Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 45.2 Feedback Regulation and Coordination With the Nervous System Are Common in Hormone Pathways
Endocrine signaling often involves feedback mechanisms that maintain homeostasis, and in many cases, the endocrine and nervous systems work together to regulate physiological responses. Hormone pathways range from simple local loops to complex hormone cascades involving multiple organs.
1. Simple Endocrine Pathways
- Endocrine cells detect a stimulus and secrete a hormone that travels through blood to target cells
- Secretin from the duodenum stimulates the pancreas to release bicarbonate, which neutralizes stomach acid
- Negative feedback reduces the original stimulus (low pH), halting further secretin release
2. Simple Neuroendocrine Pathways
- A sensory neuron detects a stimulus and signals a neurosecretory cell to release a neurohormone
- Oxytocin is released in response to suckling, causing milk release from mammary glands
- Positive feedback enhances the response until the stimulus ends (e.g., infant stops suckling)
3. Hormone Pathways in Invertebrates
- In insects, molting and metamorphosis are controlled by a neuroendocrine cascade
- PTTH, from the brain, triggers ecdysteroid release from the prothoracic gland
- Juvenile hormone (JH) determines the result: high JH → molting; low JH → metamorphosis
- Used in pest control strategies by manipulating hormone pathways
4. Vertebrate Neuroendocrine Coordination
- The hypothalamus integrates nervous and hormonal signals
- It controls the pituitary gland, which has two parts:
- Posterior pituitary: stores and releases ADH and oxytocin from hypothalamic neurons
- Anterior pituitary: an endocrine gland that releases hormones in response to hypothalamic releasing hormones
- These hormones regulate growth, metabolism, reproduction, and more
5. Hormone Cascade Pathways
- A hormone from the hypothalamus triggers hormone release from the anterior pituitary, which then stimulates another endocrine gland
- Example:
- Low T3/T4 → hypothalamus releases TRH
- TRH triggers TSH from the anterior pituitary
- TSH stimulates thyroid gland to release thyroid hormone
- Thyroid hormone provides negative feedback to shut down TRH and TSH production
6. Growth Hormone Regulation
- GH from the anterior pituitary stimulates growth directly and indirectly via IGFs from the liver
- Hypersecretion → gigantism (in children), acromegaly (in adults)
- Hyposecretion → pituitary dwarfism, treatable with human GH
7. Blood Calcium Regulation
- Parathyroid hormone (PTH) increases blood Ca²⁺ by:
- Stimulating bone breakdown
- Enhancing Ca²⁺ reabsorption in kidneys
- Activating vitamin D, which boosts Ca²⁺ uptake in intestines
- Calcitonin from the thyroid lowers Ca²⁺ by opposing PTH (less critical in humans)
In a Nutshell
Hormone pathways maintain balance in the body through feedback loops and tight integration with the nervous system. Simple loops regulate digestion and reproduction, while cascades involving the hypothalamus and pituitary coordinate complex processes like growth, metabolism, and development. Negative feedback ensures stability, while hormone interactions enable adaptive responses to internal and external changes.