Anatomy & Physiology I and II

Anatomy & Physiology Study Guide

©2018 Achieve Test Prep Page 147 of 367 Like astrocytes, oligodendrocytes possess slim cytoplasmic extensions, but the cell bodies of oligodendrocytes are smaller and have fewer processes than astrocytes. These processes form a large membranous pad that winds around the axon, forming concentric layers of plasma membrane. The membranous wrapping of electrical insulation, called myelin , increases the speed at which an action potential travels through the neuron. Many oligodendrocytes cooperate in the formation of a myelin sheath along the length of an axon. These axons are described as myelinated. Each oligodendrocyte myelinates segments of several axons. The relatively sizable areas of the axon that are thus contained in myelin are called internodes . The small gaps that separate adjacent internodes are called nodes, or nodes of Ranvier . In dissection, myelinated axons appear glossy white, primarily because of the lipids within the myelin. As a result, regions dominated by myelinated axons constitute the white matter of the CNS. Unmyelinated axons may not be completely covered by the processes of neuroglia. Areas containing unmyelinated axons have a dusky gray color, and they constitute the gray matter of the CNS. Microglia The least numerous and smallest neuroglia in the CNS are microglia. These cells are capable of traveling through neural tissue. Microglia appear early in embryonic development. Microglia migrate into the CNS as the nervous system forms. They remain isolated in neural tissue engulfing cellular debris, waste products, and pathogens. 13.6 Neuroglia Neuroglia of the Central Nervous System The central nervous system has four types of neuroglia: ependymal cells, astrocytes, oligodendrocytes, and microglia. Ependymal Cells A passageway extends along the longitudinal axis of the spinal cord and brain. This passageway is filled with cerebrospinal fluid (CSF), which also surrounds the brain and spinal cord. This fluid, which circulates continuously, provides a protective cushion and transports dissolved gases, nutrients, wastes, and other materials. The narrow passageway in the spinal cord is called the central canal . In several regions of the brain, the passageway forms enlarged chambers called ventricles . The central canal and ventricles are lined by ependymal cells , which form an epithelium known as the ependyma . Ependymal cells have slender processes that branch extensively and make direct contact with neuroglia in the surrounding neural tissue. Astrocytes Astrocytes are the biggest and most abundant neuroglia in the CNS. These cells have a variety of functions. They control the chemical exchange between the blood and interstitial fluid. They create what is known as a blood–brain barrier (BBB) that isolates the CNS from the general circulation. They make up an extensive cytoskeletal that provides a structural framework for the neurons of the brain and spinal cord. They help repair damaged neural tissue. Astrocytes in the embryonic brain appear to be involved in directing both the growth and interconnection of developing neurons. Oligodendrocytes