464 - Multicellular Animals, Especially Man 



and of touch and pressure sensations when 

 stimuli are applied on the right side of the 

 botly, below the level of the lesion; but the 

 loss of temperature and pain sensations is 

 localized on the left side of the body, like- 

 wise below the damaged level. 



Localization of Impulses in the Higher Centers, 

 Especially the Cerebral Cortex. Proprioceptive 

 impulses passing upward in the spinal cord 

 are routed mainly to the cerebellum, where 

 the complex movements of the body are 

 coordinated, but other types of impulses go 

 mainly to the cerebrum. In either case, how- 

 ever, the crossing over of all impulses is com- 

 pleted by the time they reach the higher 

 centers. Thus pain and temperature im- 

 pulses are shunted across at a relatively low 

 level in the spinal cord, but other impulses 

 pass across at a higher level, in the brain 

 stem. Each half of the cerebrum and cere- 

 bellum, therefore, is concerned with the 

 transmission of impulses that are derived 

 from the receptors on an opposite side of the 

 body. 



As is shown in Figure 25-15, the cerebral 

 hemispheres sort out the different kinds of 

 impulses and transmit the several kinds 

 through different areas of the cortex. Thus 

 sensations and reflexes initiated in the recep- 

 tors of the trunk and limbs are abolished by 



damaging specific areas in the upper mid- 

 region of the cortex; whereas sensations and 

 reflexes originating from the special sense 

 organs are definitely localized in other spe- 

 cific areas of the cortex. 



The Main Motor (Pyramidal) Tract. The main 

 motor area of the cerebral cortex, which 

 sends impulses to the muscles of the arms, 

 trunk, and legs, lies just anterior to the sen- 

 sory area of the corresponding parts of the 

 body (Fig. 25-15). Direct stimulation of this 

 motor region produces convulsive movements 

 in the different body parts, according to the 

 focal point of the applied stimulus. Such 

 movements tend to simulate the convulsions 

 that are commonly recognized in epilepsy; it 

 is generally agreed that some forms of epi- 

 lepsy represent an abnormal hyperexcitabil- 

 itv of the neurons of the motor area of the 

 cerebral cortex. 



The neurons of the motor area possess 

 very elongate axons that pass downward 

 through the brain stem to all levels of the 

 spinal cord. These axons form a compact 

 bundle of fibers, called the pyramidal tract, 

 that begin to cross over in the lower part 

 of the brain stem, at the level of the pons 

 (Fig. 25-12). Thus the pyramidal fibers from 

 one side of the cortex finally reach the motor 

 neurons of the various spinal nerves on the 



Fig. 25-15. Lateral surface 

 of the left cerebral cortex of 

 man, showing the sensory and 

 motor areas that directly con- 

 nect with the lower parts of 

 the nervous system. 



