing them to contract

Describe an action potential

Transmission of neural messages along a neurone is an electrochemical process:

An action potential is initiated when sufficient numbers of sodium ions (NA+) diffuse into the neurone

This depolarises the axon to a critical threshold level called the ‘all or none law’ which is followed by repolarisation back to the resting potential

This process forms an electrical impulse which then transmits down the neurone to make the muscle contract

In effect, this electrical impulse is conducted down the axon

The myelin sheath insulates the axon, and the action potential travels from node to node in a wave like action with the exchanges occurring at the nodes of Ranvier

The nerve action potential is followed by the muscle action potential

What is a motor unit

A motor unit is made up of a motor neurone and all of the skeletal muscle fibres supplied by the neuron’s axon terminals

All or none law

The law applies to the contraction of fibres within a motor unit. When a motor unit activates, all of the fibres within the unit contract, and at full force. The strength of the contraction depends on the number of motor units recruited. Or, none of them at all.

Synapse

A synapse is a junction where the axon of one neuron interacts with another neuron

What are the 3 types of muscle fibre

• slow oxidative (type 1)

• Fast oxidative glycolytic (type IIa)

• Fast glycolytic (type IIx)

Myoglobin

An iron-containing protein in muscle, similar to haemoglobin, that receives oxygen from the red blood cells.

ATP

The energy deprived from carbohydrates, fats and proteins is stored in bodily tissues in the form of a high energy compound called Adenosine Triphosphate.

ATP is the compound which stores energy and is therefore the energy currency linked to intensity and duration of physical activity.

ATP exists in every living tissue and its breakdown gives energy for all life functions – the includes the contraction of muscle tissue.

All muscular activity requires the availability and breakdown of ATP.

PC

An energy rich phosphate compound found in muscle cells.

In it’s chemical partnership with ATP, it is fundamental to the ability of the body to produce muscular energy.

Type 1 structural features

• Thin in diameter, therefore a short diffusion gradient = faster/greater gaseous exchange

• High capillary density – higher rate of gaseous exchange, delivery of O2, removal of waste

• Lots of myoglobin – allows better O2 carrying capacity

• High mitochondria density – allows better energy production

• Better suited to using oxygen

• Connected to slower firing nerve fibres

• Greater aerobic enzyme activity – more energy produced from aerobic system

Type 1 functional characteristics

• High resistance to fatigue

• Slow speed of force production

• Low strength of contraction

• Able to maintain force production for a long time

• High rate of aerobic energy production

Type IIa structural characteristics

• Wider in diameter than Type I, but not as wide as Type IIx (medium)

• Large amounts of myoglobin

• Large amounts of mitochondria

• Less capillaries than Type I, but more than Type IIx

• Relatively high levels of ATP and PC

type IIa functional characteristics

• Moderate resistance to fatigue

• High contraction velocity

• High capacity for regenerating ATP

• Moderate production of force

• Relatively fast nerve conduction (faster than Type I, but slower than Type IIx)

type IIx structural characteristics

• High in ATP

• High in PC (Phosphocreatine) - allows for quicker energy provision

• High actin and myosin content

• Large motor neurone size - enables muscle to produce force rapidly

• Wide in diameter

• Low in capillaries

• Large motor unit size

• Low mitochondrial density

• High in Glycogen

• Low in myoglobin

• High in Creatine Kinase

type IIx functional characteristics

• High force production = faster speed can be generated

• Low resistance to fatigue

• High glycolytic capacity - able to resynthesize ATP quickly

• High rate of relaxation

• Very high contractile speed

• Fast nerve conduction

High in PC =

high rate of contraction

High stores of PC =

maintain high rate of contraction for longer

Increased fibre sizzed strength =

Increased strength

High force production =

allows speed/power

High actin & myosin =

Allows faster contraction

At low intensity…

Type I slow twitch (Slow Oxidative) motor units are recruited first

At higher intensity…

Type IIa fast twitch (Fast Oxidative Glycolytic) motor units are recruited

At greatest intensity…

Type IIx fast twitch (Fast Glycolytic) motor units are recruited to produce powerful fast muscle contractions

Spatial summation

When the strength of a contraction changes by altering the amount of the muscles motor units. Activation is staggered, enabling a sustained contraction to be maintained. Delays fatigue

Wave summation

When there is a repeated nerve impulse with no time to relax so a smooth, powerful contraction occurs.

Example: Maximal forces required in a 100m sprint or a gymnastics vault

Tetanic contraction

A sustained powerful muscle contraction caused by a series of fast repeating stimuli during wave summation.

Example: Squat hold— muscles require sustained contraction to hold position

Twitch

a single stimulus is delivered and the muscle contracts and relaxes

Unfused (Incomplete) Tetanic

More complete twitch fusion occurs as stimuli are delivered more rapidly

Fused (Complete) Tetanic

A smooth continuous contraction without any evidence of relaxation

How does recruitment patterns of motor units enable athletes to meet demands of their events?

The strength of contractions depends upon the amount of motor units recruited. An endurance athlete would recruit more Type I Slow Twitch muscle fibres. - Such as a marathon runner.

Power athletes would recruit more Type IIx Glycolytic muscle fibres.

- Such as 100m sprinters.

Endurance athletes utilise spatial summation patterns to delay fatigue, whereas power athletes require explosive power for a short period of time.

How can training adjust the recruitment of different muscle fibres?

• Low intensity endurance training will result in asynchronous recruitment of different slow twitch fibres. Such as fartlek/continuous

• High intensity, power training will result in synchronous recruitment of fast twitch muscle fibres. For example 🡪 sprint interval & weight

• This would also increase the rate of fibre recruitment