Overview
This lecture covers synapses, the specialized sites where neurons communicate. It explains the structure and function of synapses, focusing on the differences between chemical and electrical synapses and their roles in neural circuits.
Synapse Structure and Function
- A synapse is a region where two neurons come close enough to communicate, enabling the formation of neural circuits responsible for a wide range of brain functions, from simple movement to complex cognition.
- Most neurons have thousands of synapses, and the total number of synapses in the human brain is extremely high—estimates range from hundreds of trillions to over two quadrillion.
- The most common type of synapse is the axodendritic synapse, where the axon terminals of one neuron are positioned near the dendrites of another.
- Synapses can also form between other parts of neurons, such as axon terminals and cell bodies, axon terminals and axons, or even between dendrites.
- The synaptic cleft is the tiny gap (about 20–40 nanometers wide) that separates the presynaptic and postsynaptic neurons. For perspective, a human hair is about 80,000–100,000 nanometers wide.
- The presynaptic neuron sends the signal, while the postsynaptic neuron receives it.
Chemical Synapses
- Chemical synapses are the most common type in the human nervous system.
- Communication begins when an action potential (an electrical signal) travels down the axon of the presynaptic neuron to the axon terminal (or synaptic bouton).
- This electrical signal triggers changes inside the presynaptic neuron, leading to the mobilization of yxsynaptic vesicles—tiny sacs containing thousands of neurotransmitter molecules.
- Synaptic vesicles fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft through a process called exocytosis.
- Neurotransmitter molecules diffuse across the synaptic cleft and bind to specific receptors embedded in the membrane of the postsynaptic neuron.
- Binding of neurotransmitters to receptors causes changes in the postsynaptic neuron, making it more or less likely to fire its own action potential.
- After neurotransmitters have transmitted their message, they must be removed from the synaptic cleft to prevent continuous stimulation and allow the synapse to function properly.
- Removal occurs through:
- Simple diffusion (a small percentage of neurotransmitters drift away from the cleft)
- Enzymatic degradation (enzymes break down neurotransmitters; e.g., acetylcholinesterase breaks down acetylcholine)
- Reuptake (transport proteins in the presynaptic membrane take neurotransmitters back up for recycling and repackaging into vesicles)
- Reuptake is a major target for certain drugs, such as selective serotonin reuptake inhibitors (SSRIs), which block the reuptake of serotonin, increasing its levels in the synaptic cleft and potentially improving depressive symptoms.
Electrical Synapses
- Electrical synapses are less common in the adult human nervous system but are more prevalent in embryos and invertebrates.
- At electrical synapses, neurons communicate through gap junctions, where the space between neurons is much smaller (about 2–4 nanometers).
- Gap junction channels directly connect the presynaptic and postsynaptic neurons, allowing ions (charged particles) to flow rapidly from one neuron to the next.
- This direct connection enables virtually instantaneous transmission of electrical signals, with no significant delay.
- Electrical synapses can transmit signals in both directions, allowing for bidirectional communication.
- These synapses are especially useful for synchronizing the activity of groups of neurons, such as those involved in regulating breathing in the brainstem.
Comparison: Chemical vs. Electrical Synapses
- Chemical synapses can greatly amplify signals—a small electrical current in the presynaptic neuron can trigger the release of thousands of neurotransmitter molecules, causing a strong effect on the postsynaptic neuron.
- Electrical synapses provide extremely fast, bidirectional communication, making them ideal for synchronizing neural activity.
- Most communication in the human brain occurs at chemical synapses, but both types are important for different functions.
Key Terms & Definitions
- Synapse: The junction where two neurons communicate.
- Axodendritic synapse: A synapse between an axon terminal and a dendrite; the most common type.
- Synaptic cleft: The tiny gap (20–40 nanometers wide) between neurons at a synapse.
- Presynaptic neuron: The neuron sending the signal.
- Postsynaptic neuron: The neuron receiving the signal.
- Synaptic vesicle: A small sac in the presynaptic terminal that stores neurotransmitters.
- Neurotransmitter: A chemical messenger released by neurons to transmit signals across a synapse.
- Exocytosis: The process by which synaptic vesicles fuse with the presynaptic membrane to release neurotransmitters.
- Reuptake: The reabsorption of neurotransmitters into the presynaptic neuron for recycling.
- Enzymatic degradation: The breakdown of neurotransmitters by enzymes in the synaptic cleft.
- Gap junction: A direct electrical connection between neurons, allowing ions to pass rapidly between cells.
- SSRIs (Selective Serotonin Reuptake Inhibitors): Drugs that block the reuptake of serotonin, increasing its levels in the synaptic cleft and used to treat depression.
Action Items / Next Steps
- Review the detailed process of neurotransmitter release and removal for deeper understanding.
- Prepare definitions and explanations of key terms for quizzes or flashcards.
- Consider watching additional resources, such as the referenced video on neurotransmitter release, for further clarification.