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Muscle Movements - Neurons

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Part one: When a person initiates a movement, events in the brain and the spinal cord generate action potentials in the axons of the motor neurons. Each of these axons branch out to send action potentials to many muscle fibers. Motor neurons carry information from the brain toward the spinal cord or out of the brain and spinal cord to effectors through cranial and spinal nerves. Stimulation of the effectors by motor neurons causes muscles to contract. Most nerve impulses that stimulate muscles to contract originate in the central nervous system.

There are two kinds of extensions that come from the cell body of a neuron. 1.) Dendrites receive or input information into the neuron. 2.) An axon conducts nerve impulses towards another neuron or muscle fiber. For muscle movement, neurons or neurons and an effector cell need to communicate. Where this communication takes place is called the synapse. The tips of axon terminals turn into synaptic end bulbs which contain synaptic vesicles. These sacs store chemicals called neurotransmitters. Neurotransmitters are what make communication at the synapse happen.

Neurons communicate with each other through action potentials (nerve impulses). Action potentials are the electrical signals that are transmitted along our nerve and muscle fibers. They are essential for the communication of information to, from, and within the brain. The generation of these impulses in muscle fibers or neurons depends on two things within the plasma membrane: Resting membrane potential and ion channels. When the muscles fibers and neurons are at rest they are considered resting. Ions are like electrons in a battery that carry electrical currents. Ion channels allow specific ions to diffuse across the plasma membrane which changes the membrane potential. Channels that open up in response to a change in membrane potential are used to generate and conduct ACTION POTENTIAL!

For a skeletal muscle to contract it must be stimulated by an electrical signal called a muscle action potential delivered by a motor neuron. A single motor neuron along with all of the muscle fibers it stimulates is called a motor unit. Stimulation of one motor neuron causes all the muscle fibers in that motor unit to contract at the same time. As the axon of a motor neuron enters a skeletal muscle, it divides into axon terminals (as discussed earlier). The synaptic end bulbs, again, contain vesicles which contain neurotransmitters. The synapse formed between the axon terminals of a motor neuron and the motor end plate of a muscle fiber is the neuromuscular junction (NMJ). At the NMJ a motor neuron excites a skeletal muscle fiber.

Arrival of the nerve impulse at the synaptic end bulbs releases neurotransmitters called acetylcholine (ACh). ACh then diffuses between the motor neuron and the motor end plate. When ACh binds to the motor end plate opens ion channels that allow cations like sodium ions to flow across the membrane. This inflow generates a muscle action potential. As muscle action potentials travel along the sarcolemma( the muscle fibers plasma membrane) and into the tranverse tubule system filaments begin to slide.


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