Module 1 Lesson 4 - 2

Organization of the Nervous System

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Read pages 366 - 367

The nervous system is actually two systems: Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• The central nervous system is composed of the brain and the spinal cord.
• The peripheral nervous system is everything beyond the brain and the spinal cord. It carries the sensory information to the CNS (sensory pathways) and sends information from CNS to the muscles and glands (motor pathways). 

The information sent from CNS to muscles and glands controls the body consciously and unconsciously.

• The somatic nervous system controls the voluntary movement (such as moving an arm).
• The autonomic nervous system controls the involuntary movement (such as heartbeat).
  

The involuntary autonomic nervous system is considered as the sympathetic (fight-or-flight) nervous system and the parasympathetic (rest and digest) nervous system. These complement each other to maintain homeostasis.

• The sympathetic nervous system is triggered during stressful situations to mobilize energy. It uses epinephrine to control organ responses.
  
• The parasympathetic system is triggered during relaxed states to conserve energy. It uses acetylcholine to control organ responses.

Comparison of the Autonomic Nervous System

Sympathetic Nervous System	Parasympathetic Nervous System
Increased heart rate Inhibits tears and salivation Dilates pupils Liver releases glucose (more energy available) Inhibits digest Inhibits defecation Inhibits urination	Decreased heart rate Stimulates tears and salivation Constricts pupils Gallbladder release bile (more digestion) Stimulates digestion Stimulates defecation Stimulates urination

Use this mnemonic to memorize the functions of the parasympathetic nervous system: STUDD (salivation, tears, urination, digestion, defecation).

Homeostasis

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Homeostasis is the maintenance of a nearly constant internal environment that fluctuates about a set point or an ideal point. For example, the set point for body temperature is 37℃. However, the temperature hovers around that value, either below or above by a small amount.

The nervous system is critical for maintaining homeostasis. The human nervous system is capable of regulating thousands of activities simultaneously. The general function of the nervous system is to collect information, analyze (sort, integrate, interpret) the information and to initiate an appropriate response to maintain a state of dynamic equilibrium or homeostasis. The nervous system accomplishes this function quickly and precisely. Quick action often is needed to get you out of dangerous situations.

Watch and Listen

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View the following video on the organization of the nervous system.