Help QBI research Give now. Skip to menu Skip to content Skip to footer. Site search Search. Site search Search Menu. Types of glia. Home The Brain Brain anatomy. Oligodendrocytes Oligodendrocytes provide support to axons of neurons in the central nervous system, particularly those that travel long distances within the brain. They are involved in creating cerebrospinal fluid CSF.
Radial glia : Radial glial cells are progenitor cells that can generate neurons, astrocytes and oligodendrocytes. Peripheral Nervous System Schwann cells : Similar to oligodendrocytes in the central nervous system, Schwann cells myelinate neurons in the peripheral nervous system.
They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses.
Astrocytes also form the blood-brain barrier—a structure that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown through calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses. Satellite glia provide nutrients and structural support for neurons in the PNS.
Microglia scavenge and degrade dead cells and protect the brain from invading microorganisms. Oligodendrocytes , shown in Figure 2b form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as scaffolds for developing neurons as they migrate to their end destinations.
Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal cord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal cord and the brain, and is a component of the choroid plexus. Figure 2. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia clusters of connected neurons.
These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species. Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves.
One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. The nervous system is made up of neurons , specialized cells that can receive and transmit chemical or electrical signals, and glia , cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire—it transmits a signal from one place to another.
Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Although glia have been compared to workers, recent evidence suggests that also usurp some of the signaling functions of neurons. There is great diversity in the types of neurons and glia that are present in different parts of the nervous system.
There are four major types of neurons, and they share several important cellular components. The nervous system of the common laboratory fly, Drosophila melanogaster , contains around , neurons, the same number as a lobster.
This number compares to 75 million in the mouse and million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.
Most neurons share the same cellular components. But neurons are also highly specialized—different types of neurons have different sizes and shapes that relate to their functional roles. Like other cells, each neuron has a cell body or soma that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components.
Neurons also contain unique structures, illustrated in Figure Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites.
Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections. Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon.
An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs.
Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin , which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter—from the base of the spine to the toes.
The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another as well as with other cells, like muscle cells.
Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as , other neurons. There are different types of neurons, and the functional role of a given neuron is intimately dependent on its structure. There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system and across species , as illustrated by the neurons shown in Figure
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