🧠 Endocrine System & Hormones

What is the endocrine system?

• The endocrine system is a communication system in the body that uses hormones to regulate physiological processes. It includes glands (like the pituitary and thyroid) that release hormones into the bloodstream, allowing them to act at distant target organs.

How is endocrine signaling different from other types?

• Endocrine signaling = hormones travel via blood to distant targets.

• Paracrine = signals act on neighboring cells.

• Autocrine = signals act on the same cell that released them.

• Synaptic = neurons release neurotransmitters directly to a target.

• Neuroendocrine = neurons release hormones into blood (e.g., oxytocin).

💊 Hormones

What are hormones?

• Hormones are chemical messengers secreted by endocrine glands that affect physiology and behavior at distant sites.

Tropic vs. Non-tropic hormones:

• Tropic hormones: stimulate other glands to release hormones (e.g., TSH).

• Non-tropic hormones: act directly on target tissues (e.g., insulin).

Releasing vs. Release-inhibiting hormones:

• Releasing hormones: stimulate the anterior pituitary to release hormones (e.g., GnRH → FSH/LH).

• Release-inhibiting hormones: block hormone release (e.g., somatostatin inhibits GH release).

Chemical classes of hormones:

1. Peptide/protein hormones (e.g., insulin):

   • Water-soluble, act on cell surface receptors, fast-acting.

2. Steroid hormones (e.g., cortisol, estrogen):

   • Lipid-soluble, pass through membranes, act on nuclear receptors, slower but longer-lasting.

3. Amine hormones (e.g., epinephrine, thyroxine):

   • Derived from amino acids; properties vary (epinephrine = water-soluble; thyroxine = lipid-soluble).

🏥 Major Endocrine Glands

• Hypothalamus: master controller; sends releasing hormones.

• Pituitary gland (anterior/posterior): releases tropic and direct hormones.

• Thyroid: regulates metabolism.

• Adrenal glands: stress response (cortisol, adrenaline).

• Pancreas: blood sugar (insulin, glucagon).

• Gonads (ovaries/testes): sex hormones (estrogen, testosterone).

🔁 Feedback Loops

• Negative feedback: maintains homeostasis. Hormone output reduces stimulus (e.g., high cortisol → less ACTH).

• Positive feedback: amplifies response (e.g., oxytocin in childbirth).

Why necessary? Prevent overproduction and maintain internal balance.

🐀 Castrated vs. Intact Rats in Endocrine Pathways

• Intact rats: have full hormonal feedback (e.g., testosterone inhibits LH release).

• Castrated rats: no gonadal hormones → no negative feedback → high LH/FSH.

If given a pathway diagram, use logic: remove source = feedback disrupted.

🧪 ELISA (Enzyme-Linked Immunosorbent Assay)

What is an ELISA?

• A biochemical assay used to detect and quantify antigens or antibodies using enzyme-linked detection and a colorimetric readout.

How are ELISAs used?

• Antigen-detecting ELISA: detects presence of a pathogen or viral protein.

• Antibody-detecting ELISA: checks for immune response (e.g., CoV-19 antibodies).

Steps (sandwich ELISA example):

1. Capture antibody on plate binds specific antigen.

2. Add sample (antigen binds if present).

3. Add detection antibody (binds another site on antigen).

4. Add enzyme-linked secondary antibody.

5. Add substrate → color change if enzyme present.

6. Read absorbance to quantify signal.

Each reagent step ensures specificity and detection.

🧬 Antigens and Antibodies

• Antigens: molecules recognized as foreign (e.g., virus proteins).

• Antibodies: Y-shaped immune proteins that bind specifically to antigens.

  • Produced by B cells.

  • Used for immunity and detection in assays.

😷 Why is CoV-19 so easily spread?

• High viral shedding before symptoms = asymptomatic spread.

• Aerosol transmission is efficient.

• Spike protein binds strongly to ACE2 receptor in humans.

• High mutation rate in spike = immune evasion.

⚠ ELISA Errors: False Positives & Negatives

False Positives:

• Non-specific binding (antibody binds wrong target).

• Cross-reactivity with similar antigens.

• Improper washing → excess enzyme stays.

False Negatives:

• Low antigen/antibody levels.

• Expired reagents or inactive enzyme.

• Improper incubation times/temperatures.

❓“What if ELISA went wrong...” Types of Questions

Examples:

• If no color change: enzyme may be missing, substrate not added, or no antigen present.

• If whole plate is blue: poor washing or non-specific binding.

• If low signal in positive control: faulty reagents or bad storage.

🧬 Animal Diversity III – Endocrine & Immunology Study Guide

🔔 The Endocrine System

• The endocrine system is a chemical communication network using hormones secreted into the bloodstream to regulate body functions like growth, metabolism, and reproduction.

• Works slower than nervous system signaling but has longer-lasting effects.

How it differs from other signaling:

• Endocrine: hormones travel through blood to distant cells.

• Paracrine: local signaling to nearby cells.

• Autocrine: acts on the same cell that secreted it.

• Synaptic: neurotransmitters released across synapses.

• Neuroendocrine: neurons release hormones into blood (e.g., oxytocin).

💡 Hormones Overview

What are hormones?

• Hormones are chemical messengers secreted by glands to affect distant target organs.

Tropic vs. Non-tropic:

• Tropic hormones: act on other endocrine glands (e.g., TSH → thyroid).

• Non-tropic hormones: act on non-endocrine tissues (e.g., insulin → liver/muscles).

Releasing vs. Release-inhibiting:

• Releasing hormones (e.g., GnRH): stimulate anterior pituitary hormone release.

• Release-inhibiting hormones (e.g., somatostatin): block hormone release.

Hormone classes (affect properties):

1. Peptide/Protein (e.g., insulin):

   • Water-soluble, cannot cross membranes, act on cell surface receptors, fast effects.

2. Steroid (e.g., estrogen):

   • Lipid-soluble, pass through membranes, act on nuclear receptors, slower effects.

3. Amine (e.g., epinephrine or thyroxine):

   • Derived from amino acids; properties vary by molecule.

🧠 Major Glands and Their Roles

• Hypothalamus: links nervous and endocrine systems; controls pituitary.

• Pituitary gland: "master gland" that secretes many tropic hormones.

• Thyroid: regulates metabolism and calcium levels.

• Adrenal glands: manage stress (cortisol) and acute responses (epinephrine).

• Pancreas: controls blood glucose (insulin, glucagon).

• Gonads: produce sex hormones (testosterone, estrogen).

🔁 Feedback Loops

• Negative feedback: keeps systems in balance by reducing hormone levels once the desired effect is achieved (e.g., high cortisol inhibits ACTH and CRH).

• Positive feedback: amplifies a response (e.g., oxytocin during labor).

🐭 Hormone Pathways in Lab: Castrated vs. Intact Rats

• Castrated rats: no gonads → no hormone → no negative feedback → hypothalamus/pituitary keep secreting more releasing/tropic hormones.

• Intact rats: hormone from gonads feeds back to shut off pituitary/hypothalamus stimulation.

⚡️ Phases of the Action Potential

1. Resting Potential: -70 mV maintained by Na⁺/K⁺ pump.

2. Depolarization: Na⁺ channels open → influx → membrane becomes more positive.

3. Repolarization: Na⁺ channels close, K⁺ channels open → K⁺ leaves → membrane returns negative.

4. Hyperpolarization: too much K⁺ exits → membrane dips below resting.

5. Return to Resting: Na⁺/K⁺ pump restores resting potential.

🧪 ELISA (Enzyme-Linked Immunosorbent Assay)

What is ELISA?

• A test that uses antibodies and enzymes to detect antigens or antibodies via a color change reaction.

How it works (sandwich ELISA example):

1. Capture antibody binds to antigen in sample.

2. Add sample (if antigen is present, it binds).

3. Add detection antibody.

4. Add enzyme-linked secondary antibody.

5. Add substrate → enzyme converts to colored product.

6. Measure absorbance (color intensity correlates with amount of target).

Antigen-detecting vs. Antibody-detecting:

• Antigen-detecting: tells you if a pathogen is present.

• Antibody-detecting: tells you if the immune system responded (i.e., past infection or vaccination).

Antigens vs. Antibodies:

• Antigens: molecules that trigger an immune response (often parts of pathogens).

• Antibodies: proteins made by B cells that specifically bind antigens.

🦠 Why CoV-19 Spreads Easily

• High transmission before symptoms.

• Stable binding to human ACE2 receptors.

• Aerosol transmission.

• Frequent mutations in spike protein → immune evasion.

⚠ ELISA Errors – False Positives/Negatives

False Positive:

• Non-specific binding.

• Contamination or unwashed wells.

• Cross-reactivity with similar antigens.

False Negative:

• Low antigen/antibody levels.

• Expired/inactive reagents.

• Insufficient incubation or wrong temperatures