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1. 11 Fundamentals of the Nervous System and Nervous Tissue: Part A

2. The Nervous System Central Nervous System (CNS) Peripheral Nervous System (PNS) Brain Motor Neurons Sensory Neurons Spinal Cord Somatic Nervous System Autonomic Nervous System Sympathetic Parasympathetic The nervous system chapter 12, 13 and 14

3. Histology of Nerve Tissue The two cell types of the nervous system are: Neurons– excitable cells that transmit electrical signals Supporting cells – cells that surround and wrap neurons

4. Supporting Cells: Neuroglia The supporting cells (neuroglia or glial cells): Provide a support for neurons Segregate and insulate neurons Guide young neurons to the proper connections Promote health and growth Two groups In the CNS – astrocytes, microglia, ependymal cells and oligodendrocytes In the PNS – satellite and Schwann cells

6. Neuroglia in the CNS: Astrocytes Most abundant, versatile, and highly branched glial cells In some areas of the brain they account for 90% of the tissue They cling to neurons and their synaptic endings, and cover capillaries Capillary Neuron Astrocyte http://www.ucl.ac.uk/biology/images/astrocytes.gif Figure 11.3a

7. Neuroglia in the CNS: Astrocytes functions Maintaining blood-brain barrier – preventing free access of circulating compounds to the CNS. The extensions of the astrocytes end in expanded portion that wraps capillaries

8. Why are the capillaries in the BBB less permeable? • Endothelial cells form tight junctions that prevent solutes movement between cells • Astrocytes • Selective transport properties of the endothelial cells • The BBB • Helps maintain a stable environment for the brain • Separates neurons from some bloodbornesubstances

10. Blood-Brain Barrier: Functions • Selective barrier that allows nutrients to pass freely • Is ineffective against substances that can diffuse through plasma membranes (ex. Ethanol, caffeine) • Absent in some areas: • Ex. - hormones generally do not penetrate the brain from the blood, so in order to control the rate of hormone secretion effectively, there are specialized sites where neurons can "sample" the composition of the circulating blood. At these sites, the blood-brain barrier is 'leaky‘ (pituitary gland) • Capillaries of the choroid plexus • The BBB can break down under certain conditions: • hypertension, radiation, infection and brain trauma

11. Example • In Parkinson’s disease there is a lack in the NT dopamine (neurons that produce it are either damaged or dead) • Dopamine can not be given in a pill or injection because it can’t cross the BBB • Instead, a precursor - L-dopa – is given. This is being transported across the BBB and is being used by neurons

12. Neuroglia in the CNS: Astrocytes functions • Creating a framework for the CNS with microfilaments that extend from the astrocytes • Repairing damaged neural tissue- limited structural repairs that stabilize and prevent further injury. • In the embryonic brain, the astrocyte appear to be involve in directing the growth and interconnection of developing neurons. • Secrete nerve growth factors that promote neuron growth and synapse formation

13. Neuroglia in the CNS: Astrocytes functions Control the chemical environment Regulating the concentration of sodium, potassium and carbon dioxide ions Providing system for transporting nutrients and dissolved gasses between capillaries and neurons absorb neurotransmitters to prevent excessive levels in tissue fluid; control ion concentration in the interstitial fluid

14. Neuron Microglial cell Figure 11.3b Microglia • small, ovoid cells with spiny processes that are capable of migrating throughout the CNS • Macrophages that phagocyte dead/injured neurons and invaders

15. Fluid-filled cavity Ependymal cells Brain or spinal cord tissue Ependymal Cells • Range in shape from squamous to columnar • May be ciliated • Line the central cavities of the brain and spinal column • Separate the CNS interstitial fluid from the cerebrospinal fluid in the cavities

16. Myelin sheath Process of oligodendrocyte Nerve fibers Oligodendrocytes • Branched cells • Processes wrap CNS nerve fibers, forming insulating myelin sheaths

17. Neuroglia cells in the PNS Schwann cells (neurolemmocytes) – surround fibers of the PNS Similar function as the oligodendrocytes Satellite cells surround neuron cell bodies in the PNS similar function to the astrocytes in the CNS – controlling the chemical environment

18. Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived (over 100 years) Amitotic – once achieve there roles in the system they loose the ability to divide High metabolic rate – require high supply of oxygen and glucose Their plasma membrane function in Electrical signaling

19. Cell body locations Groups of cell bodies are named according to their location: Most cell bodies are in the CNS in clusters called nuclei Some are in the PNS in clusters called ganglia

20. Neurons’ Processes location Extensions from the soma There are two types: axons and dendrites The groups of extensions are named according to their locations: In the CNS the extensions called tracts In the PNS the extensions (axons) are called nerves

21. Neuron Cell Body (Perikaryon or Soma) Posses all the components needed to produce membrane parts, synthesize enzymes and neurotransmitters: Contains the nucleus and a nucleolus rough endoplasmic reticulum/Nissle bodies for protein synthesis Golgi apparatus which packages materials into vesicles for transport Numerous mitochondria Cytoskeletal elements Has no centrioles (hence its amitotic nature) Neural stem cells are still found in the adult but are not active Cells in the hippocampus (part that is involves memory) can still dividing

22. Dendrites Short branched processes They are the receptive, or input, regions of the neuron Supply big surface for receiving signals Convey incoming messages towards the cell body. Electrical signals are conveyed as graded potentials (not action potentials)

23. Axons: Structure The axon contains the same organelles as the cell body with the exception of Nissle bodies and Golgi apparatus (protein synthesis and packaging) Depends on the cell body to supply proteins Each neuron has single axon of uniform diameter Initiate in an enlarged area called the axon hillock Numerous terminal branches (telodendria) Knoblike axon terminals (synaptic knobs or boutons) Secretory region of neuron Release neurotransmitters to excite or inhibit other cells Long axons are called nerve fibers

24. Axons: Function Generate and transmit action potentials – away from the cell The signal starts at the junction between the axon hillock and axon – trigger zone Secrete neurotransmitters from the axonal terminals in response to the impulse arriving along the axon Movement along axons (not the signal movement) occurs in two ways Anterograde — toward axonal terminal (mitochondria, cytoskeletal elements, membrane components for membrane renewal, enzymes Examples: mitochondria, membrane components, enzymes Retrograde — away from axonal terminal (organelles to be degraded, signal molecules, viruses, and bacterial toxins like polio, rabies, herpes simplex)

25. Myelin Sheath: Formation Formed by Schwann cells in the PNS Schwann cells wraps many times around the axon Myelin sheath — concentric layers of Schwann cell membrane Neurilemma—peripheral bulge of Schwann cell cytoplasm Myelin sheath functions : Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission

26. Myelin Sheath and Neurilemma: Formation http://www.jdaross.cwc.net/intronerv3.htm

27. Nodes of Ranvier (Neurofibral Nodes) Gaps in the myelin sheath between adjacent Schwann cells They are the sites where axon collaterals can emerge http://www.jdaross.cwc.net/introd13.jpg

28. Unmyelinated Axons • Thin nerve fibers are unmyelinated • One Schwann cell may incompletely enclose 15 or more unmyelinated axons

29. Unmyelinated Axons A Schwann cell surrounds nerve fibers but coiling does not take place Schwann cells partially enclose 15 or more axons http://www.bu.edu/histology/p/21201loa.htm

30. Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced

31. Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that formmyelin sheaths around CNS nerve fibers. Figure 11.3d

32. Regions of the Brain and Spinal Cord White matter – dense collections of myelinated fibers Gray matter – mostly soma (cell bodies) and unmyelinated fibers

33. Neuron Classification - Structural Multipolar — three or more processes Many extensions; many dendrites lead toward cell body, one axon leads away from cell body. Most neurons in the CNS and those whose axons carry impulses away from the CNS are multipolar. Bipolar — two processes (axon and dendrite) Two extensions; one fused dendrite leads toward cell body, one axon leads away from cell body These are rare and can be found as part of the receptor apparatus in the eye, ear and olfactory mucosa. Unipolar — single, short process One very short process from cell body that divides into central and peripheral processes. Only the distal ends of the peripheral process are dendrites and the rest act as an axon along with the central process. Nearly all neurons that conduct impulses towards the CNS are unipolar.

34. Neuron Classification - Functional Sensory (afferent) - transmit impulses towardthe CNS Most are unipolar neurons with their bodies in ganglia in the PNS. Location – sensory receptors in the internal organs, skin, skeletal muscles and joints. They sense changes in the immediate environment Motor (efferent) - carry impulses away from the CNS Carry activating impulses from CNS and to the viscera, body muscles and glands; They are multipolar and their cell bodies are in the CNS.

35. Neuron Classification - Functional Interneurons (association neurons) - shuttle signals through CNS pathways Link other neurons together (i.e. sensory neuron to interneuron to motor neuron). All bodies are in CNS and they are multipolar.