Chapter 27: The Control Systems – Nervous and Endocrine Systems

Chapter 27: The Control Systems

27.1 Nervous System

The nervous system and endocrine system are the primary control systems in animals, working together to maintain homeostasis. Both systems use chemical signals, but their mechanisms and speeds of action differ significantly.

• Nervous System: Sends rapid messages along nerve fibers, releasing neurotransmitters directly to target organs for immediate response.

• Endocrine System: Uses hormones transported via the bloodstream to target organs, resulting in slower but longer-lasting effects.

• Homeostasis: Both systems respond to internal and external changes that may threaten the body's equilibrium.

Example: The nervous system triggers a quick withdrawal from a hot surface, while the endocrine system regulates long-term processes like growth and metabolism.

27.2 Endocrine System

The endocrine system consists of glands and tissues that secrete hormones directly into the bloodstream. It is essential for regulating metabolism, growth, reproduction, and homeostasis.

• Endocrine Glands: Ductless glands that release hormones into the blood (e.g., pituitary, thyroid, adrenal glands).

• Exocrine Glands: Have ducts and secrete products into ducts leading to specific organs (e.g., salivary glands).

• Integration with Nervous System: Both systems are closely linked in maintaining homeostasis.

Hormones: Types and Mechanisms of Action

Hormones are chemical messengers that affect only target cells with specific receptors. The response to a hormone is a metabolic change, and the type of change depends on the hormone's chemical structure.

Steroid Hormones

• Structure: Lipid-soluble molecules derived from cholesterol.

• Mechanism: Pass through the plasma membrane, bind to intracellular receptors, and form a hormone-receptor complex that binds to DNA, activating gene expression.

Example: Cortisol and sex hormones (estrogen, testosterone).

Peptide Hormones

• Structure: Composed of peptides, proteins, glycoproteins, or modified amino acids.

• Mechanism: Bind to receptors on the plasma membrane. The hormone acts as a "first messenger," activating a signal transduction pathway. A "second messenger" (often cAMP) triggers an enzyme cascade, amplifying the response.

Example: Insulin, growth hormone, antidiuretic hormone (ADH).

Hypothalamus and Pituitary Gland

The hypothalamus and pituitary gland are central to the regulation of the endocrine system.

• Hypothalamus: Part of the brain; integrates nervous and endocrine functions. Contains hormone-secreting neurons that control the pituitary gland.

• Pituitary Gland: Connected to the hypothalamus by a stalk; divided into anterior and posterior lobes.

• Anterior Pituitary: Produces hormones that regulate other endocrine glands.

• Posterior Pituitary: Stores and releases hormones produced by the hypothalamus.

Thyroid and Parathyroid Glands

The thyroid gland regulates metabolism and calcium balance, while the parathyroid glands regulate blood calcium levels.

• Thyroid Hormones (T3 and T4): Increase metabolic rate in all body cells.

• Iodine Requirement: Iodine is necessary for thyroid hormone production. Deficiency can cause goiter (thyroid enlargement).

• Calcitonin: Lowers blood calcium levels.

• Parathyroid Hormone (PTH): Increases blood calcium levels by stimulating bone resorption and increasing calcium reabsorption in kidneys.

Example: Iodized salt prevents simple goiters caused by iodine deficiency.

Adrenal Glands

The adrenal glands sit atop the kidneys and consist of two regions: the adrenal medulla and adrenal cortex. Both are regulated by the hypothalamus and respond to stress.

Adrenal Medulla

• Hormones: Epinephrine (adrenaline) and norepinephrine (noradrenaline).

• Function: Mediate the "fight or flight" response; effects are rapid and short-term.

Adrenal Cortex

• Mineralocorticoids (e.g., aldosterone): Regulate salt and water balance, increasing blood volume and pressure.

• Glucocorticoids (e.g., cortisol): Regulate metabolism of carbohydrates, proteins, and fats; increase blood glucose; have anti-inflammatory effects.

• Sex Hormones: Small amounts of both male and female sex hormones are secreted in both sexes.

Adrenal Disorders

• Addison's Disease: Hyposecretion of adrenal cortex hormones; symptoms include bronzed skin, low blood pressure, dehydration, and can be fatal if untreated.

• Cushing's Syndrome: Hypersecretion of adrenal cortex hormones; symptoms include high blood glucose, hypertension, and masculinization in women.

Pancreas

The pancreas has both exocrine and endocrine functions. Its endocrine portion regulates blood glucose levels through the secretion of insulin and glucagon.

• Insulin: Lowers blood glucose by stimulating uptake of glucose by cells and promoting glycogen formation in the liver.

• Glucagon: Raises blood glucose by stimulating the liver to break down glycogen and use fats/proteins as energy sources.

Diabetes Mellitus

Diabetes mellitus is a disorder in which cells cannot take up or metabolize glucose properly, resulting in high blood sugar levels.

• Type 1 Diabetes: Insulin-dependent; caused by autoimmune destruction of pancreatic islets; requires insulin therapy.

• Type 2 Diabetes: Noninsulin-dependent; associated with obesity; cells become resistant to insulin; managed by diet and exercise.

Table 27.1 Summary of Endocrine System

This table summarizes the major hormones, their sources, and effects in the body.

Hormones	Effects in the Body
HYPOTHALAMUS AND POSTERIOR PITUITARY
Releasing and inhibiting hormones	Control anterior pituitary
Antidiuretic hormone (ADH)	Released by posterior pituitary; causes water uptake by kidneys
Oxytocin	Released by posterior pituitary; causes uterine contractions
ANTERIOR PITUITARY
Gonadotropic hormones	Stimulate gonads
Thyroid-stimulating hormone (TSH)	Stimulates thyroid
Adrenocorticotropic hormone (ACTH)	Stimulates adrenal cortex
Prolactin	Causes milk production
Growth hormone (GH)	Causes cell division, protein synthesis, bone growth
THYROID GLAND
Thyroxine (T4) and triiodothyronine (T3)	Increase metabolic rate
Calcitonin	Lowers blood calcium level
PARATHYROID GLANDS
Parathyroid hormone (PTH)	Raises blood calcium level
ADRENAL MEDULLA
Epinephrine and norepinephrine	Response to emergency situations
ADRENAL CORTEX
Mineralocorticoids (aldosterone)	Cause kidneys to reabsorb Na+
Glucocorticoids (cortisol)	Raises blood glucose level
PANCREAS
Insulin	Causes uptake of glucose by cells and formation of glycogen in liver
Glucagon	Causes liver to break down glycogen

Additional info: The endocrine system is essential for coordinating complex processes such as growth, metabolism, and stress responses. Disorders of the endocrine system can have widespread effects on health and homeostasis.