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Bio 100 – Guide 25 http://www.umich.edu/~nre562/pics/brain.jpg http://heise.nu/squid-toon.jpg http://crab-lab.zool.ohiou.edu/nervenet/squid.jpg Squid Rig http://www.thaifishingguide.com/images/techniques/saltwatertechniques/squid9.jpg Neurons are the functional units of nervous systems – Neurons are cells specialized for carrying signals • Cell body: contains most organelles • Dendrites: highly branched extensions that carry signals from other neurons toward the cell body • Axon: long extension that transmits signals to other cells – Many axons are enclosed by an insulating myelin sheath • Chain of Schwann cells • Nodes of Ranvier: points where signals can be transmitted • Speeds up signal transmission – Supporting cells (glia) are essential for structural integrity and normal functioning of the nervous system – The axon ends in a cluster of branches • Each branch ends in a synaptic terminal • A synapse is a site of communication between a synaptic terminal and another cell LE 28-2 Signal direction Dendrites Cell Body Cell body Node of Ranvier Layers of myelin in sheath Axon Schwann cell Nucleus Signal pathway Nucleus Nodes of Ranvier Myelin sheath Synaptic terminals Schwann cell NERVE SIGNALS AND THEIR TRANSMISSION • A neuron maintains a membrane potential across its membrane – A resting neuron has potential energy • Membrane potential: electrical charge difference across the neuron's plasma membrane • Resting potential: voltage across the plasma membrane of a resting neuron – The resting potential depends on differences in ionic composition inside and outside the cell • More K+ than Na+ diffuses inward through membrane channels • Sodium-potassium pumps actively transport Na+ out of cell and K+ in • The ionic gradient produces a voltage across the membrane. http:\\www.geneseo.edu\~simon\bio100\media\Resting_Potential.html LE 28-3a Voltmeter Plasma membrane –70 mV Microelectrode outside cell Microelectrode inside cell Axon Neuron • A nerve signal begins as a change in the membrane potential – Electrical changes make up an action potential, a nerve signal that carries information along an axon • Stimulus raises voltage from resting potential to threshold • Action potential is triggered; membrane polarity reverses abruptly • Membrane repolarizes; voltage drops • Voltage undershoots and then returns to resting potential –Cause of electrical changes of an action potential •Movement of K+ and Na+ across the membrane •Controlled by the opening and closing of voltage-gated channels LE 28-3b Na+ Outside of cell Na+ K+ Na+ Na+ Na+ Na+ Na+ Na+ Na+ channel K+ Plasma membrane Na+ Protein K+ Na+ Na+ Na+ Na+ -K+ pump K+ channel K+ K+ K+ K+ K+ K+ Inside of cell Na+ Na+ K+ K+ The Action Potential http://www.geneseo.edu/~simon/bio100/media/Action_Potential.html The action potential (Step 5) Na + K+ Na + K+ Additional Na + channels open, K+ channels are closed; interior of cell becomes more positive. Na + Na + channels close and inactivate. K+ channels open, and K+ rushes out; interior of cell more negative than outside. Na + A stimulus opens some Na+ channels; if threshold is reached, action potential is triggered. Membrane potential (mV) + 50 Action potential 0 – 50 The K+ channels close relatively slowly, causing a brief undershoot. Threshold Resting potential – 100 Time (msec) Neuron interior Resting state: voltage-gated Na + and K+ channels closed; resting potential is maintained. Neuron interior Return to resting state. • The action potential propagates itself along the neuron – An action potential transmits a signal in a domino effect 1. Na+ channels open, Na+ rushes inward 2. K+ channels open, K+ diffuses outward; Na+ channels are closed and inactivated 3. Membrane returns to resting potential LE 28-5 Axon Action potential Axon segment Na+ K+ Action potential Na+ K+ K+ Action potential Na+ K+ –Action potentials are propagated only from cell body to synaptic cleft •Cannot be generated where K+ is leaving axon and Na+ channels are inactivated –Action potentials are all-or-none events •Same events occur no matter how strong or weak the stimulus •Intensity of stimulus determines frequency of action potentials • Neurons communicate at synapses – The transmission of signals occurs at synapses • Junction between synaptic terminal and another cell – Electrical synapse • Electrical current passes directly from one neuron to the next • Receiving neuron stimulated quickly and at same frequency as sending neuron • Found in human heart and digestive tract – Chemical synapse 1. Action potential arrives in sending neuron 2. Vesicle containing neurotransmitter fuses with plasma membrane 3. Neurotransmitter is released into synaptic cleft 4. Neurotransmitter binds to receptor on receiving neuron – Following events vary with different types of chemical synapses The Synapse http://www.geneseo.edu/~simon/bio100/ media/Synapse.html Neuron communication (Step 1) Sending neuron Action potential arrives Vesicles Axon of sending neuron Synaptic terminal Synapse Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft Synaptic cleft Receiving neuron Receiving neuron Ion channels Neurotransmitter molecules Neurotransmitter binds to receptor Neuron communication (Step 2) Sending neuron Action potential arrives Vesicles Axon of sending neuron Synaptic terminal Synapse Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft Synaptic cleft Receiving neuron Receiving neuron Ion channels Neurotransmitter molecules Neurotransmitter Receptor Ions Ion channel opens Neurotransmitter binds to receptor Neuron communication (Step 3) Sending neuron Action potential arrives Vesicles Axon of sending neuron Synaptic terminal Synapse Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft Synaptic cleft Receiving neuron Receiving neuron Ion channels Neurotransmitter molecules Neurotransmitter Receptor Neurotransmitter binds to receptor Neurotransmitter broken down and released Ions Ion channel opens Ion channel closes • Chemical synapses make complex information processing possible – A neuron may receive information from hundreds of other neurons via thousands of synaptic terminals – Some neurotransmitters excite the receiving cell – Other neurotransmitters inhibit the receiving cell's activity by decreasing its ability to develop action potentials – If excitatory signals are strong enough to initiate an action potential, a neuron will transmit a signal Neuron communication (Step 1) Sending neuron Action potential arrives Vesicles Axon of sending neuron Synaptic terminal Synapse Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft Synaptic cleft Receiving neuron Receiving neuron Ion channels Neurotransmitter molecules Neurotransmitter binds to receptor LE 28-7 Synaptic terminals Dendrites Inhibitory Excitatory Myelin sheath Receiving cell body Axon SEM 5,500× Synaptic terminals LE 28-6 Sending neuron Action potential arrives Vesicles Axon of sending neuron Synaptic terminal Synapse Vesicle fuses with plasma membrane Neurotransmitter is released into synaptic cleft Synaptic cleft Receiving neuron Receiving neuron Ion channels Neurotransmitter molecules Neurotransmitter Receptor Neurotrans mitter binds to receptor Neurotransmitter broken down and releases Ions Ion channel opens Ion channel closes • A variety of small molecules function as neurotransmitters – Many small, nitrogen-containing molecules serve as neurotransmitters • Acetylcholine – Important in brain and at synapses between motor neurons and muscles • Biogenic amines –Important in central nervous system –Seratonin, dopamine – Amino acids • Important in central nervous system – Peptides • Substance P, endorphins influence perception of pain – Dissolved gases • NO functions during sexual arousal • Many drugs act at chemical synapses – Many psychoactive drugs act at synapses and affect neurotransmitter action • Caffeine • Nicotine • Alcohol • Psychoactive prescription drugs • Stimulants • THC (marijuana) • Opiates The End