Notes on the Nervous System and Action Potential

  • Nervous System Overview

    • Focus on physiological events in the nervous system.
    • Key concept: Action potential, the electrical current in neurons.

  • Resting Membrane Potential (RMP)

    • Definition: Membrane potential of a neuron at rest, representing stored energy.
    • Typical value: -70 millivolts (mV) inside the neuron.
    • High concentration of sodium ions (Na^+) outside the cell and high concentration of potassium ions (K^+) inside the cell.
    • Explanation of the negative charge inside the cell: large, negatively charged proteins and DNA are trapped inside the cell.

  • Ion Movement and Action Potential Generation

    • Neurons can change resting membrane potential by allowing ions to cross the cell membrane.
    • Sodium-potassium exchange pump: Moves 3 sodium ions out for every 2 potassium ions in using ATP energy.
    • The process resembles a sub pump efficiently maintaining ion balance.
    • RMP is achieved by separating positive ions (sodium outside; potassium inside) creating potential energy.

  • Gated Channels in Neurons

    • Types of channels:
    • Leak Channels: Always open for ion movement.
    • Gated Channels: Open under specific conditions (stimuli) allowing ion flow.
    • Neuron stimulation leads to sodium gated channels opening, allowing sodium to rush into the cell, making the inside more positive (depolarization).

  • Dealing with Stimuli

    • Neurons respond to various stimuli (e.g., touch, light, chemicals).
    • Dendrites receive signals from the environment and trigger graded potentials.

  • Action Potential Steps

    1. Initial Conditions: RMP at -70 mV.
    2. Stimulus Occurs: Sodium channels open; sodium enters (depolarization).
    3. Threshold Reached: At -60 mV, action potential initiated.
    4. Rapid Depolarization: Sodium continues entering (interior reaches +30 mV).
    5. Repolarization: Sodium gates close, potassium gates open; potassium exits, making the inside negative again.
    6. Hyperpolarization: Inside becomes more negative than resting potential (e.g., -90 mV).
    7. Restoration: Sodium-potassium pump restores RMP at -70 mV allowing for future action potentials.

  • Graded Potentials vs. Action Potentials

    • Graded potentials are small local changes (e.g., ripples in water); can die out without causing an action potential.
    • Action potentials are all-or-nothing signals that travel down the axon without diminishing strength.
    • The absolute refractory period prevents the action potential from reversing direction due to hyperpolarization.

  • Myelination and Speed of Conduction

    • Myelin sheath: Insulation that speeds up action potential propagation by allowing it to jump between nodes (nodes of Ranvier).
    • Resulting in a faster response time (important for reflex actions).

  • Neurotransmission

    • Action potential reaches axon terminal and triggers neurotransmitter release into the synapse.
    • Synapse: Gap between neurons where communication occurs.
    • Neurotransmitters can be excitatory (cause action potentials) or inhibitory (prevent action potentials).

  • Variety of Neurotransmitters

    • Examples include acetylcholine (excitatory for skeletal muscle), norepinephrine, epinephrine, dopamine, serotonin, and GABA (inhibitory).
    • Depending on the postsynaptic cell, neurotransmitters can produce different effects.

  • Key Terminology

    • Resting Membrane Potential (RMP): -70 mV
    • Action Potential (AP): Rapid electrical signal.
    • Threshold: -60 mV for triggering action potential.
    • Depolarization: More positive interior due to sodium influx.
    • Repolarization: Return to negative interior due to potassium efflux.
    • Hyperpolarization: Excess negativity (e.g., -90 mV).
    • Absolute Refractory Period: Time during which neuron cannot re-fire.
    • Relative Refractory Period: Period when a stronger stimulus might initiate an action potential after hyperpolarization.