Homeostasis of the internal environment is maintained through the coordinated action of two systems: the autonomic nervous system and the endocrine system (hormones). Let us accurately understand the differences in their characteristics.
Basic Knowledge
Autonomic nervous system: Commands are transmitted from central regions (the hypothalamus, brainstem, and spinal cord) to peripheral organs through nerve fibers.
・Sympathetic nervous system: “Fight-or-flight” response. Increased heart rate, pupil dilation, bronchodilation, inhibited digestion, and increased blood glucose. Neurotransmitters = noradrenaline (periphery), adrenaline (released into the blood from the adrenal medulla).
・Parasympathetic nervous system: “Rest-and-digest” response. Decreased heart rate, pupil constriction, and promoted digestion. Neurotransmitter = acetylcholine.
・The two systems act antagonistically on many organs, enabling fine regulation.
Endocrine system (hormones): Hormones are secreted from endocrine glands into the blood and transported throughout the body by the bloodstream.
・Their effects are slow but long-lasting and widespread.
・Only target cells possess receptors and respond to hormones.
・Major endocrine glands: hypothalamus, pituitary gland, thyroid gland, parathyroid glands, adrenal glands (cortex and medulla), pancreas, gonads, and kidneys.
Chemical classification of hormones:
① Peptide hormones (insulin, glucagon, ADH): Bind to receptors on the cell membrane → intracellular signal transduction.
② Steroid hormones (adrenal-cortex and sex hormones): Lipid-soluble and pass through the cell membrane → bind to intracellular receptors → directly regulate gene expression.
③ Amino-acid derivatives (thyroxine, adrenaline).
Answer: (a) Increased heart rate = sympathetic nervous system. It increases the supply of blood and oxygen to the whole body during danger. (b) Increased peristalsis of the digestive tract = parasympathetic nervous system. It promotes digestion and absorption during rest. (c) Pupil dilation = sympathetic nervous system. It widens the field of vision in darkness or during danger. (d) Increased salivary secretion = parasympathetic nervous system. It secretes salivary amylase in preparation for digestion.
- Sympathetic nervous system = fight or flight; noradrenaline (periphery)
- Parasympathetic nervous system = rest and digest; acetylcholine
- Increased sympathetic activity = increased heart rate, pupil dilation, inhibited digestion, bronchodilation
- Increased parasympathetic activity = decreased heart rate, pupil constriction, promoted digestion
- Hormones = transported throughout the body by blood; slow and long-lasting
- Peptide hormones = cell-membrane receptors; steroids = intracellular receptors
Practice
- State the neurotransmitters of the sympathetic and parasympathetic nervous systems.
- Explain how steroid hormones act on intracellular receptors.
- Compare the speed and duration of regulation by the autonomic nervous system and hormones.