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BIO Signals in nerve cells stimulate muscles The input end of a human nerve cell is connected to an output end by a long, thin, cylindrical axon. A signal at the input end is caused by a stretch sensor, a temperature sensor, contact with another cell or nerve, or some other stimulus. At the output end, the nerve signal can stimulate a muscle cell to perform a function (to contract provide information to the brain etc).
The axon of a so-called unmyelinated human nerve cell has a radius of
When an external source stimulates the input end of the nerve cell so the potential inside reaches about -50 mV, gates or channels in the membrane walls near that input open and sodium ions rush into the axon. This simulates neighboring gates to swing open and sodium ions rush into the axon farther along. This disturbance quickly travels along the axon—a nerve impulse The potential across the inside of the membrane changes in
Which answer below is closest to the resistance of the fluid inside a 0.5-m-long unmyelinated axon?
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- Assume a length of axon membrane of about 0.10 m is excited by an action potential (length excited = nerve speed pulse duration = 50.0 m/s 2.0 103 s = 0.10 m). In the resting state, the outer surface of the axon wall is charged positively with K+ ions and the inner wall has an equal and opposite charge of negative organic ions, as shown in Figure P18.43. Model the axon as a parallel-plate capacitor and take C = 0A/d and Q = C V to investigate the charge as follows. Use typical values for a cylindrical axon of cell wall thickness d = 1.0 108 m, axon radius r = 1.0 101 m, and cell-wall dielectric constant = 3.0. (a) Calculate the positive charge on the outside of a 0.10-m piece of axon when it is not conducting an electric pulse. How many K+ ions are on the outside of the axon assuming an initial potential difference of 7.0 102 V? Is this a large charge per unit area? Hint: Calculate the charge per unit area in terms of electronic charge e per squared (2). An atom has a cross section of about 1 2 (1 = 1010 m). (b) How much positive charge must flow through the cell membrane to reach the excited state of + 3.0 102 V from the resting state of 7.0 102 V? How many sodium ions (Na+) is this? (c) If it takes 2.0 ms for the Na+ ions to enter the axon, what is the average current in the axon wall in this process? (d) How much energy does it take to raise the potential of the inner axon wall to + 3.0 102 V, starting from the resting potential of 7.0 102 V? Figure P18.43 Problem 43 and 44.arrow_forwardThe speed of propagation of the action potential (an electrical signal) in a nerve cell depends (inversely) on the diameter of the axon (nerve fiber). If the nerve cell connecting the spinal cord to your feet is 1.1 m long, and the nerve impulse speed is 18 m/s, how long does it take for the nerve signal to travel this distance?arrow_forwardIntegrated Concepts Use the ECG in Figure 20.34 to determine the heart rate in beats per minute assuming a constant time between beats. Figure 20.34 A lead II ECG with corresponding arterial blood pressure. The QRS complex is created by the depolarization and contraction of the ventricles and is followed shortly by the maximum or systolic blood pressure. See text for further description.arrow_forward
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- Opening of voltage-gated channels is driven by an intramembrane electric field acting on charged voltage-sensor domains. These domains move across the membrane and generate gating (displacement) currents. When these currents are integrated over time, they produce the gating charge generated by a population of channels. What would be the gating charge passing through the external circuit when the flat array of charges moves across the distance a inside the membrane? The hydrophobic thickness of the membrane is d, the dielectric constant ɛ, the density of charges is O and the area of the membrane is A (provide an analytic solution). start end A A a d darrow_forwardConsider the model of nerve conduction in myelinated axons presented in the chapter. Suppose the distance between the nodes of Ranvier was halved for a particular axon.a. How would this affect the resistance and the capacitance of one segment of the axon?b. How would this affect the time constant for the charging of one segment?c. How would this affect the signal propagation speed for the axon?arrow_forward5. The switch in the figure below has been closed for a very long time. a. What is the charge on the capacitor? b. The switch is opened at t=0 s. At what time has the charge on the capacitor decreased to 10% of its initial value? 600 100 V 40 0 100 2.0 μFarrow_forward
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