L3 Human Physiology

🧠 Concepts Introduced

LIST
FROM "00 Archive/Objects/Concepts"
WHERE lecture = this.file.link

Learning Objectives:

1. Describe the components in the homeostatic control pathway
2. Explain how negative and positive feedback loops work in homeostasis
3. Describe the different classification of hormones and their mechanism of actions
4. Describe the endocrine system for hormonal release → link between the hypothalamus and pituitary glands

Homeostasis

Learning outcome #1

1. Describe the components in the control pathways for homeostasis
2. Describe the negative and positive feedback loops with examples

Body temperature, blood pH, sugar, sodium levels, water balance, blood pressure, etc

Control Pathways

response & feedback loop local and reflex control pathway

1. Stimulus

2. Sensor/receptor types of receptors: Cellular, CNS, Peripheral

3. Afferent pathway

4. Integrating center

5. Efferent pathway

6. Target/Effector

7. Response

Feedback loops

Positive feedback regulation

Response of system goes in the same direction as changes that sets in motion

Reinforces the stimulus that sends the variable being regulated farther than normal values

Example of positive feedback control loop:

• Regulation of blood clotting
• Uterine contractions during childbirth
• Lutenizing hormone (LH) surge trigger ovulation

Negative feedback regulation

Increase or decrease in the variable being regulated brings a response that changes the variable in the opposite direction to that of the original change.

Stabilize the variable being regulated and aid the system in maintaining homeostasis

Example of negative feedback control systems:

• Body temperature
• Blood pressure
• Blood glucose concentration

Hormones

Learning outcome #2

1. Identify the different classes of hormones and describe their mechanism of actions

• Chemical messengers
• Secreted by a cell or group of cells (e.g., gland, endocrine cells, neurons)
• Secreted into the blood circulation
• Transported to distant target tissue receptors
• Activates physiological response

Function of hormones

• Control of growth and development
• Maintain homeostatic regulation of internal environment; parameters maintained within relatively narrow limits
• Control of reproductive system processes (ovulation, menstruation, maintenance of pregnancy)
• Effects on behaviour (modification, modulation, initiation of specific patterns)

Classification of hormones

Peptide / protein hormones

• composed of linked amino acids

Steroid hormones

• derived from cholesterol

Amino-acid derived hormones

• modifications of single amino acids

Peptide / protein hormones

Cellular mechanism of action:

• membrane receptors
• signal transduction system
  • enzyme-linked receptors
  • G protein-linked

1. insulin = protein hormone

Steroid hormones

• Synthesized in the smooth ER
• Diffuse across plasma membrane
• Synthesized on demand and released immediately

unlike protein hormones, steroid hormones cannot be stored to use later

1. cholesterol
2. aldosterone
3. cortisol
4. estrogen

activate DNA transcription, translation, and protein synthesis.

Amino-acid derived hormones

Amino-acid derived hormones

Created from either tyrosine or tryptophan Catecholamine - bind to cell membrane receptors

Endocrine system

Learning outcome #3:

1. Describe the function of endocrine system including the link between hypothalamus and both anterior and posterior pituitary (Growth, Regulation, Behaviour, Reproduction)
2. Describe the role of tropic hormones and regulation of hormonal secretion (Humoral, Neuronal, Hormonal)

Consists of glands and glandular tissue involved in paracrine and endocrine communications

Endocrine glands (Vs. Exocrine)

ductless glands that secrete hormones into ECF hormones enter bloodstream and distributed to target cells

Derived from Epithelial cells

• Primary endocrine organs
• Secondary endocrine organs

heart. stomach, liver. kidney, small intestine, skin

Functions of endocrine system

Help regulate:

• extracellular fluid
• metabolism
• biological clock
• contraction of cardiac & smooth muscle
• glandular secretion
• some immune functions

Growth & development Reproduction Behaviour

Neurohormones

Endocrine reflex includes the nervous system

Neurohormones:

• chemical signals secreted from neurons
• act similarly to hormones

Major groups:

• hypothalamic neurohormones from anterior and posterior pituitary
• catecholamines in adrenal medulla

Hypothalamus & Pituitary gland (primary endocrine gland)

Hypothalamus

Master regulatory organ Integrates nervous and endocrine system

Three mechanisms of control:

1. Secrete regulatory hormones to control secretion from anterior pituitary
2. Act as endocrine organ (secretes ADH and oxytocin)
3. Has autonomic centers for neural control of adrenal medulla (neuroendocrine reflex)

Anti-diuretic hormone (ADH): This hormone increases water absorption into the blood by the kidneys.

Oxytocin: Oxytocin is involved in a variety of processes, such as orgasm, the ability to trust, body temperature, sleep cycles, and the release of breast milk.

Corticotropin-releasing hormone (CRH): CRH sends a message to the anterior pituitary gland to stimulate the adrenal glands to release corticosteroids, which help regulate metabolism and immune response.

Gonadotropin-releasing hormone (GnRH): GnRH stimulates the anterior pituitary to release follicle stimulating hormone (FSH) and luteinizing hormone (LH), which work together to ensure normal functioning of the ovaries and testes.

Growth hormone-releasing hormone (GHRH) or growth hormone-inhibiting hormone (GHIH) (also known as somatostain): GHRH prompts the anterior pituitary to release growth hormone (GH); GHIH has the opposite effect. In children, GH is essential to maintaining a healthy body composition. In adults, it aids healthy bone and muscle mass and affects fat distribution.

Prolactin-releasing hormone (PRH) or prolactin-inhibiting hormone (PIH) (also known as dopamine): PRH prompts the anterior pituitary to stimulate breast milk production through the production of prolactin. Conversely, PIH inhibits prolactin, and thereby, milk production.

Thyrotropin releasing hormone (TRH): TRH triggers the release of thyroid stimulating hormone (TSH), which stimulates release of thyroid hormones, which regulate metabolism, energy, and growth and development.

Posterior pituitary

Extension of neural tissue of the brain

Storage and release sites for two neurohormones synthesized in hypothalamus

• Antiduretic hormone (ADH)
• Oxytocin

Oxytocin and ADH stored in axon terminals

Axon terminals cause release of oxytocin and ADH into blood upon firing of hypothalamic neurons

Anterior pituitary

Control of hormone secretion

1. Humoral - in response to levels of ions or nutrients in the blood

2. Neural - stimulation by nerve fibers

3. Hormonal - stimulation received from other hormones

Endocrine control: Negative feedback loop

negative feedback

Hormone actions on target cells

Bind to specific membrane or intracellular receptors

Response depends on:

1. Blood levels of hormones

   • Rate of hormone secretion
   • Amount of hormone bound to carrier proteins
   • Rate of hormone metabolism

2. Relative number of receptors on the target cell

3. Affinity of receptors for the hormone

Target cells can change their sensitivity to hormone by changing number of receptors

Endocrine disorders

Endocrine disorders result from hormone excess or deficiency or decreased target-cell responsiveness

Hyposecretion: too little hormone is secreted

• Primary or secondary

Hypersecretion: too much hormone is secreted • Causes: tumors and immunologic factors

Abnormal target-cell responsiveness

• Possible cause: testicular feminization syndrome / androgen insensitivity syndrome