Neurons (also spelled neurones or called nerve cells) are the primary cells of the nervous system. In vertebrates, they are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system.
There are three classes of neurons: afferent neurons, efferent neurons, and interneurons.
Anatomy and histology
Many highly specialized types of neurons exist, and these differ widely in appearance. Characteristically, neurons are highly asymmetric in shape. Neurons consist of:
- The soma, or cell-body, the relatively large central part of the cell between the dendrites and the axon.
- The axon, a much finer, cable-like projection which may extend tens, hundreds, or even tens of thousands of times the diameter of the soma in length. This is the structure which carries nerve signals away from the neuron. Each neuron has only one axon, but this axon may undergo extensive branching and thereby enable communication with many target cells.
- The dendrite, a short, branching arbor of cellular extensions. Each neuron has very many dendrites with profuse dendritic branches. These structures form the main information receiving network for the neuron.
Axon and dendrites alike are typically only about a micrometre thick, while the soma is usually about 25 micrometres in diameter and not much larger than than the cell nucleus it contains. The axon of a human motoneuron can be over a metre long, reaching from the base of the spine to the toes.
Neurons communicate with one another and to other cells through synapses, where the axon tip of one cell impinges upon a dendrite or soma of another, or less commonly to an axon. Neurons of the cortex in mammals, such as the Purkinje cells , have over 1000 dendrites apiece, enabling connections with tens of thousands of other cells.
Types of signalling
Neurons communicate with one another across synapses. This communication is usually chemically mediated by rapid secretion of neurotransmitter molecules. Pre-synaptic neurons (i.e.the neurons which release the neurotransmitter) may produce in the post-synaptic neurons (i.e. the neurons being affected by the neurotransmitter) an electrical stimulation (an electrical excitation) which will spread to the axon hillock generating an action potential which then travels as a wave of electrical excitation along the axon. Arrival of an action potential at the tip of an axon triggers the release of neurotransmitter at a synaptic gap. Neurotransmitters can either stimulate or suppress (inhibit) the electrical excitability of a target cell. An action potential will only be triggered in the target cell if neurotransmitter molecules acting on their post-synaptic receptors cause the cell to reach its threshold potential.
Adaptations to carrying action potentials
The narrow cross-section of axons and dendrites lessens the metabolic expense of carrying action potentials, although thicker axons convey the impulses more rapidly, generally speaking.
Many neurons have insulating sheaths of myelin around their axons, which enable their action potentials to travel faster than in unmyelinated axons of the same diameter. Formed by glial cells in the central nervous system and Schwann cells in the peripheral nervous system. The myelin sheath in peripheral nerves normally runs along the axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier. Multiple sclerosis is a neurological disorder which results from abnormal demyelination of peripheral nerves. Neurons with demyelinated axons do not conduct electrical signals properly.
Neurons and glia make up the two chief cell types of the central nervous system. There are far more glial cells than neurons, and recent experimental results have suggested that glial cells play a vital role in information processing among neurons.
Histology and internal structure
Nerve cell bodies stained with basophilic dyes will show numerous microscopic clumps of Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl, 1860–1919), which consists of rough endoplasmic reticulum and associated ribosomes. The prominence of the Nissl substance can be explained by the fact that nerve cells are metabolically very active, and hence are involved in large numbers of protein synthesis.
The cell body of a neuron is supported by a complex meshwork of structural proteins called neurofilaments, which are assembled into larger neurofibrils. Some neurons also contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that accumulates with age).
Neurons of the brain
The nematode worm (Caenorhabditis elegans) has 302 neurons. Scientists have mapped all of the nematode's neurons.
The human brain has about 100 billion (1011) neurons and 100 trillion (1014) connections (synapses) between them.