Week 4-1 Steroid and Non-steroid hormones

The study of the Nervous and Endocrine systems emphasizes integration and coordination.
The endocrine system, along with the nervous system, helps maintain homeostasis in the body.
Composed of endocrine glands that secrete hormones into body fluids.
Organs are not always located close to one another; hormones diffuse into the bloodstream to act on distant target cells.
Some glands secrete "local hormones" that do not enter the bloodstream, known as paracrine (affecting neighboring cells) and autocrine (affecting the secreting cell itself) secretions.
Body has two major types of glands:
- Exocrine glands: secrete products into ducts, outside of internal environments.
- Endocrine glands: secrete hormones directly into the bloodstream, which influences target cells.

Major endocrine glands include:
- Pituitary gland
- Thyroid gland
- Parathyroid glands
- Adrenal glands
- Pancreas
- Pineal gland
- Reproductive glands: ovaries and testes
- Kidneys
- Thymus gland
Specialized cells in organs like the liver, heart, and gastrointestinal tract participate in hormone production but primarily belong to other systems.

Both systems communicate with target cells but employ different mechanisms:
- Endocrine System: uses hormones (chemical messengers) to communicate diverse metabolic processes across the body.
- Nervous System: uses neurotransmitters for precise communication.
- Both target specific cells located throughout the body.

Aspect	Nervous System	Endocrine System
Cells	Neurons	Epithelial & others
Chemical signal	Neurotransmitter	Hormone
Specificity of response	Receptors on postsynaptic cell	Receptors on target cell
Speed of onset	Seconds	Seconds to hours
Duration of action	Brief unless continual neuronal activity	Can be brief or last for days even after secretion ceases

Hormones are categorized into two types:
- Steroid Hormones: derived from cholesterol.
- Nonsteroid Hormones: include amines, peptides, proteins, glycoproteins produced from amino acids.

Lipid-soluble: can easily pass through cell membranes.
Receptors for steroid hormones are found inside target cells.
The hormone-receptor complex binds to DNA, activating genes that direct protein synthesis.
New proteins can function as enzymes, transport proteins, or hormone receptors, impacting cellular processes.

Water-soluble: cannot penetrate cell membranes.
Bind to surface receptors causing a cascade of activities known as signal transduction.
The first messenger (hormone) activates second messengers inside the cell, such as cAMP, leading to a series of reactions that result in cellular responses.

Hormone levels are precisely regulated via:
1. Hypothalamus control: regulates anterior pituitary hormone secretions which influence peripheral glands.
2. Nervous system influence: direct effects via nerve impulses on certain endocrine glands.
3. Direct response: other glands adjust based on changes in the internal fluid composition (e.g., glucose levels).

A gland is responsive to the concentration of the substance it regulates.
As hormone levels rise, the effects are produced, and further secretion is inhibited (negative feedback).
When concentration drops below a normal level, secretion resumes.
This system keeps hormone levels relatively constant, fluctuating within an average range.