Nervous System - Action Potential & Synapse Summary

Action Potential

Generation of Action Potentials

  • Signal enters neuron via dendrites or pre-synaptic neuron.

  • Signal converts to action potential at axon hillock and initial segment of axon (trigger zone).

Action Potential Steps

  1. Resting Membrane Potential (Polarized):

    • Outside +, Inside -.

    • Leak channels open, Na-K pump active.

    • −70-70 mV.

  2. Stimulus Arrives:

    • Graded potential.

    • If membrane potential reaches threshold, AP generated.

    • −55-55 mV = AP.

  3. Depolarization:

    • Voltage-gated Na+Na^+ channels OPEN.

    • Na+Na^+ moves INTO cell.

    • Inside +, Outside -.

    • +30+30 mV.

  4. Repolarization:

    • Voltage-gated Na+Na^+ channels CLOSE.

    • Voltage-gated K+K^+ channels OPEN.

    • K+K^+ moves OUT of cell.

    • Inside -, Outside +.

    • −70-70mV.

  5. Hyperpolarization:

    • Voltage-gated K+K^+ channels stay open too long, extra K+K^+ moves OUT.

    • Past resting potential.

    • Outside +, Inside -.

    • −85-85 mV.

  6. Resting Membrane Potential Reestablished:

    • Voltage-gated K+K^+ channels close.

    • Na-K pump moves 3 Na+Na^+ OUT, 2 K+K^+ IN.

    • −70-70 mV reestablished.

All or None Principle

  • If stimulus reaches threshold, full action potential occurs.

  • If threshold not reached, no action potential.

Factors Affecting Action Potential Speed

  1. Myelination: More myelination = more speed (Saltatory Conduction).

  2. Axon Diameter: Large diameter = more speed.

  3. Temperature: Warm = fast, Cold = slow.

Nerve Fiber Classification

  • Type A: Large diameter, myelinated, 15-120 m/s (motor and sensory neurons).

  • Type B: Medium diameter, lightly myelinated, 3-15 m/s.

  • Type C: Small diameter, unmyelinated, 2 m/s or less (ANS, internal organs).

Refractory Period

  • Decreased sensitivity to further stimulation after action potential.

  • Absolute: Complete insensitivity, no action potential possible.

  • Relative: Stronger stimulus needed for action potential.

Graded Potentials vs. Action Potentials

Feature

Graded Potentials

Action Potentials

Potential Change

EPSP (depolarizing), IPSP (hyperpolarizing)

Depolarizing ONLY

Threshold

No threshold

Yes Threshold

Stimulus Intensity

Varies (weak, medium, strong)

All or None

Signal Fading

Fades out as it spreads

Does NOT fade

Refractory Period

No refractory period

Yes Refractory Period

Location

Most cells

Neurons and Muscle Cells only

Synapse

Synapse Communication

  • Pre-Synaptic Cell: transmits signal.

  • Post-Synaptic Cell: receives signal.

Chemical Synapse Steps

  1. Action potential arrives at presynaptic terminal, voltage-gated Ca+2Ca^{+2} channels open.

  2. Ca+2Ca^{+2} diffuses in, synaptic vesicles release neurotransmitter.

  3. Neurotransmitter diffuses across synaptic cleft.

  4. Neurotransmitter binds to receptors, ligand-gated Na+Na^+ channels open.

  5. Na+Na^+ diffuses into postsynaptic cell, membrane potential changes.

Fate of Neurotransmitters

  1. Diffuse out of synaptic cleft.

  2. Broken down by enzymes (e.g., acetylcholinesterase breaks down acetylcholine).