Chapter 29, Problem 29.71PP

BIO STIMULATING THE BRAIN. Communication in the nervous system is based on the propagation of electrical signals called action potentials along axons, which are extensions of nerve cells (see the Passage Problems in Chapter 26). Action potentials are generated when the electric potential difference across the membrane of the nerve cell changes: Specifically, the inside of the cell becomes more positive. Researchers in clinical medicine and neurobiology cannot stimulate nerves (even noninvasively) at specific locations in conscious human subjects. Using electrodes to apply current to the skin is painful and requires large currents, which could be dangerous.

  Anthony Barker and colleagues at the University of Sheffield in England developed a technique called transcranial magnetic stimulation (TMS). In this widely used procedure, a coil positioned near the skull produces a time-varying magnetic field that induces in the conductive tissue of the brain (see part (a) of the figure) electric currents that are sufficient to cause action potentials in nerve cells. For example, if the coil is placed near the motor cortex (the region of the brain that controls voluntary movement), scientists can monitor muscle contraction and assess the connections between the brain and the muscles. Part (b) of the figure is a graph of the typical dependence on time t of the magnetic field B produced by the coil.

29.71    In part (a) of the figure, a current pulse increases to a peak and then decreases to zero in the direction shown in the stimulating coil. What will be the direction of the induced current (dashed line) in the brain tissue? (a) 1; (b) 2; (c) 1 while the current increases in the stimulating coil, 2 while the current decreases; (d) 2 while the current increases in the stimulating coil, 1 while the current decreases.

29.71    IDENTIFY and SET UP: Apply Lenz’s law to determine the direction of the induced current. The figure shows the current pulse in the coil is in the counterclockwise direction as viewed from above. Also, the figure shows that direction-1 for the induced current is clockwise and direction-2 is counterclockwise.

EXECUTE: As the current pulse increases, it produces an increasing upward magnetic field in the brain.

To oppose the increasing flux, the induced current must flow clockwise (direction-1). As the current pulse decreases its upward magnetic field decreases and the induced current must flow counterclockwise (direction-2) to oppose this. The correct choice is (c).

EVALUATE: Although the brain is made up of tissue, in some ways it behaves like a resistor and allows current to flow in it.