University of Toronto
BIO 304
21/09/2020
Lecture 3: Bioelectricity
1. The Study of Bioelectricity: from Galvani to optical recording
Mapping a neural connectome of an animal (golgi technique, new techniques) lets you
understand all of the pathways for information flowing in nervous system
- it’s a framework for understanding hoe the nervous system works
- it reveals a
...[Show More]
21/09/2020
Lecture 3: Bioelectricity
1. The Study of Bioelectricity: from Galvani to optical recording
Mapping a neural connectome of an animal (golgi technique, new techniques) lets you
understand all of the pathways for information flowing in nervous system
- it’s a framework for understanding hoe the nervous system works
- it reveals all pf the potential the avenues for information to flow through neural
circuits.
- 30 years ago, electron microscopy reconstruction was used to sliced through a
nematode worm and imaged each one of those slices to map the entire
connectome (302 neurons) where the neurons were at the synaptic level.
o We’ve known exactly how the nervous system works, but you’d think after
years of research we should understand the nervous system and behaviourits not that simple, its complex! THeres more we need to do to understand
how it operates.
- WE must understand both the connectivity and mechanism for the information flow
in the neural circuit (the electrical properties)
o Chemical (neurotransmitter at synapse, neuromodulators)
Ex chemical synapses- the retina and how neurons communicate with
each other. WE have a pre-synaptic terminal with various vesicles
packed with various types of excitatory neurotransmitter (ex glutamate
tends to open up ion channels and excite post synaptic cell) and
inhibitory ones (GABA open up chloride channel). That is the chemical
form of cellular communication across the synapse.
o There are also electrical components graded potential, Action potential,
electrical synapse)
Graded potential
ion channels open when neurotransmitters bind to them.
AN excitatory nerurotrasnmitter will conduct a positive cation
current into the cell that causes depolarizes. The voltage will go
up for a bit, but it will dissipate over time as the charges spread
and dilute themselves.
Graded- can have different degrees of stimulation depending on
the amount of neurotransmitter (graded amplitude)
o we have IPSPS (neurotransmitter (glycine or Gaba) open up
Cl ion channel which makes the cell more negative- more
transmitter and more Cl channels open will make IPSP
larger)
o EPSPs
Action Potentials
Voltage gated sodium and potassium channels are activated if
the graded potentials are large enough
at a threshold voltage, sodium channels open and action
potentials can occur
action potentials travel far and are very fast. Constantly
replenishing charges- used to transmit a signal.
The third form of electrical communication are electrical synapses (gap
junctions).
Connection between the cytoplasm of 2 cells by proteins called
connexons
Tunnels with channels connect to channels on the other cell so
the cytoplasm is joined. The charges move directly through the
gap junction from one cell to another without the chemical
synapse. The cells are considered electrically coupled.
This encroaches upon the reticular theory (they weren’t entirely
wrong) as because of so many gap junctions, they are effectively
work as a reticulum.
We have known about the way animals use electricity for cellular communication for the
function of the nervous system for a long time.
- Luigi Galvani is credited with the discovery of bioelectricity by exposing frog legs to
applied electrical currents. The legs twitched and moved and muscles contract and
folded when applied with electrical current.
- The process of using electricity to stimulate dead animals muscle and seeing if it
contracts was Called Galvanization by alessandro Volta.
- His nephew Giovanni Aldini used larger animals and humans,
- In 1803, did one experiment on a recently executing prisoner and performed the
galvanization technique, and then the muscles would contract in front of a large
uadience. It actually helped us figure out that we are electrical beings. The use of
equipment to study bioelectricity as pioneered by Galvani and others, is now
referred to as electrophysiology.
When is it better to have electrical synapse compared to chemical synapse
- In electrical synapse you can only pass on current in one modality in other words
o If a neuron fires an action potential those positive charges will move through
and regulate the other cells in a similar way
o In chemical we can have excitatory neurotransmitter (glutamate) or
inhibitory (gaba) which can in result illicit different responses
o Electric is quick to dissipate vs electric signal can linger and last longer
o In electrical we can either have positive or negative while in electrical
different neurons can take up different transmitter types and connections to
form neural circuits that are independent from other \
Electrical allows for lotx of variety in nexagons
o Muscle celss are actualy connected through gapjunctions *smooth musck
The way contraction can move is through gap junctions
Cruitial advancement was actually going from stimulating tissues with electricity to
actual recording the electrical actity emanating from those preparations
- Optical recording. In 1902 Julius Bernstien developed this device called the
differential rheotome
o Allowed him ti measure/sample/record membrane voltage in tissue in a very
snall time scale (microsecond scales)
o having that temporal resolution lets you record action potentials
o This allowed him to report the first action potential- he called it a negative
fluctuation (inverted going upwards). Later we look at some fundamental
advances that have allowed us to unlock the secrets of electrical signallingFundamental advances in electrophysiological recording techniques helped unlock the
secret of electrical signalling in neurons
- Hodgkin, Huxley and Cole are essential foe the invention of and application of this
electrophysiology technique called the voltage clamp technique.
o They applied this technique on the squid as they have giant axons.
- Erwin neher, and bert Sakmann created another electrophysiological technique
called the path-clamp technique. Applied to the squid.
23/09/2020
3 basic paradigms for electrophysiological recording:
1. Extracellular recording- we are looking at a single electrode or an array of them
made of glass and insulated and the tip is open. The electrode is sitting right
outside of the neuron.
a. The electrode is connected through a wire to voltage amplifier that reads
voltages between 2 inputs, a ground and the electroede.
b. We measure the difference between the tip and the ground electrode. If this
neuron fires an action potential, sodium rushes inside the neuron during
rising phase in the action potential, and we see a temporary depletion in
sodium at the place where the electrode is sitting compared to the ground
which is away from neuron at voltage amplifier.
c. The amplifier is gonna see that when this neuron fires an action potential, it
gets more negative. When K+ leaves (during repolarization), it gets more
positive outside the cell. It doesn’t go inside the cell.
d. You can’t really figure out what is causing everything unless you use multiple
electrodes and see the differences in signals. You can sample and identify a
range of neurons and monitor physiological analysis.
e. This way u can measure electrical activity without poking the neuron and
damaging it
f. cool thing is that You can take an array of this electrodes in different regions
and then record activity of multiple neurons at the same time.
g. This way u can compare the region outside the cell where ion concentration
keeps changing and compare them to the reference point
h. Can’t fully distinguish who is saying what
i. But u can use multiple electrodes near different neurons such that u
can tell the difference as the signal coming from one neuron would be
stronger at the electrode near tha neuron
ii. U can use the difference in the signals to later analyze and identify the
neural activity coming from different neurons so this method allows us
to sample and identify an array of neuron in a tissue and see how they
are behaving electrophysiologically
2. Intracellular- sharp electrode poked into the cell. The electrode measures the
activity inside the cell and is Separated from the ground electrode by the cell
membrane. The ground electrode is outside the cell membrane. We are comparing
the inside of the cell to the outside of the cell
a. We see recording of voltage/ion fluxes across the cell membrane
b. It shows us voltage changes outside the cell membrane.
c. Intracellular signalling is what gave us our action potential graph. As charges
move out, negative decline of AP and vice verce
d. . We don’t have to discern what is causing what. We can also visualize
graded and action potentials, IPSP and EPSP
e. We are now in a silent environment and don’t have to worry abput the noise
as we are now examining the inside environment of the neuron
f. We can visualize graded potential too
g. Good for understanding how electrophysiology of neuron works in great
detail
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