BIO 311D 1st EditionExam 3 Study Guide: Lectures: 10 - 31Nervous SystemNeuron Structure and Function• Most of a neuron’s organelles are in the cell body• Most neurons have dendrites, highly branched extensions that receive signals from other neurons• The axon is typically a much longer extension that transmits signals to other cells at synapses• The cone-shaped base of an axon is called the axon hillock• The synaptic terminal of one axon passes information across the synapse in the form of chemicalmessengers called neurotransmitters• A synapse is a junction between an axon and another cell• Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)• Most neurons are nourished or insulated by cells called gliaIon pumps and ion channels establish the resting potential of a neuron• Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential• The resting potential is the membrane potential of a neuron not sending signals• Changes in membrane potential act as signals, transmitting and processing informationFormation of the Resting Potential• In a mammalian neuron at resting potential, the concentration of K+ is highest inside the cell, while the concentration of Na+ is highest outside the cell • Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane• These concentration gradients represent chemical potential energy• The opening of ion channels in the plasma membrane converts chemical potential to electrical potential• A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell• The resulting buildup of negative charge within the neuron is the major source of membrane potential Choose the correct pathway of the information flow through neurons while taking a test, starting with reading a question and ending with marking an answer.A. Interneurons  motor neurons  sensory neuronsB. Interneurons  sensory neurons  motor neuronsC. Sensory neurons interneurons  motor neuronsD. Motor neurons  interneurons  sensory neuronsModeling the Resting Potential• Resting potential can be modeled by an artificial membrane that separates two chambers– The concentration of KCl is higher in the inner chamber and lower in the outer chamber– K+ diffuses down its gradient to the outer chamber– Negative charge (Cl–) builds up in the inner chamber• At equilibri um, both the electrical and chemical gradients are balancedAdding a poison that specifically disables the Na+ and aK + to a culture of neurons will cause:A. The resting membrane potential to drop to 0 mV B. The inside of the neuron to become more negative relative to the outsideC. The inside of the neuron to become positively charged relative to the outsideD. Sodium to diffuse out of the cell and potassium to diffuse into the cellModeling the Resting Potential• The equilibrium potential (Eion) is the membrane voltage for a particular ion at equilibrium and can be calculated using the Nernst equationEion = 62 mV (log[ion]outside/[ion]inside)• The equilibrium potential of K+ (EK) is negative, while the equilibrium potential of Na+ (ENa) is positive• In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steadyA(n) ____ in Na+ permeability and or an ___ in K+ permeability across a neuron’s plasma membrane could shift membrane potential from -70 mV to -80 mVA. increase; increaseB. increase; decreaseC. decrease; increaseD. decrease; increaseAt step four in the graph, it is likely thatA. Most Cl- channels closedB. Most VG-Na+ channels openedC. Most VG-K+ channels closedD. Most VG-K+ channels openedE. Na/K pumps were inactivatedGeneration of Action Potentials: A Closer Look• An action potential can be considered as a series of stages• At resting potential1. Most voltage-gated sodium (Na+) channels are closed; most of the voltage-gated potassium (K+) channels are also closed• When an action potential is generated1. Voltage-gated Na+ channels open first and Na+ flows into the cell2. During the rising phase, the threshold is crossed, and the membrane potential increases 3. During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell4. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close and resting potential is restored• During the refractory period after an action potential, a second action potential cannot be initiated• The refractory period is a result of a temporary inactivation of the Na+ channelsConduction of Action Potentials• At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane• Action potentials travel in only one direction: toward the synaptic terminals• Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwardsEvolutionary Adaptation of Axon Structure• The speed of an action potential increases with the axon’s diameter• In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase• Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNSNeurotransmitters• There are more than 100 neurotransmitters, belonging to five groups: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases• A single neurotransmitter may have more than a dozen different receptorsAcetylcholine• Acetylcholine is a common neurotransmitter in vertebrates and invertebrates• It is involved in muscle stimulation, memory formation, and learning• Vertebrates have two major classes of acetylcholine receptor, one that is ligand gated and one that is metabotropicThe use of organophosphate pesticides that inhibit acetylcholinesterase, an enzyme that breaks down acetylcholine, could cause skeletal muscles to A. Undergo more graded depolarization’s because acetylcholine would remain in the synaptic clefB. undergo more graded hyperpolarizations because acetylcholine would remain in the synaptic cleft longerC. Undergo more graded depolarizations because acetylcholine would prevent ligand gatedion