Figure 6 (below) shows tidal volume tracings (the peaks on the right) obtained f

Question

Figure 6 (below) shows tidal volume tracings (the peaks on the right) obtained from animals with various brain areas severed. The tracings illustrate the role each of the centres of the brain has in controlling breathing WARNING: This is a difficult exercise and you will have to think about what happens witlh each cut (in answering the questions below with reference to Fig 5 and the associated text). BOTH VAGUS BOTH VAGUS NERVES INTACT NERVES CUT FIRST CUT SC SUPERICR COLL ICUL SECOND CUT INFER LICULI CENTRE 2 APNBUSTIC CENTRE THIRD OUT 3 DRG et VRG FOURTH CUT 4 Figure 6: Tidal volume tracings with sectioning of the nervous system The first cut between the midbrain and pons shows that higher brain centres are not necessary for controlling respiratory movements. At the same time cutting both vagal nerves influences rhythm. What effect does it have on depth and rate? (2 Marks) Explain why this happens (1 Mark).

Explanation / Answer

The cut between the midbrain and pons does not affect the rate and depth of breathing, as long as both vagi are intact.

But, once both vagi are cut, the rate decreases and depth of respiration increases.

Explanation: The pneumotaxic center in the pons controls the "switch-off" point of the inspiratory ramp, thus controls the duration of the filling phase of the lung cycle. This primarily limits the inspiration, secondarily increase the rate of breathing.

But, the vagus serves as the input nerve for these centers and carries mechanical and chemical signals from the periphery.

So, if there is a cut between the midbrain and pons, the respiratory pattern generators (present in medulla) continue to discharge as before, thus not affecting the rate and depth of breathing.

But, if that cut is associated with bilateral vagotomy, due to lack of peripheral inputs, the inspiration continues for a longer time, and thus, decreasing the respiratory rate.

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