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Nervous System

Learn about the major structures of the nervous system including the brain, spinal cord, and sensory receptors. Understand the subdivisions of the peripheral nervous system and the different types of neurons. Explore the role of neuroglial cells in supporting neurons and maintaining the nervous system's function.

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Nervous System

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  1. Nervous System

  2. Major Structures of the Nervous System • Brain • spinal cord • cranial nerves • spinal nerves • ganglia • enteric plexuses • sensory receptors

  3. Subdivisions of the PNS • Central nervous system (CNS) • Peripheral nervous system (PNS) • Somatic (voluntary) nervous system (SNS) • Autonomic (involuntary) nervous systems (ANS) • Enteric nervous system (ENS)

  4. Central Nervous System (CNS) • The CNS consists of the brain (encephalon), which is enclosed in the skull, and the spinal cord, which is contained within the vertebral canal. Nervous tissue of the CNS does not contain connective tissue • Collagenous fibers or fibrocytes/blasts are consequently not observed, which is quite unlike other tissues. Because of the absence of connective tissue, fresh CNS tissue has a very soft, somewhat jelly-like consistency. The two major classes of cells that make up the nervous tissue are nerve cells, neurones, and supporting cells, glia.

  5. Neurones • The vast majority of neurones is generated before birth. Persisting stem cells give rise to a small number of new neurones throughout the lifetime of mammals, including humans. The permanent addition of neurones may be important for the maintenance and plasticity of some parts of the CNS, but it is insufficient to replace neurones that die because of disease or acute damage to the CNS. Neurones should last a lifetime. Mature neurones are not mitotically active, i.e. they do not divide.

  6. Neurones are generally large cellsThe keys to the understanding of the function of a neurone lies in (1) the shape of the neurone and, in particular, its processes, (2) the chemicals the neurone uses to communicate with other neurones (neurotransmitters) and (3) the ways in which the neurone may react to the neurotransmitters released by other neurones.

  7. 2 cell types: – Nerve cells (neurons) • receive or transmit impulses • interconnections (at least 1000 each) – Neuroglial cells • more numerous than neurons • support neurons in various ways • Capillaries • No lymphatics!

  8. Neurons • cell body (perikaryon or soma): – large euchromatic nucleus – Nissl bodies – neurofibrils; microtubules, neurofilaments, microfilaments • multiple dendrites: – short processes receive multiple stimuli – becomes thinner as they subdivide into branches • single axon (varying diameter, up to 1 m in length): – constant diameter – axon terminals (end bulbs-terminal boutons) form synapses to transmit the impulse to other neurons or cells

  9. Neurons • Functional unit of nervous system • Cell body • Nissl bodies • Neurofilaments • Microtubules • Lipofuscin pigment clumps • Cell processes • Dendrites • Axons

  10. Nervous Tissue • Controls and integrates all body activities within limits that maintain life • Three basic functions • sensing changes with sensory receptors • interpreting and remembering those changes • reacting to those changes with effectors

  11. Organization Sensory Integration Motor SNS (Motor) Brain Spinal cord SNS (Sensory) ANS (Sensory) ANS (Motor) ENS (Sensory)

  12. Dendrites impulse • Conducts impulses towards the cell body • Typically short, highly branched & unmyelinated • Surfaces specialized for contact with other neurons • Contains neurofibrils & Nissl bodies

  13. Axons • Conduct impulses away from cell body • Long, thin cylindrical process of cell • Arises at axon hillock • Impulses arise from initial segment (trigger zone) • Side branches (collaterals) end in fine processes called axon terminals • Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters

  14. Structural Classification of Neurons • Based on number of processes found on cell body • multipolar = several dendrites & one axon • most common cell type • bipolar neurons = one main dendrite & one axon • found in retina, inner ear & olfactory • unipolar neurons = one process only(develops from a bipolar) • are always sensory neurons

  15. Structural Classification of Neurons

  16. Structural Classification of Neurons

  17. Neuroglial Cells • Half of the volume of the CNS • Smaller cells than neurons • 50X more numerous • Cells can divide • rapid mitosis in tumor formation (gliomas) • 4 cell types in CNS • astrocytes, oligodendrocytes, microglia & ependymal • 2 cell types in PNS • schwann and satellite cells

  18. Neuroglial Cells (CNS): Astrocytes • Star-shaped cells • Form blood-brain barrier by covering blood capillaries • Metabolize neurotransmitters • Regulate K+ balance • Provide structural support

  19. Neuroglial Cells (CNS): Oligodendrocytes • Most common glial cell type • Each forms myelin sheath around more than one axons in CNS • Analogous to Schwann cells of PNS

  20. Neuroglial Cells (CNS): Microglia • Small cells found near blood vessels • Phagocytic role -- clear away dead cells • Derived from cells that also gave rise to macrophages & monocytes

  21. Neuroglial Cells (CNS): Ependymal cells • Form epithelial membrane lining cerebral cavities & central canal • Produce cerebrospinal fluid (CSF)

  22. Neuroglial Cells (PNS): Satellite Cells • Flat cells surrounding neuronal cell bodies in peripheral ganglia • Support neurons in the PNS ganglia

  23. Neuroglial Cells (PNS): Schwann Cell • Cells encircling PNS axons • Each cell produces part of the myelin sheath surrounding an axon in the PNS

  24. Myelination • Insulation of axon • Increase speed of nerve impulse

  25. Myelination: PNS • All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells • Neurilemma is cytoplasm & nucleusof Schwann cell • gaps called nodes of Ranvier • Myelinated fibers • Unmyelinated fibers Node of Ranvier

  26. Myelination: PNS • Schwann cells myelinate (wrap around) axons in the PNS during fetal development • Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath • Tube guides growing axons that are repairing themselves

  27. Myelination: CNS • Oligodendrocytes myelinate axons in the CNS • Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons • No neurilemma is formed • Little regrowth after injury is possible due to the lack of a distinct tube or neurilemma

  28. Gray and White Matter • White matter = myelinated processes (white in color) • Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color)

  29. Electrical Signals in Neurons • Neurons are electrically excitable due to the voltage difference across their membrane • Communicate with 2 types of electric signals • action potentials that can travel long distances • graded potentials that are local membrane changes only • In living cells, a flow of ions occurs through ion channels in the cell membrane

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