Neurotransmitters and Receptors Quiz

Action Potential Depolarization

• Action potentials (AP) depend on ion currents and voltage-gated channels.
• Voltage-gated sodium (Na⁺) channels and voltage-gated potassium (K⁺) channels are involved.
• At time=0, Na⁺ channels open, and Na⁺ flows in, depolarizing the membrane.
• Na⁺ channels close and inactivate.
• K⁺ channels open, and K⁺ flows out, restoring the membrane potential to its resting state.
• K⁺ leak channels are always open.

Action Potential Repolarization/Falling Phase

• AP depends on ion currents and voltage-gated channels.
• K⁺ channels open, and K⁺ flows out.
• Na⁺ channels are inactivated
• Repolarizations happens.
• Na⁺/K⁺ pump returns RMP (resting membrane potential) concentration gradient.
• Refractory period occurs after repolarization.

Hodgkin–Huxley Cycle

• AP rise phase is a positive feedback cycle.
• Initial depolarization opens Na⁺ channels and increases Na⁺ permeability.
• Increased Na⁺ flow further depolarizes the membrane.
• Further membrane depolarization opens more Na⁺ channels, amplifying the signal.

Action Potential Propagation Along an Axon

• APs are initiated at the axon hillock.
• APs are conducted unchanged along the axon membrane to the terminals.
• Dendrites and cell body have a higher concentration of K⁺ channels, reducing backpropagation into the soma (cell body).

Propagation of Action Potentials

• Action potentials move along an axon as the ion flows generated in one segment depolarize the potential in the next segment.
• Propagation can be observed in both myelinated and unmyelinated axons.

Action Potential Conduction in Unmyelinated Axons

• Reduced threshold at axon hillock (spike initiating zone).
• High concentration of Na⁺ channels.
• Current spreads along the membrane toward terminals initiating a new Action potential.
• Adjacent (downstream) Na⁺ channels reach threshold from large depolarization (new AP).
• The refractory period prevents further propagation.
• Axon diameter determines conduction speed (larger = faster; up to 40m/s).

Saltatory Conduction

• In myelinated axons, ions can flow across the plasma membrane only at nodes where the myelin sheath is interrupted.
• Action potentials skip rapidly from node to node.
• Saltatory conduction allows for thousands to millions of fast-transmitting axons to be packed into a relatively small diameter.

Action Potential Conduction in Myelinated Axons

• Myelin (protein and lipid) insulates and prevents ion flow across the membrane.
• Reduces current loss.
• Concentration of Na⁺ and K⁺ at nodes allows ions to cross the membrane.
• Axon hillock function is similar to unmyelinated neurons.
• Similar conduction process but the current quickly spreads between nodes.
• Higher conduction velocities (up to 100 m/s) are observed.

Synaptic Transmission

• Synapses are the sites where neurons communicate.
• Pre-synaptic neuron sends neurotransmitters.
• Post-synaptic neuron receives neurotransmitters.
• Chemical synapses are most common, and neurotransmitters diffuse across the synaptic cleft to post-synaptic membrane receptors.
• Ion channels open or close, creating graded potentials (PSPs).

Neurotransmitters

• Stored in vesicles within axon terminals of presynaptic neurons
• Released thru exocytosis into synaptic cleft
• Act via ligand-gated ion channels or G-protein-coupled receptors.
• Binding leads to depolarization or hyperpolarization.

Two Types of Synapses

• Electrical synapses pass directly from a presynaptic cell to the postsynaptic cell (e.g., cardiac muscle, some invertebrate neurons).
• Chemical synapses involve the release of neurotransmitters by the presynaptic cell to diffuse across the synaptic cleft and act on receptors of the postsynaptic cell, which is common in most neurons.

Vesicles Release Neurotransmitter

• Action potentials trigger Ca²⁺ influx through voltage-gated Ca²⁺ channels.
• Ca²⁺ causes vesicles to move to the plasma membrane, fuse, and release neurotransmitter into the cleft.

Postsynaptic Binding

• Neurotransmitters bind to postsynaptic receptors, opening channels that lead to depolarization (excitatory) or hyperpolarization (inhibitory).
• These signals allow for integration of multiple presynaptic inputs (up to 1,000)

Neurotransmitters Work in Two Ways

• Some bind directly to ligand-gated ion channels in the postsynaptic membrane.
• Others work more slowly by acting as first messengers and binding G protein-coupled receptors in the postsynaptic membrane and subsequently triggering second messengers.

Neurotransmitter Removal

• Neurotransmitters are removed from the synaptic cleft either by enzymatic breakdown or uptake by the presynaptic terminal or glial cells.

Types of Neurotransmitters

• Acetylcholine
• Amino acids
• Biogenic amines
• Neuropeptides
• Gases

Receptor Protein Types

• Ionotropic binding proteins
• Metabotropic binding proteins

Conclusion

• Heart 2 shows a delayed reaction to stimulation of Heart 1, implying a chemical transmission of nerve impulses.