Ch. 9

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Ch. 9
2013-11-14 22:10:20
Nervous Tissue

Nervous System
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  1. Structures of the Nervous System
    Brain: neurons enclosed within skull  Spinal cord: connects to brain and enclosed within spinal cavity  Nerves: bundles of many axons of neurons  Cranial nerves (12 pairs) emerge from brain  Spinal nerves (31 pairs) emerge from spinal cord  Ganglia: groups of neuron cell bodies located outside of brain and spinal cord  Enteric plexuses: networks in digestive tract  Sensory receptors: monitor changes in internal or external environments
  2. Functions of the Nervous System
    Sensory receptors and sensory nerves  Carry information into brain and spinal cord  Integration: information processing  Perception = awareness of sensory input  Analyzing and storing information to help lead to appropriate responses  Motor activity: efferent nerves  Signals to muscles and glands (effectors)
  3. Organization of The Nervous System
    Central Nervous System (CNS)  Brain and spinal cord  Peripheral Nervous System (PNS)  All nervous system structures outside of the CNS
  4. Histology of the Nervous System
    Neurons  Can respond to stimuli and convert stimuli to electrical signals (nerve impulses) that travel along neurons  Neuroglia cells: support, nourish and protect neurons  Neuroglia critical for homeostasis of interstitial fluid around neurons
  5. Neuronal Structure
    Dendrites: highly branched structures that carry impulses to the cell body  Axon: conducts away from cell body toward another neuron, muscle or gland  Emerges at cone-shaped axon hillock  Axon terminals: contain synaptic vesicles that can release neurotransmitters
  6. Structural Classes of Neurons
    Multipolar  Have several or many dendrites and one axon  Most common type in brain and spinal cord  Bipolar  Have one dendrite and one axon   Example: in retina of eye and inner ear  Unipolar  Have fused dendrite and axon  Sensory neurons of spinal nerves
  7. Functional Classes of Neurons
    Sensory (afferent)  Convey impulses into CAN (brain or spinal cord)  Motor (efferent)  Convey impulses from brain or spinal cord out through the PNS to effectors (muscles or glands)  Interneurons (association neurons)  Most are within the CNS  Transmit impulses between neurons, such as between sensory and motor neurons
  8. Neuroglia
    Cells smaller but much more numerous than neurons  Can multiply and divide and fill in brain areas  Gliomas: brain tumors derived from neuroglia  Functions  Do not conduct nerve impulses  Do support, nourish and protect neurons
  9. Type of Neuroglial cell
    • CNS  Astrocytes: help form blood brain barrier  Oligodendrocytes: produce myelin in CNS  Microglia: protect CNS cells from disease  Ependymal cells: form CSF in ventricles 
    • PNS  Schwann cells: produce myelin around PNS neurons; help to regenerate PNS axons  Satellite cells: support neurons in PNS ganglia
  10. Astrocytes
    help form blood brain barrier
  11. Oligodendrocytes
    produce myelin in CNS
  12. Microglia
    protect CNS cells from disease
  13. Ependymal cells
    form CSF in ventricles
  14. Schwann Cells
    produce myelin around PNS neurons hel regenerate PNS axons
  15. Satellite Cells
    support neurons in PNS ganglia
  16. Myelination
    • Axons covered with a myelin sheath
    • -many layers of lipid and protein, insulates neurons
    • -increase speed of nerve conduction
    • -appears white
    • -Nodes of Ranvier: gaps in the myelin
    • -- are important for rapid signal conduction
    • -Some disease destroy myelin
    • -multiple sclerosis
    • -tay- sachs
  17. Clusters of neuron cell bodies
  18. Ganglion: cluster of cell bodies in PNS  Nucleus: cluster of cell bodies in CNS
  19. Bundles of axons
    • Nerve: bundle of axons in PNS
    • Tract: bundle of axons in CNS
  20. Gray and White Matter
    • White matter, primarily myelinated axons
    • Gray matter: cell bodies, dendrites, unmyelinated axons, axons terminals, neuroglia
  21. Locations of gray and white matter
    Spinal cord -->white matter tracts surround centrally  located gray matter H of butterfly

    Brain-->gray matter in thin cortex surrounds white matter
  22.  Regeneration of PNS neurons
    Axons and dendrite in the PNS can be repaired if cell body is intact and Schwann cells functional. These form a regeneration tube and grow axons or dendrites if scar tissue does not fill the tube
  23.  Regeneration of CNS neurons
    • Very limited even if cell body is intact
    • inhibited by neuroglia and by lack of fetal growth-stimulators
  24.  Central nervous system (CNS) structures:
    brain, and spinal cord
  25.  Peripheral nervous system (PNS) structures:
    • cranial nerves and branches
    • spinal nerves and branches
    • ganglia
    • sensory receptors
  26. Peripheral nervous system (PNS) divisions
    • Somatic: sensory neruons from head, body wall, limbs, special sense organs
    • motor neurons to skeletal muscle, voluntary

    • Autonomic nervous system: sensory neurons from viscera, motor neurons to viscera (cardiac muscle, smooth muscle, glands involuntary)
    • -Sympathetic: fight or flight
    • -Parasympathetic rest and digest

    Enteric nervous system ENS brain of the gut
  27.  Peripheral nervous system (PNS)
    • Enteric nervous system "brain of the gut"
    • Sensory neurons monitor chemical changes and stretching of GI wall 
    • Motor neurons regulate contractions, secretions and endocrine secretions (involuntary)
  28. Action Potentials= Nerve impluses
    Require: A membrane potential,a charge differnece across cell membrane (polarization)

    Ion channels: allow ions to move by diffusion from high to low concentration
  29. Ion Channels
    Leakage channels: allow ions to leak through membrane, there are more for k+ than for Na+

    • Gated Channels
    • Open and close on command
    • Respond to changes in membrane so can generate and conduct action potentials
  30. Resting Membrane Potential
    Caused by presence of ions:  Inside (more negative) because cytosol has:  Many negative ions (too large to leak out): amino acids (in cellular proteins) and phosphates (as in ATP)  K+ that easily leaks out through many K+ channels  Outside (more positive) because interstitial fluid has:  Few negative ions  Na+ that does not leak out of cell: few Na+ channels  Membrane "pumps" that quickly pump out Na+ that does leak (diffuse) into cell
  31. Action Potential  Series of events that activate cell membrane in neuron or muscle fiber
    Triggers resting membrane to become more permeable to Na+ 

    Causes enough Na+ to enter cell so that cell membrane reaches threshold (~ –55 mv) 

    If so, the following events occur: action potential which spreads along neuron or muscle fiber
  32.  Depolarizing phase
    Na+ channels open  as more Na+ enters cell, membrane potential rises and becomes positive (–70  0  + 30 mv)
  33.  Repolarizing phase
    • K+ channels open  as more K+ leave cell, membrane potential is returned to resting value (+ 30  0  –70 mv)
    •  May overshoot: hyperpolarizing phase 

    Typically depolarization and repolarization take place in about 1 millisecond (1/1000 sec)
  34. Recovery
    Levels of ions back to normal by action of Na+/K+ pump 

    Refractory period (brief): even with adequate stimulus, cell cannot be activated
  35.  All-or-none principle
    If a stimulus is strong enough to cause depolarization to threshold level, the impulse will travel the entire length of the neuron at a constant and maximum strength.
  36. Conduction of Nerve Impluses
    Nerve impluse conduction: each section triggers the next locally as even more Na+ channels are opened
  37. Types of Conductions
    • Continous Conduction: in unmyelinated fibers; slower form of conduction
    • Saltatory Conduction: in myelinated fibers; faster as impluses "leap" between nodes of ranvier
  38. Factors that increase rate of conduction
    • -myelin
    • -larger diameter
    • -warm nerve fibers
  39. Synaptic Transmission:Similar sequence of events occurs at:
    • -Synapse (neuron-neuron)
    • -Neuromuscular junction (neuron-muscle fiber)
    • -neuroglandular junction (neuron-gland)
  40.  Components of synapse:
    Sending neuron: presynaptic neuron (releases neurotransmitter) 

    Space between neurons: synaptic cleft

    Receiving neuron: postsynaptic neuron