Homeostasis and Body Regulation

• Homeostasis maintains a dynamic equilibrium within the body's internal environment and overall balance.

Homeostatic Regulation

• A stimulus or change is detected, triggering a control system to generate a response.
• This response is then put into effect.
• Variables are detected by receptors, which communicate with a control center that directs the effector.

Achieving Homeostasis

• Homeostasis is achieved through regulatory mechanisms involving the body's organs, including the endocrine and nervous systems.

Comparing the Endocrine and Nervous Systems

• Both systems use chemical messengers known as neurotransmitters.
• Endocrine system secretions target cells to alter metabolic activities, while the nervous system transmits nerve impulses to cause muscle contraction or gland secretion.
• Endocrine responses are slow and prolonged, while nervous system responses are rapid and brief.

The Nervous System

• Sensory receptors send signals via afferent neurons to the central processor (brain), which then dispatches signals through efferent neurons to effectors like muscles or glands.

Endocrine System

• Endocrine glands or cells release hormones that act on target organs.

Classical Vertebrate Endocrine System

• The classical vertebrate endocrine system has two divisions.
• The neuroendocrine system involves neurosecretory neurons with nerve terminals that release hormones into the blood or extracellular fluid, such as the adrenal glands, pituitary gland, and hypothalamus
• The peripheral endocrine system involves non-neural tissue with no direct link to the nervous system, such as the thyroid, parathyroid, pancreas, and kidneys.

Hypothalamus-Pituitary Unit

• The hypothalamus-pituitary unit serves as a central regulator of endocrine function.
• Pituitary gland secretions are closely controlled by the hypothalamus
• All domestic animals have a hypothalamus and pituitary unit.
• Pituitary hormones are not produced at a constant rate which is dependent on the animal’s physiological needs.

Feedback Mechanisms

• Feedback mechanisms regulate hormone production
• Negative feedback occurs when a hormone reduces its own production.
• Positive feedback involves the release of additional hormone in response to an initial hormone release.

Roles of Hormones

• Hormones play a role in change in cell activity.
• Hormones stimulate the making of new proteins
• Hormones also activate/inactivate enzymes, and open/close cell membrane channels.
• Hormones cause the cell to secrete or release something.

Classifying Hormones

• Hormones are classified by their type of signalling, chemical structure, and solubility.
• Hormone classes include steroids like cortisol and estradiol, peptide hormones like insulin, amino acid derivatives like thyroid hormones, and eicosanoids like prostaglandins.

How Cells Respond to Hormones

• Cells respond to specific hormones via receptors on or inside target cells, providing specificity for hormone-cell interactions.
• Hormone receptors operate on a "lock and key" principle, where a receptor is specifically designed to bind to a particular hormone, causing a conformational change.

Solubility and Hormone Receptors

• Solubility determines the location of the hormone receptor.
• Water-soluble hormones cannot cross the cell membrane, so they bind to receptors on the outer cell surface, such as insulin and epinephrine.
• Fat-soluble hormones can cross cell membranes and bind to receptors inside the cell, such as thyroid hormones and steroid hormones.

Agonists vs Antagonists

• Agonists bind to receptors to activate a biological response.
• Antagonists bind to receptors and prevent a response from occurring.
• Melengestrol acetate (MGA) is a progesterone receptor antagonist.
• Dexamethasone is a glucocorticoid receptor agonist.

Hormonal Profile and Follicular Development

• Luteinizing Hormone (LH) triggers ovulation.
• Progesterone thickens the uterine lining.
• Prostaglandins stimulate shedding of the uterine lining.
• Estrus leads to the release of an egg ready for fertilization.

Defining Male and Female

• Sex is defined by chromosomal, gonadal, hormonal, morphological, and behavioural factors.
• Sex determination is the natural process where an individual becomes male or female.
• The process is Genetic → Gonadal → Phenotypic (physical, observable, behavioural)

Types of Sex Determination

• Sex determination can be genetic or environmental.
• Genetic sex is determined at fertilization.
• Environmental sex is determined after fertilization and dependent on chance, availability of resources, social factors, and temperature.

Testes or Ovaries

• Gonads develop based on the presence or absence of the SRY gene found on the Y chromosome.
• "Y makes the guy"
• Female is XX (default setting)
• SRY gene present = testes developed

Hormone Production

• Cells that make hormones include granulosa, Leydig, and theca cells, but NOT egg and sperm cells.

Stress

• Stress is any environmental change that disrupts homeostasis and is perceived as a threat.
• Stress response is a set of physiological and behavioural responses to reestablish homeostasis.

Kinds of Stress

• Acute stress (fight or flight) involves an alarm reaction, quick recovery, and adrenaline release from the adrenal medulla.
• Chronic stress (constant/repeated) involves failed adjustments, impaired immune system response/general health, and cortisol release from the adrenal cortex.

Stress Response Systems

• The sympathetic-adrenal-medullary system (SAM) is for acute stress.
• The hypothalamic-pituitary-adrenal system (HPA) for chronic stress.

Social Stress Factors

• Social stress factors include social isolation/introduction, feeding competition/restriction, management transportation/confinement, environment temperature/habitat loss, and poor health/sickness/neglect.

Responding to Stress

• Responses to stress can be behavioural or physiological.
• Behavioural responses can be vocalization and restlessness.
• Physiological responses can be increased heart rate and blood pressure.

Measuring Stress in Animals

• Behavioural assessments involve species-specific behaviours, learned behaviours and preferences, but are dependent on many factors like age, sex, health, and density.
• Hormonal measurements involve measuring stress hormone levels in blood
• Inducing stress will skew the levels and provide inaccurate results given obtaining blood is stressful to the animal.