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 7000+

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

0.001 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.