24/2-Nerves and Hormones: Homeostasis and Communication Systems

The lecture will discuss the roles of the nervous and endocrine systems in maintaining homeostasis.
A sneak preview of reproduction will be given.

Compare the endocrine and nervous systems.
Understand that hormones act at various distances from target cells, unlike nerves.
Introduce the nervous system.

Internal communication systems are crucial for the body's function.
Cells communicate in various ways, including hormones and nerves, as well as other methods (studied in advanced physiology courses).
The main communication avenues are the cardiovascular and nervous systems.
The quality of the signal depends on its transmission and modulation.
Receptors on target cells are necessary for hormone action.

Positive and negative feedback loops are essential for homeostasis.
Example of positive feedback: suckling of a baby, detected by sensory neurons, leads to milk release involving both the endocrine and nervous systems.
Example of negative feedback: control of pH in the duodenum. A lower pH (acidic) stimulates detector cells, leading to bicarbonate release to neutralize the acid.
Low pH indicates acidity; acids from the stomach enter the duodenum, necessitating mechanisms to prevent excessive acidity.
These feedback loops are fundamental to various systems discussed throughout the semester.

Hormones act in complex ways, with varied effects depending on the receptor type.
Epinephrine (adrenaline) example:
- Liver cells: epinephrine binds to receptors, triggering glycogen breakdown.
- Skeletal muscle blood vessels: dilation occurs via beta receptors.
- Intestinal blood vessels: constriction occurs via alpha receptors.
Different receptors elicit different responses to the same hormone, ensuring diverse functions.

Water-soluble hormones are transported directly through the bloodstream.
Lipid-soluble hormones bind to transport proteins for transport through the bloodstream.
Water-soluble hormones typically bind to receptors on the cell surface.
Lipid-soluble hormones can diffuse through the cell membrane and bind to receptors inside the cell (often in the nucleus).
*Cell membranes are made of phospholipid bilayer, contains a hydrophobic area.

Receptors for water-soluble hormones are on the cell surface.
Hormone binding triggers a cascade of intracellular events to elicit a response.
Different cells have different receptors, allowing specific responses to hormones (e.g., pancreas responding only to certain hormones).
Example: epinephrine binds to a receptor, initiating a cascade that converts glycogen to glucose but does not affect the nucleus.

Lipid-soluble hormones diffuse through the cell membrane and bind to intracellular receptors.
This complex often interacts with DNA in the nucleus and affect the DNA to produce new proteins, leading to a longer-lasting response.
Not all steroid hormones act this way; exceptions exist.

Hydrophobic (lipid-soluble) hormones require transport molecules, receptors are found in the cell.
Hydrophilic (water-soluble) hormones elicit quicker responses and the receptors are on the surface of the cell.

Transport molecules:
- Protect lipid-soluble hormones from damage during transport.
- Regulate hormone activity by mopping up excess hormones.
- Alter hormone potency.

The hypothalamus, located in the brain, controls the pituitary gland.
The pituitary gland (anterior and posterior) produces major hormones.
The hypothalamus communicates with the anterior pituitary via neurosecretory cells.

Overview of the nervous system's components.
- Central nervous system (CNS): brain and spinal cord.
- Peripheral nervous system (PNS): all other nerves.

Basic feedback loop:
- Sensory input: receptors detect stimuli.
- Integration center: processes information (often the brain).
- Motor output: response to the signal.