Final answer:
The timing of action potentials between neurons is crucial for neuronal communication. EPSPs and IPSPs resulting from this timing lead to the postsynaptic neuron reaching or not reaching the threshold to fire an action potential, with synaptic summation playing a key role in this process.
Step-by-step explanation:
The timing of action potentials between the pre- and post-synaptic neuron is critical for the transmission of signals in the nervous system. When an action potential in the presynaptic neuron occurs before the postsynaptic neuron, it causes the release of neurotransmitters. These neurotransmitters can lead to excitatory postsynaptic potentials (EPSPs) if they depolarize the postsynaptic membrane, making the neuron more likely to fire an action potential. Conversely, they can lead to inhibitory postsynaptic potentials (IPSPs) if they hyperpolarize the postsynaptic membrane, making the neuron less likely to fire.
Summation at the axon hillock is the process where multiple EPSPs, possibly from various presynaptic neurons, add up to depolarize the postsynaptic neuron to its threshold of excitation. On the other hand, if the postsynaptic neuron fires before the presynaptic neuron, there is typically no direct effect on synaptic transmission, unless there is retrograde signaling involved. In such cases, the postsynaptic neuron can influence the presynaptic neuron's activity.
Synaptic summation and threshold for excitation act together as a filter to ensure that only significant signals lead to an action potential while random 'noise' is not transmitted as important information. This fine-tuning is essential for proper neuronal communication and function.
In summary, the sequences of pre- then post- or post- then pre-firing have distinct effects on the potential and transmission of neuronal signals.