44

probably at reduced levels of sensitivity, until the child is 4 or 5 years of age. Such a time course for the critical-period of human visual system development is, of course, similar to that suggested by the behavioral studies described earlier ~,z ~5.

The anatomical results described above indicate that there are several structural changes that take place during normal and visually deprived development and that these changes are similar in both the cat and monkey. In addition, anatomical studies in the human suggest that at least some of these structural changes also occur during the development of our visual sys- tem. In fact, one of the tenets underlying these studies has been, and continues to be, that anatomical studies of the human visual system can pave the way for more direct comparisons with experimental findings derived from studies using nonhuman ani- mals. In most mammalian species compari- sons between the results of anatomical and electrophysiological experiments are poss- ible and, where similarities between the human and nonhuman visual systems are particularly striking, some of these com- parisons can be extended to the human. In this way one can analyse the structural organization and development of the human visual system itself and then make some reasonable deductions about the extent to which our visual system may share the organizational, functional and developmental features defined for other species. For the topic considered here, anatomical studies of the developing human visual system are vital because they

(1) provide evidence as to when the critical-period of development begins and ends, (2) provide some descriptions of the changes taking place during this period and (3) provide new insights into the neurolog- ical mechanisms that underly the enhanced plasticity that exists in the visual system during this period. After all, while it is this plasticity that makes the visual system par- ticularly vulnerable to changes in its envi- ronment, this same plasticity may provide a means by which our visual capabilities can be improved and, certainly, can be used to advantage by the clinician. Just as visual deprivation exerts its most profound effects when introduced early in the critical-period, so too will clinical treat- ments be more effective when begun at this time.

Acknowledgements The author's laboratory is supported by

research grants EY01338 and EY02159 from the National Eye Institute, National Institutes of Health. The laboratory is a part of a Vision Science Research Center which is supported in part by a CORE Vision Research Center Grant (EY03039), also from the National Eye Insti- tute. National Institutes of Health.

Reading List I Av, aya, S., Miyake, Y., lmaizumi, Y., Shiose, Y..

Kanda, T. and Komuro, K. (1973)Jpn. J. Oph- thalmol. 17, 69-82

2 Banks, M. S., Aslin, R. N. and Letson, R D. ( 2975) Science 190, 675~o77

3 Boothe, R. G, Greenough, W. T., Land, J. S, and Wrege. K. (1979)J. Comp. Neurol. 186,473-490

4 Cragg, B. G. (1975)J. Comp. Neurol. 260. 247-166

TINS - February 1981

5 Crawford, M. L, J. (1978) Arch. OphthalmoL Otolaryngol. 85,465-477

6 Davison. A. N. and Dobbing, 3. ( I t.~68) Applied Neurochemistrv, Contemporary Neurology Series: 4 and 5. Davis, Philadelphia, pp. 253-316

7 Deller, M. (1979) Trends NeuroSci. 2,226-218 8 Dobson. V. and Teller. D. Y. (2978) Vision Res.

28, 1469-1483 9 Fox, R.. Aslin, R. N., Shea, S. t.. and Dumais. S. 2'.

(1980) Science 207,323 20 Garey, L. J. ( 2979) Trends NeuroSci. 2, 213-226 12 VitaI-Durand, F., Garey, L. J. and Blakemore, C.

(1978) Brain Re~. 158, 45-64; Gonlieb, M., Pasik, T. and Pasik. P. (1980) Soc. Neurosci. Abstr. 6. 662

12 Hiekey, T. L I 1977) Science 298, 836-838 13 Hiekey, T. L. (1980) J. Comp. Neurol. 189,

467-48l 24 Hitchcock, P. F. and Hickey, T. L. (2981)) Brain

Res. 182, 276-279 25 Hohmann. A. and Creutzfeldt. O. 1). (2975)

Nature (London) 254, 613~o 14 26 Hubel, D. H. and Wiesel. T. (297(I) J. Physiol.

(London) 206, 419-436 17 Hubel, D. H. and Wiesel, T, N. (1972)J. Comp.

Neurol. 146, 422-450 18 Hubel, D. H., Wiesel, T. N. and LeVay, S. (1977)

Phil. Tran,~. R. Soc. London 278, 377-409 19 2keda, H. (1979) Trends NeuroSei. 2,209-213 20 KaliL R. (1978)J. Comp. Neurol. 182,265-292 21 LeVay, S., Hubel, D. H. and Wiesel. Y. N. (1975)

J. Comp. Neurol. 159, 55%576 22 LeVay, S., Wiesel, T, N. and Hubel, D, H. (198(I)

J. Comp. Neurol. 191, 1-51 23 2.and, R. D. (1978) Development and Plasticity o f

the Brain, Oxford University Press, New York 24 Rakic, P. (2977) Philos. Tran,~. R Soc. London

278, 245-260 25 Sherman, S. M. (1979) Trends NeuroSei, 2,

192-295 26 Teller, D. Y. ( 2982 ) Trends- NeuroSci. 4, 22-24

Dr Terry L. Hickey B at the School o f Optometry/The Medical Center, University o f Alabama in Birming- ham, Birmingham, Alabama 35294, U.S.A,

Sherrington's concept of proprioception

Edward V. Evarts

Proprioceptors respond naturally to active movement, but experimentally these movements must be produced by external means. Edward Evarts explains how this methodological limitation has affected our interpretation o f proprioceptor function.

Sherrington 14 defined proprioceptors as deep receptors for stimuli that 'are trace- able to actions o f the organism itself, a n d . . , s i n c e . . , the stimuli to the recep- tors are delivered by the organism itself, the deep receptors may be termed proprio- ceptors, and the deep field a field of pro- prioception', The latin wordproprius, mean- ing own, provided a prefix which called attention to the fact that the organism's own acts created the adequate stimuli for these deep receptors. Having defined

proprioception, Sherrington went on to point out a property of proprioceptive reflexes which is sometimes forgotten: the amount of muscle activity mobilized by proprioceptive inputs is relatively slight. Careful attention to Sherrington's state- ments that (1) adequate stimuli for pro- prioceptors arise from the actions of the organism itself and that (2) proprioceptive effects on muscle discharge are 'mild', will be most useful as we seek to interpret data on reflex responses to muscle and joint

t". [=lse~ier/Norlh-Holland Biomedical Press 10~.1

afferent inputs. First, such careful attention will prevent us from forgetting that passive movements (i.e. those resulting from events arising within the environment) do not affect proprioceptors in the same way as active movements by the organism itself. Unfortunately, the essential role of the organism's own active movements in generating adequate stimuli for pro- prioceptors puts neurophysiologists in a quandry: systematic laboratory investiga- tion of proprioceptive systems requires the use of externally produced changes of muscle length or tension, but these same externally produced changes (though they have marked effects on impulse frequen- cies of proprioceptive afferent fibers) do not provide proprioceptors with adequate stimuli. Perhaps the solution to this quan- dry lies in interpreting the consequences of external stimulation of proprioceptors with the same caution that we exercise in inter- preting the consequences of electrical stimulation of the nervous system. Cer-

TINS -February 1981 45

tainly, most of what we know about prop- rioceptive systems has been gained by observing the reflex consequences of externally produced changes of muscle length and tension, and the use of these inadequate stimuli does not really lead us astray until we place excessive reliance on these consequences in seeking to arrive at formulations as to the functional signifi- cance of proprioceptive reflexes.

Reflex categories

Indeed, the importance of distinguishing between reflexes elicited by natural and contrived stimuli is such that one might speak facetiously of two sorts of reflexes: natural reflexes and laboratory reflexes. The natural reflexes are the familiar ones - those whose functional significance is clear even to the person in the street, while the laboratory reflexes are the unfamiliar ones whose functional significance may be unclear even to the laboratory worker who elicits them. Examples of natural reflexes are the cough reflex, the corneal reflex, the pupillary light reflex, the vestibulo-ocular reflex (VOR) etc. The person in the street might object to calling the VOR a natural reflex, arguing that he or she hasn't the slightest idea what the VOR is. True, but one could explain to such a person that VORs occur whenever he or she moves about and that VORs provide for stability of vision by generating eye movements which exactly counterbalance head move- ments. A laboratory reflex which may be contrasted with the VOR is caloric nystag- mus resulting from irrigation of the exter- nal auditory canal with warm or cold water. Caloric nystagmus is mediated by the same pathways that mediate the VOR but is itself of no functional significance (of what possible use is it to have nystagmus when someone puts cold or warm water in one's ear?). Of course, an individual who lacks a caloric reflex will also lack a VOR and will be impaired accordingly. But this does not mean that one should seek toattribute func- tional significance to the caloric reflex per se.

This absurd discussion of laboratory and natural reflexes has been engaged in because there are certain reflexes which are actually laboratory reflexes but whose functional significance is sometimes discus- sed as if they were natural reflexes. My own work on cerebral motor cortex outputs occurring in response to externally pro- duced limb displacements must surely be classified as dealing with laboratory reflexes, and the same may be said for a variety of spinal cord reflexes (e.g. the ten- don jerks and Babinski reflexes elicited by the neurologist). The fact that these

reflexes are not natural does not mean that they are unworthy of study - far from it. But laboratory reflexes should be viewed as tools for investigating the neuronal pathways whose existence they reveal rather than as phenomena demanding a teleological explanation.

When are proprioceptive reflexes important?

Within the past few decades there have been a number of investigations aimed at assessing the extent to which propriocep- tive reflex systems mediate adaptive motor responses to large external disturbances. Unfortunately, interpretations of the results of these studies have sometimes failed to give sufficient attention to the pair of facts that Sherrington ~4 recognized so clearly: (1) muscular responses resulting from proprioceptive inputs are mild; (2) intense responses to afferent inputs arising within the environment are mediated primarily by exteroceptive inputs. Given these two facts, one should not be surprised when it is demonstrated that propriocep- tive reflexes are ineffective in achieving load compensation in the face of large external disturbances (remember Sher- rington's point that proprioceptive inputs have mild effects and that exteroceptors rather than proprioceptors underlie responses to external disturbances!). Indeed, closed-loop feedback systems come into play primarily when errors are small. Under conditions in which errors are large, open loop systems come into play and generate large movements which will reduce error to a value such that closed- loop systems can function effectively. Thus, for both segmental and the transcor- tical reflexes, a priori considerations would lead one to believe that the systems should be effective for small errors and ineffective for large errors. According to this notion, small errors would be able to elicit effective proprioceptive reflex responses by control- ling that proportion of the motoneuronal pool (or the cortico-motoneuronal pool) which is tonically active. The special capac- ity of segmental inputs to control discharge of tonic motoneurons and the differences in the properties of tonic as compared to phasic motoneurons have been documented in a number of studies since Granit et al. 8 found that axons of tonic motoneurons - as revealed by post-tetanic potentiations - were emitting smaller spikes than phasic ones. An additional study 9 showed that in both gastrocnemius and soleus muscles, spike size from indi- vidual fibers differentiated between tonic and phasic motor units. Burke s,4 found that smaller soleus motoneurons were likely to

be tonic and larger gastrocnemius motoneurons to be phasic, and the import- ance of cell size in relation to tonic and phasic properties has also been shown by Henneman et al 1'11. The phasic motoneurons which innervate fast-fatigue muscle s are at the upper end of the recruitment order and are relatively inac- cessible to excitation by segmental inputs.

Errors: internal v. external

All the above-mentioned findings point to the effectiveness of proprioceptive

,inputs in modulating tonic motoneuron discharge and to the ineffectiveness of these same proprioceptive inputs in activat- ing the phasic motoneurons innervating fast fatigue muscle which, in its brief periods of activity, generates the abrupt increases in tension necessary to react to large errors generated by external distur- bances. Numerous recent studies in man t5 and subhuman primates tz show that com- pensation for large load disturbances is not achieved by segmental reflex mechanisms operating in the closed-loop mode. But while not suited to deal with large load dis- turbances, segmental reflex mechanisms are suited to deal with smaller disturbances due to internal factors within the neuromuscular system itself. When small length changes occur during tonic muscle discharge as a subject seeks to maintain postural stability, these small length changes will modulate the discharge fre- quencies of tonically active motoneurons, but such modulation will be insufficient to achieve compensation for large external load changes, since high threshold motoneurons with phasic properties must be brought into play to re-establish a new steady state and proprioceptive inputs are unable to excite these high threshold motoneurons. For this intense excitation the motoneurons must await signals gener- ated by 'reprogramming' at spinal and/or supraspinal levels. But while inadequate to deal with large external disturbances, the increase (with lengthening) and decrease (with shortening) in tonic motoneuron dis- charge produced by proprioceptive reflex inputs points to the capacity of these inputs to modulate discharge either up or down. And though compensation for large exter- nal disturbances cannot be achieved by this feedback, modulation of tonic motoneuron discharge in relation to small internal dis- turbances can be carried out by these reflex mechanisms. Small internal disturbances (arising in muscle and/or nervous system) create errors of movement even in the absence of any external load changes, and the high dynamic sensitivity of muscle stretch receptors ~ for small length changes

46 "FINS - February 1 981

allows the segmental reflex apparatus to be effective in compensating for such small disturbances. This relatively greater effec- tiveness of the stretch reflex in dealing with small disturbances has been dealt with at length by Matthews TM. The role of reflex mechanisms in relation to internal proper- ties of muscle has also been proposed by Nichols and Houk TM in a report showing that autogenic reflexes can compensate for the marked hysteresis effects which would otherwise cause great differences in muscle tension responses during lengthening and shortening. In considering their results, Nichols and Houk stated that 'Our data do not allow us to distinguish which function is more important. The evidence rev iewed . . , favors a higher gain of force feedback in normal animals. If this is true, compensation for variations in muscle properties would be greater, whereas com- pensation for variations in load would be less."

The notion that signals from muscle spindle afferents are important in minimiz- ing the consequences of small internal dis- turbances would imply that postural instabilities should increase when inputs from spindle afferents are eliminated, and demonstration of such an effect has been provided by the work of Goodwin et al. e on control of voluntary jaw movements in the monkey. The authors note that the jaw- closing musculature affords a unique opportunity to disrupt the afferent limb of the stretch reflex with minimal damage to the rest of the sensory innervation of the region. In these studies it was found that surgical interruption of the reflex arc led to a considerable increase in the amplitude of spontaneous tremor during steady contrac- tion, suggesting an important role for mus- cle spindle afferents in 'reducing errors of muscle length produced by fluctuating levels of motor d i scharge . . . ' The work of Goodwin et al. also demonstrated that the stretch reflex made a marked contribution to muscle stiffness in response to external disturbances involving 500 micrometer (peak-to-peak 1 mm) stretches of steadily contacting muscle. In this experiment, a 1 mm jaw movement corresponded to approximately 1 of rotation at the tem- poromandibular joint, and was thus a rela- tively small movement in relation to the full range of jaw movement available to the monkey, whose jaw was held at a 6 open- ing during the voluntary force exertion of this experiment. Interruption of the affer- ent limb of the stretch reflex arc reduced resistance to stretch to less than one-half of that present in the intact animal.

Finally, recordings of spindle afferent discharge in man TM give dramatic demon-

stration of the high dynamic sensitivity of the reflex system in relation to small internally-generated irregularities of movement in such a way as to maintain smooth shortening during isotonic muscu- lar contractions. Vallbo reasoned that this same exquisitely sensitive reflex system would be relatively ineffective in compen- sating for large external load disturbances. It would thus seem that several lines of evi- dence converge in support of Sherring- ton's ~4 original notion that proprioceptive reflexes operate in relation to events aris- ing from within the muscles of the organism itself.

Reading list 1 Bizzi, E., Dev, P., Morasso, P. and Polit, A. (1978)

J. Neurophysiol. 41,542-556 2 Bizzi. E.. Polit, A. and Morasso, P. (1976)J.

Neurophysiol. 39, 435-444 3 Burke, R. E. (1968)J. Physiol. London 196,

605-630 4 Burke, R. E. (1968)J. Physiol. London 196,

631-654 5 Burke, R.E.(1973)inNewDevelopmentsinElec-

tromyography and Clinical Neurophysiology, (Desmedt, J. E.. ed.) vol. 3. pp. 69-94, Karger, Basel

6 Goodwin, G. M., Hoffman, D. and l.uschci, E. S. (1978)J. Physiol. London 279. 81-I 11

7 Granit, R. and Henatsch, H. D (1956) J. Neurophysiol. 19. 356-366

8 Granit, R., Henatsch, H.-D. and Steg, G. (1956) Acta Phy.siol, Seand. 37, 114-126

9 Granit. R., Pascoe, J. E. and Steg. (k (1957) J. Physiol. London 138, 381-400

10 Henneman, E., Somjen, G. and Carpenter, D. O. (1965) J. Neurophysiol. 28, 560-580

1 I Henneman, E., Somjen, G. and Carpenter, D. O. (1965) J. Neurophysiol. 28, 599-620

12 Matthews, P. B. C. (1972) Mammalian Muwle Receptor~ and "[heir Central Actions, Arnold, I.ondon

13 Nichols. T. R. and Houk,J.C.(1973)Seience 181, 182-184

14 Sherrington, C, S. (1906) Brain 29, 467-482 15 Vallbo, A. B. (1973)in Control of Posture and

Locomotion, Advances in Behavioral Biology, (Stein. R. B., Pearson, K. G., Smith, R, S. and Redford, J. B.. eds). vol. 7, pp. 211-226. Plenum, London

16 Vallbo. A. B. (1973) in New Developments in Electromyography and Clinical Neurophysiology, (Desmedt, J. E, ed.) vol. 3. pp. 251-262, Karger, Basel

Edward V. Evart~ is Chief of the Laboratory of Neurophysiology, National Institute of Mental Health, Bethesda. .Maryland 20205, U.S.A.

Detection and m sum nt of intraceitular calcium

A comparison of techniques J. Stinnakre

Calcium detection and measurement in living cells can be achieved using a variety o f techni- ques. Three o f these are based on the optical detection o f changes in luminescence, fluoresc- ence or absorbance o fan organic indicator produced on binding with calcium ions. How- ever, when the preparation is completely opaque, the only available technique is the calcium-selective m&ropipette which provides a direct electrical signal related to the calcium ion concentration at its tip ~,9,17.~8 (see also articles by Nicholson and by Tsien, TINS, Sep- tember, 1980). In this review Jacques Stinnakre compares and contrasts the usefulness o f these methods.

The indicators used in the optical techni- ques fall into three main categories. These are the calcium sensitive luminescent pro- teins (photoproteins) which emit light upon binding to calcium. (The two used most commonly are aequorin and obelin =, named after the jellyfish Aequorea and Obelia, from which they are isolated); cal- cium sensitive dyes such as murexide, tetra- methylmurexide TM, arsenazo IIP, anti- pyrylazo III =1 and chlorophosphonazo liP, which show large changes in their absorp- tion spectra due to Ca =+ binding; and the fluorescent Ca=+-chelators such as chlorotetracycline 6 or calcein 8 in which

* The fluorescent lanthanides (europium, terbium) will not be reviewed here for they are calcium binding site probes rather than free calcium indicators.

fluorescence, due to an exciting light, is enhanced and decreased by the presence of calcium and magnesium ions respectively*.

Since the selected indicator, or the tip of a calcium-selective microlectrode, must be inserted within the cell, application of these methods is restricted to those cells which can be impaled with at least one microelec- trode and which can withstand injection of the indicator. One exception is chlorotetra- cycline which will cross the membrane and thus can be applied externally.

In this paper, I shall compare these tech- niques and explain some of their advan- tages and disadvantages, ending with a brief description of some of the important problems which may be solved by using them. Further description of the tech-

I I,cxieriNorlh Hidland Biomedical Pres~ 19Sl