Module 7: The Nervous System and Nervous Tissue

Lesson 2: Nervous Tissue

Mô Thần Kinh

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Mỗi bài học (lesson) bao gồm 4 phần chính: Thuật ngữ, Luyện Đọc, Luyện Nghe, và Bàn Luận.
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Dưới đây là danh sách những thuật ngữ Y khoa của module The Nervous System and Nervous Tissue.
Khái quát được số lượng thuật ngữ sẽ xuất hiện trong bài đọc và nghe sẽ giúp bạn thoải mái tiêu thụ nội dung hơn. Sau khi hoàn thành nội dung đọc và nghe, bạn hãy quay lại đây và luyện tập (practice) để quen dần các thuật ngữ này. Đừng ép bản thân phải nhớ các thuật ngữ này vội vì bạn sẽ gặp và ôn lại danh sách này trong những bài học (lesson) khác của cùng một module.

Medical Terminology: The Nervous System and Nervous Tissue

absolute refractory period
time during an action period when another action potential cannot be generated because the voltage-gated Na+ channel is inactivated
action potential
change in voltage of a cell membrane in response to a stimulus that results in transmission of an electrical signal; unique to neurons and muscle fibers
activation gate
part of the voltage-gated Na+ channel that opens when the membrane voltage reaches threshold
astrocyte
glial cell type of the CNS that provides support for neurons and maintains the blood-brain barrier
autonomic nervous system (ANS)
functional division of the nervous system that is responsible for homeostatic reflexes that coordinate control of cardiac and smooth muscle, as well as glandular tissue
axon
single process of the neuron that carries an electrical signal (action potential) away from the cell body toward a target cell
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
biogenic amine
class of neurotransmitters that are enzymatically derived from amino acids but no longer contain a carboxyl group
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
brain
the large organ of the central nervous system composed of white and gray matter, contained within the cranium and continuous with the spinal cord
central nervous system (CNS)
anatomical division of the nervous system located within the cranial and vertebral cavities, namely the brain and spinal cord
cerebral cortex
outermost layer of gray matter in the brain, where conscious perception takes place
cerebrospinal fluid (CSF)
circulatory medium within the CNS that is produced by ependymal cells in the choroid plexus filtering the blood
chemical synapse
connection between two neurons, or between a neuron and its target, where a neurotransmitter diffuses across a very short distance
cholinergic system
neurotransmitter system of acetylcholine, which includes its receptors and the enzyme acetylcholinesterase
choroid plexus
specialized structure containing ependymal cells that line blood capillaries and filter blood to produce CSF in the four ventricles of the brain
continuous conduction
slow propagation of an action potential along an unmyelinated axon owing to voltage-gated Na+ channels located along the entire length of the cell membrane
dendrite
one of many branchlike processes that extends from the neuron cell body and functions as a contact for incoming signals (synapses) from other neurons or sensory cells
depolarization
change in a cell membrane potential from rest toward zero
effector protein
enzyme that catalyzes the generation of a new molecule, which acts as the intracellular mediator of the signal that binds to the receptor
electrical synapse
connection between two neurons, or any two electrically active cells, where ions flow directly through channels spanning their adjacent cell membranes
electrochemical exclusion
principle of selectively allowing ions through a channel on the basis of their charge
enteric nervous system (ENS)
neural tissue associated with the digestive system that is responsible for nervous control through autonomic connections
ependymal cell
glial cell type in the CNS responsible for producing cerebrospinal fluid
excitable membrane
cell membrane that regulates the movement of ions so that an electrical signal can be generated
excitatory postsynaptic potential (EPSP)
graded potential in the postsynaptic membrane that is the result of depolarization and makes an action potential more likely to occur
G protein
guanosine triphosphate (GTP) hydrolase that physically moves from the receptor protein to the effector protein to activate the latter
ganglion
localized collection of neuron cell bodies in the peripheral nervous system
gated
property of a channel that determines how it opens under specific conditions, such as voltage change or physical deformation
generator potential
graded potential from dendrites of a unipolar cell which generates the action potential in the initial segment of that cell’s axon
glial cell
one of the various types of neural tissue cells responsible for maintenance of the tissue, and largely responsible for supporting neurons
graded potential
change in the membrane potential that varies in size, depending on the size of the stimulus that elicits it
gray matter
regions of the nervous system containing cell bodies of neurons with few or no myelinated axons; actually may be more pink or tan in color, but called gray in contrast to white matter
inactivation gate
part of a voltage-gated Na+ channel that closes when the membrane potential reaches +30 mV
inhibitory postsynaptic potential (IPSP)
graded potential in the postsynaptic membrane that is the result of hyperpolarization and makes an action potential less likely to occur
initial segment
first part of the axon as it emerges from the axon hillock, where the electrical signals known as action potentials are generated
integration
nervous system function that combines sensory perceptions and higher cognitive functions (memories, learning, emotion, etc.) to produce a response
ionotropic receptor
neurotransmitter receptor that acts as an ion channel gate, and opens by the binding of the neurotransmitter
leakage channel
ion channel that opens randomly and is not gated to a specific event, also known as a non-gated channel
ligand-gated channels
another name for an ionotropic receptor for which a neurotransmitter is the ligand
lower motor neuron
second neuron in the motor command pathway that is directly connected to the skeletal muscle
mechanically gated channel
ion channel that opens when a physical event directly affects the structure of the protein
membrane potential
distribution of charge across the cell membrane, based on the charges of ions
metabotropic receptor
neurotransmitter receptor that involves a complex of proteins that cause metabolic changes in a cell
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
muscarinic receptor
type of acetylcholine receptor protein that is characterized by also binding to muscarine and is a metabotropic receptor
myelin
lipid-rich insulating substance surrounding the axons of many neurons, allowing for faster transmission of electrical signals
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
nerve
cord-like bundle of axons located in the peripheral nervous system that transmits sensory input and response output to and from the central nervous system
neuron
neural tissue cell that is primarily responsible for generating and propagating electrical signals into, within, and out of the nervous system
neuropeptide
neurotransmitter type that includes protein molecules and shorter chains of amino acids
neurotransmitter
chemical signal that is released from the synaptic end bulb of a neuron to cause a change in the target cell
nicotinic receptor
type of acetylcholine receptor protein that is characterized by also binding to nicotine and is an ionotropic receptor
node of Ranvier
gap between two myelinated regions of an axon, allowing for strengthening of the electrical signal as it propagates down the axon
nonspecific channel
channel that is not specific to one ion over another, such as a nonspecific cation channel that allows any positively charged ion across the membrane
nucleus
in the nervous system, a localized collection of neuron cell bodies that are functionally related; a “center” of neural function
oligodendrocyte
glial cell type in the CNS that provides the myelin insulation for axons in tracts
peripheral nervous system (PNS)
anatomical division of the nervous system that is largely outside the cranial and vertebral cavities, namely all parts except the brain and spinal cord
postsynaptic potential (PSP)
graded potential in the postsynaptic membrane caused by the binding of neurotransmitter to protein receptors
precentral gyrus of the frontal cortex
region of the cerebral cortex responsible for generating motor commands, where the upper motor neuron cell body is located
process
in cells, an extension of a cell body; in the case of neurons, this includes the axon and dendrites
propagation
movement of an action potential along the length of an axon
receptor potential
graded potential in a specialized sensory cell that directly causes the release of neurotransmitter without an intervening action potential
refractory period
time after the initiation of an action potential when another action potential cannot be generated
relative refractory period
time during the refractory period when a new action potential can only be initiated by a stronger stimulus than the current action potential because voltage-gated K+ channels are not closed
repolarization
return of the membrane potential to its normally negative voltage at the end of the action potential
resistance
property of an axon that relates to the ability of particles to diffuse through the cytoplasm; this is inversely proportional to the fiber diameter
response
nervous system function that causes a target tissue (muscle or gland) to produce an event as a consequence to stimuli
resting membrane potential
the difference in voltage measured across a cell membrane under steady-state conditions, typically -70 mV
saltatory conduction
quick propagation of the action potential along a myelinated axon owing to voltage-gated Na+ channels being present only at the nodes of Ranvier
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
sensation
nervous system function that receives information from the environment and translates it into the electrical signals of nervous tissue
size exclusion
principle of selectively allowing ions through a channel on the basis of their relative size
soma
in neurons, that portion of the cell that contains the nucleus; the cell body, as opposed to the cell processes (axons and dendrites)
somatic nervous system (SNS)
functional division of the nervous system that is concerned with conscious perception, voluntary movement, and skeletal muscle reflexes
spatial summation
combination of graded potentials across the neuronal cell membrane caused by signals from separate presynaptic elements that add up to initiate an action potential
spinal cord
organ of the central nervous system found within the vertebral cavity and connected with the periphery through spinal nerves; mediates reflex behaviors
stimulus
an event in the external or internal environment that registers as activity in a sensory neuron
summate
to add together, as in the cumulative change in postsynaptic potentials toward reaching threshold in the membrane, either across a span of the membrane or over a certain amount of time
synapse
narrow junction across which a chemical signal passes from neuron to the next, initiating a new electrical signal in the target cell
synaptic cleft
small gap between cells in a chemical synapse where neurotransmitter diffuses from the presynaptic element to the postsynaptic element
synaptic end bulb
swelling at the end of an axon where neurotransmitter molecules are released onto a target cell across a synapse
temporal summation
combination of graded potentials at the same location on a neuron resulting in a strong signal from one input
thalamus
region of the central nervous system that acts as a relay for sensory pathways
thermoreceptor
type of sensory receptor capable of transducing temperature stimuli into neural action potentials
threshold
membrane voltage at which an action potential is initiated
tract
bundle of axons in the central nervous system having the same function and point of origin
unipolar
shape of a neuron which has only one process that includes both the axon and dendrite
upper motor neuron
first neuron in the motor command pathway with its cell body in the cerebral cortex that synapses on the lower motor neuron in the spinal cord
ventricle
central cavity within the brain where CSF is produced and circulates
voltage-gated channel
ion channel that opens because of a change in the charge distributed across the membrane where it is located
white matter
regions of the nervous system containing mostly myelinated axons, making the tissue appear white because of the high lipid content of myelin
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Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.
Neurons are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.

A. Parts of a Neuron

As you learned in the first section, the main part of a neuron is the cell body, which is also known as the soma (soma = “body”). The cell body contains the nucleus and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—meaning that information flows in this one direction. Figure 1 shows the relationship of these parts to one another.

Where the axon emerges from the cell body, there is a special region referred to as the axon hillock. This is a tapering of the cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. As the axon hillock narrows, it transitions into the beginning of the axon called the initial segment. Action potentials are generated in the trigger zone, which is a combination of the axon hillock and initial segment.

Many axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. Each gap is called a node of Ranvier and is important to the way that electrical signals travel down the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an axon segment. At the end of the axon is the axon terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic end bulb. These bulbs are what make the connection with the target cell at the synapse.

B. Types of Neurons

There are many neurons in the nervous system—a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron’s polarity (Figure 2).

Unipolar cells have only one process emerging from the cell. True unipolar cells are only found in invertebrate animals, so the unipolar cells in humans are more appropriately called “pseudo-unipolar” cells. Invertebrate unipolar cells do not have dendrites. Human unipolar cells have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Unipolar cells are exclusively sensory neurons and have two unique characteristics. First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.

Bipolar cells have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar cells are not very common. They are found mainly in the olfactory epithelium (where smell stimuli are sensed), and as part of the retina.

Multipolar neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). With the exception of the unipolar sensory ganglion cells, and the two specific bipolar cells mentioned above, all other neurons are multipolar. Some cutting edge research suggests that certain neurons in the CNS do not conform to the standard model of “one, and only one” axon. Some sources describe a fourth type of neuron, called an anaxonic neuron. The name suggests that it has no axon (an- = “without”), but this is not accurate. Anaxonic neurons are very small, and if you look through a microscope at the standard resolution used in histology (approximately 400X to 1000X total magnification), you will not be able to distinguish any process specifically as an axon or a dendrite. Any of those processes can function as an axon depending on the conditions at any given time. Nevertheless, even if they cannot be easily seen, and one specific process is definitively the axon, these neurons have multiple processes and are therefore multipolar.

Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 3). For example, a multipolar neuron that has a very important role to play in a part of the brain called the cerebellum is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangelista Purkinje, 1787–1869).
Glial cells, or neuroglia or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means “glue,” and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: “This connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.” Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.

There are six types of glial cells. Four of them are found in the CNS and two are found in the PNS. Table 1 outlines some common characteristics and functions.

A. Glial Cells of the CNS

One cell providing support to neurons of the CNS is the astrocyte, so named because it appears to be star-shaped under the microscope (astro- = “star”). Astrocytes have many processes extending from their main cell body (not axons or dendrites like neurons, just cell extensions). Those processes extend to interact with neurons, blood vessels, or the connective tissue covering the CNS that is called the pia mater (Figure 4). Generally, they are supporting cells for the neurons in the central nervous system. Some ways in which they support neurons in the central nervous system are by maintaining the concentration of chemicals in the extracellular space, removing excess signaling molecules, reacting to tissue damage, and contributing to the blood-brain barrier (BBB). The blood-brain barrier is a physiological barrier that keeps many substances that circulate in the rest of the body from getting into the central nervous system, restricting what can cross from circulating blood into the CNS. Nutrient molecules, such as glucose or amino acids, can pass through the BBB, but other molecules cannot. This actually causes problems with drug delivery to the CNS. Pharmaceutical companies are challenged to design drugs that can cross the BBB as well as have an effect on the nervous system.

Like a few other parts of the body, the brain has a privileged blood supply. Very little can pass through by diffusion. Most substances that cross the wall of a blood vessel into the CNS must do so through an active transport process. Because of this, only specific types of molecules can enter the CNS. Glucose—the primary energy source—is allowed, as are amino acids. Water and some other small particles, like gases and ions, can enter. But most everything else cannot, including white blood cells, which are one of the body’s main lines of defense. While this barrier protects the CNS from exposure to toxic or pathogenic substances, it also keeps out the cells that could protect the brain and spinal cord from disease and damage. The BBB also makes it harder for pharmaceuticals to be developed that can affect the nervous system. Aside from finding efficacious substances, the means of delivery is also crucial.

Also found in CNS tissue is the oligodendrocyte, sometimes called just “oligo,” which is the glial cell type that insulates axons in the CNS. The name means “cell of a few branches” (oligo- = “few”; dendro- = “branches”; -cyte = “cell”). There are a few processes that extend from the cell body. Each one reaches out and surrounds an axon to insulate it in myelin. One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. The function of myelin will be discussed below.

Microglia are, as the name implies, smaller than most of the other glial cells. Ongoing research into these cells, although not entirely conclusive, suggests that they may originate as white blood cells, called macrophages, that become part of the CNS during early development. While their origin is not conclusively determined, their function is related to what macrophages do in the rest of the body. When macrophages encounter diseased or damaged cells in the rest of the body, they ingest and digest those cells or the pathogens that cause disease. Microglia are the cells in the CNS that can do this in normal, healthy tissue, and they are therefore also referred to as CNS-resident macrophages.

The ependymal cell is a glial cell that filters blood to make cerebrospinal fluid (CSF), the fluid that circulates through the CNS. Because of the privileged blood supply inherent in the BBB, the extracellular space in nervous tissue does not easily exchange components with the blood. Ependymal cells line each ventricle, one of four central cavities that are remnants of the hollow center of the neural tube formed during the embryonic development of the brain. The choroid plexus is a specialized structure in the ventricles where ependymal cells come in contact with blood vessels and filter and absorb components of the blood to produce cerebrospinal fluid. Because of this, ependymal cells can be considered a component of the BBB, or a place where the BBB breaks down. These glial cells appear similar to epithelial cells, making a single layer of cells with little intracellular space and tight connections between adjacent cells. They also have cilia on their apical surface to help move the CSF through the ventricular space. The relationship of these glial cells to the structure of the CNS is seen in Figure 4.

Glial Cells of the PNS

One of the two types of glial cells found in the PNS is the satellite cell. Satellite cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons. This accounts for the name, based on their appearance under the microscope. They provide support, performing similar functions in the periphery as astrocytes do in the CNS—except, of course, for establishing the BBB.

The second type of glial cell is the Schwann cell, which insulate axons with myelin in the periphery. Schwann cells are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment. The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in Figure 5.

Myelin The insulation for axons in the nervous system is provided by glial cells, oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.

The appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog for “pigs in a blanket” or a similar food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath. During development, the glial cell is loosely or incompletely wrapped around the axon (Figure 6a). The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed.

Myelin sheaths can extend for one or two millimeters, depending on the diameter of the axon. Axon diameters can be as small as 1 to 20 micrometers. Because a micrometer is 1/1000 of a millimeter, this means that the length of a myelin sheath can be 100–1000 times the diameter of the axon. Figure 1, Figure 4, and Figure 5 show the myelin sheath surrounding an axon segment, but are not to scale. If the myelin sheath were drawn to scale, the neuron would have to be immense—possibly covering an entire wall of the room in which you are sitting.

OpenStax. (2022). Anatomy and Physiology 2e. Rice University. Retrieved June 15, 2023. ISBN-13: 978-1-711494-06-7 (Hardcover) ISBN-13: 978-1-711494-05-0 (Paperback) ISBN-13: 978-1-951693-42-8 (Digital). License: Attribution 4.0 International (CC BY 4.0). Access for free at openstax.org.

The major parts of the neuron are labeled on a multipolar neuron from the CNS.

Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites.

Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong.

CNS gliaPNS gliaBasic function
AstrocyteSatellite cellSupport
OligodendrocyteSchwann cellInsulation, myelination
Microglia–Immune surveillance and phagocytosis
Ependymal cell–Creating CSF

The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways.

The PNS has satellite cells and Schwann cells.

Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. EM × 1,460,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

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Script:
  1. Nervous tissue contains two major cell types, neurons and glial cells.
  2. Neurons are the cells responsible for communication through electrical signals.
  3. Glial cells are supporting cells, maintaining the environment around the neurons.
  4. Neurons are polarized cells, based on the flow of electrical signals along their membrane.
  5. 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.
  6. Many axons are insulated by a lipid-rich substance called myelin.
  7. Specific types of glial cells provide this insulation.
  8. 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.
  9. In the central nervous systems, astrocytes, oligodendrocytes, microglia, and ependymal cells are found.
  10. Astrocytes are important for maintaining the chemical environment around the neuron and are crucial for regulating the blood-brain barrier.
  11. Oligodendrocytes are the myelinating glia in the central nervous systems.
  12. Microglia act as phagocytes and play a role in immune surveillance.
  13. 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 blood-brain barrier.
  14. In the peripheral nervous systems, satellite cells are supporting cells for the neurons, and Schwann cells insulate peripheral axons.
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