Defense Date


Document Type


Degree Name

Doctor of Philosophy


Physiology and Biophysics

First Advisor

Alfred J. Szumski


Until the mid to late 1800's, the cerebral hemispheres were considered to be only the sites of mentation and intellectual activity. On the basis of his observations of epileptic patients, HughIings Jackson (cited from Walshe, l935a) theorized that the cerebral hemispheres were also sites of motor control and sensory experience. The electrical excitability of the cerebrum was first demonstrated by Fritsch and Hitzig in 1870. Since then the motor representation of the various muscles has been mapped out and the types of movement in response to stimulation of the motor cortex and the pyramidal tract have been thoroughly studied.

Mammalian muscles contain sensory receptors which supply the central nervous system information on muscle length and rate of change of muscle length. One important feature of muscle spindle receptors is that they receive an efferent (gamma) innervation from the central nervous system through which their sensitivity to stretch can be changed. There is little direct evidence for pyramidal and cortical control of the gamma motor system on the basis of experiments using electrical stimulation. Most of the evidence for the control of the gamma motor system by the cortex and pyramidal tract stems from studies in which the motor cortex or pyramidal tract are ablated or lesioned and the resulting changes in posture are observed. The basis of these posture changes is the stretch or myotatic reflex (Liddell and Sherrington, 1924, 1925). When the muscle belly or tendon is lengthened, the receptor region of the muscle spindle is stretched. This results in a volley of Ia impulses being sent into the spinal cord to the motoneurons of the stretched muscle. As a result of the Ia excitation, the motoneurons discharge causing the stretched muscle to contract, tending to return to its original length.

Sherrington (1898, 1915, 1924) demonstrated the stretch reflex to be the basis of posture and resistance to gravity. Here the stretch is due to gravity and the resulting contraction of the extensor muscles results in the maintenance of an upright posture. The sensitivity of the spindle to stretch, hence the response of the muscles to stretch, can be influenced by the presence or absence of gamma motor activity. An increase of gamma motor activity can result in an increased stretch reflex and pathologically to rigidity or spasticity, while a decrease in gamma activity can result in hypotonus. The literature in this area is confusing. The clinical data suffers from the lack of precision in lesion localization and absence of detailed histological follow-up of the patients. Though the ability to produce more discrete lesions is greater in experimental animals, damage to areas peripheral to the intended lesion site cannot be prevented. Thus, these lesions are not truly restricted to a well defined site. Furthermore, in many studies there have not been adequate descriptions of the areas lesioned or ablated (Fulton, 1935).

The purpose of the present experiments was to study the effect of electrical stimulation of the motor cortex and pyramidal tract on the gamma motor system of the cat.


Scanned, with permission from the author, from the original print version, which resides in University Archives.


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