Understanding Homeostasis

• Homeostasis involves maintaining a dynamic equilibrium of the internal body environment
• This sustains balance within all bodily systems

Homeostatic Regulation

• A stimulus or change is detected, leading to a signal to the control system, which then generates a response
• The response is then put into play
• The process goes: Variable → receptor → control center → effector

Achieving Homeostasis

• Achieved through regulatory mechanisms involving organs
• Both the endocrine and nervous systems are crucial for this

Comparing Endocrine and Nervous Systems

• Both systems use chemical messengers like neurotransmitters

• Endocrine system secretions target cells to change metabolic activities in specific cells; it is slow and prolonged

• The nervous system transmits nerve impulses, causing muscular contraction or gland secretion; it is rapid and brief

Nervous System Function

• It uses this pathway: Sensory receptors → afferent neurons → central processor (brain) → efferent neurons → effectors (muscles or glands)

Endocrine System Function

• Works via this pathway: Endocrine gland/cell → hormone → target organ

Classical Vertebrate Endocrine System

• It has two divisions: neuroendocrine and peripheral endocrine

• Neuroendocrine system uses neurosecretory neurons with nerve terminals that release hormones into the blood or ECF such as adrenal glands, pituitary gland, and the hypothalamus

• The peripheral endocrine system involves non-neural tissue with no direct link to the nervous system like the thyroid, parathyroid, pancreas, and kidneys

Hypothalamus-Pituitary Unit

• Serves as the central regulator of endocrine function
• Pituitary gland secretions are closely controlled by the hypothalamus
• All domestic animals possess a hypothalamus and pituitary unit
• Pituitary hormones aren't produced at a constant rate; their production depends on the animal's physiological needs

Feedback Mechanisms

• These regulate hormones

• Negative feedback occurs when a hormone feeds back to DECREASE its own production

• Positive feedback involves the release of additional hormone in response to an original hormone release

Roles of Hormones

• Include changes in cell activity
• Can stimulate the making of new proteins
• Can activate or inactivate enzymes
• They open or close cell membrane channels
• It causes the cell to secrete/release something

Classifying Hormones

• Classified based on the type of signalling, chemical structure, and solubility

Hormone Classes

• Steroids (sex and stress) like cortisol and estradiol
• Peptide Hormones like insulin
• Amino acid derivatives like thyroid hormones
• Eicosanoids like prostaglandins

Cell Response to Hormones

• Determined through receptors on/inside target cells that provide specificity for hormone-cell interactions

Hormone Receptors

• Function like a "Lock and Key", where a conformational change occurs and the receptor is specific to a hormone

Solubility Implications

• Dictates where the hormone receptor is located

• Water-soluble hormones cannot cross the cell membrane, so the receptor is outside of the cell, as seen with insulin and epinephrine

• Fat-soluble hormones can cross cell membranes, enabling the receptor to be inside the cell, as seen with thyroid and steroid hormones

Agonists vs Antagonists

• Agonists bind to a receptor to activate a biological response
• Antagonists bind to a receptor and prevent a response from occurring

Examples of Agonists and Antagonists

• Melengestrol acetate (MGA) functions as a progesterone receptor antagonist
• Dexamethasone functions as a glucocorticoid receptor agonist

Hormonal Profile and Follicular Development

• LH triggers ovulation
• Progesterone thickens the uterine lining
• Prostaglandins stimulate shedding of the uterine lining
• Estrus releases the egg to be fertilized

Defining Male or Female

• Defined by chromosomal, gonadal, hormonal, morphological, and behavioural sex

Sex Determination

• The natural event where an individual of a species becomes male or female
• Follows the process: Genetic → Gonadal → Phenotypic (physical, observable, behavioural)

Types of Sex Determination

• Genetic: sex is determined at fertilization by the combination of genes
• Environmental: sex is determined after fertilization by environmental factors, depending on chance, resource availability, social factors, and temperature

Testes or Ovaries Determination

• Gonads develop based on the presence or absence of the SRY gene on the Y chromosome
• 'Y makes the guy'
• Female is the default setting (XX)
• If SRY is present, male organs develop (testes)

Hormone Production

• Cells that produce hormones include granulosa, Leydig, and theca cells
• Egg and sperm cells do not produce hormones

Stress Definition

• Any environmental change that disrupts homeostasis and that animals perceive as a threat

Stress Response

• This is a suite of physiological and behavioural responses to reestablish homeostasis

Types of Stress

• Acute (fight or flight) involves an alarm reaction, triggers recovery quickly, and causes adrenaline release from the adrenal medulla
• Chronic stress (constant/repeated stress) has the adjustments fail to compensate for the stress, impairs the immune system and general health, and triggers cortisol release from adrenal cortex

Stress Response Systems

• Sympathetic-Adrenal-Medullary System (SAM) for acute stress
• Hypothalamic-Pituitary-Adrenal System (HPA) for chronic stress

Social Stress Factors

• Include social factors like isolation and introduction
• Include feeding factors like competition and restriction
• Include management factors like transportation and confinement
• Include environmental factors like temperature and habitat loss
• Include poor health factors like sickness and neglect

Responding to Stress

• Responses can be behavioural or physiological

Examples of Reactions to Stress

• Behavioural - vocalization, restlessness.
• Physiological - increased heart rate, blood pressure

Measuring Stress in Animals

• Use behavioural assessments to measure species-specific and learned behaviours, and animal preferences
  • Downfall: dependent on many factors: age, sex, health, density
• Use hormonal measurements to measure stress hormone levels in blood
  • Downfall: inducing stress will skew the levels and provide inaccurate results, also obtaining blood is stressful to the animal within itself