Neurophysiology: Motor Unit & Contractility

The random slides in the lecture have a few key points to make:

This picture shows the strength at which muscles contract for different starting/’resting’ lengths. The important line is NOT actually the red line, but the active force line (the red line represents the tension generated in both active (i.e. contraction) and passive (due to the non-contractile component of the sarcomere- connectin/tritin). The active force line shows that:

At (A): there is no active force because the myosin and actin do not touch
Between (A&B): there is a linear increase in contractility as more myosin comes into contact with actin (and linear increase in binding sites)
At optimal length between (B&C): there is a relatively stable point. This is because the middle of the thick (myosin) filament has no binding sites on it.
With further contraction (C&D): overlap of the thin (actin) filaments reduces binding and the tension capacity
Once the thick (myosin) filament collide with the Z-disc (D-E)- they act like springs, opposing further contraction

NB The passive action is brought about by muscle stretch and is largely linear. It is due to the presence of elastic connectin (titin) fibres that hold the two Z-plates together.

Motor unit

One action potential in one muscle fibre causes a twitch- on its own has little value. There are ways the body gets around this:

Principle of divergence/convergent neuronal signalling

Convergence/Divergence — The motor neurons are in yellow; the sensory neurons in orange/red.

The image on the left shows divergence and is common in motor signalling- i.e. one signal is sent to more than one motor neuron. The image on the right occurs, for example, in reflexes when stretch sensed by sensory neurons need to synapse with specific motor neurons responsible for extension. In general (and there is a slight difference):

motor neurons receive input from many different inputs
sensory neurons send signals to many different target neurons

Motor innervation is to multiple motor fibres (motor units)

A motor unit is a group of muscle fibres innervated by a single motor neuron.
- They vary in size: large motor units (i.e. lots of fibres per neuron) for gross motor actions; and small motor units (some as small as a few fibres per neuron) for fine movement. These produce large and small forces, respectively.
In order for a productive force to be generated, often more than one motor unit will need to be active- Motor unit recruitment or muscle fibre summation
- For long term muscle use e.g. in the antigravity muscles- this recruitment is asynchronous to prevent fatigue.
- Another mechanism for reducing fatigue is by recruiting motor units that are appropriate (most muscles will contain a mixture of different kinds) (see below)

Twitch summation

As well as muscle fibre summation, the physical contraction of an individual fibre can be increased by a process called twitch summation. A single AP lasts around 1-2ms. The resulting twitch (from an AP) lasts around 100ms. Therefore, if more than one AP fires during this time, their contractions can effectively add up to produce a much stronger contraction. The point of maximal contraction occurs when a muscle is stimulated so much that it is unable to relax. This is tetany.

Fibre types

There are 3 main types of muscle fibres (note a motor unit will only ever contain one of these types) innervated by the α-motor neurons (efferent motor neurons that are responsible for all voluntary movement):