C2.2.13 Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials (HL) Notes

Inhibitory Neurotransmitters Reduce Neuronal Firing by Generating IPSPs

1. Neurotransmitters play a critical role in transmitting signals between neurons.
2. While some neurotransmitters are excitatory and increase the likelihood of action potentials, others are inhibitory and reduce the likelihood of neuronal firing by generating inhibitory postsynaptic potentials (IPSPs).

Inhibitory Neurotransmitters Hyperpolarize Neurons to Prevent Action Potentials

1. Instead of depolarizing the postsynaptic membrane (as excitatory neurotransmitters do), inhibitory neurotransmitters make the membrane potential more negative, a process known as hyperpolarization.
2. This occurs through the opening of channels that allow negatively charged ions to enter or positively charged ions to leave the neuron.

How Inhibitory Neurotransmitters Generate IPSPs

1. Neurotransmitter Release: When an action potential reaches the presynaptic terminal, it triggers the release of inhibitory neurotransmitters into the synapse.
2. Binding to Postsynaptic Receptors: These inhibitory neurotransmitters bind to specific receptors on the postsynaptic membrane.
   1. Common inhibitory neurotransmitters include gamma-aminobutyric acid (GABA) and glycine.
3. Opening of Ion Channels: Upon binding to their receptors, the neurotransmitters cause the opening of chloride (Cl-) channels or potassium (K+) channels:
   1. Chloride ions (Cl-): When GABA binds to its receptor, it often opens chloride channels, allowing Cl- ions to flow into the postsynaptic neuron.
   2. Potassium ions (K+): In some cases, the neurotransmitter can open potassium channels, allowing K+ ions to flow out of the neuron.
4. Hyperpolarization: The movement of negatively charged ions (Cl-) into the neuron or positively charged ions (K+) out of the neuron makes the inside of the cell more negative, causing a hyperpolarization of the postsynaptic membrane.
   1. This makes it more difficult for the neuron to reach the threshold potential required for an action potential, reducing the likelihood of firing.

Why Hyperpolarization Matters

1. Resting Potential: Neurons at rest have a membrane potential of about $-70 mV$.
2. Threshold for Action Potential: To fire an action potential, the membrane potential must reach a threshold of about $-50 mV$.
3. Hyperpolarization: Inhibitory neurotransmitters make the membrane potential even more negative (e.g., $-75 mV$), moving it further from the threshold.
4. Result: The neuron becomes less likely to fire, effectively suppressing the signal.

Balancing Excitation and Inhibition

Neurons receive inputs from both excitatory and inhibitory neurotransmitters. The net effect determines whether the neuron fires.

1. Excitatory Inputs: Move the membrane potential closer to the threshold.
2. Inhibitory Inputs: Move the membrane potential further away from the threshold.
3. Integration: The neuron integrates these signals to decide whether to fire an action potential.

Why Are Inhibitory Neurotransmitters Important?

1. Preventing Overactivity: Without inhibition, neurons could fire uncontrollably, leading to conditions like seizures.
2. Fine-Tuning Responses: Inhibition helps the nervous system filter out unnecessary signals, allowing focus on important tasks.
3. Maintaining Balance: A balance between excitation and inhibition is essential for normal brain function.