Sample 1Medium
A figure shows a short length of an axon. At point X the outer surface of the membrane is marked positive and the inner surface negative. At point Y, where a stimulus has just been applied, the outer surface is marked negative and the inner surface positive. Which statement explains the state at Y?
- A
Y is depolarised, because K+ has rushed into the axon
- B
Y is the resting state, kept by the sodium-potassium pump
- C
Y is depolarised, because the membrane there has become freely permeable to Na+ and Na+ has rushed in
- D
Y is polarised, because the pump moves 3 K+ in for every 2 Na+ out
Show the answer
The answer is C.A resting axon is polarised: positive outside and negative inside. At the site of a stimulus the membrane becomes freely permeable to Na+, which rushes in and reverses the polarity. This depolarised state is the action potential.
- A.
Depolarisation is caused by a rapid influx of Na+. K+ moves out during repolarisation.
- B.
At rest the outer surface is positive and the inner surface negative, as at X. Y shows the reversed polarity of an action potential.
- D.
The pump moves 3 Na+ out for every 2 K+ in, and this keeps the resting (polarised) state shown at X, not Y.
NCERT: Class 11 Biology, Chapter 18
Sample 2NEET level
Match the parts of the human brain in List I with their features in List II. List I A. Hypothalamus B. Corpus callosum C. Medulla D. Pons List II I. A tract of nerve fibres connecting the left and right cerebral hemispheres II. Centres that control body temperature and the urge for eating and drinking III. Fibre tracts that interconnect different regions of the brain IV. Centres that control respiration, cardiovascular reflexes and gastric secretions Choose the correct answer from the options given below.
- A
A-II, B-I, C-IV, D-III
- B
A-I, B-II, C-IV, D-III
- C
A-II, B-I, C-III, D-IV
- D
A-II, B-III, C-IV, D-I
Show the answer
The answer is A.The hypothalamus has centres controlling body temperature, eating and drinking; the corpus callosum connects the two cerebral hemispheres. The medulla controls respiration, cardiovascular reflexes and gastric secretions, and the pons consists of fibre tracts that interconnect different regions of the brain.
- B.
Swaps the hypothalamus and the corpus callosum.
- C.
Swaps the medulla and the pons.
- D.
Gives the corpus callosum the pons's feature and the pons the corpus callosum's.
NCERT: Class 11 Biology, Chapter 18 (Neural Control and Coordination), Central Neural System