Given their diversity of functions performed in different parts of the nervous system, there is a wide variety in their shape, size, and electrochemical properties. Neurons are highly specialized for the processing and transmission of cellular signals. However, strong evidence supports generation of substantial numbers of new neurons in the hippocampus and olfactory bulb. Neurogenesis largely ceases during adulthood in most areas of the brain. In most cases, neurons are generated by neural stem cells during brain development and childhood. Synaptic signals may be excitatory or inhibitory, increasing or reducing the net voltage that reaches the soma. This potential travels rapidly along the axon, and activates synaptic connections as it reaches them. If the voltage changes by a large enough amount over a short interval, the neuron generates an all-or-nothing electrochemical pulse called an action potential. Neurons are electrically excitable, due to maintenance of voltage gradients across their membranes. The signaling process is partly electrical and partly chemical. However, synapses can connect an axon to another axon or a dendrite to another dendrite. At the majority of synapses, signals cross from the axon of one neuron to a dendrite of another. Most neurons receive signals via the dendrites and soma and send out signals down the axon. The term neurite is used to describe either a dendrite or an axon, particularly when the cell is undifferentiated. Neurons may lack dendrites or have no axon. At the farthest tip of the axon's branches are axon terminals, where the neuron can transmit a signal across the synapse to another cell. It branches but usually maintains a constant diameter. The axon leaves the soma at a swelling called the axon hillock, and travels for as far as 1 meter in humans or more in other species. Dendrites typically branch profusely and extend a few hundred micrometers from the soma.
The axon and dendrites are filaments that extrude from it. A group of connected neurons is called a neural circuit.Ī typical neuron consists of a cell body ( soma), dendrites, and a single axon. Interneurons connect neurons to other neurons within the same region of the brain or spinal cord. Motor neurons receive signals from the brain and spinal cord to control everything from muscle contractions to glandular output. Sensory neurons respond to stimuli such as touch, sound, or light that affect the cells of the sensory organs, and they send signals to the spinal cord or brain. Neurons are typically classified into three types based on their function.
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