18. COMPARISON OF CONTROL BY THE NERVOUS AND ENDOCRINE SYSTEMS

18. COMPARISON OF CONTROL BY THE NERVOUS AND ENDOCRINE SYSTEMS

• The nervous and endocrine systems act together to coordinate functions of all body systems. 

•  the nervous system acts through nerve impulses (action potentials) conducted along axons of neurons. 
• At synapses, nerve impulses trigger the release of mediator (messenger) molecules called neurotransmitters. 

• The endocrine system also controls body activities by releasing mediators, called hormones,
• but the means of control of the two systems are very different.

• A hormone (hormon to excite or get moving) is a mediator molecule that is released in one part of the body 
• but regulates the activity of cells in other parts of the body. 
• Most hormones enter interstitial fluid and then the bloodstream. 
• The circulating blood delivers hormones to cells throughout the body. 

• Both neurotransmitters and hormones exert their effects by binding to receptors on or in their “target” cells. 

• Several mediators act as both neurotransmitters and hormones.
• One familiar example is norepinephrine, which is released as 
• a neurotransmitter by sympathetic postganglionic neurons and 
• as a hormone by chromaffin cells of the adrenal medullae.

• Responses of the endocrine system often are slower than responses of the nervous system; 
• although some hormones act within seconds, most take several minutes or more to cause a response. 

• The effects of nervous system activation are generally briefer than those of the endocrine system. 

• The nervous system acts on specific muscles and glands.

•  The influence of the endocrine system is much broader; 
• it helps regulate virtually all types of body cells.

• We will also have several opportunities to see how the nervous and endocrine systems function together as an interlocking “super- system.”

 For example, 

• certain parts of the nervous system stimulate or inhibit the release of hormones by the endocrine system.

Comparison of Control by the Nervous and Endocrine Systems
CHARACTERISTIC	NERVOUS                  SYSTEM	ENDOCRINE            SYSTEM
Mediator molecules	Neurotransmitters released locally in response to nerve impulses.	Hormones delivered to tissues throughout the body by the blood.
Site of mediator action	Close to site of release, at a synapse; binds to receptors in postsynaptic membrane.	Far from site of release (usually);  binds to receptors on or in target cells.
Types of target cells	Muscle (smooth, cardiac, and skeletal) cells,  gland cells, other neurons.	Cells throughout the body.
Time to onset of action	Typically within milliseconds (thousandths of a second).	Seconds to hours or days.
Duration of action	Generally briefer (milliseconds).	Generally longer (seconds to days).