The Nervous System, Part 3 - Synapses!: Crash Course Anatomy & Physiology #10

Key Concepts

• Synapse: The junction between two neurons where communication occurs.
• Action Potential: An electrical signal that travels down a neuron's axon.
• Electrical Synapse: A fast synapse where ions flow directly between neurons through gap junctions.
• Chemical Synapse: A slower, more controlled synapse that uses neurotransmitters to transmit signals across a synaptic cleft.
• Neurotransmitters: Chemical messengers that transmit signals across the synaptic cleft.
• Presynaptic Neuron: The neuron sending the signal.
• Postsynaptic Neuron: The neuron receiving the signal.
• Synaptic Vesicles: Sacs in the presynaptic terminal that contain neurotransmitters.
• Receptor Region: Area on the postsynaptic neuron that binds to neurotransmitters.
• Synaptic Cleft: The gap between the presynaptic and postsynaptic neurons.
• Voltage-gated Calcium (Ca2+) Channels: Channels in the presynaptic terminal that open when an action potential arrives, allowing calcium to enter the neuron.
• Reuptake: The process by which neurotransmitters are reabsorbed by the presynaptic neuron.
• Excitatory Neurotransmitters: Neurotransmitters that depolarize the postsynaptic neuron, making it more likely to fire.
• Inhibitory Neurotransmitters: Neurotransmitters that hyperpolarize the postsynaptic neuron, making it less likely to fire.
• Serotonin: A neurotransmitter that regulates mood, appetite, circadian rhythm, and sleep.
• Dopamine: A neurotransmitter that influences emotion, attention, and pleasure.
• Norepinephrine: A neurotransmitter that triggers the fight-or-flight response.

The Synapse: The Communication Hub of the Nervous System

The synapse, the meeting point between two neurons, is crucial for turning the nervous system's structure into a functional system. Neurons are connected through synapses, which are essential for communication. The word "synapse" originates from the Greek term meaning "to clasp or join," representing a junction or crossroads.

• Synapse Numbers: The human brain contains approximately 100 billion neurons, each with 1,000 to 10,000 synapses, resulting in 100 to 1,000 trillion synapses in the brain.
• Synapse Function: Each synapse acts as a tiny computer, capable of running multiple programs simultaneously and adapting to neuron firing patterns, strengthening or weakening over time based on usage.
• Synapse Importance: Synapses are vital for learning, memory, and are implicated in psychiatric disorders and drug addiction. They facilitate communication of experiences like euphoria, hunger, desire, confusion, and boredom through electrochemical signals.

Electrical vs. Chemical Synapses: Two Communication Settings

Nerve cells use two main types of synapses for communication, depending on the speed required: electrical and chemical.

Electrical Synapses: The Immediate Group Text

• Mechanism: Electrical synapses involve direct ion current flow from one nerve cell to another through gap junctions.
• Speed: They are extremely fast because the signal remains in its pure electrical state.
• Synchronization: One cell can trigger thousands of others to act in synchrony, as seen in heart muscle cells.
• Control Issues: The direct connection means an action potential in one neuron will automatically generate one in the connected cells. This lack of control is why not all synapses are electrical. If every synapse activated all surrounding neurons, the nervous system would be in constant "group text" mode, leading to exhaustion and system shutdown (death).

Chemical Synapses: The Controlled and Selective Message

• Abundance and Characteristics: Chemical synapses are more abundant, slower, and more precise than electrical synapses.
• Mechanism: They use neurotransmitters, chemical signals that diffuse across the synaptic gap.
• Signal Conversion: Chemical synapses convert the signal from electrical to chemical and back to electrical, allowing for signal modification, amplification, inhibition, or splitting.
• Components:
  • Presynaptic Neuron: The sending cell, transmitting through the presynaptic terminal (usually the axon terminal). This terminal contains synaptic vesicles loaded with neurotransmitters.
  • Postsynaptic Neuron: The receiving cell, accepting neurotransmitters in its receptor region (usually on the dendrite or cell body).
  • Synaptic Cleft: A tiny gap (less than five millionths of a centimeter) between the neurons.

The Chemical Synapse Process: A Step-by-Step Breakdown

Communication via chemical synapses involves a specific chain of events:

1. Action Potential Arrival: An action potential reaches the presynaptic terminal.
2. Calcium Channel Activation: Voltage-gated calcium (Ca2+) channels open, releasing calcium into the neuron's cytoplasm.
3. Vesicle Fusion and Neurotransmitter Release: Calcium ions cause synaptic vesicles to fuse with the cell membrane and release neurotransmitters into the synaptic cleft.
4. Neurotransmitter Diffusion and Binding: Neurotransmitters diffuse across the synaptic gap and bind to receptor sites on the postsynaptic neuron.
5. Signal Conversion Back to Electrical: Neurotransmitter binding to receptors causes ion channels to open, converting the signal back to electrical.
6. Excitation or Inhibition: Depending on the neurotransmitter and receptor, the postsynaptic neuron is either excited (depolarized, more likely to fire) or inhibited (hyperpolarized, less likely to fire).
7. Summation of Signals: A neuron's likelihood of firing depends on the sum of all excitatory and inhibitory signals it receives.

Neurotransmitters: The Body's Chemical Messengers

• Diversity: There are over a hundred different naturally-occurring neurotransmitters in the body.
• Functions: They regulate movement, organ function, mood, alertness, hunger, sleep, and feelings of pleasure.
• Short-Lived Effects: Neurotransmitters only bind to receptors for a few milliseconds before detaching.
• Recycling and Degradation: After delivering their message, neurotransmitters are either reabsorbed by the sending neuron (reuptake), broken down by enzymes, or diffused away from the synapse.

Drug Effects on Synapses: Exploiting the Electrochemical System

Drugs, both legal and illegal, exploit the neurotransmitter system to create desired effects.

• Mechanisms of Action: Drugs can affect neurotransmitter production, release, reuptake, or mimic neurotransmitters.
• Cocaine Example: Cocaine targets serotonin, dopamine, and norepinephrine.
  • Serotonin: Regulates mood, appetite, circadian rhythm, and sleep.
  • Dopamine: Influences emotion, attention, and pleasure.
  • Norepinephrine: Triggers the fight-or-flight response.
• Cocaine's Impact: Cocaine blocks the reuptake of these neurotransmitters, especially dopamine, causing them to accumulate in the synaptic cleft, leading to euphoria, paranoia, and jitteriness.
• Depletion and Adaptation: Prolonged cocaine use depletes neurotransmitter supplies and causes the brain to adapt by losing receptors, requiring more of the drug to function normally.

Synaptic Dysfunction: When Things Go Wrong

Understanding how synapses can malfunction provides insight into their normal function. Healthy synapses maintain a balance between excitation and inhibition, using electrical and chemical signals appropriately. Drug-induced imbalances in this electrochemical system can lead to various dysfunctions.

Conclusion

Synapses are the fundamental communication units of the nervous system, enabling learning, memory, and a wide range of physiological and psychological processes. Understanding the differences between electrical and chemical synapses, the mechanisms of neurotransmitter action, and the impact of drugs on synaptic function is crucial for comprehending brain function and neurological disorders.