Nervous Tissue Lecture Notes PDF

Nervous Tissue CHAPTER 12 Introduction The nervous system is one of great complexity Nervous system is the foundation of our conscious experience, personality, and behavior Neurobiology combines the behavioral and life sciences Overview of the Nervous System Endocrine and nervous systems maintai...

Nervous Tissue CHAPTER 12 Introduction The nervous system is one of great complexity Nervous system is the foundation of our conscious experience, personality, and behavior Neurobiology combines the behavioral and life sciences Overview of the Nervous System Endocrine and nervous systems maintain internal coordination ◦ Endocrine system: communicates by means of chemical messengers (hormones) secreted into to the blood ◦ Nervous system: employs electrical and chemical means to send messages from cell to cell  Overview of the Nervous System Nervous system carries out its task in three basic steps ◦ Sense organs receive information about changes in the body and the external environment, and transmit coded messages to the spinal cord and the brain ◦ Brain and spinal cord process this information, relate it to past experiences, and determine what response is appropriate to the circumstances ◦ Brain and spinal cord issue commands to muscles and gland cells to carry out such a response  Overview of the Nervous System Nervous system has two major anatomical subdivisions ◦ Central nervous system (CNS) ◦ Brain and spinal cord enclosed in bony coverings ◦ Enclosed by cranium and vertebral column ◦ Peripheral nervous system (PNS) ◦ All the nervous system except the brain and spinal cord; composed of nerves and ganglia ◦ Nerve—a bundle of nerve fibers (axons) wrapped in fibrous connective tissue ◦ Ganglion—a knotlike swelling in a nerve where neuron cell bodies are concentrated Overview of the Nervous System Peripheral nervous system has two major functional subdivisions ◦ Sensory (afferent) division: carries sensory signals from various receptors to the CNS ◦ Informs the CNS of stimuli within or around the body ◦ Somatic sensory division: carries signals from receptors in the skin, muscles, bones, and joints ◦ Visceral sensory division: carries signals from the viscera of the thoracic and abdominal cavities ◦ Heart, lungs, stomach, and urinary bladder Overview of the Nervous System Motor (efferent) division—carries signals from the CNS to gland and muscle cells that carry out the body’s response ◦ Effectors: cells and organs that respond to commands from the CNS ◦ Somatic motor division: carries signals to skeletal muscles ◦ Output produces muscular contraction as well as somatic reflexes— involuntary muscle contractions  Overview of the Nervous System Visceral motor division (autonomic nervous system) ◦ Carries signals to glands, cardiac muscle, and smooth muscle ◦ Involuntary, and responses of this system and its receptors are visceral reflexes ◦ Sympathetic division ◦ Tends to arouse body for action ◦ Accelerating heart beat and respiration, while inhibiting digestive and urinary systems ◦ Parasympathetic division ◦ Tends to have calming effect ◦ Slows heart rate and breathing ◦ Stimulates digestive and urinary systems Subdivisions of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous Peripheral nervous system (CNS) system (PNS) Brain Spinal cord Nerves Ganglia Subdivisions of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous system Peripheral nervous system Spinal Sensory Motor Brain cord division division Visceral Somatic Visceral Somatic sensory sensory motor motor division division division division Sympathetic Parasympathetic division division  Universal Properties of Neurons Excitability (irritability) ◦ Respond to environmental changes called stimuli Conductivity ◦ Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations Secretion ◦ When electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell Functional Classes of Neurons Three general classes of neurons based on function Sensory (afferent) neurons ◦ Specialized to detect stimuli ◦ Transmit information about them to the CNS ◦ Afferent—conducting signals toward CNS Interneurons (association neurons) ◦ Lie entirely within the CNS ◦ Receive signals from many neurons and carry out the integrative function ◦ Process, store, and retrieve information and “make decisions” that determine how the body will respond to stimuli Motor (efferent) neuron ◦ Send signals out to muscles and gland cells (the effectors) ◦ Motor because most of them lead to muscles ◦ Efferent neurons conduct signals away from the CNS  Classes of Neurons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peripheral nervous system Central nervous system 1 Sensory (afferent) neurons conduct signals from receptors to the CNS. 2 Inter neurons 3 Motor (efferent) (association neurons conduct neurons) are signals from the CNS confined to to effectors such as the CNS. muscles and glands.  Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Soma—the control center of the Dendrites neuron – Also called neurosoma, cell Soma Nucleus body, or perikaryon Nucleolus – Has a single, centrally located Trigger zone: Axon hillock nucleus with large nucleolus Initial segment – Cytoplasm contains Axon Axon collateral mitochondria, lysosomes, a Golgi complex, numerous inclusions, Direction of signal transmission and extensive rough endoplasmic Internodes reticulum and cytoskeleton Node of Ranvier – Cytoskeleton consists of dense Myelin sheath Schwann cell mesh of microtubules and neurofibrils (bundles of actin filaments) Terminal arborization Compartmentalizes rough ER into dark-staining Nissl bodies Synaptic knobs – No centrioles: no further cell (a) division  Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Soma Dendrites—vast number of branches coming Nucleus Nucleolus from a few thick branches from the soma Trigger zone: ◦ Resemble bare branches of a tree in Axon hillock Initial segment winter Axon collateral ◦ Primary site for receiving signals from Axon other neurons Direction of ◦ The more dendrites the neuron has, the signal transmission Internodes more information it can receive and incorporate into decision making Node of Ranvier Myelin sheath ◦ Provide precise pathway for the reception Schwann cell and processing of neural information Terminal arborization Synaptic knobs (a)  Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites Axon (nerve fiber)—originates from a mound on one side of the soma Soma Nucleus called the axon hillock Nucleolus ◦ Cylindrical, relatively unbranched for Trigger zone: Axon hillock most of its length Initial segment ◦ Axon collaterals—branches of axon Axon Axon collateral ◦ Branch extensively on distal end Direction of ◦ Specialized for rapid conduction of signal transmission Internodes nerve signals to points remote to the Node of Ranvier soma Myelin sheath Schwann cell Terminal arborization Synaptic knobs (a)  Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Axon (nerve fiber) (cont.) Dendrites ◦ Axoplasm: cytoplasm of axon ◦ Axolemma: plasma membrane of axon Soma Nucleus Nucleolus ◦ Only one axon per neuron Trigger zone: ◦ Schwann cells and myelin sheath enclose Axon hillock Initial segment axon Axon collateral Axon ◦ Distal end, axon has terminal arborization: extensive complex of fine Direction of signal transmission branches Internodes ◦ Synaptic knob (terminal button)—little Node of Ranvier swelling that forms a junction (synapse) Myelin sheath with the next cell Schwann cell ◦ Contains synaptic vesicles full of neurotransmitter Terminal arborization Synaptic knobs (a) Fig. 12.4a Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Trigger zone: Initial segment Axon collateral Direction of signal transmission Internodes Schwann cell Terminal arborization (a)  Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Multipolar neuron ◦ One axon and multiple dendrites ◦ Most common Dendrites ◦ Most neurons in the brain and spinal cord Axon Multipolar neurons Bipolar neuron ◦ One axon and one dendrite Dendrites ◦ Olfactory cells, retina, inner ear Unipolar neuron Axon ◦ Single process leading away from the soma Bipolar neurons ◦ Sensory from skin and organs to spinal cord Dendrites Anaxonic neuron Axon Unipolar neuron ◦ Many dendrites but no axon ◦ Help in visual processes Dendrites Anaxonic neuron Supportive Cells (Neuroglia) About 1 trillion (1012) neurons in the nervous system Neuroglia outnumber the neurons by as much as 50 to 1 Neuroglia or glial cells ◦ Support and protect the neurons ◦ Bind neurons together and form framework for nervous tissue ◦ In fetus, guide migrating neurons to their destination ◦ If mature neuron is not in synaptic contact with another neuron it is covered by glial cells ◦ Prevents neurons from touching each other ◦ Gives precision to conduction pathways  Types of Neuroglia Four types occur only in CNS ◦ Oligodendrocytes ◦ Form myelin sheaths in CNS ◦ Each armlike process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction ◦ Ependymal cells ◦ Line internal cavities of the brain ◦ Secretes and circulates cerebrospinal fluid (CSF) ◦ Clear liquid that bathes the CNS Types of Neuroglia Microglia ◦ Small, wandering macrophages ◦ Thought to perform a complete checkup on the brain tissue several times a day ◦ Wander in search of cellular debris to phagocytize Types of Neuroglia Astrocytes ◦ Most abundant glial cell in CNS ◦ Cover entire brain surface and most non-synaptic regions of the neurons in the gray matter of the CNS ◦ Diverse functions ◦ Form a supportive framework of nervous tissue ◦ Have extensions (perivascular feet) that contact blood capillaries that stimulate them to form a tight seal called the blood–brain barrier ◦ Convert blood glucose to lactate and supply this to the neurons for nourishment Types of Neuroglia Two types occur only in PNS ◦ Schwann cells ◦ Envelope nerve fibers in PNS ◦ Wind repeatedly around a nerve fiber ◦ Produce a myelin sheath similar to the ones produced by oligodendrocytes in CNS ◦ Assist in the regeneration of damaged fibers ◦ Satellite cells ◦ Surround the neurosomas in ganglia of the PNS ◦ Provide electrical insulation around the soma ◦ Regulate the chemical environment of the neurons  Neuroglial Cells of CNS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Capillary Neurons Astrocyte Oligodendrocyte Perivascular feet Myelinated axon Ependymal cell Myelin (cut) Cerebrospinal fluid Microglia Figure 12.6 12-33  Glial Cells and Brain Tumors Tumors—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 ◦ Often glial cells that are mitotically active throughout life Gliomas grow rapidly and are highly malignant ◦ Blood–brain barrier decreases effectiveness of chemotherapy ◦ Treatment consists of radiation or surgery  Myelin Myelin sheath—an insulating layer around a nerve fiber – Formed by oligodendrocytes in CNS and Schwann cells in PNS – Consists of the plasma membrane of glial cells 20% protein and 80% lipid Myelin Sheath in PNS Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Schwann cell Axoplasm nucleus Axolemma Neurilemma (c) Myelin sheath Nodes of Ranvier and internodes  Myelin Myelin sheath is segmented ◦ Nodes of Ranvier: gap between segments ◦ Internodes: myelin-covered segments from one gap to the next ◦ Initial segment: short section of nerve fiber between the axon hillock and the first glial cell ◦ Trigger zone: the axon hillock and the initial segment ◦ Play an important role in initiating a nerve signal Diseases of the Myelin Sheath Degenerative disorders of the myelin sheath ◦ Multiple sclerosis ◦ Oligodendrocytes and myelin sheaths in the 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