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2015-‐04-‐13 Nervous System Dr. Dinithi Peiris Dept. of Zoology Brain Teaser 8 15 14 9 1 6 3 Which number comes next in the sequence? 4,7,10 or 12 ? 3 IntroducGon • For survival, animal must respond to the outside environment – External – Internal • To detect changes animals have – Receptors 4 1 2015-‐04-‐13 IntroducGon • To response animals have – Effectors • Lower forms-‐ receptor & effectors are the same cell • In higher form they are interconnected via • Nervous system • Endocrine system • Circulatory system 5 IntroducGon • FuncGon smoothly • Nervous system is rapid, fast & do not last long • Endocrine system is slow & prolong • Nervous & endocrine systems act independently or together. 6 Development of Nervous tissue • NS develops from the ectoderm • Ectoderm along mid-‐dorsal side of the embryo thickness to from the neural plate • Cells of neural tube give rise to enGre CNS 7 2 2015-‐04-‐13 Introduc:on • 3 structural elements – Nerve cells or neurons – SupporGng cells – ConnecGve Gssues (join nerve cells and supporGng cells) 8 Nervous tissue Two types of neural cells in the nervous system: § Neurons / Nerve cells -‐ For processing, transfer, and storage of informaGon (nerve impulses) § Neuroglia – Associate with neurons. For support, regulaGon & protecGon of neurons 9 Nervous tissue Two Anatomical Divisions ¨ Central nervous system (CNS) § § Brain Spinal cord Peripheral nervous system (PNS) All the neural Gssue outside CNS § § Afferent division (sensory input) Efferent division (motor output) § § SomaGc nervous system Autonomic nervous system 10 3 2015-‐04-‐13 Nervous tissue PNS CNS Brain Sensory division Spinal cord Somatic sensory division Visceral sensory division Motor division Visceral motor division Sympathetic division Somatic motor division Parasympathetic division 11 Neurons Neurons : microscopic nerve cells that make up the brain, spinal cord, and nerves -‐ 30,000 neurons can fit on a pinhead 12 Nerve cell • Very specialized to conduct messages • Cannot divide • Damages are irreversible • Metabolism is very simple and will die within 5-‐6 min without oxygen • Extremely delicate 13 4 2015-‐04-‐13 Neuron Consists of 3 parts • • Cell body • Cell processes DendriGe • Axon • • Distal porGon of the axon is branched. 14 Nerve cell • what is the main defining characteristic of neurons? • have the property of electrical excitability - ability to produce action potentials or impulses in response to stimuli 15 Nerve cell • Each neuron contains: -‐ Cell body with nucleus -‐ Processes 1 Dendrites : fibers that receive messages from other neurons 2. Axons : fibers that send messages to other neurons 16 5 2015-‐04-‐13 Cell body • Ovoid or irregular structure • Size varies from 4-‐5 μm up to 150 μm • Nucleus & nucleoli large • The center is called Trophic center & involve in metabolism • Nissal granules can be seen. • Large neurons – more Nissal granules than small neurons 17 Cell body • Nissal granules increases when nerves used for a long Gme • During an injury granules reduces. 18 Dendrites • In Greek – large branching. • Project from the cell body & are branched • Diameter is not uniform increases when go out from the cell wall 19 6 2015-‐04-‐13 Dendrites • All organelles are present except nucleus & Golgi bodies • No sheath around as found in axon. • Always conduct impulses towards the cell body 20 Axon • Thin & extremely long (may exceed 100m) • Diameter is constant • In vertebrates axon is surrounded by a sheath. • Found in blocks • In between the blocks, plasma membrane is exposed – Nodes of Ranvier • One neuron – one axon 21 Axon • At terminal end – branches out • Branches end in a terminal bueon • Contains vesicles with neurotransmieers • Ach – Cholinergic neuron • Adrenaline or nor-‐adrenaline – adrenergic neurons • Serotonin – Seratonergic neurons 22 7 2015-‐04-‐13 Axon Schwann cell nucleus Neurilemma Axoplasm Axolemma Myelin sheath 23 Axon • Neurons do NOT touch; there is a gap between them called a synapse • Messages are sent across the synapses by special chemicals called neurotransmieers 24 Structural ClassificaGon of Neurons • Mul:polar neuron – most common • Bipolar neuron – one dendrite/one axon • Unipolar neuron – Ex. sensory from skin to spinal cord directly • Anaxonic neuron – many dendrites/no axon – Ex. help in visual processes 25 8 2015-‐04-‐13 A Structural ClassificaGon of Neurons 26 12.4 Figure Func:onal Classifica:on of Neurons • Sensory neurons: Most unipolar, few bipolar • Transmit sensory informaGon from receptors of PNS towards CNS • Motoneurons : Mul:polar • transmit motor informaGon from the CNS to effectors (muscles/glands/adipose Gssue) in the periphery of the body. • Internurons: Mul:polar • transmit informaGon between neurons within the CNS; analyze inputs, coordinate outputs • Most common type of neuron (20 billion) 27 Comparison of Neurons 28 9 2015-‐04-‐13 Comparison of Neurons 29 Neuroglia (glial cells) • Neurons are out numbered by neuroglia (1:50) • CNS neuroglia • Astrocytes • Oligodendrocytes • Microglia • Ependymal cells • PNS neuroglia. Schwann cells • Satellite cells • 30 Astrocytes (CNS) • most abundant glial cells • Provide structural support for neurons • Largest & star shape, project from the cell body • Astrocytes with few long processes – Fibrous ast. Found in white maeer • Ast. With many short branched processes in grey maeer – Protoplasmic ast. • Help & maintain BBB 31 10 2015-‐04-‐13 Neuroglia Cells (CNS) Ependymal cells • Low columnar or cuboidal cells • Apical ends of some cells have cilia to facilitate the movement of CSF • No basal lamina. Instead the basal ends are elongated & extended branch processes 32 Neuroglia Cells (CNS) Oligodendrocytes • Create myelin sheath around axons of neurons in the CNS. • Myelinated axons transmit impulses faster than unmyelinated axons • Most common • Form a supporGve networmk Microglia • Brain macrophages • Phagocyte cellular wastes & pathogens 33 Myelin Sheath • WhiGsh, faey (protein-‐lipid), segmented sheath around most long axons • FuncGon: • ProtecGon of the axon • Electrically insulaGng fibers from one another • Increase the speed of nerve impulse transmission 34 11 2015-‐04-‐13 Neuroglia Cells (PNS) Schwann cells • Surround all axons of neurons in the PNS creaGng a neurilemma around them. Neurilemma allows for potenGal regeneraGon of damaged axons • creates myelin sheath around most axons of PNS Satellite cells • Support groups of cell bodies of neurons within ganglia of the PNS 35 Myelin Sheath Myelin sheath of CNS Myelin sheath of PNS 36 CNS • 3 regions • Cerebrum • Cerebellum • Spinal cord • Show regions of grey & white maeer • Cerebral cortex has 3 layers • Molecular layer: outer layer • Purkinje cells: very large neurons • Granule layer: inner layer 37 12 2015-‐04-‐13 CNS 38 Blood Brain Barrier • Main structural component is capillary endothelium • Cells are Gghtly sealed together • Basal lamina of capillaries are enveloped by preivascular feet of astrocytes. • BBB allows stable composiGon & constant balance of ions and fluid 39 PNS • 3 components • Nerves: bundles of nerve fibers surrounded by glial cells and CT • Ganglia • Nerve endings 40 13 2015-‐04-‐13 Nerve Fibers • Consist of axons enclosed within a sheath. • P. nerves contains groups of nerve fibers; where axons are sheathed by Schwann cells. • Axons of small diameter are unmyelinated nerve fibers. • Thicker axons are sheathed by numerous concentric wrappings of enveloping cells forming myelin 41 sheath. Nerves • Nerve fibers are grouped into bundles to form nerves. • Axon & Schwann cells are enclosed within concentric layers of nerve fibers. • Consists of 3 layers • Epineruium: dense fibrous coat • Perineurium: surrounds each nerve fiber bundle • Endoneurium: consists of spare layer of loose CT 42 Ganglia • Ovoid structures containing neural cell bodies & glial cells supported by CT • 2 types • Sensory ganglia • Receive afferent impulses • Associated with dorsal root of the spinal & cranial nerves • Autonomic ganglia • Effects the acGvity of skeletal muscles 43 14 2015-‐04-‐13 ClassificaGon of synapses • CNS -‐ Nerve & nerve only • Excitatory • Inhibitory • PNS • Nerve & muscles (Smooth, skeletal, cardiac) • Nerve & nerve (Autonomic ganglia) • Nerve & receptors (e.g. rods & bipolar nerve cells in the eye) 44 15
