7 December 2014 CHANNELS OF THE NEURON: ACTING ON IMPULSE

 By the end of the lesson I can:  Describe the pathway of an action potential across a neuron.  Explain the mechanism by which axonal depolarization occurs.  Explain with a diagram and words the changes in voltage through an instance of action potential. (Paying particular attention to the behavior of the voltage channels) LEARNING OUTCOMES

PIECES OF THE PUZZLE

 Leakage channels are non-gated channels found throughout the neuron. They are always open and contribute to the resting membrane potential. LEAKAGE CHANNELS

© 2013 Pearson Education, Inc. K + leakage channels Cell interior –90 mV Suppose a cell has only K + channels... K + loss through abundant leakage channels establishes a negative membrane potential. The permeabilities of Na + and K + across the membrane are different.

© 2013 Pearson Education, Inc. Now, let’s add some Na + channels to our cell... Na + entry through leakage channels reduces the negative membrane potential slightly. Cell interior –70 mV

© 2013 Pearson Education, Inc. Finally, let’s add a pump to compensate for leaking ions. Na + -K + pumps maintain the concentration gradients, resulting in the resting membrane potential. Cell interior –70 mV Na + -K + pump

 Ligand-gated channels are found on dendrites and the cell body and open or close in response to the presence of neurotransmitters (ligands).  Example: Acetylcholine LIGAND CHANNELS

Chemical or Ligand-gated channels Open in response to binding of the appropriate neurotransmitter Receptor Closed Neurotransmitter chemical attached to receptor Open Chemical binds

 Voltage-gated channels open or close in response to changes in membrane potential. Many of these are voltage-gated Na + and K + channels that are important for the initiation and propagation of action potentials along the axon  Found along the axon and synapse.  The action potential moves along the axon in one direction because of the refractory period. The action potential travels continuously along the length of the axon like a wave. VOLTAGE GATED CHANNELS

© 2013 Pearson Education, Inc. 1 Sodium channel Potassium channel Resting state Activation gates Inactivation gate

© 2013 Pearson Education, Inc. 2 Depolarization

Depolarization spreads: Opposite charges attract each other. This creates local currents (black arrows) that depolarize adjacent membrane areas, spreading the wave of depolarization.

© 2013 Pearson Education, Inc. 3 Repolarization

© 2013 Pearson Education, Inc. 4 Hyperpolarization

Resting state Depolarization Action potential Repolarization Hyperpolarization Threshold Time (ms) Membrane potential (mV) –70 – The big picture

1. Absolute Refractory Period occurs 1-2 msec after the initiation of the action potential. During this time an action potential cannot be generated. 2. Relative Refractory Period immediately follows the absolute refractory period and lasts msec. During this time a second action potential can be generated but only if a stronger stimulus is used. REFRACTORY PERIODS

MYELINATED VS. UNMYELINATED

GATE CONFIGURATION