The human nervous system, responsible for every thought and movement, depends on neuronal communication. Billions of specialized cells called neurons form a network that operates through a combination of electrical and chemical signals. Understanding how these signals are generated and transmitted is foundational to comprehending the functions of the brain and the nervous system as a whole.
The Structure of a Neuron
Each neuron consists of three main parts specialized for sending and receiving signals: the dendrites, the cell body, and the axon. Dendrites are tree-like extensions that branch from the cell body and receive information from other neurons. Their branched structure increases the surface area for receiving signals, with some neurons receiving information from thousands of other cells.
The cell body, or soma, contains the nucleus and organelles that maintain the neuron and provide energy for its activities. The axon is a long fiber extending from the cell body that carries signals away to other neurons, muscles, or glands. At the end of the axon are the axon terminals, which send the signal to the next cell.
The Electrical Signal Within a Neuron
Communication within a single neuron is an electrical event centered on the action potential, a rapid change in the electrical charge across the neuron’s membrane. In its resting state, a neuron maintains a negative charge inside the cell compared to the outside. This resting potential is maintained by regulating sodium and potassium ions on either side of the cell membrane.
When a neuron receives a signal at its dendrites, gated channels on the membrane open. This allows positively charged sodium ions to flow into the cell, which reduces the negative charge inside in a process called depolarization. If the signal is strong enough to reach a specific threshold, it triggers an action potential. This is an “all-or-none” event, meaning the action potential will fire with its full strength once the threshold is reached.
The action potential is a wave of depolarization that travels down the axon. Voltage-gated sodium channels open in sequence, allowing an influx of sodium ions and propagating the signal toward the axon terminal. Following this wave, potassium channels open, allowing potassium ions to exit the cell. This repolarizes the membrane and returns it to its negative resting state.
Some axons are covered in a fatty substance called myelin. This acts as an insulator and allows the electrical signal to travel much faster by jumping between gaps in the myelin sheath called nodes of Ranvier.
The Chemical Signal Between Neurons
When the action potential reaches the axon terminal, the message is passed to the next neuron at a junction called a synapse. This is a tiny gap between the sending neuron’s axon terminal and the receiving neuron’s dendrite. Communication across this synaptic cleft is a chemical process involving molecules called neurotransmitters, converting the electrical signal to a chemical one for controlled communication.
The arrival of the action potential at the axon terminal opens calcium channels. The influx of calcium causes synaptic vesicles, small sacs filled with neurotransmitters, to fuse with the terminal’s membrane. This fusion releases the neurotransmitters into the synaptic cleft in a process known as exocytosis.
Neurotransmitter molecules travel across the gap and bind to specific receptor proteins on the receiving neuron’s dendrites. This binding is specific, like a key in a lock, and causes ion channels on the receiving neuron to open. This action changes the electrical state of the receiving neuron, either exciting or inhibiting it. After the signal is delivered, neurotransmitters are cleared from the synapse through enzymatic degradation or reabsorbed by the sending neuron in a process called reuptake.
Types of Neuronal Messages
Messages between neurons can be either excitatory or inhibitory. This distinction depends on the neurotransmitter released and the receptor it binds to on the receiving neuron. The balance between these “go” and “stop” signals allows the nervous system to perform complex calculations and regulate bodily functions.
Excitatory neurotransmitters increase the likelihood that the receiving neuron will fire an action potential. When these neurotransmitters bind to their receptors, they open channels that allow positive ions, like sodium, to enter the cell. This influx of positive charge depolarizes the neuron, moving its membrane potential closer to the firing threshold. Glutamate is the most common excitatory neurotransmitter in the brain, involved in learning and memory.
Inhibitory neurotransmitters decrease the likelihood that the receiving neuron will fire. When they bind to their receptors, they open channels that allow negatively charged ions, such as chloride, to enter the cell or positive ions, like potassium, to exit. This process, called hyperpolarization, makes the neuron’s inside more negative and moves its membrane potential further from the firing threshold. Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the brain, helping regulate brain activity and prevent excessive firing of neurons.
Impact of Disrupted Communication
When the precise signaling process between neurons is disrupted, it can lead to various neurological and psychiatric disorders. Imbalances in neurotransmitters, damage to neurons, or problems at the synapse can all impair these communication pathways.
Parkinson’s disease is characterized by the loss of dopamine-producing neurons in a brain region that controls movement. The depletion of this neurotransmitter disrupts signals for smooth muscle control, leading to symptoms like tremors, stiffness, and slow movement. The disease also impacts mood and cognition due to the disruption of dopamine and serotonin levels.
Mood disorders like depression are linked to disruptions in neuronal communication. An imbalance in neurotransmitters such as serotonin, which regulates mood, sleep, and emotion, contributes to the symptoms of depression. Similarly, Alzheimer’s disease involves the buildup of plaques around synapses, which impairs their function and leads to memory loss and cognitive decline.