endocrine system

• Endocrine vs nervous system

  Similarities:

  • Both use the brain and hypothalamus
  • Some molecules are used as both neurotransmitters and hormones
  • Both regulate bodily functions
  • Some neurons secrete hormones (neurohormones)

  Differences:

  • Mode of transport - endocrine secretes hormones into bloodstream, nervous system secretes neurotransmitters directly onto target cells
  • Speed of response - endocrine is slower
  • Duration of response - hormones remain in bloodstream for minutes, days, weeks, nervous system is active as long as action potentials are sent
• Classes of Chemical Messengers

  • Autocrine: secreted from an individual cell and influences the cell it is secreted from
  • Paracrine: produced by a wide variety of tissue and secreted into extracellular fluid and has a localised effect
  • Neurotransmitter: produced by neuron and secreted into the hypnotic cleft and travels a short distance to influence postsynaptic cells
  • Endocrine: hormones secreted into the bloodstream and travel some distance to target cells
• Hormone Secretion Patterns


  • Chronic: a relatively stable concentration is maintained over a long period of time
  • Acute: hormone concentration alters dramatically and irregularly and changes with each stimulus
  • Episodic: hormones are secreted at regular intervals and concentrations
• Control of hormone release

  Increased demand for a hormone causes appropriate stimuli to induce the endocrine gland to secrete a hormone. When the particular hormone has done its required job it needs to be:

  • Removed from circulation - broken down and removed
  • The endocrine gland must receive an inhibitory message.

  Regulation of this stimuli and inhibition in order to maintain homeostatic levels of hormones are done by two major mechanisms:

  • Positive feedback
  • Negative feedback
• Positive feedback


  When a hormone:

  • Stimulates their target cell AND
  • Promotes the synthesis and secretion of the hormone = further secretion of the hormone
• Negative feedback


  Most common regulatory mechanism. Hormone secretion is inhibited buy itself when there is adequate hormone levels in the blood.
• Receptors


  Hormones bind to receptors (proteins) on the target cells. Generally hormones can only stimulate those cells that have the matching receptor. Some hormones can bind to many (family) receptors which have similar structures.
• Hormone regulation


  Neural activation

  Hormonal activation

  Humoral control
• Neural activation


  A neuron releases a neurotransmitter that signals the endocrine gland to release a hormone.

  • A stimulus causes neural action potentials to release neurotransmitters into the synapse of hormone-producing cells which stimulate the release of a hormone.

  ▪ If the stimulus stops then the neural stimulation will stop = hormone release will stop.
• Neural activation


  Some neurons secrete chemical messengers into the blood, these hormones are called neurohormones (neuropeptides). These can then cause the secretion of releasing hormones, primarily from the hypothalamus.

  Inhibitory hormones are also released in this manner.
• Hormonal activation


  A hormone (typically called a hormone-releasing factor) will stimulate an endocrine organ/gland to release a different hormone.

  Tropic hormones - stimulate the secretion of other hormones from other endocrine glands or organs.

  Inhibition - some hormones prevent the secretion of other hormones.
• Humoral control


  Circulating blood-borne molecules stimulate the release of hormones. Hormones are sensitive to changes in blood levels of substance.

  Inhibition - often a hormone with opposing effects is secreted to inhibit the effects of the stimulation hormone.

  Hormones work together to maintain homeostasis.
• Hypothalamus


  A major control site of the nervous and endocrine systems. It receives information from hormones, emotions and the central nervous system. The hypothalamus is connected to the pituitary gland by an extension = infundibulum.
• Hypothalamus hormones


  Major releasing and inhibiting neurohormones are secreted by the hypothalamus which travel in the portal system to the anterior pituitary gland.The hypothalamus tis a neural tissue, secretion from the hypothalamus are neural hormones.
• Hypothalamus hormones


  Growth hormone-releasing hormone -> increased growth hormone secretion

  Growth hormone-inhibiting hormone -> decreased growth hormone secretion

  Thyrotropin-releasing hormone -> increase thyroid-stimulating hormone secretion

  Corticotropin-releasing hormone -> increased adrenocorticotropic hormone secretion

  Gonadotropin-releasing hormone -> increased secretion of luteinising hormone and follicle-stimulating hormone
• Pituitary gland


  Made up of two portions:

  Posterior pituitary gland

  • Extension of the hypothalamus
  • No blood portal system
  • Neuron axons from the hypothalamus travel down the infundibulum and terminate in the posterior pituitary
  • Secretes neurohormones that enter the circulatory system
• Pituitary gland

  Anterior pituitary gland:

  • Not made from neural tissues, hormones are not neurohormones
  • Blood supply between hypothalamus and anterior pituitary is through a specialised system = hypothalamohypophysial portal system
  • Neurohormones move from a primary capillary network in the hypothalamus down the infundibulum to the anterior pituitary gland. Hormones are produced in response and carried by a secondary capillary network
  • Veins carry hormones form the secondary capillary network to the general circulation
• Posterior pituitary gland


  Stimuli from the nervous system causes an increase or decrease in action potentials from hypothalamic neurons.

  Action potentials move through axons down the hypothalamohypophysial tract to the posterior pituitary.

  Causes the release of stored neurohormones from the pituitary gland.

  Neurohormones circulate in the blood and travel to their target tissues.
• Posterior pituitary hormones

  Antidiuretic hormone (ADH) - secretion is dependent on changes in blood osmolality and volume.

  Stimulation:

  • Osmoreceptors increases as the concentration of solutes in a solution increase = increase in action potentials in both osmoreceptors and ADH neurosecretory neurons and increase in ADH secretion.

  Target tissue:

  • Kidneys

  Response:

  • Increased water reabsorption = decrease in urine output and blood osmolality
  • Works in reverse if blood osmolality decreases.
• Posterior pituitary hormones

  Antidiuretic hormone (ADH)

  Baroreceptors (situated in the blood vessels) respond to changes in blood pressure:

  • Decrease in blood pressure usually due to a decrease in blood volume
  • Increase in action potentials frequency
  • = Increase in ADH secretion
  • = Kidneys retain water

  Therefore it slows any reduction in blood volume and causes vasoconstriction which increases blood pressure. ADH is influenced by small changes in blood osmolality but quite large changes are required in blood pressure to alter ADH secretion.
• Anterior pituitary hormones

  The anterior pituitary gland secretions are influence by releasing and inhibiting hormones from the hypothalamus. Majority of secretions are tropic hormones which stimulate the secretion of other hormones from other endocrine glands or organs.
• Anterior pituitary hormones


  Growth hormone (GH) - somatotropin

  Stimulation:

  • Stressors (low blood glucose) stimulates the release of GH releasing hormone from the hypothalamus = increase secretion of GH.

  Target tissue:

  • Most tissues

  Response:

  • Stimulate growth
  • Regulates metabolism
  • Regulates blood nutrient levels during fasting and post meal - increase protein synthesis and protein growth

  Regulated by negative feedback and GH inhibiting hormone deceases the secretion of GH.
• Anterior pituitary hormone

  Thyroid-stimulatin hormone (TSH) - thyrotropin

  Stimulation:

  • Controlled by the secretion of thyroid releasing hormone (TRH) and thyroid hormone.

  Target tissue:

  • Thyroid gland

  Response:

  • Stimulates the synthesis and secretion of thyroid hormone

  Inhibited by thyroid hormones

  Secreted in an episodic manner
• Anterior pituitary hormones

  Adrenocorticotropic hormone (ACTH)

  Stimulation:

  • Released in response to corticotropin-releasing hormone - hypothalamus

  Target tissue:

  • Adrenal cortex

  Response:

  • Increased secretion of glucocorticoid hormones
• Thyroid gland

  One of the largest endocrine glands in the body, inferior to pharynx and wraps around the trachea.

  Secretes:

  • Thyroid hormones (90%)
  • Calcitonin (10%)
• Thyroid gland hormones 


  Thyroid hormones

  Stimulation:

  • Thyroid releasing hormone (TRH) and thyroid stimulating hormone (TSH)

  Target tissues:

  • Most cells in the body

  Response:

  • Regulates metabolism, alters the metabolic rate
  • Affects the body temperature as a result of influencing mechanisms that create heat production e.g. metabolism, mitochondria activity
  • Growth and maturation of orange (bone, hair, nervous tissue, connective tissue)
  • Decrease lovelies of TSH decrease thyroid hormone release
  • Thyroid hormone in the blood provide negative feedback which cause a decrease in secretion
• Thyroid gland hormone


  Calcitonin

  Stimulation:

  • Secreted in response to high blood calcium levels - blood wants to offload calcium to the bones

  Target tissues:

  • Bone

  Response:

  • Decreases osteoclast (break down of bone) activity - osteoclast cells = bone reabsorption
  • Lengthens the life of osteoblast - osteoblast cells = assists bone formations
  • = Decrease in calcium (and phosphate) levels
• Parathyroid glands


  Embedded in the posterior of the thyroid gland. Two one each side. Two types of cells:

  • Cheif cells - secretes parathyroid hormone (PTH)
  • Oxyphils cells - function unknown
• Parathyroid gland hormone


  Parathyroid hormone - assists with calcitonin/regulation of calcium levels

  Stimulation:

  • A decrease in blood calcium levels

  Target tissue:

  • Bone, kidneys, small intestine
• Parathyroid gland hormone


  Response:

  Binds to osteoblast receptors which

  • Increases osteoclast activity and number
  • = bone reabsorption = release of calcium
  • = increase in blood calcium levels

  Stimulates calcium uptake within the kidneys

  • Less calcium in the urine
  • = increase in blood calcium levels

  Increase absorption int he small intestine of calcium

  • = increase in blood calcium levels

  Inhibited by increase calcium levels and is secreted in response to the humeral activation - blood born molecule stimulation the release of a hormone.
• Adrenal gland

  Situated on the superior portion of the kidneys, adrenal glands are compromised of an; inner medulla and outer cortex. Adrenal cortex is compromised of small cells in 3 layers:

  • Zona glomeulosa
  • Zona fasciculata
  • Zona reticularis
• Adrenal medulla hormones

  Catecholamine - neurohormones, neuron stimulates release. Epinephrine (80%), norepinephrine (20%)

  Stimulation:

  • Emotional excitement
  • Injury
  • Exercise
  • Stress
  • Low blood glucose

  Target tissue:

  • Heart, liver, blood vessels and adipose tissue
• Adrenal medulla hormones


  Stress, physical activity, and low blood glucose levels act as stimuli to the hypothalamus, resulting in increased sympathetic nervous system activity. An increased frequency of action potentials conducted through the sympathetic division of the autonomic nervous system stimulates the adrenal medulla to secrete epinephrine and some norepinephrine into the blood. Epinephrine and norepinephrine act on their target tissues to produce response.
• Adrenal medulla hormone


  Fight or flight hormones

  • Increase blood pressure
  • Decrease blood flow to internal organs that are not necessary
  • Mobilise substrates
  • Get glucose and fatty acids and break them down
  • Increase metabolic rate of skeletal msucles
  • Increase HR and blood flow to skeletal muscles and heart
• Adrenal medulla hormones


  Epinephrine and norepinephrine in the target tissues:

  • Increase the release of glucose from the liver into the blood
  • Increase the release of fatty acids from adipose tissue into the blood
  • Increase heart rate
  • Decrease blood flow through blood vessels of most internal organs
  • Increase blood flow through blood vessels of skeletal muscle and the heart
  • Increase blood pressure
  • Decrease the function of visceral organs
  • Increase the metabolic rate of skeletal muscles
• Adrenal cortex hormone


  Zona glomerulosa - secrete mineralocorticoids. Mainly the hormone aldosterone

  Stimulation:

  • Low blood pressure

  Target tissue:

  • Kidney

  Response:

  • Increases sodium reabsorption = increase blood sodium
  • Causes increase in water reabsorption = increase blood volume, increase blood pressure
• Adrenal cortex hormone


  Zona fasciculata - secrete glucocorticoid hormones. Major one is cortisol

  Stimulation:

  • Low blood glucose and stress stimulate corticotropin-releasing hormone (CRH)
  • CRH stimulates ACTH which travels to the adrenal cortex = increase cortisol secretion

  Target tissue:

  Most tissues - skeletal, liver, adipose tissue, immune tissues
• Adrenal cortex hormone


  Zona fasciculata

  Response:

  Metabolic

  • Increase protein and lipid breakdown
  • Increase blood glucose levels

  Developmental

  • Maturation of tissues

  Anti-inflammatory

  • Decrease the intensity of the inflammatory and immune response therefore the number of inflammatory chemicals and white blood cells

  Negative feedback of ACTH and cortisol inhibit CRH secretion. High levels of cortisol inhibit ACTH.
• Adrenal cortex hormone


  Zona reticularis - secret androgens.

  • Steroid hormones
  • Function as weak androgens
  • Most androgens are secrete y reproductive system
  • Some adrenal androgens are secreted and converted in peripheral tissues to testosterone

  Target tissue:

  • Many tissues

  Response:

  • Androgen causes the development of male secondary sex characteristics
  • Development of secondary sex characteristics in females
• Pancreas


  Lies under the peritoneum and between the stomach and duodenum. Both an endocrine and exocrine gland.

  Exocrine gland:

  • Secretions are secreted via a duct external to the gland either onto a bodily surface or inside the body
  • Acinin cells produce pancreatic juice which is carried to the small intestine

  Endocrine gland:

  • Islets of langerhans make up the endocrine portion
  • Alpha cells (20%) secrete glucagon
  • Beta cells (75%) secrete insulin