The Endocrine system: Mechanisms

Week 8

Endocrine system functions

• Core function: coordinates long term bodily processes to maintain internal balance (homeostasis)

• Growth and development: regulates cellular tissue, differentiation, bone elongation and maturation

• Metabolism and energy: controls BMR, nutrient absorption and lipolysis

• Fluid and electrolyte balance: direct kidney reabsorption of water, sodium and calcium ions

• Adaptive stress response: orchestrates short-term fight or flight and chronic resistance pathways

Exocrine overall

• Exo = outside

• Exocrine glands: has a duct (e.g., sweat, saliva)

• Secretes onto epithelial surfaces

• Chemicals produced by the glands

Endocrine overall

• Endo = inside

• secretes without ducts

• secretes into interstitial fluid

• Hormones are secreted into the blood in the capillaries

• Uses hormones to signal and control actions and responses around the body

Neural

• Axons and synapses

• electrical impulses (neurotransmitters)

• milliseconds

• brief and transient

• hyperlocalised

Hormonal

• Used by the endocrine

• Blood stream

• chemical hormones

• seconds to hours

• sustained and long lasting

• broad/systemic

The hormone types

1. Water soluble

2. Lipid soluble

Water soluble steps

1. Hormone (1st messenger) binds to outside receptor

2. Receptors activates G protein

3. G protein activates 2nd messenger system

4. 2nd messenger system triggers the action wanted

Lipid soluble steps

1. Hormone diffuses through the plasma membrane to bind to intracellular receptor

2. Receptor-hormone complex enters the nucleus

3. Complex binds to specific DNA region

4. Binding initiates transcription of the genes to mRNA

What is specific to water soluble?

• Peptides, proteins and amides

• Transports free in plasma

• Receptors on plasma membranes

• Uses seconds messengers cascades

• Fast response to hormone arrivals (sec to mins)

• Brief and short-lived responses

What is specific to lipid soluble?

• Steroids, thyroid hormones

• Binds to transport proteins

• Receptors in cytosol or nucleus

• Directs gene expressions and protein synthesis itself

• Slow response to hormone arrival (hours to days)

• Sustaining and long-lasting

How do water soluble hormones act?

hormones are able to dissolve into blood for transport but require receptors on the membrane that act through second-messenger systems

How do lipid soluble hormones act?

hormones require transport through the blood but act on receptors in the plasma and nucleus, directly causing changes in gene expression

Humoral control

• Humoral → bodily fluids

• Responds to triggers and level changes within the blood

• e.g., high/low glucose and calcium, dehydration

Neural control

• Neural uses nerve fibres

• Responds to neural stimuli such as activation of receptors or stress

• e.g., adrenalin release, oxytocin during childbirth

Hormonal control

• meaning releases are caused or triggered by other hormones → tropic hormones

• e.g., hypothalamic pituitary axis

Negative feedback

• keeps things stabilised in a range

• works to remove the stimulus

• returns to baseline

• e.g.., healthy blood glucose level

Positive feedback

• Amplifies to original stimulus

• Ends up with a result at the end

• e.g., childbirth, milk, bloodclots

Nervous system modulation

it is possible for system to modify the turn-on and turn-off factors

Hypothalamic pituitary axis

Connections between the hypothalamus and the pituitary

• the hypothalamus acts as a starting point for lots of hormonal pathways

What is the pituitary divided into

Anterior and posterior

Hypothalamus and the anterior pituitary

• vascular connection

• Hypothalamus produces tropic hormones:

  • releasing hormones (4x)

  • inhibiting hormones (4x)

• Hormones released into the hypophyseal portal system

• Hypothalamic hormones acts on tissue of the anterior pituitary

• New hormones produced by anterior pituitary and released into the blood

Hypothalamus and the posterior pituitary

• Neural connection

• Hypothalamus produces hormones that are stored into posterior pituitary:

  • Oxytocin

  • ADH

• Hormones travel via hypothalamic-hypophyseal tract

• Hormones stored in vesicles in the axon terminals

• Hormones released into the blood in response to neural signals

Comparing anterior to posterior communication

• Anterior:

  • Blood connection

  • Hypophyseal portal system

  • New hormones produced

• Posterior:

  • Neural connections

  • Hypothalamic-hypophyseal tract

  • Hypothalamic hormones stored

Hypothalamus is connected to the anterior pituitary via

hypophyseal portal system

Hypothalamus connected to the posterior pituitary via

hypothalamic-hypophyseal tract

The anterior pituitary is responsible for

producing it’s own hormones

The posterior pituitary is responsible for

storing and releasing hormones made in hypothalamus

Hormones in the pineal gland

• Melanin → helps regulate sleep

Releasing hormones in the hypothalamus (posterior)

• released into the hypophyseal-portal system to activate anterior pituitary to produce hormones

• growth hormone releasing hormone

• thyrotropin releasing hormone

• corticotropin releasing hormone

• gonadotropin releasing hormone

Inhibitory hormones into the hypothalamus (posterior)

• released into the hypophyseal-portal system, activating posterior pituitary not to make hormones

• Growth hormone inhibiting hormone

• Prolactin inhibiting hormone

Hypothalamic hormones (anterior)

• Made in hypothalamus but stored in the posterior pituitary axon terminal of neurons

• Oxytocin

• ADH

Hormones in anterior pituitary

• Thyroid stimulating hormone (TSH)

• Luteinising hormone (LH)

• Follicle stimulating hormone (FSH)

• Adrenocorticotropic hormone (ACTH)

• Prolactin

• Growth hormone

Thyroid gland

• T3 and T4 work together to control and regulate metabolism

• Calcitonin → works to control and regulate calcium

Parathyroid gland

• sits on top of the thyroid gland

• specialised organ to regulate calcium via parathyroid hormone

Hormones in the pancreas

• Releases insulin and glucagon to respond to blood glucose levels for regulation

Hormones in the adrenal glands

• produces adrenalin

• Medulla: adrenalin

• Cortex: cortisol and aldosterone

Hormones in the kidneys

renin

Hormones in the gonads

• includes testes and ovaries

• Estrogen, Progesterone, testosterone

Hormones in the digestive system

• Leptin (released from fat cells)

• Ghrelin (released from the stomach)

• GLP-1 (released from the intestine and the brain stem)