Neurophysiology and Brain Anatomy: Action Potentials and the Midbrain

Electrophysiology of the Neuron: The Action Potential

Resting Membrane Potential (RMP): * The specific value for the resting membrane potential is $-70\,mV$ . * In the provided transcript, this is abbreviated as "RNP."
Threshold Potential: * The critical level to which a membrane potential must be depolarized to initiate an action potential is $-55\,mV$ . * This is the threshold for the firing of the nerve impulse.
Depolarization Phase: * Directionality: Depolarization occurs anytime the membrane potential moves in a positive direction from the resting state toward the threshold and beyond. * Mechanism: The influx of Sodium ( $Na^+$ ) ions into the cell. * Channel Involvement: This process involves the opening of channel proteins. Specifically, if a "light gated channel" is mentioned incorrectly in place of the appropriate channel, it results in a loss of points in the grading rubric. * Grading Criteria (Virtual Points): * Two points are awarded for correctly labeling the axes and the specific data points ( $-70\,mV$ and $-55\,mV$ ). * Three points are awarded for correctly illustrating the general shape of the action potential curve, identifying the ions involved ( $Na^+$ ), the direction of ion movement (inward), and the specific type of channel being used.
Repolarization and Hyperpolarization: * Repolarization: The phase where the membrane potential returns toward the negative resting potential after the peak of the action potential. * Hyperpolarization: A state where the membrane potential becomes more negative than the resting membrane potential ( $-70\,mV$ ). * Mechanism of Hyperpolarization: This involves the activity of Voltage-Gated (VG) Potassium ( $K^+$ ) channels.

Anatomy of the Midbrain and Fluid Passageways

Midbrain Location: * The lecture context focuses on the midbrain region of the brainstem. * A specific passageway, the cerebral aqueduct (referred to as the "aqueduct"), should be visible in a high-quality transverse cut of the midbrain.
Protection and the Barrier System: * The brain requires a sophisticated barrier system to protect neural tissue from harmful substances while ensuring the entry of necessary nutrients. * Vascular Components: * Arteries: Responsible for bringing oxygen-rich blood into the system. * Veins: Responsible for carrying deoxygenated blood away. * Capillaries: The site of exchange between blood and tissue.
The Blood-Brain Barrier (BBB) and Ependymal Cells: * Ependymal cells are a type of glial cell that resembles epithelial cells. * These cells are characterized by the presence of "extra tight junctions" which facilitate the barrier function. * Choroid Plexus: The specific location where ependymal cells and capillaries interact to produce cerebrospinal fluid. The tight junctions here are essential for maintaining the integrity of the barrier.

Course Administration and Laboratory Context

Assignments: * An assignment posted on Canvas that was turned in on a prior Friday is being finalized for grading.
Laboratory Work: * The material discussed, particularly the anatomy of the midbrain and the cerebral aqueduct, correlates directly with the laboratory session held in the previous week. * Participation in the lab is essential for understanding the practical application of these anatomical structures.

Questions & Discussion

Discussion on Potential Values: * The speaker asks what the negative potential value is, confirming it is $-70\,mV$ . * The speaker asks for the name of the $-55\,mV$ value, which the students correctly identify as "Threshold."
Discussion on Ion Movement: * The speaker prompts the students to identify what is happening during the blue part of the graph: "What's happening? Ion moving what direction? What's open? What kind of channel?" * Correct identification of Sodium ( $Na^+$ ) moving in through voltage-gated channels is required for full marks.
Final Notes: * The session concludes with a brief mention of California and a break in the lecture.