Overview
This lecture explains synaptic transmission, detailing how neurons communicate via electrical signals and chemical messengers, key steps in signal transmission, and related terminology.
Neuron Structure & Synapse
- Neurons have dendrites (receive signals), a soma (cell body), and an axon (sends signals away).
- The end of the axon is called the axon terminal or synaptic bulb.
- The gap between two neurons is the synaptic cleft.
- The neuron sending the signal is the presynaptic neuron; the receiving neuron is the postsynaptic neuron.
- Dendrites of the postsynaptic neuron receive the chemical message.
Electrical Charge & Action Potential
- Neurons are negatively charged inside and positively charged outside at rest (resting membrane potential: -70 mV).
- An action potential is the electrical charge that travels down the neuron.
- Stimulation causes the neuron to become more positive; if it reaches -55 mV (threshold potential), an action potential fires (point of no return).
- Voltage-gated sodium channels open at threshold, allowing sodium ions to enter, depolarizing the cell to about +40 mV.
Signal Propagation & Neurotransmitter Release
- Depolarization travels down the neuron as an action potential.
- When the action potential reaches the axon terminal, voltage-gated calcium channels open, allowing calcium ions to enter.
- Calcium causes synaptic vesicles (containing neurotransmitters) to fuse with the presynaptic membrane.
- Neurotransmitters are released into the synaptic cleft by exocytosis.
Neurotransmitter Binding & Signal Transmission
- Neurotransmitters cross the synaptic cleft and bind to receptors on the postsynaptic neuron.
- These receptors are ligand-gated ion channels that open when neurotransmitters bind, allowing sodium to enter and depolarize the next neuron.
- If depolarization is sufficient, a new action potential is triggered in the postsynaptic neuron.
Neurotransmitter Removal and Recycling
- Not all neurotransmitters bind; some are broken down, diffuse away, or are reabsorbed by the presynaptic neuron.
- Reabsorption and repackaging of neurotransmitters is called reuptake.
Repolarization
- After transmission, potassium exits the cell, repolarizing the neuron (restoring negative charge inside).
Key Terms & Definitions
- Synaptic Transmission — process by which neurons send signals to each other.
- Resting Membrane Potential — the baseline negative charge inside a neuron (~ -70 mV).
- Action Potential — rapid electrical impulse traveling down the neuron.
- Threshold Potential — the critical level (-55 mV) needed to trigger an action potential.
- Depolarization — when the neuron’s charge becomes positive due to sodium influx.
- Synaptic Vesicle — small sacs containing neurotransmitters.
- Exocytosis — process of neurotransmitter release into the synaptic cleft.
- Ligand-Gated Channel — membrane channel that opens when a specific chemical binds.
- Reuptake — reabsorption of neurotransmitters by the presynaptic neuron.
Action Items / Next Steps
- Review the action potential process for deeper understanding.
- Study the definitions of key terms listed above.