Homeostasis and Body Regulation

Homeostasis

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

Homeostatic Regulation

• A stimulus or change is detected, which is sent to a control system
• This generates a response which is then put into play
• Process: Variable → receptor → control center → effector

Achieving Homeostasis

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

Comparing Endocrine and Nervous Systems

Similarities

• Both systems use chemical messengers, specifically neurotransmitters

Differences

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

The Nervous System

• Sensory receptors pass through afferent neurons to a central processor (brain)
• They then pass through efferent neurons to effectors (muscles or glands)

Endocrine System

• An endocrine gland/cell releases a hormone which targets a specific organ

Classical Vertebrate Endocrine System

• The system has two divisions

Neuroendocrine system

• Neurosecretory neurons release hormones with nerve terminals into the blood or ECF
• Adrenal glands, pituitary gland, and the hypothalamus are examples

Peripheral Endocrine System

• Non-neural tissue has no direct link to the nervous system
• Thyroid, parathyroid, pancreas, and kidneys are examples

Hypothalamus-Pituitary Unit

• The hypothalamus acts as a 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 are not produced at a constant rate
• Hormone production depends on the animal's physiological needs

Feedback Mechanisms

• Hormones are regulated by feedback mechanisms
• Negative feedback occurs when a hormone decreases its own production
• Positive feedback occurs when additional hormone is released in response to an original hormone release

Roles of Hormones

• Hormones cause changes in cell activity
• They stimulate the production of new proteins
• They activate or inactivate enzymes
• They open or close cell membrane channels
• They cause cells to secrete or release substances

Classifying Hormones

• Hormones are 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

• Cells respond to specific hormones through receptors on or inside target cells
• This provides specificity for hormone-cell interactions

Hormone Receptors

• The "lock and key" model describes how a conformational change occurs when a receptor binds to a specific hormone

Solubility

• Solubility dictates where the hormone receptor is located
• Water-soluble hormones cannot cross the cell membrane
• The receptor is located outside the cell (outer surface), ex: insulin and epinephrine
• Fat-soluble hormones can cross cell membranes
• The receptor is located inside the cell, ex: 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

• Luteinizing hormone (LH) triggers ovulation
• Progesterone thickens the uterine lining
• Prostaglandins stimulate shedding of the uterine lining
• Estrus is the release of an egg to be fertilized

Defining Male or Female

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

Sex Determination

• The natural event through which an individual becomes male or female
• The process is Genetic → Gonadal → Phenotypic (physical, observable, behavioral)
• Two main types of sex determination are genetic and environmental
• Genetic sex determination occurs at fertilization through gene combination
• Environmental sex determination occurs post-fertilization and depends on chance, resources, social factors, and temperature

Testes or Ovaries

• Gonads develop based on the presence or absence of the SRY gene on the Y chromosome, which is Y-linked
• The phrase "'Y' makes the guy" refers to this
• Female is the default XX setting
• If SRY is present, male organs (testes) develop

Hormone Production

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

Stress

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

Stress Response

• Physiological and behavioral responses re-establish homeostasis

Kinds of Stress

Acute Stress (fight or flight)

• Involves an alarm reaction
• Recovery follows quickly
• Adrenaline is released from the adrenal medulla

Chronic Stress (constant/repeated)

• Adjustments fail to compensate
• Impairs immune system response/general health
• Cortisol is released from the adrenal cortex

Stress Response Systems

Sympathetic-Adrenal-Medullary System (SAM)

• Responds to acute stress

Hypothalamic-Pituitary-Adrenal System (HPA)

• Responds to 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
• Poor health factors include sickness and neglect

Responding to Stress

• Responses can be behavioural or physiological

Examples

• Behavioural: vocalization and restlessness
• Physiological: increased heart rate and blood pressure

Measuring Stress in Animals

Behavioural Assessments

• Measuring species-specific, learned, and animal preferences
• Downfall: dependent on age, sex, health, and density

Hormonal Measurements

• Measuring stress hormone levels in blood
• Downfall: inducing stress can skew levels and results

Ways of Measuring Cortisol

Blood

• Considered the 'Gold standard'
• Cortisol is secreted directly into the bloodstream
• Downfall: drawing blood is stressful, skewing cortisol levels

Saliva

• There is a linear relationship between free cortisol in blood and saliva
• It is a less stressful sampling procedure
• Only the free cortisol portion is measured

Fecal Matter

• No direct interaction with the animal
• Less accurate due to processes that occur, causing more variability in measurements

Urine

• Measures free cortisol levels
• It can be difficult to collect a sample
• Sampling time needs to be consistent

Hair

• Acts as a biomarker of chronic stress
• It can be cut or retrieved from the environment, meaning no stress to the animal
• It can be transported without compromising samples

Shiverdecker Study

• Proved 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
• Human interactions decreased cortisol levels