14th lecture February 25, 2016
* Nervous TissueControls 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
* Major Structures of the Nervous System
Brain spinal cord cranial nerves spinal nerves ganglia enteric plexuses sensory receptors* Organization of the Nervous System
CNS is brain and spinal cord PNS is everything else* Subdivisions of the PNS
Central nervous system (CNS) Peripheral nervous system (PNS) a) Somatic (voluntary) nervous system (SNS) neurons from cutaneous and special sensory receptors to the CNS motor neurons to skeletal muscle tissueSubdivisions of the PNS
b) Autonomic (involuntary) nervous systems (ANS) sensory neurons from visceral organs to CNS motor neurons to smooth & cardiac muscle and glands sympathetic division (speeds up heart rate) parasympathetic division (slow down heart rate) c) Enteric nervous system (ENS) involuntary sensory & motor neurons control GI tract neurons function independently of ANS & CNS
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Neurons
* Functional unit of nervous system Have capacity to produce action potentials electrical excitability* Neurons
Cell body: single nucleus with prominent nucleolus Nissl bodies (chromatophilic substance) rough ER & free ribosomes for protein synthesis neurofilaments give cell shape and support microtubules move material inside cell lipofuscin pigment clumps (harmless aging) 2. Cell processes = dendrites & axons* Parts of a Neuron
Nucleus with NucleolusAxons or Dendrites
Cell body
Neuroglial cells
* Dendrites
Conducts impulses towards the cell body Typically short, highly branched & unmyelinated Surfaces specialized for contact with other neurons Contains neurofibrils & Nissl bodiesimpulse
* 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* 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* 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* 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* Association or Interneurons
Named for histologist that first described them or their appearance* 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* Neuroglial Cells (CNS): Astrocytes
Star-shaped cells Form blood-brain barrier by covering blood capillaries Metabolize neurotransmitters Regulate K+ balance Provide structural support* 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* 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* Neuroglial Cells (CNS): Ependymal cells
Form epithelial membrane lining cerebral cavities & central canal Produce cerebrospinal fluid (CSF)* Neuroglial Cells (PNS): Satellite Cells
Flat cells surrounding neuronal cell bodies in peripheral ganglia Support neurons in the PNS ganglia* Neuroglial Cells (PNS): Schwann Cell
Cells encircling PNS axons Each cell produces part of the myelin sheath surrounding an axon in the PNS
* Myelination
Insulation of axon Increase speed of nerve impulse* Myelination: PNS
All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells Neurilemma is cytoplasm & nucleus of Schwann cell gaps called nodes of Ranvier Myelinated fibers Unmyelinated fibersNode of Ranvier
* 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* 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* Regeneration & Repair
Plasticity maintained throughout life sprouting of new dendrites synthesis of new proteins changes in synaptic contacts with other neurons Limited ability for regeneration (repair) PNS can repair damaged dendrites or axons CNS no repairs are possible