Overview
This lecture explores synapses—the connections between neurons—detailing their structure, function, transmission methods, and the effects of neurotransmitters and drugs on neural communication.
Synapse Structure and Function
- Synapses are the junctions where two neurons communicate.
- The brain contains trillions of synapses, each acting like a tiny, adaptable computer.
- Synapses enable learning, memory, and are involved in psychiatric disorders and drug effects.
- The term "synapse" means "to clasp or join," highlighting its role as a connection point.
Types of Synapses
- There are two main synapse types: electrical and chemical.
- Electrical synapses use ion currents through gap junctions for instant, synchronized communication.
- Electrical synapses are fast but lack selectivity and control; mainly found in the heart and some neural circuits.
- Chemical synapses are slower but more common and precise, using neurotransmitters to send messages to specific neurons.
Chemical Synapse Transmission
- The presynaptic neuron sends signals via axon terminals containing synaptic vesicles with neurotransmitters.
- The synaptic cleft is the tiny gap separating neurons at the synapse.
- An action potential opens voltage-gated calcium channels, causing neurotransmitter release into the synaptic cleft.
- Neurotransmitters bind to receptors on the postsynaptic neuron, opening ion channels and creating an excitatory or inhibitory response.
Neurotransmitters and Their Effects
- Excitatory neurotransmitters depolarize the postsynaptic neuron, making it more likely to fire.
- Inhibitory neurotransmitters hyperpolarize the postsynaptic neuron, making it less likely to fire.
- The net effect on a neuron depends on the sum of excitatory and inhibitory signals received.
- Over 100 neurotransmitters exist, with roles in mood, movement, organ function, and alertness.
Neurotransmitter Removal and Drug Effects
- Neurotransmitters are removed by reuptake into the presynaptic neuron, enzyme breakdown, or diffusion.
- Drugs like cocaine block neurotransmitter reuptake, especially dopamine, leading to temporary euphoria followed by depletion and dysfunction.
- Prolonged drug use can change synaptic sensitivity and lower natural receptor numbers, causing dependence.
Key Terms & Definitions
- Synapse — Junction between two neurons where communication occurs.
- Action Potential — Electrical signal transmitted down a neuron.
- Electrical Synapse — Synapse where signals pass directly via ion currents through gap junctions.
- Chemical Synapse — Synapse using neurotransmitters to send messages across a synaptic cleft.
- Neurotransmitter — Chemical messenger that transmits signals between neurons.
- Presynaptic Neuron — The neuron sending the signal.
- Postsynaptic Neuron — The neuron receiving the signal.
- Synaptic Cleft — The gap between two communicating neurons.
- Reuptake — Process of reabsorbing neurotransmitters back into the presynaptic neuron.
Action Items / Next Steps
- Review the mechanisms of action potentials and synaptic transmission.
- Study the effects of specific neurotransmitters (serotonin, dopamine, norepinephrine).
- Understand the impact of drugs on synaptic transmission for upcoming exams.