Homeostasis and Regulation

Homeostasis

• Dynamic equilibrium is maintained within the body's internal environment through homeostasis
• Homeostasis maintains balance within the body systems

Homeostatic Regulation

• A stimulus or change is detected, sent to a control system, and generates a response that is then put into play
• A variable goes to a receptor which informs a control center, which then influences an effector

Achieving Homeostasis

• Homeostasis is achieved through regulatory mechanisms that involve organs of the body
• These mechanisms include both the endocrine and nervous systems

Comparing Endocrine and Nervous Systems

• Both systems use chemical messengers called neurotransmitters

Differences between the Endocrine and Nervous Systems

• Endocrine system secretions target cells to change metabolic activities in specific cells
• The nervous system transmits nerve impulses to cause muscular contraction or gland secretion
• Endocrine responses are typically slow and prolonged
• Nervous system responses are rapid and brief

The Nervous System

• Sensory receptors send signals via afferent neurons to the central processor (brain)
• Efferent neurons then carry signals to effectors, such as muscles or glands

Endocrine System

• An endocrine gland or cell releases a hormone that travels to a target organ

Classical Vertebrate Endocrine System

• The classical vertebrate endocrine system has two divisions
• Neuroendocrine system
• Peripheral Endocrine System

Neuroendocrine System

• Neurosecretory neurons with nerve terminals release hormones into the blood or extracellular fluid (ECF)
• Examples include adrenal glands, pituitary gland, and the hypothalamus

Peripheral Endocrine System

• Non-neural tissue with no direct link to the nervous system
• Examples include the thyroid, parathyroid, pancreas, and kidneys

Hypothalamus-Pituitary Unit

• A central regulator of endocrine function
• The hypothalamus closely controls pituitary gland secretions
• All domestic animals have a hypothalamus and pituitary unit
• Pituitary hormones are not produced at a constant rate
• Production depends on animals' physiological needs

Feedback Mechanisms

• Hormonal regulation involves feedback mechanisms
• 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

• Hormones can stimulate the production of new proteins
• Hormones can activate or inactivate enzymes
• Hormones can open or close cell membrane channels
• Hormones can cause a cell to secrete or release substances

Classifying Hormones

• Hormones can be classified by type of signalling, chemical structure, and solubility

Hormone Classes

• Steroids (sex and stress hormones) e.g., cortisol and estradiol
• Peptide Hormones e.g., insulin
• Amino acid derivatives e.g., thyroid hormones
• Eicosanoids e.g., prostaglandins

How Cells Respond to Hormones

• Cells respond to specific hormones via receptors on or inside target cells
• These receptors provide specificity for hormone-cell interactions

Hormone Receptors

• Hormone receptors exhibit a "lock and key" mechanism, where a conformational change occurs and the receptor is specific to a hormone

Solubility and Hormone Receptors

• Solubility dictates where the hormone receptor is located
• Water-soluble hormones cannot cross the cell membrane, so the receptor is outside the cell
  • Examples include insulin and epinephrine
• Fat-soluble hormones can cross cell membranes, so the receptor can be inside the cell
  • Examples include thyroid hormones 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
• Melengestrol acetate (MGA) is a progesterone receptor antagonist
• Dexamethasone is 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

• Male or female is defined by chromosomal, gonadal, hormonal, morphological, and behavioural sex

Sex Determination

• The natural event by which an individual of a species becomes male or female
• Process: Genetic information leads to Gonadal development that leads to Phenotypic expression (physical, observable, behavioural)

Types of Sex Determination

• Genetic factors determined at fertilization combine
• Environmental factors determined after fertilization, dependant on chance, availability of resources, social factors, and temperature

Testes or Ovaries

• Gonads develop based on the presence or absence of the SRY gene
  • SRY gene is found on the Y chromosome
  • 'Y makes the guy'
• Female by default is XX
• If SRY is present, male organs such as testes develop

Hormone Production

• Granulosa, Leydig, and theca cells make hormones
• Egg and sperm cells do not

Stress

• An environmental change that disrupts homeostasis and is perceived as a threat

Stress Response

• A suite of physiological and behavioural responses to reestablish homeostasis

Kinds of Stress

• Acute (fight or flight) results in an alarm reaction characterized by the quick release of adrenaline from the adrenal medulla and is followed by quick recovery
• Chronic stress (constant or repeated stress) results in an impaired immune system response and general health
• Adjustments fail to compensate for stress and Cortisol is released from the adrenal cortex

Stress Response Systems

• Acute stress uses the Sympathetic-Adrenal-Medullary System (SAM)
• Chronic stress uses the Hypothalamic-Pituitary-Adrenal System (HPA)

Social Stress Factors

• Social, through isolation or introduction
• Feeding, through competition or restriction
• Management, through transportation or confinement
• Environment, through temperature or habitat loss
• Poor health, through sickness or neglect

Responding to Stress

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

Measuring Stress in Animals

• Stress can be measures through behavioural assessments
• Stress can be measured through hormonal measurements

Behavioural Assessments

• Measuring species-specific, learned behaviours, and animal preferences
• Downfall: can be dependent on many factors such as age, sex, health, and density

Behavioural Assessments

• Stress can be measured through measuring stress hormone levels in blood
• Downfall: inducing stress will skew the levels and provide inaccurate results

Measuring Cortisol

• Blood is the gold standard
  • Cortisol is secreted directly into the bloodstream
  • Downfall: drawing blood is stressful to the animal, and skews cortisol level results
• Saliva has a linear relationship with free cortisol in blood
  • Blood cortisol increases, saliva cortisol increases
  • Less stressful sampling procedure, but only the free cortisol portion is measured
• Fecal matter analysis has no interaction with animal directly
  • Not as accurate as blood because of the variety of processes it has been through
  • Much more variability in measurements, further away from original blood concentration
• Urine measures free cortisol levels
  • Sampling time needs to be kept consistent
  • Collection can be difficult
• Hair is a biomarker of chronic stress
  • Can be cut or retrieved without stress to the animal
  • Can be transported without compromising samples

Shiverdecker Study

• Found that human interaction decreased stress levels in shelter dogs
• Each dog was its own control where blood samples were taken before and after interactions to measure plasma cortisol levels
• All three interactions decreased cortisol levels