Neurophysiology Refractory Periods Quiz

Refractory Periods

• Absolute Refractory Period: Occurs during depolarization and most of repolarization. No second action potential can be triggered, regardless of stimulus strength. This is caused by the inactivation of Na⁺ channels, preventing them from reopening until the membrane potential returns to a certain level.
• Relative Refractory Period: Occurs during the latter part of repolarization and undershoot. A second action potential can be initiated, but only with a stronger-than-normal stimulus. This is caused by open K⁺ channels leading to hyperpolarization, requiring a stronger stimulus to reach the threshold.

Sodium-Potassium Pump

• The sodium-potassium pump continuously works to maintain potassium and sodium distributions, restoring the initial ion concentrations after an action potential.
• It pumps 3 sodium ions out of the cell and 2 potassium ions into the cell against their concentration gradients.
• This requires a significant amount of ATP as it works against the concentration gradient.

Action Potential Origin and Travel

• Action potentials originate at the axon hillock (trigger zone).
• They travel down the axon toward the axon terminals by depolarizing sections of the membrane in a wave-like manner.
• Sodium (Na+) ions enter the cell, while potassium (K+) ions exit during depolarization.

Myelin and Saltatory Conduction

• Myelin is a fatty substance that insulates axons.
• It is produced by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS).
• Myelinated axons conduct action potentials faster through saltatory conduction, where the signal jumps between Nodes of Ranvier.
• Voltage-gated sodium channels are only located at the Nodes of Ranvier.

Electrical vs. Chemical Synapses

• Electrical synapses: Direct ion flow between cells via gap junctions. They are fast but less flexible and simpler than chemical synapses. More common in invertebrates.
• Chemical synapses: Use neurotransmitters to transfer signals across a synaptic cleft. They are slower but highly adaptable and act on specific receptors on the receiving cell's surface.

Neurotransmitter Release: Proteins and Ions

• Calcium ions (Ca²⁺): Trigger vesicle fusion.
• SNARE proteins: (syntaxin, SNAP-25, synaptobrevin) are essential for vesicle docking and neurotransmitter release. Found on the membrane of the vesicles and the neuronal membrane.

Neurotransmitter Release: Sequence of Events

• Action potential reaches the axon terminal.
• Voltage-gated Ca²⁺ channels open, allowing calcium to enter.
• Synaptic vesicles fuse with the membrane, releasing neurotransmitters.
• Neurotransmitters diffuse across the synapse and bind to receptors.

Ligands and Receptors

• Ligand: A molecule that binds to a receptor (e.g., neurotransmitter).
• Receptor: A protein on the cell surface that ligands bind to, initiating a response.

Ionotropic vs. Metabotropic Receptors

• Ionotropic receptors: The receptor itself is an ion channel. Ions rapidly flow through when it is open (depending on driving force). These are also called "transmitter-gated ion channels".
• Metabotropic receptors: The receptor activates a signaling pathway, indirectly affecting channels or other cellular processes. Slower acting, modulatory effects.

Glia Types and Functions

• Astrocytes: Found in the CNS. They provide structural support, regulate the blood-brain barrier, help deliver nutrients, clear excess neurotransmitters and metabolic waste, and maintain ion balance.
• Oligodendrocytes: Found in the CNS. They produce myelin to insulate axons.
• Schwann cells: Found in the PNS. They produce myelin to insulate axons.

Neuron vs. Other Cell Types

• Excitability: Neurons can generate and propagate electrical impulses (action potentials).
• Synaptic transmission: Communicate via neurotransmitter release at synapses.
• Unique structure: Dendrites (receive signals), axon (transmit signals), soma (cell body).
• Polarity: Signals flow in one direction from dendrites to axon terminal.

Neuron Parts

• Dendrites: Receive signals from other neurons.
• Cell body (soma): Contains nucleus and organelles, integrates signals.
• Axon: Transmits electrical impulses away from the soma.
• Axon hillock: Where the axon connects to the soma; action potentials are initiated.
• Myelin sheath: Insulates axon, speeding up signal transmission.
• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated..
• Axon terminal: Releases neurotransmitters to communicate with other neurons.

Test your understanding of the absolute and relative refractory periods in neurophysiology. This quiz covers key concepts like sodium and potassium ion dynamics, as well as the role of the sodium-potassium pump in maintaining ion balance during action potentials.