Skeletal Muscle Contraction
Neuromuscular Junctions (NMJ)
The neurons that stimulate skeletal muscle fibres to contract are called Somatic Motor Neurons . Each SMN has an axon that extends from the CNS to a group of skeletal muscle fibres. As noted earlier, a muscle fibre contracts in response to one or more action potentials propagating along its sarcolemma and through its system of T-tubules. Muscle action potentials arise at the Neuromuscular Junction (NMJ), the Synapse between a somatic motor neuron and a skeletal muscle fibre. Synapse : a region where communication occurs between two neurons, or between a neuron and a target cell (SMN and a muscle fibre). In most cases there is a small gap at the point of communication called a Synaptic Cleft. The first cell communicates with the other by releasing a chemical called a Neurotransmitter . Synaptic end bulbs : at the NMJ, the end of the motor neuron called the Axon Terminal divides into a cluster of synaptic end bulbs. Suspended in the cytosol within each synaptic end bulb are hundreds of membrane-enclosed sacs called Synaptic Vesicles . Inside each vesicle are 1000s of molecules of Acetylcholine (ACh) the neurotransmitter released at the NMJ. Motor end plate is the region of the sarcolemma opposite the synaptic end bulbs and is the muscle fibre part of the NMJ. Within each motor plate are 30-40 million ACh receptors, which are integral transmembrane proteins that bind specifically to ACh. Therefore, an NMJ includes: all the synaptic end bulbs on one side of the synaptic cleft, plus the motor end plate of the motor fibre on the other side.
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