biol 3410 11/12 lec

Overview of Membrane Potentials

Depolarizing Greater Potential: Conditions under which depolarization occurs
- Sodium Ions Enter the Cell: Sodium influx results in a more positive internal environment.
- Mechanically Gated Sodium Channels Open: Physical deformation allows sodium to enter and initiate depolarization.
- Ligand Gated Sodium Channels Open: Binding of ligands causes these channels to open, permitting sodium entry.

Graded Potentials: Localized changes in membrane potential
- Characteristics: Can be of varying magnitudes (big or small), and only affect a small area of the membrane.
Action Potentials: Rapid and uniform electrical events that propagate along the entire membrane.
- Function: Essential for muscle contraction and nerve signaling.
- Characteristics: All-or-none response.

Causes of Hyperpolarization:
- Ligand Gated Potassium Channels Open: Potassium exits, making the interior more negative.
- Potassium Exits the Cell: Due to concentration gradient, enhances negative membrane potential.
- Chloride Ions Enter the Cell: Chloride is negatively charged; its influx reduces potential.

Increased Sodium Permeability:
- Greatest Depolarization: Sodium influx compared to potassium efflux contributes to more significant depolarization.
Decreased Extracellular Sodium Concentration:
- Impact on Resting Membrane Potential: Membrane becomes more negative; potassium exits more readily.

Effect of Pyrethrin:
- Sodium Channels Remain Open: Cells continually depolarize as sodium enters, unable to respond to new action potentials.
- Blocker Example: Depolarizing blocker disrupts cellular response.

Refractory Periods:
- Impact on Action Potential Directionality: Impedes backward conduction, ensuring signal travels unidirectionally from dendrites to synaptic terminals.
Continuous Conduction:
- Definition: Occurs in unmyelinated axons where the entire membrane depolarizes sequentially.
- Speed: Slower action potential travel but allows every axonal part to participate.
Saltatory Conduction:
- Definition: Occurs in myelinated axons; action potentials 'leap' between nodes of Ranvier.
- Efficiency: Faster and more energy-efficient as fewer areas require repolarization.

Electrical Synapses: Ions pass directly via gap junctions for rapid signaling (e.g., in cardiac muscle).
Chemical Synapses: Utilize neurotransmitters to transmit signals across synaptic clefts, involving calcium-triggered neurotransmitter release.

Acetylcholine: Key neurotransmitter for signal transmission. It does not cross membranes directly but binds to receptors.
Catecholamines: Includes epinephrine (adrenaline), norepinephrine, and dopamine, functioning in various physiological processes including stress responses and mood regulation.
GABA (Gamma-Aminobutyric Acid): Principal inhibitory neurotransmitter in the CNS.
Serotonin: Linked with mood regulation; targeted by many antidepressants.

EPSP (Excitatory Postsynaptic Potential): Graded depolarization bringing membrane closer to action potential threshold.
IPSP (Inhibitory Postsynaptic Potential): Graded hyperpolarization moving the potential further from threshold.
Temporal Summation: Increased frequency of EPSPs leads to threshold potential and depolarization.
Spatial Summation: Integrative responses from multiple presynaptic inputs reinforcing excitation.

Spinal Cord Structures: Meninges (pia mater, arachnoid mater, dura mater) protect the spinal cord.
Neural Integration: Interaction between excitatory and inhibitory signals determines overall neuronal response, critical in coordinated movements and reflexes.
Spinal Nerves: Nerve root organization and plexuses ensure coordinated signaling across body regions.