Nervous System Physiology and Electrical Activity (Fall 2022) PDF

Chapter 11 Electrical Signals in the Nervous System 11-1 ©2020 McGraw-Hill Education  11.5 Electrical Signals Cells produce electrical signals called action potentials...

Chapter 11 Electrical Signals in the Nervous System 11-1 ©2020 McGraw-Hill Education  11.5 Electrical Signals Cells produce electrical signals called action potentials. Transfer of information from one part of body to another. Membrane potential is the result from ionic concentration differences across plasma membrane and permeability of membrane. Ion concentrations across membrane are a result of two processes: the Na+/K+ pump and membrane permeability. High concentrations of Na+ and Cl− ions outside of cell; high concentrations of K+ and proteins on inside of cell. Steep concentration gradient of Na+ and K+, but in opposite directions. Permeability Characteristics of the Plasma Membrane Proteins: synthesized inside cell. Large and negatively charged. Don't dissolve in phospholipids of membrane. Cl− repelled by proteins and they exit thru open, nongated Cl− channels. Gated ion channels open and close in response to a stimulus. When they open, they change the permeability of the cell membrane. 11-2 ©2020 McGraw-Hill Education  Establishing the Resting Membrane Potential 1 Intracellular fluid and extracellular fluid are electrically neutral, but there is a charge across the membrane due to uneven distribution of anions/cations. Opposite charges across the membrane, so membrane is polarized. Potential difference: unequal distribution of charge exists between the immediate inside and immediate outside of the plasma membrane: −70 to −90 mV. The resting membrane potential exists in an unstimulated (resting) cell, due to: Permeability characteristics of membrane. Differences in ion concentrations on each side of membrane. 11-3 ©2020 McGraw-Hill Education  Establishing the Resting Membrane Potential 2 Membrane more permeable to K+ due to many leak channels, and K+ diffuses from inside to outside the cell. Positive charges accumulate outside the membrane. Negatively charged proteins cannot diffuse with K+, so as K+ diffuses out of cell, inside of membrane becomes more negative. Na+, Cl−, and Ca2+ do not have a great affect on resting potential since there are very few leakage channels for these ions. If leakage channels alone were responsible for resting membrane potential, in time Na+ and K+ ion concentrations would eventually equalize. But they are maintained by the Na+/K+ pump. For each ATP that is consumed, three Na+ moved out, two K+ moved in. Outside of plasma membrane slightly positive. 11-4 ©2020 McGraw-Hill Education  Establishing the Resting Membrane Potential 11-5 ©2020 McGraw-Hill Education  Changing the Resting Membrane Potential: Depolarization Depolarization: inside of cell becomes more positive; for example, from −70mV to −55mV. Sodium ions. Most common way neurons become depolarized. If gated sodium channels open, sodium diffuses into cell down its concentration gradient, and inside of cell becomes more positive. Calcium ions. Calcium entry also causes depolarization, as seen in cardiac muscle. Other roles: Regulation of voltage-gated sodium channels: closed voltage-gated sodium channels stabilized by calcium; low levels of extracellular calcium can cause channels to open spontaneously. Regulation of neurotransmitter secretion at presynaptic terminal. Potassium ions. Normally, potassium diffuses out of the cell, but changes in the extracellular concentration of potassium can affect the resting membrane potential. If extracellular potassium levels increase, intracellular stays inside cell (less concentration gradient), and membrane becomes depolarized. 11-6 ©2020 McGraw-Hill Education  Changing the Resting Membrane Potential: Hyperpolarization 1 Hyperpolarization: inside of cell becomes more negative; for example, from −70mV to −90mV. Potassium ions. Most common way neurons become hyperpolarized. If gated potassium channels open, potassium diffuses out of cell down its concentration gradient, and inside of cell becomes more negative. Chloride ions. Chloride is in higher concentration outside cell, so opening of ligand-gated chloride channels causes chloride to diffuse into cell. Adding negative charges into the cell hyperpolarizes it. 11-7 ©2020 McGraw-Hill Education  Depolarization and Hyperpolarization of the Resting Membrane Potential 11-8 ©2020 McGraw-Hill Education  Graded Potentials Result from. Ligands binding to receptors. Changes in charge across membrane. Mechanical stimulation. Temperature changes. Spontaneous change in permeability. Graded - Magnitude varies from small to large depending on stimulus strength or frequency. Can be hyperpolarizing or depolarizing. Can summate or add onto each other. Spread (are conducted) over the plasma membrane in a decremental fashion: rapidly decrease in magnitude as they spread over the surface of the plasma membrane. Can cause generation of action potentials. 11-9 ©2020 McGraw-Hill Education  Table 11.4 11-10 ©2020 McGraw-Hill Education  Action Potentials Way that neurons communicate. Graded potentials summate at trigger zone, reaching threshold. Depolarization phase followed by repolarization phase. Depolarization: sodium enters through voltage-gated channels; membrane potential becomes more positive. Repolarization: potassium leaves through voltage-gated channels; membrane potential becomes more negative may get afterpotential [slight hyperpolarization]. All-or-none principle. No matter how strong the stimulus, as long as it is greater than 11-11 ©2020 McGraw-Hill Education  Table 11.5 11-12 ©2020 McGraw-Hill Education  Voltage-Gated Ion Channels and the Action Potential 1 Resting membrane potential. Na+ channels (pink) and most, but not all, K+ channels (purple) are closed. The outside of the plasma membrane is positively charged compared to the inside. Depolarization. Na+ channels open. K+ channels begin to open. Depolarization results because the inward movement of Na+ makes the inside of the membrane more positive. 11-13 ©2020 McGraw-Hill Education  Voltage-Gated Ion Channels and the Action Potential 2 Repolarization. Na+ channels close and additional K+ channels open. Na+ movement into the cell stops, and K+ movement out of the cell increases, caused repolarization. End of repolarization and afterpotential. Voltage-gated Na+ channels are closed. Closure of the activation gates and opening of the inactivation gates reestablish the resting condition for Na+ channels (see step 1). Diffusion of K+ through voltage-gated channels produces the afterpotential. Return to Resting Potential The resting membrane potential is reestablished after the voltage-gated K+ channels close. 11-14 ©2020 McGraw-Hill Education  Refractory Period Sensitivity of area to further stimulation decreases for a time. Parts. Absolute. Complete insensitivity exists to another stimulus. From beginning of action potential until near end of repolarization. No matter how large the stimulus, a second action potential cannot be produced. Relative. A stronger-than-threshold stimulus can initiate another action potential. 11-15 ©2020 McGraw-Hill Education  Production of an Action Potential 11-16 ©2020 McGraw-Hill Education  Action Potential Frequency Number of potentials produced per unit of time to a stimulus. Subthreshold stimulus: does not cause a graded potential that is great enough to initiate an action potential. Threshold stimulus: causes a graded potential that is great enough to initiate an action potential. Maximal stimulus: just strong enough to produce a maximum frequency of action potentials. Submaximal stimulus: all stimuli between threshold and the maximal stimulus strength. Supramaximal stimulus: any stimulus stronger than a maximal stimulus. These stimuli cannot produce a greater frequency of action potentials than a maximal stimulus. 11-17 ©2020 McGraw-Hill Education  Propagation of Action Potentials Threshold graded current at trigger zone causes action potential. Continuous conduction: Action potential in one site causes action potential at the next location. Cannot go backwards because initial action potential site is depolarized yielding one-way conduction of impulse. saltatory conduction - Myelinated axons 11-18 ©2020 McGraw-Hill Education  Saltatory Conduction 1. An action potential (orange) at a node of Ranvier generates local currents (black arrows). The local currents flow to the next node because the Schwann cell insulates the axon of the internode. 2. When the depolarization caused by the local currents reaches threshold at the next node, a new AP is produced (orange). 3. Action potential propagation is rapid in myelinated axons because the action potentials are produced at successive nodes of Ranvier (1 to 5) instead of at every part of the membrane along the axon. 11-19 ©2020 McGraw-Hill Education  Speed of Conduction Faster in myelinated than in non-myelinated. In myelinated axons, lipids act as insulation forcing ionic currents to jump from node to node. In myelinated, speed is affected by thickness of myelin sheath. Diameter of axons: large-diameter conduct more rapidly than small-diameter. Large have greater surface area and more voltage-gated Na+ channels. Nerve Fiber Types Type A: large-diameter, myelinated. Conduct at 15 to 120 m/s. Motor neurons supplying skeletal and most sensory neurons. Type B: medium-diameter, lightly myelinated. Conduct at 3 to 15 m/s. Part of ANS. Type C: small-diameter, unmyelinated. Conduct at 2 m/s or less. Part of ANS. 11-20 ©2020 McGraw-Hill Education  11.6 The Synapse Junction between two cells. Site where action potentials in one cell cause action potentials in another cell. Types of cells in synapse. Presynaptic: cell that transmits signal toward the synapse. Postsynaptic: target cell receiving the signal. Two types of synapses. Electrical. Chemical. Electrical Synapses Cells connected by gap junctions that allow graded current to flow between adjacent cells. Connexons: protein tubes in cell membrane. Found in cardiac muscle and many types of smooth muscle. Important where contractile activity among a group of cells important. 11-21 ©2020 McGraw-Hill Education  Chemical Synapses Components. Presynaptic terminal. Synaptic cleft. Postsynaptic membrane. Neurotransmitters released by action potentials in presynaptic terminal. Synaptic vesicles: action potential causes Ca2+ to enter cell that causes neurotransmitter to be released from vesicles. Diffusion of neurotransmitter across synapse. Postsynaptic membrane: when neurotransmitter binds to receptor, ligand-gated ion channels open. 11-22 ©2020 McGraw-Hill Education  Receptor Molecules in Synapses Neurotransmitter only "fits" in one receptor. Neurotransmitter affects only the cells with receptors for that neurotransmitter. Neurotransmitters are excitatory in some cells and inhibitory in others. Some neurotransmitters (for example, norepinephrine) can also attach to the presynaptic terminal and modulate it’s own release. Neurotransmitter Removal Method depends on neurotransmitter/synapse. ACh: acetylcholinesterase splits ACh into acetic acid and choline. Choline recycled within presynaptic neuron. Norepinephrine: recycled within presynaptic neuron or diffuses away from synapse. Enzyme monoamine oxidase (MAO). Absorbed into circulation, broken down in liver. 11-23 ©2020 McGraw-Hill Education  Neurotransmitters and Neuromodulators Chemical messengers secreted by neurons. Neurons can secrete more than one type. At least 100 different types. Some major chemical classes of neurotransmitters: Acetylcholine (ACh): best understood; acetic acid + choline. Biogenic amines: catecholamines and indoleamines. Amino acids: examples include glycine and glutamate. Purines: nitrogen-containing compounds derived from nucleic acids; examples include adenosine and ATP. Neuropeptides: short chains of amino acids. Gases and lipids: examples include nitric oxide (gas), carbon monoxide (gas), and endocannabanoids (lipid-derived). 11-24 ©2020 McGraw-Hill Education  Responses at the Postsynaptic Cells: Excitatory and Inhibitory Postsynaptic Potentials 1 Excitatory postsynaptic potential (EPSP). Depolarization occurs and response is stimulatory. Depolarization might reach threshold producing an action potential and cell response. Inhibitory postsynaptic potential (IPSP). Hyperpolarization and response is inhibitory. Decrease likelihood of action potential by moving membrane potential farther from threshold. 11-25 ©2020 McGraw-Hill Education  Neuromodulation Neuromodulators: influence likelihood of an AP being produced in postsynaptic cell. Axoaxonic synapses: axon of one neuron synapses with the presynaptic terminal (axon) of another. Common in CNS. Presynaptic inhibition: reduction in amount of neurotransmitter released from presynaptic terminal. Endorphins can inhibit pain sensation. Presynaptic facilitation: amount of neurotransmitter released from presynaptic terminal increases. Glutamate facilitating nitric oxide production. 11-26 ©2020 McGraw-Hill Education  Spatial and Temporal Summation A single postsynaptic potential is not sufficient to reach threshold. Many postsynaptic potentials combine in summation at the trigger zone. If threshold is reached, and action potential is triggered. Two types of summation: Spatial. Temporal. Spatial Summation Action potentials 1 and 2 cause the production of graded potentials at two different dendrites. These graded potentials summate at the trigger zone to produce a graded potential that exceeds threshold, resulting in an action potential. 11-27 ©2020 McGraw-Hill Education  Temporal Summation Two action potentials arrive in close succession at the presynaptic membrane. The first action potential causes the production of a graded potential that does not reach threshold at the trigger zone. The second action potential results in the production of a second graded potential that summates with the first to reach threshold, resulting in the production of an action potential. Combined Spatial and Temporal Summation An action potential is produced at the trigger zone when the graded potentials produced as a result of the EPSPs and IPSPs summate to reach. 11-28 ©2020 McGraw-Hill Education  11.7 Neuronal Pathways and Circuits Organization of neurons in CNS varies in complexity. Convergent pathways: many converge and synapse with smaller number of neurons. for example, synthesis of data in brain. Divergent pathways: small number of presynaptic neurons synapse with large number of postsynaptic neurons. for example, important information can be transmitted to many parts of the brain. Reverberating circuit: outputs cause reciprocal activation. For example, rhythmic activities such as breathing. Parallel after-discharge circuit: neurons stimulate several neurons in parallel organization, which converge upon a common output cell. for example, complex data processing in brain. 11-29 ©2020 McGraw-Hill Education