The autonomic nervous system (ANS) regulates heart rate, breathing-related processes, and thermoregulation largely without conscious control. It maintains homeostasis by linking sensory information, central control centres, and effectors through sympathetic and parasympathetic pathways.
The Control Mechanisms
| Function | Control mechanism | Main response |
|---|---|---|
| Heart rate | Cardiovascular centre in the medulla oblongata | Sympathetic activity increases heart rate; parasympathetic activity decreases it |
| Breathing | Respiratory centres in the medulla oblongata and pons | Adjust breathing rate and depth; autonomic pathways also alter airway diameter |
| Temperature | Thermoregulatory centre in the hypothalamus | Activates heat-loss or heat-conservation responses |
For heart rate, baroreceptors and chemoreceptors send sensory information to the cardiovascular centre. Sympathetic nerves release noradrenaline at the sinoatrial node, increasing heart rate and force of contraction. The parasympathetic vagus nerve releases acetylcholine, decreasing heart rate.
Breathing rhythm is generated by respiratory centres in the medulla oblongata and modified by the pons. Chemoreceptors detect changes in blood carbon dioxide, pH, and oxygen; rising carbon dioxide usually increases breathing rate and depth. Sympathetic activity causes bronchodilation, while parasympathetic activity promotes bronchoconstriction and mucus secretion.
For temperature control, thermoreceptors in the skin and hypothalamus detect deviations from the body’s set range. When body temperature rises, the hypothalamus promotes sweating and reduces sympathetic vasoconstrictor activity, increasing skin blood flow. When temperature falls, sympathetic activity causes vasoconstriction and piloerection, reducing heat loss.
A common misconception is that the ANS directly contracts the diaphragm. Respiratory centres regulate breathing automatically, but the diaphragm and intercostal muscles are activated through somatic motor neurons; breathing can also be voluntarily controlled. Similarly, shivering uses somatic motor pathways, although it is coordinated by the hypothalamus.
IB Exam Technique
For A.1.1 inter-system communication, identify the receptor, control centre, nervous pathway, effector, and response. Distinguish sympathetic from parasympathetic effects and explain how negative feedback restores homeostasis.
