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1.6. The Neuromuscular Junction
What is a Neuromuscular Junction?
As you will have seen in Section 1.5. Reflex Arcs, a motor neuron running from the spinal cord must be able to stimulate, or excite, a skeletal muscle, so that it can contract and perform its function. A motor neuron communicates with a skeletal muscle using a particular combination of neurotransmitters, ions and a type of synapse called a ‘neuromuscular junction’. This process of communication is described below.
Communication at a Neuromuscular Junction
Stage 1
As a motor neuron approaches a skeletal muscle, it divides into lots of branches. Each branch forms a neuromuscular junction with a muscle fibre. This means that one motor neuron innervates a number of muscle fibres.
As you might expect, the process begins with an action potential arriving at the presynaptic terminal. Ca2+ ions enter the presynaptic terminal and stimulate neurotransmitter-containing vesicles to migrate towards the presynaptic membrane. Communication with skeletal muscles requires the use of a specific neurotransmitter: acetylcholine. The vesicles exocytose the acetylcholine into the neuromuscular junction. This is quite similar to a regular synapse.
Stage 2
The membrane of a muscle fibre is referred to as ‘the sarcolemma’, and it contains receptors for acetylcholine and Na+ ion channels. The acetylcholine molecules bind to their receptors in the sarcolemma (they have a complementary shape) and the receptors open the Na+ ion channels, allowing a rush of Na+ ions into the sarcolemma and the generation of an action potential in the muscle.
Stage 3
Now, individual muscle fibres are made up of myofibrils. To ensure that the action potential gets deep down into the muscle fibre and close to all of the myofibrils, the sarcolemma has special tubes that run deep through the muscle fibre. These special tubes are called T-tubules, and the action potential runs continues down these too.
Stage 4
The T-tubules don’t lead directly to the myofibrils. Instead, they lead to the sarcoplasmic reticulum, a unique feature of muscle fibres. The sarcoplasmic reticulum holds a large quantity of Ca2+ ions and, when the action potential reaches it via the T-tubules, it releases these Ca2+ ions into the myofibrils.
Stage 5
The myofibrils then use the Ca2+ ions to enable their filaments to slide over each other and shorten – this is a muscular contraction.
As a motor neuron approaches a skeletal muscle, it divides into lots of branches. Each branch forms a neuromuscular junction with a muscle fibre. This means that one motor neuron innervates a number of muscle fibres.
As you might expect, the process begins with an action potential arriving at the presynaptic terminal. Ca2+ ions enter the presynaptic terminal and stimulate neurotransmitter-containing vesicles to migrate towards the presynaptic membrane. Communication with skeletal muscles requires the use of a specific neurotransmitter: acetylcholine. The vesicles exocytose the acetylcholine into the neuromuscular junction. This is quite similar to a regular synapse.
Stage 2
The membrane of a muscle fibre is referred to as ‘the sarcolemma’, and it contains receptors for acetylcholine and Na+ ion channels. The acetylcholine molecules bind to their receptors in the sarcolemma (they have a complementary shape) and the receptors open the Na+ ion channels, allowing a rush of Na+ ions into the sarcolemma and the generation of an action potential in the muscle.
Stage 3
Now, individual muscle fibres are made up of myofibrils. To ensure that the action potential gets deep down into the muscle fibre and close to all of the myofibrils, the sarcolemma has special tubes that run deep through the muscle fibre. These special tubes are called T-tubules, and the action potential runs continues down these too.
Stage 4
The T-tubules don’t lead directly to the myofibrils. Instead, they lead to the sarcoplasmic reticulum, a unique feature of muscle fibres. The sarcoplasmic reticulum holds a large quantity of Ca2+ ions and, when the action potential reaches it via the T-tubules, it releases these Ca2+ ions into the myofibrils.
Stage 5
The myofibrils then use the Ca2+ ions to enable their filaments to slide over each other and shorten – this is a muscular contraction.
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