Neuromuscular System

Autonomic Nervous System and the Neuromuscular System

The autonomic nervous system controls the body's internal, involuntary functions, including some skeletal muscle activity – it works automatically, without conscious control.

It is part of the peripheral nervous system and has two branches, which act to prepare the body for exercise and then return it to rest:

• The sympathetic nervous system prepares the body for exercise – often called the 'fight or flight' response
• The parasympathetic nervous system has the opposite effect, calming the body and reducing high-energy functions – often called 'rest and relax'

Types of Muscle Fibre

Skeletal muscle contains three main types of muscle fibre: slow oxidative (type I / slow twitch), fast oxidative glycolytic (type IIa), and fast glycolytic (type IIx).

Every muscle contains a mixture of all three fibre types, but not in equal proportions – the mix is largely genetically determined.

• The proportions vary between individuals: endurance athletes tend to have more type I fibres in their leg muscles, while sprinters tend to have more type II fibres
• Postural muscles, which must sustain contraction over long periods, tend to have a higher proportion of type I fibres
• Type I (slow twitch) fibres contract relatively slowly and are suited to lower-intensity, longer-duration exercise; they produce energy mainly aerobically
• Type II (fast twitch) fibres contract quickly and generate greater force, but fatigue rapidly; they produce energy mainly anaerobically and are used for short, intense efforts
• Type IIa fibres are more fatigue-resistant than type IIx, suited to sustained but intense efforts such as middle-distance events (e.g. 1500m)
• Type IIx fibres fatigue fastest of all three types and are suited to very short, explosive efforts such as sprinting

Characteristics of Muscle Fibre Types

Each fibre type has a distinct set of characteristics, which can be split into two categories:

• Functional characteristics – what the fibre does, e.g. contraction speed, force produced, fatigability, aerobic/anaerobic capacity
• Structural characteristics – the physical make-up of the fibre, e.g. motor neurone size, mitochondrial density, myoglobin content, capillary density

Type I fibres: slow contraction speed, small motor neurones, slow conduction, low force production, low fatigability, high mitochondrial density, high myoglobin content, high capillary density, high aerobic capacity, low anaerobic capacity, low enzyme activity.

Type IIa fibres: fast contraction speed, large motor neurones, fast conduction, high force production, medium fatigability, medium mitochondrial density/myoglobin/capillary density, medium aerobic capacity, high anaerobic capacity, high enzyme activity.

Type IIx fibres: fast contraction speed, large motor neurones, fast conduction, high force production, high fatigability, low mitochondrial density/myoglobin/capillary density, low aerobic capacity, very high anaerobic capacity, very high enzyme activity.

Effect of Training on Fibre Type

Fibre type is largely fixed by genetics, and training cannot convert one fibre type into another.

However, training can increase the size of existing muscle fibres – a process called hypertrophy – which leads to greater muscular strength.

Motor Units and the All-or-None Law

A motor unit consists of a single motor neurone and all the muscle fibres it supplies; every fibre within one motor unit is of the same type.

Motor neurones carry electrical nerve impulses from the brain to the muscle, meeting the fibres at the neuromuscular junction.

• Motor unit size varies: muscles used for fine motor control (e.g. eye muscles) have small motor units with few fibres per neurone
• Muscles used for gross motor control (e.g. the quadriceps) have large motor units, with a single neurone supplying hundreds of fibres

The all-or-none law states that once stimulation reaches the required threshold, all fibres in that motor unit contract fully; below the threshold, none contract at all – a motor unit cannot partially contract.

The brain selectively recruits slow twitch motor units for low-intensity activity (e.g. jogging) and fast twitch motor units when greater force is required (e.g. sprinting, lifting).

Increasing the Strength of Muscle Contraction

The nervous system varies the force of a whole-muscle contraction using two mechanisms: wave summation and spatial summation.

• Wave summation – repeated, high-frequency stimulation of a motor neurone, with insufficient time between impulses for the muscle to relax, causes calcium to build up in the muscle cell, producing a stronger, sustained contraction
• Where wave summation is continuous, the result is a smooth, sustained tetanic contraction rather than a series of individual twitches
• Spatial summation – increasing the force of contraction by recruiting additional and progressively larger motor units within the muscle

Together, wave and spatial summation allow the nervous system to grade the strength of contraction to match the demands of the task, from a light movement up to a maximal effort.

Proprioceptors: Muscle Spindles and Golgi Tendon Organs

Proprioceptors are sensory receptors within muscles, tendons and joints that provide the central nervous system with information about movement and body position.

• Muscle spindles lie between skeletal muscle fibres and detect the speed and extent to which a muscle is being stretched, sending excitatory signals to the central nervous system
• The central nervous system responds with the stretch reflex, causing the muscle to contract to resist further stretching and reduce the risk of injury
• Golgi tendon organs are located between the muscle fibres and the tendon and detect the level of tension within a muscle
• When a muscle contracts isometrically, golgi tendon organs sense the rise in tension and send inhibitory signals to the brain, causing the muscle to relax and lengthen – a response known as autogenic inhibition

Proprioceptive Neuromuscular Facilitation (PNF)

PNF is an advanced stretching technique regarded as one of the most effective methods for increasing flexibility and range of motion. The most commonly used form is the CRAC technique (contract-relax-antagonist-contract).

1. Passive stretch – a partner moves the limb into a stretched position until tension is felt; this stretch is detected by the muscle spindles, which may trigger a stretch reflex if the muscle is stretched too far
2. Isometric contraction – the individual contracts the stretched muscle isometrically against their partner's resistance for around ten seconds; this activates the golgi tendon organs, whose inhibitory signals override the muscle spindles' excitatory signals, delaying the stretch reflex
3. Further stretch – as the limb is moved further, autogenic inhibition (triggered by the golgi tendon organs) allows the target muscle to relax and lengthen further than in the initial passive stretch

This contract-relax cycle can be repeated, allowing further gains in range of motion each time, until no further increase is achievable.