Which graph in Fig. P29.74 best represents the time t dependence of the current i induced in the brain tissue, assuming that this tissue can be modeled as a resistive circuit? (The units of i are arbitrary.)

(a) A;

(b) B;

(c) C;

(d) D.

Figure P29.74

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.

Transcribed Image Text:

B, t (ms) 1.0 t (ms) 1.0 0.5 0.5 t (ms) Л.0 0.5 t (ms) 1.0 D 0,5 (a) (b) B (T) 4 3 t (ms) 0.2 0.4 0.6 0.8 1.0