Saturday, April 23, 2011

You need to step up on a step to reach an object on a high shelf over your head. Your starting point should be anatomical position and your finishing point should be the position you are in once your hand has reached the object overhead. Include the steps involved in transmitting the impulse “microscopically” through an individual neuron, from one neuron to another, and then from the neuron to the muscle fiber. Include the steps involved in transmitting the impulse “microscopically” through an individual neuron, from one neuron to another, and then from the neuron to the muscle fiber.

An action potential in a neuron is initialized in its axon hillock, along the axon and its terminals. The impulse is conducted through propagation. An action potential occurs when there is a stimulus, in this case the conscious thought of voluntary muscle movement. When the stimulus - conscious thought of the movement reaches threshold, the neuron’s membrane depolarizes: the gate for NA allowing the sodium to flow outside the cell, causing the membrane to reach a voltage of 30mV. At this point, an impulse is conducted through the axon plasma membrane. Then, the K gate opens, allowing the potassium inside the cell, repolarizing the cell: this is the end of the impulse, the neuron returns to its resting state.
The communication between two neurons occurs through the nerve action potential. The axon will conduct the impulse to the next neuron through releasing a neurotransmitter at its terminals, to the receiving dendrites of the neighboring neuron: This is where the synapse occurs. When the transmission occurred, the presynaptic neuron returns to its polarized state, and the postsynaptic one depolarizes to further conduct the impulse. The axon of the postsynaptic neuron conducts the action potential further through its terminals to the following cell, and so on. These axons are myelinated in order to coordinate fast, voluntary movements; therefore a saltatory conduction takes place: Impulses are saltatory conducted from the level of the nodes of Ranvier.

The plasma membranes of two neurons do not touch, as they are separated by the synaptic cleft. Because nerve impulses do not transmit through the synaptic cleft, the synapse between the two neurons happens when the depolarization opens the voltage-gated Ca2+ channels, allowing the calcium to flow into the synaptic end bulbs of the presynaptic neuron’s axon. As the calcium level increases, synaptic vesicles release neurotransmitter molecules into the synaptic cleft. These molecules diffuse and bind to the neurotransmitter receptors in the plasma membrane of the postsynaptic neuron. The process opens ion channels; when ions start flowing, voltage changes to create depolarization. When depolarization occurs, threshold is reached; the postsynaptic neuron generates a nerve impulse.

The final neuron in the row reaches the muscle. Since this is the last neuron, there is no postsynaptic neuron that follows it, so the synapse happens with the muscle fiber: this is the neuromuscular junction. Here, the neuron excites the muscle fiber that will contract. When the nerve impulse reaches the synaptic end bulbs, ACh is released and diffused into the synaptic cleft between the axon terminals of the neuron and the motor end plate of the muscle fiber. Sodium flows into the cell and generates a muscle action potential.


Resource:

Tortora, G.J., Derrickson B. (2010).  Introduction to the Human Body:  The Essentials of Anatomy and Physiology (8th Ed.).  Hoboken, NJ: John Wiley & Sons, Inc. 

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