The Nervous System
 

 

SYMPATHETIC AND PARASYMPATHETIC BRANCHES OF THE AUTONOMIC DIVISION OF THE NERVOUS SYSTEM

The autonomic nervous system includes two separate systems of autonomic control.

The parasympathetic system is active while we are at rest, and is primarily associated with digestive system responses.

The sympathetic system operates under conditions of fear, anger, excitement, action, or stress. It stimulates circulatory, respiratory and muscular functions that are typical of these conditions. Sympathetic responses also suppress the activity of the parasympathetic branch.

 

SEE TABLE COMPARING PARASYMPATHETIC AND SYMPATHETIC ACTIONS.

 

While the differences in the functions of the two systems are more well known, the differences between the motor pathways of the parasympathetic and sympathetic nerves are no less significant. Motor fibers of the parasympathetic branch stem from some of the cranial nerves (III, VII, IX, and X) and sacral nerves (S2, S3, and S4), and for this reason the parasympathetic system is sometimes called the craniosacral division. Motor fibers of the sympathetic branch have their origins in spinal nerves from T1 to L2, making the sympathetic branch the thoracolumbar division.

Preganglionic fibers of the parasympathetic branch are longer than those of the sympathetic branch. This is because the ganglia of the parasympathetic system are generally situated close to, or within the tissue to be affected.

Sympathetic ganglia are usually situated near the central nervous system. Their postganglionic fibers then extend a longer distance to the intended organ.

While the neurotransmitter of choice at the parasympathetic effector is acetylcholine, norepinephrine is more commonly employed at the effector of a sympathetic function. In the sympathetic division there are exceptions to this. Acetylcholine is used at a few sympathetic effectors, and it should be known that the adrenal medulla, which releases epinephrine to the blood, receives direct innervation from sympathetic preganglionic motor fibers.

 

 

 

 

 

 

STUDY TOPICS IN SEQUENCE

Overview of the Nervous System

Anatomical development

Meninges

Neurocoel, ventricles, and CSF

Spinal cord and reflex arc

Sympathetic and Parasympathetic