Galvanism 1790 Luigi Galvani & “animal electricity” Contraction of a muscle that is stimulated by an electric current

Frankenstein and Galvanism 1831 Mary Shelley

Action Potential Electrochemical message that can stimulate or inhibit another neuron

Resting Potential -70 milliVolts High concentration of sodium ions outside of cell; relatively low concentration of sodium ions inside of cell Low concentration of potassium ions outside of cell; relatively high concentration of potassium ions inside of cell

Threshold Potential -55 millivolts Sodium channels open and sodium ions cross membrane Potassium channels open later, allowing K + to leave As such, interior becomes positive relative to exterior Reversal of polarity = depolarization

Propagation of Action Potential The voltage change in an adjacent region of plasma membrane triggers the opening of sodium (Na + ) channels in another area

Refractory Period A second stimulus less than second after the first will not trigger another impulse. The membrane is depolarized and the neuron is in its refractory period. Not until the −70 mv polarity is reestablished will the neuron be ready to fire again. In some human neurons, the refractory period lasts only 0.001–0.002 second. As such, the neuron can transmit 500–1000 impulses/sec.

Sodium/Potassium Pump Concentration of sodium ions outside of plasma membrane is greater than that of potassium ions. This is due to the Na + /K + pump. Three sodium are pumped out for every two potassium pumped in As such, interior appears negative relative to exterior at resting potential

Schwann Cells & Myelin The axons of many neurons are wrapped in glial tissue called Schwann cells Schwann cells produce a fatty tissue called myelin that wraps around regions of the axon The spaces between the wrapped layers of myelin are called Nodes of Ranvier

Saltatory Conduction The myelin sheaths insulate the axon, preventing excessive leakage of K + ions Hence, a depolarization at one Node of Ranvier is sufficient to propagate the depolarization at an adjacent Node. As less gated channels need to be opened and closed, the effective speed of the action potential is greater

Axon Hillock & Integration of Action Potentials Integration of Excitatory Impulses (EPSP) and Inhibitory Impulses (IPSP) occurs here Has no synaptic connections of its own Lowest threshold potential of neuron

Summation of Action Potentials One EPSP is insufficient to reach the threshold of the neuron. EPSPs created in quick succession, however, add together ("summation"). If summation reaches threshold, an action potential is generated. The EPSPs created by separate excitatory synapses (A + B) can also be added together to reach threshold. Activation of inhibitory synapses (C) makes the resting potential of the neuron more negative. The resulting IPSP may also prevent what would otherwise have been effective EPSPs from triggering an action potential.