Neuromuscular system

What are the different muscle fibres?

Slow twitch = type I

Fast twitch = type IIa and type IIx

What are the characteristics of type I muscle fibres?

Structural

• motor neurone size = small

• capillary density = high

• myoglobin content = high

• mitochondrial density = high

Functional

• force produced = low

• contraction speed = slow

• motor neurone conduction capacity = slow

• fatigability = low

• aerobic capacity = high

• anaerobic capacity = low

• myosin ATPase enzyme = low

What are the characteristics of type IIa muscle fibres?

Structural

• motor neurone size = large

• capillary density = medium

• myoglobin content = medium

• mitochondrial density = medium

Functional

• force produced = high

• contraction speed = fast

• motor neurone conduction capacity = fast

• fatigability = medium

• aerobic capacity = medium

• anaerobic capacity = high

• myosin ATPase enzyme = high

What are the characteristics of type IIx muscle fibres?

Structural

• motor neurone size = large

• capillary density = low

• myoglobin content = low

• mitochondrial density = low

Functional

• force produced = high

• contraction speed = fast

• motor neurone conduction capacity = fast

• fatigability = high

• aerobic capacity = low

• anaerobic capacity = very high

• myosin ATPase enzyme = very high

What is a motor unit?

A motor unit consists of a motor neurone and its muscle fibres. Only one type of muscle fibre can be found on one particular motor unit

Muscle fibres work with the nervous system so that contraction can occur, the motor neurone transmits impulses to the muscle fibre. Each motor neurone have branches that end in the neuromuscular junction on the muscle fibre

What is the all or none law?

Once the motor neurone stimulates the muscle fibres either all of them contract of non of them contract. A minimum amount of stimulation called the threshold is required to start a contraction. if the sequence of impulses is equal to or more than the threshold, all the muscle fibres in a motor unit will contract. however if the sequence of impulses is less than the threshold then no muscle action will occur.

When are slow twitch or fast twitch motor units recruited?

the brain will recruit slow twitch motor units for low intensity activity such as jogging or long distance swimming. If greater force of contraction is needed, the brain will recruit fast twitch motor units for activities such as sprinting or power lifting.

What is wave summation?

Wave summation is when there is repeated nerve impulse with no time to relax sent to the muscle fibre, this builds up tension in the muscle this then leads to a greater potential force of contraction which creates a forceful, sustained smooth contraction which is referred to as tetanic contraction

What is spatial summation?

this occurs when impulses are received at the same time at different places on the neurone which add up to fire the neurone. It is the recruitment of additional and bigger motor units within a muscle to develop more force.

When the strength of contraction changes by alternating the number and size of muscles motor units.

How to increase the strength of contraction?

• Increase the number of muscle fibres recruited

• increase the size of the muscle fibres recruited

• type of muscle fibres recruited (type IIx produce a larger force of contraction)

• wave summation

• spatial summation

What are the proprioceptors?

these include muscle spindles which detect how far the muscle is being stretched and produce a stretch reflex and the Golgi tendon organs which are activated when there is tension in a muscle

How muscle spindles work?

these lie between the skeletal muscle fibres. They provide information to the central nervous system about how fast and how far a muscle is being stretched. The central nervous system then sends an impulse back to the muscle telling it to contract to prevent overstretching and injury and the stretch reflex is initiated.

How do Golgi tendon organs work?

these are found between the muscle fibres and the tendon. They detect levels of tension in a muscle. When the muscle is contracted isometrically in PNF, they sense the increase in muscle tension and send inhibitory signals to the brain which allows the antagonist muscle to relax and lengthen and the stretch reflex Is delayed, this is known as autogenic inhibition.