Neurons are made up of a cell body, dendrites that receive messages from other neurons, and an axon, which transmits messages.
The brain is made up of cells called neurons and glial cells. A neuron, or nerve cell, is a cell that transmits electrical and chemical signals throughout the body. A glial cell is the type of cell that makes up most of the brain tissue. It is responsible for providing nutrients to neurons and removing debris. Glial cells also hold neurons in place and insulate (protect) them from one another. Neurons are made up of a cell body, dendrites that receive messages from other neurons, and an axon, which transmits messages to other neurons. Neurons are covered in myelin, an insulating substance that helps neurons send electrical signals. Myelinated neurons send messages electrochemically in a process called saltatory conduction. There are periodic gaps in the myelin, called nodes of Ranvier, which allow ions (electrically charged molecules) to flow in and out of the cell. The electrochemical signal seems to jump from one node of Ranvier to the next. The thicker the myelin sheath, the faster nerve impulses travel and the faster the brain processes information. Processing speed is a strong predictor of performance on intelligence tests. Genes impact the degree of myelination and brain processing speed. These factors are more strongly correlated among identical twins than fraternal twins. This correlation supports the impact of genes, as identical twins share all of their genes whereas fraternal twins share only about half.
When a neuron is not sending a signal, the inside of the neuron has a negative charge relative to the outside. The difference in the voltage between the inside and outside of the neuron is its resting potential. The typical resting potential across the neuron's membrane is –70 millivolts (mV). The neuron maintains this electrical difference by pumping positively charged sodium ions (Na+) out of the cell. When a neuron receives enough excitatory inputs, the electrical potential changes, and the nerve transmits an electrical impulse along the axon away from the cell body. This signal sent by the neuron once inputs exceed a threshold is called an action potential. As the neuron receives excitatory inputs, positively charged sodium ions rush in, changing the electrical charge (depolarization). If the cell reaches a threshold for firing (usually around –55 millivolts), the electrical potential of the neuron will change rapidly, briefly reaching a positive electrical potential. To return to the usual negative resting potential, the neuron allows positively charged potassium (K+) to rush out of the cell. This processes is called repolarization. Usually the neuron will briefly dip below the usual resting potential. Once a neuron fires, it enters the refractory period, a period during which it cannot fire again until 1–2 milliseconds have passed.
Neurons fire according to the all-or-none principle. The all-or-none response is the principle that the strength of a neural response does not depend on the strength of the stimulus. Neurons either fire or do not fire in response to stimulation; there are no gradations. In other words, a neuron cannot fire strongly sometimes and weakly at other times. However, depending on inputs, a neuron may fire more or less frequently.
Neurotransmitters are molecules used to transmit messages between neurons by making a neuron more or less likely to fire. Many drugs work by influencing or mimicking neurotransmitter activity.
Because neurons do not touch one another, they communicate chemically rather than electronically. After a neuron fires, the axon terminal at the end of the neuron transmits the signal it has received by releasing neurotransmitters. A neurotransmitter is a hormone released into the gap between the axon of a neuron and another neuron's dendrites. This gap between two nerve cells, across which neurotransmitters travel, is called the synapse. Neurotransmitters can be divided into two broad groups based on their effects. An agonist is a neurotransmitter that excites or stimulates neurons, and an antagonist is a neurotransmitter that inhibits or suppresses neurons. There are over 100 kinds of neurotransmitters. Major ones include serotonin (which influences mood), dopamine (which induces feelings of reward and pleasure), histamine (which regulates immune response), and norepinephrine (which is released when a stressful event occurs). The level of neurotransmitters in the brain differs among individuals due to their genetic makeup and life events. For example, people prone to thrill-seeking tend to have high baseline levels of dopamine. Studies suggest that this sensation-seeking trait is 60% genetic and linked to a specific dopamine-producing gene.
Psychoactive drugs work by influencing or mimicking neurotransmitter activity. Depressants (such as alcohol and morphine) reduce neural activity and slow bodily functions, while stimulants (such as cocaine and amphetamines) have the opposite effect. Opiates (such as oxycodone) reduce pain. Hallucinogens (psychedelics, such as LSD) distort perceptions and trigger hallucinations.
Long-term potentiation (LTP) is an increase in the strength of a synapse that lasts from minutes to several days. Synaptic strength is a measure of how easy it is for one neuron to be affected by inputs from another neuron. LTP means that after a given neuron (Neuron A) stimulates another neuron (Neuron B), Neuron A will need less signal strength to stimulate Neuron B in the future. Repeated stimulation leads to a neural circuit as the receiving neuron adds more neurotransmitter receptors. Neuroscientists have the saying, "Neurons that fire together, wire together."
Neurotransmitters that suppress activity in the receiving neuron can produce long-term depression rather than LTP of the synapse. This makes the receiving neuron less likely to fire when it receives inputs. Any kind of learning consists of modifying synapses—making them more or less likely to receive and send signals in the presence of particular inputs. Synaptic changes can result from external inputs (sensation and perception) or internal activity (thinking).