Unit 4: Functional Organization of Nervous Tissue

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Unit 4: Functional Organization of Nervous Tissue
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  1. Functions

    *Figure 11.2, p. 371
    • Sensory – recognize changes in environment (stimuli) 
    • Integration – analyze sensory information, stores information, makes decisions
    • Motor – initiates impulses to effectors (muscles or glands)
    • Maintaining homeostasis
    • •Establishing & maintaining mental activity 
  2. Organization

    *Figure 11.1, p. 370

    *Figure 11.3, p. 372
    • •Central Nervous System (CNS) 
    •     –brain and spinal cord
    • •Peripheral Nervous System (PNS)
    •    -Sensory (afferent) division
    •    –Motor (efferent) division
    • •Somatic Nervous System
    • •Autonomic Nervous System (ANS)
    •    –Sympathetic division
    •    –Parasympathetic division
    •    –Enteric NS
  3. Review: Cells of the Nervous System
    • •Neurons = nerve cells 
    • •Neuroglia = glial cells = nonneural cells 

    • Neurons (Nerve Cells) 
    • •most are amitotic (no cell division) 
    • •high metabolic rates (aerobic respiration) 
    • •long-lived 
    • •produce impulses to transfer information 
    • •communicate with each other at synapses 
    •    –junction of nerve cell with another cell
  4. Review: Neurons

    *Figure 11.4, p. 373
    • Consists of: - Cell body 
    • - Dendrites 
    • - Axons 

    • Cell body (perikaryon) 
    • •contains nucleus, mitochondria, neurofilaments,  & microtubules 
    • •rich in ribosomes 
    • •extensive rough ER (Nissl bodies) & Golgi apparatus 
    •    –protein synthesis & export to axon or dendrite 
    • no myelin 
  5. Review: Neuron Processes

    *Figure 11.4, p. 373
    • Dendrites 
    • •short, highly branched with dendritic spines (extensions)
    • •bring depolarization (message) toward cell body
    • no myelin

    • Axons
    • •take action potential (impulse) away from cell body
    • •myelinated or unmyelinated
    • •contain trigger zone (axon hillock & initial segment)
    • •form presynaptic terminals
  6. Types of Neurons
    • Classification: 
    • Structural– number of processes extending from cell body
    •    –multipolar
    •    –bipolar
    •    –unipolar

    • Functional – type and direction of information
    •    –sensory
    •    –motor
    •    –interneurons
  7. Structural Classification

    *Figure 11.5, p. 374
    • Multipolar neurons 
    • •most abundant - ~99% of all neurons
    • •single axon & many dendrites 
    •    –may be myelinated or unmyelinated 
    • •most neuron in CNS (interneurons) & motor neurons 
  8. Structural Classification

    *Figure 11.5, p. 374
    • Bipolar neurons 
    • •two processes: one axon & one dendrite
    • •sensory 
    • •in sensory organs (retina of eye & olfactory mucosa)
  9. Structural Classification

    *Figure 11.5, p. 374
    • Pseudounipolar neurons 
    • •single process extending from cell body
    •   –divides into 2 branches: 
    •     •to CNS 
    •     •to periphery – dendritelike sensory receptors (conduct action potential toward cell body) receptive
    •     •sensory neurons in ganglia of PNS
  10. Practice Question
    • •Bipolar cells are commonly: 
    • a. motor neurons
    • b. called neuroglia
    • c. found in ganglia
    • d. found in the retina of the eye
  11. Functional Classification
    • •Sensory: 
    •    –most unipolar or bipolar 
    •    –afferent (brings sensory info to CNS) 
    • •Interneurons (Association): 
    •    –most multipolar 
    •    –integration between sensory & motor in CNS 
    • •Motor: 
    •    –most multipolar 
    •    –efferent (goes toward effector)
  12. NS Cells: Neuroglia (Glial cells)
    • •more numerous than neurons 
    • •support and protect neurons 
    • Neuroglia of CNS 
    •    –astrocytes 
    •    –oligodendrocytes 
    •    –ependymocytes (ependymal cells) 
    •    –microglia 
    • **Tumors due to abnormal divisions of glial cells of CNS 

    • Neuroglia of PNS
    •    –satellite cells  
    •    –Schwann cells (neurolemmocytes)
  13. Neuroglia of CNS

    *Figure 11.6, p. 375
    • Astrocytes - “star cells” 
    • •most abundant
    • •form foot processes that cover surfaces of blood vessels, neurons & pia mater (blood-brain barrier)  
    •    –control composition of interstitial fluid (regulate ions & gases)
    • •may influence synaptic sygnaling by secreting/ removing neurotransmitters (essential for learning &  memory)
  14. Neuroglia of CNS

    *Figure 11.8, p. 375
    • Microglia 
    •    –specialized macrophages 
    •    –phagocytize necrotic tissue & foreign substances 
    •    –*cells of immune system can’t gain access to CNS
  15. Neuroglia of CNS

    *Figure 11.7, p. 375
    • Ependymal cells 
    •    –epithelial lining of brain ventricles and central canal of spinal cord
    •    –choroid plexus = ependymal cells + blood vessels
    • •produce CSF (cerebrospinal fluid)
  16. Neuroglia of CNS

    *Figure 11.9, p. 376
    • Oligodendrocytes 
    •    –form cytoplasmic extensions that surround axons (produce myelin sheath)
    •    –single oligodendrocyte may form myelin sheath around portions of several axons
  17. Neuroglia of  PNS

    *Figure 11.10, p. 376
    • Satellite cells 
    •    –surround cell bodies in sensory ganglia
    •    –provide support & nutrients to cell bodies
    •    –protect neurons from heavy-metal poisons (mercury, lead)
  18. Neuroglia of PNS

    *Figure 11.4, p. 373

    *Figure 11.11, p. 376
    • Schwann cells (neurolemmocytes) 
    •    –form myelin sheaths around larger nerve fibers 
    •    –play role in regeneration of nerve fibers 
    •    –may be useful to treat damaged regions of spinal cord 
    •       •Schwann cell transplants are tried in lab experiments
  19. Let’s apply
    •Predict the effect on the part of a severed axon that’s no longer connected to its neuron cell body. 

    •Explain
  20. Let’s apply: Answer
    •Distal portion --> detached from cell body --> no access to enzymes & proteins (for repair) -->axon degenerates & dies 

    •Proximal portion --> attached to cell body --> nucleus & new proteins available --> remains alive -->may grow & replace severed distal axon
  21. Myelin Sheath: Importance
    • •myelin protects & electrically insulates axons from one another à more rapid spread of action potential 
    • • formed by:
    •    –oligodendrocytes in CNS
    •    –Schwann cells in PNS – guide neuron regeneration

    Multiple sclerosis – destruction of myelin sheath in CNS --> diminishes impulse conduction

    What cells of the nervous system might be affected?
  22. Myelin Sheath - Importance

    *Figure 11.11, p. 376
    • Myelinated axons 
    • •white appearance (lipids & proteins)
    • nodes of Ranvier = gaps between sheath cells
    • •allow faster speed of impulse conduction (less energy required)

    • Unmyelinated axons
    • •axons rest in invaginations of oligodenrocytes or Schwann cells
    •    –plasma membrane surrounds but not wraps around axon many times
  23. Regeneration of Neurons
    cell body - if cell body is damaged à cell dies, neurons “downstream” from damaged neuron may die as well 

    • peripheral axons (PNS)
    • •portion of axon distal to site of damage is degraded within 1 week
    • •neurolemma usually remains intact (depends on severity of damage)
    • •axon regrows at about 1-5 mm per day
    • •when two ends of injured axon are not aligned in close proximity --> regeneration is unlikely
  24. Neuron Regeneration in PNS - Steps
    1.axon & its myelin sheath distal to site of injury disintegrate 

    2.macrophages enter area to phagocytize debris & release mitosis-stimulating chemicals

    • 3.Schwann cells:
    • •proliferate in response to mitosis-stimulating chemicals & nerve growth factor (NGF)
    • •form column of cells which axon follows during regeneration
    • •Schwann cells that form cord will eventually remyelinate regenerated axon
  25. Regeneration of Neurons in CNS
    • •limited & poor compared to nerves of PNS
    •  
    • oligodendrocytes 
    •    –cell body a distance from the axons they myelinate 
    •    –fewer oligodenrocytes than Schwann cells 
    •    –myelin sheaths (oligodendrocytes) of nearby axons secrete inhibitory proteins or die 

    •nearby astrocytes (reactive astrocytes) may proliferate to form wall around the injury --> scar-forming astrocytes limit regeneration

    •CNS macrophages - phagocytize debris more slowly than peripheral macrophages do
  26. Regeneration of Neurons in CNS
    • •in experiments 
    •    –macrophages transplanted to CNS can secrete proteins that inhibit inhibitory proteins
    •    –ALS - mice or rat models are being used to replace damaged motor neurons with stem cells
  27. Organization of Nervous Tissue
    Nerve: bundle of neuron fibers in PNS 

    Tract: bundle of neuron fibers in CNS

    Ganglion (ganglia): cluster of cell bodies in PNS

    Nucleus (nuclei): cluster of cell bodies in CNS

    • White matter: myelinated neuron fibers in CNS
    •    –outside in spinal cord; inside in brain

    • Gray matter: cell bodies, dendrites, and unmyelinated neuron fibers in CNS
    •    –inside in spinal cord, outside in brain
  28. Types of Nerves
    • Cranial nerves – originate from brain 
    •    –12 pairs 

    • Spinal nerves – originate from spinal cord
    •    –31 pairs 
    •    –Sensory nerves - carry afferent fibers only
    •    –Motor nerves - carry efferent fibers only
    •    –Mixed nerves - carry both kinds of fibers
  29. Practice Questions
    1.The oligodendrocytes can myelinate several axons. 

    TRUE/FALSE

    2.In the neuron, the rough ER is also known as Nissl bodies.

    TRUE/FALSE

    3. After axonal injury, regeneration in peripheral nerves is guided by:

    • a.Oligodendrocytes
    • b.Schwann cells
    • c.Dendrites
    • d.Golgi organs
  30. Let’s apply
    •Multiple sclerosis (MS) is a disease in which the myelin sheaths are destroyed. 

    With what process does this interfere and what would be the consequence?
  31. Let’s apply: Answer 
    Demyelination results in interference with salutatory conduction (requires Nodes of Ranvier) which would result in a slowing down of nerve impulse propagation.

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