Muscle spindle afferent fibers |
The primary and secondary muscle spindle afferent fibers both arise from a specialized structure within the muscle, the muscle spindle, a fusiform structure 4-7 mm long and 80-200 m in diameter. The spindles are located deep within the muscle mass, scattered widely through the muscle body, and attached to the tendon, the endomysium or the perimysium, so as to be in parallel with the extrafusal or regular muscle fibers. Although spindles are scattered widely in muscles, they are not found throughout.
- primary and secondary 근방추 구심섬유는 근육내에 특별한 구조이고, 방추상 형태로 길이는 4-7mm, 직경은 80-200 ??임.
- 근방추는 근육내에 깊이 자리하고, 근육체에 넓게 흩어져 있고, 힘줄에 연결되어 있고, 근섬유(추외근섬유)와 평행하게 존재함.
- 근방추가 근육내에서 넓게 흩어져 있을지라도, 모든곳에서 발견되지는 않음.
Figure 11-1 shows the distribution of spindles in the medial gastrocnemius of the cat, in dorsal (A), and midsagittal projections (B), and for comparison the location of Golgi tendon organs (C).
The aponeuroses of the muscle are indicated by shaded areas. It can be seen that nearly 40% of the muscle is devoid of spindles, especially the marginal areas. A drawing of a muscle spindle is shown in Figure 11-2.
- 근육의 건막은 shaded 지역이 있음. 근육의 40%는 근방추가 없음. 특히 가장자리쪽에..
It contains two types of intrafusal muscle fibers (intrafusal = inside the fusiform spindle): the nuclear bag fibers and the nuclear chain fibers. The nuclear bag fibers are thicker and longer than the nuclear chain fibers, and they receive their name from the accumulation of their nuclei in the expanded bag-like equatorial region-the nuclear bag. The nuclear chain fibers have no equatorial bulge; rather their nuclei are lined up in the equatorial region-the nuclear chain. This distinction is illustrated in Figure 11-3. A typical spindle contains two nuclear bag fibers and 4-5 nuclear chain fibers.
- 추내근 섬유에는 핵대 섬유와 핵사슬섬유 두가지가 있음.
- 핵대섬유는 핵사슬 섬유보다 길고 두꺼움.
- 전형적인 근방추는 2개의 핵대섬유와 4-5개의 핵사슬섬유를 가짐.
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Fig. 11-2. Drawing of a muscle spndle to show the nature of attachment, the arrangement of the intrafusal fibers, and how the afferent and efferent fibers enter the spindle. |
The sensory innervation of the muscle spindle arises from both group Ia and group II afferent fibers. As shown in Figure 11-3, a single, large group Ia fiber coils around the equatorial regions of both nuclear bag and nuclear chain fibers, forming the annulospiral endings or primary muscle spindle receptors.
- 근방추의 감각신경지배는 Ia and group II 구심성 섬유로 구성됨.
- Ia 섬유는 핵대섬유와 핵사슬섬유의 팽대부를 코일처럼 감싸고 있고, annulospiral endings or primary muscle spindle receptors를 이룸.
There appears to be only one group Ia afferent fiber per spindle, but every intrafusal muscle fiber within that spindle receives innervation from that fiber. Current thought is that all group Ia afferent fibers form annulospiral endings, and therefore the terms primary muscle spindle afferent fiber and group Ia afferent fiber are used interchangeably.
The smaller group II fibers terminate at either end of the nuclear region primarily on the nuclear chain fibers (there is apparently some innervation of bag fibers by secondary muscle spindle afferent fibers, but there is disagreement as to how much); they form flower-spray endings or secondary muscle spindle receptors. There usually are several group II fibers innervating each spindle. Not all group II fibers form such endings, so these terms are not synonymous.
- II2 섬유는 핵사슬섬유위 nuclear 지역의 끝에 종지함. - (논란이 있지만 secondary근방추에 의해서 핵대섬유에 신경지배 하고 있음)
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Fig. 11-3. A nuclear bag and a nuclear chain fiber showing their innervation by group Ia and Group II afferent fibers and gamma motoneurons. (Matthews PBC: Physiol Rev44:219-288, 1964) |
The intrafusal fibers are striated muscle fibers that receive innervation from the fusimotor neurons or gamma-motoneurons. Activity in fusimotor neurons produces a contraction of the striated, polar regions of the bag and chain fibers, putting stretch on the equatorial region (where the receptor regions are) that has few myofibrils and therefore, has little contractility.
- 추내근 섬유는 줄무늬 근섬유로서 감마운동신경원(fusimotor 신경원)으로부터 신경지배
- fusimotor 신경원 활성은 추내근 핵대섬유와 핵사슬섬유의 극성부위의 수축을 일으킴. 핵사슬섬유와 핵대섬유의 중앙부에 약간의 근원섬유가 있어 약간의 수축능을 가짐.
It is apparently the stretching of this central region, regardless of how it is accomplished, that is the adequate stimulus for both primary and secondary spindle receptors.
- 근방추 중앙부위의 늘어남은 primary and secondary 근방추 수용기에 최적의 자극임.
The fusimotor neurons or gamma-motoneurons should not be confused with the larger skeletomotor neurons or alpha-motoneurons, whose activity produces contraction of the extrafusal fibers that do the work of the muscle.
- 감마운동신경원과 알파운동신경원은 전혀 다름.
The difference in diameter of fusimotor (gamma-motoneurons) and skeletomotor fibers (alpha-motoneurons) is illustrated in Figure 11-4. The former average about 5 microns in diameter, the latter 13 microns. Contraction of all the intrafusal fibers at once does not produce any measurable tension in the muscle. The intrafusal fibers are much shorter than the extrafusal fibers, 4-7 millimeters compared with 3-50 centimeters. Extrafusal muscle fibers can shorten by as much as 40% of their resting length, which for a 20-cm muscle would be 8 cm. The largest intrafusal fiber, shortening by the same percentage, would only change length by 2.8 mm. The extrafusal fiber is therefore capable of a 30-fold greater change in length. The average human striated muscle fiber has a diameter of about 60 micrometers; the intrafusal fiber averages about 10 micrometers.
- 감마운동신경원과 알파운동신경원의 직경차이는 11-4 그림에 표시.
- 감마 운동신경원의 직경은 평균 5 마이크론(100만분의 1 미터), 알파운동신경원은 평균 13마이크론
- 모든 추내근섬유 수축은 단번에 측정가능한 근수축 장력을 생산하지 않음.
- 추내근섬유는 4-7mm, 추외근 섬유는 3-50cm길이임.
- 추외근 섬유는 안정길이의 40%만큼 길이가 줄어들 수 있음. 가장 큰 추내근섬유는 같은 비율로 길이가 줄어들 수 있는데, 7mm의 40%인 2.8mm 가능함.
- 추외근 섬유 직경은 60마이크로미터 (100만분의 1미터), 추내근섬유는 10micrometers임.
참고) <물리> 미터법에 의한 길이의 단위. 1마이크로미터는 1미터의 100만분의 1이다. 1967년 국제 도량형 총회에서 폐지된 ‘미크론’ 대신에 사용한다. 기호는 ㎛.
Because the force produced by a skeletal muscle is proportional to its cross-sectional area, the extrafusal fiber is capable of producing at least 36 times more force than the intrafusal fiber. Add these factors to the relative numbers of intrafusal (in cat soleus muscle about 300) and extrafusal fibers (again in cat soleus muscle about 25,000), and it is not hard to see why intrafusal fibers do not generate much force.
- 골격근에 의해 생성되는 힘은 골격근의 단면적 비율이기 때문에, 추외근섬유는 추내근섬유보다 최소한 36배의 힘을 더 생산할 수 있음.
- 게다가 추내근섬유는 300개, 추외근 섬유는 2만5천개(쥐의 가자미근기준)이므로 추내근섬유가왜 그렇게 큰힘을 내지 못하는가는 쉽게 이해할 수 있음.
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Fig. 11-4. Fiber spectrum of efferent portion of a muscle nerve. Indicated are A alpha and A gamma fibers, fusimotor and skeletomotor fibers. (Boyd IA, Davey MR: Composition of Peripheral Nerves. Edinburgh, Livingstone, 1968) |
It also appears that the mechanism for generating force in intrafusal muscle fibers may be different than in extrafusal fibers. There are no action potentials in intrafusal fibers as there are in extrafusal fibers (see Chapter 14), with a consequence that one striated end of an intrafusal fiber may contract without the other end doing so. This cannot happen in normal extrafusal fibers.
- 추내근섬유 힘생산 기전은 추내근섬유와는 다름.
- 추내근섬유는 추외근섬유와 같은 활동전위가 없음.
Stretching the equatorial region of the muscle spindle, the adequate stimulus for the receptor, may be accomplished by gamma-motoneuron activation and intrafusal muscle contraction. Another way to stretch the equatorial region of the spindle is to stretch the muscle (Fig. 11-5) and thereby stretch the entire spindle, because it is attached in parallelwith extrafusal muscle fibers.
- 근방추의 중앙부위 늘어남은 근방추 수용기에 적절한 자극이고 감마운동신경원을 활성화시켜 추내근섬유 수축을 일으킴.
- 근방추 중앙부위 늘어남을 일으키는 다른 방법은 근육이 늘어나는 것임. 추외근 섬유와 근방추는 수평배열이므로 근육이 늘어나면 전체 근방추가 늘어남.
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Fig. 11-5. Drawing depicting the stretch of the nuclear region of the spindle caused by stretching the muscle (upper pair) and by stimulation of gamma motoneurons, causing the striated intrafusal fibers to contract (lower pair). |
Muscle spindle receptors respond to stretch of the muscle and signal muscle length and rate of change of length to the central nervous system. Both primary and secondary spindle afferent fibers give static or length-sensitive responses to stretch, i.e., they respond to maintained stretch in a sustained (tonic) fashion at a discharge frequency proportional to the length of the muscle (Fig. 11-6). Both primary and secondary muscle spindle afferent fibers usually discharge tonically when the muscle is at its resting length. When the muscle is stretched and held at some new length (left side of figure, lengthening is an upward deflection of stimulus trace), both types increase their discharge rates and maintain a discharge for as long as the new muscle length is maintained (an example of a slowly adapting response).
- 근방추 수용기는 근육의 늘어남에 반응하고, 단일 근육길이와 길이변화비율이 대뇌에 전달됨.
- primary and secondary 근방추 구심섬유는 정적인 또는 스트레치에 의한 길이-민감성 반응을 제공함.
- 근방추 구심섬유는 등장성 상황에서 유지되는 늘어남에 반응함. 11-6 그림과 같이 근육길이에 따른 방전주파수 비율에 의해..
- 근방추섬유는 근육이 안정길이에 있을때 일반적으로 discharge됨.
- 근육이 늘어나고 새로운 근육길이에서 유지할때, .. 새로운 근육길이가 유지하는 만큼 .. 그림과 같음..
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Fig. 11-6. Responses of spindle afferent fibers to muscle stretch. A monitor of the stretch (lengthening, upward deflection) is shown in the upper trace. The response of a primary spindle afferent fiber is shown in the second trace, that of a secondary spindle afferent fiber in the third. (Matthews PBC: Physiol Rev 44:219-288, 1964) |
When the muscle is returned to its resting length (right side of figure), the primary spindle afferent fiber is observed to stop discharging briefly and then begin again at a frequency appropriate for its new resting length, whereas the secondary spindle fiber discharge does not stop, but just slows to the appropriate rate.
- 근육이 안정길이로 되돌아갈때, primary 근방추 구심섬유는 잠깐 discharging을 멈추는 것이 관찰됨. 그리고 다시 이어짐. 반면에 secondary 근방추 섬유는 멈춤이 없고, 비율이 줄어듬.
This is illustrated in Figure 11-6, where the monitor of the muscle length is shown in the top trace and the responses of a primary and secondary spindle afferent fiber are shown in the second and third traces. The greater the muscle length, the greater is the stretch on the spindle and the greater is the static discharge of either type of spindle afferent fiber. In fact, for static conditions, there is an approximately linear relationship between the rate of discharge of the afferent fibers and the length of the muscle.
- 위 그림에서 보는 바와같이 근육길이의 모니터는 ..
- 근육길이가 크면 클수록 근방추는 늘어나고, 근방추 구심섬유타입에 의해 정적 discharge가 있음.
- 사실 정적인 상황에서 근육길이와 구심섬유의 discharge 비율사이에는 직선적인 관계가 있음.
This is illustrated in Figure 11-7, which is a plot of discharge frequency versus length of the muscle, for a primary (filled circles) and a secondary spindle afferent fiber (filled squares)(2). The frequency of discharge increases monotonically and linearly with increases in muscle length. The slope of these relations is called the position-sensitivity and is usually about the same for primary and secondary spindle receptors.
- 아래 그림은 ..
- discharge 빈도수는 근육길이의 증가와 함께 직선적...
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Fig. 11-7. A plot of the frequency of static discharge of a primary and secondary muscle spindle against the length of the muscle. Recordings were made with ventral roots cut. Primary resposes are plotted with filled circles. (Jansen JKS, Matthews PBC: Acta Physiol Scand55:376-386, 1962) |
Only the primary muscle spindle afferent fiber gives a dynamic or velocity-sensitive response to muscle stretch. As the muscle length is being changed, the primary ending signals the rate at which it is being changed. The faster the muscle is stretched, the higher is the rate of discharge of the ending.
- 오직 primary 근방방추 구심섬유는 동적인 또는 속도-민감성 반응을 제공함.
- 근육길이가 변화함에 따라 primary end 신호 비율은 변화함.
- 근육스트레치가 빠르면, 근방추 섬유 말단의 discharge비율이 높아짐.
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Fig. 11-8. The dynamic response. The monitors of the stretch at two different rates (A and B) are shown in the upper traces. Note that both stretches start and end at the same muscle length. The responses of a primary ending are shown in the second traces; those of a secondary ending are shown in the third traces. Note the higher frequency of discharge of the primary ending at the higher rate of stretch (B). Note that the length of the muscle is changing in phase 2 but is constant in phases 1 and 3 of these records. |
Figure 11-8 shows the responses of both primary and secondary spindle endings to two stretches of the muscle at different rates. A monitor of the muscle length is shown in the upper traces, the response of the primary ending is shown in the second traces, and the response of the secondary ending is shown in the third traces of both A and B.
- 위그림은 두 근육 스트레치속도 비율에 따른 primary and secondary 말단의 반응을 보여줌.
Notice that the primary ending responds with a higher frequency during the faster stretch in B, even though the initial and final lengths are the same in each case. Also note that the rate of discharge decreases from its peak after the final length has been reached. This decrease defines the dynamic index, which is the difference between the dynamic response frequency for that rate of stretch and the static response frequency for that final length and serves as an indicator of rate sensitivity.
- B처럼 빠르게 스트레칭되는 동안 primary 말단이 높은 비율주파수반응을 보여주는 것을 주목
- 초기에는 비슷함.
- final 길이에 도달한 후 discharge의 비율이 파크에서 감소됨.
- 이 감소는 dynamic index로 정의됨. 이는 늘어남 비율의 동적반응 주파수와 final leng를 위한 정적반응주파수와의 차이임. 그리고 비율민감도의 지침이 됨.
When the velocity of stretch is zero, that is, when the muscle length is constant (phases 1 and 3 of Fig. 11-8), the discharge of the cell will be signaling only the length of the muscle. The primary spindle codes zero velocity with zero "velocity" discharge, but with an appropriate "length" discharge.
- 근육길이가 일정할때(스트레치 속도가 0일때), 세포의 discharge는 오직 근육길이 신호가 될것임.
When the muscle is changing length, that is, when the velocity is not zero, there will be a velocity response, as well as a length response that is appropriate for the length of the muscle at each given instant of time. Very near the end of phase 2 in Figure 11-8, the muscle has nearly reached its final length as in phase 3, but it is still changing length. Because its length is nearly that in phase 3, they can be taken as equal for a first approximation.
- 근육길이가 변화할때(스트레치 속도가 0이 아닐때), 속도반응이 있을 것임. 속도반응은 각각 주어진 순간에 근육의 길이에 적절하게 반응하는 것과 같이..
- 위 그림과 같이 phase 2의 끝에 근육은 final 길이에 도달함. 그러나 여전히 길이는 변화함.
This gives the following conditions:
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Thus, the dynamic index can be used as an indicator of the velocity response. Figure 11-9 is a plot of the dynamic index versus velocity (or rate) of stretch for both primary and secondary endings. The curve for secondary endings is flat compared with that for primary endings, indicating that this receptor has little sensitivity to the velocity of stretch.
- 그래서 dynamic index는 속도반응의 계시기으로 사용될 수 있음.
- 아래 그림은 dynamic index 대 늘어남의 속도 도표임.
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Fig. 11-9. A plot of the dynamic index versus the rate of stretch for a primary and a secondary muscle spindle afferent fiber. Note the flatness of the secondary curve. (Matthews PBC: J Physiol (Lond) 168:660-678, 1963) |
It is the dynamic response capacity of the primary muscle spindle ending that provides for the vigorous response to tapping the muscle tendon as the physician does in reflex testing (Fig. 11-10). The tap rapidly stretches the spindles (upper trace) and the primary endings respond to this rapid rate of stretching (second trace). The secondary endings, because they lack dynamic sensitivity, respond little if at all to the tap, which produces only a small change in the length of the muscle (third trace). After a brief stretch, the spindle returns to its original length, and the discharge of the primary ending stops and then it returns to its prestretch rate of discharge. This behavior will be important in our consideration of the tendon tap reflex(3)(Chapter 15).
- 아래 그림과 같이 개의 반사검사로 힘줄을 tap하면 근방추의 동적 반응능력을 보여줌.
- primary ending은 tapping 순간 빠르게 근방추가 늘어나고 그림과 같이 반응함.
- secondary endings은 동적 민감도가 부족하기 때문에 힘줄을 가격해도 반응이 없음.
- 짧은 늘어남 후에 근방추는 원래 길이로 돌아오고, primary ending의 discharge는 멈추고 discharge 비율의 스트레치 전단계로 돌아감.
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Fig. 11-10. Responses of primary and secondary muscle spindle afferent fibers to tapping the tendon of the muscle that they innervate. Monitor of change in length caused by tap is shown in upper trace; upward is an increase. (Matthews PBC: Physiol Rev 44:219-288, 1964) |
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Fig. 11-11. Effect of gamma motoneuron discharges on spindle sensitivity. A. Monitor of the muscle length during a stretch. B. Response of a primary spindle ending to stretch. C. Response of the ending to the stretch during continuous stimulation of a single static fusimotor neuron at 70/sec. D. Response of the ending to the stretch during continuous stimulation of a single dynamic fusimotor neuron at 70/sec. (Crowe A, Matthews PBC: J Physiol (Lond) 174:109-131, 1964) |
Fusimotor effects on spindle afferent fiber discharges
Fusimotor neurons are physiologically separable into two groups based on their effects on the dynamic and static responses of the spindle afferent fibers. Activity in static fusimotor fibers increases the static responses of both primary and secondary spindle afferent fibers, whereas activity in dynamic fusimotor fibers increases the dynamic response in primary spindle receptors. These effects are illustrated in Figure 11-11.
- 감마운동신경원은 근방추 구심섬유의 동적, 정적 반응효과에 기초하여 두 그룹으로 나눔.
- 정적 감마운동 섬유 활성은 primary 와 secondary 근방추 구심섬유의 정적반응이 증가시킴. 반면에 동적 감마운동 섬유활성은 primary 근방추 수용기의 반응을 증가시킴. 11-11참고.
Trace A is again the monitor of muscle length and trace B is the response of a single primary spindle afferent fiber to the stretch. The effect is similar to that illustrated in Figure 11-5. During trace C, a single static fusimotor fiber was stimulated at 70/sec. The effect on the static stimulation was so marked that the cell increased its discharge at the initial length; it discharged more vigorously at its final length (but still signaled both lengths in its discharge); and the dynamic response was masked by the increase in the static response, resulting in a decrease in the dynamic index, the indicator of the velocity response. The same procedure, this time while stimulating a dynamic fusimotor fiber, produced a very marked change in the dynamic discharge, but had much less effect on the static response, although there was some increase (Fig. 11-11D).
- 그림 A선은 근육길이의 모니터임, 그림 B선은 단일 primary 근방추 구심섬유가 스트레치되는 반응임.
- 그림 C선은 단일 동적 감마운동신경섬유가 초당 70으로 자극됨. 정적 자극 효과는 .. 그림참조..
Such changes in the discharge properties show up more clearly perhaps when the muscle is stretched and relaxed in a sinusoidal fashion rather than a linear fashion. Records made of the discharge of a primary muscle spindle afferent fiber are shown in Figure 11-12. The first and fourth traces show the action potentials discharged by the fiber. The second and fifth traces are a monitor of the stretch (upward deflection) and relaxation (downward deflection) of the muscle at 1 mm peak-to-peak at 3 Hz.
- 근육이 스트레치 또는 이완될때 직선적인 반응이라기 보다는 사인파 형식을 보임.
This sinusoidal stimulus was applied to the muscle so that the zero position was approximately at the resting length, the muscle being stretched and relaxed equally around it. The two remaining traces show the period during the record when a dynamic fusimotor neuron (third trace) and a static fusimotor neuron (sixth trace) were stimulated (i.e., stimulation is on when the line is present, off when it is absent).
At the beginning of each record there is a period of time when there was no fusimotor stimulation. The group Ia fiber discharges during the stretch of the muscle and ceases its discharge when the muscle is relaxed. The average length of the muscle was not sufficiently great in this experiment to provoke a continuous discharge from the fiber. The effect of dynamic fusimotor activity was to increase the rate of discharge during the stretch portion of the stimulus, but it did not induce the cell to discharge during the relaxation phase of the sine wave stimulus. The static fusimotor activity, on the other hand, converted the bursting response into a continuous response that occurred at a frequency appropriate for the average length of the muscle during the stimulus. A similar effect could have been achieved by increasing the length of the muscle before the sinusoidal stimulus was applied, i.e., by setting the zero position of the sinusoid at some length longer than the resting length.
The effects of fusimotor activity on spindle receptors are summarized in Table 11-1. From the distribution of the primary and secondary spindle afferent fibers and the nature of the effects it is possible to deduce that dynamic fusimotor fibers innervate primarily nuclear bag fibers, whereas static fusimotor neurons innervate nuclear chain or nuclear bag fibers or both. In a study of single chain and bag fibers, it was found that stimulation of a single dynamic gamma-motoneuron caused contraction of only bag fibers; chain fibers were not affected. Static gamma-motoneurons caused contraction of bag fibers only, chain fibers only, or both depending upon the static gamma-motoneuron.
- 감마운동신경 활성의 효과는 아래 테이블에 정리됨.
Phenomenon observed | Dynamic neuron | Static neuron |
Static response of primary ending | Increase | Slightly larger increase |
Static response of secondary ending | No effect | Increase |
Dynamic response of primary ending | Increase | Decrease in dynamic index, though rate may increase |
Dynamic response of secondary ending | No effect | Remains small |
Frequency of occurrence | One-fourth of -fibers | Three-fourths of -fibers |
Intrafusal fiber type affected | Nuclear bag | Nuclear chain or bag or both |
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Fig. 11-12. The effect of fusimotor stimulation on the respnsiveness of the mammalian primary ending to a sinusoidal stretching of medium extent (1 mm peak-to-peak movement at 3 Hz). (Crowe A, Matthews PBC: J Physiol (Lond) 174:132-151, 1964) |
The important physiological effect of the fusimotor innervation of the spindles seems to be to alter the bias of the spindle afferent endings. Stimulation of a static fusimotor neuron increases the length discharge of both primary and secondary muscle spindle afferent fibers for any muscle length. This is shown in Figure 11-13A. The discharge rate without the fusimotor activity is plotted against muscle length as the lower line, as in Figure 11-7. With fusimotor activity a new relationship is generated parallel to, but slightly above the lower line. Actually, a family of such parallel lines would be generated by different rates of discharge in the gamma motoneuron. For any muscle length, the rate of discharge is greater with gamma activity than without, and it is greater by a constant amount.
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Fig. 11-13. Effects of fusimotor neuron activity on the sensitivity of the primary or secondary muscle spindle afferent fibers to length of the muscle (A) and of primary muscle spindle afferent fibers to rate of change of length of the muscle (B). |
Likewise, dynamic fusimotor neuron stimulation increases the velocity discharge of a primary muscle spindle afferent fiber for any velocity of stretch. Figure 11-13B shows two parallel curves relating velocity of stretch to the velocity discharge rate, expressed as the dynamic index.
The upper curve is the discharge of the spindle under the influence of a gamma-motoneuron discharge; the lower is without such influence. Again, there would be a family of such parallel curves for different rates of gamma-motoneuron discharge. The velocity discharge rate is greater for any velocity with gamma-motoneuron stimulation, and it is higher by a constant amount. The effect of both static and dynamic fusimotor neurons on the muscle spindle receptors is to increase their discharge, in the one case, their discharge for the length of the muscle (static fusimotor neurons on both primary and secondary spindle afferent fibers) and in the other case, their discharge for rates of change of the length of the muscle (dynamic fusimotor neurons on the primary spindle afferent fiber).
Many people are tempted to say that the gamma-motoneuron discharge increases the sensitivity of the spindle receptors, but actually the change is in a parameter called bias. A change in sensitivity would actually result in a change in the slope of the discharge rate-extension relation. The slopes of the relations with and without gamma-motoneuron activity are the same, the curves are parallel. The kind of change actually seen, i.e., that in Fig. 11-13A, is called a change in bias. Similarly, that seen in Fig. 11-13B is a change in bias.
It appears that there is ongoing fusimotor activity (gamma tone or gamma bias). Figure 11-14 shows the response of a primary spindle receptor to stretch (monitor in lower trace) when the ventral roots of the spinal cord are intact (second trace) and when they have been cut (upper trace). Recall that the gamma-motoneurons are efferent fibers, and they exit the spinal cord through ventral roots. The discharge of the primary spindle ending, both dynamic and static, is reduced by cutting the ventral root, suggesting that this fusimotor modulation is an ongoing thing, and that both dynamic and static fusimotor fibers are active in the absence of movement or stimulation. Position-discharges (the slope of the lines in Fig. 11-13A) can fall from as high as 10 impulses/sec/mm to four impulses/sec/mm when the ventral roots are cut. In addition, spindle receptors become silent at minimum lengths (see Fig. 11-7), a condition never obtained with ventral roots intact. Except during rapid shortening, primary endings discharge some spikes even at shortest muscle lengths in the presence of gamma bias.
It is reasonable to ask why sensory receptors like the muscle spindle receptors would have something like gamma bias. One likely reason can be seen by referring back to Fig. 11-13A. In the absence of gamma-motoneuron activity, the muscle spindle receptor would have a discharge of zero at a 1-mm length (extrapolate the lower curve back to the x-axis). In that condition, it would not be signaling anything to the brain about muscle length. But the job of the spindle receptors is to continuously signal muscle length.
The brain can make the receptor signal something even at these shorter lengths if it makes the gamma-motoneurons active. In the presence of gamma-motoneuron activity, the receptor would have a non-zero discharge (read upward to the upper curve at 1 mm). So, what the gamma activity does is to force the spindle receptors to work in the center of their operating ranges, not at the ends. But, you might ask, isn't the information that the brain is getting about length false information? In a way, it is. But remember that it was the CNS that sent out the gamma signal. It knows how large that signal was, and presumably it knows what effect that would have on the spindle receptors. It might be just a matter of subtracting a length corresponding to the gamma signal.
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Fig. 11-14. Response of a primary spindle ending to stretching the muscle. Lower trace is amonitor of the muscle length. Middle trace shows the response to the same stretch with ventral roots intact; upper trace, with ventral roots cut. (Jansen JKS, Matthews PBC: J Physiol (Lond) 161:357-378, 1962) |
Golgi tendon organs |
The third muscle receptor of concern to us is the Golgi tendon organ. This receptor lies near the muscle-tendon junction (Fig. 11-15) or buried deep within the tendon itself. The receptor consists of specializations of the terminals of the group Ib afferent fiber itself, with a delicate capsule that surrounds the nerve which, in turn, surrounds several fascicles of tendon. The nerve fibers lie between fascicles in such a way that they can be "pinched" between them as the force is increased. This is how this receptor is thought to be activated. Group Ib seems to be composed entirely of Golgi tendon organ afferent fibers, so these terms are used interchangeably.
- 세번째 근육 수용기는 골지건기관
- 골지건기관 수용기는 Ib 구심섬유 그룹으로 구성됨.
The Golgi tendon organ signals to the central nervous system the tension developed by the muscle during contraction or exerted on it during a stretch. For many years, it was thought that these receptors had high thresholds to muscle tension and participated in controlling muscle activity only at extremes of tension, functioning as a protective device. Actually, the tendon organs are relatively insensitive to tension applied to the muscle by stretching it, but they are extremely sensitive to tension developed by the muscle when it contracts. The reason for this is that the tension on tendon organs is different under passive and active conditions.
- 골지건기관 신호는 근육 수축이나 힘을 냄에 의해 발생하는 장력을 대뇌에 전달함.
- 오랫동안 골지건기관 수용기는 근육장력과 장력의 극단에 반응하여 근육을 보호하는 기관으로 이해됨.
- 사실, 골지건기관은 스트레칭에 의해서 근육에 주어진 압력에는 상대적으로 민감하지 않음. 하지만 근육이 수축할때 발생하는 장력에 대해서는 매우 민감함.
A muscle like the cat's soleus muscle has many extrafusal muscle fibers in it (about 25,000), but only a few Golgi tendon organs (about 45). Each tendon organ is arranged in series with a few muscle fibers (average 7-10). When the muscle is stretched, the tension applied is distributed across all 25,000 muscle fibers, but the seven fibers associated with each tendon organ only experience 7/25,000 to 10/25,000 of it-quite a small amount.
- 쥐의 가자미근은 약 2만 5천개의 추외근 섬유를 가지고, 골지건기관은 약 45개를 가짐.
- 각각 골지건기관은 몇개의 근섬유(평균 7-10개)에 연결적으로 이어짐.
- 근육이 늘어날때 장력은 2만 5천개 근섬유에 분산됨. 그러나 7개 섬유는 ....
Also, the material of the tendon in which the Golgi tendon organs are located is stiffer at rest than the muscle is. When the resting muscle is stretched, much of the tension applied is used in stretching the relatively compliant muscle fibers; the stiffer tendon is not much affected. In addition, the tension caused by stretching the muscle is exerted on the tendons at an angle to the Golgi tendon organs, so that only the component of the force parallel to the muscle fibers actually stretches the tendon organ. This can be small, especially in pennate muscles.
- 또한, 골지건기관이 존재하는 힘줄은 안정상황에서 근육이 그러한 것보다 더 단단함.
- 근육이 늘어날때, 장력적용의 양은 근섬유 늘어남을 이용함. 단단한 힘줄은 덜 영향받음.
- 게다가, 근육 스트레칭에 의해 야기된 장력은 ...
On the other hand, if the muscle contracts, the tension developed by the extrafusal fibers is transferred directly to the tendon organ associated with them. Not every tendon is innervated by a Golgi tendon organ. It appears that the system of tendon organs "samples" the average tension in various parts of the muscle in the same way spindle afferent fibers sample its length.
- 반면에, 만약 근육이 수축한다면 추외근섬유에 의해 생성된 장력은 골지건기관에 직접적으로 전달됨. 모든 힘줄이 골지건기관에 의해 신경지배되지는 않음.
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Fig. 11-15. The location of the Golgi tendon organ at the muscle-tendon junction. |
The response to muscle stretch (monitor in lower trace) of a Golgi tendon organ (second trace) is compared with that of a primary spindle ending (upper trace) in Figure 11-16. The tendon organ has both a dynamic and a static response, but it, of course, is signaling tension and rate of change of tension, rather than the length or rate of change of length that the primary muscle spindle signals. Because of the series elastic properties of the muscle, the tendon organ does not begin to discharge until long (in neurophysiological time) after the spindle ending has begun to discharge. The real distinction between the response of the two receptors is seen when an isometric twitch contraction is elicited in the muscle (Fig. 11-16).
- 골지건기관의 근육 스트레치 반응은 primary 근방추 ending에 비견함.
- 골지건기관은 동적 그리고 정적 반응을 가짐. 그러나 primary 근방추 신호와 관련된 길이변화 비율보다는 장력변화와 관련함.
When the extrafusal fibers shorten (monitor in upper trace), the spindle is unloaded because it is attached in parallel, and the equatorial region relaxes. The result is that the primary spindle ending stops discharging (middle trace). On the other hand, when the muscle contracts isometrically, it develops tension, and the in-series-attached Golgi tendon organ increases its discharge (lower trace).
- 추외근 섬유가 짧아질때, 근방추는 부하가 줄어듬. 왜냐하면 그것은 추외근섬유와 평행하고 중앙부위가 이완되기 때문.
- 반면에 근육이 등척성 수축을 할때, 장력이 발생하고, 골지건기관은 그것의 discharge가 증가함.
The differences between the three types of muscle receptors are summarized in Table 11-2. Although differences in location and sensitivity to different forms of stimulation exist, these receptors work together in controlling the activity of the muscles with which they are associated, as we shall see in another chapter.
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Fig. 11-16. Comparison of the responses of a primary spindle ending and a tendon organ. A monitor of the muscle stretch is shown in the lower trace and the response of the spindle ending in the upper trace. The response of the tendon organ is shown in the middle trace. Note the later start in the discharge of the tendon organ and the less vigorous response. (Matthews BHC: J Physiol (Lond) 78:1-53, 1933) |
Anatomical evidence suggests that among muscle fascicles that contain muscle spindles, Golgi tendon organs are usually present at the muscle-tendon junction. This is shown in Figure 11-1B and C. This relationship suggests an even closer cooperation than was previously expected.
- 해부학적 증거는 근육섬유 다발이 근방추, 골지건기관을 포함하는데, 그것은 근-건 접합에 존재함.
Property | Primary Ending | Secondary Ending | Golgi Tendon Organ |
1. Location | Mid-equatorial region of bag and chain fibers in spindles | Juxta-equatorial region of chain fibers in spindles | Muscle-tendon junction |
2. Afferent fiber | Large, group Ia | Small, group II | Large, group Ib |
3. Efferent control | Both static and dynamic fusimotor | Static fusimotor | None known |
4. Response to ramp stretch with plateau | Dynamic and static (signals length) | Static (signals length) | Dynamic and static (signals tension) |
5. Response to release of stretch | Abrupt silence | Progressive decrease | Abrupt silence |
6. Response to tendon tap | Low threshold, vigorous | High threshold, little | High threshold, vigorous if threshold is exceeded |
7. Sensitivity to small stretches | High, especially if rapid | Low | Low |
8. Response to twitch contractions | Abrupt silence | Abrupt silence | Vigorous discharge |
9. Signals | Muscle length and rate of change of length | Muscle length | Muscle tension and rate of change of tension |
Summary |
Most mammalian striated muscles are supplied with three types of receptors that signal the contractile state of the muscle.
The first is the annulospiral ending or the primary muscle spindle ending that innervates the equatorial region of the intrafusal muscle fibers of the muscle spindles. The second is the flower-spray ending or secondary muscle spindle ending that innervates the ends of the nuclear regions of primarily the nuclear chain, intrafusal muscle fibers. Both primary and secondary muscle spindle receptors signal the length of the muscle in a linear fashion as a frequency code. Primary spindle receptors also signal the rate of change of muscle length. Primary spindle endings are terminals of group Ia afferent fibers, whereas secondary endings are terminals of fibers in the group II range.
The bias of both types of spindle receptors is controlled by the fusimotor neurons or gamma-motoneurons that innervate the muscular portions of the intrafusal muscle fibers and cause them to contract. Dynamic fusimotor neurons affect mainly the dynamic or rate responses of primary endings, whereas static fusimotor neurons affect mainly the static or length responses of primary and secondary spindle endings. Fusimotor neurons discharge tonically resulting in gamma bias or gamma tone.
The third receptor in muscle is the Golgi tendon organ that innervates the tendon near the muscle-tendon junction and signals muscle tension and rate of change of muscle tension.
As a self-test, try filling in the following table. Use only I for increase discharge, D for decrease discharge, or N for no change.
Activity | Primary Spindle | Secondary Spindle | Golgi Tendon Organ |
Passive stretch | |||
Isometric1 contraction | |||
Isotonic1 contraction | |||
Lengthening2 contraction |
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