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Download Lecture 048 - Neurons and Nervous Systems
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Colonie High AP Biology DeMarco/Goldberg Nervous System Cells Neuron a nerve cell dendrites signal direction Chapter 45 cell body Neurons and Nervous Systems Structure fits function many entry points for signal one path out transmits signal axon signal direction synapse dendrite cell body axon terminal branches Types of Neurons Fun Facts About Neurons Most specialized cell in cell body sensory neuron “afferent” cell body axon dendrites interneuron “associative” dendrites cell body animals Longest cell blue whale neuron giraffe axon human neuron 10-30 meters 5 meters 1-2 meters motor neuron “efferent” Transmission of a Nerve Signal Think dominoes! start the signal knock down line of dominoes by tipping 1st one trigger the signal propagate the signal Nervous system allows for ~ millisecond response times Transmission of a Nerve Signal Neuron has similar system protein channels are set up once first one is opened, the rest open in succession all or nothing response do dominoes move down the line? no, just a wave through them! re-set the system before you can do it again, have to set up dominoes again reset the axon a “wave” action travels along neuron have to re-set channels so neuron can react again Colonie High AP Biology DeMarco/Goldberg Cells: Surrounded by Charged Ions Cells live in a sea of charged ions Cells have voltage! Opposite charges on opposite sides of anions cell membrane more concentrated within the cell Cl-, charged amino acids (aa-) charge gradient more concentrated in the extracellular fluid K+, Na+ K+ Na+ Na+ K+ aa- K+ stored energy (like a battery) + + + + + + + + + + + + + + + Na+ aaCl- Na+ ClK+ Na+ aa- Na+ K+ aa- K+ Na+ ClCl- Na+ Na+ Na+ Na+ Cl- aa- – – – – – – – – – – – – – – – – – – – – – – – – – – – – – channel leaks K+ membrane is polarized negative inside; positive outside cations + + + + + + + + + + + + + + + + aa- Cl- Measuring Cell Voltage How does a nerve impulse travel? Stimulus: nerve is stimulated reaches threshold potential open Na+ channels in cell membrane Na+ ions diffuse into cell charges reverse at that point on neuron positive inside; negative outside cell becomes depolarized – + + + + + + + + + + + + + + + – – – – – – – – – – – – – – Na+ + – – – – – – – – – – – – – – – + + + + + + + + + + + + + + unstimulated neuron = resting potential of ~60 mV How does a nerve impulse travel? Wave: nerve impulse travels down neuron How does a nerve impulse travel? Re-set: 2nd wave travels down neuron Gate + change in charge opens + – + next Na+ gates down the line “voltage-gated” channels channel Na+ ions continue to diffuse into cell channel closed open “wave” moves down neuron = action potential K+ channels open K+ ions diffuse out of cell charges reverse back at that point K+ channels up more slowly than Na+ channels negative inside; positive outside K+ – – – + + + + + + + + + + + + + + + – – – – – – – – – – – – + – – – – + + + + + + + + + + – + + + + – – – – – – – – – – Na+ Na+ + + + – – – – – – – – – – – – – – – + + + + + + + + + + + + wave – + + + + – – – – – – – – – – + – – – – + + + + + + + + + + wave Colonie High AP Biology DeMarco/Goldberg How does a nerve impulse travel? Combined waves travel down neuron wave of opening ion channels moves down neuron signal moves in one direction flow of K+ out of cell stops activation of Na+ channels in wrong direction How does a nerve impulse travel? Action potential propagates wave = nerve impulse, or action potential brain finger tips in milliseconds! K+ K+ + + + – – – – + + + + + + + + – – – + + + + – – – – – – – – + + + + + + + – – – – + + + + – – – – – – – + + + + – – – – Na+ Na+ – – – + + + + – – – – – – – – + + + – – – – + + + + + + + + – – – – – – – + + + + – – – – + + + + + + + – – – – + + + + wave Voltage-gated Channels Ion channels open & close in response to changes in charge across membrane Na+ channels open quickly in response to depolarization & close slowly K+ channels open slowly in response to depolarization & close slowly wave How does the nerve re-set itself? After firing a neuron has to re-set itself Na+ needs to move back out K+ needs to move back in both are moving against concentration gradients need a pump!! K+ Na+ K+ – – – – – – – – – + + + – – – + + + + + + + + + – – – + + + Na+ Na+ active transport protein in membrane requires ATP Na+ 3 pumped out + 2 K pumped in re-sets charge across membrane ATP Na+ Na+ Na+ + K K+ Na+ + +Na Na+ Na Na+ K+ K+ Na+ K+ K+ + Na+ K+ Na+ K+ K Na+ K+ Na+ – – – – – – – – – – + + + + – + + + + + + + + + + – – – – + wave wave How does the nerve re-set itself? Na+ / K+ pump K+ K+ + + + + + + + – – – – + + + + – – – – – – – – – – + + + + – + + + + + + + + + – – – + + + – – – – – – – – – + + + – – – Na+ Na+ Neuron is ready to fire again… Na+ Na+ Na+ K+ aa- aaNa+ Na+ Na+ K+ Na+ Na+ K+ Na+ aa- K+ Na+ Na+ Na+ Na+ K+ aaNa+ Na+ Na+ K+ Na+ Na+ Na+ Na+ K+ aa- aa- K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ resting potential + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + Na+ Colonie High AP Biology DeMarco/Goldberg Action Potential Graph Myelin Sheath Axon coated by Schwann cells 1. Resting potential 2. Stimulus reaches 40 mV 4 10 mV Depolarization Na+ flows in 0 mV –10 mV 3 –20 mV saltatory conduction Repolarization K+ flows out 5 –30 mV –40 mV –50 mV Threshold –60 mV 2 –70 mV –80 mV 1 insulate axon speeds signal signal hops from node to node 20 mV Hyperpolarization (undershoot) Resting potential 150 m/sec vs. 5 m/sec (330 mph vs. 11 mph) 6 Resting myelin sheath saltatory conduction What happens at the end of the axon? Impulse has to jump the synapse! Na+ myelin + + 30 mV action potential axon signal direction Membrane potential threshold potential 3. Depolarization Na+ channels open; K+ channels closed 4. K+ channels open; Na+ channels close; 5. Repolarization reset charge gradient 6. Undershoot: K+ channels close slowly + + + – – junction between neurons has to jump quickly from one cell to next Na+ Multiple Sclerosis immune system (T cells) attack myelin sheath loss of signal The Synapse Events at synapse axon terminal action potential synaptic vesicles synapse Ca++ receptor protein neurotransmitter acetylcholine (ACh) muscle cell (fiber) action potential depolarizes membrane opens Ca++ channels neurotransmitter vesicles fuse with membrane release neurotransmitter to synaptic cleft neurotransmitter binds with protein receptor ion-gated channels open neurotransmitter degraded or reabsorbed synapse Acetylcholinesterase Enzyme which breaks down acetylcholine neurotransmitter neurotoxins = inhibitors snake venom, sarin, insecticides neurotoxin in green active site in red acetylcholinesterase snake toxin blocking acetylcholinesterase active site Colonie High AP Biology DeMarco/Goldberg Nerve Impulse in Next Neuron Post-synaptic neuron Neurotransmitters Acetylcholine triggers nerve impulse in next nerve cell chemical signal opens ion-gated channels Na+ diffuses into cell K+ binding site Na+ Na+ ACh diffuses out of cell K+ K+ Na+ + – – – – – – – – – – – – – – – + + + + + + + + + + + + + + Neurotransmitters Weak point of nervous system! any substance that affects neurotransmitters or mimics them affects nerve function gases: nitrous oxide, carbon monoxide mood altering drugs: stimulants Na+ – + + + + + + + + + + + + + + + – – – – – – – – – – – – – – widespread in brain lack of dopamine in brain associated with Parkinson’s disease excessive dopamine linked to schizophrenia pleasure & reward pathways Serotonin ion channel transmit signal to skeletal muscle Dopamine widespread in brain affects Glutamate excites neurons into action GABA inhibits passing of information Questions to ponder… Why are axons so long? Why have synapses at all? How do “mind altering drugs” work? caffeine, alcohol, nicotine, marijuana… Do plants have a nervous system? amphetamines, caffeine, nicotine Do they need one? depressants hallucinogenic drugs Prozac poisons Muscle Contraction Nerve signal stimulates muscle cell’s sarcoplasmic reticulum (SR) to release stored Ca+2 Ca+2 Triggers Muscle Action At rest, tropomyosin blocks myosin-binding sites on actin Ca+2 binds to troponin complex shape change causes movement of tropomyosintroponin complex exposes myosinbinding sites on actin Colonie High AP Biology DeMarco/Goldberg How it all works… How Ca+2 Controls Muscle Action potential causes Ca+2 release from SR Sliding filament model Troponin moves tropomyosin uncovering exposed actin binds to myosin fibers slide past each other ATP myosin binding site on actin Myosin binds actin ATP muscle doesn’t relax until Ca+2 is pumped back into SR requires ATP 1 uses ATP to "ratchet" each time releases, "unratchets" & binds to next actin Myosin pulls actin chain along Sarcomere shortens shorten muscle cell muscle contraction ratchet system Ca+2 binds to troponin Z lines move closer together Whole fiber shortens contraction! Ca+2 pumps restore Ca+2 to SR relaxation! ATP ATP Put it all together… pumps use ATP Cephalization = Brain Evolution Cephalization = clustering of neurons in “brain” at front (anterior) end of bilaterally symmetrical animals where sense organs are 2 associative neurons 3 ATP nerve cords nerve ribs 7 nerve net 4 Cnidarian 6 5 Cephalization = Brain Evolution Flatworm Platyhelminthes Echinoderm Simplest nervous system no control of complex actions ATP radial nerve More organization but still based on nerve nets; supports more complex movement Simplest, defined central nervous system more complex muscle control Evolution of Vertebrate Brain increase in interneurons in brain region central nervous system peripheral nerves forebrain giant axon brain brain ventral nerve cords forebrain dominant cerebrum Shark hindbrain Frog Crocodile Earthworm Cat Mollusk Arthropod More complex brains connected to all other parts of body by peripheral nerves More complex brains in predators most sophisticated invertebrate nervous system Further brain development ganglia = neuron clusters along CNS Human Spinal cord Hind: Medulla oblongata Hind: Cerebellum Optic tectum Midbrain Fore: Cerebrum Olfactory tract forebrain Bird