Explain how the nervous system and the digestive system are integrated during the ingestion and early digestion of a meal.

IB DP Biology Standard Level (2023 syllabus) — C3.1 Integration of body systems · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Sensory receptors in the mouth and nose detect food stimuli such as taste, smell and texture and generate nerve impulses.
These impulses travel via afferent fibres to the brainstem (medulla oblongata) where they are processed.
The brain then sends efferent signals through the vagus nerve (parasympathetic nervous system) to the digestive organs.
The vagal signals stimulate the salivary glands to secrete saliva, which contains amylase and mucus, thereby starting chemical and mechanical digestion in the mouth.
They also stimulate gastric glands in the stomach wall to release gastric juice (hydrochloric acid and pepsinogen), preparing the stomach for protein digestion before the food arrives.
Thus the nervous system coordinates early digestion by initiating saliva production and priming the stomach for the later hormonal phase.

Examiner tips

  • Mention sensory receptors and nerve impulses first, then the brainstem, then vagus nerve, then salivary glands and stomach glands in that order.
  • Use the exact terms ‘vagus nerve’, ‘parasympathetic nervous system’, ‘salivary glands’, ‘gastric juice’, ‘amylase’, ‘hydrochloric acid’, ‘pepsinogen’.
  • Show the sequence: detection → brain processing → efferent signals → salivary secretion → gastric secretion.
  • Keep answer concise; 4 marks allow one short point per bullet above.

Common mistakes

  • Mixing up afferent and efferent pathways; forgetting the vagus nerve; using ‘autonomic nervous system’ without specifying parasympathetic; omitting the role of saliva or gastric juice.

Mark scheme (4 marks)

  1. Sensory receptors (e.g. in the mouth/nose) detect food stimuli such as taste, smell or texture and generate nerve impulses
  2. Nerve impulses are transmitted to the brain (medulla oblongata / brainstem) where they are processed and efferent signals are sent via the vagus nerve / parasympathetic nervous system to the digestive organs
  3. Efferent nerve signals stimulate the salivary glands to secrete saliva (containing amylase and mucus), initiating chemical and mechanical digestion in the mouth
  4. Nerve signals also stimulate gastric glands in the stomach wall to secrete gastric juice (hydrochloric acid and pepsinogen), preparing the stomach for protein digestion before food arrives — this nervous phase precedes (and complements) the later hormonal/gastric phase

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