Graded Potentials and Action Potentials

Definition: Graded potentials are changes in membrane potential that vary in size (amplitude) and are essential for neuronal signaling.
Purpose: Understanding how graded potentials work is crucial for grasping how neurons communicate through action potentials.

Neurons send electrical signals through changes in membrane potential, particularly through resting membrane potential.
Important terms introduced:
- Resting Membrane Potential: The difference in electric charge inside the neuron compared to outside at approximately -70 millivolts (mV)
- Threshold Potential: The level of depolarization needed to fire an action potential, specifically -55 mV.

Graded Potentials vs. Action Potentials:
- Graded Potentials: Localized changes in membrane potential that can be depolarizing or hyperpolarizing.
- Action Potentials: Large, uniform depolarizations that propagate along the axon.

Stimulus → opens ion channels on the neuron’s membrane.
Ion movement → changes membrane potential (because ions moving in/out changes the charge inside).
- The intermediate about proteins is mostly the pumps and channels maintaining ion balance, but that’s ongoing background work — it’s not the main driver of the immediate voltage change.
If the membrane potential reaches threshold, an action potential is triggered.
The action potential travels down the axon:

Generator Potentials: Occur in sensory receptors in response to stimuli (e.g., fine touch, pain).
Postsynaptic Potentials: Occur at synapses; consist of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs).
End Plate Potentials: Large graded potentials at the neuromuscular junction.
Pacemaker Potentials: Occur in the heart contributing to automaticity.

→ graded nature

→ decremental nature

→ summative nature

Definition: Graded potentials change in amplitude with the strength of the stimulus.
- Small stimulus → small depolarization.
- Large stimulus → larger depolarization.

Definition: Graded potentials diminish in strength as they spread away from the site of stimulation.
Analogy: Similar to waves created by a dog jumping into a pool; large splashes diminish as they move outward.

Definition: Graded potentials can add together.
Importance: Multiple small graded potentials can combine to reach the threshold necessary for firing an action potential

carried out by two types of channels:
→ ionotropic= cell membrane protein with the dual function of receptor and channel
→ metabotropic= receptor has a g protein intermediate that communicates with another ion channel and makes it close/open.
EPSP/Excitatory Postsynaptic Potential: Occurs when the membrane potential becomes less negative (depolarization). Depolarizes the postsynaptic cell, increasing likelihood of action potential.
IPSP/Inhibitory Postsynaptic Potential: Occurs when the membrane potential becomes more negative (hyperpolarization). Hyperpolarizes the postsynaptic cell, decreasing likelihood of action potential..

Sodium (Na⁺): Typically enters the cell upon opening channels, leading to depolarization.
Chloride (Cl⁻): Enters cell via specific channels; leads to hyperpolarization due to its negative charge.
Calcium (Ca²⁺): Also enters the cell, contributing to depolarization; plays a role in intracellular signaling so it cant fluctuate too much, thus other ions have a bit more of a role.
Potassium (K⁺): Leaves the cell via specific channels, causing hyperpolarization.

Sodium channels open when a neurotransmitter binds, causing Na⁺ ions to flow into the cell.
Non-specific monovalent cation channels contribute by allowing Na⁺ influx, quickly raising the membrane potential towards the threshold.

Chloride channels open allowing Cl⁻ influx, or potassium channels open allowing K⁺ efflux, both move the membrane potential further from threshold.

Different types of graded potentials (generator, postsynaptic, end plate, pacemaker) determine if an action potential will occur based on their functional roles.
Neurotransmitters modulate the opening and closing of ion channels influencing graded potentials.

Graded potentials are crucial for neuronal communication and can vary in effect based on stimulus intensity and ion channel manipulation.
Both types of postsynaptic potentials (EPSPs and IPSPs) have distinct mechanisms and consequences for neuronal excitability.