Biopsych lecture 4

Membrane Potential and Action Potentials

  • Membrane Potential (V_m)

    • Defined as the difference in electric charge across a neuronal membrane, measured in millivolts (mV).

    • Resting membrane potential is approximately -70 mV.

  • Action Potential Duration

    • The process of an action potential from depolarization back to resting state takes about 2 milliseconds.

    • This implicates the maximum firing rate of a neuron, calculated as:

      • extFiringRate=rac1000extms2extms=500extHzext{Firing Rate} = rac{1000 ext{ ms}}{2 ext{ ms}} = 500 ext{ Hz}

    • Neurons can fire action potentials at a rate of up to 500 times per second.

  • Frequency Coding

    • Neurons cannot fire as fast as high-frequency sounds (e.g., sounds at 18,000-20,000 Hz) due to the physiological limit of action potential firing rates.

    • Implications for sound frequency perception: up to 500 Hz can be accurately perceived through firing rates, but higher frequencies cannot be coded accurately, as neurons cannot distinguish between 500 Hz and 20,000 Hz in terms of action potential firing.

Action Potential Characteristics

  • Key Thresholds and Potentials

    • Resting Membrane Potential: -70 mV

    • Threshold Potential: -55 mV

    • Peak Potential: +30 mV

  • Stimuli and Action Potential Generation

    • Stimulus may include neurotransmitter release (e.g., glutamate as an excitatory neurotransmitter).

    • Glutamate will bind to its receptors, allowing sodium ions (Na^+) to enter the neuron, leading to depolarization.

  • Ion Channels

    • Voltage-Gated Sodium Channels: Open at -55 mV, allowing Na^+ to flow in, causing rapid depolarization.

    • Voltage-Gated Potassium Channels: Open later, allowing K^+ to flow out and initiating repolarization.

    • Aquaporins: Water channels allowing passive flow of water through the membrane.

    • Ligand-Gated Channels: Open when a neurotransmitter binds, allowing ions to flow based on concentration gradients.

      • E.g., Ligand-gated sodium channels allow Na^+ influx when a neurotransmitter binds.

Conducting an Action Potential

  • Phases of Action Potential

    • Depolarization Phase: From threshold to peak, caused by Na^+ influx.

    • Repolarization Phase: From peak back to resting potential, caused by K^+ efflux.

    • Hyperpolarization: Occurs when membrane potential overshoots resting levels due to prolonged K^+ outflow.

  • States of Voltage-Gated Sodium Channels

    • Closed and Ready: Channels can open with stimuli.

    • Open: Channels allow Na^+ to enter, leading to depolarization.

    • Closed and Refractory: Channels return to a closed state, preventing immediate reactivation due to the refractory period.

Toxins and Their Effects

  • Tetrodotoxin (TTX)

    • Found in pufferfish, blocks voltage-gated sodium channels.

    • Effect: Prevents action potentials from occurring, causing symptoms like numbness due to loss of neural signals.

  • Bacotoxin

    • Blocks voltage-gated potassium channels, disrupting repolarization.

    • Effect: Prevents neurons from repolarizing; prolonged depolarization leads to eventual equilibrium and can result in excessive neuronal activation.

Neurotransmitters and Their Categories

  • Classification

    • Monoamines: Derived from single amino acids.

      • Indolamines: Serotonin (5-hydroxytryptamine = 5-HT).

      • Catecholamines: Dopamine, Norepinephrine (noradrenaline), and Epinephrine (adrenaline).

    • Amino Acid Neurotransmitters: Major excitatory neurotransmitter is glutamate, major inhibitory neurotransmitter is GABA.

  • Enzymatic Processes

    • Enzymes convert precursor amino acids into neurotransmitters (e.g., Tyrosine to L-Dopa to Dopamine).

    • Enzymes like MAO and COMT metabolize monoamines, breaking them down into inactive metabolites.

Synthesis and Degradation of Neurotransmitters

  • Serotonin Pathway:

    • Tryptophan converted to serotonin through the action of Tryptophan Hydroxylase, followed by degradation via MAOA.

  • Dopamine Pathway:

    • Tyrosine converted to L-Dopa via Tyrosine Hydroxylase, then to dopamine. Subsequent conversions yield norepinephrine and epinephrine, regulated by their respective enzymes (DBH and PNMT).

Recap of Receptor Dynamics

  • Broad Classes of Receptors

    • Ionotropic Receptors: Directly control ion channels (e.g., 5-HT3 for serotonin).

    • Metabotropic Receptors: GPCRs that lead to intracellular signaling cascades (e.g., dopamine receptors can either activate or inhibit cell activity).

  • Medications

    • Effective medications must cross the blood-brain barrier to target neurotransmitter systems directly.

Study Recommendations

  • Memorization is key, especially concerning neurotransmitter nomenclature, structures, and pathways.

  • Utilize diagrams and repeated writing exercises to facilitate retention.