NEUR 2600 Lecture 4: Ch4Neur2600
CHAPTER 4: NOW DO NEURONS USE ELECTRICAL SIGNALS TO TRANSMIT
INFO
●Early clues that linked electricity and neuronal activity
○Electrical stimulation studies
■Galvani (18th century)
●Electrical current applied to a dissected nerve induced a twitch in
the muscle connected to the nerve; Galvani concluded that
electricity flows along the nerve
●Electrical stimulation
○Passing an electrical current from the tip of an electrode
through brain tissue, resulting in changes in the electrical
activity of the tissue
■Fritsch and Hitzig (mid-nineteenth century)
●Electrical stimulation of the neocortex causes movement (arms and
legs)
■Bartholow (1874)
●First report of human brain stimulation: “Passed an insulated
needle into the left posterior lobe so that the non-insulated portion
rested entirely in the substance of the brain. The reference was
placed in contact with the dura mater. When the circuit was closed,
muscular contraction in the right upper and lower extremities
ensued.”
○Electricity and electrical stimulation
■Electricity
●A flow of electrons from a body that contains a higher charge
(more electrons) to a body that contains a lower charge (fewer
electrons)
■Negative pole
●The source of electrons; higher charge
■Positive pole
●Location to which electrons flow; lower charge
○Electrical recording studies
■Caton (early nineteenth century)
●First attempt to measure electrical currents of the brain using a
voltmeter and electrodes on the skull
■Electroencephalogram
●Graph that records electrical activity through the skull or from the
brain and represents graded potentials of many neurons
■Von Helmholtz
●Flow of information in the nervous system is too slow to be a flow
of electricity
○Nerve conduction: 30-40 meters/second
○Electricity-light: 299 792 258m/s
●It is not the ions themselves that travel along the axon, but rather a
wave of change (Bernstein, 1886)
○Tools for measuring a neuron’s electrical activity
■Electrical potential
●An electrical charge measured in volts; the ability to do work
through the use of stored potential electrical energy
●Difference between the two poles
■Volt
●A measure of a difference in electrical potential
■Voltmeter
●A device that measures the difference in electrical potential
between two bodies
■Giant axon of the squid
●Much larger in diameter than human axons
○Humans: 1-20 micrometers
○Squid: upto 1 millimeter (1000 micrometers)
●Easier subject of experiments
○Used by Hodgkin and Huxley in the 1930s and 1940s
●A giant axon could be dissected out of the squid and kept
functional in a bath of salty liquid approximating body fluids
●Hodgkin and Huxley described the neuron’s electrical activity and
won the Nobel prize
■The oscilloscope
●Device that serves as a sensitive voltmeter
●Used to record voltage changes on an axon
■Microelectrodes
●A set of electrodes small enough to place on or in an axon
●Can be used to
○Measure a neuron’s electrical activity
○Deliver an electrical current to a single neuron (stimulation)
●Use of microelectrodes
○Measure voltage across the membrane
■Tip of one microelectrode placed on the surface of
an axon
■A second microelectrode, used as the reference,
inserted into the axon
○Patch clamp
■Place microelectrode tip on the neuron’s membrane
and apply a little back suction until the tip seals to a
patch of the membrane
■Recording will be made from only the small patch
of membrane sealed in the perimeter of the
microelectrode tip
○Neurons can convey information as a wave, induced by stimulation on the cell
body, that travels down the axon to its terminal
■A voltmeter detects the passage of the wave
○How ion movement produces electrical charges
■Cations
●Positively charged ions
○Examples: sodium (Na+), potassium (K+)
■Anions
●Negatively charged ions
○Examples: chloride (Cl-), protein molecules (A-)
■Diffusion
●Movement of ions from an area of higher concentration to an area
of lower concentration through random motion
■Concentration gradient
●Differences in concentration of a substance among regions of a
container allow the substance to diffuse from an area of higher
concentration to an area of lower concentration
■Voltage gradient
●Difference in charge between two regions that allows a flow of
current if the two regions are connected
○Opposite charges attract
○Similar charges repel
●Ions will move down a voltage gradient from an area of higher
charge to an area of lower charge
Document Summary
Chapter 4: now do neurons use electrical signals to transmit. Early clues that linked electricity and neuronal activity. Electrical current applied to a dissected nerve induced a twitch in the muscle connected to the nerve; galvani concluded that electricity flows along the nerve. Passing an electrical current from the tip of an electrode through brain tissue, resulting in changes in the electrical activity of the tissue. Electrical stimulation of the neocortex causes movement (arms and legs) First report of human brain stimulation: passed an insulated needle into the left posterior lobe so that the non-insulated portion rested entirely in the substance of the brain. The reference was placed in contact with the dura mater. When the circuit was closed, muscular contraction in the right upper and lower extremities ensued. A flow of electrons from a body that contains a higher charge (more electrons) to a body that contains a lower charge (fewer electrons)
