Homeostasis

Physiological control systems keep regulated variables within a desired range during homeostasis

Components of control pathway

• Stimulus

• Sensor/receptor

• Afferent pathway

• Integrating center

• Efferent pathway

• Target/Effector

• Response

GREEN = INPUT

BLUE = OUTPUT

Feedback Loops

• Negative feedback

  The response counteracts with the stimulus, shutting off the response loop

• Positive feedback

  The response reinforces the stimulus, sending the variable further from the set point

Negative feedback regulation

• Increase or decrease in the variable being regulated brings a response that changes variable in the opposite direction to that of the original change.

• Stabilise the variable being regulated and aid the system in maintaining homeostasis

• Example:

  • Body temperature

  • Blood pressure

  • Blood glucose concentration

    Glucose^, insulin is secreted

    Glucose declines, glucagon is broken down to glucose

Positive feedback regulation

• The response of system goes in the same direction as changes that sets in motion

• Reinforces the stimulus that sends the variable beings regulated farther than normal values

• Example:

  • Regulation of blood clotting

  • Uterine contractions during childbirth

  • Luteinising hormones (LH) surge trigger ovulation

Introduction to endocrine system

Hormones

• Chemical messengers

• Secreted by a cell or group of cells (e.g. gland, endocrine cells, neurons)

• Secreted into the blood circulation

• Transported to distant target tissue receptors

• Activates physiological response

Functions of hormones

• Control of Growth and development

• Maintain homeostatic Regulation of internal environment; parameters maintained within relatively narrow limits

• Control of Reproductive system processes (ovulation, menstruation, maintenance of pregnancy)

• Effects on Behaviour (modification, modulation, initiation of specific patters)

GRRB

Classification of Hormones

• Peptide/protein hormones

  • composed of linked amino acids

  • Prepropeptide (inactive) → Propeptide (inactive) → Peptide (active)

  • Unable to enter target cells as they are lipophobic, so they must combine with membrane receptors (triggers signalling pathway, refer to previous notes)

• Steroid hormones

  • ALL derived from cholesterol, so are lipophilic (e.g. testosterone)

  • Synthesised in smooth ER, diffuse across plasma membrane

  • Steroid hormones enter the nucleus after binding to membrane receptor. A receptor-hormone complex is formed and gene transcription is triggered.

  • Controls gene expression (e.g. growth spurts)

• Amino-acid derived hormones

  • modifications of single amino acids (lipophilic, affects transcription of genes)

  • Created from either tyrosine or tryptophan

  • Catecholamine - bind to cell membrane receptors

Endocrine System

• Consists of glands and glandular tissue involved in paracrine and endocrine communications

• Endocrine glands (vs. exocrine)

  • ductless glands that secrete hormones into ECF

  • hormones enter bloodstream and distributed to target cells

• Derived from epithelial cells

• Primary endocrine organs (expresses hormones)

• Secondary endocrine organs (does other stuff but also secrete hormones)

Functions of endocrine system

• Help regulate:

  • extracellular fluid

  • metabolism

  • biological clock

  • contraction of cardiac & smooth muscle

  • glandular secretion

  • some immune functions

• Growth & Development

• Reproduction

• Behaviour

Neurohormones

• Endocrine reflex includes the nervous system

• Neurohormones

  • chemical signals secreted from neurons

  • act similarly to hormones

• Major groups:

  • hypothalamic neurohormones from anterior and posterior pituitary

  • catecholamines (dopamine, norepinephrine) in adrenal medulla

Primary endocrine organs:

Hypothalamus & Pituitary gland

Anterior pituitary is a true endocrine gland whereas posterior pituitary is just an extension of the neural tissue.

Infundibulum connects pituitary gland to hypothalamus

Hypothalamus

• Master regulatory organ

• Integrates nervous and endocrine systems

• Three mechanisms of control:

  1. Secrete regulatory hormones to control secretion from anterior pituitary

  2. Act as endocrine organ (secretes ADH and oxytocin)

  3. Has autonomic centers for neural control of adrenal medulla (neuroendocrine reflex)

Hypothalamic-Pituitary Portal System

Hypothalamic neurons synthesise hormones.

The anterior pituitary either stimulate or inhibit the release of hormones.

In response to releasing the hormones, the anterior pituitary secretes hormones into the secondary capillary plexus. This in turn empties into the general circulation.

Primary hormones secreted by hypothalamus

Secreted by posterior pituitary

• Anti-diuretic hormone (ADH)

• Oxytocin

Secreted by anterior pituitary

• Corticotropin-releasing hormone (CRH)

• Gonadotropin-releasing hormone (GnRH)

• Growth hormone-releasing hormone (GHRH) or growth hormone-inhibiting hormone (GHIH)

• Prolactin-releasing hormone (PRH) or prolactin-inhibiting hormone (PIH)

• Thyrotropin releasing hormone (TRH)

Posterior Pituitary

• Extension of neural tissue of the brain

• Storage and release sites for two neurohormones synthesised in hypothalamus

  • ADH

  • Oxytocin

• Oxytocin and ADH stored in axon terminals

• Axon terminals cause release of oxytocin and ADH into blood upon firing of hypothalamic neurons

Control of Hormone Secretion

Humoral - in response to levels of ions or nutrients in the blood

Neural - stimulation by nerve fibres (very fast)

Hormonal - stimulation received from other hormones

Hormone actions on target cells

• Bind to specific membrane or intracellular receptors

• Response depends on:

  1. Blood levels of hormones

     • Rate of hormones secretion

     • Amount of hormone bound to carrier proteins

     • Rate of hormone metabolism

  2. Relative number of receptors on the target cell

  3. Affinity of receptors for the hormone

• Target cells can change their sensitivity to hormone by changing number of receptors

Endocrine disorders

• Endocrine disorders result from hormone excess or deficiency or decreased target-cell responsiveness

  • Hyposecretion: too little hormone is secreted

  • Hypersecretion: too much hormone is secreted

  • Abnormal target-cell responsiveness

Summary

• Describe the components in the homeostatic control pathway

• Explain how negative and positive feedback loops work in homeostasis

• Identify the different classes of hormones and their mechanism of actions

• Describe the primary endocrine organs and the control of hormonal secretion