CENTRAL CONTROL OF RECEPTORS AND SENSORY TRANSMISSION SYSTEMS 



743 



Efferenl Cunlinl nf Irivetiehrale Stretch Rcajitors 



Another central control mechanism relating to 

 peripheral afferent nerve discharge has been dem- 

 onstrated in crustacean stretch receptors by Kuffler & 

 E\zaguirre (49)- They have shown that the stretch- 

 sensitive muscle afferent in the crayfish tail is itself 

 innervated by an efferent inhibitory ner\e fiber which 

 can diminish or arrest the activity of the afferent fiber. 

 The afferent ner\e discharge that is ordinarily elicited 

 by a given muscle stretch can be decreased or oblit- 

 erated depending on the rate and number of impulses 

 delivered to the inhibitory fiber. Presumabh' this sort 

 of control can be effected by central ganglia in the 

 intact cravfish. 



Efferenl Crjiitrol oj Mainmalnin Stretch Receptors 



The rate of discharge of the large mammalian 

 muscle-spindle afferent apparentK' depends upon the 

 degree of tension developed by a small intrafusal 

 mu.sclc fiber contained within the spindle. This intra- 

 fusal fiber can be passively stretched or relaxed along 

 with lengthening or shortening of the surrounding 

 skeletal muscle. In addition, it has its own motor con- 

 trol by way of the small ventral root gamma efferents 

 (40, 50-52). Thus, the discharge of spindle afferents, 

 which play such an important role in proprioception, 

 is determined both by the state of the skeletal muscle 

 and by the rate of discharge of the gamma efferents. 



The gamma efferents enable the spindle afferents 

 to have a full range of discharge rates for any given 

 muscle length, the end result being a better accom- 

 modation of different loads and rates of movements. 

 It can readily be appreciated that this peripheral 

 feedback or loop-gain system provides an exceedingly 

 important measure of central control over sensory 

 input. 



Remote Central Control of Stretch Receptors 



Granit & Kaada (30) discovered that the gamma 

 efferents controlling muscle-spindle afferents are in 

 turn regulated by a number of remote central struc- 

 tures. As shown in figure i, muscle-spindle discharges 

 are readily accelerated by stimulating the mesence- 

 phalic and diencephalic reticular formation — the 

 brain-stem facilitatory region of Magoun (28, 30). 

 When these structures are activated, a mu.scle-spindle 

 afferent will continue to show facilitation for up to 

 half a minute or more following discontinuation of 

 the brain-stem excitation. Similar but less uniform 



13 



16 



FIG I. Effect of brain-stem reticular (midbrain tegmentum) 

 stimulation on a gastrocnemius muscle spindle afferent dis- 

 charge. Above: Contraction of 134 gm at low myograph sensi- 

 tivity to demonstrate silent period of the large muscle spindle 

 afferent unit. Initial tension throughout, 52 gm. Light Dial- 

 chloralose anesthesia. / to ./: Control before reticular stimula- 

 tion. 5 to //: During stimulation. 12 to 31: After stimulation. 

 Consecutive sweeps at 2 sec. intervals. Myograph (M) alongside 

 film. Distance M-B (base line) corresponds to 10 gm. Note that 

 stimulation of the brain-stem reticular formation, without 

 altering the muscle tension, accelerates the spindle's rate of 

 firing and that this effect persists more than half a minute. 

 [From Granit & Kaada (30).] 



