MNSR 14 Flashcards
Passive spread of a local signal no ion channels required happens when…
The membrane has a capacitance.
The cytosole has a resistance.
Ions can move inside the cytosol along dendrites or axons.
What are the characteristics of passive propagation?
Charges want to be distributed evenly over the inner membrane due to charge-charge repulsion (Coulomb force) and due to the concentration difference (Diffusion force).
This leads to an ionic flow along the dendrite/axon away from the source.
This current flow goes hand-in-hand with the traveling of a voltage signal due to the charging of the membrane capacitance.
If the source is not maintained, the signal quickly decreases in amplitude.
Two types of synapses exist:
chemical or electrical.
All synapses have in common that the arrival of an AP leads to…
an influx of ions into the postsynaptic cell.
This influx causes a local rise of membrane voltage, the so called…
excitatory postsynaptic potential (EPSP).
How does the signal travel from the synapse to the soma?
Dendrites contain few voltage-gated ion channels, i.e. very few Na+ channels.
Importantly, a single action potential in a presynaptic cell does not produce an EPSP large enough to reach threshold and trigger an action potential.
The EPSP thus passively spreads along the dendrite and decays.
Dendritic summation: Temporal
If multiple APs are fired in the presynaptic cell, the corresponding multiple excitatory potentials can add up through a process called…
temporal summation to reach threshold and triggering an AP
Dendritic summation: Spatial
In an analogue way, excitatory inputs from many synapses add up along the…
dendrites (spatial summation) and might exceed the threshold for an AP.
Essentially, neurons are like adding machines.
A single neuron can receive inputs from hundreds of others. The AP is generated at the axon inital segment (AIS; also called axon hillock) which is the…
the part of the axon very close to the soma. The AIS has the highest density of voltage-gated Na+ channels along the neuron.
The shape of the AP is always the same as long as the stimulus was…
above threshold.
Why is the AP shape so reproducible?
Ion Channel Gating Mechanics: The action of voltage-gated ion channels, which open and close in response to specific voltage changes, ensures a consistent and predictable pattern of ion flow across the neuron’s membrane.
All-or-None Principle: Once the threshold is reached, an action potential is always triggered with the same magnitude, following an all-or-none response. This principle guarantees that every action potential has a similar shape and size.
Defined Ion Movements and Refractory Periods: The specific stages of depolarization and repolarization are governed by the controlled movement of sodium and potassium ions. Additionally, refractory periods prevent the immediate initiation of another action potential, contributing to the uniform timing and shape of each action potential.
What is the refractory period?
Inactivated Na+ channels cannot re-open directly.
Next AP needs higher threshold stimulus (relative) or cannot be triggered at that time (absolute).
The refractory period limits the maximum rate at which neurons can ‘fire‘ or how many AP per time can travel down an axon.
What characterizes the resting potential state of a neuron?
During resting potential, the inside of the neuron is negatively charged relative to the outside due to the distribution of ions, with more sodium ions (Na+) outside and more potassium ions (K+) inside.
How does an action potential begin in an unmyelinated axon?
An action potential begins when a stimulus causes a localized influx of Na+ ions, changing the permeability of the neuronal membrane and leading to depolarization.
How does the action potential propagate along an unmyelinated axon?
The action potential propagates as the influx of Na+ ions in one region of the axon causes depolarization of the adjacent region, sequentially activating voltage-gated ion channels along the axon.
