Neuron
Molecular evidence suggests that the ability to generate electric signals first appeared in evolution some 700 to 800 million years ago, during the Tonian period. Predecessors of neurons were the peptidergic secretory cells. They eventually gained new gene modules which enabled cells to create post-synaptic scaffolds and ion channels that generate fast electrical signals. The ability to generate electric signals was a key innovation in the evolution of the nervous system. Neurons are the main components of nervous tissue in all animals except sponges and placozoans. Plants and fungi do not have nerve cells.
The soma or cell body is a compact structure containing the nucleus, mitochondria, and Golgi bodies. Dendrites typically branch profusely and extend a few hundred micrometers from the soma. 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. At the farthest tip of the axons branches are axon terminals, where the neuron can transmit a signal across the synapse to another cell. The longest axon of a human motor neuron can be over a meter long, reaching from the base of the spine to the toes. Sensory neurons can have axons that run from the toes to the posterior column of the spinal cord, over 1.5 meters in adults. Giraffes have single axons several meters in length running along the entire length of their necks.
Neurons are electrically excitable due to the maintenance of voltage gradients across their membranes. 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. This potential travels rapidly along the axon and activates synaptic connections as it reaches them. Synaptic signals may be excitatory or inhibitory, increasing or reducing the net voltage that reaches the soma. In most cases, neurons receive signals via the dendrites and soma and send out signals down the axon. At the majority of synapses, signals cross from the axon of one neuron to the dendrite of another. However, synapses can connect an axon to another axon or a dendrite to another dendrite.
Most neurons can be anatomically characterized as unipolar, bipolar, multipolar, Golgi I, Golgi II, or anaxonic. Unipolar cells are exclusively sensory neurons with cell bodies always found in ganglia. Bipolar neurons contain 1 axon and 1 dendrite and are found mainly in the olfactory epithelium and retina. Multipolar neurons have 1 axon and 2 or more dendrites. Camillo Golgi grouped neurons into two types: type I with long axons used to move signals over long distances and type II with short axons. Type I cells include pyramidal cells, Purkinje cells, and anterior horn cells. Granule cells serve as the best example for Golgi II neurons whose axonal process projects locally.
The German anatomist Heinrich Wilhelm Waldeyer introduced the term neuron in 1891 based on the ancient Greek word for sinew or cord. The neuron's place as the primary functional unit of the nervous system was first recognized in the late 19th century through the work of the Spanish anatomist Santiago Ramón y Cajal. In 1888 Ramón y Cajal published a paper about the bird cerebellum stating that he could not find evidence for anastomosis between axons and dendrites. He called each nervous element an autonomous canton which became known as the neuron doctrine. To make the structure of individual neurons visible, Ramón y Cajal improved a silver staining process developed by Camillo Golgi. This improved process involves a technique called double impregnation and is still in use today.
Alzheimer's disease is characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms. The most striking early symptom is loss of short-term memory which usually manifests as minor forgetfulness that becomes steadily more pronounced with illness progression. Parkinson's disease is a degenerative disorder of the central nervous system that often impairs motor skills and speech. It is characterized by muscle rigidity, tremor, and a slowing of physical movement. Multiple sclerosis is a neurological disorder that results from the demyelination of axons in the central nervous system. Charcot-Marie-Tooth disease affects 36 in 100,000 people and is one of the most common inherited neurological disorders.
In most cases, neurons are generated by neural stem cells during brain development and childhood. Neurogenesis largely ceases during adulthood in most areas of the brain. Fully differentiated neurons are permanently postmitotic however stem cells present in the adult brain may regenerate functional neurons throughout the life of an organism. Peripheral axons can regrow if they are severed but one neuron cannot be functionally replaced by one of another type. Studies of the age of human neurons suggest that this process occurs only for a minority of cells and that the vast majority of neurons in the neocortex form before birth and persist without replacement. Researchers found a way to transform human skin cells into nerve cells using transdifferentiation where cells are forced to adopt new identities.
Common questions
When did the ability to generate electric signals first appear in evolution?
Molecular evidence suggests that the ability to generate electric signals first appeared in evolution some 700 to 800 million years ago during the Tonian period. Predecessors of neurons were peptidergic secretory cells that eventually gained new gene modules enabling them to create post-synaptic scaffolds and ion channels.
Who introduced the term neuron and when was it coined?
The German anatomist Heinrich Wilhelm Waldeyer introduced the term neuron in 1891 based on the ancient Greek word for sinew or cord. The place of the neuron as the primary functional unit of the nervous system was first recognized in the late 19th century through the work of Santiago Ramón y Cajal.
How long can axons grow in humans and other species?
Axons travel for as far as 1 meter in humans or more in other species with the longest human motor neuron axon reaching over a meter from the base of the spine to the toes. Sensory neurons can have axons running from the toes to the posterior column of the spinal cord over 1.5 meters in adults while giraffes possess single axons several meters in length along their necks.
What are the main structural components of a neuron soma?
The soma or cell body is a compact structure containing the nucleus mitochondria and Golgi bodies. Dendrites typically branch profusely and extend a few hundred micrometers from the soma while the axon leaves at a swelling called the axon hillock.
Which diseases affect neurons and what are their symptoms?
Alzheimer's disease is characterized by progressive cognitive deterioration together with declining activities of daily living and neuropsychiatric symptoms including loss of short-term memory. Parkinson's disease impairs motor skills and speech through muscle rigidity tremor and slowing of physical movement while multiple sclerosis results from demyelination of axons in the central nervous system.