M11

• Chemical Messengers


  Allow cells to communicate with each other to regulate body activities
• Four Classes of Chemical Messengers (based on source & mode of transport)

  • Autocrine
  • Paracrine
  • Neurotransmitter
  • Endocrine
• Autocrine


  Stimulates the cell that originally secreted it, and sometimes nearby cells of same type (ex: WBC)
• Paracrine


  Local messengers secreted by one cell type but affect neighboring cells of different type; do not travel in circulation but secreted into extracellular fluid (ex: histamine)
• Neurotransmitter


  Secreted by neurons to activate another neuron, muscle cell or glandular cell; secreted into a synaptic cleft (ex: acetylcholine)

  
• Endocrine


  Secreted into bloodstream by certain glands & cell; affect cells distant from source (ex: epinephrine)
• Main Regulatory Functions of Endocrine System

  • Metabolism
  • Control of food intake & digestion
  • Tissue development
  • Ion regulation
  • Water balance
  • Heart rate and blood pressure regulation
  • Control of blood glucose & other nutrients
  • Control of reproductive functions
  • Uterine contractions & milk release
  • Immune system regulation
• Endocrine System


  System of glands that secrete hormones into the bloodstream
• Hormones


  Chemical messengers secreted by endocrine glands that regulate various bodily functions

  
• Chemical Nature of Hormones

  • Lipid-Soluble Hormones
  • Water-Soluble Hormones
• Lipid-Soluble Hormones


  • Nonpolar; include steroid hormones, thyroid hormones, and fatty acid derivatives hormones, such as eicosanoids
  • Small molecules; insoluble in water-based fluids (plasma); therefore, attached to binding proteins for transport through bloodstream & protection of hormones
  • Degraded slowly & not rapidly eliminated from circulation; life span ranges from days to several weeks
  • Breakdown products are excreted via urine or bile
• Water-Soluble Hormones


  • Polar; includes protein, peptide, & most amino acids derivative hormones
  • Many circulate as free hormones; large molecules diffuse from blood into tissue spaces slowly; small molecules attached to binding protein to avoid being filtered out
  • Regulate activities that have rapid onset and short duration
  • Relatively short half-lives due to rapid degradation of enzymes (proteases) within bloodstream; hormone breakdown products are then excreted in urine; however, some hormones have chemical modifications, such as adding of carbohydrate groups, which prolongs their life span
• Types of Stimuli Regulating the Hormone Release

  • Humoral Stimuli
  • Neural Stimuli
  • Hormonal Stimuli

  
• Humoral Stimuli


  Directly by blood-borne chemicals to hormones that are sensitive to blood levels of particular substances (insulin)
• Companion hormones


  Secreted with release of humoral stimuli; oppose the effect of secreted hormone & counteract its actions (glucagon)
• Neural Stimuli


  Stimulated by action potential, releasing neurotransmitter from neurons into synapse with cell producing hormones (epinephrine)

  
• Neuropeptide


  Chemical messengers secreted by neurons directly to blood
• Releasing Hormones


  Specialized neuropeptides that stimulate hormone secretion from other endocrine cells

  
• Hormonal Stimuli


  Hormone is secreted to stimulate the secretion of other hormones (tropic hormones of anterior pituitary gland)
• Negative Feedback


  Prevents further hormone secretion once a set point is achieved
• Positive Feedback


  Self-promoting system whereby the stimulation of hormone secretion increases over time
• Classes of Receptors


  • Nuclear Receptors
  • Membrane-Bound Receptors
• Nuclear Receptors


  • Often found in cell nucleus where the lipid-soluble hormones bind; cannot respond immediately because it takes time to produce mRNA and the protein
  • Hormone-Response Elements - fingerlike projections that recognize & bind specific nucleotide sequence in DNA; found in receptors that bind to DNA
  • Transcription Factor - forms from combination of hormone and its receptors
  • The hormone-receptor complex activates genes, which in turn activate the DNA to produce mRNA
  • The mRNA increases the synthesis of certain proteins that produce the target cell's response

  
• Membrane-Bound Receptors


  • Proteins that extend across the cell membrane where water-soluble hormones bind; with hormone-binding sites exposed on cell membrane's outer surface
  • Activate responses in two ways: Alter the activity of G proteins (α, β, and γ) at the inner surface of cell membrane, or Alter the activity of intracellular enzymes directly
  • These activations elicit second messengers that activates the cellular response referred to as second-messenger systems which act rapidly because they act on already existing enzymes
  • Has a signal amplification which means that a single hormone activates many second messengers, each of which activates enzymes that produce enormous amount of final product

  
• Pituitary Gland / Hypophysis


  Small gland that rests in sella turcica which is controlled by hypothalamus; connected to hypothalamus by infundibulum; divided into two parts: Anterior Pituitary and Posterior Pituitary

  
• Anterior Pituitary


  1. Made of epithelium from embryonic oral cavity

  2. Secretions are controlled by hormones that pass through Hypothalamic-Pituitary Portal System (capillary beds and veins that transport the releasing and inhibiting hormones)

  
• Posterior Pituitary


  1. Extension of brain & composed of nerve cells

  2. Hormone secreted are controlled by action potentials carried by axons that pass from the hypothalamus (Direct Innervation)