Define motor unit. Describe the innervation ratio and the size principle.

Definition

Innervation Ratio

• Small innervation ratio = finer control
• All fibres of one motor unit are of the same type (all type I or all type II)
• Fibres of one unit are scattered through the muscle, not clustered

Properties

• The motor unit obeys the all-or-none law — when the neurone fires, all its fibres contract maximally
• Graded contraction of the whole muscle is therefore achieved by two mechanisms only:

– Recruitment — more motor units activated

– Rate coding — ↑ frequency of firing of each unit

Size Principle (henneman)

Weak effort → Small motor units recruited first (type I, fatigue-resistant) → Increasing effort → Large motor units recruited (type II, powerful)

• Small motor neurones have a higher input resistance → lower threshold
• Advantage — smooth, graded increase in force and economy of energy

Applied Aspects

• Electromyography (EMG) records motor unit potentials

– Neurogenic lesion → large, long, polyphasic potentials (surviving neurones sprout and adopt orphaned fibres → giant motor units)

– Myopathic lesion → small, short, polyphasic potentials

• Fasciculation — spontaneous discharge of a whole motor unit; seen in anterior horn cell disease such as motor neurone disease
• Fibrillation — contraction of a single denervated fibre; detectable only on EMG