Electrical Signals in Neurons Flashcards Preview

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Flashcards in Electrical Signals in Neurons Deck (15)
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1
Q

Describe the resting distributions of physiologically important ions (Na+, K+, Cl- and Ca2+) in intracellular and extracellular environments, identify where concentration gradients exist between these compartments and define the direction for these gradients.

A
2
Q

Explain the relationship between concentration gradients and electrical gradients for Na+ and K+ in defining equilibrium (Nernst) potentials, also known as reversal potentials (Eion).

A

For K+, its concentration gradient tends to move it out of the cell, leading to a more positive extracellular conditon. Since large anions are impermeable to the membrane, the intracellular conditon becomes more negative. The resulting electrical gradient opposes the outward movement of K+; ending with no further net movement of K+ at equilibrium potential of -90 mV.

For Na+, the case is similar other than the fact the its concentration is much larger outside the cell than inside the cell. Its Nerst Potential is +60 mV.

3
Q

Explain the relative importance of internal and external ion concentrations (Na+, K+, Cl- and Ca2+) and their membrane permeabilities in setting resting membrane potential (Vm)

A

RMP is around -70 mV.

90% of RMP is due to passive diffusion of K+ and Na+ through leak channels. However, since the RMP is much closer to the reverse potential of K+ than to Na+, K+ leak channels has a higher conductance compared to Na+ when cells are at rest.

10% of RMP is due to Na+/K+ pump, which employs active transport to maintain Na+ and K+ gradients. It is electrogenic as it moves 3 Na+ out for every 2K+ inside.

Other non-penetrating ions, such as protein and ATP, also contributed to the membrane potential.

4
Q

Describe the features of the axon hillock.

A

Axon hillock is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon. It is also considered as the usual site of initiation of action potentials.

5
Q

Describe the mechanism of current flow during graded potential changes.

A

A stimulus (<15mV) triggers the opening of a number of voltage-gated Na+ channels, leading to an influx of Na+ ions. The original active area then become deoplarized due to the Na+ influx.

These Na+ ions then spread to adjacent areas, depolarizing those areas as well. This is characterized by a a local current flow between teh active and adjacent areas. The membrane potential charge diminsihes away from the site of initiation.

6
Q

Describe the mechanism of changes in MP during an AP in regards to voltage-gated Na+ and K+ channels.

A

If the stimulus exceeds ~15-20 mV, it will trigger an invariant AP response.

VG-Na+ channels has multiple functional states: closed, open (activated), closed and not capable of opening (inactivated). Activated, open channels are only possible above threshold and open their activation gates, increasing Na+ permeability.

This depolarization event is self-reinforcing (positive feedback) as it will cause more VG-Na+ channels to be activated and hence larger influx of Na+.

The slower opening VG-K+ channels starts to open, increasing K+ permeability but still below Na+ permeability. As the Na+ channels start to spontaneously close and inactivate, the K+ peremeability takes over leading to repolarization.

Hyperpolarization occurs due to extra potassium conductance as the VG-K+ takes longer to close. Note: The states are triggered by the same initiating event as the one acting on the Na+ channel

7
Q

Explain the conduction of an AP.

A

Voltage change of the MP caused by the AP will activate adjacent voltage-sensitive Na+ channels. The initial trigger of AP will occur in the axon hillock (initial axonal segment). Local current flow due to Na+ influx from intial sites will lead to depolarisation in adjacent areas to the point it retrigger the AP in that area.

The AP then propagates along the axon, with the hyperpolarisation in past areas preventing any repolarisation and backtracking of AP.

8
Q

Mention the factors that influence speed of AP conduction. Explain how myelin sheath helps increase conduction speed.

A

Two factors:

  • Diameter of Axon
  • Resistance of Axonal Membrane to Ion Leakage

The myelination process refers to the development of electrically insulating layer known as a myelin sheath over neurons. This increases the diameter of the neuron as well as providing insulation that prevents current loss. The presence of the sheath increases rate of conduction.

9
Q

Describe the process of saltatory conduction.

A

The attenuation effect of the AP signal in neurons prevent the neurons to be completely encased in myelin. Nodes of Ranvier are present between myelin sheaths to provide the refiring of AP potential along the axon through the rapid influx of Na+.

This method of AP conduction is called saltatory conduction. Where the propagation of action potentials occurs along myelinated axons from one node of Ranvier to the next node.

10
Q

Differ between the Divergent and Convergent Pathway.

A
  • Divergent pathway: one presynaptic neuron branches to affect a larger number of postsynaptic neurons
  • Convergent pathway: many presynaptic neurons converge to influence a smaller number of postsynaptic neurons
11
Q

Explain the general mechanism of synaptic communication

A

Action potential depolarizes the axon terminal, which has no Na+ or K+ channels. On the other hand, it triggers the opening of the voltage-gated Ca2+ channels leading to influx of Ca2+.

Ca2+ entry triggers exocytosis of synaptic vesicle contents (neurotransmitters), which diffuses across the synaptic cleft and binds with receptors on the postsynaptic cells. Neurotransmitter-binding then initiates a response in the postsynaptic cell (electrical/chemical).

12
Q

Explain the concept of spatial summation of potentials.

A

Potential can be differentiated to EPSP and IPSP. EPSP is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential, while the opposite for IPSP.

A post-synaptic neuron may receive multiple potentials from multiple neurons (convergence pathway). The summation of these action potentials may either lead to AP induction or not in the post-synaptic neuron.

13
Q

Describe Henneman’s Size Principle.

A

Henneman’s size principle states that under load, motor units are recruited from smallest to largest – less ions/signals are required to reach the threshold potential

In practice, this means that slow-twitch, low-force, fatigue-resistant muscle fibers are activated before fast-twitch, high-force, less fatigue-resistant muscle fibers.

14
Q

Describe absolute and relative refractory period.

A

Absolute refractory period: no stimulus can cause action potential => due to majority of VG-Na+ channel being in an inactive site

Relative RP: a greater than normal stimulus is required to inititate AP => due to VG-K+ channels close slowly leading to extra K+ conductance

15
Q

Voltage-gated Na+ and K+ Channels are triggered to open at the same instant.

True or False

A

True