Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 289 



Responses, Unlearned and Learned. Reflexes are the prompt responses of 

 muscle, either voluntary or involuntary (Fig. 16.11). Naturally they are un- 

 learned actions, such as the wink of the eyelids at a flash of light. Learned 

 responses may be established through the conditioning of unlearned ones, 

 and are then called conditioned reflexes or responses. Human behavior is 

 largely made up of reflexes. They begin at birth with the first breathing, a 

 response to an accumulation of carbon dioxide and lack of oxygen in the 



Brain 



sing of 

 motor pathway. 



Medulla 



Lower leve 

 of cord - 



An area of cord 

 not sfiown 



Sensory fiber 

 from skin 



Crossing of a 

 sensory pathway 



Muscle t;i^L 



Fig. 16.11. Diagram showing the courses of nerve fibers within sections of 

 spinal nerves, and in nerve cord and brain. A reflex response is represented on 

 the lower right side. It involves: a sensory cell carrying an impulse from the skin 

 to the cord; an adjustor cell carrying the impulse from the dorsal or sensory horn 

 of the gray matter of the cord to a cell in the ventral or motor horn; and, a motor 

 cell conducting the impulse to the muscle which then contracts. This response 

 may occur without association with the brain. The diagram also shows the course 

 of responses that are associated with the brain. An impulse enters the cord on 

 a sensory fiber. In the cord this passes to the fiber of another cell, and upward 

 over a succession of cells to the cortex of the brain. There, it passes to the fiber 

 (dendrite) of another cell and on through a succession of cells finally reaching 

 the one that bears it along to the muscle. The way upward is the sensory path- 

 way; the way downward is the motor pathway. The diagram shows the crossing 

 of cell fibers from the right to the left side of the nerve cord in the sensory 

 (ascending) pathway and from the left to the right in the motor (descending) 

 pathway. The right foot is pricked and moves but the order to do this comes from 

 the left side of the brain. 



