Overview
This lecture explains the structure of neurons, how they generate and transmit electrical signals, and the mechanisms underlying action potentials and signal propagation.
Neuron Structure and Signal Flow
- Neurons have four main parts: dendrites, cell body, axon, and axon terminal.
- Dendrites receive incoming information; the cell body integrates information.
- The axon transmits electrical signals long distances; axon terminals pass signals to the next cell.
- A bundle of axons is called a nerve.
Resting Membrane Potential and Ionic Gradients
- At rest, more sodium ions (Na⁺) are outside the neuron, while more potassium ions (K⁺) are inside.
- This unequal distribution creates both chemical and electrical gradients across the membrane, forming the electrochemical gradient.
- The resting membrane potential is about -70 millivolts, inside less positive than outside.
- The sodium-potassium pump maintains these gradients by moving 3 Na⁺ out and 2 K⁺ in, using ATP.
Ion Channels and Graded Potentials
- Ions move across the neuron membrane via ion channels.
- Some ion channels are always open; others open in response to voltage, ligand binding, or mechanical force.
- Most channels are selective for specific ions.
- Small changes in membrane potential from ion movement are called graded potentials; they are transient and variable.
Action Potentials and Signal Transmission
- If stimulus raises membrane potential to -55 mV (threshold), an action potential is triggered at the axon hillock.
- Voltage-gated sodium channels open, Na⁺ rushes in, causing depolarization (membrane becomes more positive).
- Depolarization leads to overshoot (+30 mV); sodium channels inactivate.
- Voltage-gated potassium channels open, K⁺ exits, causing repolarization (membrane becomes negative again).
- Slow closing of potassium channels causes hyperpolarization (more negative than resting potential).
- Sodium-potassium pump restores resting ion distribution and membrane potential.
Refractory Periods and Signal Properties
- Absolute refractory period: neuron cannot fire another action potential regardless of stimulus strength.
- Relative refractory period: neuron can fire only with a stronger stimulus due to hyperpolarization.
- Action potentials are all-or-nothing responses; only frequency (not amplitude) varies with stimulus strength.
Signal Conduction and Myelination
- Myelin sheaths increase conduction speed via saltatory conduction, where action potentials jump between nodes of Ranvier.
- Schwann cells form myelin in the peripheral nervous system; oligodendrocytes form it in the central nervous system.
Key Terms & Definitions
- Neuron — nerve cell that transmits electrical signals.
- Dendrite — receives incoming signals.
- Axon — carries signals away from the cell body.
- Action Potential — rapid, all-or-nothing electrical signal in a neuron.
- Resting Membrane Potential — voltage difference across membrane at rest (about -70 mV).
- Sodium-Potassium Pump — protein that actively transports Na⁺ out and K⁺ into the cell.
- Depolarization — membrane potential becomes less negative (more positive).
- Repolarization — membrane potential returns to resting level.
- Hyperpolarization — membrane potential becomes more negative than resting.
- Absolute Refractory Period — time when neuron can't fire another action potential.
- Relative Refractory Period — time when neuron requires a stronger stimulus to fire.
- Myelin Sheath — insulating layer speeding up signal transmission.
Action Items / Next Steps
- Review neuron structure diagrams.
- Study the steps and phases of action potential generation.
- Practice identifying different ion channels and their triggers.