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Summary Block 4 The Human Body Problem 3
- Instelling
- Erasmus Universiteit Rotterdam (EUR)
Summary for Problem 3 in Block 4 of 1st year psychology
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Voorbeeld 2 van de 6 pagina's
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Voorbeeld van de inhoud
Problem 3Kalat 11E, Pinel 9E
1) Action Potential information flow within neurons via electrical impulses
Membrane: All parts of a neuron are covered by membrane. It’s composed of two layers of
phospholipid molecules (contains fatty acids and a phosphate group). Among these
phospholipids are cylindrical protein channels that allows chemicals to pass. The structure of
the membrane allows flexibility and firmness and also controls what goes in and out of the
membrane. The membrane is selectively permeable.
Membrane potential: difference between electrical charge between the inside and the
outside of a cell membrane. To record the membrane potential, you position one tip of an
electrode inside and one tip of an electrode outside the membrane. These intercellular
electrodes are called microelectrodes. A steady potential is about -70 mV which means that
the inside of the membrane is 70 less than the outside of the neuron. This voltage is resting
potential. In a resting neuron, more sodium ions are outside of the cell and more potassium
ions are inside the cell. Oscilloscopes are also used to measures voltages.
Resting Potential: When there’s no disturbance the membrane has an electrical gradient
known as polarization. There is a difference in electrical charge between the inside and the
outside of the membrane. The inside of the membrane is negative and the outside is
positive. This difference in voltage is called the resting potential. The resting potential
prepares the neurons to respond rapidly.
Factors that influence ion discharge during resting potential:
1. Sodium Potassium Pump: The membrane is selectively permeable, so oxygen, water
urea etc. can pass freely but large molecules and charged ions can’t. Some
biologically important ions are potassium, sodium, chlorine and calcium, and they use
channels to pass the membrane. When the membrane is at rest, these ions are
prevented from moving across the membrane because the channels are closed.
The sodium-potassium pump is a protein complex that transports 3 sodium ions out
of the cell and 2 potassium ions into the cell. This pump is an active transport unit so
it needs ATP (energy) to work. Because of this pump the sodium ions are more
concentrated outside of the membrane and potassium ions are more concentrated
inside the membrane. So the inside of the membrane is more negative (-70) (Chloride
ions are also on the outside of the membrane)
2. Electrical gradient (electrostatic pressure): Sodium is positively charged and inside of
the cell is negatively charged (-70). Since opposite electrical charges attract, this
electrical gradient pulls the sodium ions inside the cell. For potassium, it’s the same;
potassium is also positively charged so the electrical charges attract it to get inside
the membrane.
, 3. Concentration gradient – random motion: Sodium is more concentrated on the
outside so it wants to get inside. Sodium is more likely to enter the cell then leave it.
For potassium, since potassium is more concentrated on the inside, it wants to go
outside.
- The sodium channels are closed during resting potential to prevent the electrical and
concentration gradient from getting sodium inside. The sodium is only pushed out of
the cell with the sodium-potassium pump.
- For potassium, the electrical and concentration gradient act as competing forces. The
potassium channels are open but only a few of them leak outside because they are
held inside by the negative electrical gradient. But at the same time the sodium-
potassium pump pulls more potassium inside the cell really fast so that they’re not
balanced and the inside has more potassium ions.
Action potential: Electrical impulse/messages sent by axons. Movement of ions across the
cell membrane of the axon of a neuron to cause a potential difference. Massive but
momentary reversal of the membrane potential from -70 to +50 mV
When the membranes potential rises and a certain potential level is reached (threshold
potential, generally around -55 mV) the sodium channels open and driven by the electrical
and concentration gradients the sodium enters the membrane. This is called depolarization
(rising phase). This rapid change also triggers the opening of voltage gated channels: both
for sodium and potassium channels.
At the peak of the sodium flow, the sodium channels close and potassium channels open.
Potassium ions flow out of the cell and the neuron becomes polarized again. This is called
repolarization (falling phase). Once repolarization is reached the potassium channels start
to close. Because they gradually close, too many potassium ions flow out of the neuron and
this causes hyperpolarization.
The All or None Law: The amplitude and the velocity of the action potential are independent
from the intensity of the stimulus that initiated it, provided that the stimulus reaches a
threshold. (Like flushing a toilet: You have to put a certain amount of pressure to flush
(threshold) but doing it any harder won’t make the toilet flush faster.) They occur to their
full extent or don’t occur at all.
To signal the difference between a weak or strong stimulus, the axon can’t send bigger or
faster action potentials. All it can do it change timing and the rate of neural firing.
The amplitude, velocity and shape of an action potential also varies from one neuron to
another.
Rate Law: The principle that the variations in the intensity of a stimulus being transmitted in
an axon are represented by variations in the rate at which that axon fires.
Refractory Period: Immediately after an action potential, the neuron is in a refractory
period during which it resists the production of further action potentials.
1- Absolute refractory period: the membrane can’t produce any more action potentials
regardless of stimulation (you can’t flush the toilet again right after you’ve flushed it)
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