Two broad families of cells
Brain tissue is built mainly from two kinds of cells: neurons and glia. Neurons are specialized for rapid signaling. Glia, whose name comes from a Greek word for glue, were long regarded as passive support cells, but research has shown they do far more than hold things together. Estimates of cell numbers vary, but a commonly cited figure is around 86 billion neurons in the human brain, with glia present in a number of the same general order of magnitude. Earlier claims that glia outnumber neurons by a large factor are no longer widely accepted.
The neuron
A typical neuron has three main parts.
- The cell body contains the nucleus and the machinery that keeps the cell alive.
- Dendrites are branching extensions that receive signals from other neurons.
- The axon is a long fiber that carries signals away from the cell body toward other cells. Axons can be very short or, in the case of nerves that run down the spinal cord to the foot, extremely long.
Neurons come in many shapes and sizes, and they differ in the chemicals they release. Some neurons detect sensations, some drive muscles, and most are in between, connecting other neurons together. This is the cellular groundwork for the larger systems described in the discussion of neural circuits.
The electrical signal
Neurons maintain a difference in electrical charge across their outer membrane. Proteins that act as channels and pumps keep certain charged particles, or ions, in different concentrations inside and outside the cell. When a neuron receives enough input to push its membrane voltage past a threshold, channels open in a sequence and produce a brief electrical pulse called an action potential. The pulse travels along the axon.
Action potentials are largely all-or-nothing: they either happen or they do not, and they have a roughly consistent size. Information is thought to be carried by how often neurons fire and when, rather than by the size of each pulse. Many axons are wrapped in a fatty insulating layer called myelin, which speeds conduction. Diseases that damage myelin, such as multiple sclerosis, can interfere with signaling.
The synapse
Neurons usually do not touch each other directly. They communicate across a tiny gap called a synapse. At the most common type of synapse, the signal is chemical.
- An action potential arrives at the end of the sending neuron’s axon.
- This triggers the release of chemical messengers, called neurotransmitters, into the gap.
- The neurotransmitters bind to receptors on the receiving neuron.
- Depending on the type of receptor, the receiving cell becomes more or less likely to fire.
- The neurotransmitter is cleared away, either taken back up or broken down, so the signal ends.
Different neurotransmitters have different typical effects. Glutamate is the brain’s main excitatory messenger and GABA its main inhibitory one. Others, such as dopamine, serotonin and acetylcholine, modulate activity over wider areas and are involved in processes such as movement, mood and attention. Some synapses also pass signals electrically through direct connections between cells.
A single neuron may receive thousands of inputs. It effectively adds up excitation and inhibition and then decides whether to fire. The strength of synapses can change with experience, which is the cellular basis for the learning discussed under synaptic strengthening.
Glial cells
Several kinds of glia have been described, each with identifiable roles.
- Astrocytes support neurons, help regulate the chemical environment around synapses and the blood supply, and appear to take part in signaling.
- Oligodendrocytes produce myelin in the brain and spinal cord.
- Microglia act as the nervous system’s resident immune cells, clearing debris and responding to injury or infection.
Researchers are actively studying how glia influence synapses, development and disease. Some of the work is promising, but it is an evolving area and the full picture is not settled.
Chemistry and medicine
Because so much signaling depends on neurotransmitters and receptors, many medicines act on these systems. Examples include drugs for epilepsy, depression, pain and Parkinson’s disease. The effects are often complex because the same chemical has various roles in different parts of the brain, so a drug that helps one symptom may cause side effects elsewhere. Anyone considering medications should rely on a health professional rather than general articles.
Keeping the details in perspective
The cellular story explains much, but it does not by itself explain behavior. Millions of cells interact to produce even simple experiences, and knowing the properties of one neuron does not predict what a network will do. Much of the field works to bridge that gap. Still, understanding the basics of neurons and synapses gives a solid foundation for anything else in neuroscience, from how sleep affects thought to why certain drugs change mood.