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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 200,000 other neurons.

Art connection

Illustration shows a neuron. The main part of the cell body, called the soma, contains the nucleus. Branch-like dendrites project from three sides of the soma. A long, thin axon projects from the fourth side. The axon branches at the end. The tip of the axon is in close proximity to dendrites of an adjacent nerve cell. The narrow space between the axon and dendrites is called the synapse. Cells called oligodendrocytes are located next to the axon. Projections from the oligodendrocytes wrap around the axon, forming a myelin sheath. The myelin sheath is not continuous, and gaps where the axon is exposed are called nodes of Ranvier.
Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.

Which of the following statements is false?

  1. The soma is the cell body of a nerve cell.
  2. Myelin sheath provides an insulating layer to the dendrites.
  3. Axons carry the signal from the soma to the target.
  4. Dendrites carry the signal to the soma.

Glia

While glia are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that they also play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be disastrous—most brain tumors are caused by mutations in glia.

Types of glia

There are several different types of glia with different functions, two of which are shown in [link] . Astrocytes , shown in [link] a make contact with both capillaries and neurons in the CNS. 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 [link] b 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 for the choroid plexus.

Illustration A shows various types of glial cells surrounding a multipolar nerve of the central nervous system. Oligodendrocytes have an oval body and protrusions that wrap around the axon. Astrocytes are round and slightly larger than neurons, with many extensions projecting outward to neurons and other cells. Microglia are small and rectangular, with many fine projections. Ependymal cells have small, round bodies lined up in a row. Long extensions connect with an astrocyte. Illustration B shows a pseudounipolar cell of the peripheral nervous system. Schwann cells wrap around the branched axon, and satellite cells surround the neuron cell body.
Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons.
Astrocytes, fluorescently labeled green, are irregularly shaped with long extensions that provide support to nerve cells. Oligodendrocytes, also labeled green, are round with long, branched extensions that form the myelin sheath of nerve cells.
(a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system. (credit a: modification of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt Russell)

Section summary

The nervous system is made up of neurons and glia. Neurons are specialized cells that are capable of sending electrical as well as chemical signals. Most neurons contain dendrites, which receive these signals, and axons that send signals to other neurons or tissues. There are four main types of neurons: unipolar, bipolar, multipolar, and pseudounipolar neurons. Glia are non-neuronal cells in the nervous system that support neuronal development and signaling. There are several types of glia that serve different functions.

Art connections

[link] Which of the following statements is false?

  1. The soma is the cell body of a nerve cell.
  2. Myelin sheath provides an insulating layer to the dendrites.
  3. Axons carry the signal from the soma to the target.
  4. Dendrites carry the signal to the soma.

[link] B

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Source:  OpenStax, Bmcc 103 - concepts of biology. OpenStax CNX. Aug 06, 2015 Download for free at https://legacy.cnx.org/content/col11855/1.2
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