GENERAL PHYSIOLOGY OF NERVE-TiSSl E 



101 



and the duration of a movement, the resistance offered and the posture 

 of the body or of its individual parts. 



PHYSIOLOGIC PROPERTIES OF NERVES 



Nerve Irritability or Excitability and Conductivity. These terms 

 are employed to express that condition of a nerve which enables it to develop 

 and to conduct nerve impulses from the center to the periphery, or from 

 the periphery to the center, in response to the action of stimuli. A nerve 

 is said to be excitable or irritable so long as it possesses these capabilities or 

 properties. For the manifestation of these properties the nerve must 

 retain a state of physical and chemic integrity; it must undergo no change 

 in structure or chemic composition. The irritability of an efferent nerve is 

 demonstrated by the contraction of a muscle, by the secretion of a gland, or 

 by a change in the caliber of a blood-vessel, whenever a corresponding nerve 

 is stimulated. The irritability of an afferent nerve is demonstrated by the 

 production of a sensation or a reflex action whenever it is stimulated. The 

 irritability of nerves continues for a certain period of time after separation 

 from the nerve-centers and even after the death of the animal, the time 

 varying in different classes of animals. In the warm-blooded animals, in 

 which the nutritive changes take place with great rapidity, the irritability 

 soon disappears a result due to disintegrative changes in the nerve, caused 

 by the withdrawal of the blood-supply and other non-physiologic conditions. 

 In cold-blooded animals, on the contrary, in which the nutritive changes 

 take place relatively slowly, the irritability lasts, under favorable conditions, 

 for a considerable time. Other tissues besides nerves possess irritability, 

 that is, the property of responding to the action of stimuli e.g., glands and 

 muscles, which respond by the production of a secretion or a contraction. 



Independence of Tissue Irritability. The irritability of nerves is 

 distinct and independent of the irritability of muscles and glands, as shown 

 by the fact that it persists in each a variable length of time after their histo- 

 logic connections have been impaired or destroyed by the introduction of 

 various chemic agents into the circulation. Curara, for example, induces 

 a state of complete paralysis by modifying or depressing the conductivity of 

 the end-organs of the nerves just where they come in contact with the mus- 

 cles, without impairing the irritability of either nerve-trunks or muscles. 

 Atropin induces complete suspension of gland activity by impairing the 

 terminal organs of the secretor nerves just where they come into relation with 

 the gland-cells, without destroying the irritability of either gland-cell or nerve. 

 Nerve Stimuli. Nerves do not possess the power of spontaneously 

 generating and propagating nerve impulses; they can be aroused to activity 

 only by the action of an external stimulus. In the physiologic condition the 

 stimuli capable of throwing the nerve into an active condition act for the 

 most part on either the central or peripheral end of the nerve. In the case 

 of motor nerves the stimulus to the excitation, originating in some molecular 

 disturbance in the nerve-cells, acts upon the nerve-fibers in connection with 

 them. In the case of sensor or afferent nerves the stimuli act upon the pecul- 

 iar end-organs with which the sensor nerves are in connection, which in 

 turn excite the nerve-fibers. Experimentally, it can be demonstrated that 

 nerves can be excited by a sufficiently powerful stimulus applied in any 

 part of their extent. 



