Prelim 4 Biog1440 Flashcards
Types of Neurons
- Model Neuron
- Sensory Neuron (sensory nerve responds to chemical or physical stimuli)
- Motor neuron (originates from the central nervous system)
- Local interneurons (communicate between 2 neurons)
- Project interneuron
- Neuroendocrine cell (responsible for producing hormones and released into circulation)
Excitation
The cells are able to respond to a certain number or stimuli
Membrane Potential
The difference in charges between inside and outside a cell
Resting membrane potential
At resting state of a neuron, this membrane potential is negative (-65 millivolts)
When is the cell polarized?
The cell is polarized when anything that changes the resting membrane potential resulting from ion flow brings the resting membrane potential towards 0 or the positive side. Then the cell is capable of responding to a stimulus.
–> basis of excitation in the neuron
Sodium Potassium ATPase
For every 3 molecules of Na+ that moves outside, 2 molecules of K+ is brought into the cell through active transport
Intercellular and Extracellular values for K+, Na+, Cl-, and A-
K+ Intracellular: 140 Extracellular: 5 Na+ Intracellular: 15 Extracellular: 150 Cl- Intracellular: 10 Extracellular: 120 Large anions (A-) Intracellular: 100 Extracellular: Not applicable
Why is the overall charge negative?
The large anions give it the negative charge and thus, the negative membrane potential
Potassium channels (leak channels)
They are known as leak channels because they have a random switching rate between open and closed states to allow potassium to flow out of the cell
Sodium channels
Most of which are not really open, allow sodium to flow into the cell
Chemical force
The differentials in concentration between the extracellular and intracellular environment for the Na+ & K+ creates a concentration gradient. Diffusion would favor the neutralization of the number of molecules on both sides of the membrane, however, this asymmetry exists and this chemical force exist in parallel with electrical forces.
Electrical force
Attraction between opposite charges in an effort to remain neutral
ex. Works to keep K+ inside the cell. The net flow of K+ out of the neuron (which is natural because it wants to reach a state of equilibrium) should be kept in to stay neutralized. They do not want K+ to escape to the outside of the cell despite the K+ channels
ex. Na+ and Cl- are oppositely charged so they’re at equilibrium
The result of the electric force
No net flow across the membrane (able to maintain resting membrane potential at around -60). Also called the equilibrium potential
Nernst equation
E ion = 62(log [ion]out/[ion]in)
units in mV
For each ion, given that the concentration differs from outside vs. inside the cell…
There will be an electrical force that exactly balances the chemical force
Permeability of K+, Na+, and Cl-
K+>Na+>Cl-
Graded hyperpolarization
If the resting potential was to decrease or become more negative, the cells can become more resistant to stimuli. These are called the graded hyperpolarization and can occur if certain stimulus produces and increased membrane permeability to K+.
Graded depolarization
If the resting potential was to increase, then the cell can be more easily excited than these transient temporal shifts. These are called the graded depolarization that are produced by stimuli that increase membrane permeability to sodium (called graded potentials)
If the graded depolarization that brings an increase in sodium into the cell reaches a particular threshold…
The voltage-gated sodium channels will open and will result in an action potential. There is a huge influx of sodium.
What is the threshold in mV?
Voltage gated channels open when the resting membrane potential reaches the threshold, -55 mV
Steps of an Action Potential
Resting state: The gated Na+ and K+ channels are closed, Ungated channels maintain the resting potential (ligand gated channels and the Na+ K+ ATPase)
- A stimulus is necessary to trigger an action potential and the stimulus brings a change to the resting membrane potential
Depolarization: A stimulus opens some Na+ channels. Na+ inflow through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential
Rising phase of the action potential: Depolarization opens most sodium channels while potassium channels remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside
-Peak of the curve is when the Na+ channels begin to close and K+ channels open. There is a small delay even though they are activated at the same threshold.
Falling phase of the action potential: Most Na+ channels become inactivated, blocking Na+ inflow. Most K+ channels open, permitting K+ outflow, which makes the inside of the cell negative again.
Undershoot: The sodium channels close, but some potassium channels are still open. As these potassium channels close and the sodium channels become unlocked (though still closed), the membrane returns to its resting state (with the help of ligand-gated channels and the sodium potassium ATPase)
Speed of voltage gated channels
Voltage gated Na+ channels are quick to open and quick to close.
Voltage gated K+ channels are slow to open and slow to close
Importance of the undershoot and propagation of action potential
Due to a depolarization event occuring in one location, the location right adjacent to that is also activated. When the adjacent region is activated, the region that was previously excited is undergoing the undershoot so it is not able to activate again. This provides the directionality for the flow across an axon up the nerves
* Not possible for an action potential to go backwards
Structure of typical neurons and communication networks
The signal is received in the neuronal cell body, integrated in the axon hillock, communicated or transmitted through the axon, and then ends in the synaptic terminals, which might stimulate elicit specific function