Lecture 5 Physiology PDF

 Membrane potential It is the voltage difference between the inside and outside of a cell Chemical compositions of extracellular and intracellular fluids  The Resting Membrane Potential The steady potential of an unstimulated cell. Is the electrical potenti...

 Membrane potential It is the voltage difference between the inside and outside of a cell Chemical compositions of extracellular and intracellular fluids  The Resting Membrane Potential The steady potential of an unstimulated cell. Is the electrical potential difference across the plasma membrane of most cells, with inside negative to outside at rest. Recording: Done by the use of cathode ray oscilloscope when one of its electrode is introduced inside the cells and the other electrode is placed on the outer surface. The Resting Membrane Potential -70 mV in nerve cells -90 mV in muscle cells -10 mV in RBCs -30 m.v in liver cells.  Why RMP is –ve? 3 Na+ are pumped out for every 2 K+ pumped in. There is a greater flux of K+ out of the cell than Na+ into the cell. This is because in a resting membrane there is a greater permeability (more leak channels) to K+ than there is to Na+. Because there is greater net efflux than influx of positive ions during this step, a significant negative membrane potential develops  Causes of RMP 1- A tiny excess of negative ions inside the cell and an excess of positive ions outside. Na+ and K+ generally make the most important contributions in generating the resting membrane potential, but in some cells Cl− is also a factor. Na+ and Cl− concentrations are lower inside the cell than outside, and that the K+ concentration is greater inside the cell. Distribution of Major Mobile Ions Across the Plasma Membrane of a Typical Neuron  Causes of RMP 2- Na / K pump: The concentration differences for Na+ and K+ are established by the action of the sodium/potassium-ATPase pump that pumps Na+ out of the cell and K+ into it. Without this pump, Na+ will enter and K+ will leave the cell and the resting membrane potential will disappear or markedly diminished. The magnitude of the resting membrane potential depends mainly on two factors: (1) differences in specific ion concentrations in the intracellular and extracellular fluids; and (2) differences in membrane permeabilities to the different ions, which reflect the number of open channels for the different ions in the plasma membrane. (3) a direct contribution from ion pumps, has a very minor role. This is a Polarization of This is a stimulation of the resting membrane resting membrane by due to distribution of net movement of ions (+) and (-) charged ion across membrane  Action potentials occur in several types of animal cells, called excitable cells, which included: Neurons They play a central role in cell- to-cell communication Muscle cells Is the first step in the chain of events leading to contraction Endocrine cells Their main function is to activate intracellular processes An action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory.  Action potential Action potentials are very different from graded potentials. They are large alterations in the membrane potential; the membrane potential may change by as much as 100 mV. For example, a cell might depolarize from −70 to +30 mV, and then repolarize to its resting potential. Action potentials are generally very rapid (as brief as 1–4 milliseconds) and may repeat at frequencies of several hundred per second. The propagation of action potentials down the axon is the mechanism the nervous system uses to communicate from cell to cell over long distances. The terms depolarize, repolarize, and hyperpolarize are used to describe the direction of changes in the membrane potential relative to the resting potential in an excitable cell. The resting membrane potential is “polarized,” simply meaning that the outside and inside of a cell have a different net charge. The membrane is depolarized when its potential becomes less negative (closer to zero) than the resting level. Overshoot refers to a reversal of the membrane potential polarity—that is, when the inside of a cell becomes positive relative to the outside. When a membrane potential that has been depolarized returns to the resting value, it is repolarized. The membrane is hyperpolarized when the potential is more negative than the resting level.  Action potentials are all-or-none Action potentials either occur maximally or they do not occur at all. Stimuli stronger than those required to reach threshold elicit action potentials, the action potentials resulting from such stimuli have exactly the same amplitude as those caused by threshold stimuli. This is because once threshold is reached, membrane events are no longer dependent upon stimulus strength. Rather, the depolarization generates an action potential because the positive feedback cycle is operating. Threshold Potential is the membrane potential at which an action potential is initiated Stimuli that are just strong enough to depolarize the membrane to this level are threshold stimuli  Behavior of voltage-gated Na+ and K+ channels. Depolarization of the membrane causes Na+ channels to rapidly open, then undergo inactivation followed by the Opening of K+ channels. When the membrane repolarizes to negative voltages, both channels return to the closed state.  Refractory Period During the action potential, a second stimulus, no matter how strong, will not produce a second action potential. That region of the membrane is then said to be in its absolute refractory period. APs are propagated without any change in size from one site to another along a membrane. In myelinated nerve fibers, APs are regenerated at the nodes of Ranvier in saltatory conduction.  Block voltage-gated Na+ channels The generation of action potentials is prevented by local anesthetics such as procaine (Novocaine) and lidocaine (Xylocaine) because these drugs block voltage-gated Na+ channels, preventing them from opening in response to depolarization. Without action potentials, graded signals generated in sensory neurons—in response to injury, for example—cannot reach the brain and give rise to the sensation of pain.  It is a n area of communication between 2 neurons. It is an anatomically specialized junction between two neurons, at which the electrical activity in a presynaptic neuron influences the electrical activity of a postsynaptic neuron. 30  Synapse Functional Anatomy of Synapses A. electrical synapse consist of gap junctions that allow current to flow between adjacent cells. Allow current carrying ions to flow directly from one neuron to the next Found in the brain, limited in adulthood Most are replaced by chemical synapses  B. chemical synapse neurotransmitter molecules are stored in synaptic vesicles in the presynaptic axon terminal, and when released transmit the signal from a presynaptic to a postsynaptic neuron. 1- Excitatory synapse brings the membrane of the postsynaptic cell closer to threshold (Depolarization). 2-Inhibitory synapse prevents the postsynaptic cell from approaching threshold by hyperpolarizing or stabilizing the membrane potential.  Synaptic transmitter (neurotransmitter) A neurotransmitter is a chemical substances that is released by a neuron ( called presynaptic cell ) , crosses the synaptic cleft , and binds to a receptor located on the membrane ( postsynaptic membrane ) of another cell. Mechanisms of Neurotransmitter Release I. Depolarization of the axon terminal increases the Ca2+ concentration within the terminal, which causes the release of neurotransmitter into the synaptic cleft. II. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic cell; the activated receptors usually open ion channels.  It is recording of electrical activities of brain along the scalp EEG measures voltage fluctuations resulting from ionic current flows within the neurons of the brain EEG was first analyzed systematically by the German psychiatrist Hans Berger and so the waves of EEG are referred as Berger’s waves There are Two types of electrical activity of brain may be recorded : Spontaneous activity Evoked Potentials EEG These are the electrical This activity appears to a Potentials that are caused raise without any Obvious by stimulation of sensory stimulation Receptors or nerve fibers EEG is useful in the diagnosis of neurological disorders and sleep disorders The common neurological disorders in which the EEG pattern is altered are: Epilepsy Subdural hematoma  Brain Tumor Disorders of midbrain affecting the ascending reticular activating system EEG waveforms are generally classified according to their frequency, amplitude, and shape, as well as the sites on the scalp at which they are recorded. The most familiar classification uses EEG waveform frequency (eg, alpha, beta, theta, gamma and delta).In normal persons, EEG is characterized by three frequency bands namely Alpha rhythm Theta rhythm Beta rhythm Gamm rhythm Delta rhythm  It includes: low frequency high voltage waves occurring at the rate of 0.5 to 3/second with an amplitude of 20 to 200 µV This commonly occurs in: Early childhood during waking hours. In adults, it appears mostly during deep sleep. Presence of delta waves in adults during condtions other than sleep, indicates the pathological process in brain like: Tumor Epilepsy Increased intracranial pressure Mental deficiency or depression : Its includes low frequency wives of 3 to 8/second Theta brainwaves occur most often in sleep but are also dominant in the deep meditation. In theta we are in a dream and information beyond our normal conscious awareness. Potential occurring at : a frequency of 8 to 12 waves/second with an amplitude of 50 µV This is obtained in : Inattentive brain or mind as in drowsiness light sleep or narcosis with closed eyes. Waves with a frequency of 7 Hz or less often are classified as abnormal in awake adults, although they normally can be seen in children or in adults who are asleep. : Its includes high frequency wives of 12 to 38/second the amplitude is low, i.e. 5 to 10 µv. This is recorded during mental activity or mental tension or arousal state. It is not affected by opening the eyes Beta is a ‘fast’ activity, present when we are alert, attentive, engaged in problem solving, judgment, decision making, and engaged in focused mental activity. : Its includes highest frequency wives of 38 to 42/second Gamma brainwaves are the fastest of brain waves (high frequency, like a flute), and relate to simultaneous processing of information from different brain areas.