Exam 2 - Action Potential and Neuro Intro Flashcards
Describe the propagation of an electrical signal by fast sodium channels?
A stimulus depolarizes a section of the cell that opens fast sodium channels. The singal travels both directions along the cell depolarizing the membrane and opening more sodium channels.
Descibe the how the propagation of a signal in repolarization occurs?
Fast Na+ channel close quickly and potassium channels open, repolarizing the cell in the same direction as the depolarization.
What type of feedback is used during opening of Na+ channels during depolarization?
Positive feedback
Na+ opening causes depolarization and more Na+ channels to open.
What structure(s) allow communication between the brain and muscles?
Motor neurons and release of neurotransmitters at the neuromuscular junction (NMJ).
Describe the nACh receptor/channel?
A protein at the NMJ that has 2 binding sites for acetylcholine (neurotransmitter). The inside of the protein is lined with negatively charged amino acids that repel negatively charged ions (positvely charged ion specific). The majority of the current is Na+ that starts the action potential. Small amount of K+ leaks out but is mitigated by Na+ influx. Ca+ also can come in and depolarize the cell, but is large and doesn’t come in as readily as Na+.
Nicotine can simulate Ach and causes tremors.
Describe how nAch receptors can lead to depolarization of the cell?
2 Ach have to bind to open the negatively charged pore and allow the positive charge in (usually Na+). This sets off the fast Na+ channels the are next to the Ach receptor.
Why are NMJ inportant for us to know about?
This is where our paralytics work.
Describe mAch receptors and their function during hyperpolarization?
Found in the heart and mediates pumping by hyperpolarization. They are GPCRs and are activated by acetylcholine, the alpha subunit activates K+ channels which causes K+ to leave the cell making it more electronegative. This makes Vrm more negative (hyperpolarized) and makes it take longer to reach an action potential.
Named because they respond to a substance called muscarine.
In the heart, this process maintains HR and with vagal stimulation slows HR down even more.
How are the AV and SA node innervated?
The left vagus nerve attaches to the AV and the right vagus nerve attaches to the SA.
How do antimuscarnics work? What effect do the have in the heart?
They antagonize mAch receptors, which prevents K+ channels from opening making Vrm more positive, leading to shorter time to fire an action potential and more BPM.
Ex: Atropine - blocks vagus activity in the HR
Describe how physical pressure can turn a neuron on?
There are baroreceptors that sense pressure in cells. Increased pressure causes widening of pressure sensitive Na+ channels that leads to influx of Na+. If cell become positive enough, it will lead to an action potential that goes back to the brain/brainstem for processing.
More pressure = more Na+ influx
Define threshold?
The charge that a stimulus must depolarize the cell to in order to fire an action potential.
Not all cell’s have same threshold
What is sub-threshold?
A stimulus that does not overcome the cell’s threshold and will not fire an action potential.
What would an action potential look like for a weak stimulus that barely meets threshold?
There can be a delay in firing the action potential; slower firing of the action potential.
What is supra-threshold?
A strong stimulus that depolarizes the cell well above threshold, leading to a faster action potential.
How do action potentials in the heart differ from other action potentials?
The action potentials take longer and are sustained (plateau) to allow effective pumping. This is due to slow Ca++ channels.
What is the role of Cl- in the nervous system?
Permeability is adjusted to hyperpolarize or supress excitable cells. This occurs via GABA receptors that open chloride channels in neurons; prevents overactivity like seizures.
“Brakes” of the nervous system
Describe calcium’s role in action potentials?
Calcium is large, has double positive charge, and huge concentration gradient. So, it sits near the cell wall and will block Na+ leak channels, blunting cell electrial activity.
Results in a “calming effect”, by stabilizing membrane potentials
What effects would hypocalcemia have on the cells membrane potential?
Hypocalcemia results in an increase in Na+ influx through Na+ leak channels (because less Ca ions are blocking the channel). This makes the cell less polar and makes the membrane potential more positive (more excitable).
What happens to membrane potential during hyperkalemia? How can this be corrected?
K+ will not leave the cell as much leading to a more positive membrane potential.
Giving Ca++ will block Na+ leak channels make the cell less positive, opposing the effects of hyperkalemia.
Wy doesn’t Ca++ also block K+ leak channels?
Because K+ is leaving the cell, so Ca++ will just be moved out of the way.
What effect does hypocalcemia have on motor neurons? What clinical sign does this cause?
Leads to a more positive resting potential and will cause hyperactivity of the motor neuron, leading to tetany.
Trusseau sign and Chvostek sign
What other compound can aid in reducing cell hyperactivity (increased membrane potential)?
Mg++ helps to hyperpolarize the cell; provides similar effects to Ca++, but mechanism is not clear.
What effects the rate of electrial propagation in nerve fibers?
- Length of the nerve (longer nerve takes more time for signal to reach destination)
- Diameter of the nerve (Wider = Faster; less resistance)
- Insulation (myelin sheath = faster)
Where can myelin be derived from?
The sphingomyelin in the cell wall.
What cell is involved in myelination in the periphery? How does it work?
Schwann cells. They wrap around the nerve in layers and as it does, all of the water is pushed out of the cell leaving a lipid compound that provides protection, speed, and efficency for neurons.
How is an action potential spread?
Through opening of fast Na+ channels. Na+ channels open causing depolarization that cause more Na+ channels to open.
What are the effects of myelination of nerve fibers?
- Speeds up rate of signal by blocking Na+/K+ pumps, preventing loss of Na+ and continuation of the signal.
- Reduces cell energy requirements because Na+/K+ pumps are not running as much. Less prone to ischemia.
- Protection from injuries or other forces.
What are the openings in between myleinated nerve fibers called?
Nodes of Ranvier
What is important about the nodes of ranvier?
There is a high density of fast Na+ channels that help ensure the continuation of the signal to the next myleinated nerve fiber. (saltatory conduction)
What is one difference between myelinated and unmyelinated neurons?
Unmyelinated neurons have much more fast Na+ channels along them than unmyelinated neurons.
Although they may not have more overall.
Why would a myelinated nerve fiber require MORE local anesthetic to block than and unmyelinated nerve?
Because of the high density of fast Na+ channels at the nodes of ranvier.
What is saltatory conduction?
The “jumping” of an action potential from one node of ranvier to the next, due to Na+ being prevented from leaving the cell under the myelinated sections.
What cells preserve and maintain myelin in the CNS?
Oligodendrocytes
These cells are not good at replacing lost myelin
What cells preserve and maintain myelin in the PNS?
Schwann cells
Are able to replace some lost myelin much better than oligodendrocytes
Describe what happens with demyelinating diseases?
When myelin is present the channels underneath the myelin will disappear beacuse they are not being used. When the myelin degrades, the Na+ and K+ channels are not replenished but, the Na/K/ATP pump is. So, during an action potential the Na channels are still far apart and the Na/K pump is in between pumping all of the Na back out, leading to inhibition of the signal which can result in paralysis.
What are some demyelinating diseases and their causes?
MS, Optic Neuritis, Guillain-Barre Syndrome
Causes: Genetics, infection, autoimmune hyperactivity
What are gap junctions?
Gap junctions are small pores/channels that allow movement of small ions (mainly Na+) from one cell to a neighboring cell (both forwards and backwards). This is used in rapid transduction of an electrical signal. The more gap junctions, the faster the signal is moved from one cell to the next.
The bidirectionality can be bad in the heart when a rogue electrical current can be allowed to move easily though gap juntions.
What is the structure of a gap junction?
Comprised of 6 connexin proteins that make up a single connexon. 2 connexons lined up together make up the gap junction.
What are the postsynaptic and presynaptic terminals?
Presynaptic is the sending cell. Postsynaptic is the recieving cell.
What are the myelination classifications?
A: Myelinated
B: Lightly myelinated
C: Non-myelinated
How does importance of a signal relate to nerve size?
The more important signals (motor functions) are sent through bigger, myelinated, neurons. Not so important signals are sent through smaller, unmyelinated neurons.
What are the categories of A fibers? How are they classifed?
Alpha, beta, gamma, and delta. Classifed by size from largest to smallest.
Describe a multipolar nueron?
They are decision making cells. Decide wheter to fire AP or not. (Motor neuron)
Describe a bipolar neuron?
Used in special organs for specialized senses. (Photoreceptors in retina and optic nerve).
Describe a pseudounipolar neuron?
Majority of sensory function. Primarily just passing of information. Doesn’t need to make decisions.
What is the function of transverse tubules?
Crosses the muscle fibers transversely allowing the action potential to travel deep into the tissue via fast Na+ channels and DHP receptors.
What are the names of the indicated structures?
Primary clefts and secondary clefts
Describe the location of ACh receptors and fast Na+ channels in the NMJ?
Acetylcholine receptors are situated near the top of the clefts and the VG Na+ channels are towards the bottom of cleft as well as spanning the length of the muscle fiber.
