Nervous tissue contains two major cell types, neurons and glial cells. Neurons are the cells responsible for communication through electrical signals. Glial cells are supporting cells, maintaining the environment around the neurons.
Neurons are polarized cells, based on the flow of electrical signals along their membrane. Signals are received at the dendrites, are passed along the cell body, and propagate along the axon towards the target, which may be another neuron, muscle tissue, or a gland. Many axons are insulated by a lipid-rich substance called myelin. Specific types of glial cells provide this insulation.
Several types of glial cells are found in the nervous system, and they can be categorized by the anatomical division in which they are found. In the CNS, astrocytes, oligodendrocytes, microglia, and ependymal cells are found. Astrocytes are important for maintaining the chemical environment around the neuron and are crucial for regulating the blood-brain barrier. Oligodendrocytes are the myelinating glia in the CNS. Microglia act as phagocytes and play a role in immune surveillance. Ependymal cells are responsible for filtering the blood to produce cerebrospinal fluid, which is a circulatory fluid that performs some of the functions of blood in the brain and spinal cord because of the BBB. In the PNS, satellite cells are supporting cells for the neurons, and Schwann cells insulate peripheral axons.
astrocyte
glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier
axon hillock
tapering of the neuron cell body that gives rise to the axon
axon segment
single stretch of the axon insulated by myelin and bounded by nodes of Ranvier at either end (except for the first, which is after the initial segment, and the last, which is followed by the axon terminal)
axon terminal
end of the axon, where there are usually several branches extending toward the target cell
axoplasm
cytoplasm of an axon, which is different in composition than the cytoplasm of the neuronal cell body
bipolar
shape of a neuron with two processes extending from the neuron cell body—the axon and one dendrite
blood-brain barrier (BBB)
physiological barrier between the circulatory system and the central nervous system that establishes a privileged blood supply, restricting the flow of substances into the CNS
cerebrospinal fluid (CSF)
circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood
choroid plexus
specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain
ependymal cell
glial cell type in the CNS responsible for producing cerebrospinal fluid
initial segment
first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated
microglia
glial cell type in the CNS that serves as the resident component of the immune system
multipolar
shape of a neuron that has multiple processes—the axon and two or more dendrites
myelin sheath
lipid-rich layer of insulation that surrounds an axon, formed by oligodendrocytes in the CNS and Schwann cells in the PNS; facilitates the transmission of electrical signals
node of Ranvier
gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon
oligodendrocyte
glial cell type in the CNS that provides the myelin insulation for axons in tracts
satellite cell
glial cell type in the PNS that provides support for neurons in the ganglia
Schwann cell
glial cell type in the PNS that provides the myelin insulation for axons in nerves
synapse
narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell
synaptic end bulb
swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse
unipolar
shape of a neuron which has only one process that includes both the axon and dendrite
ventricle
central cavity within the brain where CSF is produced and circulates
Visit this site to learn about how nervous tissue is composed of neurons and glial cells. The neurons are dynamic cells with the ability to make a vast number of connections and to respond incredibly quickly to stimuli and to initiate movements based on those stimuli. They are the focus of intense research as failures in physiology can lead to devastating illnesses. Why are neurons only found in animals? Based on what this article says about neuron function, why wouldn’t they be helpful for plants or microorganisms?
Neurons enable thought, perception, and movement. Plants do not move, so they do not need this type of tissue. Microorganisms are too small to have a nervous system. Many are single-celled, and therefore have organelles for perception and movement.
View the University of Michigan WebScope at http://virtualslides.med.umich.edu/Histology/EMsmallCharts/3%20Image%20Scope%20finals/054%20-%20Peripheral%20nerve_001.svs/view.apml?listview=1& to see an electron micrograph of a cross-section of a myelinated nerve fiber. The axon contains microtubules and neurofilaments, bounded by a plasma membrane known as the axolemma. Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain that makes them the deep, dark, black color, such as the multiple layers that are the myelin sheath?
Lipid membranes, such as the cell membrane and organelle membranes.
1. What type of glial cell provides myelin for the axons in a tract?
A) oligodendrocyte
B) astrocyte
C) Schwann cell
D) satellite cell
A
2. Which part of a neuron contains the nucleus?
A) dendrite
B) soma
C) axon
D) synaptic end bulb
B
3. Which of the following substances is least able to cross the blood-brain barrier?
A) water
B) sodium ions
C) glucose
D) white blood cells
D
4. What type of glial cell is the resident macrophage behind the blood-brain barrier?
A) microglia
B) astrocyte
C) Schwann cell
D) satellite cell
A
5. What two types of macromolecules are the main components of myelin?
A) carbohydrates and lipids
B) proteins and nucleic acids
C) lipids and proteins
D) carbohydrates and nucleic acids
C
1. Multiple sclerosis is a demyelinating disease affecting the central nervous system. What type of cell would be the most likely target of this disease? Why?
The disease would target oligodendrocytes. In the CNS, oligodendrocytes provide the myelin for axons.
2. Which type of neuron, based on its shape, is best suited for relaying information directly from one neuron to another? Explain why.
Bipolar cells, because they have one dendrite that receives input and one axon that provides output, would be a direct relay between two other cells.