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Nervous Tissue Suzanne D'Anna 1 Nervous Tissue Consists of two main cell types: - neurons - sensory control, and regulation - neuroglia - support, protection, and homeostasis Suzanne D'Anna 2 Parts of a Neuron cell body dendrites axon Suzanne D'Anna 3 Cell Body contains nucleus, surrounded by granular cytoplasm contains organelles (i.e., lysosomes, mitochondria, Golgi complex, etc.) Nissl bodies (chromatophilic substance) - arrangement of rough endoplasmic reticulum (site of protein synthesis) neurofibril - provides support and shape for cell Suzanne D'Anna 4 Two Kinds of Nerve Fibers dendrites - neurons usually contain many axons - neurons contain only one Suzanne D'Anna 5 Dendrites dendro = tree highly branched processes not myelinated extend out from cell body receive stimuli carry a nerve impulse toward the cell body Suzanne D'Anna 6 Axon long, thin, cylindrical projection may or may not be myelinated contains mitochondria and neurofibrils but no rough ER; no protein synthesis cytoplasm - axoplasm surrounded by axolemma (lemma = sheath or husk) carries a nerve impulse away to other cells i.e., (nerves, muscles, glands) Suzanne D'Anna 7 Parts of an Axon long nerve cell fibers are covered with myelin - fatty substance - protects and insulates fibers - increases transmission rate of - nerve impulses - forms myelin sheath Suzanne D'Anna 8 Parts of an Axon (cont.) neurolemma sheath surrounds myelin Schwann cells are surrounded by neurolemma - Schwann cells are also called neurolemmocytes narrow gaps in myelin sheath between adjacent Schwann cells are called nodes of Ranvier Suzanne D'Anna 9 Parts of an Axon (cont.) axon hillock - cone-shaped elevation where axon joins cell body initial segment - first portion of axon trigger zone - junction of the hillock and initial segment where nerve impulses arise Suzanne D'Anna 10 Parts of an Axon (cont.) axon collateral - side branch along length of axon axon terminals - fine branched end portions of an axon and its collaterals - when impulse reaches axon terminal, neurotransmitter is released Suzanne D'Anna 11 Parts of an Axon (cont.) neurons are close to each other but separated by synaptic gap - this junction is called a synapse Suzanne D'Anna 12 Parts of an Axon (cont.) synaptic end-bulbs - bulb-shaped structures - tips of axons synaptic vesicles - membrane-enclosed sacs within synaptic end-bulbs - store neurotransmitters (different neurons have different neurotransmitters) Suzanne D'Anna 13 Neuroglia Suzanne D'Anna 14 Neuroglia constitute one-half of volume of CNS outnumber neurons 5-50 times can multiply and divide, unlike neurons Functions: - nerve glue - supports - insulates - protects Suzanne D'Anna 15 Types of Neuroglial Cells astrocytes oliogodendrocytes microglia ependymal cells Schwann cells satellite cells Suzanne D'Anna 16 Astrocytes (astro = star) star-shaped many processes participate in metabolism of neurotransmitters maintain potassium (K+) balance for generation of nerve impulses participate in brain development aid in formation of blood-brain barrier link between neurons and blood vessels Suzanne D'Anna 17 Oliogodendrocytes (oligo = few, dendro = tree) few processes smaller than astrocytes form supporting network by twining around neurons produce a lipid and protein wrapping called a myelin sheath Suzanne D'Anna 18 Microglia (micro = small, glia = glue) small protect CNS from disease phagocytic - engulf microbes and clear away debris Suzanne D'Anna 19 Ependymal Cells (ependyma = upper garment) shape ranges from squamous to columnar may be ciliated line brain ventricles and central canal of spinal cord ventricles are spaces that produce and circulate cerebrospinal fluid Suzanne D'Anna 20 Schwann Cells (Neurolemmocytes) found in peripheral nervous system form myelin sheath support, protect, and nourish neurons plasma membrane of neurolemmocytes forms myelin sheath around axon in PNS myelin sheath insulates speed of impulse transmission Suzanne D'Anna 21 Satellite Cells found in peripheral nervous system support neurons in ganglia (clusters of neuron cell bodies) of PNS Suzanne D'Anna 22 Neuron Classification structural classification - based on number of processes extending from cell body functional classification - based on direction in which a neuron transmits a nerve impulse Suzanne D'Anna 23 Structural Classification unipolar (pseudounipolar) - have a single process extending from cell body - always are sensory neurons - originate in embryo as bipolar - during development, axon and dendrite fuse together into a single process - single process divides into two branches a short distance from cell body Suzanne D'Anna 24 Structural Classification bipolar - have one dendrite and one axon - found in retina, internal ear, and olfactory area of brain multipolar - several dendrites and one axon - most neurons in brain and spinal cord are of this type Suzanne D'Anna 25 Functional Classification based on the direction in which a neuron transmits a nerve impulse Two types of neurons in PNS: - afferent (toward CNS) - efferent (away from CNS) Suzanne D'Anna 26 Afferent carry impulses (sensory information) toward CNS also known as sensory neurons Efferent carry impulses away from CNS affect activity of muscles or glands also known as motor neurons Suzanne D'Anna 27 Sensory = Afferent Motor = Efferent Suzanne D'Anna 28 Afferent or Sensory Neurons Four categories: - general somatic - special somatic - general visceral - special visceral Suzanne D'Anna 29 Afferent or Sensory Neurons (cont.) general somatic - carry impulses from skin, skeletal muscles, and joints (pain, temperature, touch, vibration, and pressure) Suzanne D'Anna 30 Afferent or Sensory Neurons (cont.) special somatic - carry or relay impulses from retina and internal ear (vision, hearing, and balance) Suzanne D'Anna 31 Afferent or Sensory Neurons (cont.) general visceral - carry impulses from internal organs and receptors of body fluids (distension of internal organs and chemical conditions in body) Suzanne D'Anna 32 Afferent or Sensory Neurons (cont.) special visceral - carry impulses from tongue and olfactory mucosa (taste and smell) Suzanne D'Anna 33 Efferent or Motor Neurons general somatic - carry impulses to most skeletal muscles general visceral - carry impulses to smooth muscle, cardiac muscle, and glands special visceral - carry impulses to skeletal muscles that control facial expression Suzanne D'Anna 34 Associated Neurons located in CNS carry impulses from one neuron to another inside brain and spinal cord establish interrelationship with other neurons forming neuronal circuits Suzanne D'Anna 35