© 2012 pearson education, inc. nervous system all organisms must sense and react to their...
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© 2012 Pearson Education, Inc.
Nervous system
•All organisms must sense and react to their environment•Hydra – simplest nervous system with a nerve net•Arthropods and annelids – ganglia•We have neurons•Neurons consist of a cell body, dendrites and axon
© 2012 Pearson Education, Inc.
Types of neurons
1. Sensory – receive impulses from the environment and bring them to the body
2. Motor – transmits the impulse to the muscles or glands in response
3. Interneurons – link sensory and motor neurons – they are found in the brain and spinal cord
© 2012 Pearson Education, Inc. Figure 7.4a
Dendrite Cellbody
Nissl substance
Axonhillock
Neurofibrils Collateralbranch
OneSchwann cell
Node ofRanvier
Schwann cells,forming the myelin sheath on axon
Nucleus
Axonterminal
Mitochondrion
Axon
(a)
© 2012 Pearson Education, Inc. Figure 7.6
Dendrites Peripheralprocess (axon)
Ganglion
Cellbody
Sensoryneuron
Central process (axon)
Spinal cord(central nervous system)
Motor neuron
Interneuron(associationneuron)
Afferenttransmission
Peripheralnervous system
Receptors
To effectors (muscles and glands)
Efferent transmission
Reflex arc
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Nerve Impulses
•Resting neuron•The plasma membrane at rest is polarized•Fewer positive ions are inside the cell than outside the cell
© 2012 Pearson Education, Inc. Figure 7.9, step 1
Resting membrane is polarized. In the resting state, theexternal face of the membrane is slightly positive; its internalface is slightly negative. The chief extracellular ion is sodium(Na+), whereas the chief intracellular ion is potassium (K+).The membrane is relatively impermeable to both ions.
[Na+]
++++ + + + +[K+]
– ––– – – – –
++++ + + +– ––– – – –
1
© 2012 Pearson Education, Inc. Figure 7.9, step 2
Na+
++++ + + + +– ––– – – – –
++++ + + +– ––– – – –+
2 Stimulus initiates local depolarization. A stimuluschanges the permeability of a local "patch" of the membrane,and sodium ions diffuse rapidly into the cell. This changes thepolarity of the membrane (the inside becomes more positive;the outside becomes more negative) at that site.
© 2012 Pearson Education, Inc. Figure 7.9, step 3
Na+
++–– + + + ++ ––+ – – – –
++–– + + ++ ––+ – – –+
3 Depolarization and generation of an action potential.If the stimulus is strong enough, depolarization causesmembrane polarity to be completely reversed and an actionpotential is initiated.
© 2012 Pearson Education, Inc. Figure 7.9, step 4
–––– – + + ++ +++ + – – –
–––– + + ++ +++ – – –
Propagation of the action potential. Depolarization ofthe first membrane patch causes permeability changes in theadjacent membrane, and the events described in step arerepeated. Thus, the action potential propagates rapidly alongthe entire length of the membrane.
4
2
© 2012 Pearson Education, Inc. Figure 7.9, step 5
+++++ – – – –
– ––– + + + +
–+++ – – –– +–– + + +K+
Repolarization. Potassium ions diffuse out of the cell asthe membrane permeability changes again, restoring thenegative charge on the inside of the membrane and thepositive charge on the outside surface. Repolarization occursin the same direction as depolarization.
5
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Diagram on Pg. 193 in Cliffs
© 2012 Pearson Education, Inc. Figure 7.9, step 6
Initial ionic conditions restored. The ionic conditionsof the resting state are restored later by the activity of thesodium-potassium pump. Three sodium ions are ejected forevery two potassium ions carried back into the cell.
6
Cellexterior Na+
Na+Na+
K+
Dif
fusi
on
Na+
Dif
fusi
on
Na+ – K+
pump
Plasmamembrane
K+
K+
K+K+
Cellinterior
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Parts of the Nervous System
Central Nervous System
•Brain and spinal cord
Peripheral Nervous System
•All other nerves
•Somatic and autonomic
•Somatic – voluntary activities – skeletal muscle contraction
•Autonomic – involuntary activities – digestion/heartbeat
•These systems sometimes work together
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Nervous Tissue: Neurons
•Neurons = nerve cells
•Cells specialized to transmit messages
•Major regions of neurons
•Cell body—nucleus and metabolic center of the cell
•Processes—fibers that extend from the cell body
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Nervous Tissue: Neurons
•Cell body
•Nissl bodies
•Specialized rough endoplasmic reticulum
•Neurofibrils
•Intermediate cytoskeleton
•Maintains cell shape
•Nucleus with large nucleolus
© 2012 Pearson Education, Inc. Figure 7.4a
Dendrite Cellbody
Nissl substance
Axonhillock
Neurofibrils Collateralbranch
OneSchwann cell
Node ofRanvier
Schwann cells,forming the myelin sheath on axon
Nucleus
Axonterminal
Mitochondrion
Axon
(a)
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Nervous Tissue: Neurons
•Processes outside the cell body
•Dendrites—conduct impulses toward the cell body
•Neurons may have hundreds of dendrites
•Axons—conduct impulses away from the cell body
•Neurons have only one axon arising from the cell body at the axon hillock
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Nervous Tissue: Neurons
•Axons •End in axon terminals•Axon terminals contain vesicles with neurotransmitters
•Axon terminals are separated from the next neuron by a gap•Synaptic cleft—gap between adjacent neurons
•Synapse—junction between nerves
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Nervous Tissue: Neurons
•Myelin sheath—whitish, fatty material covering axons
•Schwann cells—produce myelin sheaths in around axons (PNS)
•Nodes of Ranvier—gaps in myelin sheath along the axon – saltatory conduction
•Oligodendrocytes—produce myelin sheaths around axons of the CNS
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Transmission of a Signal at Synapses
•When the action potential reaches the axon terminal, the electrical charge opens calcium channels
© 2012 Pearson Education, Inc. Figure 7.10, step 1
Axon oftransmittingneuron
Receivingneuron
Actionpotentialarrives.
Dendrite
Vesicles
Synapticcleft
Axon terminal
1
© 2012 Pearson Education, Inc. Figure 7.10, step 2
Vesicle fuses with plasma membrane.
2 Transmitting neuron
Synapticcleft
Ion channels
Neurotransmittermolecules
Receiving neuron
© 2012 Pearson Education, Inc. Figure 7.10, step 3
Vesicle fuses with plasma membrane.
2
3
Transmitting neuron
Neurotrans-mitter isreleased into synaptic cleft.
Synapticcleft
Ion channels
Neurotransmittermolecules
Receiving neuron
© 2012 Pearson Education, Inc. Figure 7.10, step 4
Vesicle fuses with plasma membrane.
4
Transmitting neuron
Neurotrans-mitter isreleased into synaptic cleft.
Neurotrans-mitter bindsto receptoron receivingneuron's membrane.
Synapticcleft
Ion channels
Neurotransmittermolecules
Receiving neuron
2
3
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Transmission of a Signal at Synapses
• If enough neurotransmitter is released, graded potential will be generated• Inhibitory or excitatory•Eventually an action potential (nerve impulse) will occur in the neuron beyond the synapse
© 2012 Pearson Education, Inc. Figure 7.10, step 5
Receptor
Ion channel opens.
Neurotransmitter
Na+
5
© 2012 Pearson Education, Inc. Figure 7.10, step 6
Neurotransmitter isbroken down andreleased.
Ion channel closes.
Na+
6
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Transmission of a Signal at Synapses
•The electrical changes prompted by neurotransmitter binding are brief •The neurotransmitter is quickly removed from the synapse
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Acetylcholine
•Neurotransmitter that is released from the axon as a result of Ca2+ entering the axon terminal•Stimulates muscle contraction•Active in parasympathetic nervous system•Norepinephrine and GABA are other neurotransmitters
© 2012 Pearson Education, Inc. Figure 7.27
Centralnervous system Peripheral nervous system Effector organs
Somatic nervous system
Sympatheticdivision
Autonomicnervoussystem
Parasympatheticdivision
KEY:
Preganglionic axons(sympathetic)
Postganglionic axons(sympathetic)
Myelination Preganglionic axons(parasympathetic)
Postganglionic axons(parasympathetic)
Acetylcholine
Acetylcholine
Acetylcholine
Epinephrine andnorepinephrine
Bloodvessel
Adrenal medullaAcetylcholine
Ganglion
Ganglion
Norepinephrine
Skeletalmuscle
Smooth muscle(e.g., in stomach)
Glands
Cardiacmuscle
© 2012 Pearson Education, Inc. Figure 6.4a(a)
Spinal cord
Motor unit 1
Motor unit 2
Axon terminals at neuromuscular junctions
Nerve
Axon ofmotorneuron
Motor neuroncell bodies
Muscle Muscle fibers
© 2012 Pearson Education, Inc. Figure 6.5, step 4
Action potential reaches axonterminal of motor neuron.
Calcium (Ca2+) channelsopen and Ca2+ enters the axon terminal.
Ca2+ entry causes somesynaptic vesicles to release theircontents (acetylcholine, a neurotransmitter) by exocytosis.
Acetylcholine diffuses acrossthe synaptic cleft and binds toreceptors in the sarcolemma.
Synaptic vesicle containing ACh
Axon terminal of motor neuron
Mitochondrion
Ca2+
Fusing synapticvesicleSarcoplasmof muscle fiber
Folds ofsarcolemma
Ca2+
AChreceptor
ACh
1
2
3
4
SarcolemmaSynapticcleft
© 2012 Pearson Education, Inc. Figure 6.5, step 5
ACh binds and channels openthat allow simultaneous passageof Na+ into the muscle fiber and K+ out of the muscle fiber. MoreNa+ ions enter than K+ ions leaveand this produces a local changein the electrical conditions of themembrane (depolarization), whicheventually leads to an actionpotential.
5Ion channel insarcolemma opens;ions pass.
Na+ K+
© 2012 Pearson Education, Inc. Figure 6.5, step 6
ACh effects are ended by itsbreakdown in the synaptic cleft bythe enzyme acetylcholinesterase.
6
Ion channel closed;ions cannot pass.
Acetylcholinesterase
Na+
Degraded AChACh
K+
© 2012 Pearson Education, Inc. Figure 9.13
Short term
Spinal cord
Catecholamines(epinephrine and norepinephrine)
Adrenalmedulla
1. Increased heart rate2. Increased blood pressure3. Liver converts glycogen to glucose and releases glucose to blood4. Dilation of bronchioles5. Changes in blood flow patterns, leading to increased alertness and decreased digestive and kidney activity6. Increased metabolic rate
1. Retention of sodium and water by kidneys2. Increased blood volume and blood pressure
1. Proteins and fats converted to glucose or broken down for energy2. Increased blood sugar3. Suppression of immune system
Short-term stress response Long-term stress response
Preganglionicsympatheticfibers
Nerve impulses
Hypothalamus
More prolongedStress
Releasing hormones
Corticotropic cells ofanterior pituitary
ACTH Adrenalcortex
Mineralocorticoids Glucocorticoids
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Brain
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Brain
•Cerebrum – convoluted folds, two halves
•Corpus callosum – allows the cerebral hemispheres to communicate
•Left side – dominant in math, language, logic
•Right – nonverbal thinking and image recognition
•Hypothalamus and pituitary gland – hormone release, limbic system (thirst, pain, sexual desire)
•Cerebellum – coordination and muscle movement
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Skeletal Muscle Functions
•Produce movement
•Maintain posture
•Stabilize joints
•Generate heat
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Microscopic Anatomy of Skeletal Muscle
•Sarcolemma—specialized plasma membrane
•Myofibrils—long organelles inside muscle cell
•Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum
© 2012 Pearson Education, Inc. Figure 6.3a
Sarcolemma
Myofibril
Dark(A) band
Light(I) band
Nucleus
(a) Segment of a muscle fiber (cell)
© 2012 Pearson Education, Inc. Figure 6.3b
(b) Myofibril or fibril (complex organelle composed of bundles of myofilaments)
Z disc H zone Z disc
I band A band I band M line
Thin (actin) filament
Thick (myosin) filament
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Microscopic Anatomy of Skeletal Muscle
•Sarcomere—contractile unit of a muscle fiber
•Organization of the sarcomere
•Myofilaments
•Thick filaments = myosin filaments
•Thin filaments = actin filaments
© 2012 Pearson Education, Inc. Figure 6.3c
Z disc
Sarcomere
M lineZ disc
Thin (actin) filament
Thick (myosin) filament
(c) Sarcomere (segment of a myofibril)
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The Nerve Stimulus and Action Potential
•Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract
•Motor unit—one motor neuron and all the skeletal muscle cells stimulated by that neuron
© 2012 Pearson Education, Inc. Figure 6.4a(a)
Spinal cord
Motor unit 1
Motor unit 2
Axon terminals at neuromuscular junctions
Nerve
Axon ofmotorneuron
Motor neuroncell bodies
Muscle Muscle fibers
© 2012 Pearson Education, Inc. Figure 6.5
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The Sliding Filament Theory of Muscle Contraction
•Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament
•Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere
•This continued action causes a sliding of the myosin along the actin
•The result is that the muscle is shortened (contracted)
© 2012 Pearson Education, Inc. Figure 6.7a–b
Myosin Actin
Z H
I
Z
A I
(a)
(b)
Z
I A I
Z
© 2012 Pearson Education, Inc. Figure 6.8a
Protein complex
Myosinmyofilament
Actinmyofilament(a)
In a relaxed muscle cell, the regulatory proteins formingpart of the actin myofilaments prevent myosin binding(see a). When an action potential (AP) sweeps along itssarcolemma and a muscle cell is excited, calcium ions(Ca2+) are released from intracellular storage areas (thesacs of the sarcoplasmic reticulum).
© 2012 Pearson Education, Inc. Figure 6.8b
Myosin-binding siteCa2+
Upper part of thick filament only
(b)
The flood of calcium acts as the final trigger forcontraction, because as calcium binds to the regulatoryproteins on the actin filaments, the proteins undergo a change in both their shape and their position on the thinfilaments. This action exposes myosin-binding sites onthe actin, to which the myosin heads can attach (see b),and the myosin heads immediately begin seeking out binding sites.
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The Cross Bridge cycle