12.3 Supportive Cells

12.3 Supportive Cells 1 Expected Learning Outcomes: • Name the six types of cells that aid neurons, and state their respective functions. • Describe the myelin sheath that is found around certain axons, and explain its importance. • Describe the relationship of unmyelinated axons to their supportive cells. • Explain how damaged axons regenerate. 26 Supportive Cells 2 Approximately 86 billion neurons in the adult brain, and a roughly equal number of neuroglia, or glial cells—nonneuronal supportive cells • Overview of functions: • Bind neurons together • Form supportive tissue framework • In fetus, guide migrating neurons to their destination • Cover mature neurons (except at synapses) • Prevents neurons from touching each other • Gives precision to conduction pathways 27 12.3a Types of Glia 1 Approximately 86 billion neurons in the adult brain, and a roughly equal number of neuroglia, or glial cells—non-neuronal supportive cells • Four types of glia occur in the CNS: • Oligodendrocytes—form myelin sheaths in CNS • Ependymal cells—line internal cavities of brain; secrete and circulate cerebrospinal fluid (CSF) • Microglia—macrophages; engulf debris, provide defense against pathogens • Astrocytes—most abundant type; wide variety of functions • Provide framework for nervous tissue; extensions (perivascular feet) contact capillaries and stimulate formation of blood-brain barrier; adjust blood flow; supply neurons with lactate for energy; secrete nerve growth factors; influence synaptic signaling; regulate composition of tissue fluid; form scar tissue when neurons are damaged, a process called astrocytosis or sclerosis 28 Neuroglia of the Central Nervous System Figure 12.6 29 Types of Glia 2 Remaining types are peripheral glia cells, found only in the peripheral nervous system • Two types: • Schwann cells, or neurolemmocytes—envelop axons of PNS, form myelin sheath, and assist in regeneration of damaged fibers • Satellite cells—surround nerve cell bodies in ganglia of PNS; provide insulation around cell body and regulate chemical environment 30 Glial Cells and Brain Tumors Tumors are masses of rapidly dividing cells Mature neurons have little or no capacity for mitosis and seldom form tumors Brain tumors arise from: • Meninges (protective membranes of CNS) • Metastasis from nonneuronal tumors in other organs • Glial cells that are mitotically active throughout life Gliomas—tumors of glial cells; grow rapidly and are highly malignant • Blood–brain barrier decreases effectiveness of chemotherapy • Treatment consists of radiation or surgery 31 Brain Tumor Figure 12.7 32 12.3b Myelin 1 Myelin sheath—spiral layers of insulation around an axon • Formed by Schwann cells in PNS, oligodendrocytes in CNS • Sheath consists of the plasma membrane of these cells, 20% protein and 80% lipid • Production of sheath is called myelination • Begins during fetal development, proceeds rapidly during infancy, complete by late adolescence 33 Myelin 2 Myelination in the PNS: • A Schwann cell spirals repeatedly around a small section of a single axon • Lays down as many as 100 layers of its membrane with no cytoplasm between the layers • These layers are the myelin sheath • Neurilemma—thick, outermost coil • Contains Schwann cell nucleus and most of its cytoplasm • External to neurilemma: • Basal lamina • Endoneurium—a thin layer of fibrous connective tissue 34 Myelination 1 Figure 12.8a 35 Myelination 2 Figure 12.8c 36 Myelin 3 Myelination in the CNS: • Each oligodendrocyte extends several processes that wrap around small portions of many axons in its immediate vicinity • During myelination, nucleus cannot migrate around the axon like a Schwann cell does • Must push newer layers of myelin under the older ones, so myelination spirals inward toward axon • No neurilemma 37 Myelination 3 Figure 12.8b 38 Myelin 4 In both the PNS and CNS, the myelin sheath is segmented • Many Schwann cells (PNS) or oligodendrocytes (CNS) are needed to myelinate one axon • Each segment of axon wrapped by different glial cell • Myelin sheath gap (node of Ranvier)—gap between segments • Internodal segments—myelin-covered segments • Initial segment—bare section of axon between the axon hillock and the first glial cell • Trigger zone—axon hillock and initial segment • Plays important role in initiating nerve signal 39 Diseases of the Myelin Sheath 1 Multiple sclerosis (MS): • Oligodendrocytes and myelin sheaths in CNS deteriorate • Myelin replaced by hardened scar tissue • Nerve conduction disrupted (double vision, tremors, numbness, speech defects) • Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis • Cause may be autoimmune triggered by virus 40 Diseases of the Myelin Sheath 2 Tay–Sachs disease: • Hereditary disorder seen mainly in infants of Eastern European Jewish ancestry • Usually fatal before age 4 • Abnormal accumulation of glycolipid called GM2 in the myelin sheath • Normally decomposed by lysosomal enzyme • Enzyme missing in individuals homozygous for Tay–Sachs allele • Accumulation of ganglioside (GM2 ) disrupts conduction of nerve signals • Causes blindness, loss of coordination, dementia 41 12.3c Unmyelinated Axons 1 Many axons in CNS and PNS are unmyelinated • In PNS, Schwann cells hold small unmyelinated axons in surface grooves • Membrane folds once around each axon; does not spiral repeatedly around it • This wrap serves as the neurolemma • Basal lamina surrounds Schwann cell along with its axons 42 Unmyelinated Axons 2 Figure 12.9 43 12.3d Conduction Speed of Axons Speed at which a nerve signal travels down an axon depends on two factors: • Diameter: larger axons have more surface area and conduct signals more rapidly • Presence or absence of myelin: myelin speeds signal conduction • Examples: • Small, unmyelinated fibers: about 0.5 to 2.0 m/s • Small, lightly myelinated fibers: 3 to 15.0 m/s • Large, myelinated fibers: up to 120 m/s 44 12.3e Nerve Regeneration 1 Regeneration of damaged PNS nerve fiber (axon) can occur if nerve cell bodies are intact and at least some neurilemma remains Steps of regeneration: • Axon distal to the injury degenerates, macrophages clean up tissue debris • Cell body swells, ER breaks up, and nucleus moves off center • Changes due to loss of nerve growth factors from target cell • Axon stump sprouts multiple growth processes • Schwann cell neurolemma, endoneurium, and basal lamina form a regeneration tube • Guides regrowth to original destination 45 Nerve Regeneration 2 Steps of regeneration (continued): • Once contact is reestablished with original target, the neurosoma shrinks and returns to its original appearance • Nucleus returns to normal shape • Atrophied muscle fibers regrow • Regeneration is not fast, perfect, or always possible • Slow regrowth; process may take 2 years • Some axons connect with wrong muscle fibers; some die • Damaged CNS axons usually unable to regenerate 46 Regeneration of a Damaged Axon 1 Figure 12.10(1,2) 47 Regeneration of a Damaged Axon 2 Figure 12.10(3,4) 48 Regeneration of a Damaged Axon 3 Figure 12.10(5,6) 49