SUPPORT, MOTION, AND SENSATION 



341 



the medullary sheath, while other periplieral nerves possess both a 

 medullary sheath and an outer neurilemma. 



The manner in which neurons operate depends upon their 

 "hook up." Contact without fusion (synapse) is made between 

 the end organ of one neuron and the dendrite of another, resulting 

 in continuity from the physiological point of view. 



The Physiological Unit — Reflex Arc 



The physiological unit of the nervous system is a reflex arc. Such 

 arcs are made up of two or more neurons and a muscle or gland ele- 

 ment. A simple arc consists of a receptor neuron, the dendrites of 



recsptoT- 



stimttlus- 



"Sensory neuron. 



synocpss 



epi 



fbeli 



lum. 



I 



2. 



csffectox^ 



Simpk$t form reflex ai 



of a reflex: arc J vith 



]^~ ne,u.ro-ns 



re$ponss-« 



association 



syn<xp^e 



associat ior> 

 neixrorL ' 



■muscle fibers 



Diagram of reflex arcs. Explain why this is often called the "physiological unit 



of the nervous system." 



which receive the stimulus and transmit it via the axon to the spinal 

 cord where a synapse with the dendrites of an effector cell occurs. 

 The impulse is then transmitted by means of the hitter's axon to 

 the muscle or gland cell. Reflex arcs generally require one or more 

 adjuster neurons in the circuit between the receptor and effector 

 cells. Such adjuster cells are usually located in the spinal cord or 

 in the brain. 



Even so brief a discussion of reflexes cannot be concluded without 

 mention of the compound reflex arcs which are formed by a single 

 receptor neuron and two or more effectors that may be widely sepa- 

 rated in the body, or l)y two or more receptors and a single effector. 

 Varying complexities of these latter types are made by the inter- 



