Midterm 2 Review Notes

Midterm 2 Logistics

Midterm 2 is next Wednesday.
The material covered up to today will be on the midterm.
Molecular medicine content may appear as minor multiple-choice questions related to ethics or context, but not as full questions.
The exam will be similar to homework problems, focusing on problem-solving rather than pure memorization.
Some memorization is required, like knowing what SRP is and the importance of classrooms.
The exam format will be on paper.

When the voltage gets above the threshold voltage (V_t), voltage-gated sodium channels open, and sodium rushes in.
Voltage-gated potassium channels open, and potassium rushes out, returning the cell to its resting potential.

Neurons receive signals from neighboring neurons via dendrites.
At synapses, neurotransmitters are secreted to transmit signals between cells that are very close but not touching.
Neurotransmitters bind to receptors on the receiving end, triggering the opening of ligand-gated ion channels.
Sodium entering through these channels increases the voltage.
Other synapses secrete neurotransmitters that lower the voltage.
The neuron integrates hundreds of inputs, some excitatory (increasing voltage) and some inhibitory (decreasing voltage).
If the sum of positive signals exceeds negative signals sufficiently, the threshold is crossed, triggering an action potential.

The action potential fires in the main body of the neuron.
The axon transmits the signal to the next neuron or to a muscle (e.g., in the leg).
The axon can be several feet long.
The action potential perturbs the voltage in the neighboring region of the axon, triggering the next action potential.
This creates a wave of action potentials propagating down the axon.
In the brain, distances are short, but axons can be long when signaling down the leg.
Myelin, formed by cells wrapping around the axon, provides insulation and capacitance to speed up transmission.

It's crucial to understand how to establish the equilibrium potential.
Across a membrane, there is a concentration gradient for ions (e.g., calcium).
If calcium ion channels open, calcium is driven into the cell due to the concentration gradient (diffusion).
This brings positive charge into the cell, leaving negative charge outside, establishing an electric field.
The electric field counteracts the diffusive flow of calcium.
The electric field grows until it perfectly balances diffusion, reaching equilibrium.
At equilibrium, the electric field perfectly counteracts the concentration gradient.
A stronger concentration gradient requires a larger voltage to counteract it.

Opening ion channels doesn't significantly change the overall concentration gradients because the amount of ions needed to change the electric field is a tiny fraction of their total amount.
Even during action potentials, where there are large voltage swings and significant ion flow, the concentration gradients remain essentially constant.
Only a tiny fraction of the total amount of sodium and potassium ions are needed to fire action potentials rapidly.