The Architecture of Thought: Part 3—Neural Coding and Neurotransmitters

So far, in our series on the architecture of thought, we’ve dissected the conductive power of neurons and the vast interconnected web of the peripheral nervous system. The nervous system is an incredibly complex machine, so you may be feeling overwhelmed by now (if you’re not overwhelmed, consider this: your nervous system is currently trying to understand the nervous system). Before we dive into the next section, let’s take a moment to ground ourselves using my favorite grounding exercise. Take a deep breath and settle into the space around you. Now, look around you and name five things you can see, four things you can hear, three things you can feel, two things you can smell, and one thing you can taste. Not only does this exercise soothe anxiety by helping to trigger the parasympathetic division of the autonomic nervous system, but it is also a useful demonstration of the range of sensory information our nervous system manages from moment to moment. During the exercise, you forced your brain to consciously focus on fifteen discrete pieces of sensory information. But at any given moment, your nervous system collects thousands of individual information “packets” from your various sensory organs. In order to transmit this cache of raw information to the brain, neurons have to convert it into a complex binary language of electrical signals and chemical neurotransmitters. These signals influence how you perceive those sensations—whether they all dissolve into an overwhelming jumble of stimuli or if one stimulus rises to the front of your consciousness. And the way we perceive the world can even change day to day, moment to moment, based on the levels of certain neurotransmitters in the brain and the complex interplay of neurons.

At the level of an individual neuron, excitation is an all-or-nothing affair—either the neuron reaches the threshold potential and completely depolarizes, or it doesn’t. But that excitation doesn’t have to all come at once or from the same source. Smaller excitations—called graded potentials—can sum together in succession or from different points of contact to create a full depolarization. On the sensory level, a single physical or chemical stimulus from the environment may not be enough to trigger full depolarization, but if it is persistent, the summation of the potentials over time will generate an action potential. This is called temporal summation, where graded potentials build up over time to trigger the neuron. Temporal summation can also affect how signals travel from one neuron to the next. Repetition of a relatively weak signal from an adjacent neuron can sum together to create an ongoing action potential. Graded potentials from multiple inputs—spatial summation—can also generate an action potential. Depending on their function, neurons can often form vast connection webs where their dendrites interface with multiple adjacent axons. An incoming signal from just one of those neurons may not be strong enough to trigger ongoing change, but signals from multiple sources can sum together to generate the full action potential. Temporal and spatial summation often work together, allowing the nervous system to filter stimuli and coordinate the electrical propagation of information. They also help to minimize the risk of random misfiring by preventing the propagation of isolated signals.

Graded potentials can come in many different shapes, strengths, and flavors. Stronger stimuli push the membrane potential up more than weaker stimuli. More consistent stimuli may result in a longer, more drawn out graded potential. Some stimuli can even result in graded potentials that are hyperpolarizing—they reduce the membrane potential. In the summation of signals, hyperpolarizing graded potentials act as subtractors, reducing the likelihood of an action potential. Hyperpolarization helps neurons carefully orchestrate the patter of firing neurons, which influences how a sensation is perceived. But you may be wondering how neurons modulate and control the movement of graded potentials. While some of these potentials are conducted electrically, the real nuance of graded potentials comes from the chemical interactions of neurotransmitters.

When an action potential reaches the end of the axon, it triggers the release of packaged neurotransmitters into the synaptic cleft between the presynaptic axon and the postsynaptic dendrite. As neurotransmitters flood this intercellular space, they diffuse and bind to specific receptors on the dendrite. Different receptors can have different responses to different neurotransmitters, so changing the receptor makeup of its dendrites is one way a neuron can modulate how it receives signals. There are hundreds of diverse neurotransmitters that affect neuron signaling, but many of them fall into a few general categories.

One of the primary neurotransmitter systems of peripheral motor neurons is the cholinergic system, which involves the binding of the organic cation acetylcholine to specific dendrite receptors. In most cases, acetylcholine triggers depolarization by opening ion channels. But when it binds to specific receptors that are not associated with ion channels, the same neurotransmitter can trigger hyperpolarization instead. The release of acetylcholine by motor neurons directly interfacing with muscle cells triggers muscle contraction. In the central nervous system, acetylcholine also acts as a neuromodulator—a chemical signal that modulates the activity of many local neurons. As a neuromodulator, acetylcholine helps us maintain attention and focus on specific stimuli.

Another broad grouping of neurotransmitters is amino acids—the small molecule building blocks of proteins. Outside of their role in building proteins, amino acid neurotransmitters like glycine, glutamate, and the glutamate derivative GABA can excite or inhibit neurons by binding to specific receptors. These amino acids are important for promoting or inhibiting signals in the central nervous system. Ion-channel receptors for glutamate trigger depolarization, while those for glycine and GABA trigger hyperpolarization that can slow a signal down. Action potential inhibition can be crucial in the tightly packed neural net of the central nervous system. Decreased GABA activity can be responsible for the rampant excitation that causes seizure disorders like epilepsy. And benzodiazepines—drugs prescribed for anxiety disorders—function by enhancing the activity of GABA that slows down neuron firing in the brain.

In addition to amino acids, amino acid precursors called biogenic amines are another diverse group of neurotransmitters. This is one of the most diverse groups of neurotransmitters in terms of function. The biogenic amine dopamine is responsible for coordinating movement—dopamine malfunction causes the motor dysfunction characteristic of Parkinson’s disease. Dopamine also acts to reinforce and reward certain behaviors, which is why certain drugs that stimulate dopamine can become addictive. A similar pair of biogenic amines norepinephrine and epinephrine (also known as adrenaline) double as hormones. These versatile molecules are responsible for triggering functions of the sympathetic nervous system that respond to perceived threats—real or imaginary. Histamine is another type of biogenic amine that promotes arousal and attention in the central nervous system. Immune cells also release histamine in response to infection or allergens, stimulating inflammation. Many people take antihistamines during allergy season to curb this response. If the antihistamine is capable of passing through the blood-brain barrier, it can interfere with histamine’s neurotransmitter function—causing drowsiness and inattention. Lastly, the biogenic amine serotonin has a wide array of functions including regulation of sleep, appetite, and endocrine activity. Serotonin dysfunction has been linked to many mood disorders like depression (although this connection is disputed), so many drugs for treating depression are selective serotonin reuptake inhibitors (SSRIs) that keep serotonin floating around in the synaptic cleft for longer.

Neurotransmitters and their specialized receptors help neurons finely modulate the way signals are propagated, which is crucial for controlling how a signal is perceived and processed. In a simplified sense, the nervous system computes information about the outside world using a binary code of firing and non-firing neurons. The information is encoded in the rate and pattern of neuron firing—called neural coding. Neural coding is a remarkably complex computational system that straddles the line between analog and digital. There is still so much we don’t know about how the nervous system encodes information, which makes interpreting and manipulating brain activity difficult. Understanding the intricacies of the neural code is the primary hurdle we need to overcome to replicate neural pathways in mechanical systems—like prosthetics or artificial intelligence.

In next month’s series, we’ll delve deeper into how neural coding and neurotransmitters relate sensation to perception. But first, we need to take a look at the last level of nervous system organization—the architecture of the brain. More on that next week! For now, check out last month’s series on influential Black scientists of history. Comment on this post or email me at contact@anyonecanscience.com to let me know what you think about this week’s blog post and tell me what sorts of topics you want me to cover in the future. And subscribe below for weekly science posts sent straight to your email!