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Thursday, November 1, 2007

Nursing Reference: Nerve Impulse

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Nursing Reference: Nerve Impulse Slide Transcript

Slide 2: • Neurons are specifically designed to conduct nerve impulses • Nerve impulses are only conducted when the neuron has recovered from conducting its last nerve impulse – Must also be sufficiently stimulated to conduct a new one

Slide 3: Three states of a neuron • Resting potential – The state during which no nerve impulse is being conducted although the neuron is capable of doing so • A ction potential – The state during which the neuron is actively involved in conducting a nerve impulse • Recovery/Refractory potential – The state during which the neuron is unable to conduct a nerve impulse since the neuron must “ recover” following the last nerve impulse

Slide 4: 1. Resting Potential • The state of the neuron when no nerve impulse is being conducted • During resting potential there is an ion displacement between the inside and the outside of the neuron (i.e. on either side of the neuron cell membrane) as follows: – There are more Na+ ions on the outside than on the inside – There are more K+ ions on the inside than on the outside – There are many large organic anions (-ve charged ions) locked inside since they are too big to pass through the neuron’s cell membrane

Slide 5: • Due to this difference in ion displacement there is a NE T CHA RGE difference across the cell membrane = M EMB RA NE POTENTIA L • This membrane potential when the neuron is at rest is called the RE S TING POTE NTIA L =-70mV • This difference in ion displacement and thus the resting potential is largely maintained by a protein channel called the Na+/ + PUM P K

Slide 6: Na+/K+ PUMP • Powered by A TP this pump actively “ pumps” Na+ ions out of the cell and K+ ions into the cell • A s a result of this active transport, the cytoplasm of the neuron contains more K+ ions and fewer Na+ ions than the surrounding medium. • The cell membrane also has 2 other separate protein channels, one that “ leaks” K+ ions and one that “ leaks” Na+ ions down their electrochemical gradient (combo of concentration and electrical). • There are more K+ channels than Na+ channels which means more K+ ions leak out of the cell as opposed to Na+ leaking into the cell • A s a result, K+ ions leak out of the cell to produce a negative charge on the inside of the membrane. • This charge difference is known as the resting potential of the neuron. The neuron, of course, is not actually "resting" because it must produce a constant supply of A TP to fuel active transport.

Slide 8: • A t rest, the inside of a neuron's membrane has a negative charge. • A s the figure shows, a Na+ / K+ pump in the cell membrane pumps sodium out of the cell and potassium into it. • However, because the cell membrane is a bit leakier to potassium than it is to sodium, more potassium ions leak out of the cell. • A s a result, the inside of the membrane builds up a net negative charge relative to the outside.

Slide 9: Review Questions • Why are sodium ions exposed to a greater driving force than potassium ions at rest? • What is meant by the resting membrane potential? • What creates the membrane potential in neurons? • What would happen to a neuron’s ability to conduct a nerve impulse if the resting potential didn’t exist?

Slide 10: Action Potential • A n action potential occurs when a neuron is conducting a nerve impulse • In order for an action potential to occur, the neuron must receive sufficient stimulation to open enough Na gates to reach the threshold level • If sufficient Na gates are opened to reach the threshold level, other Na and K gates will be stimulated to open • This results in a self-propagating wave of action potentials and Na and K gates opening along the entire length of the neuron and an action potental and nerve impulse occur • S ince an action potential will only occur if the membrane threshold level is reached, an action potential can also be described as an all or none response • A ction potential can be divided into 2 phases: depolarization & repolarization

Slide 11: Depolarization (upswing) • If a neuron received sufficient stimulation to reach the membrane threshold, successive Na gates along the entire neuron membrane will open • The opening of the Na gates allows Na ions to move into the neuron • The movement of Na ions into the neruon causes the membrane potential to change from -70mV to +40mV • A s the membrane potential becomes more positive, Na gates begin to close. A t the end of depolarization, the Na gates are all closed

Slide 12: Repolarization (Down-swing) • A t the end of the depolarization phase, K gates begin to open, allowing K to leave the neuron • These K gates are activated at the +ve membrane potential value of about +40mV • The movement of K ions out of the neuron produces a change in membrane potential such that the potential becomes more –ve • F ollowing repolarization, the K gates close slowly

Slide 13: • During the conduction of a nerve impulse, each successive section of a neuron’s membrane will undergo an action potential consisting of depolarization followed by repolarization • Thus the nerve impulse is the movement of the action potential along the neuron cell membrane

Slide 14: Recovery/Refractory Potential • Immediately following an action potential, a neuron is unable to conduct a nerve impulse until it has recovered because its Na gates won’t open • A neuron which is undergoing recovery is said to be refractory since it cannot conduct a nerve impulse

Slide 15: • During the recovery phase the following events are occurring: 1. The K gates are closing 2. The Na/ pump is returning the Na ions to the K outside and K ions to the inside of the neuron 3. The membrane potential is returning to its resting value of -70mV • Once the recovery phase is complete, the neuron is no longer in its refractory period and is ready to conduct another nerve impulse

Slide 16: S alatory Conduction • S ome neurons have no myelin coating and are described as unmyelinated • In unmyelinated neurons, an action potential must pass through each point along the neuron cell membrane which makes the conduction of the nerve impulse relatively slow • M ost neurons in humans are (since they are enclosed in a myelin coat formed by S chwann cell membranes wrapped around the neuron) • In myelinated neurons, an action potential does not occur along sections of the neuron which are wrapped in myelin • Ions are unable to cross the nerve cell membrane in these sections

Slide 17: • The gaps in myelin are called Nodes of Ranvier • These gaps are the site of an action potential • Thus, in myelinated neurons, the action potential jumps from one node of Ranvier to the next in a process called salatory conduction • Salatory conduction is very rapid, allowing the nerve impulse to travel very rapidly along the neuron

Slide 18: Review Questions • Define the following: – Membrane Threshold – A ction potential • E xplain why the 1 st part of an action potential is called “ depolarization” • E xplain why the 2 nd part of an action potential is called “ repolarization”

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