Temporal summation is primarily caused by a rapid sequence of action potentials from a single presynaptic neuron, where each subsequent postsynaptic potential arrives before the previous one has fully decayed, leading to their cumulative effect on the postsynaptic neuron's membrane potential.
Understanding Temporal Summation
In the intricate network of the brain and nervous system, neurons communicate through electrical and chemical signals. When a presynaptic neuron transmits a signal to a postsynaptic neuron, it generates a postsynaptic potential (PSP). These PSPs can be either excitatory (EPSPs), making the postsynaptic neuron more likely to fire an action potential, or inhibitory (IPSPs), making it less likely. Temporal summation is a crucial mechanism that allows these individual PSPs to add up over time.Key Factors Contributing to Temporal Summation
Temporal summation arises from a combination of specific conditions within the synaptic transmission process:-
High Frequency of Presynaptic Action Potentials:
When a single presynaptic neuron fires action potentials at a rapid rate, it repeatedly releases neurotransmitters into the synaptic cleft. This high-frequency bombardment ensures that a series of postsynaptic potentials are generated in quick succession. -
Prolonged Postsynaptic Potential (PSP) Duration:
A critical condition for temporal summation is that the duration of a postsynaptic potential (whether an EPSP or IPSP) is longer than the interval between incoming action potentials. This means that before one PSP fully fades away, another one arrives and overlaps with it.Example: Imagine a series of small waves hitting a beach. If the waves arrive slowly, each wave dissipates before the next one hits. But if they arrive rapidly, each new wave builds on the energy of the previous one, creating a larger overall surge. Similarly, in a neuron, rapid, successive PSPs accumulate because the previous potential has not yet fully decayed, leading to an amplified effect.
Factor Description Contribution to Summation Presynaptic Firing Rate A single presynaptic neuron sends impulses (action potentials) at a very high frequency. Ensures a rapid succession of neurotransmitter release and thus, postsynaptic potentials. PSP Duration vs. Interval The period during which a postsynaptic potential is active is longer than the time gap between individual incoming action potentials. Allows successive postsynaptic potentials to overlap and add to one another, instead of dissipating independently.
The Result: Reaching Threshold
This cumulative effect is vital for neuronal integration. If enough excitatory postsynaptic potentials (EPSPs) summate temporally, they can collectively depolarize the postsynaptic neuron's membrane potential to its **threshold potential**. Once the threshold is reached, the postsynaptic neuron will fire its own action potential, propagating the signal further. Conversely, inhibitory postsynaptic potentials (IPSPs) can summate to hyperpolarize the neuron, making it less likely to fire.Understanding temporal summation provides insight into how the nervous system processes information by integrating multiple sub-threshold inputs from a single source over time to generate a significant output.
