Resting, Action, Graded Potential, Salt Cond, Propagation

Overview of Neuron Excitability and the Cellular Environment

• Ions and Membrane Charge: The cell membrane of a neuron maintains distinct groups of ions on either side, creating an electrical gradient.

• Resting Membrane State: When a neuron is at rest, the outside of the cell has an overall positive charge, while the inside of the cell has an overall negative charge.

• Voltage Measurement: The difference in charge is measured in millivolts ($mV$). A common signpost for the inside of a resting neuron is $-70\,mV$, though the exact number varies.

• Transmembrane Proteins: Various types of proteins span the membrane to facilitate the movement of ions.

The Role of Active Transport and the Sodium-Potassium Pump

• Mechanism of the Pump: The sodium-potassium pump is a specific transmembrane protein powered by adenosine triphosphate ($ATP$).

• Function: It moves sodium ($Na^+$) from the inside to the outside of the cell and potassium ($K^+$) from the outside to the inside, moving these ions against their concentration gradients.

• Phosphorylation: The pump requires $ATP$ to function. When the protein is phosphorylated (receives a phosphate group), it changes its conformation to move ions; it changes back once the phosphate group is lost.

• Ion Distribution: Because of this pump, there is a high concentration of sodium ions outside the cell and a high concentration of potassium ions inside the cell.

Types of Ion Channels and the Concept of Transduction

• General Channel Function: Unlike pumps, channels do not require $ATP$; they allow ions to flow down their concentration gradients under specific circumstances.

• Ligand-Gated Channels: These open or close in response to a specific signaling molecule (a ligand).   - Example: At the neuromuscular junction, the neurotransmitter acetylcholine ($ACh$) binds to transmembrane proteins. This opens the channel, allowing sodium and calcium ($Ca^{2+}$) to enter the cell while potassium leaves.

• Mechanically-Gated Channels: These channels respond to physical vibration or pressure.   - Example: In afferent sensory neurons in the finger, pressure against a wall opens these channels in the dendrites. According to the transcript, in this specific modality, potassium comes in and calcium goes out.

• Voltage-Gated Channels: These respond to direct changes in the membrane potential (the voltage across the membrane).   - Structural Closing Mechanisms: These channels can close in two ways: the two sides of the protein stick together, or a mechanism described as a "ball and chain" doohickey physically plugs the channel.

• Leakage Channels: These channels open and close randomly without a specific stimulus.

• Transduction: This is the process of converting real-world sensations (like pressure, taste, or light) into an electrical form that the nervous system can process.   - Camera Metaphor: Transduction is compared to a camera phone's $CCD$ (charge-coupled device) sensor, which converts light hitting the lens into digital data.   - Frequency of Stimulation: Stronger sensations (e.g., strong taste or high pressure) are perceived because the neurons generate action potentials at a higher frequency.

Electrochemical Theory of Membrane Potential

• Electronegativity and Affinity: Elements like chlorine ($Cl$) on the right of the periodic table have high electron affinity (negativity). Sodium ($Na$) and potassium ($K$) on the left have low affinity and give up electrons easily.

• Elemental Comparison: Sodium is a row higher than potassium on the periodic table; therefore, it has an even lower electron affinity than potassium, making sodium "more positively positive" than potassium.

• Charge Distribution at Rest:   - Outside: High sodium concentration makes it more positive.   - Inside: High potassium concentration, along with negative chlorine ions and negatively charged proteins, makes it more negative.

• Defining Voltage: Voltage is the measurement of the potential for electrical current to flow. It represents the attraction a negative electron would feel toward the positive side of the membrane and the repulsion it would feel from the negative side.

• Resting Potential: The specific membrane potential when a neuron is not being stimulated and is not passing a message.

Variations in Membrane Potential: Depolarization and Hyperpolarization

• Hyperpolarization: This occurs if potassium channels are opened. Potassium ions diffuse down their gradient out of the cell. This makes the outside more positive and the inside even more negative than the resting state.

• Depolarization: This occurs if sodium channels are opened. Sodium ions zip into the cell because they are attracted to the negative interior and are moving down their concentration gradient. This reverses the charge, making the inside less negative (or even positive) and the outside more negative.

• Relative Change: The student is not responsible for exact voltage numbers, but rather the relative direction of change (e.g., closer to zero or more negative).

• Graded Potentials: These are smaller stimuli that cause varying degrees of depolarization. In the real world, these often occur in the dendrites as they receive input from multiple other neurons.   - Summation: Similar to twitches in a muscle, multiple graded potentials can add together to create a larger wave of depolarization.

The Action Potential: Thresholds and Phases

• Threshold: The specific limit of depolarization required to trigger an action potential. Once the membrane potential hits this "doorway," the graded potential becomes an action potential.

• All-or-Nothing Principle: Once the threshold is reached, an action potential occurs at full strength. Increasing the stimulus size will not make the individual action potential larger or longer; it only increases the frequency of action potentials.

• Phases of the Action Potential:   1. Resting State: Voltage is stable at the resting membrane potential.   2. Depolarizing Phase: Threshold is reached, voltage-gated sodium channels open, and $Na^+$ pours into the cell, making the interior positive.   3. Repolarizing Phase: Sodium channels inactivate, and voltage-gated potassium channels open. Potassium ($K^+$) leaves the cell, bringing the voltage back toward negative.   4. Hyperpolarization: The potassium channels stay open slightly longer, causes the voltage to "overshoot" the resting potential and become more negative than it was originally.   5. Return to Rest: The sodium-potassium pumps restore the original ionic conditions.

Propagation and Saltatory Conduction

• One-Way Travel: The action potential propagates in only one direction down the axon.

• Refractory Period: The period during which a neuron cannot generate another action potential, ensuring the signal moves forward.   - Absolute Refractory Period: Occurs during depolarization and the start of repolarization when sodium channels are inactivated by a "plug."   - Relative Refractory Period: Occurs during hyperpolarization; a very strong stimulus is required to start a new potential because the potassium is still diffusing out.

• Saltatory Conduction: This occurs in myelinated axons.   - Mechanism: Sodium enters at a node (Node of Ranvier) and moves rapidly through the cytoplasm (axoplasm) and via diffusion to the next node.   - Speed: This "jumping" from node to node is much faster than the opening of individual channels along the entire length of the membrane.

Synaptic Transmission and Postsynaptic Potentials

• Synapse Interaction: The action potential travels to the end of the presynaptic neuron, opening calcium channels.

• Neurotransmitter Release: Calcium entry triggers vesicles to release neurotransmitters into the synaptic cleft, which then act as ligands on the postsynaptic neuron.

• Postsynaptic Potential Types:   - Excitatory Postsynaptic Potential (EPSP): The neurotransmitter opens ligand-gated sodium channels. This causes depolarization, moving the neuron closer to its threshold.   - Inhibitory Postsynaptic Potential (IPSP): The neurotransmitter opens ligand-gated potassium or chlorine channels. This causes hyperpolarization, making the neuron less likely to fire.

• Neurotransmitter Fate: After signaling, the neurotransmitter may diffuse away, be broken down by enzymes, or be pumped back into the cell (reuptake).

Summation and Synaptic Learning

• Decision to Fire: Whether a neuron produces an impulse depends on how the excitatory and inhibitory potentials add together in both space and time.

• Synaptic Strengthening:   - If a synapse repeatedly receives excitatory signals ("yes, yes, yes"), the synapse gets "tighter" or stronger.   - If a synapse repeatedly receives inhibitory signals ("no, no, no"), that inhibitory connection also gets stronger.

• Learning via Inhibition: Inhibition is viewed as a form of learning. This is compared to a school play where a student must learn not just their lines, but also to wait behind the curtain until called.

Future Directions: Neuroanatomy and Function

• Brain Study Approach: The lecture will move toward studying brain anatomy first to help organize thinking. Function will be integrated later.

• Occipital Lobe: Specifically mentioned as the area of the brain involved with vision.