NERVE-TERMINATIONS. 101,5 



Endings in Involuntary Muscle. — The terminations of the axones of the 

 sympathetic neurones supplying the nonstriated muscle are comparatively simple. 

 The neurones contributing the immediate fibres of distribution 

 usuallv occupy the nodal points of plexuses from which bundles Fig. 865. 



of nonmedullated nerve-fibres extend to and enclose the muscle 

 fasciculi. Entering the latter the nerve-fibres divide into 

 delicate varicose threads that pass between the muscle-cells, 

 parallel with their long axes. As they course within the 

 intercellular substance, the varicose fibrils give off short lateral 

 branches that end, as does also the parent fibre, in minute 

 terminal knots on the surface of the muscle-cells, often in the 

 vicinity of the nucleus. Probably by no means every muscle- 

 cell individually receives a nerve-ending, a longitudinal group 

 including three or four rows of muscle-cells lying between 

 two adjoining terminal nerve-fibrils { Huber). 



Endings in Cardiac Muscle. — These, also the termi- 

 nations of sympathetic neurones, have been studied by, among Nerve-endin"- in involuntary 

 others, Cajal, Retzius, Huber and Smirnow. According to muscle." (Huber.) 



the last-named investigator, the varicose nerve-fibrils may be 



followed between the muscle-cells, during which course side branches arise that, as 

 well as the main fibril, terminate on the muscle elements in endings of varying com- 

 plexity. In some cases these are merely minute simple end-knots, resembling those 

 found in involuntary muscle ; in other cases they are more elaborate and consist of a 

 group of secondary fibrillae bearing nodular endings, the whole recalling somewhat 

 the motor end-plates in striped muscle. It is probable that most of the cardiac 

 muscle-cells are in direct relation with nerve-endings (Huber). 



Sensory Nerve-Endings. 



Since the sensory endings are the peripheral terminal arborizations of the 

 neurones whose cell-bodies lie in the spinal and other sensory ganglia, such teloden- 

 dria are functionally the beginnhigs of the paths conducting the sensory stimuli to 

 the central nervous system. According to their relations to the surrounding tissue, 

 the sensory endings are broadly grouped into free and ejicapsiilated. 



Free Sensory Endings. — These endings include vast numbers of nerve- 

 terminations found in the skin and the mucous membranes, chiefly within the 

 epithelium but to some extent also within the connective tissue strata. As a rule 

 the sensory (afferent) nerve-fibres do not branch to any extent until near their 

 peripheral destination, where they undergo repeated divisions, always at a node of 

 Ranvier and in various directions. The medullary sheath of the main fibre is 

 retained until close to its termination, although some of its branches may course 

 as nonmedullated fibres for a considerable distance before ending or entering the 

 epithelium. In the skin — and the same general plan applies to the mucous mem- 

 branes — the fibres destined for the epidermis lose their myelin coat beneath the 

 basement membrane and enter the epithelium as vertically coursing nonmedullated 



fibrils. Within the epidermis they break up into 

 Y iG. 866. numerous delicate fibrils which undergo further divi- 



sion into still finer varicose threads that ramify 

 between the cells of the stratum germinativum and 

 terminate in minute free end-knobs (Fig. 866). 

 Although an intracellular position of these nerve- 

 endings has been described by various writers, ii 

 is probable that the endings are extracellular and lie 

 upon the surface of and not within the epithelial 

 Free sensory endings within epidermis elements. Similar, but far Icss numerous, free end- 



t1rmtna'^e^n"end-knote^.'^(S.^1.7r^''""'' ^"§^^' ^aricosc and club-like in form, occur within 



the connective tissue layers of the skin and the 

 tunica propria of mucous membranes. Within the integument, conspicuous end- 

 ramifications of sensory neurones surround the hair folUcles, lying upon the outer 

 surface of the glassy membrane. 



