RECEPTORS AND EFFECTORS 



lOo 



fil)er.s. 'I'lic distal end of an allVrent iier\e breaks up into fine 

 thread-like branehes which are coiled about these special fibers. 

 When the nmscle as a whole has contracted, the sensory nerve of 

 the muscle spindle is stimulated, and this is relaxed to a nerve 

 center. In this way we get information of the position of the liml)s, 

 for exani])le, the degree of flexion. 



Distributed widely through fibroelastic connective tissue, are 

 special sense organs called Pacinian corpuscles. These are small 

 ovoid structures consisting of concentric o\'erla])ping layers of 

 connective tissue covering an inner core of semifluid plasm in which 



Fig. 60. — Photograph of neuromuscular spindle in skeletal muscle of the cat. 

 The terminal arborization of a nerve fiber is wrapped about skeletal muscle fibers 

 and acts as a sensoreceptor. Silver impregnation. 



is embedded the flat end-process of a ner\'e. These are a few of the 

 numerous types of sensory end-organs. 



Motor Endings. — A motor nerve as it ends in a skeletal muscle 

 passes in through the external and internal perimysia to the muscle 

 fibers. The end of each nerve fiber breaks up into telodendria and 

 myelin disappears. The neurolemma at the end of the fiber appears 

 to merge with the sarcolemma of the muscle fiber to which that 

 particular telodendrion is connected. The group of neurofibrils 

 passes into a shallow pool of sarcoplasm just under the sarcolemma, 

 where the nerve process forms short irregular branches, each ending 

 in a small knob. The whole apparatus is known as a motor end- 

 plate. The nerve impulse reaching this organ causes the muscle to 

 contract. P^ach muscle fiber has at least one motor end-plate. 



