Neurons Lecture Notes PDF

Neurons Textbook Chapter 48 https://pollev.com/barbaraakum216 1 Topics to be covered A. Neurons and electrochemical gradient generation B. Resting and membrane potential changes C. Action potentials D. Conduction of the action potential E. Transmission of acti...

Neurons Textbook Chapter 48 https://pollev.com/barbaraakum216 1 Topics to be covered A. Neurons and electrochemical gradient generation B. Resting and membrane potential changes C. Action potentials D. Conduction of the action potential E. Transmission of action potentials: synapses 2 How does a neuron transmit information? 3 Coordination and Control: nervous system (b) Signaling by neurons STIMULUS The nervous system transmits information between specific Cell body of neuron locations Nerve Axon impulse Signal travels The information conveyed depends to a specific on a signal’s pathway, not the type location. of signal Nerve impulse Nerve signal transmission is very fast Axons Response 4 A. Neurons and electrochemical gradient generation The neuron is a cell that exemplifies the close fit between form and function NEURON STRUCTURE Cell body - Most of organelles Dendrites, highly branched extensions that receive signals from other neurons The axon is often a much longer extension that transmits signals to other cells at synapses The cone-shaped base of an axon is called the axon hillock 5 Synapse A synapse is a junction between an axon and another cell. The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters. Short distance: Chemical signal 6 Structural diversity of neurons Sensory neurons transmit information about external stimuli such as light, touch, or smell Interneurons integrate (analyze and interpret) the information Motor neurons transmit signals to muscle cells, causing them to contract Figure 48.5 7 Transmitting a signal Long distance: Electrical Signal direction Synapse Short distance: Chemical Neurotransmitter 8 B. Resting and membrane potential changes 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 can be graded or action potentials. Figure 48.6 9 Formation of the Resting Potential In most neurons, the concentration of K+ is higher inside the cell, while the concentration of Na+ is higher 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. 10 Ion Concentrations Inside and Outside of Mammalian Neurons 11 Modeling a mammalian neuron Membrane selectively Membrane selectively permeable to K+ permeable to Na+ 12 Ion Channel diversity in Neurons 1. Nongated Channels - responsible for the resting membrane potential. 2. Voltage-gated Channels - responsible for generation and propagation of the action potential, the outgoing signal from the neuron. 3. Chemically-gated Channels - responsible for synaptic potentials, the incoming signals to the neuron. 13 1. Non gated Ion channels Resting potential dominated by K+ 14 Neuronal resting potential: -70mV Technique Microelectrode Voltage Key recorder Na+ K+ Sodium- Reference potassium pump electrode OUTSIDE OF CELL Potassium channel Sodium channel -70 mV INSIDE OF CELL 15 2. Voltage-gated ion channel open or close in response to a change in voltage across the plasma membrane Figure 48.9 16 Depolarization Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential. For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell Voltage-gated Na+ Channels 17 Graded Potentials Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus. When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in magnitude of the membrane potential 18 Graded Potentials When gated Na+ channels open, Na+ diffuses in, making the inside of the cell more positive This is depolarization, a reduction in the magnitude of the membrane potential 19 C. Action Potentials If a depolarization shifts the membrane potential sufficiently, it results in a massive change in membrane voltage called an action potential. Action potentials have a constant magnitude, are all-or-none, and transmit signals over long distances. They arise because some ion channels are voltage-gated, opening or closing when the membrane potential passes a certain level called threshold. 20 Voltage-gated ion channel http://cnx.org/ 21 5 4 3 1 2 Action Potential: Role of voltage-gated ion channels Key Na+ Na Na K+ + + K+ Falling phase of the action potential Rising phase of the action potential +50 Action potential Membrane potential 0 (mV) Threshold −50 Depolarization Resting potential −100 Time OUTSIDE OF CELL Sodium Potassium channel channel INSIDE OF CELL Inactivation loop K+ Resting state Undershoot 22 Hyperpolarization When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in magnitude of the membrane potential 23  2 3 Depolarization 4 Repolarization Resting potential 1 5 Hyperpolarization 24 Action potential Review For a neuron with an initial membrane potential at -70 mV, an increase in the movement of potassium ions out of that neuron's cytoplasm would result in: Log into PollEV now https://pollev.com/barbaraakum216 25 Action potential Review The "undershoot" phase of after-hyperpolarization is due to A. slow opening of voltage-gated sodium channels. B. sustained opening of voltage-gated potassium channels. C. rapid opening of voltage-gated calcium channels. D. slow restorative actions of the sodium-potassium ATPase. E. ions that move away from their open ion channels. Log into PollEV now https://pollev.com/barbaraakum216 26 D. Action potential conduction 27 Action potentials “travel” along the axon Time 28 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. Refractory period 29 Conduction of an action potential 30 Figure 48.13 Myelinated axons Myelin sheath - The electrical insulation that surrounds vertebrate axons Produced by gliaL cells ○ Oligodendrocytes in the CNS ○ Schwann cells in the PNS. During development, these specialized glia wrap axons in many layers of membrane. The membranes forming these layers are mostly lipid, which is a poor conductor of electrical current and thus a good insulator Action potentials traveling down the axon "jump" from node to node Saltatory conduction 31 Action potential conduction A toxin that binds specifically to voltage-gated sodium channels in axons would be expected to A. prevent the hyperpolarization phase of the action potential. B. prevent the depolarization phase of the action potential. C. prevent graded potentials. D. have most of its effects on the dendritic region of a neuron. Log into PollEV now https://pollev.com/barbaraakum216 32 E. Transmitting Transmitting a signal: a signal: Synapse Synapse At electrical synapses, the electrical current flows from one neuron to another through gap junctions. At chemical synapses, a chemical neurotransmitter carries information between neurons. Most synapses are chemical synapses. What happens when the signal reaches the end of the axon? 33 Voltage-gated Ca2+ channels Axon Voltage-gated Plasma Ca2+ channel Action potential membrane Ions Na+ Cytosol Change in membrane potential Action potential (voltage) K+ Na+ Ion K+ channel Action potential + K Gate open: Ions flow Gate closed: No ions through channel. flow across membrane. Na+ K+ 34 Action potential causes the release of the neurotransmitter The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal. The neurotransmitter diffuses across the synaptic cleft and is received by the postsynaptic cell. 35 Neurotransmitters A single neurotransmitter may bind specifically to more than a dozen different receptors. A single neurotransmitter could excite postsynaptic cells expressing one receptor and inhibit postsynaptic cells expressing a different receptor. 36 Generation of Postsynaptic Potentials Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell. Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential. 37 Metabotropic receptor In some synapses, a neurotransmitter binds to a receptor that is metabotropic In this case, movement of ions through a channel depends on one or more metabolic steps 38 Generation of Postsynaptic potentials Postsynaptic potentials fall into two categories: 1. Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold. 2. Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold. Types of summation: Temporal vs spatial Summation 39 Termination of Neurotransmitter Signaling After a response is triggered, the chemical synapse returns to its resting state. The neurotransmitter molecules are cleared from the synaptic cleft. Two mechanisms of terminating neurotransmission Enzymatic breakdown of NT Reuptake of NT Figure 48.18 40 Nerve gas sarin Blocking this process can have severe effects. The nerve gas sarin triggers paralysis and death due to inhibition of the enzyme that breaks down the neurotransmitter controlling skeletal muscles. Simpson, Beth. “Sarin Nerve Gas - Or How I Learned to Stop Worrying and Love Pon-1.” (2004). Figure 48.18 41 The connection among neurons make neuronal circuits Presynaptic cell Dendrites Stimulus Axon hillock Nucleus Cell body Axon Signal direction Synapse Synaptic terminals Synaptic terminals Postsynaptic cell Neurotransmitter 42 Flow of information Integration Sensory input Sensor Motor output Effector 43