BIO 311D 1st Edition Exam 3 Study Guide Lectures 10 31 Nervous System Neuron 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 chemical messengers 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 glia Ion 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 information Formation 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 neurons B Interneurons sensory neurons motor neurons C Sensory neurons interneurons motor neurons D Motor neurons interneurons sensory neurons Modeling 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 balanced Adding 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 outside C The inside of the neuron to become positively charged relative to the outside D Sodium to diffuse out of the cell and potassium to diffuse into the cell Modeling the Resting Potential The equilibrium potential Eion is the membrane voltage for a particular ion at equilibrium and can be calculated using the Nernst equation Eion 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 steady A 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 mV A increase increase B increase decrease C decrease increase D decrease increase At step four in the graph it is likely that A Most Cl channels closed B Most VG Na channels opened C Most VG K channels closed D Most VG K channels opened E Na K pumps were inactivated Generation of Action Potentials A Closer Look An action potential can be considered as a series of stages At resting potential 1 Most voltage gated sodium Na channels are closed most of the voltage gated potassium K channels are also closed When an action potential is generated 1 Voltage gated Na channels open first and Na flows into the cell 2 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 cell 4 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 channels Conduction 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 backwards Evolutionary 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 PNS Neurotransmitters 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 receptors Acetylcholine 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 metabotropic The 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 clef B undergo more graded hyperpolarizations because acetylcholine would remain in the synaptic cleft longer C Undergo more graded depolarizations because acetylcholine would prevent ligand gated ion channels from opening Amino Acids Amino acid neurotransmitters are active in the CNS and PNS Known to function in the CNS are Glutamate Gamma aminobutyric acid GABA Glycine Biogenic Amines Biogenic amines include Epinephrine Norepinephrine Dopamine Serotonin They are active in