Exam 5:How Do Neurons Communicate and Adapt? Part B

Correct Answer:

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Excitatory and inhibitory synapses are two types of connections formed between neurons in the nervous system. They play crucial roles in the modulation of neuronal activity and are essential for various brain functions, including processing of information, learning, and memory. Here are the key differences between them:

1. Function:
- Excitatory synapses promote the firing of an action potential in the postsynaptic neuron. They increase the likelihood that the neuron will pass on the electrical signal.
- Inhibitory synapses decrease the likelihood that the postsynaptic neuron will fire an action potential. They help to dampen or modulate neuronal activity.

2. Neurotransmitters:
- Excitatory synapses typically use neurotransmitters such as glutamate, which is the most common excitatory neurotransmitter in the central nervous system.
- Inhibitory synapses often use neurotransmitters like gamma-aminobutyric acid (GABA) or glycine, which are prevalent inhibitory neurotransmitters in the brain and spinal cord, respectively.

3. Receptor Types:
- Excitatory synapses usually involve ionotropic receptors like AMPA and NMDA receptors for glutamate, which, when activated, lead to the influx of positively charged ions such as sodium (Na+) and calcium (Ca2+) into the postsynaptic neuron.
- Inhibitory synapses often involve ionotropic receptors such as GABA_A and glycine receptors, which typically allow the influx of chloride ions (Cl-) or the efflux of potassium ions (K+), leading to hyperpolarization of the postsynaptic membrane.

4. Postsynaptic Potentials:
- Excitatory synapses generate excitatory postsynaptic potentials (EPSPs), which are small depolarizations that bring the membrane potential closer to the threshold for firing an action potential.
- Inhibitory synapses generate inhibitory postsynaptic potentials (IPSPs), which are hyperpolarizations that take the membrane potential further away from the threshold, making it less likely for an action potential to occur.

5. Synaptic Location:
- Excitatory synapses are often located on the dendritic spines of neurons, which are small protrusions that extend from the dendritic shaft and are specialized for receiving synaptic input.
- Inhibitory synapses are more commonly found on the cell body (soma) or the proximal dendrites of neurons, where they can effectively control the overall excitability of the neuron.

6. Plasticity:
- Excitatory synapses are typically more plastic, meaning they can undergo long-term changes in strength, such as long-term potentiation (LTP) or long-term depression (LTD), which are believed to be cellular mechanisms underlying learning and memory.
- Inhibitory synapses also exhibit plasticity, but the mechanisms and implications of plastic changes at inhibitory synapses are less well understood and are an active area of research.

In summary, excitatory and inhibitory synapses have distinct roles in neuronal communication, with excitatory synapses promoting action potential generation and inhibitory synapses suppressing it. The balance between excitation and inhibition is critical for the proper functioning of neural circuits and the overall activity of the brain.

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The transmission of information across a chemical synapse involves a complex sequence of events that can be broken down into four main steps:

1. **Action Potential Arrival**: The process begins with an action potential (an electrical signal) reaching the axon terminal of the presynaptic neuron. This electrical impulse is generated by the movement of ions across the neuron's membrane and travels along the axon to the synapse.

2. **Neurotransmitter Release**: When the action potential arrives at the axon terminal, it triggers the opening of voltage-gated calcium channels. The influx of calcium ions into the terminal causes synaptic vesicles filled with neurotransmitters (chemical messengers) to fuse with the presynaptic membrane. This fusion is facilitated by a group of proteins known as SNAREs. As a result, neurotransmitters are released into the synaptic cleft, the small gap between the presynaptic and postsynaptic neurons.

3. **Neurotransmitter Binding**: The released neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic neuron's membrane. These receptors are typically specific to the neurotransmitter released, ensuring that the signal is accurately transmitted. The binding of neurotransmitters to these receptors causes them to change shape, which can either directly or indirectly open ion channels in the postsynaptic membrane.

4. **Signal Initiation and Termination**: The opening of ion channels leads to changes in the postsynaptic neuron's membrane potential. If the neurotransmitter is excitatory, the postsynaptic neuron is depolarized, bringing it closer to the threshold for firing an action potential. If the neurotransmitter is inhibitory, the postsynaptic neuron is hyperpolarized, moving it further from the threshold. After the neurotransmitter has exerted its effect, it is removed from the synaptic cleft to terminate the signal. This can occur through reuptake into the presynaptic neuron, enzymatic degradation, or diffusion away from the synapse.

These four steps constitute the basic mechanism by which neurons communicate with each other at chemical synapses, allowing for the complex signaling required for brain function and the coordination of body activities.