The human nervous system operates on a balance of opposing signals, which act as “go” and “stop” commands for every action, thought, and sensation. This interplay between activating and quieting messages forms the foundation of all brain activity. One signal encourages a nerve cell to become active, while the other signal suppresses that activity. This equilibrium allows the nervous system to perform its vast range of functions, from the automatic rhythm of our heartbeat to forming a memory.
The Neural Basis of Communication
The nervous system’s communication network is built from specialized cells called neurons. Each neuron has a cell body, dendrites that receive signals, and an axon that sends signals. When a neuron is sufficiently stimulated, it generates an electrical impulse known as an action potential. This charge travels down the axon, carrying the message toward the next cell.
Neurons do not physically touch each other, but are separated by a microscopic gap called a synapse. When an action potential reaches the end of the axon, it triggers the release of chemical messengers into this gap. These chemicals travel across the synapse and bind to the dendrites of the neighboring neuron, influencing its electrical state.
The signals a neuron receives can be either excitatory or inhibitory. An excitatory postsynaptic potential (EPSP) is a charge that makes the receiving neuron more likely to fire an action potential. Conversely, an inhibitory postsynaptic potential (IPSP) is a charge that makes the neuron less likely to fire.
Whether a neuron fires is determined by the sum of all signals it receives. If the total excitatory input outweighs the inhibitory input enough to reach a specific threshold, the neuron will generate an action potential and pass the signal along. This process of summing up inputs allows for control within neural circuits.
Key Chemical Messengers
The chemical messengers that travel across the synapse are called neurotransmitters. While the body uses many different types, the vast majority of rapid communication in the brain is handled by just two: glutamate and GABA. Their interaction represents the primary “on” and “off” switches for brain activity.
Glutamate is the most abundant excitatory neurotransmitter in the nervous system. When a neuron releases glutamate into a synapse, it binds to receptors on the next neuron, causing an influx of positively charged ions. This influx generates an excitatory postsynaptic potential (EPSP), making the receiving neuron more electrically positive and thus more likely to fire its own action potential. This process is important for neural pathways involved in learning and memory.
In contrast, gamma-aminobutyric acid, or GABA, is the main inhibitory neurotransmitter. When GABA is released into a synapse, it binds to different types of receptors on the postsynaptic neuron. This binding opens channels that allow negatively charged chloride ions to enter the cell, generating an inhibitory postsynaptic potential (IPSP). This makes the neuron more negatively charged and therefore less likely to reach the threshold for firing an action potential.
While glutamate and GABA are the primary actors, other neurotransmitters like dopamine and serotonin can also have excitatory or inhibitory effects, often in a more modulatory role that fine-tunes the overall circuit activity.
The Role of Balance in Brain Function
The balance between excitation and inhibition allows the brain to function effectively, performing complex tasks. This equilibrium is not static; it shifts dynamically to meet the demands of different situations. This coordination enables everything from physical movement to cognitive processes.
Consider bending your arm. To contract your bicep muscle, motor neurons send excitatory signals to it. Simultaneously, inhibitory signals are sent to the opposing tricep muscle, causing it to relax. Without this inhibition, both muscles would contract at the same time, resulting in rigid movements instead of a smooth action.
This principle extends to cognitive functions like learning and memory. When you learn something new, excitatory signals strengthen the synaptic connections between specific neurons, a process known as long-term potentiation. At the same time, inhibitory signals quiet down irrelevant neural activity and sharpen the focus of the active neural circuit. This inhibition filters out distracting sensory information, allowing the brain to encode information more effectively.
Consequences of Imbalance
Disruptions in the equilibrium between excitation and inhibition can have significant consequences for brain health. When this balance is thrown off, it can lead to neurological and psychiatric conditions. The nature of the condition depends on whether the imbalance favors excessive excitation or inhibition.
Epilepsy is an example of what happens when there is too much excitation in the brain. It is characterized by seizures, which are storms of uncontrolled, synchronized electrical activity spreading through neural networks. This state can arise from genetic factors or brain injury that lead to an overabundance of excitatory signaling or a failure of inhibitory mechanisms.
Conversely, an excess of inhibition can also lead to problems. Many sedative drugs, such as benzodiazepines and alcohol, work by enhancing the effect of the inhibitory neurotransmitter GABA. This boost in inhibitory signaling quiets the brain, leading to effects like reduced anxiety, muscle relaxation, and sedation. While beneficial in a therapeutic context, excessive inhibition can impair cognitive function and slow reaction times.