Cell Transport Mechanisms Quiz

Action Potential Propagation and Transmission

• Action potentials propagate from the axon hillock to the axon terminal.
• The propagation is one-way due to the absolute refractory period following the action potential.
• The axon hillock has the highest density of voltage-gated Na+ channels.
• Conduction velocity differs in myelinated and unmyelinated neurons.

Session Learning Objectives (February 7th, 2025)

• Students will be able to describe the process of action potential propagation in unmyelinated and myelinated neurons.
• They will differentiate the conduction velocity difference between these neuron types and
• recall, and draw the events of synaptic transmission in the presynaptic cell,
• describe the types of synapses (electrical and chemical),
• recall post-synaptic potentials (PSPs) and the ions responsible for depolarization (excitatory) and hyperpolarization (inhibitory),
• including excitatory post-synaptic potentials (EPSP) and
• inhibitory post-synaptic potentials (IPSP).

Action Potential Propagation

• Action potential propagation from the axon hillock to the axon terminal is typically one-way.
• This is because the absolute refractory period follows along the "wake" of the moving action potential.
• The axon hillock has the highest density of voltage-gated Na+ channels.

Action Potential Propagation: Unmyelinated Axons

• Action potentials begin at the "trigger zone" (axon hillock) and propagate to the axon terminal.
• Propagation is slow.
• Depolarization at one site triggers depolarization of the adjacent site in a wave-like manner (initiation and propagation).
• Conduction is relatively slow in unmyelinated axons.

Action Potential Propagation in Myelinated Neurons

• Myelin is created by oligodendrocytes in the CNS and Schwann cells in the PNS.
• Myelin increases membrane resistance to current flow, so ions flow along the axon interior instead of against the high resistance membrane.
• Propagation is much faster, and occurs via saltatory conduction.
• Saltatory conduction is where action potentials jump from Node of Ranvier to Node of Ranvier.
• Large diameter axons have faster conduction velocity due to a higher density of voltage-gated channels.
• Myelin further increases conduction velocity.

AP Propagation

• One-way propagation (from axon hillock to axon terminal) starts at the axon hillock and moves toward the axon terminal.
• The refractory period prevents it from moving in the retrograde direction after initiation.
• Velocity increases with axon diameter and myelination (saltatory conduction).
• Relevant pathologies include multiple sclerosis and diabetes mellitus.

Synapses

• Neurons are functionally associated with other neurons, and effector organs (muscles or glands).
• Two types of synapses exist:
• Electrical synapses.
• Chemical synapses.

Synaptic Transmission: Electrical

• Two (or more) excitable cells are linked together by gap junctions.
• Transmission is very rapid.
• Multiple cells in a tissue/organ can "behave as one".

Synaptic Transmission: Chemical

• Slower than electrical synapses.
• More complex and modifiable.
• Communication from one neuron to other neurons, to muscle cells, gland cells, and other cells is via chemical synapses.

Chemical Synaptic Transmission: Presynaptic Events

• AP propagates to axon terminal.
• Voltage-gated Ca2+ channels open.
• Rapid influx of Ca2+ activates vesicle exocytosis, allowing vesicles to fuse with plasma membrane.
• Neurotransmitter diffuses across synaptic cleft (15 nm) onto postsynaptic cell.

Postsynaptic Events at Excitatory Synapse

• Neurotransmitter (NT) binds to a post-synaptic receptor, opening ligand-gated channels.
• Cations (mainly Na+ or Ca2+) flow into the cell.
• The net effect is depolarization (EPSP).

Postsynaptic Events at Inhibitory Synapse

• NT binds to a post-synaptic receptor, opening ligand-gated channels.
• Either K+ flows out or Cl- flows into the cell.
• The net effect is hyperpolarization (IPSP).

Chemical Synaptic Transmission

• Postsynaptic potentials are brief because neurotransmitters (NT) rapidly bind and unbind to receptors.

Chemical Signals: Neurotransmitters

• There are many different types of neurotransmitters.
• The receptor determines whether an IPSP or EPSP develops.

Chemical Synaptic Transmission: Summary

• AP propagates to axon terminal.
• Voltage-gated Ca2+ channels open.
• Ca2+ initiates vesicle exocytosis.
• Neurotransmitter crosses across the synapse.
• NT binds to a receptor.
• Ligand-gated channels open.
• Ions flow.
• A graded potential (EPSP or IPSP) occurs.

Postsynaptic Potential

• Net effect could be depolarization (EPSP) or hyperpolarization (IPSP) .

Membrane, graded, and action potentials

• Key difference between a graded potential and an action potential
• Types of channels used
• Phases of an action potential

In-class Activity

• Predicting net flux of Cl- at various membrane potentials.

Ionic Basis of Action Potentials

• Action Potentials (AP) occur in response to graded potentials, summing up to reach threshold.
• APs are initiated at the trigger zone.
• An AP typically consists of three phases.
• Understanding the changing permeability of Na+ and K+ is essential to understanding APs.

Phase 1: Rapid Depolarization

• Voltage-gated Na+ channels open, rapidly depolarizing the membrane as sodium enters.
• Voltage-gated K+ channels remain closed.

Voltage-Gated Na+ Gating Properties

• Fast to open, fast to inactive/close

Phase 2: Repolarization

• The decrease in Na+ permeability (sodium channels inactivate) causes a rapid decrease in the positive charge entering the cell.
• Lots of positive charge (K+) leaves the cell.

Voltage-Gated K+ Gating Properties

• Slow to open, slow to close

Phase 3: Afterhyperpolarization

• Voltage-gated Na+ channels are inactivated, preventing further Na+ entry into the cell.
• Voltage-gated K+ channels are slow to close, allowing further K+ outflow, hyperpolarizing the membrane to a potential more negative than the RMP.
• Eventually, the voltage-gated K+ channels close, and the NA+/K+-ATPase pumps and leak channels bring the cell back to RMP.

Flow Chart of Channel Events

• Graded potentials depolarize the membrane, reaching threshold.
• Voltage-gated Na+ channels open and membrane rapidly depolarizes.
• Voltage-gated Na+ channels inactivate while Voltage-gated K+ channels open, resulting in membrane repolarization.

Testing Your Understanding

• Comparing changes in ion channel permeability to the change in membrane potential.

Refractory Periods

• During and immediately after an action potential, the membrane is less excitable than at rest.
• Absolute refractory period: it's impossible for a cell to generate a second action potential regardless of stimulus size.
• Voltage-gated Na+ channels are not available to open during this time.
• Relative refractory period: possible to generate a second AP only with a larger than normal stimulus.
• Some voltage-gated Na+ channels have reentered the "closed" state and are available to open.

Action Potentials

• All-or-none: Once the membrane is depolarized to threshold, the amplitude is independent of the stimulus size.
• Not graded by stimulus size: ECF conditions such as higher ECF Na+ concentration can influence AP properties.
• No summation due to refractory period: Absolute and relative refractory periods prevent summation.
• Do not decrease with distance: Propagate over long distances.

Equilibrium Potential (Nernst Potential)

• For any given concentration gradient of a single ion, the membrane potential that exactly opposes the concentration gradient is known as the equilibrium potential.
• At the equilibrium potential, the movement of an ion across the membrane due to its concentration gradient is opposed by the movement of the ion in the opposite direction due to its electrical gradient.
• This is not the resting membrane potential.

Ion Concentrations

• Intracellular and Extracellular fluid concentration of different ions

In-Class Question

• Equilibrium potential of K+.
• Net flux of K+ in a resting cell

Membrane Potential

• When a cell is permeable to more than one ion (e.g., K+ and Na+), both affect the resting membrane potential in proportion to their relative permeabilities and conductances.
• Vm is a weighted average of each ion's flux.
• Relative permeability determines which ion (K+ is the primary determinant) dominates in determining RMP.

Signaling through Dynamic Changes in Membrane Potential

• Changes in membrane potential are the basis of cellular communication, allowing cells to receive and send information.

Electrical Signals

• Neurons communicate via electrical signals—changes in membrane potential.
• Ion channels open and close to change membrane potential.

Types of Ion Channels

• Leak channels
• Ligand-gated channels
• Voltage-gated channels
• Mechanically-gated channels

Two Types of Electrical Signals

• Graded potentials are small and short distance; action potentials are large and long distance.
• Graded potentials amplitude varies with stimulus intensity and may be depolarizing or hyperpolarizing. Their amplitude decreases with distance. Summation is possible.
• Action potentials are all-or-none, have similar amplitude, are initiated by depolarization, and do not summate.

Graded Potentials

• Are proportional to the size of the stimulus.
• Decrease with distance from the stimulus site.
• Are short-distance signals.
• Can be depolarizations or hyperpolarizations.
• Summate with each other. AKA sensory receptor potentials or generator potentials.

Osmolarity

• Total solute concentration of a solution per unit volume.
• Normal osmolarity of ECF and ICF in most physiological solutions is approximately 300 mOsm.
• Tonicity describes the relative concentration of nonpenetrating solutes between two solutions, either (hypertonic, hypotonic, or isotonic).

Ion Channels, Active Transport, and Vesicular Transport

• Leak channels
• Ligand-gated channels
• Voltage-gated channels
• Primary active transport
• Secondary active transport
• Vesicular transport or transport through vesicles

Transport Rate Across the Membrane

• Simple diffusion: rate increases with concentration.
• Carrier-mediated transport: rate reaches a maximum (Vmax) before saturation.

Primary Active Transport

• Requires energy (usually ATP) to transport substances directly across the cell membrane.
• Uses transmembrane proteins (pumps).
• Pumps against concentration gradient.

Secondary Active Transport

• Uses energy from another molecule's concentration gradient to transport substances against their concentration gradient.
• Uses transmembrane proteins.
• Two main types: cotransport (symport), and countertransport (antiport).

Vesicular Transport: Endocytosis and Exocytosis

• Endocytosis: Molecules from the ECF enter the cell via vesicles formed from the plasma membrane.
• Exocytosis: Intracellular vesicles fuse with plasma membrane, releasing contents into the ECF.

Proteins in the Phospholipid Bilayer

• Allow the cell to be selectively permeable.

The Cell Membrane

• Dynamic: many membrane processes are always occurring., such as cardiac muscle.

Physiology 3051

• Basics of physiology, homeostasis, and various body functions.

In-Class Activity (Various)

• In-class activities were presented to students to apply their knowledge of diffusion, chemical driving forces, and membrane potentials.

Neurophysiology

• Specific aspects of neurophysiology (related to nerve cells, action potentials, and related mechanisms, and processes).

Penetrating and Non-penetrating Substances

• Classification of substances based on their ability (or inability) to cross cell membranes.

Learning Objectives(various)

• Overview of different physiologically relevant topics and learning objectives.