Homeostasis and Body Regulation

Homeostasis

• Dynamic equilibrium is maintained within the body's internal environment
• Balance is maintained within all body systems

Homeostatic Regulation

• Stimulus or change triggers a detection of change, which is sent to a control system that generates a response, which is then put into play
• Variable goes to receptor, which goes to the control center, which goes to the effector

Achieving Homeostasis

• Achieved through regulatory mechanisms involving body organs
• Involves the endocrine and nervous systems

Comparing Endocrine and Nervous Systems

• Both use chemical messengers (neurotransmitters)
• The endocrine system involves secretions to target cells to change metabolic activities in specific cells
• The nervous system transmits nerve impulses to cause muscular contraction or gland secretion
• The endocrine system's effects are slow and prolonged
• The effects of the nervous system are rapid and brief

The Nervous System

• Works through sensory receptors to afferent neurons to a central processor (brain) to efferent neurons to effectors (muscles or glands)

Endocrine System

• Works through an endocrine gland/cell to a hormone to a target organ

Classical Vertebrate Endocrine System

• There are 2 divisions: neuroendocrine and peripheral endocrine systems

Neuroendocrine System

• Uses neurosecretory neurons with nerve terminals that release hormones into the blood or ECF
• Examples include adrenal glands, pituitary gland, and hypothalamus

Peripheral Endocrine System

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

Hypothalamus-Pituitary Unit

• Acts as a central regulator of endocrine function
• Pituitary gland secretions are closely controlled by the hypothalamus
• Present in all domestic animals
• Pituitary hormones are not produced at a constant rate
• Production depends on an animal's physiological needs

Feedback Mechanisms

• These are for the regulation of 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 (Changes in Cell Activity)

• Stimulates the making of new proteins
• Activates or inactivates enzymes
• Opens or closes cell membrane channels
• Causes the cell to secrete or release something

Classifying Hormones

• Classified by type of signaling, chemical structure, and solubility

Hormone Classes

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

Cell Response to Hormones

• Determined through receptors on or inside target cells, providing specificity for hormone-cell interactions

Hormone Receptors

• Operate on a "lock and key" principle, where a conformational change occurs and the receptor is specific to a hormone

Solubility

• Dictates where the hormone receptor is located.
• Water-soluble hormones cannot cross the cell membrane
• The receptor is located outside of the cell on the outer surface
• Examples include insulin and epinephrine
• Fat-soluble hormones can cross cell membranes
• The receptor is 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

Examples of Agonists and Antagonists

• Melengestrol acetate (MGA) acts as a progesterone receptor antagonist
• Dexamethasone acts 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 Sex, Gonadal Sex, Hormonal Sex, Morphological Sex, and Behavioral Sex

Sex Determination

• The natural event by which an individual of a species becomes male or female
• Process: Genetic to Gonadal to Phenotypic (physical, observable, behavioral)

Types of Sex Determination

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

Determining Testes or Ovaries

• 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 (testes) will develop

Hormone Production

• Cells that make hormones include granulosa, Leydig, and theca cells
• Egg and sperm cells DO NOT make hormones

Stress

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

Stress Response

• A suite of physiological and behavioral 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 stress) 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) is for chronic stress

Social Stress Factors

• Social factors include isolation and introduction
• Feeding factors include competition and restriction
• Management factors include transportation and confinement
• Environmental factors include temperature and habitat loss
• Can also be caused by poor health, sickness, or neglect

Responding to Stress

• Can be behavioral or physiological responses

Examples of Responses

• Behavioral responses can include vocalization and restlessness
• Physiological responses can include increased heart rate and blood pressure

Measuring Stress in Animals

• Can be done through behavioral assessments and hormonal measurements

Behavioral Assessments

• Measured by species-specific behaviors, learned behaviors, and animal preferences
• Downfall: Dependent on many factors, like age, sex, health, and density

Hormonal Measurements

• Measures stress hormone levels in the blood
• Inducing stress will skew the levels and provide inaccurate results
• Obtaining blood is stressful to the animal

Ways of 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

Measuring Saliva

• Has a linear relationship with free cortisol in blood so as blood cortisol increases, saliva cortisol increases
• Is a less stressful sampling procedure
• Only the free cortisol portion is measured

Measuring Fecal Matter

• There is no interaction with animal directly
• It is not as accurate because it has been through a variety of processes, so there is much more variability in measurements
• Further away from original blood concentration

Measuring Urine

• Measures free cortisol levels
• Can be difficult to collect a sample
• Sampling time needs to be kept consistent

Measuring Hair

• Hair is a biomarker of chronic stress
• Can be cut or retrieved from the environment, meaning no stress to the animal
• Can be transported without compromising samples

Shiverdecker Study

• Showed that human interaction decreased stress levels in shelter dogs
• Blood samples were taken before and after interactions to measure plasma cortisol levels
• Each dog was used as its own control
• All 3 human interactions decreased cortisol levels