nervous tissue chapter 12. nervous system controls and integrates all body activities basic...
TRANSCRIPT
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Nervous Tissue
Chapter 12
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Nervous System
Controls and integrates all body activities
Basic functions:Sense changeInterpret and remember changeReact to changes
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Nervous vs Endocrine System
Nervous system
electrical
fast
local
Endocrine system
chemical
slow
general
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Nervous System
Processing
Sensory Motor
input output
Integration
stimulus response
CNS
PNS
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Major Structures of the Nervous System
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Organization of Nervous System
Peripheral Nervous System - PNSsomatic (SNS)
sensorymotor
autonomic (ANS)sensorymotor parasympathetic
sympatheticenteric (ENS)
Central Nervous System – CNSbrain spinal cord
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Organization of Nervous System
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Transmit electrical impulses (action potentials)
Neurons
Functional unit of the Nervous System
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Structural Classes of Neurons
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Functional Classes of Neurons
Afferent
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Efferent
Functional Classes of Neurons
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Interneurons
Functional Classes of Neurons
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Supportive and protective role
CNS
Astrocytes
Oligodendrocyte
Microglia
Ependymal cells
PNS
Schwann cells
Neuroglia
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Schwann Cells
Node of Ranvier: exposed axon between Schwann cells
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Myelination and Schwann Cells
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Gray and White Matter
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Overview of Nervous Function
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Ion Channels
Leakage channels
Voltage-gated channels
Ligand-gated channels
Mechanically gated channels
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Ion Channels
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Ion Channels
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Resting Membrane Potential
-70 mV difference between outside and inside of the membrane, maintained by active transport
High concentration of Na+ on outside
High concentration of K+ on inside
Cannot diffuse freely because gates of their
channels are closed
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• Negative ions along inside of cell membrane & positive ions along outside– potential energy difference at rest is -70 mV
• Resting potential exists because– concentration of ions different inside & outside
• extracellular fluid rich in Na+ and Cl-
• cytosol full of K+, organic phosphate & proteins
– membrane permeability differs for Na+ and K+
• 50-100x’s greater permeability for K+
• inward flow of Na+ can’t keep up with outward flow of K+
• Na+/K+ pump removes Na+ as fast as it leaks in
Resting Membrane Potential
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Resting Membrane Potential
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Resting Membrane Potential
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Graded Potential
Typically on dendrites or cell body
Graded means that potential varies in amplitude.
Stronger the stimulus, greater the amplitude.
Stronger the stimulus the farther it will travel.
Decreases as it gets farther away from the stimulus point.
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Graded Potential
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Graded Potential
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Graded Potential
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Graded Potential
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Action Potential
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Action Potential
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Action Potential Summary
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• Origin– GPs arise on dendrites and cell bodies– APs arise only at trigger zone on axon hillock
• Types of Channels– AP is produced by voltage-gated ion channels– GP is produced by ligand or mechanically-
gated channels • Conduction
– GPs are localized (not propagated)– APs conduct over the surface of the axon
Comparison of Graded & Action Potentials
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Comparison of Graded & Action Potentials
• Amplitude– amplitude of the AP is constant (all-or-none)– graded potentials vary depending upon stimulus
• Duration– The duration of the GP is as long as the stimulus lasts
• Refractory period– The AP has a refractory period due to the nature of
the voltage-gated channels, and the GP has none.
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Comparison of Graded & Action Potentials
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Continuous vs Saltatory Conduction
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Speed of Impulse PropagationThe propagation speed of a nerve impulse is not
related to stimulus strength. – larger, myelinated fibers conduct impulses faster due
to size & saltatory conduction
Fiber types– A fibers: largest (5-20 µm & 12-130 m/sec)
• myelinated somatic sensory & motor to skeletal muscle
–B fibers: medium (2-3 µm & 15 m/sec)
• myelinated visceral sensory & autonomic preganglionic
–C fibers: smallest (0.5-1.5 µm & 0.5-2 m/sec)
• unmyelinated sensory & autonomic motor
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Encoding of Stimulus Intensity
How do we differentiate a light touch from a firmer
touch?
– frequency of impulses• firm pressure generates impulses at a higher frequency
– number of sensory neurons activated• firm pressure stimulates more neurons than does a light touch
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Signal Transmission at Synapses
2 Types of synapses– electrical
• ionic current spreads to next cell through gap junctions
• faster, two-way transmission & capable of synchronizing groups of neurons
– chemical• one-way information transfer from a presynaptic neuron to
a postsynaptic neuron– axodendritic -- from axon to dendrite
– axosomatic -- from axon to cell body
– axoaxonic -- from axon to axon
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Chemical Synapse
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Postsynaptic Potential
Excitatory postsynaptic potential (EPSP)
Na+ and K+ gates open at the same time, Na+ diffuses faster results in a depolarizing potential
Inhibitory postsynaptic potential (IPSP)
Membrane made more permeable to K+ and Cl-, Na+ not affected results in a hyperpolarization
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Postsynaptic potentials
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Removal of Neurotransmitter
Neurotransmitter must be removed from the synapse for
normal synaptic function.
- Diffusion
- Enzymatic degradation
- Uptake by cell
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Summation
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Summation
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Summary
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Neurotransmitters
Acetylcholine
Amino Acids glutamate and aspartateGABA and glycine
Biogenic amines norepinephrineepinephrinedopamineserotonin
ATP and Other Purines
Nitric oxide
Neuropeptidesendorphins
enkephalindynorphins
substance P
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Neural Circuits
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Regeneration