University:
Glendale Community College
Course:
BIO160 | Introduction to Human Anatomy and Physiology
Academic year:

2020
Views:

311

Pages:

36
Author:

5kra3gecc

Electrical Potentials and Currents • Nerve pathways are not continuous “wires” but a series of separate cells that relay signals. • Neuronal communication is based on mechanisms for producing electrical potentials and currents. – electrical potential - difference in concentration of charged ions across a membrane measured in millivolts (mV) – electrical current - flow of ions across a membrane measured in milliamps (mA) • Living cells have polarized membranes. – Ions are maintained by the cell at different concentrations inside and outside of the cells. – Membrane polarity of a resting (inactive) neuron is about -70 mV because of a relatively negative charge inside of the cell compared to the outside of the cell. Electrical potentials of living cells are measured using a sensitive voltmeter with tiny glass electrodes. One electrode is placed inside the cell (intracellular) and one electrode is placed just outside the cell (extracellular). Neurophysiologists make extremely thin glass recording electrodes from glass tubes with instruments that precisely control the heat and tension on the tube. These hollow glass needles are then filled with a conducting salt solution. note: a sheet of paper is about 100 m thick! Resting Membrane Potential (RMP) • RMP is a voltage difference across the membrane of an inactive neuron and is usually about -70 mV. • RMP is caused by: – unequal ion distribution between Extracellular Fluid (ECF) and the Intracellular Fluid (ICF) caused by selective permeability of plasma membrane and active transport. – Na+/K+ pumps transport Na+ and K+ in a 3:2 ratio 3 Na+ out of the neuron and 2 K+ into the neuron • pumps work continuously and require ATP • high use of ATP means glucose and oxygen must be supplied continuously to nerve tissue – large cytoplasmic anions do not escape (anionic proteins, PO42-, SO42-) so inside of cell is negative. Local Potentials • Dendrite and soma membranes start to depolarize in a particular location when a neuron is stimulated by ligands (hormones or neurotransmitters from another cell), light, heat or a mechanical disturbance. – membrane depolarizes due to opening of gated Na+ channels that let Na+ rush in according to its concentration and electrical gradients • Local Potentials: – are graded (vary in magnitude with stimulus strength) – are decremental (get weaker the farther they spread) – are reversible (as K+ leaks out of cell and the Na+/K+ pump works to restore membrane polarity) – can lead to an action potential Generation of a Local Membrane Potential Summation of Depolarizations from Local Potentials can lead to an Action Potential Local depolarizing potentials are excitatory. Local potentials that bring the cell closer to threshold are called EPSPs (excitatory postsynaptic potentials). Depolarization must reach a threshold to trigger an action potential Local hyperpolarizing potentials would be inhibitory. Synapse Postsynaptic Potentials • Excitatory postsynaptic potentials (EPSP) cause a positive voltage change in the postsynaptic cell making it more likely to fire (depolarize). – result from Na+ flowing into the cell – noradrenaline and glutamate are excitatory neurotransmitters • Inhibitory postsynaptic potentials (IPSP) cause a voltage change in the postsynaptic cell that makes it less likely to fire because it is hyperpolarized. – results from Cl- flowing into the cell or K+ leaving the cell – glycine and GABA (gamma aminobutyric acid) are examples of inhibitory neurotransmitters • Some neurotransmitters, like ACh and norepinephrine, can be either excitatory or inhibitory depending on the type of membrane receptor it binds to. Excitatory (a) and Inhibitory (b) Postsynaptic Potentials Summation of Postsynaptic Potentials • Temporal Summation occurs when a single cell receives many EPSPs in a short period of time. • Spatial Summation occurs when a single cell receives many EPSPs from more than one presynaptic cell. Summation of EPSP’s brings a cell to Threshold Summation of IPSP’s Inhibit Neurons synapse • Inhibitory Neuron “I” suppresses presynaptic neuron “S” by releasing an inhibitory neurotransmitter like glycine. • Inhibitory neurotransmitters can block voltage-gated calcium channels and open K+ channels in neuron “S” dropping its membrane potential so it will not release neurotransmitter onto neuron “R” . Action Potential • Action Potential plotted on a realistic timescale looks like a “spike”. • Initial depolarization events are very subtle and happen very quickly. • Characteristics of an AP – follows an all-or-none law • voltage gates either open or they don’t – irreversible (once started, it goes to completion and can not be stopped) Action Potential at the Axon Hillock 1. Sodium influx due to local potential spreads to the high density of voltage-gated channels at the trigger zone (500 channels/m2 at hillock vs 50 channels/m2 on soma). 2. If threshold potential (-55mV) is reached, voltage-gated Na+ channels open (more Na+ enters causing more depolarization). 3. Voltage-gated Na+ channels open quickly. Incoming Na+ further depolarizes membrane which opens more Na+ channels (positive feedback). Voltage gated K+ channels slowly start to open. 4. Depolarization peaks at+35mV Action Potential at the Axon Hillock 4. 5. 6. 7. Na+ channels are inactivated and close above 0mV. Membrane is now positive on the inside. Voltage-gated K+ channels fully open, K+ diffuses out. K + outflow and the Na+/K+ pump repolarize the membrane. K+ channels stay open longer than Na+ channels so more K+ leaves the cell and the membrane voltage hyperpolarizes below the resting potential. Ion diffusion through K+ leak channels in the membrane or astrocyte scavenging of K+ from the interstitial fluid restores resting membrane potential. Phases of the Action Potential Membrane Potential (mV) Phases of the Action Potential 1 2 3 4 5 1 Time (milliseconds) TIME (msec) Phase 1 2 3 4 5 Name of Phase Resting Membrane Graded Local Potential Depolarization Repolarization Hyperpolarization Na+/K+ pump Na+ out K+ in Na+ out K+ in Na+ out K+ in Na+ out K+ in Na+ out K+ in K+ leak channels K+ out K+ out K+ out K+ out K+ out voltage-gated Na+ channel Closed and ready to open Closed and ready to open Open Closed and not able to open Closed and ready to open voltage-gated K+ channel Closed Closed Slowly opening Open Slowly Closing Ion primarily responsible for the membrane voltage K+ (out) Na+ (in) Na+ (in) K+ (out) K+ (out) ligand-gated Na+ channels voltage-gated Na+ channels voltage-gated K+ channels membrane channel(s) primarily responsible for membrane voltage Na+/K+ pump channels K+ leak voltage-gated K+ channels Na+/K+ pump voltage-gated voltage-gated voltage-gated voltage-gated Membrane Potential (mV) Depolarization Repolarization Hyperpolarization 0 Time (milliseconds) Saladin Text page 460: “A traveling nerve signal is an electrical current, but it is not the same as a current traveling through a wire. A current in a wire travels millions of meters per second and is decremental – it gets weaker with distance. A nerve signal is much slower (not more than 2m/sec in unmyelinated fibers), but is nondecrimental. Even in the longest axons, the last action potential generated at a synaptic knob has the same voltage as the first one generated at the trigger zone. …We can compare the nerve signal to a burning fuse. When a fuse is lit, the heat ignites powder immediately in front of this point and this repeats itself in a selfpropagating fashion until the end of the fuse is reached. At the end, the fuse burns just as hotly as it did at the beginning. In a fuse the combustible powder is the source of potential energy that keeps the process going in a nondecremental fashion. In an axon, the potential energy comes from the ion gradient across the plasma membrane. Thus, the signal does not grow weaker with distance; it is selfpropagating, like the burning of a fuse. The Refractory Period • The Refractory Period is when a cell is resistant to stimulation. • Assures one way conduction of the impulse because membrane channels are temporarily deactivated. • • No stimulus can start an action potential during the Absolute Refractory Period because the voltage-gated Na+ channels are closed. Another action potential can be started by a stronger than normal stimulus because some voltage-gated Na+ channels are reactivated. The Refractory Period During the Refractory Period, voltage-gated Na+ channels are inactivated by a protein tail that temporarily “plugs” the channel. Impulse Conduction in Unmyelinated Fibers • All healthy axons are covered by glial cells. • Unmyelinated Axons are covered by a single layer of Schwann cell membrane (in the PNS) or oligodendrocyte membrane (in the CNS). • Myelinated Axons are wrapped with many layers of glial cell membrane. • Threshold voltage in the trigger zone (axon hillock) starts the impulse down the axon. • Nerve signal (impulse) is a chain reaction of sequentially opening voltage-gated Na+ channels down entire length of axon. Impulse Conduction in Unmyelinated Fibers Speed of Nerve Signals • Signal speed depends on: – diameter of the axon • larger diameter = faster signal conduction because of increased membrane surface area for signal conduction – myelination • myelinated axons are faster because of saltatory conduction • axons can be fast and thin (saves space) if they are myelinated • Speeds: – slow, unmyelinated fibers conduct at 0.5-2 m/sec (1-4 mph) • slow pain fibers (burning, aching, throbbing pain) from a sprain, sun burn or stubbing a toe take a relatively long time to reach the CNS and last a long time. – fast, myelinated fibers conduct at 120 m/sec (268 mph) • fast pain fibers (sharp, pricking pain like stepping on a thorn) are conducted to the CNS quickly to help prevent further injury • fast signals are also to skeletal muscles or from sensory organs for vision and balance Saltatory Conduction in Myelinated Fibers • Voltage-gated channels at Nodes of Ranvier – fewer than 25 per m2 in myelin-covered regions – up to 12,000 per m2 in nodes of Ranvier • Fast Na+ diffusion into axon occurs between nodes depolarizing the membrane and generating a local current flow that quickly spreads to the next node. Saltatory Conduction in Myelinated Fiber • The action potentials jump from node of Ranvier to node of Ranvier. (Saltator (L) a leaper, dancer) Clinical Correlation • Multiple Sclerosis – Myelin sheaths formed by oligodendrocytes in the CNS deteriorate and are replaced by scar tissue. – Deterioration may be caused by an immune disorder triggered by a virus in genetically susceptible individuals. – Nerve conduction is disrupted. Specific symptoms depend upon the part of the CNS is involved. – Symptoms can include double vision, blindness, speech defects, spontaneous muscle cramps, tremors, numbness. Myelination and Brain Maturation Age 4 Age 8 Age 12 Age 16 Age 20 “Few axons are covered with myelin at birth. More are insulated over time from the back of the cerebral cortex to the front. Basic functional areas such as vision (back) are completed before age 4, followed by language and last, self-control (forehead). Myelin is laid down until age 25 or so, one reason teenagers do not have adult decision- making abilities” R. Douglas Fields “Why White Matter Matters” Scientific American, March 2008.