Difference Between Temporal And Spatial Summation

Temporal and spatial summation are two fundamental processes by which neurons integrate multiple synaptic potentials to generate an action potential. But understanding these mechanisms is crucial to comprehending how the nervous system processes information. But both involve adding together excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) to determine whether a neuron will fire. On the flip side, they differ significantly in how these potentials are accumulated and processed.

Temporal Summation: Summing Over Time

Temporal summation occurs when a single presynaptic neuron fires multiple times in quick succession, causing successive postsynaptic potentials to overlap and add together.

The Underlying Mechanism

At the heart of temporal summation lies the principle of synaptic potentials having a finite duration. Day to day, this change, either an EPSP (depolarization) or an IPSP (hyperpolarization), isn't instantaneous. When a neurotransmitter is released from a presynaptic neuron, it binds to receptors on the postsynaptic neuron, causing a change in the postsynaptic membrane potential. It takes time for the ion channels to open, ions to flow across the membrane, and the potential to return to its resting state.

If the presynaptic neuron fires again before the first postsynaptic potential has completely decayed, the second potential adds to the remaining potential from the first firing. If this process repeats rapidly enough, the summed potential can reach the threshold required to trigger an action potential at the axon hillock.

Factors Influencing Temporal Summation

Several factors can influence the effectiveness of temporal summation:

• Time Constant (τ): The time constant of a neuron is a measure of how quickly its membrane potential decays. Neurons with a longer time constant are more effective at temporal summation because the postsynaptic potentials linger for a longer duration, increasing the likelihood of overlap.
• Firing Frequency of the Presynaptic Neuron: The higher the firing frequency of the presynaptic neuron, the more closely spaced the postsynaptic potentials will be, and the greater the summation.
• Synaptic Delay: The synaptic delay, the time it takes for the neurotransmitter to be released, diffuse across the synaptic cleft, and bind to receptors, can also affect temporal summation. A shorter synaptic delay allows for more rapid succession of postsynaptic potentials.
• Membrane Resistance (Rm): Higher membrane resistance means fewer ions leak out of the cell, allowing the potential change to last longer and summate more effectively.

Example of Temporal Summation

Imagine a sensory neuron receiving input from a pressure receptor in your skin. Now, if the pressure is weak and the sensory neuron fires slowly, each postsynaptic potential in the connected interneuron might be too small to trigger an action potential on its own. That said, if the pressure increases and the sensory neuron fires more rapidly, the successive postsynaptic potentials in the interneuron can summate over time, eventually reaching the threshold and triggering an action potential. And that's what lets you perceive even subtle changes in pressure.

Spatial Summation: Summing Over Space

Spatial summation occurs when multiple presynaptic neurons fire simultaneously or nearly simultaneously, causing postsynaptic potentials at different locations on the postsynaptic neuron to sum together.

The Underlying Mechanism

Neurons receive input from thousands of other neurons, and these synapses are distributed across the dendrites and soma of the postsynaptic neuron. When multiple presynaptic neurons fire around the same time, each generates a postsynaptic potential at its respective synapse. These potentials then spread passively towards the axon hillock.

Spatial summation depends on the cable properties of the neuron, specifically how effectively electrical signals travel through the dendrites. Plus, if the EPSPs generated at different locations arrive at the axon hillock close enough in time, they can add together. Similarly, IPSPs can cancel out EPSPs, reducing the likelihood of the postsynaptic neuron firing.

Factors Influencing Spatial Summation

Several factors can influence the effectiveness of spatial summation:

• Distance from the Axon Hillock: Synapses closer to the axon hillock have a greater influence on the neuron's firing. Postsynaptic potentials generated at more distant synapses are more likely to decay before reaching the axon hillock.
• Dendritic Diameter: Dendrites with larger diameters offer less resistance to current flow, allowing postsynaptic potentials to travel more efficiently towards the axon hillock.
• Branching of Dendrites: Extensive dendritic branching can increase the surface area available for synapses, but it can also increase the distance signals must travel to reach the axon hillock.
• Simultaneous Activity of Multiple Synapses: The more synapses that are active simultaneously, the greater the likelihood of spatial summation.
• Location of Inhibitory Synapses: Inhibitory synapses located close to the soma or axon hillock can effectively veto excitatory signals arriving from more distant synapses.

Example of Spatial Summation

Consider a motor neuron receiving input from multiple interneurons in the spinal cord. Neither input alone might be strong enough to trigger an action potential in the motor neuron. One interneuron might be activated by a stretch receptor in a muscle, while another is activated by a pain receptor in the skin. That said, if both interneurons fire simultaneously, the EPSPs they generate can sum together spatially, depolarizing the motor neuron sufficiently to reach threshold and cause a muscle contraction.

Key Differences Between Temporal and Spatial Summation

While both temporal and spatial summation serve the same overall purpose – integrating synaptic potentials to determine neuronal firing – they differ in several key aspects:

The Interplay of Temporal and Spatial Summation

In reality, temporal and spatial summation rarely occur in isolation. Most neurons receive a complex barrage of synaptic inputs that vary in timing and location. So, the decision of whether a neuron will fire is usually based on a combination of both temporal and spatial summation.

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To give you an idea, a neuron might receive weak excitatory input from several different presynaptic neurons. While the individual EPSPs generated by these neurons might be too small to trigger an action potential on their own, they can sum together spatially. If one of these presynaptic neurons also fires at a high frequency, the temporal summation of its EPSPs can further boost the overall depolarization, potentially pushing the neuron over the threshold.

The interplay between temporal and spatial summation allows neurons to perform complex computations and integrate diverse sources of information. It provides a flexible mechanism for the nervous system to respond to a wide range of stimuli and generate appropriate behavioral outputs.

The Significance of Summation in Neural Processing

Temporal and spatial summation are fundamental to neural processing for several reasons:

• Signal Amplification: They allow weak or subthreshold synaptic inputs to trigger an action potential. This is crucial for detecting subtle changes in the environment and responding to faint stimuli.
• Noise Reduction: By requiring the simultaneous or rapid activation of multiple synapses, summation reduces the likelihood of spurious firing due to random fluctuations in membrane potential or background noise.
• Integration of Information: Summation allows neurons to integrate information from multiple sources, weighing the relative importance of different inputs and making complex decisions.
• Neural Plasticity: The efficacy of synaptic transmission can be modified by experience, leading to changes in the strength of synaptic connections. Summation provides a mechanism for these changes to influence neuronal firing and contribute to learning and memory.

Clinical Relevance

Understanding temporal and spatial summation is also relevant to various clinical conditions:

• Epilepsy: In some forms of epilepsy, there is an imbalance between excitation and inhibition in the brain, leading to excessive neuronal firing. Understanding how summation contributes to neuronal excitability can help in developing new treatments for epilepsy.
• Neuropathic Pain: Chronic pain conditions can result from changes in synaptic transmission and neuronal excitability. Understanding the role of summation in pain pathways can lead to more effective pain management strategies.
• Neurodegenerative Diseases: Diseases like Alzheimer's and Parkinson's disease can affect synaptic function and neuronal communication. Understanding how summation is impaired in these diseases can provide insights into their underlying mechanisms and potential therapeutic targets.
• Drug Action: Many drugs that affect the nervous system, such as antidepressants and antipsychotics, work by modulating synaptic transmission and neuronal excitability. Understanding how these drugs influence summation can help to optimize their therapeutic effects and minimize side effects.

Further Elaboration on Neuronal Cable Properties

As noted, spatial summation is highly dependent on the cable properties of neurons. These properties describe how electrical signals passively spread through the dendrites and soma. Here's a more closer look:

• Membrane Resistance (Rm): This refers to the resistance of the neuronal membrane to the flow of ions. A higher membrane resistance means fewer ions leak out of the cell, allowing the potential change to last longer and travel further.
• Axial Resistance (Ra): This refers to the resistance of the cytoplasm inside the dendrite to the flow of current. A lower axial resistance allows current to flow more easily along the dendrite.
• Membrane Capacitance (Cm): This refers to the ability of the neuronal membrane to store electrical charge. A higher membrane capacitance slows down the rate of change of the membrane potential.
• Length Constant (λ): The length constant is a measure of how far a signal can travel along a dendrite before it decays to 37% of its original amplitude. It is determined by the membrane resistance, axial resistance, and membrane capacitance. A longer length constant allows postsynaptic potentials to travel further and summate more effectively.

The cable properties of neurons vary depending on the type of neuron and its location in the nervous system. As an example, neurons with large-diameter dendrites and high membrane resistance will have longer length constants and be more effective at spatial summation.

Computational Models of Summation

Computational neuroscientists have developed various mathematical models to simulate temporal and spatial summation. These models can be used to:

• Investigate the effects of different parameters: Models can be used to study how changes in the time constant, cable properties, or synaptic strength affect neuronal firing.
• Test hypotheses about neural processing: Models can be used to test hypotheses about how the nervous system integrates information and generates behavior.
• Develop new treatments for neurological disorders: Models can be used to simulate the effects of drugs or other interventions on neuronal activity and identify potential therapeutic targets.

These models range from simple integrate-and-fire models to more complex compartmental models that simulate the electrical properties of individual dendrites. By using these models, researchers can gain a deeper understanding of the complex processes underlying temporal and spatial summation.

Conclusion

Temporal and spatial summation are essential mechanisms that enable neurons to integrate synaptic inputs and generate action potentials. In practice, both processes are influenced by various factors, including the time constant, cable properties, and firing frequency of the presynaptic neurons. Temporal summation involves the summation of postsynaptic potentials over time, while spatial summation involves the summation of postsynaptic potentials across different locations on the neuron. Understanding these mechanisms is crucial for comprehending how the nervous system processes information, responds to stimuli, and generates behavior. What's more, a deeper understanding of temporal and spatial summation is vital for addressing neurological disorders and developing targeted therapeutic interventions.