Dynamic properties of cat tenuissimus muscle

Some of the factors which influence the development of tension in cat tenuissimus muscle were studied quantitatively. Under isometric conditions, it was shown that the dynamic properties of the relationship between the tension of the muscle and its electrical stimulation depend on the mean rate of stimulation. This non-linear effect cannot be explained on the basis of the dependence of muscle tension on instantaneous rate of stimulation since the tension due to a stimulus following closely a previous stimulus is augmented, but the time course of the twitch response is unaltered. The interaction between the tension due to active contraction and that due to the viscoelastic properties of the muscle was investigated by independently varying muscle length and the rate of stimulation. Within the limits of resolution of the data, it was concluded that these two components of tension are additive and that muscle stiffness is related to the instantaneous tension of the muscle.     

Multiple-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 300 complete and incomplete cat muscle spindles were examined in serial transverse sections of tenuissimus muscles in search of spindles with more than two nuclear bag intrafusal muscle fibers. Several histochemical and histological stains were used to identify the intrafusal fibers and assess their motor and sensory innervation. About 13% of the spindles contained either three or four bag fibers rather than the usual two. Every multiple-bag-fiber spindle possessed at least one nuclear bag1 and one nuclear bag2 fiber. The supernumerary bag fibers were either another bag1 and/or bag2 fiber, or a mixed bag fiber. The extra bag fibers had the usual morphologic and histochemical properties of cat nuclear bag fibers. All multiple-bag spindles received primary sensory innervation, and most had secondary sensory endings in addition. Their motor pattern was similar in the number, appearance and disposition of intrafusal motor endings to that of the usual two-bag-fiber spindles. Bag fibers of the same kind shared motor nerve supply in three multiple-bag spindles in which tracings of individual motor axons were obtained histologically. It is unclear whether any functional advantage is conveyed to a muscle spindle by its having more than one bag1 and one bag2 fiber.     

One-bag-fiber muscle spindles in tenuissimus muscles of the cat

Over 150 complete and 139 incomplete single muscle spindles were examined in serial transverse sections of cat tenuissimus muscles in search for spindles lacking one of the two types of nuclear bag intrafusal fiber. Several histochemical reactions were used to type the intrafusal muscle fibers and assess the spindle motor and sensory innervation. One complete spindle lacked a bag1 fiber, and another spindle lacked a bag2 fiber. Several incomplete spindles also lacked bag1 fibers. In addition, ten double tandem spindles contained one capsular unit each that lacked the bag1 fiber, and one triple tandem spindle had two such capsules. All one-bag-fiber spindles had primary sensory innervation, but none had secondary sensory innervation. Their motor innervation was similar to that of the usual two-bag-fiber spindles in the number and disposition of intrafusal motor endings. It is unclear whether the one-bag fiber spindles, either single or tandem-linked, are products of an aberrant spindle development or represent a true anatomical and functional subcategory of the cat muscle spindle.     

One-bag-fiber muscle spindles in tenuissimus muscles of the cat

Summary Over 150 complete and 139 incomplete single muscle spindles were examined in serial transverse sections of cat tenuissimus muscles in search for spindles lacking one of the two types of nuclear bag intrafusal fiber. Several histochemical reactions were used to type the intrafusal muscle fibers and assess the spindle motor and sensory innervation. One complete spindle lacked a bag₁ fiber, and another spindle lacked a bag₂ fiber. Several incomplete spindles also lacked bag₁ fibers. In addition, ten double tandem spindles contained one capsular unit each that lacked the bag₁ fiber, and one triple tandem spindle had two such capsules. All one-bag-fiber spindles had primary sensory innervation, but none had secondary sensory innervation. Their motor innervation was similar to that of the usual two-bag-fiber spindles in the number and disposition of intrafusal motor endings. It is unclear whether the one-bag fiber spindles, either single or tandem-linked, are products of an aberrant spindle development or represent a true anatomical and functional subcategory of the cat muscle spindle.     

Morphometric studies on tenuissimus muscle spindles in the cat

Muscle spindles were studied histochemically in serial transverse sections of 42 cat tenuissimus muscle specimens. Staining for myofibrillar adenosine triphosphatase was employed to identify nuclear bag 1, nuclear bag 2, and nuclear chain intrafusal muscle fibers. The nuclear chain fibers were further subdivided into three categories according to their polar length and the intensity of their staining for nicotinamide adenine dinucleotide tetrazolium reductase. A total of 430 spindle poles were surveyed. The mean spindle content of bag 1, bag 2, and chain fibers was established. The mean polar length of intrafusal fibers as well as that of the intracapsular and extracapsular spindle regions was determined. A cholinesterase (ChE) staining technique was used to demonstrate the termination sites of motor axons along intrafusal fibers. Two types of circumscribed ChE deposits. The "rim" and the "plate," occurred on the fibers. The nuclear chain fibers usually carried both the ChE rims and plates, while most nuclear fibers displayed only the plates. The ChE plates were assessed in term of their appearance, staining intensity, length, and location along the fibers. The mean number of ChE plates found along the fibers was established for each of the various intrafusal fiber types. These histochemical observations are discussed with regard to the current concepts of cat spindle morphology and motor innervation. The results suggest a degree of predictability in the spindle fiber content and in the distribution of motor nerve terminals along intrafusal muscle fibers, at least in the tenuissimus muscle.     

The organization of muscle spindles in the tenuissimus muscle of the cat during late development

Intrafusal muscle fibers in the tenuissimus muscle of the cat develop as two separate groups; one being a single nuclear bag fiber while the other comprises a second nuclear bag fiber along with all the nuclear chain fibers. The groupings are very distinctive in the late fetus (55 days gestation) and remain so until 18 days of age. In the adult, the grouping is less distinctive but can often be recognized and followed for considerable distances within the capsular region of the spindle. Each group develops under its own basement membrane and is separated from the other by fibrocytes. ATPase histochemistry indicates the isolated single nuclear bag fiber is slow twitch while the fibers of the other group, the second bag and all the nuclear chains, are fast twitch. The organization of intrafusal fibers in late development into two groups of different fiber types is discussed in relation to their selective innervation by γ fibers.     

Observations on the primary sensory ending of tenuissimus muscle spindles in the cat

Summary The arrangement of preterminal and terminal axon branches in the primary sensory endings of cat tenuissimus muscle spindles was studied using whole-mount and serial-section techniques. Although in every case one firstorder preterminal branch was supplied exclusively to the bag₁ type of intrafusal muscle fibre, the preterminal branching patterns differed considerably in detail.Terminals varied widely in size and location. Their precise form varied according to their position on the intrafusal muscle fibres rather than their relationship to preterminal branches. Terminals derived from separate preterminal branches remained separate and did not fuse with themselves or each other. Individually bag₁ fibres had most terminals, chain fibres least. The surface of the muscle fibres were differentially indented by the terminals, least in bag₁ fibres and most in chain fibres.The results are discussed in relation to mechanosensory transduction and to the factors involved in determining the form of the primary ending.     

Motor innervation of the cat muscle spindle studied by the cholinesterase technique

Summary Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. Myofibrillar adenosine triphosphatase staining reaction was used to identify nuclear bag₁, nuclear bag₂ and nuclear chain intrafusal muscle fibers. Typical chain fibers and long chain fibers were distinguished, the latter extending for more than 1,000 m beyond the termination of the spindle capsule. Simple rim and more elaborate plate deposits were demonstrated histochemically along the poles of the typical chain fibers in staining for cholinesterases. They were considered to correspond, respectively, to the trail and plate motor nerve terminals. Most long chain fibers and the majority of nuclear bag fibers had their motor innervation limited to plate-type endings. In addition, faint diffuse cholinesterase staining occurred along the spindle capsule and the surface of some intrafusal fibers. These histochemical observations are discussed with regard to the current concepts concerning the morphological and functional organization of the motor innervation of the cat muscle spindle.     

The topography of long nuclear chain intrafusal fibers in the cat muscle spindle

Summary Muscle spindles were studied histochemically in serial transverse sections of specimens of the cat tenuissimus muscle. The nuclear chain intrafusal muscles fibers were separated into three subtypes, called long, intermediate and typical. The long chain and intermediate chain fibers tended to assume a particular position within the axial bundle of intrafusal fibers. The fibers were usually located in that layer of chain fibers that was positioned farthest away from the bag₂ fiber. Furthermore, they were usually situated adjacent to the bag₁ fiber throughout much of the extent of the spindle pole. Some long chain and intermediate chain fibers had several fiber nuclei abreast at the equator rather than a single row of central nuclei, as in most nuclear chain fibers. The relative position of intrafusal fibers within the cat spindle may reflect their order of formation during development, with the fibers retaining, to a variable degree, their association with the bag₂ fiber which acted as template. Thus, the axial position of long chain and intermediate chain fibers suggests that they are among the first nuclear chain fibers to form. This may play a role in the known preferential innervation of these chain fibers by skeleto-fusimotor axons.     

The topography of long nuclear chain intrafusal fibers in the cat muscle spindle

Muscle spindles were studied histochemically in serial transverse sections of specimens of the cat tenuissimus muscle. The nuclear chain intrafusal muscle fibers were separated into three subtypes, called long, intermediate and typical. The long chain and intermediate chain fibers tended to assume a particular position within the axial bundle of intrafusal fibers. The fibers were usually located in that layer of chain fibers that was positioned farthest away from the bag2 fiber. Furthermore, they were usually situated adjacent to the bag1 fiber throughout much of the extent of the spindle pole. Some long chain and intermediate chain fibers had several fiber nuclei abreast at the equator rather than a single row of central nuclei, as in most nuclear chain fibers. The relative position of intrafusal fibers within the cat spindle may reflect their order of formation during development, with the fibers retaining, to a variable degree, their association with the bag2 fiber which acted as template. Thus, the axial position of long chain and intermediate chain fibers suggests that they are among the first nuclear chain fibers to form. This may play a role in the known preferential innervation of these chain fibers by skeleto-fusimotor axons.     

Ultrastructure of dynamic and static skeletofusimotor endings in a cat muscle spindle

Summary Endings of four skeletofusimotor axons in a spindle of the cat tenuissimus muscle were examined in semithin (1-m thick) and ultrathin transverse serial sections. Two (dynamic) axons terminated on the nuclear bag₁ intrafusal muscle fiber and on extrafusal fibers of the dark type. Two (static) axons terminated on the nuclear chain intrafusal fibers and extrafusal fibers of the intermediate type. The degree of indentation of axon terminals into the muscle surface, thickness of the sole plate and extent of folding of subjunctional membranes differed among intrafusal and extrafusal terminations of the same axon. Endings of axons on the bag₁ and chain fibers were also morphologically dissimilar. Motor axons may not determine ending morphology. Rather the form and structure of a bag₁ or chain ending may be determined by the type of intrafusal fiber on which the ending lies and the ending's distance from the primary sensory axon.     

Tetanic depression in fast motor units of the cat gastrocnemius muscle.

Ability of muscle fibers to generate force is decreased when higher frequency of stimulation of motor units immediately follows lower frequency. This phenomenon called tetanic depression was found in rat medial gastrocnemius. However, it was not clear whether tetanic depression occurred only in rat muscle or it concerns all mammals. This study was conducted on motor units of cat medial gastrocnemius. Analyses were made at three successive trains of stimulation: 30 Hz, 20 and 30 Hz and again 30 Hz (the first pattern) or 40 Hz, 25 and 40 Hz and 40 Hz (the second pattern). In all fast units force generated within the middle tetanus was lower than force generated at the same, but constant frequency of stimulation applied earlier or later. The mean tetanic depression in 30 Hz tetani amounted to 10.9% for fast fatigable (FF) and 15.9% for fast resistant (FR) motor units, whereas in 40 Hz tetani mean values were 5.6% and 7.3% for FF and FR motor units, respectively. In slow motor units tetanic depression was not observed. These results proved the existence of tetanic depression in the feline muscle and indicated that its intensity depends on the fusion of tetanus. It has been concluded, that the tetanic depression is a general property of fast motor units in mammals.     

Discharge characteristics of spindle receptors located in different parts of the cat soleus muscle

Discharge characteristics of 48 receptors of passive muscle spindles during gradual stretching of the soleus muscle were studied in anesthetized cats. The position of the receptor was determined by electrical stimulation of the muscle surface, by pressure on the muscle, by stretching of individual bundles of fibers, by intramuscular electrical stimulation, and also by coagulation of areas of muscle leading to disappearance of discharges of the monitored unit. Primary and secondary endings were found in which dynamic and static sensitivity could be distinguished and were independent of localization. Primary endings with identical functional properties were found in both the proximal and middle parts of the muscle. It is postulated that lower or higher dynamic and static sensitivity of receptors depends not on the location of muscle spindles in the soleus muscle, but on differences in the density of afferent terminal contacts with three types of intrafusal fibers.     

Force-velocity properties of fatigue-resistant units in cat fast-twitch muscle after fatigue

The isometric and force-velocity properties of an identified and uniform population of fast-twitch, fatigue-resistant (FR) fibers within the flexor digitorum longus (FDL) muscle were investigated before, immediately after, and during recovery from a fatiguing repetitive isometric stimulus regime (40 Hz for 330 ms every s for 180 s) in the anesthetized cat. It was necessary to determine the smallest fraction of muscle that had the same force-velocity properties as the whole muscle. This was approximately 15% for FDL; if the fraction was less, the maximum speed of shortening was depressed and the a/Po value increased. Motor units were enlarged by partial denervation of the muscle, causing the intact motoneurons to sprout and incorporate more muscle fibers; FR units showed the greatest increase. Immediately after the fatigue regime, maximum isometric tetanic tension declined to 67% but subsequently recovered to 90% of the control value by the end of the 60-min recovery period. Maximum speed of shortening dropped to 71% of the control but after 30 min had recovered and did not differ significantly from control values. It is concluded that the capacity for recovery from fatigue is greater for FR units than for a whole muscle, which also contains fast-fatiguable units, and that the mechanisms involved in the recovery of the maximum isometric tension and maximum speed of shortening are independently regulated.     

The occurrence of "mixed" nuclear bag intrafusal fibers in the cat muscle spindle

A total of 147 muscle spindles was studied histochemically in serial transverse sections of 42 cat tenuissimus muscle specimens. Nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers were distinguished by the differential staining resulting from the reactions for myosin adenosine 5'-triphosphatase and nicotinamide adenine dinucleotide tetrazolium reductase. The majority of intrafusal fibers were of the same histochemical type at both fiber poles. However, seven muscle spindles contained one nuclear bag fiber each that presented as a bag1 in one pole and as a bag2 in the other pole. These "mixed" nuclear bag fibers were found in spindles that also contained at least one bag1 and one bag2 fiber of equivalent histochemical presentation in both fiber poles. The "mixed" bag fibers displayed differences of apparent fiber diameter and relative polar length between the two fiber poles. The motor innervation pattern, as revealed by staining for cholinesterase, was also dissimilar between the two poles of "mixed" bag fibers. The study indicates that the spindle equatorial region may in some instances serve as a boundary between two morphologically and histochemically different poles of the same intrafusal fiber.     

Plane of vertebral movement eliciting muscle lengthening history in the low back influences the decrease in muscle spindle responsiveness of the cat

Proprioceptive feedback is thought to play a significant role in controlling both lumbopelvic and intervertebral orientations. In the lumbar spine, a vertebra's positional history along the dorsal-ventral axis has been shown to alter the position, movement, and velocity sensitivity of muscle spindles in the multifidus and longissimus muscles. These effects appear due to muscle history. Because spinal motion segments have up to 6 degrees of freedom for movement, we were interested in whether the axis along which the history is applied differentially affects paraspinal muscle spindles. We tested the null hypothesis that the loading axis, which creates a vertebra's positional history, has no effect on a lumbar muscle spindle's subsequent response to vertebral position or movement. Identical displacements were applied along three orthogonal axes directly at the L(6) spinous process using a feedback motor system under displacement control. Single-unit nerve activity was recorded from 60 muscle spindle afferents in teased filaments from L(6) dorsal rootlets innervating intact longissimus or multifidus muscles of deeply anesthetized cats. Muscle lengthening histories along the caudal-cranial and dorsal-ventral axis, compared with the left-right axis, produced significantly greater reductions in spindle responses to vertebral position and movement. The spinal anatomy suggested that the effect of a lengthening history is greatest when that history had occurred along an axis lying within the anatomical plane of the facet joint. Speculation is made that the interaction between normal spinal mechanics and the inherent thixotropic property of muscle spindles poses a challenge for feedback and feedforward motor control of the lumbar spine.     

Interspecies differences of motor units properties in the medial gastrocnemius muscle of cat and rat

The purpose of the study was to analyze the interspecies differences of motor unit contractile properties in two most frequently studied mammals: cats and rats. A total sample of 166 motor units (79 in cats and 85 in rats) was investigated in the medial gastrocnemius muscle. Considerable differences were found in composition of the studied muscle. In cats, fast fatigable, fast resistant and slow units formed 68, 18 and 14% of the investigated population, whereas in rats 36, 52 and 12%, respectively. The contraction and relaxation times of motor units in the cat muscle were evidently longer than in the rat and the border values for fast/slow motor units division in these species were 44 and 20 ms, respectively. The mean values of twitch and tetanic forces appeared to be 7-8 times lower in rats, for fast, while 2-5 times for slow motor units. Also variability between the strongest and the weakest units within each type revealed differences 10-60 times in cats, whereas only 3.5-14 times in rats. The summation of twitches into tetanus for fast units was comparable in both species, but for S units was evidently more effective in the cat. In fast motor units' tetanic contractions evident interspecies differences concerned sag appearance and profiles of unfused tetani of FF and FR units. Differences in contractile properties described in the study may depend on the size, number and innervation ratio of motor units in the muscle of cat and rat, as well as their biochemical variability. Differences in composition of motor unit types and uneven mechanisms of force development may reflect biological adaptation to variable behaviour of cats and rats.