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 ‘all or none’ law as mentioned above also applies to the contraction of fibres within a motor unit. This causes depolarisation of the motor endplate and puts the sliding filament theory of muscular contraction into practice. 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). Where the synaptic knobs of the neuron meet the muscle fibres is known as the neuromuscular junction.
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. A motor unit can contain anywhere between 10 and thousands of muscle fibres. Motor UnitsĪ motor unit is described as a single motor neurone and all of the muscle fibres it innervates. This repolarisation is achieved by the movement of K+ (Potassium) ions out of the cell, restoring the internal negative charge. This ensures that each stimulus is kept separate. Prior to another action potential occurring the resting membrane potential must be restored. Without reaching this level, no impulse will be propagated. The ‘all or none’ law states that there must be a minimum level of depolarisation for an action potential to occur. When this reaches a threshold, an action potential is established and the impulse can travel along the neuron. 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. In order for an impulse to travel along the neuron, the resting membrane potential must change and become depolarised. This negative charge is the resting membrane potential and in this state, the neuron polarises. When not under impulse a nerve has a negative charge compared to its surroundings. Nerve propagation is the way in which an impulse transmits along the nerve. The picture below shows the structure of a motor neuron. The impulse jumps from one node to the next, allowing more rapid conduction.
The sheath is not continuous however and contains breaks, known as nodes of Ranvier. Surrounding the Axon is a fatty covering called the Myelin sheath, which acts to insulate the nerve. In the end, the axon branches into axon terminals and ends at synaptic knobs which have contact with the muscle. The axon (long thin part of the neuron) carries the electrical impulses away from the cell body and towards the muscle. 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. In order to understand nerve propagation, it is important to understand the structure of a motor neurone (nerve).Įach neuron contains a cell body and an axon.
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