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The Nervous System 11/14 Lab 8B-BIO 105 Divisions • Central nervous system (CNS) – Brain and spinal cord (dorsal body cavity) – Integration and command center • Interprets sensory input and dictates motor output • Peripheral nervous system (PNS) – Portion of nervous system outside CNS; brings information from body – Consists mainly of nerves that extend from brain and spinal cord • Spinal nerves to and from spinal cord • Cranial nerves to and from brain BIO L 105—lab 8B-Nerve Histology-----1 Peripheral Nervous System (PNS) • Two functional divisions – Sensory (afferent) division • Somatic sensory fibers—convey impulses from skin, skeletal muscles, and joints to CNS • Visceral sensory fibers—convey impulses from visceral organs to CNS – Motor (efferent) division • Transmits impulses from CNS to effector organs – Muscles and glands • Two divisions – Somatic or Voluntary nervous system – Autonomic nervous system BIO L 105—lab 8B-Nerve Histology-----2 Motor Division of PNS: Somatic Nervous System • Voluntary nervous system – Conscious control of skeletal muscles • Conducts impulses from CNS to skeletal muscle BIO L 105—lab 8B-Nerve Histology-----3 Motor Division of PNS: Autonomic Nervous System • Involuntary nervous system – Regulates smooth muscle, cardiac muscle, and glands – Visceral motor nerve fibers • Two functional subdivisions which work in opposition to each other • Sympathetic • Parasympathetic BIO L 105—lab 8B-Nerve Histology-----4 Histology of Nervous Tissue • Two principal cell types – Neurons (nerve cells)—excitable cells that transmit electrical signals – Neuroglia or Glia – small cells that surround, wrap, and generally support neurons BIO L 105—lab 8B-Nerve Histology-----5 • Neuroglial cells: – Provide a supportive scaffolding for neurons – Insulate neurons – Guide young neurons so they can make proper connections – Promote health and growth BIO L 105—lab 8B-Nerve Histology-----6 Neuroglia • • • • • • Astrocytes (CNS) Microglial cells (CNS) Ependymal cells (CNS) Oligodendrocytes (CNS) Satellite cells (PNS) Schwann cells (PNS) BIO L 105—lab 8B-Nerve Histology-----7 Astrocytes • Most abundant, highly branched cell • Cling to neurons, synaptic endings, and covers capillaries • Functions include – Support neurons and help maintain position – Play role in exchanges between capillaries and neurons; help with nutrient supply – Control chemical environment around neurons BIO L 105—lab 8B-Nerve Histology-----8 Microglial Cells • Small cells with processes that monitor neurons • Migrate toward injured neurons • Can transform to phagocytize microorganisms and neuronal debris BIO L 105—lab 8B-Nerve Histology-----9 Ependymal Cells • May be ciliated – Cilia help CerebroSpinalFluid circulate throughout CNS • Line central cavities of brain and spinal column BIO L 105—lab 8B-Nerve Histology-----10 Myelin Sheath • Whitish, protein-lipoid segmented sheath around most long or large-diameter axons – Myelinated fibers • Function of myelin – Protects and electrically insulates axon – Increases speed of nerve impulse transmission • Nonmyelinated fibers conduct impulses more slowly BIO L 105—lab 8B-Nerve Histology-----11 Oligodendrocytes • Branched cells that wrap CNS nerve fibers and form insulating myelin sheaths • Can wrap up to 60 axons at once • Have Nodes of Ranvier (gaps in myelin) • No neurilemma BIO L 105—lab 8B-Nerve Histology-----12 Satellite Cells and Schwann Cells: in PNS • Schwann cells – Surround all peripheral nerve fibers and form myelin sheaths – Wrap around axon in jelly roll fashion – One cell forms one segment of myelin sheath • Similar function as oligodendrocytes – Vital to regeneration of damaged peripheral nerve fibers BIO L 105—lab 8B-Nerve Histology-----13 • Neurilemma – Outermost layer of Schwann cell containing nucleus and most of cytoplasm – Essential for axon healing and repair • Nodes of Ranvier – Myelin sheath gaps between adjacent Schwann cells BIO L 105—lab 8B-Nerve Histology-----14 Neurons – Structural units of nervous system--form gray matter of CNS – Highly specialized cells that conduct impulses – Extreme longevity – High metabolic rate—requires continuous supply of oxygen and glucose – All have cell body, axon and one or more dendrites • Plasma membrane functions in: – Electrical signaling BIO L 105—lab 8B-Nerve Histology-----15 Neuron Cell Body (Perikaryon or Soma) • Contains nucleus and nucleolus • Major biosynthetic center – Synthesizes proteins, membranes, and other chemicals – Rough ER is very active; also called nissl bodies • Synthesizes neurotransmitters • Plasma membrane receives information from other neurons BIO L 105—lab 8B-Nerve Histology-----16 Dendrites – short, tapering, diffusely branched processes – Increase surface area of cell so can receive messages • Receptive (input) region of neuron • Convey incoming electrical signals toward cell body as graded potentials (short distance signals) not action potentials BIO L 105—lab 8B-Nerve Histology-----17 The Axon • Generates and transmits nerve impulses along axolemma (neuron cell membrane) to axon terminal – Neurotransmitters released into extracellular space • Either excite or inhibit neurons with which axons are in close contact • Carries on many conversations with different neurons at same time BIO L 105—lab 8B-Nerve Histology-----18 Structural Classification of Neurons • Grouped by number of processes • Three types – Multipolar – 3 or more processes • 1 axon, other processes dendrites • Most common; major neuron in CNS – Bipolar – 2 processes • Rare, e.g., Retina and olfactory mucosa – Unipolar – 1 short process – One process –sensory or afferent receptor – Other process – motor or efferent branch BIO L 105—lab 8B-Nerve Histology-----19 Functional Classification of Neurons • Grouped by the direction in which impulse travels in relation to CNS • Sensory or afferent – Transmit impulses from sensory receptors toward CNS – Almost all are Unipolar • Motor or efferent – Carry impulses away from CNS to effectors – Multipolar • Interneurons (association neurons) – Lie between motor and sensory neurons – Shuttle signals through CNS pathways; most are entirely within CNS BIO L 105—lab 8B-Nerve Histology-----20 Structure of a Nerve • Bundle of myelinated and non-myelinated axons enclosed by connective tissue • Cell bodies lie in CNS or in ganglia near CNS • Connective tissue coverings include – Endoneurium—loose connective tissue that encloses axons and their myelin sheaths – Perineurium—coarse connective tissue that bundles fibers into fascicles – Epineurium—tough fibrous sheath around a nerve BIO L 105—lab 8B-Nerve Histology-----21 LABWORK 1. Explain organization of nervous system. 2. Describe neuroglia and process of myelin formation. 3. Describe and identify parts of a neuron and the different classifications of neurons on models and microscope slides. 4. Describe and identify structure of a nerve (microscope slides and models). BIO L 105—lab 8B-Nerve Histology-----22