C2.2.14 Summation of the effects of excitatory and inhibitory neurotransmitters (HL) Notes

Inhibitory Neurotransmitters Hyperpolarize Neurons to Prevent Action Potentials

1. Neural communication relies on the integration of signals from multiple presynaptic neurons to determine whether a postsynaptic neuron will generate an action potential.
2. This integration occurs through a process known as summation, where the combined effects of excitatory neurotransmitters (which depolarize the membrane) and inhibitory neurotransmitters (which hyperpolarize the membrane) determine the postsynaptic response.
3. If the threshold potential is reached, the postsynaptic neuron will fire an action potential, illustrating the all-or-nothing principle.

How Summation Works

1. Spatial Summation

1. Spatial summation occurs when multiple presynaptic neurons send simultaneous signals to a single postsynaptic neuron.
2. The combined effect of these signals is evaluated by the postsynaptic neuron at the same time.

How does it work?

1. Signals can come from multiple synapses located at different places on the postsynaptic neuron's dendrites or cell body.
2. If enough excitatory signals reach the postsynaptic neuron at the same time, the membrane potential will depolarize and, if it reaches the threshold potential, will trigger an action potential.
3. Inhibitory signals can counteract the excitatory signals. If there are enough inhibitory signals, they may hyperpolarize the postsynaptic neuron, preventing it from firing.

2. Temporal Summation

1. Temporal summation occurs when a single presynaptic neuron fires action potentials rapidly in a short period.
2. The effects of the signals build up over time, as the postsynaptic neuron doesn’t have time to fully return to its resting potential before the next signal arrives.

How does it work?

1. A single presynaptic neuron releases neurotransmitters at a very fast rate, leading to repeated depolarization of the postsynaptic neuron.
2. If the action potentials arrive in quick succession, their effects can add up, leading to a larger depolarization of the postsynaptic membrane.
3. This process is particularly important in rapid signaling, like in reflexes or other processes that require fast, sustained neuronal firing.

All-or-Nothing Consequences of Postsynaptic Depolarization

1. Neurons follow an all-or-nothing principle in terms of action potential generation.
2. This means that once the threshold potential is reached, the neuron will always fire an action potential. There is no partial firing.
3. The strength of the signal transmitted down the axon is constant, what varies is the frequency of action potentials, not their size.

Why Summation Matters

1. Integration of Information: Neurons receive inputs from multiple sources, and summation allows them to weigh these inputs and make decisions.
2. Fine-Tuned Control: The balance between excitatory and inhibitory signals ensures that the nervous system responds appropriately to stimuli.
3. Complex Processing: Summation underlies higher-order functions like decision-making and sensory processing in the brain.