Endocrine System Overview and Signaling

Endocrine System Overview

• The endocrine system uses hormones secreted into extracellular fluid to reach target cells through the bloodstream
• Functions include: maintaining homeostasis, mediating environmental responses, regulating growth and development, and triggering sexual maturity and reproduction.

Paracrine and Autocrine Signaling

• Local regulators are molecules that act over short distances by diffusion
• Paracrine signaling: target cells are near the secreting cell
• Autocrine signaling: target cells are the same as the secreting cell
• Local regulators help with blood pressure regulation, nervous system function, and reproduction.

Synaptic and Neuroendocrine Signaling

• Neurohormones are hormones secreted by neurosecretory cells, diffusing into the bloodstream
• Many regulate other endocrine signaling
• Different types of signaling involve different mechanisms:
  • Endocrine: Hormones secreted into bloodstream to act on distant target cells
  • Paracrine: Hormones secreted locally to act on nearby target cells
  • Autocrine: Hormones secreted locally to act on the secreting cell itself
  • Synaptic: Specialized chemical signals (neurotransmitters) released by neurons act on nearby target cells across a synapse
  • Neuroendocrine: Neurohormones released by neurons directly into the bloodstream influencing distant target cells.

Classes of Hormones

• Hormones fall into three major chemical classes:
  • Polypeptides
  • Steroids
  • Amines

Cellular Hormone Response Pathways

• Water-soluble hormones: Cannot diffuse through plasma membranes, bind to cell-surface receptors, and trigger changes in cytoplasmic molecules, sometimes altering gene transcription.
• Lipid-soluble hormones: Diffuse into target cells, bind to intracellular receptors (cytoplasm or nucleus), and trigger changes in gene transcription.

Response Pathways to Water-Soluble Hormones

• Hormone binding to a cell-surface receptor protein triggers a cellular response (e g., enzyme activation, changes in molecular uptake/secretion, or cytoskeleton rearrangement)
• Signal transduction: The conversion of an extracellular chemical signal to an intracellular response.

Response Pathways to Lipid-Soluble Hormones

• Hormone binding to a receptor directly triggers the cell's response, often involving changes in gene expression.

Multiple Responses to a Single Hormone

• Hormones can elicit different responses in different target cells due to variations in receptor type or signaling molecules.

Endocrine Tissues and Organs

• Endocrine cells can be found within organs or grouped into endocrine glands
• Endocrine glands are ductless, secreting hormones directly into the surrounding fluid.
• Exocrine glands have ducts that carry secreted substances onto body surfaces or into body cavities.

Simple Endocrine Pathways

• In simple endocrine pathways, endocrine cells respond directly to a stimulus by releasing hormones, carried by the bloodstream to target cells, triggering a specific response. Stimulus-response-feedback loops regulate the pathway.

Simple Neuroendocrine Pathways

• The stimulus is received by a sensory neuron and triggers a neurosecretory cell to release a neurohormone, carried into the bloodstream to target cells.

Feedback Regulation

• Negative feedback: Response reduces the initial stimulus (prevents excessive pathway activity)
• Positive feedback: Response increases the initial stimulus (reinforces the stimulus to a greater response)

Vertebrates

• The hypothalamus heavily coordinates endocrine signaling by receiving nerve input and initiating responses, including neuroendocrine signals
• The pituitary gland (hypophysis) has a posterior and anterior lobe, each with differing roles in hormone regulation

Posterior Pituitary Hormones

• Antidiuretic Hormone (ADH): Influences kidney function and blood solute concentration; social behavior
• Oxytocin: Regulates milk secretion, uterine contractions, maternal care, pair bonding, and sexual activity

Anterior Pituitary Hormones

• Hypothalamic releasing and inhibiting hormones control the anterior pituitary's release of hormones
• Anterior pituitary releases tropic hormones (affect other endocrine glands) and nontropic hormones (affect non-endocrine targets). Examples include FSH, LH, TSH, ACTH, prolactin, MSH, and GH, with corresponding target cells.

Parathyroid Hormone

• The parathyroid hormone (PTH) regulates calcium levels in blood by targeting bones, kidneys, and promoting vitamin D production.

Adrenal Hormones

• Adrenal glands, located above the kidneys, consist of the adrenal medulla (producing epinephrine and norepinephrine involved in stress responses) and the adrenal cortex (producing mineralocorticoids and glucocorticoids)
• These glands regulate various biological processes, mediating responses to stress (fight-or-flight responses), regulating mineral balance, influencing metabolism, and suppressing immune responses.

Sex Hormones

• Gonads (testes and ovaries) produce androgens (e.g., testosterone), estrogens (e.g., estradiol), and progesterone, influencing reproduction, sexual development, and behavior.

Hormones and Biological Rhythms

• Melatonin, produced by the pineal gland, is associated with biological rhythms, particularly those related to reproduction and daily activity levels; regulated by the suprachiasmatic nucleus (SCN) in the hypothalamus.

This quiz covers the fundamentals of the endocrine system, including hormone functions and the different types of signaling: endocrine, paracrine, autocrine, synaptic, and neuroendocrine. You will learn how these signaling mechanisms play crucial roles in processes like homeostasis, growth, and reproduction.