Notes on Dendritic Integration and Postsynaptic Potentials

Dendritic reception and synaptic signaling

In brain and behaviour, dendrites on a postsynaptic neuron act as the reception zone for information from other neurons. The postsynaptic neuron’s dendrite receives synapses from presynaptic neurons (illustrated by red and blue cells).
When presynaptic neurons fire, they release neurotransmitters into the synaptic cleft. These neurotransmitters bind to receptors on the postsynaptic dendrite.
Binding of neurotransmitters to postsynaptic receptors changes the membrane potential of the postsynaptic cell. This small shift in membrane potential is called a postsynaptic potential (PSP).
PSPs can be excitatory or inhibitory, depending on the neurotransmitter type and receptor type:
- EPSP: excitatory postsynaptic potential, depolarizes the membrane (makes the neuron more likely to fire).
- IPSP: inhibitory postsynaptic potential, hyperpolarizes the membrane (makes the neuron less likely to fire).
Example from the slide:
- The pink presynaptic neuron produces an EPSP, depolarizing the postsynaptic cell.
- The blue presynaptic neuron produces an IPSP, inhibiting the postsynaptic cell.
The net effect on the postsynaptic neuron is determined by the balance between EPSPs and IPSPs.
The relationship between input and output is not a one-to-one mapping; it depends on how many EPSPs/IPSPs the neuron receives and how they sum over time and space.

EPSP and IPSP definitions and roles

EPSP (excitatory postsynaptic potential): depolarizes the postsynaptic membrane toward the threshold for firing an action potential.
IPSP (inhibitory postsynaptic potential): hyperpolarizes the postsynaptic membrane, moving away from the firing threshold.
Both EPSPs and IPSPs cause small shifts in the membrane potential, and their combined effect determines whether the neuron reaches the firing threshold.
Neurons can receive inputs from many presynaptic neurons via multiple receptor types, leading to complex integration at the dendrites.

Integration: spatial and temporal summation

Spatial summation: inputs arriving at different locations on the dendritic tree can summate at the soma to influence the cell body membrane potential. If enough EPSPs occur across locations, their combined depolarization can reach threshold and trigger an action potential.
If many IPSPs occur, they can hyperpolarize the membrane and counteract excitation, making firing less likely.
Temporal summation: inputs arriving close in time at the same location (or nearby) sum more effectively because the individual PSPs overlap in time before decay, increasing the chance of reaching threshold.
In the presence of both EPSPs and IPSPs from different presynaptic neurons, the overall neuronal response is a balance of excitation and inhibition, shaping whether an action potential is generated and how frequently it fires.
If IPSPs outweigh EPSPs, the neuron may hyperpolarize and fail to fire, even with excitatory input present.
If EPSPs outweigh IPSPs, the neuron may reach threshold and fire, producing an action potential whose strength is effectively the same once threshold is crossed (an all-or-none event for the spike itself).

Firing threshold and firing rate

Whether an action potential is generated depends on achieving threshold depolarization at the soma.
The firing rate of the postsynaptic neuron reflects the integration of inputs: more excitation or better temporal alignment can increase firing rate; substantial inhibition can decrease it.
The action potential is typically all-or-none once the threshold is reached, meaning the same spike property (e.g., amplitude) irrespective of the exact subthreshold input prior to firing.
In summary: the cell’s response is governed by the balance of excitation and inhibition across time and space, which shapes the probability and rate of firing.

Dendritic complexity and integration across the dendritic tree

Dendrites can be highly branched, receiving EPSPs and IPSPs from many locations and at many times.
Signals generated at different dendritic locations can sum at the soma to affect the overall membrane potential.
The closer in time two PSPs are, the more likely they are to sum effectively, amplifying their influence on the somatic membrane potential.
The more complex the dendritic tree, the richer the temporal and spatial integration landscape, enabling nuanced processing of synaptic inputs.

Sensory system example: from touch to firing rate

Neuron in a sensory pathway may receive input from receptors in skin (e.g., pressure sensors).
Light touch: lower firing rate (fewer action potentials).
Higher force: higher firing rate (more action potentials per unit time).
This firing rate may reflect not only the intensity of touch but also inputs from nearby skin patches and different skin receptors (e.g., stretch sensors), allowing more complex interpretation of tactile stimuli.
The rate of firing encodes aspects of stimulus magnitude and potentially quality (through integration with other sensory inputs).

Neuroscience measurement techniques and what they reveal

Electrophysiology techniques study electrical activity within cells in living brain tissue or in cultured neurons:
- Very fine electrodes can measure single-neuron activity in vivo or in vitro.
- Neurons can be kept alive on a dish to study their properties and responses to stimuli.
In humans, invasive single-neuron recordings are rare and typically used in clinical contexts; more common are noninvasive methods:
- Electroencephalography (EEG): records electrical activity from the scalp, reflecting the summed activity of vast numbers of neurons across cortex.
- EEG electrodes on the scalp measure the combined electrical potential of many neurons, not single cells, but can reveal patterns associated with states like sleep or epileptic seizures.
Practical applications and contexts:
- EEG is used in epilepsy diagnosis (look for abnormal, uncontrolled electrical activity in cortex).
- EEG is used in sleep studies to identify characteristic patterns of activity at different sleep stages.
The field relies on both invasive and noninvasive approaches, and cross-species extrapolation is common due to ethical and practical constraints of human invasive experiments.

Clinical relevance: epilepsy, sleep, and brain monitoring

Epilepsy testing often involves EEG to detect abnormal synchronization of neural activity, which can manifest as seizures.
Sleep studies use EEG to analyze patterns that occur during different sleep stages and transitions.
These clinical contexts illustrate how large-scale neuronal firing patterns relate to observable physiology and behavior.

Research context and ethical considerations

Much of our understanding of neuronal activity comes from single-neuron studies and animal research due to the invasive nature of measuring electrical activity directly from neurons.
In humans, ethical considerations limit invasive recording; hence noninvasive approaches like EEG are essential, even though they measure aggregated activity rather than single neurons.
Cross-species generalization is a key assumption grounded in fundamental principles of neuronal function, supported by observed similarities across the animal kingdom and humans.
Ethical and philosophical implications include balancing scientific knowledge with animal welfare, the limitations and benefits of invasive human research, and the importance of noninvasive methods for clinical and ethical reasons.

Mathematical notes: how integration is represented

Let V_m represent the membrane potential at the soma.
The net postsynaptic potential is the sum of individual PSPs:
Vm = \sumi PSP_i.
An action potential is triggered when the membrane potential reaches a threshold value Vth: $ext{If } Vm \ge V_{th}, \text{ then an action potential is fired}.$
Temporal summation of a single input at time ti with decay constant τ can be represented as a decaying postsynaptic potential: $PSPi(t) = Ai \, e^{-\frac{t - ti}{\tau}} \quad (t \ge ti),$ where Ai is the peak amplitude of the i-th PSP at its time of occurrence t_i.
The combined effect of multiple PSPs over time yields the instantaneous membrane potential, influencing firing probability and rate.

Connections to foundational concepts and real-world relevance

Builds on core ideas of membrane potential, ion channel activity, and action potentials interpreted in previous lectures.
Demonstrates how microscopic synaptic changes scale up to macroscopic signals measured in EEG and observed in behavior.
Highlights how neural coding (firing rate) links neural activity to sensory perception, decision-making, and motor output.

Summary and takeaways

Dendrites collect inputs via EPSPs and IPSPs from multiple presynaptic neurons. The postsynaptic neuron's output depends on the balance and timing of these inputs.
Spatial summation (across locations) and temporal summation (over time) determine whether the neuron reaches threshold to fire an action potential.
The firing rate of a neuron encodes information about stimulus intensity and other contextual inputs, contributing to complex perception and behavior.
Neuroscience uses a range of techniques, from single-cell recordings to EEG, to understand neural activity in humans and animals. Clinical tools like EEG have practical uses in epilepsy and sleep research.
Ethical considerations shape how research is conducted across species and how findings translate to humans.