ACCESS TO SURGERY: 500 SINGLE BEST ANSWER ... - PasTest

ACCESS TO SURGERY: 500 SINGLE BEST ANSWER QUESTIONS IN GENERAL AND SYSTEMIC PHYSIOLOGY1.21Answer: AThe cell will depolarise if its membrane potential ispositive with respect to the equilibrium potential forchloride ionsgeneralAlthough electrogenic pumps may contribute to the membranepotential of certain cells, the major determinants of membranepotential are the external and internal concentrations of permeantions and their relative permeabilities in the membrane. Decreasingthe conductance causes the membrane potential to move awayfrom the equilibrium potential for that ion. So, a decrease in theconductance of a membrane to chloride ions causes the cells todepolarise – that is, become more positive – if the membranepotential is positive with respect to the chloride equilibriumpotential. Conversely, increasing the conductance for an ion causesthe membrane potential to approach the equilibrium potentialfor that ion. External and internal ion chloride concentrations areneeded to calculate the Nernst potential for this ion, but a simplecomparison of these two values does not allow predictions aboutthe change in membrane potential.1.22Answer: EReflex inhibition of motor neuronesThe stimulation of receptors in the Golgi tendon organs leads to theinverse stretch reflex. This reflex is responsible for the relaxationthat is observed when a muscle is subjected to a strong stretch.Impulses from the organs travel in type Ib fibres to the spinal cord,where they activate inhibitory interneurones. These in turn suppressthe activity of motor neurones and therefore lead to relaxation ofthe extrafusal muscle fibres attached to the tendons. The state ofcontraction of intrafusal fibres, the gamma-efferent discharge rateand the activity in group II afferent fibres control the stretch reflex,which, distinct from the inverse stretch reflex, is mediated by theGolgi tendon organs.216