838 THE NERVOUS SYSTEM 



Fig. 358. General structural features of spinal nerves, and of nerve fibers terminating 

 in muscle tissue. (A) Diagrammatic representation of a spinal nerve in the region of the 

 mammalian diaphragm showing functional components. Three facts are evident relative 

 to the components of a typical spinal nerve, viz., ( 1 ) The somatic efferent motor neuron 

 lies within the central nerve tube; its fiber extends peripherad to the effector organ. One 

 neuron therefore is involved in the somatic efferent system (see Fig. 352A). (2) Unlike 

 the somatic efferent system, the visceral efferent (motor) system is composed of a chain 

 of two neurons, a preganglionic neuron whose cell body lies within the central nerve 

 tube, and a postganglionic neuron whose cell body lies in one of the peripheral ganglia. 

 (3) The somatic afferent (sensory) and visceral afferent (sensory) fibers both possess 

 but one neuron whose cell body lies within the dorsal root ganglion. The somatic afferent 

 fiber connects with a sense or receptor organ lying somewhere between the viscera and 

 the external surface (i.e., cutaneous surface) of the body, whereas the visceral afferent 

 fiber contacts the structural makeup of the visceral structures. (B) A spinal nerve in 

 the region of the brachial plexus. The main difference between this type of nerve and 

 the typical spinal nerve resides in the fact that the ventral ramus proceeds into the limb 

 and not into the body wall. Before proceeding into the limb it inosculates with the 

 ventral rami of other nerves to form the brachial plexus. (C) Portion of a transverse 

 section of the sciatic nerve of a newborn showing groups of nerve fibers joined together 

 into bundles. Each nerve-fiber bundle is surrounded by connective tissue, the perineurium, 

 and is partly divided by septa of connective tissue, the endoneurium. External to the 

 perineurium is the epineurium, or the connective tissue which holds the entire nerve 

 together (Redrawn from Maximow and Bloom, 1942, A Textbook of Histology, W. B. 

 Saunders Co., Philadelphia, after Schaffer. ) (D) Diagram of the autonomic efferent 

 system of neurons and ganglia. The parasympathetic (craniosacral) outflow is shown in 

 heavy black lines with white spaces; the sympathetic (thoracicolumhar) outflow is repre- 

 sented by ordinary black lines. (Adapted from Ranson, 1939, The Anatomy of the 

 Nervous System, Philadelphia, Saunders, after Meyer and Gottlieb.) 



G. cerv. sup. = superior cervical ganglion 



G. stellatum = inferior cervical or stellate ganglion 



G. mes. sup. = superior mesenteric ganglion 



G. mes. inf. = inferior mesenteric ganglion 



G. pelv. = pelvic ganglion 

 Neurohumoral substances are produced at the terminal (effector) tips of the various 

 autonomic nerve fibers. A substance similar to adrenalin appears to be produced at the 

 tips of the sympathetic nerves proper, whereas in the case of the parasympathetic fibers 

 the substance is acetylcholine. These humoral substances stimulate the effector structures. 

 (E, F, and G) Nerve endings associated with muscle tissue. (E) Effector (motor) nerve 

 endings associated with cardiac or smooth muscle. Sympathetic motor endings terminate 

 in small swellings. This figure portrays sympathetic motor endings on a smooth muscle 

 cell of an artery of the rabbit's eye. (Redrawn from Maximow and Bloom, 1942, A 

 Textbook of Histology, Philadelphia, Saunders, after Retzius.) (F) Another example of 

 the termination of sympathetic nerve fiber endings on smooth muscle fibers. In this in- 

 stance the bronchial musculature is the effector organ. (Redrawn from Maximow and 

 Bloom, 1942, A Textbook of Histology, Philadelphia, Saunders, after Larsell & Dow.) 

 (G and G') Nerve endings in striated muscle. (G redrawn from Ranson, 1939, The 

 Anatomy of the Nervous System, Philadelphia, Saunders, after Huber & De Witt; G' 

 redrawn from Maximow and Bloom, 1942, A Textbook of Histology, Philadelphia, 

 Saunders, after Boeke.) (G) Represents a neuromuscular end organ of a sensory nerve 

 fiber terminating within a muscle spindle in striated muscle from a dog. These muscle 

 spindles are in the form of a connective tissue capsule which invests spindle-shaped 

 bundles of muscle fibers. Within this capsule, large myelinated nerve fibers terminate 

 in non-myelinated branches which spiral around the muscle fibers or end in flattened 

 discs. (G') Represents a somatic motor (efferent) nerve fiber terminating in a motor 

 plate within a striated muscle fiber. The motor plate is composed of an irregular mass 

 of sarcoplasm below the sarcolemma of the muscle fiber. This motor plate receives the 

 naked terminal ramifications of the nerve fiber. 



