The Endocrine System Cellular coordination Endocrine communication Endocrine vs. Exocrine Target Cells & Receptors Hormones Endocrine Glands Thyroxine The Pituitary & Hypothalamus Hormone Action

How do multicellular organisms differ from single celled organisms? One way is coordination. Complex events (like flowering) require cells to work together.

Life cycle transitions in insects require the coordinated activities of tens of thousands of cells. How do all these cells “know” that it’s time to molt or time to emerge from a chrysalis?

A series of chemical signals known as hormones are released from glands within the insect’s body, regulating growth and development. ecdysone

Neurons signal other cells by the release of chemical messengers (neurotransmitters) at synapses Direct cell-to-cell contact between neuron and effector (in this case, skeletal muscle)

Endocrine signaling uses the bloodstream and acts over long distances Synaptic signaling in neurons is one example of local cell-to-cell signaling. Paracrine signaling is another.

Endocrine cells release chemical signals, or “Hormones” that travel throughout the body, where they affect the actions of “target cells.” Hormones affect only those cells that have specific receptors for a particular hormone.

Endocrine glands release their secretions into the bloodstream. Exocrine glands release their secretions outside the body (or into the digestive system) Endocrine glands release their secretions into the bloodstream. Exocrine gland Endocrine gland “Endocrine” endo - inside krinein - to judge or separate

Types of Hormones Steroid hormones pass through the cell membrane. Most other hormones interact with cell surface receptors.

The Human Endocrine System consists of a series of glands at various positions in the body ....

... that produce a wide variety of hormones. © 2011 Pearson Education, Inc.

Thyroxine is synthesized from the amino acid tyrosine Thyroxine is synthesized from the amino acid tyrosine. It is the reason we need a small amount of iodine in our diets. Cells synthesize thyroxine and store it in compartments in the thyroid gland.

If iodine is insufficient in the diet, the thyroid tries to compensate by producing more cells to extract more iodine from the bloodstream.

Answer: It doesn’t. “Somebody” else decides that. The Pituitary. Thyroxine (T4 or T3) • increases metabolic rate • increases heart rate • increases breathing rate Question: How does the thyroid “know” whether to release more thyroxine? Answer: It doesn’t. “Somebody” else decides that. The Pituitary.

The Posterior descends from the Hypothalamus. The Anterior Pituitary forms from the nasal lining during development.

The Posterior Pituitary consists of the terminals of neurosecretory cells from the Hypothalamus. When these cells are stimulated in the Hypothalamus, they release hormones (ADH and oxytocin) into the bloodstream.

Sensory input to the brain stimulates the posterior pituitary to release oxytocin, producing the milk letdown reflex.

The Anterior Pituitary is a little more complicated... Neurosecretory cells in the Hypothalamus dump small amounts of “releasing hormones” into vessels leading to the pituitary. These hormones target endocrine cells in the pituitary, causing them to release pituitary hormones.

Five major “tropic” hormones are released from the Anterior Pituitary. Including TSH (Thyroid Stimulating Hormone), which stimulates the release of thyroxine from the Thyroid.

1) Low temp or low Thyroxine (T4 or T3) level? 2) Hypothalamus releases TRH 3) TRH causes release of TSH 5) Thyroxine increases metabolism, warms the body. 4) TSH causes release of Thyroxine from thyroid

1) Low temp or low Thyroxine (T4) level? 2) Hypothalamus releases TRH 3) TRH causes release of TSH A classic negative feedback loop! 5) Thyroxine increases metabolism, warms the body. 4) TSH causes release of Thyroxine from thyroid

Similar feedback loops exist for the other pituitary hormones. Example: ACTH (Adrenocorticotropic hormone) and Cortisol. Cortisol is a stress-response hormone. It increases blood sugar levels and suppresses the immune system (relieving inflammation).

How does a hormone act at the cellular level? That depends on whether it’s a steroid hormone or a peptide hormone.

Hormone Action - Steroids Because they are lipids, steroids can cross cell membranes easily.

Target cells contain steroid hormone Receptor Proteins Steroid hormones bind to their receptors, and the Hormone-Receptor Complex enters the nucleus. The complex binds to DNA sequences known as Hormone Response Elements and activates transcription.

Activation of transcription by Steroid Hormones can cause profound long-termchanges in cellular activity - including molting and metamorphosis in insects.

Peptide Hormones like epinephrine don’t enter the cell. They bind to receptors at the cell surface, which in turn release “second messengers” inside the cell. Example: cAMP. These receptors are often coupled to proteins that bind GTP, known as G-proteins.

One effect of epinephrine binding is the activation of an enzyme called Phosphorylase, which releases glucose into the cell (and bloodstream). How does this happen?

The production of cAMP starts a “cascade” of events that amplifies the signal. This amplification enables just a few hormone molecules to elicit a powerful response from the target cell.

Example: cardiac muscle cells’ response to epinephrine. These “second-messenger: hormones can produce very rapid changes in cellular activity. Example: cardiac muscle cells’ response to epinephrine.

Such as antler growth in male deer. Because they affect transcription, the effects of steroid hormones are generally slower - but can be much more profound. Such as antler growth in male deer.

Which prepares us for Wednesday’s topic. SEX