n chapter 48 ~ nervous system. u/animations/actionpotential.sw f n downloads\actionpotential.swf...
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Chapter 48 ~ Nervous System
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http://outreach.mcb.harvard.edu/animations/actionpotential.swf
http://outreach.mcb.harvard.edu/animations/actionpotential.swf
downloads\actionpotential.swf
downloads\nerve action potential.swf downloads\animations.htm
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downloads\animationsRaven test.htm
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Nervous System OverviewNervous System Overview Sensory Input Integration Motor Output-signal conducted from processing
center to effector cells
Signals Conducted by Nerves-extensions of nerve cells
Nervous System Composition:Neurons and Glia (supporting cells)
Neurons communicate information via electrical and chemical signals
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Both Divisions of the Nervous System InvolvedBoth Divisions of the Nervous System Involved
1. Central nervous system (CNS)~ brain and spinal cord; Integration
2. Peripheral nervous system (PNS)~ sensory (input) and motor neurons (output)
Effector cells~ muscle or gland cells
Nerves~ bundles of neurons wrapped in connective tissue
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Neuron structureNeuron structure Neuron- structural and functional unit
– Cell body- nucelus and organelles
– Dendrites- signals to cell body. Short, numerous
– Axons- away from cell body. Long, Myelin sheath- supporting, insulating layer produced by Schwann Cells Schwann cells-PNS support cells; surround axons Axon hillock-Hillock-axon extends from here Synaptic terminals~ neurotransmitter releaser
Synapse- gap / neuron junction
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3 Classes of neurons3 Classes of neurons
1. Sensory neuron: receive & convey from sensory environment information to spinal cord
2.Interneurons: information integration; located in CNS. Synapse only with other neurons.
3. Motor neurons: convey impulses from CNS to effector cell. (muscle or gland)
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Neurons Grouped into Nerve CircuitNeurons Grouped into Nerve Circuit
The Reflex Arc– Simplest : – Knee-Jerk Reflex (Patellar
Reflex)– Stretch receptor– simple response; sensory
to spinal cord to motor neurons—knee contracts
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Neural SignalingNeural Signaling Signal transduction depends on voltages across neuron plasma
membranes.– Membrane Potential: voltage differences across the plasma membrane).
Net negative charge of about -70mV
Ions Intracellular ( -) ; K+ principal cation Large organic ions- anions Extracellular (less negative) Na+- principal cation Cl- main anion. Ion channels- ungated, gated; all selective
K+ diffuses out (Na+ in); large anions cannot follow….selective permeability of the plasma membrane
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Creating & Maintaing the Membrane PotentialCreating & Maintaing the Membrane Potential
Na + - K + Pumps --pump against their conc. gradients
ATP
K+ pumped back in
Na+ pumped back out
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Changes in membrane potential key to neural transmission
Changes in membrane potential key to neural transmission
Only neurons and muscle cells can change their membrane potentials in response to stimuli – Excitable Cells– Sensory neurons-environmental stimuli– Interneurons stimuli transmitted via other neurons
– Resting Potential: M.P. of excitable cell at rest.– Change due to flow of ions as gated ion channels open.– stimuli cause ion channels to open
Stimuli that open K+ channels HYPERPOLARIZE the neuron
Stimuli that open NA+ channels DEPOLARIZE the neuron
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Graded Potentials –these voltage changes Graded Potentials –these voltage changes
1- Hyperpolarization (outflow of K+); increase in electrical gradient; cell becomes more negative
2- Depolarization (inflow of Na+); reduction in electrical gradient; cell becomes less negative
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MylenationMylenation
Electrical insulation—lipid is poor conductor
– Increasing speed of nerve impulse propagation
Multiple Sclerosis: myelin sheaths deteriorated-los of coordination
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Normal Membrane PotentialNormal Membrane Potential
Resting Potential: Resting Neuron -70 mV Cytoplasm is negatively charged relative to cell
interior
Resting potentialResting potential~ the membrane potential of the unexcited nerve. – A change in voltage MAY result in an
electrical impulse.
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When the Threshold potential is reached, usually sl. More positive (-50 to -55 mV)….
The action potential is triggered….
– The rapid change in membrane potential in an excitable cell
– b/c stimulus triggered the selective opening and closing of voltage-gated ion channels
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Action Potential-All Or None change in the Membrane Potential Phases
Action Potential-All Or None change in the Membrane Potential Phases
1. Resting stage •both channels closed
2-Depolarization: •a stimulus opens some Na+ channel gates
Na+ influx reverses membrane polarity.
Threshold reached. (cell interior sl. positive)
Action potential generated .
3-Repolarization •Na+ channels close. K+ channels open; K+ leaves
cell returns to resting potential—then ..
4-Undershoot •K+ channels still open-temporarily HYPERPOLAR.
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The Action PotentialThe Action Potential
Followed by a Refractory period~ insensitive to stimulus.
Amplitude not affected by stimuli Intensity
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Action Potentials are self-propagatingAction Potentials are self-propagating
Action Potential regenerated along axon membrane begins at Axon Hillock “Travel” of the action potential is self-propagating One direction only.
Nodes of Ranvier-action potential jumps from one node to the next– Gaps, ion sensitive channels concentrated here, extracellular fluid
contact here
Forward direction only
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Action potential speed:Action potential speed:1) Axon diameter (larger = faster; 100m/sec)
2) Saltatory Conduction:
– Mylenation
– Nodes of Ranvier (concentration of ion channels in gaps of the myelin).
– A.P. “jumps” from node to node. 120m/sec
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Chemical or Electrical Communication between cells occurs at synapses
Chemical or Electrical Communication between cells occurs at synapses Synapse-tiny gap
Presynaptic cell: transmitting cell Postsynaptic cell: receiving cell
1) Electrical Synapses-via gap junctions; no delay or less in signal strength; less common; fish tail-swim away quickly from predator
2) Chemical Synapses: synaptic cleft separates pre and post-synaptic cells.
Not electrically coupled
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Synaptic communicationSynaptic communication Synaptic cleft: separation gap Synaptic vesicles: neurotransmitter
releasers
When an Action Potential arrives at synaptic terminal of presynaptic cell
Causes Ca++ influx; Synaptic vesicles fuse with presynaptic membrane and release…. Neurotransmitter
Neurotransmitters quickly degraded
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Neurotransmitter maydo one of the followingNeurotransmitter maydo one of the following
1. Excite the membrane by depolarization
Or
2. Inhibit the postsynaptic cells by hyperpolarization
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Types of NeurotransmittersTypes of Neurotransmitters Acetylcholine (most common)
– may be excitatory or inhibitatory– skeletal muscle
Biogenic amines (derived from amino acids)•norepinephrine , epinephrine•dopamine •serotonin (from tryptophan)
Amino acids– GABA—most abundant inhibitory transmitter in brain
Neuropeptides (short chains of amino acids)•endorphin-natural analgesics for the brain
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Gaseous Signals of the Nervous SystemGaseous Signals of the Nervous System
NO (nitric oxide)—blood vessel dilation.
– Acetylcholine stimulates blood vessel walls to release NO; neighboring smooth muscles relax & dilate heart’s blood vessels.
– Nitroglycerine is converted to NO—similar response
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Nervous system organization tends to corrolate with body symmetry
Nervous system organization tends to corrolate with body symmetry
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Vertebrate PNSVertebrate PNS
Cranial nerves (brain origin)
Spinal nerves (spine origin)
Sensory division Motor division
•somatic system voluntary, conscious control •autonomic system √parasympathetic
conservation of energy √sympathetic
increase energy consumption
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The Vertebrate BrainThe Vertebrate Brain Forebrain
•cerebrum~memory, learning, emotion•cerebral cortex~sensory and motor nerve cell bodies •corpus callosum~connects left and right hemispheres •thalamus; hypothalamus
Midbrain •inferior (auditory) and superior (visual) colliculi
Hindbrain •cerebellum~coordination of movement •medulla oblongata/ pons~autonomic, homeostatic functions