Today you will: Describe a resting membrane

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Presentation transcript:

Today you will: Describe a resting membrane Describe a membrane with action potential and explain what the action potential is Explain the role of the sodium-potassium pump

Nerve Impulse Conduction Compare electricity to nerve transmission: Electric Current Nerve Transmission Much faster. Core of nerve cell is resistant. Current diminishes during transmission. Impulse remains same strength (ATP used). Wire acts as conduit, current provided externally. Cells are conduit and source of current. A current is generated by the movement of ions through the nerve cell membrane.

In the early 1900s Julius Bernstein suggested that nerve A bit of history, In the early 1900s Julius Bernstein suggested that nerve Impulses were an electrochemical message created By the movement of ions through the nerve cell membrane. 1939 the theory was proven with tiny electrodes and a squid.

How do particles transport across a membrane? Plasma membranes, including neurons are composed of a phospholipid bilayer. Membranes, are selectively permeable  ions cannot cross by simple diffusion, they use facilitated diffusion.

Resting membrane (resting potential) a non-conducting situation, the nerve cell is not being stimulated a steady charge difference across the cell membrane, Usually -70mv In a resting neuron the ions want to diffuse. Potassium will diffuse out (follow concentration gradient) because the membrane has a high potassium ion permeability. Sodium ions want to diffuse in (follow concentration gradient), but the membrane has a low sodium ion permeability. Therefore, inside the membrane becomes negative relative to the positive area Outside.

Potassium gates are open, sodium gates are closed. To try and balance the excess of positive ions outside the cell, negative ions in the cell accumulate along the membrane edge. The membrane is said to be polarized. The potential difference (PD) indicates the charge difference inside the cell (-70mV inside)

Resting membrane

a nerve impulse occurs, nerve becomes excited Action Potential a nerve impulse occurs, nerve becomes excited a reversal of charge occurs across the cell membrane, the charge inside becomes +40mV. lasts for only a few milliseconds, then returns to -70mV When a neuron receives a stimulus, the membrane becomes more permeable to sodium than potassium. Potassium gates are closed, sodium gates are open. Highly concentrated sodium ions rush into the cell by diffusion and by the charge attraction. The rapid flow of sodium ions reverses the charge on both sides of the membrane. This reversal of charges is called depolarization. A nerve impulse is an action potential.

Action Potential

As soon as the internal charge becomes positive, the sodium gates slam shut. The potassium gates reopen and potassium will start to move out again to balance the charge. Eventually the cell returns to no polarity but sodium (inside) and potassium (outside) are on opposite sides of the membrane. The sodium-potassium pumps get to work, using ATP to move sodium out and potassium in. This process is called repolarization. This process takes 0.001 s! NOTE: After being depolarized, the nerve cells can’t transmit again until it is repolarized. This time is called the refractory period. It usually Lasts 1 to 10 milliseconds. Hyperpolarization – condition in which the inside of the nerve cell membrane has a greater negative charge than the resting membrane. It is caused by excessive diffusion of potassium ions of the cell.

Movement of the Action Potential As an area of the neuron undergoes depolarization, the ions are attracted to ions in an adjacent section, causing a wave motion to occur. The neuron undergoes a wave of depolarization, and then a wave of refractory periods. diagram

In myelinated axons, depolarization happens only at the nodes of Ranvier (gate channels are concentrated there). Action potentials need to jump from node to node therefore, speeding it up. diagram

What is a polarized membrane? What causes the inside of a neuron to become negatively charged? Why does the polarity of a cell membrane reverse an action potential? Why do nerve impulses move faster along myelinated nerve fibres?