Much of what we know about nerve impulses came from experiments by Hodgkin and Huxley in the 1940s and 1950s on the giant axon of the squid. Its large diameter (up to about 1 mm) made it possible to insert a fine electrode inside it.The potential difference across the membrane is measured using two microelectrodes. One, a fine glass tube filled with a conducting salt solution, is inserted inside the axon. The other is a reference electrode in the fluid outside the axon. The electrodes are connected through an amplifier to an oscilloscope (or a computer data logger), which displays the potential difference in millivolts against time in milliseconds.A stimulating electrode delivers small electrical pulses to the axon. By varying the strength of the stimulus, scientists showed that:• the unstimulated axon has a resting potential of about −70 mV (inside negative)
• weak stimuli below threshold produce no action potential
• stimuli above threshold produce action potentials of the same height, peaking at about +40 mVPlacing two recording electrodes a known distance apart allows the conduction velocity to be calculated from the time difference between the two traces.
Hodgkin and Huxley also changed the ion concentrations in the fluid around the axon. Lowering the Na+ concentration outside made the action potential smaller, showing that the rising phase is caused by Na+ entering the axon.Reading a trace: if each horizontal square represents 1 ms and an action potential spans 3 squares, the action potential lasts 3 ms.
• weak stimuli below threshold produce no action potential
• stimuli above threshold produce action potentials of the same height, peaking at about +40 mVPlacing two recording electrodes a known distance apart allows the conduction velocity to be calculated from the time difference between the two traces.
Hodgkin and Huxley also changed the ion concentrations in the fluid around the axon. Lowering the Na+ concentration outside made the action potential smaller, showing that the rising phase is caused by Na+ entering the axon.Reading a trace: if each horizontal square represents 1 ms and an action potential spans 3 squares, the action potential lasts 3 ms.