PSYB65H3 Chapter Notes - Chapter 4: Luigi Galvani, Hermann Von Helmholtz, Richard Caton

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Published on 8 Nov 2015
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Brain & Behaviour
Ch. 4 How do Neurons use Electrical Signals to Transmit Information?
Epilepsy is the most common neurological disease worldwide: 1 person in 20 experiences an
epileptic seizure in his or her lifetime.
Synchronous stimuli can trigger a seizure; thus, a strobe light is often used indiagnosis.
Some epileptic seizures can be linked to a specific symptom, such as infection, trauma,
tumor, or other damage to a part of the brain.
Others appear to arise spontaneously. Three symptoms are common to many kinds of epilepsy:
1. An aura, or warning, of an impending seizure, which may take the form of a sensation, such as
an odor or sound, or may simply be a “feeling”
2. Abnormal movements such as repeated shaking; twitches that start in a limb and spread across
the body; and in some cases, a total loss of muscle tone and postural support causes the person to
collapse
3. Loss of consciousness and later unawareness that the seizure happened.
If seizures occur repeatedly and cannot be controlled by drug treatment, surgery may be
performed.
The goal of surgery is to remove damaged or scarred tissue that serves as the focal point
of a seizure.
Removing this small area prevents seizures from starting and spreading to other brain
regions.
The condition of epilepsy reveals that the brain is normally electrically active and that if
this activity becomes abnormal, the consequences are severe.
Descartes believed that CSF carried information through nerve tubes. Descartess theory was
inaccurate, but he isolated the three basic questions that underlie a behavioral response to
stimulation:
1. How do our nerves detect a sensory stimulus and inform the brain about it?
2. How does the brain decide what response should be made?
3. How does the brain command muscles to move to produce a behavioral response?
4.1 Searching for Electrical Activity in the Nervous System
The first hints about how the nervous system conveys its messages came in the eighteenth
century, following the discovery of electricity.
Early discoveries about the nature of electricity quickly led to proposals that it plays a role in
conducting information in the nervous system.
Early Clues that Linked Electricity and Neuronal Activity
In 1731, Stephen Gray conducted an experiment:
He rubbed a rod with a piece of cloth to accumulate electrons on the rod.
Then he touched the charged rod to the feet of a boy suspended on a rope and brought a
metal foil to the boys nose.
The foil was attracted to the boy’s nose and bent on approaching it, and as foil and nose
touched, electricity passed from the rod through the boy to the foil.
Therefore, Gray speculated that electricity might be the messenger that spreads
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information through the nervous system.
Electrical Stimulation Studies
When scientist Luigi Galvani, observed that frogs’ legs hanging on a wire in a market twitched
during a lightning storm, he assumed that sparks of electricity from the storm were activating the
muscles.
Luigi found that, if an electrical current is applied to a dissected nerve the muscle connected to
that nerve contracts.
Galvani had discovered the technique of electrical stimulation: passing an electrical current from
the uninsulated tip of an electrode onto a nerve produces behavior—a muscular contraction.
Electrical stimulation Passage of an electrical current from the uninsulated tip of an
electrode through tissue, resulting in changes in the electrical activity of the tissue.
Galvanis technique was to produce muscle contraction.
Gustave Theodor Fritsch and Eduard Hitzig, demonstrated that electrical stimulation of the
neocortex causes movement.
They studied several animal species, including rabbits and dogs, and may even have
stimulated the neocortex of a person whom they were treating for head injuries sustained
on a Prussian battlefield.
They observed movements of the arms and legs of their subjects in response to the
stimulation of specific parts of the neocortex.
Electrical Recording Studies
Another, less invasive line of evidence that the flow of information in the brain is partly electrical
in nature came from the results of electrical recording experiments.
Richard Caton, was the first to measure the electrical currents of the brain with a sensitive
voltmeter, a device that measures the flow and the strength of electrical voltage by recording the
difference in electrical potential between two bodies.
Caton reported that, when he placed electrodes on the skull of a human subject, he could
detect fluctuations in his voltmeter recordings.
Today, this type of brain recording, the electroencephalogram (EEG), is a standard tool
used to monitor sleep stages and record waking activity as well as to diagnose
disruptions, such as those that occur in epilepsy.
Voltmeter Device that measures the flow and the strength of electrical voltage by recording the
difference in electrical potential between two bodies.
Electroencephalogram (EEG) Graph that records electrical activity through the skull or from
the brain and represents graded potentials of many neurons.
The results of all these studies provide evidence that neurons send electrical messages, but
concluding that nerves & tracts carry conventional electrical currents proved difficult.
Hermann von Helmholtz, stimulated a nerve leading to a muscle and measured the time the
muscle took to contract.
The nerve conducted information at the rate of only 30 to 40 meters per second, whereas
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electricity flows along a wire at the much faster speed of light (3X108 meters per second).
The flow of information in the nervous system is too slow to be a flow of electricity
To explain the electrical signals of a neuron, Julius Bernstein suggested in 1886, that the
chemistry of neurons produces an electrical charge.
He also proposed that the charge can change and act as a signal.
Bernsteins idea was that successive waves of electrical change constitute the message
conveyed by the neuron.
It is not the electrical charge but the wave that travels along the axon.
To understand the difference, consider other kinds of waves.
If you drop a stone into a pool of still water, the contact produces a wave that travels
away from the site of impact.
The water itself does not travel.
Only the change in pressure moves, changing the height of the surface of the water and
creating the wave effect.
Wave Effect: Waves created by dropping a stone into still water do not entail the forward
movement of the water but rather differences in pressure that change the height of the surface of
the water.
Bernsteins idea was that waves of chemical change travel along an axon to deliver a neuron’s
message.
Tools for Measuring a Neurons Electrical Activity
The waves that carry nervous-system messages are very small and are restricted to the surfaces of
neurons.
We can measure these waves using electrical stimulation and electrical recording techniques and
determine how they are produced.
If a single axon is stimulated, it produces a wave of excitation, and if an electrode
connected to a voltmeter is placed on a single axon, the electrode can detect a change in
electrical charge on that axon’s membrane as the wave passes.
Neurons can convey information as a wave induced by stimulation on the cell body
traveling down the axon to its terminal.
A voltmeter detects the passage of the wave.
Recording a wave and explaining how it is produced requires a neuron large enough to record, a
recording device sensitive enough to detect a small electrical impulse, and an electrode small
enough to place on the surface of a single neuron.
The discovery of the giant axon of the squid, the invention of the oscilloscope, and the
development of microelectrodes met all these requirements.
Giant Axon of the Squid
The neurons of most animals, including humans, are tiny, on the order of 1 to 20 micrometers in
diameter, too small to be seen by the eye and too small on which to perform experiments easily.
The British zoologist J. Z. Young, when dissecting the North Atlantic squid, Loligo vulgaris,
noticed that it has giant axons, as much as a millimeter (1000 micrometers) in diameter.
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