Quick Membrane Review 1

2 Interfere with the neurons ability to transfer electrical impulses Over loads nervous system volts Taser Tasers

Impulse Transmission Is an electrochemical reaction (the movement of ions – mostly Na+ and K+) The actual impulse is caused by a movement of ions into and out of the neuron An impulse requires an input of energy (ATP) An impulse is started by a stimulus - electric shock, pH, pinch, light, sound, heat, etc. 3

Every cell has a voltage, or membrane potential, across its plasma membrane Voltage is electrical potential energy – a separation of opposite charges. Charge attraction and concentration gradients lead to this “potential energy” The cytoplasm is negatively charged and the extracellular fluid is positively charged. Voltage = membrane potential A neuron in its resting state (not transmitting an electrical signal) usually has a membrane potential of -70mV; the minus sign indicates the inside of the cell is negative in charge with respect to the outside. 4

5 Electrochemical Impulse or Nerve Impulse or Action Potential The electrochemical impulse results from small changes in sodium and potassium ions in the dendrites and then the axon of the neuron. These changes “move down” the axon. The 3 Stages of Nerve Impulse Include: 1.Resting Potential (aka Polarization) 2.Action Potential (aka Depolarization) 3.Refractory Period (Repolarization)

1. The Resting Membrane outside the neuron’s membrane is a high concentration of sodium ions causing it to be positively charged compared to the inside inside the neuron is a high concentration of potassium ions and chloride ions (- ions) there is an electrical potential difference between the inside and outside of about - 70mV there are ion channels in the lipid bilayer membrane that allow certain ions to diffuse in and out (only certain ion channels let sodium move and different ones let potassium move). 6

1. The Resting Membrane at resting membrane potential, more potassium ions have the potential to move out (because there are more open potassium ion channels), than sodium ions have the potential to move in (almost no sodium ion channels open) also, another membrane protein called the sodium potassium pump moves 3 sodium out while moving 2 potassium in (ATP must be used for this to happen = active transport) 7

8 Resting Membrane Potential

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The Action Potential Nerve impulses only occur at the Nodes of Ranvier in myelinated neurons a nerve impulse is an action potential. An action potential begins when the dendrites are stimulated with certain chemicals or an electric jolt. When the neuron receives a stimulus, the cell membrane becomes more permeable to Na+ than K+ (Na+ channels open, and the K+ channels close). The highly concentrated Na+ ions rush into the cell by diffusion and charge attraction. The rapid inflow of Na+ reverses the charges on both sides of the membrane (approximately +40mV). This charge reversal is referred to as Depolarization 10

11 Animation

3.Refractory/Recovery voltage inside the cell becomes positive = the Na+ channels slam closed K+ channels then open = K+ ions rush out. Restores + outside and – inside neuron Once action potential has passed, sodium is actively pumped out of the cell while potassium is pulled back in (Na+/K+ Pump) (Na+/K+ Pump) the membrane will go through a brief hyperpolarization before returning to its RMP 12

3.Refractory/Recovery Refractory period is the period of time the nerve will not respond to a stimulus (needs to return to resting potential) – Period of time immediately after an impulse when the neuron will not respond to a stimulus – cannot conduct another impulse until it has been repolarized – Duration 1-10 milliseconds 13

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15 Schematic of an Action Potential Depolarization of one region in a neuron stimulates depolarization in adjacent regions and allows nerve impulses to move along the membrane of the neuron. The region of the axon behind the location of the depolarization undergoes repolarization and refractory.

Nerve impulse 16

17 An Action Potential Can Be Represented in Graphical Form

18 An Action Potential Can Be Represented in Graphical Form

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Step by Step Action potential and membrane animation Action Potential and Na/K Gates Action Potential 20

21 Saltatory Conduction In myelinated axons, depolarization occurs only at the Nodes of Ranvier. The flow of ions can only happen at those nodes so action potentials have to “jump” from node to node

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23 All or None Response For an impulse to be generated, the stimulus must be strong enough to “jolt” the membrane to allow sodium ions to rush in. This level of intensity is called the “threshold level”. Different neurons have different threshold levels. If the stimulus is strong enough to generate an action potential, the impulse is transmitted along the entire neuron.

24 However, if the stimulus is much stronger than the threshold, the neuron will not fire any stronger (the graph stays the same) Thus, neurons fire maximally or not at all. This is the all-or-none response. All or None Response cont…

25 Strength & Type of Stimulus How do you distinguish between different stimuli? For instance, how is it that your body can tell the difference between the touch from a feather or a rock? There are 2 ways: 1.A stronger stimulus causes more frequent impulses along a neuron. 2. A stronger stimulus causes more neurons to be stimulated because the threshold of many neurons are reached.

26 Concept Review: Action Potential Animation

Draw on Board Example of Graph of action potential What is happening on the membrane 27

28 Check Your Understanding 1.What is a polarized membrane? 2.What causes the inside of a neuron to become negatively charged? 3.Why does the polarity of a cell membrane reverse during an action potential? 4.Why do nerve impulses move faster along myelinated nerve fibers? 5.Some people report that they have a high pain tolerance. Explain this in terms of threshold levels. 6. What is the “all or none response”?