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Anatomy and Physiology - Nervous Tissue

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    Study Guide Anatomy and Physiology – Nervous Tissue 1. Neuroglia (Glial Cells) Neuroglia , or glia , are special cells that support, protect, and help neurons function properly . Unlike neurons, glial cells do not mainly focus on sending nerve impulses. Instead, they act like the “helper team” of the nervous system. Neuroglia are found in both: • the CNS (Central Nervous System: brain and spinal cord) • the PNS (Peripheral Nervous System: nerves outside the CNS) 1.1 Neuroglia in the CNS (4 Types) 1. Astrocytes Astrocytes have many branching extensions, giving them a star - shaped appearance. Main function: • maintain the ion balance around neurons • control exchange of materials between blood vessels and neurons 2. Oligodendrocytes Oligodendrocytes have fewer processes than astrocytes. Main function: • wrap their cytoplasmic processes around neurons to form a myelin sheath The myelin sheath acts as an insulating layer that helps nerve signals travel faster. 3. Microglia Microglia act like immune - defense cells in the CNS.

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    Study Guide Main function: • they are phagocytic macrophages • they protect by engulfing: o microorganisms o damaged cells o cellular debris 4. Ependymal Cells Ependymal cells form a lining inside the CNS. Main features: • they line the fluid - filled cavities of the brain and spinal cord • many are ciliated , helping with movement of fluid 1.2 Neuroglia in the PNS (2 Types) 1. Schwann Cells (Neurolemmocytes) Schwann cells wrap around axons in the PNS. Main function: • form a myelin sheath around axons • provide the same role in the PNS that oligodendrocytes provide in the CNS 2. Satellite Cells Satellite cells are found in ganglia (clusters of neuron cell bodies in the PNS). Main function: • surround and support the cell bodies of neurons

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    Study Guide 2. Quiz: Neuroglia 1. Question Glial cells that form a myelin sheath around axons in the CNS are called __________. Answer Choices • microglia • astrocytes • oligodendrocytes Correct Answer oligodendrocytes Why This Is Correct In the central nervous system (CNS) , oligodendrocytes form the myelin sheath around neuron axons. Myelin acts like insulation and helps signals travel faster. 2. Question Which glial cells act as phagocytes to protect CNS neurons? Answer Choices • microglia • astrocytes • oligodendrocytes Correct Answer microglia

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    Study Guide Why This Is Correct Microglia are the “immune cells” of the CNS. They act as phagocytes , meaning they clean up debris and destroy pathogens to protect neurons. 3. Question Which statement is true? Answer Choices • Neurolemmocytes form a myelin sheath around axons in the PNS, and oligodendrocytes form a myelin sheath around axons in the CNS. • Astrocytes form a myelin sheath around axons in both the PNS and CNS. • Schwann cells are the same as oligodendrocytes, and both act as phagocytic cells. Correct Answer Neurolemmocytes form a myelin sheath around axons in the PNS, and oligodendrocytes form a myelin sheath around axons in the CNS. Why This Is Correct • Neurolemmocytes (Schwann cells) = myelin in the PNS • Oligodendrocytes = myelin in the CNS Astrocytes do not make myelin, and Schwann cells are not the same as oligodendrocytes. 4. Question Which glial cell is found in the peripheral nervous system (PNS)? Answer Choices • ependymal cells • neurolemmocytes (Schwann cells) • oligodendrocytes

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    Study Guide Correct Answer neurolemmocytes (Schwann cells) Why This Is Correct Neurolemmocytes (Schwann cells) are found in the PNS and help form the myelin sheath around axons outside the brain and spinal cord. 5. Question The myelin sheath wraps around the __________ of a neuron to provide insulation. Answer Choices • axon • soma • dendrite Correct Answer axon Why This Is Correct The myelin sheath covers the axon , helping electrical signals travel faster and protecting the nerve fiber. 3. Myelination Myelination is the process of forming a myelin sheath , which is a fatty insulating covering around an axon. This insulation helps nerve impulses travel faster and more efficiently, and it also prevents signals from interfering with nearby neurons. Myelination happens differently in the:

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    Study Guide PNS (Peripheral Nervous System) CNS (Central Nervous System) 3.1 Myelination in the PNS (Schwann Cells) In the PNS , the myelin sheath is formed by Schwann cells . How Schwann cells form myelin Each Schwann cell wraps around an axon many times . This creates a thick, multilayered insulation called the myelin sheath . Neurilemma The final outer wrap of the Schwann cell membrane is called the neurilemma . Unlike the tight myelin layers, the neurilemma is looser and contains: • the Schwann cell’s nucleus • cytoplasm • organelles 3.2 Nodes of Ranvier (Neurofibral Nodes) The myelin sheath is not continuous along the entire axon. Small gaps occur between Schwann cells, leaving uninsulated areas called: • nodes of Ranvier (also called neurofibral nodes) 3.3 Saltatory Conduction (“Jumping” Signals) Because myelin insulates the axon, the nerve impulse cannot travel smoothly along the entire surface. Instead: • electrical current moves through the inside of the axon • action potentials happen only at the nodes of Ranvier

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    Study Guide This makes the signal appear to “jump” from node to node. This fast method is called saltatory conduction . Why Myelin is Useful The myelin sheath helps by: • speeding up impulse conduction • insulating nearby nerve fibers • preventing one nerve impulse from crossing over to a neighboring axon 3.4 Myelination in the CNS (Oligodendrocytes) In the CNS , myelin is formed by oligodendrocytes , not Schwann cells. How oligodendrocytes form myelin An oligodendrocyte sends out a process that wraps around an axon to form myelin. One important difference from the PNS: A single oligodendrocyte can myelinate many axons , using multiple processes . White Matter vs Gray Matter The brain and spinal cord have white matter and gray matter , based on the presence of myelin. White Matter White matter contains: myelinated axons The white color comes from the myelin sheath .

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    Study Guide Gray Matter Gray matter contains: • neuron cell bodies • dendrites • axon terminals • unmyelinated axons • neuroglia Gray matter is where most processing and neuron communication occurs. 4. Quiz: Myelination 1. Question Which of the following best describes saltatory conduction? Answer Choices • An impulse moves from one neurofibril node to the next neurofibril node along the length of an axon. • An impulse moves from one Schwann cell to the next Schwann cell along the length of the axon. • An impulse moves from the axon of one neuron, across the synapse, and to the dendrite of the next neuron. Correct Answer An impulse moves from one neurofibril node to the next neurofibril node along the length of an axon. Why This Is Correct Saltatory conduction means the nerve impulse “jumps” from one node (node of Ranvier) to the next. This makes conduction much faster than traveling continuously down the whole axon.

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    Study Guide 2. Question The neurofibril nodes are areas of the axon that __________. Answer Choices • consist of nuclei for the neuron • are covered by the neurolemmocytes • are not covered by the neurolemmocytes Correct Answer are not covered by the neurolemmocytes Why This Is Correct Neurofibril nodes (nodes of Ranvier) are gaps in the myelin sheath. They are the spots where the axon is not covered by Schwann cells, allowing the impulse to “jump” node to node. 3. Question If a neurolemmocyte makes 10 wraps around an axon, the nucleus of the neurolemmocyte can be found in wrap number __________. Answer Choices • 1 • 5 • 10 Correct Answer 1 Why This Is Correct The nucleus and cytoplasm of the Schwann cell stay in the outermost layer (the first wrap). The inner wraps become mostly myelin, which is tightly packed and doesn’t contain the nucleus.

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    Study Guide 5. Transmission of Nerve Impulses A nerve impulse travels along a neuron because of electrical changes across the neuron’s cell membrane . These changes happen when ions move in and out of the neuron. The neuron’s membrane acts like an electrical barrier, and when it is not being stimulated, it stays in a special charged condition called polarization . 5.1 Polarization and Resting Membrane Potential An unstimulated neuron is polarized , meaning: • the outside and inside of the membrane have different electrical charges • the inside is negative compared to the outside This charge difference is maintained mainly by ion concentration differences: • more sodium ions (Na ⁺ ) are kept outside the cell • more potassium ions (K ⁺ ) are kept inside the cell 5.2 Ion Leakage and the Na ⁺ / K ⁺ Pump Even at rest, Na ⁺ and K ⁺ constantly “leak” across the membrane through leakage channels . To fix this leakage and maintain the proper ion balance, the membrane uses Na ⁺ / K ⁺ pumps . These pumps actively restore: • Na ⁺ back to the outside • K ⁺ back to the inside 5.3 Why the Inside Stays More Negative The most important reason for the resting membrane potential is that: the resting membrane is much more permeable to K ⁺ than to Na ⁺ .

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