Types of Neuroglia

Dorn Spinal Therapy

Spine Healing Therapy

Get Instant Access

There are about a trillion (1012) neurons in the nervous system—10 times as many neurons in your body as there are stars in our galaxy! Yet the neurons are outnumbered as much as 50 to 1 by supportive cells called neuroglia (noo-ROG-lee-uh), or glial (GLEE-ul) cells. Glial cells protect the neurons and aid their function. The word glia, which means "glue," implies one of their roles— they bind neurons together and provide a supportive framework for the nervous tissue. In the fetus, glial cells form a scaffold that guides young migrating neurons to their destinations. Wherever a mature neuron is not in synaptic contact with another cell, it is covered with glial cells. This prevents neurons from contacting each other except at points specialized for signal transmission, and thus gives precision to their conduction pathways.

There are six kinds of neuroglia, each with a unique function (table 12.1). Four types occur in the central nervous system (fig. 12.6):

  1. Oligodendrocytes17 (OL-ih-go-DEN-dro-sites) somewhat resemble an octopus; they have a bulbous body with as many as 15 armlike processes. Each process reaches out to a nerve fiber and spirals around it like electrical tape wrapped repeatedly around a wire. This spiral wrapping, called the myelin sheath, insulates the nerve fiber from the surrounding extracellular fluid. For reasons explained later, it speeds up signal conduction in the nerve fiber.
  2. Astrocytes18 are the most abundant and functionally diverse glia in the CNS and constitute over 90% of the tissue in some areas of the brain. They are many-branched and have a somewhat starlike shape. Astrocytes cover the entire brain surface and most nonsynaptic regions of the neurons in the gray matter of the CNS. They form a supportive framework for the nervous tissue. They issue numerous extensions, called perivascular feet, that contact the endothelial cells of the blood capillaries and stimulate them to form tight junctions. These junctions contribute to a blood-brain barrier that strictly controls which substances are able to get from the bloodstream into the brain tissue (see chapter 14). Astrocytes convert blood glucose to lactate and supply this to the neurons for nourishment. They secrete growth factors that promote neuron growth and synapse formation. They communicate electrically with neurons and may influence future synaptic signalling between neurons. Astrocytes also regulate the chemical composition of the tissue fluid—when neurons transmit signals, they release neurotransmitters and potassium ions; astrocytes absorb these substances and prevent them from reaching excessive levels in the tissue fluid. When neurons are damaged, astrocytes form hardened masses of scar tissue and fill space formerly occupied by neurons. This process is called astrocytosis or sclerosis.
  3. Ependymal19 (ep-EN-dih-mul) cells resemble a cuboidal epithelium lining the internal cavities of the brain and spinal cord. Unlike epithelial cells, however, they have no basement membrane and they exhibit rootlike processes that penetrate into oligo = few + dendro = branches + cyte = cell sastro = star + cyte = cell ,aependyma = upper garment oligo = few + dendro = branches + cyte = cell sastro = star + cyte = cell ,aependyma = upper garment

Table 12.1

Types of Glial Cells

Types

Functions

Neuroglia of CNS

Oligodendrocytes

Form myelin in brain and spinal cord

Astrocytes

Cover brain surface and nonsynaptic regions of neurons; form supportive framework in CNS; induce formation of blood-brain barrier; nourish neurons; produce growth factors that stimulate neurons; communicate electrically with neurons and may influence synaptic signalling; remove neurotransmitters and K+ from ECF of brain and spinal cord; help to regulate composition of ECF; form scar tissue to replace damaged nervous tissue

Ependymal cells

Line cavities of brain and spinal cord; secrete and circulate cerebrospinal fluid

Microglia

Phagocytize and destroy microorganisms, foreign matter, and dead nervous tissue

Neuroglia of PNS

Schwann cells

Form neurilemma around all PNS nerve fibers and myelin around most of them; aid in regeneration of damaged nerve fibers

Satellite cells

Surround somas of neurons in the ganglia; function uncertain

Saladin: Anatomy & 12. Nervous Tissue Text © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 12 Nervous Tissue 451

Chapter 12 Nervous Tissue 451

Nervous System Tissue
Figure 12.6 Neuroglia of the Central Nervous System.

Insight 12.1 Clinical Application the underlying nervous tissue. Ependymal cells produce cerebrospinal fluid (CSF), a clear liquid that bathes the CNS and fills its internal cavities. They have patches of cilia on their apical surfaces that help to circulate the CSF. Ependymal cells and CSF are considered in more detail in chapter 15.

4. Microglia are small macrophages that develop from white blood cells called monocytes. They wander through the CNS and phagocytize dead nervous tissue, microorganisms, and other foreign matter. They become concentrated in areas damaged by infection, trauma, or stroke. Pathologists look for clusters of microglia in brain tissue as a clue to sites of injury.

The other two types of glial cells occur in the peripheral nervous system:

  1. Schwann20 (shwon) cells envelop nerve fibers of the PNS. In most cases, a Schwann cell winds repeatedly around a nerve fiber and produces a myelin sheath similar to the one produced by oligodendrocytes in the CNS. There are some important differences between the CNS and PNS in the way myelin is produced, which we consider shortly. In addition to myelinating peripheral nerve fibers, Schwann cells assist in the regeneration of damaged fibers, which also is discussed later.
  2. Satellite cells surround the neuron cell bodies in ganglia of the PNS. Little is known of their function.

2QTheodor Schwann (1810-82), German histologist

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Yusef Gebre
    What are the types of neuroglia and their functions?
    6 years ago

Post a comment