Skip to main content
Back

Cell Biology Study Notes: Membrane Potential, Action Potentials, Synaptic Transmission, and Neurotransmitters

NotesPracticeVideo lessons

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

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

Neurotransmitter

Structure

Functional Class

Receptor

Acetylcholine

CH3COOCH2CH2N(CH3)3

Excitatory

Nicotinic, muscarinic

Dopamine

C8H11NO2

Excitatory or inhibitory

D1-D5

GABA

C4H9NO2

Inhibitory

GABAA, GABAB

Glutamate

C5H9NO4

Excitatory

NMDA, AMPA, kainate

Serotonin

C10H12N2O

Excitatory or inhibitory

5-HT receptors

Endocannabinoids

Various lipid derivatives

Inhibitory

CB1, CB2

Additional info: Table entries inferred from standard neurotransmitter classification.

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.

Pearson Logo

Study Prep