Membrane Potential and Maintenance

Overview of Membrane Potential

The membrane potential is the voltage difference across a cell's plasma membrane, resulting from the unequal distribution of ions. This potential is essential for the function of excitable cells such as neurons and muscle cells.

• Resting membrane potential is typically negative inside the cell relative to the outside, maintained by ion pumps and channels.

• The sodium-potassium pump (Na+/K+ ATPase) actively transports 3 Na+ out and 2 K+ in, contributing to the negative resting potential.

• Leak channels allow passive movement of ions, further stabilizing the membrane potential.

• Formula for equilibrium potential (Nernst equation):

Action Potentials

Phases and Ion Channel Actions

Action potentials are rapid, transient changes in membrane potential that propagate electrical signals along excitable cells.

• Depolarization: Membrane potential becomes less negative, reaching the threshold due to Na+ influx.

• Repolarization: K+ channels open, K+ exits the cell, restoring negativity.

• Hyperpolarization: Membrane potential temporarily becomes more negative than resting.

• Action potentials are all-or-none events.

• Graphical representation shows a sharp rise (depolarization), fall (repolarization), and undershoot (hyperpolarization).

Patch Clamping and Molecular Techniques

Patch clamping is a technique used to study the activity of single ion channels in excitable cells. It allows measurement of ionic currents and analysis of channel properties.

• Excitable cells exhibit action potentials measurable by patch clamp.

• Action potentials involve rapid changes from negative to positive membrane potential and back.

Voltage-Gated and Ligand-Gated Ion Channels

Channel Structure and Function

• Voltage-gated ion channels open in response to changes in membrane voltage.

• Voltage-gated Na+ and K+ channels are essential for action potential generation.

• Ligand-gated ion channels open when specific molecules (ligands) bind to them.

Categories and Structure

• Voltage-gated potassium channels: Multimeric proteins with four subunits.

• Voltage-gated sodium channels: Large monomeric proteins with four domains.

• Each domain/subunit contains six transmembrane α helices.

Channel Specificity

• Channel specificity is determined by the size of the central pore and its interaction with ions.

• Oxygen atoms in amino acids at the channel center form a selectivity filter for ions.

Channel Gating and Inactivation

• Channel gating: Channels open rapidly in response to stimuli and close again; states are all-or-none.

• Helix S4 acts as a voltage sensor.

• Channel inactivation: Channels enter a closed state and cannot reopen immediately; caused by an inactivating particle blocking the pore.

Propagation of Action Potentials

Mechanism of Propagation

• Depolarization to threshold initiates an action potential.

• Inward Na+ movement followed by outward K+ movement.

• Opening and closing of voltage-gated channels control ion flow.

• Action potential travels along the membrane by propagation.

Rapid Changes in Membrane Potential

• Membrane potential rises to about +40 mV during action potential.

• Falls to about -75 mV (undershoot/hyperpolarization).

• Stabilizes at resting potential (~ -60 mV).

Signal Transmission Steps

1. Stimulation causes depolarization and Na+ influx.

2. Membrane polarity reverses and depolarization spreads.

3. K+ channels open, K+ exits, restoring resting state.

4. Depolarization spreads, repeating the sequence.

Nerve impulse moves only away from the initial site.

Transmission of Signals in Neurons

Synapses and Signal Initiation

• Signals are transmitted at synapses (contact points between neurons).

• Depolarization spreads passively to the axon hillock, where action potentials are initiated.

Myelination and Nodes of Ranvier

Consequences of Myelination

• Myelination decreases membrane capacitance, allowing faster and farther spread of impulses.

• Action potentials are renewed at nodes of Ranvier.

Saltatory Propagation

• Nodes of Ranvier are spaced to allow action potential renewal.

• Action potentials jump from node to node (saltatory propagation), increasing speed.

Synaptic Transmission and Signal Integration

Electrical Synapses

• Presynaptic and postsynaptic neurons connected by gap junctions.

• Ions move directly between cells; transmission is instantaneous.

Chemical Synapses

• Neurons separated by a synaptic cleft.

• Signals are transmitted chemically via neurotransmitters.

Neurotransmitters

Storage and Release

• Stored in synaptic boutons of presynaptic neurons.

• Released upon arrival of an action potential.

• Diffuse across the cleft and bind to postsynaptic receptors.

• Converted to electrical signals in the postsynaptic cell.

Types of Neurotransmitter Receptors

• Ionotropic receptors: Ligand-gated ion channels.

• Metabotropic receptors: Indirectly affect ion channels via messenger systems.

Neurotransmitter Function

• Excitatory receptors cause depolarization.

• Inhibitory receptors cause hyperpolarization.

Classification of Neurotransmitters

• Over 100 neurotransmitters, grouped as:

  • Acetylcholine: Excitatory, cholinergic synapses.

  • Catecholamines: Dopamine, norepinephrine, epinephrine (adrenergic synapses).

  • Amino acids: Histamine, serotonin (excitatory), γ-aminobutyric acid (GABA, inhibitory), glycine (inhibitory), glutamate (excitatory).

  • Neuropeptides

  • Gases, lipids

• Each neurotransmitter may have multiple receptor types.

Table: Different Kinds of Neurotransmitters

Acetylcholine

Role and Function

• Most common neurotransmitter in vertebrates outside the CNS and at neuromuscular junctions.

• Excitatory neurotransmitter.

• Synapses using acetylcholine are called cholinergic synapses.

Endocannabinoids

Role in Neural Activity

• Lipid derivatives that inhibit presynaptic neuron activity.

• Main receptor (CB1) is also activated by THC from Cannabis plants.

• THC is responsible for marijuana's effects.

Calcium and Neurotransmitter Secretion

Role of Calcium in Synapse

• Neurotransmitter secretion is controlled by Ca2+ concentration in synaptic boutons.

• Action potential arrival opens voltage-gated calcium channels, increasing Ca2+.

• Neurotransmitters are stored in neurosecretory vesicles.

Vesicle Mobilization and Fusion

• Calcium release mobilizes vesicles for rapid release.

• Ready vesicles dock and fuse with the plasma membrane in the bouton region.

Docking and Fusion Mechanism

• Docked vesicles fuse with the membrane, mediated by t-SNARE and v-SNARE proteins.

• Ca2+ binds synaptotagmin, triggering SNARE interaction and vesicle fusion.

Additional Activities and Applications

Botulinum Toxin and SNARE Proteins

• Clostridium botulinum produces botulinum toxin A, which cleaves SNARE proteins.

• Cleaving SNAREs prevents vesicle fusion, causing paralysis.

• Endocytosis of the toxin is required for its function inside neurons.

• Botox uses the toxin to block neurotransmitter release, reducing muscle activity.

Enzyme Kinetics Activity

• Enzyme kinetics plots: Michaelis-Menten and Lineweaver-Burk.

• Axes: [S] (substrate concentration) vs. V0 (initial velocity).

• Equation (Michaelis-Menten):

• Vmax: Maximum velocity; Km: Substrate concentration at half Vmax.

• At low [S], velocity is limited by substrate amount.

tRNA Sequence Activity

• tRNA sequence includes a CCA end (3' terminus).

• Anticodon is complementary to the start codon (AUG).

• Start codon (AUG) encodes methionine.

Vesicle Trafficking Complexes

• Three main protein complexes mediate vesicle trafficking:

  • COPI: Transports cargo from Golgi to ER.

  • COPII: Transports cargo from ER to Golgi.

  • Clathrin: Transports cargo from plasma membrane and trans-Golgi to endosomes.

• Complexes are composed of coat proteins (e.g., clathrin, Sec proteins for COPII).

Additional info: Academic context and table entries inferred to ensure completeness and clarity for cell biology exam preparation.