Sunday, April 28, 2024
How action potentials propagate down the axon
Action potentials are the electrical signals that allow neurons to communicate with each other.
- Let’s delve into how these fascinating events propagate down an axon:
Depolarization and Na+ Channels:
At the start of an action potential, the membrane potential of the neuron depolarizes due to the opening of voltage-gated Na+ channels.
As more Na+ channels open, the Hodgkin cycle kicks in, leading to the regeneration of the action potential.
This sequential process causes the action potential to move down the axon.
Propagation Down the Axon:
The propagating action potential depolarizes the axon segment in front of it, bringing it to threshold.
Once the threshold is reached, this segment generates its own action potential.
The process repeats as the action potential travels along the axon, like a wave of depolarization1.
Refractory Period and Unidirectional Propagation:
The refractory period plays a crucial role in ensuring unidirectional propagation.
After an action potential, an area of the membrane remains refractory, preventing the action potential from moving backward.
This ensures that the signal travels only in one direction along the axon1.
Myelination and Saltatory Conduction:
Myelin, a fatty substance, wraps around some axons.
Myelination decreases membrane capacitance and increases resistance, resulting in less current loss during propagation.
The action potential “jumps” from one Node of Ranvier (unmyelinated region) to the next, a process called saltatory conduction.
This leads to faster propagation speeds compared to non-myelinated axons1.
In summary, the coordinated opening and closing of ion channels, refractory periods, and myelination all contribute to the efficient and precise propagation of action potentials in neurons.🌟
Graded potentials
Graded potentials are brought about by external stimuli (in sensory neurons) or by neurotransmitters released in synapses, where they cause graded potentials in the post-synaptic cell.
Action potentials are triggered by membrane depolarization to threshold.
Graded potentials are responsible for the initial membrane depolarization to threshold.
Graded potentials
1.Definition: Graded potentials are typically generated in the dendrites of a neuron, where voltage-gated channels are not present. They occur in response to stimuli, such as neurotransmitters binding to receptors.
2. Definition: A change in the electrical potential on the membrane of an excitable cell (e.g. a nerve cell) in response to a stimulus, and where the magnitude of change is proportional to the strength of the stimulus.
Chegg :
- Graded potentials in neurons are present in the dendrites following synaptic activity.
- While the usual membrane potential change is seen as an all-or-nothing response, graded potentials introduce variability in membrane potential.
quiz
distribution of charge across the cell membrane, based on the charges of ions
- Size exclusion
- Potential
- Membrane potential
- Activation gate
Gated
property of a channel that determines how it opens under specific conditions, such as voltage change or physical deformation.
- Leakage channel
- Resistance
- Potential
- Gated
Ligand-gated ion channels
Ligand-gated ion channels (LICs, LGIC), also commonly referred to as ionotropic receptors, are a group of transmembrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter.
Ligand-gated channel
Neurotransmitter receptor that acts as an ion channel gate, and opens by the binding of the neurotransmitter. Another name for an ionotropic receptor
Excitable membrane
cell membrane that regulates the movement of ions so that an electrical signal can be generated is called _________________
- Excitable membrane
- resting membrane
- resting membrane potential
- refractory period
mechanically gated channel
A mechanically gated channel opens because of a physical distortion of the cell membrane. Many channels associated with the sense of touch (somatosensation) are mechanically gated. For
Ion movement
When an ion channel is open, ions can diffuse down their respective electrochemical gradients.
The movement of ions across the membrane creates an electrochemical charge.
For example, the sodium/potassium pump actively transports sodium ions (Na⁺) out of the cell and potassium ions (K⁺) into the cell, maintaining ion concentration gradients on both sides of the membrane.
In summary, ion channels facilitate the movement of specific ions across the cell membrane, allowing cells to generate electrical signals such as action potentials.
These channels are essential for nerve impulses, muscle contractions, and other physiological processes.
wiki-https://en.wikipedia.org/wiki/Ion_channel
Kenhub: https://www.kenhub.com/en/library/anatomy/ion-channels-and-gradients
Cell Membrane Structure:
Cell Membrane Structure:
The cell membrane consists of a phospholipid bilayer. It primarily regulates what can cross the membrane.
Charged particles (ions) are hydrophilic and cannot pass through the hydrophobic core of the membrane without assistance.
Transmembrane channel proteins, including ion channels, allow ions to move across the membrane2.
Medulla oblongata
Medulla oblongata control involuntary movement of respiratory , circulatory and digestive system.
- contribute hearing , balance and taste
- played crucial role in homeostasis regulate blood pressure , respiration , digestion and excretion
- autonomic reflex swallowing , coughing , sneezing
What are the two forces that go into how ions move across membranes?
It turns out that you can measure the charge difference across the membrane. (In other words, you can find out how different the charge inside vs outside is). This difference is measured as a voltage (units = mV).
If we only had the Na+/K+ pump involved, the charge difference of the cell = -0.5mV. But when we measure a real neuron, we find a charge difference of -70mV! This is way more negative than we expect and it means we don't have all the information regarding a cell at rest.
Please watch two videos next to better understand what else is going on. First, you'll learn about electrochemical gradients and then you'll review the whole picture of a cell at rest.
Watch and Review electrochemical gradients videoLinks to an external site. (6 minutes)
What are the two forces that go into how ions move across membranes?
- physical (mechanical) force
- THE FORCE (like what Yoda uses)
- electrical force (+ vs - charges)
- chemical force (concentration gradients)
- ATP
Based on the Na+/K+ pump and your answer above, we find that there is a charge difference of the ions across the cell membrane.
Based on the Na+/K+ pump and your answer above, we find that there is a charge difference of the ions across the cell membrane.
What is true of the cell at rest?
(This is based on our handout activity! So please review your notes from lecture!)
A leakage channel
A leakage channel is randomly gated, meaning that it opens and closes at random, hence the reference to leaking. There is no actual event that opens the channel; instead, it has an intrinsic rate of switching between the open and closed states. Leakage channels contribute to the resting transmembrane voltage of the excitable membrane
voltage-gated channel
A voltage-gated channel is a channel that responds to changes in the electrical properties of the membrane in which it is embedded. Normally, the inner portion of the membrane is at a negative voltage. When that voltage becomes less negative, the channel begins to allow ions to cross the membrane
cerebrospinal fluid (CSF)
circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood
thermoreceptor
Found in the skin of your fingers or toes is a type of sensory receptor that is sensitive to temperature, called a thermoreceptor.
When you place your hand under the shower , the cell membrane of the thermoreceptors changes its electrical state (voltage).
The amount of change is dependent on the strength of the stimulus (how hot the water is). This is called a graded potential.
If the stimulus is strong, the voltage of the cell membrane will change enough to generate an electrical signal that will travel down the axon. You have learned about this type of signaling before, with respect to the interaction of nerves and muscles at the neuromuscular junction.
The voltage at which such a signal is generated is called the threshold, and the resulting electrical signal is called an action potential.
Graded potential
At the level of the spinal cord at which this axon makes a synapse, a graded potential occurs in the cell membrane of a lower motor neuron.
precentral gyrus
The upper motor neuron is in this region, called the precentral gyrus of the frontal cortex, which has an axon that extends all the way down the spinal cord.