Hormones and Integrating Systems

• Hormones are chemicals produced in tissues or organs which affect the growth and/or function of other target tissues or organs.
• Hormones have varying structures from a single amino acid to complex proteins, carbohydrates, or lipids.
• Hormones are involved in regulating:
  • Metabolism
  • Growth and development
  • Muscle and fat distribution
  • Fluid and electrolyte balance
  • Sexual development and reproduction
  • Stress response

Integrating Systems

• Hormones are commonly associated with the endocrine system.
• The endocrine system is a group of tissues capable of secreting hormones.
• The pancreas and thyroid are examples of endocrine glands.
• Neurons can synthesize and release hormones.
• Neurotransmitters (epinephrine, dopamine, serotonin, norepinephrine) are chemical messengers, synthesized by neurons, which rapidly stimulate a neural response.
• Inflammatory and immune cells release chemical mediators (cytokines, leukotrienes, prostaglandins) which act like hormones to trigger defense mechanisms.
• Tumor cells can synthesize and release ectopic hormones which can cause homeostatic imbalance.
• The neurologic, inflammatory, immune, and endocrine systems collaborate in a unified system.
• Neurotransmitters act quickly, endocrine hormones take longer to have an effect.
• Regulatory processes integrate to protect the body from injury and maintain homeostasis.

Regulating Hormones

• Hormone synthesis and release is controlled by tissues and organs.
• The hypothalamic–pituitary axis in the brain is an important control center for many hormones
• Hormones exhibit predictable patterns of secretion, metabolism, and elimination
• Hormones adjust based on negative or positive feedback mechanisms
• Hormones exhibit two primary functions:
  • To act on target tissues to achieve an effect
  • To act on glands to produce another hormone
• To exert an effect, hormones must locate and attach onto target cells of specific tissues

The Hypothalamic–Pituitary Axis

• The hypothalamic–pituitary axis controls the synthesis and secretion of many hormones.
• The hypothalamus contains neurons that synthesize releasing and inhibiting hormones which act on the anterior pituitary gland.
• Releasing hormones include:
  • Growth hormone–releasing hormone (GHRH)
  • Thyrotropin-releasing hormone (TRH)
  • Corticotropin-releasing hormone (CRH)
  • Gonadotropin-releasing hormone (GnRH)
• Inhibiting hormones include:
  • Somatostatin (inhibits growth hormone and thyroid-stimulating hormone [TSH])
  • Dopamine (inhibits prolactin)
• The pituitary gland responds to hypothalamic triggers.
• Prolactin, releasing hormones, and inhibiting hormones travel through the hypophyseal portal system (blood vessels) to get from the hypothalamus to the anterior pituitary while:
• The hypothalamus produces antidiuretic hormone (ADH) and oxytocin, which then travel along nerve axons to the posterior pituitary for unchanged release into the systemic circulation.
• Melanocyte-stimulating hormone (MSH) comes from the pars intermedia between the anterior and posterior portions of the pituitary gland.

Feedback Mechanisms

• Neurotransmitters, injury and the resulting release of chemical mediators, and neuroendocrine signals provide input to the hypothalamus to initiate hormone release.
• Negative feedback is a common mechanism like an internal thermostat.
• The hypothalamus and pituitary act as sensors that gauge hormone levels.
• When levels rise above the expected range, stimulation, production, or secretion of hormone is decreased.
• When levels fall, stimulation, production, or secretion of hormone is increased.
• The negative feedback mechanism is affected by environmental and body temperature, stress, nutrition, and specific body substances.
• Aldosterone levels adjust based on sodium and potassium levels.
• ADH is responsive to body fluid levels.
• Positive feedback stimulates increased production of the same hormone until there is an interruption of the cycle.
• Oxytocin is one example of a hormone regulated by positive feedback.

Hormone Secretion, Metabolism, and Elimination

• Hormones exhibit predictable patterns of secretion.
• Estrogens, progesterone, luteinizing hormone, and follicle-stimulating hormone (FSH) regulate the female menstrual cycle.
• Growth hormone levels increase during sleeping hours and decrease during waking hours.
• Hormone accumulation is prevented through inactivation (enzymes that break down the hormone or inactivation in the liver) and elimination commonly through the urine or along with bile in the feces.

Receptor Binding

• Receptor binding allows hormones to act selectively on certain cells.
• The number of receptors on each cell can reach up to 100,000 or more.
• Hormones attach to specific receptors on the cell surface or within the cell interior via a hormone-receptor complex.
• A surface receptor requires a second messenger to elicit a response from the cell.
• Receptor binding is altered when the number of receptors is reduced or when the affinity for the hormone is reduced due to genetics, hormone levels, and body fluid pH.

Mediating Cell-to-Cell Communication

• Five major pathways characterize cell-to-cell communication, where hormones move from production and secretion, to response
  • Paracrine pathway: hormones are produced in a cell, secreted, and act directly on nearby receptive cells.
  • Autocrine pathway: the same as the paracrine pathway except that the receptor cells are also secretory cells so, in essence, the cell is able to produce the hormone and exert an effect on itself.
  • Endocrine pathway: hormones are produced in a cell, secreted, and travel through blood vessels to distant cells, attach to receptors, and act on that cell.
  • Synaptic pathway: hormones (neurotransmitters) are produced in the neuron, secreted, and travel along the axon to the synapse where they are released and taken up by a nearby neuron with the appropriate receptors to exert an effect.
  • Neuroendocrine pathway: hormones (neurohormones) are produced in a neuron, secreted, travel along the axon to the synapse, are released, are taken up into the vascular system, and travel to distant cells with the appropriate receptors to exert an effect.