Animal Endocrine System and Hormones

• Cell-to-cell communication is vital for organizing specialized cells with different functions within a biological system.

Animal Endocrine System

• The endocrine system uses chemical communication to maintain cellular interaction.
• Hormones are the endocrine system's chemical messengers, distinct from neurotransmitters in the neural system.
• Neurotransmitters offer immediate, short-distance communication at synapses.
• Hormones travel through the circulatory system, reaching cells with the appropriate receptors over a greater distance.
• Graded potential relates to hormonal events over longer durations, such as seconds to hours.
• These mediate long-term processes through the circulatory system and are slower than action potentials.
• Action potential facilitates instantaneous, short-distance communication in the neural system.

Three Main Hormone Groups

• Steroids, peptides, and amines constitute the three primary categories of hormones.
• Identifying the group of a hormone is necessary.

Steroids

• Steroids' names often end in '-one'.
• Steroids are derived from cholesterol molecules, making them fat-soluble.
• Steroids are hydrophobic (water-repelling) and lipophilic (fat-attracting).
• Carrier proteins aid in steroid transportation through the watery circulatory system.
• Steroids easily penetrate the cell membrane due to its phospholipid composition since it is not a barrier.
• Steroids have either nuclear or cytoplasmic receptors inside the cell.
• When a steroid reaches a cell, it detaches from its carrier protein and binds to an intracellular nuclear receptor.

Peptides

• Peptide hormones are protein-based, consisting of polypeptide chains.
• Peptides are hydrophilic (water-attracting) and lipophobic (fat-repelling).
• Peptide hormones do not require carrier proteins for transport.
• Peptide hormones interact with membrane-bound receptors due to their inability to cross the cell membrane.

Amines

• Amine hormones, or monoamine hormones, derive from a single amino acid.
• Tyrosine and tryptophan are the two primary amino acids involved.
• Tyrosine produces catecholamines (dopamine, norepinephrine, epinephrine) and iodothyronines (thyroxine [T4], triiodothyronine [T3]).
• Tryptophan gives rise to melatonin.

Thyroid Gland

• The thyroid gland's exclusive function is endocrine.
• The thyroid gland regulates metabolic rate and calcium levels.
• It regulates basal metabolic rate, development, and the timing of metamorphosis in amphibians by regulating oxidative rate in tissues.
• Amine hormones regulate metabolic rate, while peptide hormones regulate calcium levels.
• Thyroxine 3 and 4 are major regulators of metabolic rate.
• Calcitonin, a peptide hormone, encourages calcium incorporation into bone.
• The parathyroid hormone stimulates calcium release from bone and absorption by the gut and kidney.

Pancreas

• The pancreas is not solely endocrine in function.
• It produces digestive enzymes.
• Islets of Langerhans are the region of the pancreas that produces peptide hormones.
• Alpha cells produce glucagon, which elevates glucose levels, while beta cells produce insulin, which lowers glucose levels.

Adrenal Gland

• The adrenal gland has an exclusive endocrine function.
• It produces steroid and amine hormones.
• The medulla, the adrenal gland's inner core, responds first by releasing epinephrine and norepinephrine (amines/monoamines) to control fight or flight.
• The cortex, the adrenal gland's outer layer, is secondary to the medulla, it produces steroids like cortisol to increase blood sugar, mobilize it to cells, and suppress the immune system, plus aldosterone to regulate blood sodium.

Gonads

• Gonads are not solely endocrine in function.
• Gonads produce steroids such as testosterone, dihydrotestosterone, estrogen, and progesterone.
• Gonads regulate puberty, reproductive behavior, and gamete production.

Pineal Gland

• The pineal gland is not solely endocrine in function.
• It produces amines, including melatonin, which regulates circadian rhythms.

Hypothalamus

• The hypothalamus is not solely endocrine in function.
• The hypothalamus produces peptides.
• It regulates body temperature, responses to stress, and feeding/mating behaviors.
• Peptide release activates steroid release during puberty to foster secondary sexual characteristics and reproductive function in adulthood.

Pituitary Gland

• The pituitary gland has a dedicated endocrine function.
• The anterior pituitary receives peptide signals from the hypothalamus to regulate stress and reproduction.
• The posterior pituitary receives neural projections from the hypothalamus, releasing oxytocin and vasopressin to regulate labor, milk release, water balance, and blood pressure.

Hormone Cascade

• External or internal conditions trigger hormone release.
• The hypothalamus releases releasing hormones like GnRH or CRF.
• The anterior pituitary releases tropic hormones like LH, FSH, or ACTH into the bloodstream.
• These subsequently affect endocrine glands, like the gonads or adrenals, which release hormones.
• Hormones exert negative feedback on the anterior pituitary and hypothalamus.

Stress System

• The autonomic nervous system innervates the adrenal medulla.
• The adrenal medulla releases norepinephrine and epinephrine.
• In the HPA axis, the hypothalamus releases CRH.
• The anterior pituitary releases ACTH.
• The adrenal cortex produces cortisol (in humans) or corticosterone (in other animals).
• High cortisol levels inhibit CRH and ACTH release.

Reproductive Hormone Cascade

• The hypothalamus releases GnRH (peptide hormone).
• The anterior pituitary releases LH and FSH (peptide hormones).
• The gonads release steroid hormones.

Plant Hormones

• Plants need to monitor their environment and adjust their growth and reproduction.
• Plant hormones control growth and development using environmental cues, receptors like photoreceptors, hormones, and regulatory proteins/enzymes.
• Each plant hormone can play multiple regulatory roles with interactions that can be complex and overlapping.

Gibberellins

• Gibberellins promote cell elongation.
• Gibberellins promote fruit growth.
• Gibberellins mobilize seed reserves by activating enzymes that break down starches, proteins, and other monomers.

Auxins

• Auxins control plant growth up and are common in sunflowers.
• Auxins are involved in root initiation, causing roots to develop and grow into new plants.
• Auxins inhibit abscission and anti-intuitively control phototropism and gravitropism.
• Auxins accumulate on the shaded side of a plant, increasing cell division and causing the plant to grow towards the light.
• Apical buds inhibit the growth of axillary buds, while branches that are exposed to lower amounts of auxin tend to branch more than apex branches.

Cytokinins

• Cytokinins inhibit stem elongation.
• Cytokinins in conjuction with auxins control outward growth in auxiliary buds.
• Cytokinins delay senescence of leaves.

Ethylene

• Ethylene is a gas produced by all sections of a plant.
• Ethylene promotes senescence and speeds ripening of fruit.
• Ethylene stimulates its own production.

Brassinosteroids

• Brassinosteroids have diverse effects.
• Brassinosteroids inhibit root elongation.
• Brassinosteroids promote xylem differentiation, pollen tube growth, seed germination, apical dominance, and leaf senescence.
• Brassinosteroids enhance cell elongation and cell division in shoots.