The axon is a finer, cable-like projection that can extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length.This is where the majority of input to the neuron occurs via the dendritic spine. This overall shape and structure is referred to metaphorically as a dendritic tree. The dendrites of a neuron are cellular extensions with many branches.The nucleus can range from 3 to 18 micrometers in diameter. As it contains the nucleus, most protein synthesis occurs here. For instance, the soma of a neuron can vary from 4 to 100 micrometers in diameter. 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. Bundles of axons in the central nervous system are called tracts. In vertebrates, the majority of neurons belong to the central nervous system, but some reside in peripheral ganglia, and many sensory neurons are situated in sensory organs such as the retina and cochlea.Īxons may bundle into fascicles that make up the nerves in the peripheral nervous system (like strands of wire make up cables). The nervous system is made up of the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which includes the autonomic and somatic nervous systems. Neurons are the primary components of the nervous system, along with the glial cells that give them structural and metabolic support. ( December 2020) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this section by adding citations to reliable sources. 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 soma is a compact structure and the axon and dendrites are filaments extruding from the soma. When multiple neurons are connected together they form what 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.
Plants and fungi do not have nerve cells. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses.
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