Neuronal Action Potentials and Protein Transport

Na+ Channels and Inactivation

• Automatic Inactivation Mechanism: Na+ channels quickly reclose due to an automatic inactivation process.
  • Remain inactivated until membrane potential returns to a negative state.
  • Inactivation gate: A flexible loop between the third and fourth domains acts as a plug.

K+ Channels and Repolarization

• Delayed K+ Channels: Essential for membrane repolarization during action potential.
  • Open in response to membrane depolarization but are slower than Na+ channels.
  • Open during the action potential's falling phase to restore the resting potential by allowing K+ efflux.

Action Potential Propagation

• Propagation Mechanism:
  • Intracellular electrodes measure voltages along the axon; Na+ and delayed K+ channels play crucial roles in action potential generation and propagation.
  • Action potential travels unidirectionally due to Na+ channel inactivation, ensuring it moves away from the depolarization site.

Neuronal Stimulation and Signal Transmission

• Action Potential Trigger:
  • Initiated at the axon hillock after passive depolarization from a dendritic stimulus.
  • Passive spread occurs over the cell body to the axon.

Myelination and Action Potential Speed

• Myelin Sheath:
  • Formed by glial cells wrapping around axons to insulate them.
  • Increases the speed of action potentials significantly via saltatory conduction, jumping between nodes of Ranvier.
• Energy Efficiency: Action potential energy usage is minimized as active excitation happens only at nodal regions.

Chemical Synapses and Signal Conversion

• Transmission Process:
  • At synapses, action potentials lead to voltage-gated Ca2+ channels opening, causing neurotransmitter release via exocytosis.
  • Neurotransmitters diffuse across the synaptic cleft, binding to postsynaptic receptors to induce electrical responses.

Neurotransmitter Removal and Precision

• Recycling Mechanisms:
  • Neurotransmitters are cleared from the synaptic cleft by enzymes or uptake by presynaptic or glial cells.
  • This process ensures effective signaling and prepares the synapse for subsequent neurotransmitter releases.

Ion Channel Selectivity and Effects

• Types of Ion Channels:
  • Excitatory neurotransmitters typically open nonselective cation channels (e.g., Na+, Ca2+, K+), promoting depolarization.
  • Inhibitory neurotransmitters activate Cl- channels, making it harder for depolarization to occur.
  • Some transmitters can be either excitatory or inhibitory, highlighted by acetylcholine's dual role depending on receptor subtype.

Neuromuscular Junction and Transmission

• Muscle Cell Activation:
  • Triggered by nerve impulses causing Ca2+ influx at the nerve terminal, releasing acetylcholine into the synaptic cleft.
  • Acetylcholine interacts with muscle cell receptors, causing Na+ influx and subsequent depolarization leading to muscle contraction.

Protein Transport Mechanisms

• Synthesis and Sorting:
  • Most proteins begin synthesis in the cytosol, directed to their final locations by sorting signals recognized by specific receptors.
  • Categories of transport include:
  • Protein Translocation: Direct transport into compartments.
  • Gated Transport: Movement through nuclear pores.
  • Vesicular Transport: Use of membrane-bound vesicles for transport.
  • Engulfment: Involving processes like autophagy.

Endoplasmic Reticulum Functions

• Structural Characteristics:
  • The ER is composed of branching tubules and flattened sacs, critical for lipid and protein biosynthesis.
  • Distinct regions: Rough ER (with ribosomes) and Smooth ER (without ribosomes).

Protein Translocation Mechanisms in the ER

• Signal Sequences and Recognition:
  • SRP directs ER signal sequences to translocators for protein entry into the ER.
  • The signal is recognized, and protein translocation happens through a gated channel in the translocator.
  • This process occurs concurrently with translation, ensuring prompt delivery into the ER lumen.