GLOSSARY

The autonomic nervous system is the part of the nervous system that involuntarily controls the internal organs, heart, smooth muscle (of vessels, gut, bladder, and airways), and glands, maintaining the body’s homeostasis.

Divisions

• Sympathetic: “fight or flight”, preparing the body for activity and stress.
• Parasympathetic: “rest and digest”, conserving energy and restoring the body.
• Enteric nervous system: about 400–600 million neurons in the gut wall, a “second brain” that can act independently.

Characteristics

• Two-neuron pathway: preganglionic neuron (from the CNS) → autonomic ganglion → postganglionic neuron (to the organ).
• Control centres: the hypothalamus (the main centre), brainstem, and spinal cord.
• Dual innervation: most organs are supplied by both divisions, whose effects are usually opposing but coordinated.
• Background tone: both divisions are constantly active, and the balance between them shifts.

A Scientific Note

The model “sympathetic = stress, parasympathetic = calm” is a simplification; the two divisions are often active simultaneously and in an organ-specific way.

Relevance to Sport

• Response to exercise: at the start of exercise, withdrawal of parasympathetic tone raises heart rate to about 100, and sympathetic activation then raises it further; after exercise, parasympathetic reactivation lowers the heart rate.
• Heart rate recovery: a fall of fewer than 12 beats in the first minute after exercise signals a poorer cardiovascular prognosis in the general population.
• Practical example: a faster drop in heart rate after the same exercise compared with a few weeks earlier is a sign of improved fitness and parasympathetic tone.