Neuroscientists are delving deep into the complex interactions between three types of neurons in the brain: excitatory, inhibitory, and the rarer neuromodulatory neurons.
Understanding how these cells work together is crucial, as it may lead to insights on how to reestablish balance within neural networks.
This imbalance can occur due to various reasons, including the natural aging process.
At the heart of this exploration are excitatory and inhibitory neurons, which communicate through neurotransmitters that traverse synapses, the tiny gaps between neurons.
The primary distinction between these two types of neurons lies in the neurotransmitter they release.
Excitatory neurons predominantly release glutamate, which triggers the influx of positive ions into a neighboring neuron.
This influx raises the neuron’s internal voltage, prompting it to fire an action potential—a significant electric impulse that travels down its fiber.
This action potential enables the neuron to communicate with others.
Conversely, inhibitory neurons release GABA, which prompts negatively charged ions to enter the next neuron or positive ions to exit.
As a result, the internal voltage of the inhibited neuron decreases, preventing it from firing.
Inhibitory neurons effectively serve as brakes in the brain’s signaling system, maintaining balance and ensuring that messages are correctly transmitted and received throughout the network.
In the mammalian cortex, there is a notable imbalance, with excitatory neurons significantly outnumbering their inhibitory counterparts.
However, throughout mammalian brain evolution, inhibitory neurons have become more diverse and numerous, indicating their vital role in higher cognitive functions.
Historically, inhibitory neurons have been viewed as having supportive roles, primarily because excitatory neurons are easier to study.
For instance, excitatory place cells in the hippocampus can be observed firing in response to specific locations, making their functions clear.
In contrast, the activity of inhibitory neurons has often appeared less focused, making it challenging to identify the precise signals they inhibit.
Recent research highlights that inhibitory neurons are more responsive and specific than previously acknowledged.
A study published in Nature established that these neurons play an essential role in helping mice quickly learn and remember food locations.
By reducing their firing frequency as the mice approached certain spots, inhibitory neurons amplify desired signals, facilitating memory formation about those critical sites.
This finding suggests that inhibitory neurons are far more active and influential in cognitive processes than once believed.
Moreover, the conceptualization of inhibitory neurons as generalists, providing indiscriminate inhibition over their adjacent neurons, has also been challenged.
As part of the Microns project, Nuno Maçarico da Costa and his team have discovered that many inhibitory neurons are selective in choosing which neurons to inhibit based on unique connectivity patterns.
The brain’s neural circuits consist of a harmonious mix of inhibitory and excitatory cells, engaged in various intricate interactions.
Different types of inhibitory neurons may target distinct regions of other neurons, such as dendrites or cell bodies.
Some inhibitory cells may even collaborate to inhibit certain other neurons, resulting in a layered and multifaceted system.
This complexity is essential for orchestrating our thoughts, actions, memories, and consciousness.
While the speed of neuronal communication can occur in mere milliseconds, the cognitive effects often unfold over much longer timescales.
Neurotransmitters can act within fractions of a second, but the behavioral and cognitive outputs they generate are generally much slower.
This disparity between rapid signal transmission and prolonged cognitive outcomes presents a significant puzzle for neuroscientists.
In this context, neuromodulatory neurons could play a critical role in bridging the timing gap.
These rarer neurons work on slower timescales, yet their impacts tend to be more widespread and longer-lasting.
Unlike traditional transmitters, which act on direct synapses, neuromodulators disperse their molecules throughout a broader area, influencing multiple synapses and leading to significant changes in neuronal activities.
For instance, the neuromodulator norepinephrine is known to enhance emotional memory.
When released, it strengthens synaptic connections between neurons that form memory, thereby influencing which memories are retained and how they are reinforced.
Despite the clear categorizations of excitatory, inhibitory, and neuromodulatory neurons, the boundaries between these roles can often blur.
Some neurons may function as both excitatory and inhibitory, releasing a combination of GABA and glutamate, thereby showcasing dual characteristics.
Additionally, under conditions of stress or other chronic situations, some neurons may even switch from one type to another, altering their roles within the neural network.
As research continues to unfold, the diversity present within these neuronal categories reveals profound implications for understanding brain function and dysfunction.
Neuroscientists are just beginning to uncover the ramifications of imbalances within these networks, a quest that has the potential to enhance treatment approaches for neurological disorders.
As Cembrowski emphasizes, advancing this understanding could significantly improve individuals’ quality of life, with broad implications for society as a whole.
image source from：quantamagazine
