A motor unit comprises a motor neurone and all the muscle fibres it supplies. Nerve propagation is the way in which a nerve transmits an electrical impulse.
In order to understand nerve propagation, it is important to understand the structure of a motor neurone (nerve).
Each neuron contains a cell body and an axon. The cell body contains a nucleus which is the centre of operation for the neuron and dendrites or branched projections that act to conduct electrical impulses towards the nucleus.
The axon (long thin part of the neuron) carries the electrical impulses away from the cell body and towards the muscle. In the end, the axon branches into axon terminals and ends at synaptic knobs which have contact with the muscle.
Surrounding the Axon is a fatty covering called the Myelin sheath, which acts to insulate the nerve. The sheath is not continuous however and contains breaks, known as nodes of Ranvier.
The impulse jumps from one node to the next, allowing more rapid conduction. The picture below shows the structure of a motor neuron.
Nerve propagation is the way in which an impulse transmits along the nerve. When not under impulse a nerve has a negative charge compared to its surroundings. This negative charge is the resting membrane potential and in this state, the neuron polarises. In order for an impulse to travel along the neuron, the resting membrane potential must change and become depolarised.
This occurs because the stimulus allows a surge of Na+ ions (sodium) into the cell, which changes the charge, making the inside positive compared to its surroundings. This is depolarisation. When this reaches a threshold, an action potential is established and the impulse can travel along the neuron.
The ‘all or none’ law states that there must be a minimum level of depolarisation for an action potential to occur. Without reaching this level, no impulse will be propagated.
Prior to another action potential occurring the resting membrane potential must be restored. This ensures that each stimulus is kept separate. This repolarisation is achieved by the movement of K+ (Potassium) ions out of the cell, restoring the internal negative charge.
A motor unit is described as a single motor neurone and all of the muscle fibres it innervates. A motor unit can contain anywhere between 10 and thousands of muscle fibres. Muscles that produce large powerful movements contain motor units with large numbers of fibres, and those for small intricate movements contain only a few fibres per motor unit.
Where the synaptic knobs of the neuron meet the muscle fibres is known as the neuromuscular junction. When an impulse reaches the neuromuscular junction, a neurotransmitter called Acetylcholine is released which filters across the synaptic cleft (microscopic space between the synaptic knob and motor endplate). This causes depolarisation of the motor endplate and puts the sliding filament theory of muscular contraction into practice.
All or none law
The ‘all or none’ law as mentioned above also 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, there is no strong or weak contraction. The strength of the resultant whole muscular contraction depends upon the number of motor units recruited.
Another way of increasing the strength of a muscle contraction is by decreasing the time between impulses so that the muscle fibres do not have time to relax, resulting in a continuous wave of contractions known as wave summation.
To produce a strong contraction all motor units in the muscle are recruited, but only for a short time. In order to increase the length of a contraction a kind of rotation system is implemented whereby some units contract while others rest and continuously alternate. This is known as spatial summation or tetanus.