Case
Imaging
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- Institution
This case consists of all the techniques relevant for this course as well as how neurons communicate.
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Task 2:Imaging
1. How do neurons communicate?
Neurons have the ability to generate electrical impulses and send messages to other cells. Their
function is thus as information units. Neurons are also plastic, they have the capacity to change,
allowing them to serve as memory units.
Overview of a neuron
A neuron has 1 to 20 dendrites which have small protrusions called
dendritic spines that increase the cell’s surface area. Each neuron has
a single axon that may branch into axon collaterals. These axon
collaterals may divide a number of smaller branches called
teleodendria before contacting the dendrites of another neuron. At
the end of each teleodendrion is a knob called an end foot, or terminal
button. This terminal button is very close to the dendritic spine but
doesn’t touch it. That space of almost touching is called the synapse.
The cell body fuels the cell and houses in its nucleus the chromosomes
that carry genetic instructions. Extending from the nucleus is the axon
hillock that forms at the beginning of the neuron’s axon. The axon
hillock sums up all the excitatory and inhibiting signals and decides
with whether it will fire depending if the voltage reach a threshold or
not.
Information flows through a neuron from the dendrites to the cell body to the axon hillock and
through the axon to its teleodendria and their terminal buttons. At each terminal button,
information is conveyed to the next neuron.
- Some synapses are inhibitory: they decrease the neuron’s ability to pass information along
to other neurons
- Some synapses are excitatory: they increase the neuron’s ability to pass information along to
other neurons
Information travels through a neuron on a flow of electrical current that begins on the dendrites and
travels along the axon to the terminals. In the axon, the summated flow consists of discrete electrical
impulses. As each impulse reaches the terminal buttons, they release a chemical message. This
neurotransmitter, carries the signal across the synapse to influence the target cell’s electrical activity
and pass the information along.
The neuron’s electrical activity
The resting potential
When electrodes are placed at an axon’s membrane, the difference in charge is about 70mV. This
charge is the membrane’s resting potential. Four kinds of charged particles interact to produce the
resting potential; sodium (Na+), chloride (Cl-), potassium (K+) and
large protein anions (A-).
Embedded in the cell membrane are protein molecules that serves
as channels and pumps to regulate the resting potential.
Protein anions remain in the cell because no membrane channels
are large enough for them. Their charge contributes to the negative
charge on the inside of the cell membrane. To balance the negative
, charge of the large protein anions in the intracellular fluid, cells accumulate K + ions. Potassium enter
the cell through open potassium channels in the membrane.
Not quite enough potassium ions are able to enter the cell to balance the negative charge of protein
anions. There is a limit on the number of potassium ions that accumulate in the cell because, when
the intracellular potassium concentration becomes higher, K + start moving out of the cell.
If Na+ ions were free to move across the membrane, they could diffuse into the cell and reduce the
transmembrane charge. The sodium channels in the membrane are mostly closed. The high
concentration of Na+ ions outside relative to inside is maintained by the action of a sodium-
potassium pump, a protein molecule embedded in the membrane that shunts Na + ions out of the
cell and K+ ions into it. This pump is there to stabilize and remain stable, that’s because most of the
times there is leakage where sodium goes out of that potassium goes is.
Chloride ions ordinarily contribute little to the resting potential. They move in and out the cell
through open chloride channels. At equilibrium, the chloride concentration gradient equals the
chloride voltage gradient at approximately the membrane’s resting potential.
As summarized, the unequal distribution of anions (negative charged ions) and cations (positively
charged ions) leave a neuron’s intracellular fluid negatively charged. Three aspects of
semipermeable membranes contribute to this resting potential:
1) Large, negatively charged protein molecules remain
inside the cell.
2) Gates keep out Na+ ions and channels allow K+ and Cl-
ions to pass more freely.
3) Sodium-potassium pumps extrude Na+ from the
intracellular fluid.
Graded potential
Slight decreases or increases in an axon’s membrane voltage are
graded potentials, highly localized and restricted to the vicinity on
the axon where they are produced. A decrease in the voltage on a
membrane is termed depolarization and an increase is termed
hyperpolarization.
For a graded potential to rise, an axon must receive some
stimulation that changes the ion flow. Stimulating the axon
electrically through a microelectrode is one way to increase or
decrease the membrane voltage and produce a graded potential.
Hyperpolarization: it is due to an efflux of K+, making the
extracellular side of the membrane more positive. An
influx of Cl- can also produce hyperpolarization.
Depolarization: it is due to an influx of Na+ through Na+
channels.
Action potential
Electrical stimulation of the cell membrane at resting potential produces localized graded potentials.
An action potential however, is a brief but extremely large reversal in the polarity of the axon’s
membrane, lasting about 1 ms. In the action potential, the signal changes from an electrical signal to
a chemical signal.
An action potential is triggered when the cell membrane is depolarized to
about -50 mV. At this threshold (= drempel) potential, the membrane charge
undergoes a remarkable change with no further stimulation. A class of gated
sodium and potassium channels called voltage-sensitive channels are
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