Chapter 18
The Endocrine System
The body contains roughly 30 chemical messengers known as hormones, which regulate activities such as sleep, body temperature, hunger, and stress management.
Intracellular communication like direct communication which occurs between two cells of the same type-- Through Gap Junctions.
Gap junctions: 1. Coordinates ciliary movement among epithelial cells 2. Coordinate contractions of cardiac muscle cells 3. Facilitate the propagation of action potentials from one neuron to the next at electrical synapses

Paracrine communication-the use of chemical messengers to transfer information from cell to cell within a single tissue 1. Chemicals involved are called paracrine factors or local hormones 2. Example is prostaglandins 3. Enter bloodstream but with low concentrations
Difference between paracrine factors and hormones is a matter of degree. Paracrine factors can diffuse out of their tissue of origin and have widespread effects. Hormones can affect their tissues of origin as well as distant cells

Target cells-specific cells that have the receptors needed to bind and read the hormonal message when it arrives.

Hormone may: 1. Stimulate the synthesis of an enzyme or a structural protein not already present in the cytoplasm by activating appropriate genes in the cell nucleus 2. Increase or decrease the rate of synthesis of a particular enzyme or other protein by changing the rate of transcription or translation; or 3. Turn an existing enzyme or membrane channel “on” or “off” by changing its shape or structure

A single hormone can alter the metabolic activities of multiple tissues and organs at the same time. These effects may be slow to appear, but they typically persist for days.
Hormonal regulation is quite suitable for directing gradual, coordinating processes, but cannot handle situations requiring split-seconds responses (That type of crisis management is handled by the nervous system)
The nervous system sends messages through action potential propagation. Synaptic communication

Mechanisms of Intercellular Communication | Mechanism | Transmission | Chemical Mediators | Distribution of Effects | Direct communication | Through gap junctions | Ions, small solutes, lipid-soluble materials | Usually limited to adjacent cells of the same type that are interconnected by connexons | Paracrine communication | Through extracellular fluid | Paracrine factors | Primarily limited to a local area, where paracrine factor concentrations are relatively high. Target cells must have appropriate receptors | Endocrine communication | Through the bloodstream | Hormones | Target cells are primarily in other tissues and organs and must have appropriate receptors | Synaptic communication | Across synaptic clefts | Neurotransmitters | Limited to very specific area, target cells must have appropriate receptors |

* The nervous and endocrine system share many chemical messengers. For example, norepinephrine and epinephrine are called hormones when released into the bloodstream, but neurotransmitters when released across synapses. * Both systems are regulated mainly by negative feedback control mechanisms * The two systems share a common goal: to preserve homeostasis by coordinating and regulating the activities of other cells, tissues, organs and systems

Endocrine cells are glandular secretory cells that release their secretions into the extracellular fluid differently from exocrine cells which secrete their products onto epithelial surfaces, generally by way of ducts

Classes of Hormones
We divide hormones into three groups on the basis of their chemical structure 1. Amino acid derivatives
Amino acid derivatives, sometimes known as biogenic amines, are relatively small molecules that are structurally related to amino acids, the building blocks of proteins.
These hormones are synthesized from the amino acids tyrosine and