Homeostasis and Regulation

• Homeostasis is maintaining a dynamic equilibrium in the body's internal environment.
• Homeostatic regulation is achieved through the endocrine and nervous systems.
• The homeostatic regulation process involves several steps:
  • Stimulus or change detection.
  • Sending signals to a control system.
  • Generating a response.
  • Sending out the response.
  • Putting the response into action.

Endocrine System

• It uses chemical neurotransmitters.
• It changes metabolic activities in specific cells.
• Its action is relatively slow.
• Its effects are prolonged.

Nervous System

• It uses chemical neurotransmitters.
• It causes muscular contraction or gland secretion.
• Its action is rapid.
• Its effects are brief.
• The nervous system's elements are:
  • Sensory receptors that collect information.
  • Afferent neurons that sends information.
  • A central processor that interprets the information.
  • Efferent neurons.
  • Effectors that cause a response.

Endocrine System Components

• Endocrine gland/cell: secretes hormones.
• Hormone: chemical products released upon stimulation.
• Target organ: expresses hormone-specific receptors and shows a biological response.

Classical Vertebrate Endocrine System

• Neuroendocrine System: employs neurosecretory neurons with nerve terminals to release hormones into the blood or extracellular fluid (ECF).
  • Examples include adrenal glands, the pituitary gland, and the hypothalamus.
• Peripheral Endocrine System: found in higher invertebrates and vertebrates, it consists of non-neural tissue.
  • Non-neural tissue has no direct link to the nervous system.
  • Examples include the thyroid, pancreas, and kidneys.

Neuroendocrine Control of Hormones

• Hormone secretions from the pituitary gland are closely controlled by the brain, especially the hypothalamus.
• The hypothalamus-pituitary unit is a central regulator of endocrine function.
• All domestic animals have a hypothalamus-pituitary unit.

Pituitary Hormones

• Pituitary hormones are not produced all the time and are regulated by feedback mechanisms.

Feedback Mechanisms

• Negative feedback involves a hormone reducing its own production.
• Positive feedback involves the release of additional hormone in response to an initial hormone release.

Roles of Hormones

• Stimulating new protein production.
• Activating or inactivating enzymes.
• Opening or closing cell membrane channels.
• Triggering cells to secrete or release substances.

Hormone Classifications

• Hormone classifications are based on:
  • Type of signaling.
  • Chemical structure.
  • Solubility.

Classes of Hormones

• Steroids (e.g., stress and sex hormones).
• Peptides.
• Amino acid derivatives.
• Eicosinoids.

Cell Responses

• Cell responses are determined through receptors on or inside the target cell.
• The 'Lock & Key' principle provides specificity for hormone-cell interactions.
• Conformational changes enable interaction with other cell components.

Solubility

• The solubility of hormones determines receptor location.
• Water-soluble hormones (hydrophilic):
  • Cannot cross the cell membrane.
  • Bind to receptors on the outer surface.
  • Examples include insulin and epinephrine.
• Fat-soluble hormones (lipophilic):
  • Can easily cross the membrane.
  • Examples include thyroid hormones and steroid hormones.

Hormone Actions at Receptor

• Agonists bind to a receptor and activate a biological response.
• Antagonists bind to a receptor and prevent activation.
• Both agonists and antagonists can disrupt the endocrine system.

Examples

• Melengestrol Acetate (MGA): progesterone receptor ANTAGONIST.
• Dexamethasone: glucocorticoid receptor AGONIST.
• Agonists and antagonists are very similar to the target hormone, needing to bind to the same active site.

Cow Hormonal Profile

• GnRH agonists have a crucial role in estrus synchronization.
  • They induce a surge in LH.
  • This triggers ovulation.
• GnRH agonists can allow for more consistent and predictable breeding schedules.
  • This is beneficial for managing large herds.

GnRH as Contraceptives

• GnRH vaccines stimulate the production of specific antibodies that neutralize GnRH activity.
  • This inhibits LH and FSH secretion, causing infertility.

Sex

• Sex can be defined by:
  • Chromosomes.
  • Gonads.
  • Hormones.
  • Morphology.
  • Behavior.
• Sex determination is a natural event where an individual becomes male (XY) or female (XX).
• This involves a process that begins with the fusion of an egg and sperm, leading to:
  • Genetic sex.
  • Gonadal sex.
  • Phenotypic sex (observable physical and behavioral traits).

Sex Determination Types

• Genetic.
• Environmental.

Genetic Sex Determination

• Mammals: XX or XY.
• Birds/Amphibians: ZZ (male) or ZW (female).
  • Note: sex chromosomes are opposite to those of humans.

Environmental Sex Determination

• Sex is determined by external factors after fertilization.
  • Determined by chance, resource availability, social cues, or temperature.

Male vs Female

• The Y chromosome makes an individual male (XY).
• Female (XX) is the default setting.
• The Y chromosome contains the SRY gene.

Sexually Indifferent Stage

• In this stage, cells have not yet differentiated into testes or ovaries.
• The presence or absence of the Y chromosome determines cell fate.

Hormone Production

• Eggs and sperm do not make hormones.
• Gonad hormone production cells:
  • Granulosa cells.
  • Theca cells.
  • Leydig cells.

Stress

• Stress refers to any environmental change that disrupts homeostasis and is perceived as threatening.
• Stress response involves physiological and behavioral responses that help re-establish homeostasis.

Types of Stress

• Acute Stress: "fight-or-flight" response.
  • It involves an alarm reaction.
  • Bronchial dilation is stimulated by adrenaline.
• Chronic Stress: constant or repeated long-term stress.

Stress Response Systems

• Acute: Sympathetic-adrenal-medullary (SAM) system.
  • It has a nervous connection.
• Chronic: Hypothalamic-pituitary-adrenal (HPA) system.
  • It has an endocrine pathway.

Adrenal Glands & Stress Hormones

• Cortisol: the adrenal cortex produces cortisol.
• Adrenaline: the adrenal medulla produces adrenaline.

Stress Response

• Perceiving the stimuli via:
  • Visual.
  • Tactile.
  • Olfactory senses.
• Autonomic response to stress:
  • It includes an alarm stage.
  • It activates the SAM system for a 'fight or flight' response.
• Endocrine response to stress:
  • It involves a resistance stage.
  • It uses the HPA system.
  • It mobilizes resources to solve the problem.
• Post-stress exhaustion:
  • The body is in a depleted state.

Animal Stress Factors

• Social factors: isolation or introducing new animals.
• Feeding factors: competition, feed restriction, or diet change.
• Management factors: confinement or transportation.
• Environmental factors: cold or heat stress, and habitat loss.
• Poor health: sickness, neglect, or pregnancy.

Responding to Stress

• Physiological responses include adrenaline and cortisol production.
• Behavioral responses include active or passive behaviors.

How Animals Show Stress

• Behavioral signs:
  • Vocalization.
  • Restlessness.
  • Fight or freeze response.
• Physiological signs:
  • Increased heart rate.
  • Increased blood pressure.
  • Increased adrenaline/cortisol levels.
• Immune responses:
  • Increased WBC's.
  • Increased infectious diseases.
• Performance:
  • Decreased milk yield.
  • Decreased feed intake.
  • Decreased fertility.

Measuring Stress in Animals

• Behavioral Assessments:
  • Observing species-specific behaviors.
  • Learned behaviors.
  • Animal preference tests.
• Hormonal Measurements:
  • Increased hormone (cortisol) levels in the blood indicate stress.

Measuring Cortisol Levels

• Free cortisol + bound cortisol = total cortisol concentration.
  • 'Free' cortisol can affect different tissues.
  • 'Bound' cortisol is lipo-soluble and bound to carrier proteins in the blood.