A neuron is a cell that produces an electrical pulse. This pulse travels along an axon and activates synaptic connections with other neurons.
There are three main kinds of neurons in the human brain: sensory, motor, and interneurons. Sensory neurons receive information from outside the body; motor neurons send signals to the spinal cord or other parts of the brain; and interneurons connect one neuron with another within a circuit.
Neurons
The human brain is composed of 86 billion neurons and about as many nonneuronal cells. It has just as many neurons as a generic primate brain of its size, the same distribution of neurons between the cerebral cortex and cerebellum (despite a relative enlargement of the latter), and costs the same amount of energy per neuron as a primate.
The number of neurons in the brain also varies across different regions. For example, the hippocampus has fewer neurons than the prefrontal cortex and the cingulate gyrus has less of them.
The fact that the number of neurons in a species’ brain can increase dramatically with absolute brain size suggests that there are certain processes that can drive these increases in intelligence. Such changes may be accompanied by the expansion of specific cortical areas, and, more generally, the establishment of new neural circuits to interact with them.
Dendrites
Dendrites are branched protoplasmic extensions of neurons that receive electrical stimulation from upstream neurons via synapses. The dendrites then pass the signal to the cell body of the neuron, or soma, where it is integrated into the overall output of the neuron.
The number of dendrites varies among neurons, depending on their function. Some, such as Purkinje cells in the cerebellum, have over 1000 dendritic branches that make connections with tens of thousands of other neurons; other neurons have only one or two.
But the majority of the brain’s 100 billion neurons do not yet have connections in networks. Instead, at birth, they each have only a single dendrite and a single axon.
During childhood, these neurons grow and expand, forming “dendrite trees” that can receive signals from many other neurons. This proliferation of dendrites accounts for some of the brain’s rapid growth as a child develops.
Axons
At birth, babies’ brains contain 100 billion neurons – as many nerve cells as there are stars in the Milky Way. The number of neurons and “synapses” (connections between the brain cells) is determined by heredity, but during the first three years of life, a child’s experiences and interactions build up the brain’s network.
Until recently, neuroscience textbooks promoted the idea that glial cells, which support nerve cells, outnumber neuron cells 10 to one in the human brain. But when Suzana Herculano-Houzel devised a new way to count brain cells, she came up with a different number — 86 billion.
Her research is a fine example of the type of consistent skepticism that is needed to produce scientific knowledge. It also demonstrates the importance of a scientific mindset that is open to change and not afraid to question long-held assumptions.
Synapses
The brain consists of hundreds of billions of nerve cells that communicate through 100 trillion synapses. The density of connections is so great that it could take a lifetime to untangle its network.
The communication between neurons is exquisitely regulated as a balance between excitatory and inhibitory influences. This is the result of a process called synaptic integration.
Each neuron has hundreds of inputs on its dendrites and cell body, mainly from other neurons. These inputs add and subtract in a pattern that changes depending on what the brain is thinking.
In the axon, this input is transformed into an electrical signal called an action potential. This signal travels rapidly along the axon and activates synapses as it reaches them.
The axon can extend up to one meter in humans or tens of meters in other species, and can branch profusely. At the end of the axon, it emerges from the soma in a swelling known as an axon hillock. The axon hillock is the most easily excited part of the neuron, and it contains voltage-dependent sodium channels.