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Chapter 45 Neurons and Nervous Systems Nervous System Cells Neuron signal direction a nerve cell dendrites cell body 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 sensory neuron “afferent” cell body cell body axon dendrites dendrites motor neuron “efferent” interneuron “associative” cell body Fun Facts About Neurons Most specialized cell in animals Longest cell blue whale neuron 10-30 meters giraffe axon 5 meters human neuron 1-2 meters Nervous system allows for ~ millisecond response times 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 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 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 a “wave” action travels along neuron have to re-set channels so neuron can react again Cells: Surrounded by Charged Ions Cells live in a sea of charged ions anions more concentrated within the cell Cl-, charged amino acids (aa-) more concentrated in the extracellular fluid K+, Na+ K+ Na+ Na+ K+ aa- K+ Na+ aaCl- Na+ ClK+ Na+ aa- Na+ K+ aa- K+ Na+ ClCl- Na+ aa- Na+ Na+ Na+ Claa- Cl- – channel leaks K+ cations + Cells have voltage! Opposite charges on opposite sides of cell membrane membrane is polarized negative inside; positive outside charge gradient stored energy (like a battery) + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + Measuring Cell Voltage unstimulated neuron = resting potential of ~60 mV 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 The 1st domino goes down! – + + + + + + + + + + + + + + + – – – – – – – – – – – – – – Na+ + – – – – – – – – – – – – – – – + + + + + + + + + + + + + + How does a nerve impulse travel? Wave: nerve impulse 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 The rest of the dominoes fall! – – – + + + + + + + + + + + + + + + – – – – – – – – – – – – Na+ + + + – – – – – – – – – – – – – – – + + + + + + + + + + + + wave How does a nerve impulse travel? Re-set: 2nd wave travels down neuron K+ channels open K+ channels up more slowly than Na+ channels K+ ions diffuse out of cell charges reverse back at that point negative inside; positive outside Set dominoes back up quickly! K+ + – – – – + + + + + + + + + + – + + + + – – – – – – – – – – Na+ – + + + + – – – – – – – – – – + – – – – + + + + + + + + + + wave 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 K+ + + + – – – – + + + + + + + + – – – + + + + – – – – – – – – Na+ – – – + + + + – – – – – – – – + + + – – – – + + + + + + + + wave How does a nerve impulse travel? Action potential propagates wave = nerve impulse, or action potential brain finger tips in milliseconds! In the blink of an eye! K+ + + + + + + + – – – – + + + + – – – – – – – + + + + – – – – 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 K+ + + + + + + + + + – – – + + + – – – – – – – – – + + + – – – Na+ – – – – – – – – – + + + – – – + + + + + + + + + – – – + + + 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!! A lot of work to do here! Na+ + Na+ + K K Na+ K+ + Na Na+ Na+ K+ Na+ +Na + Na Na K+ + + + + + + + – – – – + + + + – – – – – – – – – – + + + + – Na+ Na+ K+ K+ + + + Na K K+ + Na+ K+ Na K+ K Na+ Na+ K+ – – – – – – – – – – + + + + – + + + + + + + + + + – – – – + wave Na+ + How does the nerve re-set itself? Na+ / K+ pump active transport protein in membrane requires ATP 3 Na+ pumped out + 2 K pumped in re-sets charge across That’s a lot of ATP! membrane You need some sugar, and quick! Wait a second… I’m nothing more than crystallized high fructose syrup! uh-oh… ATP 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+ K+ aa- aa- K+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ resting potential + + + + + + + + + + + + + + + – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – + + + + + + + + + + + + + + + Action Potential Graph 1. Resting potential 2. Stimulus reaches 40 mV 4 30 mV 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 20 mV 10 mV Depolarization Na+ flows in 0 mV –10 mV 3 –20 mV Repolarization K+ flows out 5 –30 mV –40 mV –50 mV Threshold –60 mV 2 –70 mV –80 mV 1 Resting potential Hyperpolarization (undershoot) 6 Resting Myelin Sheath Axon coated by Schwann cells signal direction insulate axon speeds signal signal hops from node to node saltatory conduction 150 m/sec vs. 5 m/sec (330 mph vs. 11 mph) myelin sheath action potential saltatory conduction Na+ myelin axon + + + + + – – Na+ Multiple Sclerosis immune system (T cells) attack myelin sheath loss of signal What happens at the end of the axon? Impulse has to jump the synapse! junction between neurons has to jump quickly from one cell to next How does the wave jump the gap? synapse 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 We switched… from an electrical signal to a chemical signal ion-gated channels open neurotransmitter degraded or reabsorbed 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 Nerve Impulse in Next Neuron Post-synaptic neuron triggers nerve impulse in next nerve cell chemical signal opens ion-gated channels Na+ diffuses into cell K+ diffuses out of cell binding site Na+ Na+ ACh ion channel Here we go again! K+ K+ Na+ – + + + + + + + + + + + + + + + – – – – – – – – – – – – – – Na+ + – – – – – – – – – – – – – – – + + + + + + + + + + + + + + Neurotransmitters Acetylcholine transmit signal to skeletal muscle Dopamine widespread in brain lack of dopamine in brain associated with Parkinson’s disease excessive dopamine linked to schizophrenia pleasure & reward pathways Serotonin widespread in brain affects Glutamate excites neurons into action GABA inhibits passing of information 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 amphetamines, caffeine, nicotine depressants hallucinogenic drugs Prozac poisons 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? Do they need one? 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 How Ca+2 Controls Muscle Sliding filament model exposed actin binds to myosin fibers slide past each other ATP ratchet system shorten muscle cell muscle contraction muscle doesn’t relax until Ca+2 is pumped back into SR ATP requires ATP How it all works… Action potential causes Ca+2 release from SR Ca+2 binds to troponin Troponin moves tropomyosin uncovering ATP myosin binding site on actin Myosin binds actin uses ATP to "ratchet" each time releases, "unratchets" & binds to next actin Myosin pulls actin chain along Sarcomere shortens Z lines move closer together Whole fiber shortens contraction! Ca+2 pumps restore Ca+2 to SR relaxation! ATP pumps use ATP 1 Put it all together… 2 3 ATP 7 4 6 ATP 5 Cephalization = Brain Evolution Cephalization = clustering of neurons in “brain” at front (anterior) end of bilaterally symmetrical animals where sense organs are associative neurons nerve cords nerve net Cnidarian Simplest nervous system no control of complex actions nerve ribs radial nerve Echinoderm More organization but still based on nerve nets; supports more complex movement Flatworm Platyhelminthes Simplest, defined central nervous system more complex muscle control Cephalization = Brain Evolution increase in interneurons in brain region central nervous system peripheral giant axon brain brain ventral nerve cords nerves Earthworm 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 Evolution of Vertebrate Brain forebrain forebrain dominant cerebrum Shark hindbrain Frog Crocodile Cat Human Spinal cord Hind: Medulla oblongata Hind: Cerebellum Optic tectum Midbrain Fore: Cerebrum Olfactory tract forebrain Bird Any Questions??