With approximately 86 billion neurons in the human brain, it is a complex piece of machinery in charge of movement, sensation, decision-making, and more. Behaviors cannot arise from the work of a single neuron, but instead rely on neuronal communication to orchestrate behaviors, whether simple or complex. Functionally, neurons are connected through synapses: the “pre-synaptic” neuron communicates through the synaptic junction to the “post-synaptic” neuron to propagate information. Through chains of synaptic connections, information can be speedily relayed through the brain, at speeds of up to 100 m/s – for reference, that’s about ten times faster than the fastest man on Earth, Usain Bolt, can run!
If the number of neurons in a human brain seems staggering, arranging precise connections between them is an even more impressive feat. Within the nearly 100 billion neurons there are an estimated 100 trillion synapses to connect these neurons together!
The brain represents quite possibly the most precise and efficient communication web imaginable, capable of generating impressive behaviors through transmission between these basic building blocks of the nervous system.
Within a single neuron, information is relayed through electric signals. Ions of either positive or negative charge cross the cell membrane, creating a potential across the cell membrane. When a neuron is resting, the charge created across the membrane is called the resting potential, and is usually about -70 mV. This means that the inside of the neuron is negative compared to the outside. Signals from other neurons will cause a shift in ions, or a movement of charge. Thus, the neuron will veer away from its resting potential to become depolarized— less negative— or hyperpolarized— more negative. When a neuron depolarizes sufficiently, it sends signals onto the next neuron! This is the nervous system’s check – a neuron will not propagate information until it is sufficiently depolarized.
Communication between neurons is achieved primarily through neurotransmitters, or small molecules that are released into the synapse. There are as many as 100 different neurotransmitters in the nervous system, and most neurotransmitters can act upon multiple receptors, making communication extremely precise. Some neurotransmitters are excitatory, and cause the post-synaptic neuron to depolarize, or become less negative. The primary excitatory neurotransmitter is glutamate, which can be found in the majority of synaptic connections. Other neurotransmitters are inhibitory, and cause the post-synaptic neuron to hyperpolarize. One such inhibitory neurotransmitter is gamma-aminobutyric acid, or GABA. A number of anesthetics work by targeting GABA receptors, causing those neurons to inactivate or shut down. When those neurons shut down, behaviors including pain perception, movement, and memory formation shut down!
Many other neurotransmitters work through more complex mechanisms, including the neurotransmitter serotonin. Serotonin, sometimes mistakenly thought of as the “feel-good neurotransmitter” or the “calming hormone,” is released centrally in the brain and distributed widely to affect many brain regions involved in different types of functions. Serotonin is involved in a number of regulatory systems, including sleep and wakefulness, and is a target for treating depression and anxiety. Neurotransmitters like serotonin still facilitate communication between neurons, but their actions play a large and direct role in our complex behaviors.
From ions crossing a neuronal membrane to serotonin regulation across the entire brain, neuronal communication is critical for the nervous system to function properly. With nearly 100 billion neurons, 100 trillion synapses, and up to 100 types of neurotransmitters, the nervous system uses what is possibly the most complex network known to humankind!
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Molecular and Cellular Physiology of Neurons. Gordon L. Fain
Neuroscience, Fifth Edition. Dale Purves.
General Anesthetic Actions on GABAA Receptors. Paul S Garcia. Curr Neuropharmacol, 2010, 8(1):2 – 9.