Cell Transport Processes Part 2 Flashcards
Electrochemical gradient
+ —–> - Electrochemical gradient when voltage and concentration gradients work in same direction
- ——> + When voltage and concentration gradients work in opposite directions
Resting Potential
Resting potential is the electrical charge difference across a cell membrane when the cell is not actively sending signals.
Definition: It’s typically around -70 mV in neurons, where the inside of the cell is more negative compared to the outside.
Cause: Maintained by ion gradients (mainly Na⁺ and K⁺) and the sodium-potassium pump, with selective permeability to K⁺ through leak channels.
Importance: It sets the stage for action potentials and proper cell signaling
Neuron Structure and Function
Dendrites:
Structure: Branch-like extensions from the cell body.
Function: Receive signals (electrical or chemical) from other neurons or sensory receptors.
Cell Body (Soma):
Structure: Contains the nucleus and organelles.
Function: Processes incoming signals and maintains cellular function.
Axon:
Structure: A long, thin extension from the cell body, often covered in a myelin sheath.
Function: Transmits electrical signals (action potentials) to other neurons or muscles.
Myelin Sheath:
Structure: Fatty insulating layer around the axon, made by Schwann cells or oligodendrocytes.
Function: Increases the speed of signal transmission.
Nodes of Ranvier:
Structure: Gaps in the myelin sheath along the axon.
Function: Allow saltatory conduction, where signals jump between nodes for faster transmission.
Axon Terminals (Synaptic Boutons):
Structure: Endings of the axon.
Function: Release neurotransmitters to communicate with the next neuron, muscle, or gland.
Stages of Action Potential (GRAPH)
Resting Potential:
The neuron is at rest, with a charge of about -70 mV.
Sodium (Na⁺) is concentrated outside the cell, and potassium (K⁺) inside.
Maintained by the sodium-potassium pump and K⁺ leak channels.
Depolarization:
A stimulus opens voltage-gated Na⁺ channels.
Na⁺ rushes into the cell, making the inside more positive.
The membrane potential rises to around +30 mV.
Repolarization:
Voltage-gated Na⁺ channels close, and voltage-gated K⁺ channels open.
K⁺ exits the cell, making the inside more negative again.
Hyperpolarization:
K⁺ channels stay open a bit longer, causing the membrane potential to drop below resting potential (more negative than -70 mV).
This ensures the neuron doesn’t immediately fire another action potential.
Return to Resting Potential:
K⁺ channels close, and the sodium-potassium pump restores the original ion balance.
The neuron is ready to fire again.
Brief Summary of Action Potential
1 - A stimulus from a sensory cell or another neuron depolarizes the target neuron to its threshold potential
2 - Na+ channels in the axon open, allowing positive ions to enter the cell
Once the sodium channels open, the neuron completely depolarizes to a membrane potential of about +40 mV
3 - Voltage-gated K+ channels open, allowing K+ to leave the cell.
As K+ ions leave the cell, the membrane potential once again becomes negative
4 – Two things happens,
-Na Channels start a refractory period as K channels open,
-K Channels close after a brief period of hyperpolarization
Propagation of an Action Potential Along an Axon
An action potential begins at the axon hillock when the membrane potential reaches the threshold due to depolarization.
Voltage-gated sodium (Na⁺) channels open, and Na⁺ rushes into the cell.
Depolarization Spreads:
The influx of Na⁺ causes the adjacent section of the axon to depolarize, opening voltage-gated Na⁺ channels in that region.
This creates a wave of depolarization traveling along the axon.
Repolarization Behind the Wave:
After depolarization, voltage-gated potassium (K⁺) channels open, and K⁺ exits the cell.
This repolarizes the membrane and restores a negative charge inside the axon.
Refractory Period Ensures Unidirectional Flow:
The absolute refractory period (when Na⁺ channels are inactivated) prevents the action potential from traveling backward.
The relative refractory period (during hyperpolarization) ensures a stronger stimulus is needed to trigger another action potential.
Saltatory Conduction (in Myelinated Axons):
In myelinated axons, the action potential jumps between Nodes of Ranvier (gaps in the myelin sheath).
This increases the speed of propagation significantly compared to unmyelinated axons.
Arrival at Axon Terminals:
The action potential reaches the axon terminals, triggering the release of neurotransmitters into the synapse, facilitating communication with the next neuron or target cell.
Myelin Sheath
Glial cells form a membraneous
sheath surrounding axons
called myelin, thereby
insulating the axon.
This myelination, as it is
called, can greatly increase
the speed of signals
transmitted between
neurons
