Membrane Potentials (And Neuromuscular Junctions)
Skeletal Muscles Irritability & contractility Motor neurons & motor units Muscle cells have two important and unique properties: They are irritable and they are contractile. Irritability means that they can respond to chemical signals. Contractility means that they respond by becoming shorter (and, thus, pulling on tendons, which pull on bones, which causes movement). Both muscle and nervous cells are irritable, but only muscle cells are contractile. Each motor neuron stimulates several muscle cells (all in the same muscle, though). The entire thing: a motor neuron plus all of the muscle cells it stimulates is called a motor unit. Motor units are all-or-nothing units. Either the whole thing (all of its muscle cells) is stimulated, or none of it is.
Association sites of motor neurons and muscles. Synapses Axon terminal Post-synaptic synaptic membrane Synaptic cleft Synapses The place where a motor neuron and a muscle cell are attached is called a neuromuscular junction. It is important to realize, though, that motor neurons are usually not physically connected. There is a gap (a very narrow one) between them. This gap is called a synaptic cleft. The way a motor neuron stimulates a muscle cell, then, is that it sends a chemical signal molecule across the synaptic gap, causing a reaction in the muscle cell. The ending of the motor neuron is called the axon terminal. From here, signal molecules are released into the synaptic cleft. They diffuse across the synaptic cleft and bind to receptors on the post-synaptic membrane (the part of the sarcolemma at the neuromuscular junction. This binding ultimately results in muscle contraction.
Motor Neuron Axon terminal Vesicles with neurotransmitters Acetylcholine (ACh) A closer look at the presynaptic side (the motor neuron side): Inside the axon terminal of any neuron, including motor neurons, there are many vesicles that are filled with signal molecules, called neurotransmitters. These vesicles sit in the axon terminal and wait until an impulse travels down the motor neuron and reaches the terminal. When this happens, the vesicles are mobilized. They move to the end of the axon terminal and dump their neurotransmitters into the synaptic cleft. Although the nervous system has over a hundred different kinds of neurotransmitters, motor neurons only use a chemical called acetylcholine, abbreviated ACh, as a neurotransmitter. In other words, all of the vesicles in a motor neuron contain acetylcholine.
Post-synaptic synaptic membrane Motor end plate ACh receptors Ligand-gated gated sodium channels Now a closer look at the muscle cell side of the synapse: The region of the post-synaptic membrane that responds to acetylcholine when it comes across the synaptic cleft is called the motor end plate. It has several receptors that are specially-tuned to acetylcholine, and to which only acetylcholine (or a very similar molecule) can bind. Each receptor is associated with a ligandgated sodium channel. This is basically a gate that is closed most of the time, but opens when acetylcholine (the ligand ) binds to its receptor. When open, the ligand-gated sodium channel, let sodium ions rush into the cell, which is the first electrochemical event leading to muscle contraction. Note that the ligand-gated sodium channels are open only when acetylcholine is bound to their receptors and that only sodium can pass through them. (Sodium goes in instead of out for reasons we ll look at in the next lecture).
Try to identify each part in this figure. 1 is the motor neuron, ending in the axon terminal. 2 is the post-synaptic membrane, the sarcolemma of the muscle cell. 3 is a vessicle containing acetylcholine. 4 is an acetylcholine receptor associated with a ligand-gated sodium channel. 5 is a mitochondrion. Here s a summary of the steps of muscle contraction we ve examined so far: A nerve impulse from the central nervous system arrives at the axon terminal of a motor neuron. This causes vesicles filled with acetylcholine in the axon ending to mobilize and move to the end of the axon terminal. The vesicles dump their acetylcholine into the synaptic cleft and it diffuses across the gap. At the opposite side, the acetylcholine binds to ACh receptors at the motor end plate in the muscle cell sarcolemma. This binding causes the ligand-gated sodium channels to open, and sodium begins to flood into the muscle cell. Simple, isn t it?