The role of the autonomic nervous system
in extraocular muscle function
Kenneth E. Eakins and Ronald L. Katz
The purpose of this study was to determine the possible role of the autonomic nervous system
in the function of mammalian extraocular muscle. Experiments were carried out on the cat
anesthetized with pentobarbital. Stimulation of the cervical sympathetic nerve and the injection of epinephrine increased the tension of the superior rectus muscle. These responses were
unaffected by sympathetic ^-receptor blocking agents, potentiated by cocaine, and antagonized
by sympathetic a-receptor blocking agents. Similar results were observed with the nictitating
membrane. The superior rectus muscle and nictitating membrane differed in that atropine
blocked the response of the nictitating membrane to epinephrine, but not that of the superior
rectus muscle. The implications of these remits are discussed.
.he histological observations of Boeke1
and Wolter2 indicating that the striated
muscle fibers of extraocular muscles have
a double innervation, motor and autonomic, led Alpern and Wolter3 to advance
the view that the slow vergence movements of the eye were under autonomic
control. This aspect of the control of extraocular muscle activity has since received
little scientific attention. For this presentation, I would like to deal exclusively with
some results we have obtained in our
laboratory concerning the effect of cervical
sympathetic stimulation and systemically
administered epinephrine on extraocular
muscle tension. No attempt will be made
to assess the relative importance of the role
of the parasympathetic nervous system in
extraocular muscle function at this stage,
except to point out that we have repeatedly
observed in our laboratory that the contracture of the extraocular muscle produced by acetylcholine is unaffected by
atropine, which would seem to preclude the
involvement of parasympathetic muscarinic
receptors in the response.
In an earlier study,4 we observed that
the intravenous injection of epinephrine in
the cat resulted in a small but fairly wellmaintained contracture of the superior and
lateral rectus muscles. This observation
was of interest to us in that, as long ago as
1930, Duke-Elder and Duke-Elder1 had
drawn attention to the fact that mammalian
extraocular muscles behave in many respects like denervated mammalian'skeletal
muscle; for example, both the external muscles of the eye and chronically denervated
leg muscles respond to acetylcholine with
From the Departments of Ophthalmology Research
and Anesthesiology, College of Physicians and
Surgeons, Columbia University, New York,
N. Y.
Supported by Fight-for-Sight Grant-in-Aid G 303C-2-C3 of the National Council to Combat
Blindness, Inc., New York, and United States
Public Health Service National Institutes of
Health Grant GM 09069.
Figs. 1, 2, 5, 6, and 7 are reprinted from and
with the permission of The Journal of Pharmacology and Experimental Therapeutics. In press.
253
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Investigative Ophthalmology
June 1967
254 Eakins and Katz
a contracture, and, in contrast to the lack
of effect of sympathetic amines on normal
peripheral skeletal muscle, these compounds will produce a slowly developing
reversible increase in resting tension several weeks after section of the motor nerve.
Bowman and Raper in 1965° reported
that the response of the chronically denervated tibialis anterior and soleus muscles
of the cat to epinephrine corresponded on
AhlquistV 1948 classification to a sympathetic /3-receptor effect since it was selectively blocked by sympathetic /3-receptor,
but not by a-receptor antagonists.
In this presentation, we will describe
some experiments that we have carried out
to determine whether the increased tension
of extraocular muscles observed after systemic administration of epinephrine represents yet another similarity between extraocular and chronically denervated skeletal
muscle. We have examined the response
of the superior rectus muscle of the cat to
epinephrine and sympathetic stimulation
and its modification by a variety of agents.
In addition, we determined the response of
the nictitating membrane under these conditions to monitor the behavior of the orbital smooth muscle.
Methods
Cats of either sex, weighing between 2.5 and
3.5 kilograms, were used in these experiments.
The animals were anesthetized with sodium pentabarbital (36 mg. per kilogram) given by intra-
peritoneal injection. The trachea was cannulated
and artificial respiration employed. Arterial blood
pressure was recorded from the right femoral
arteiy with a Statham (P23Db) pressure transducer coupled to a Grass Model 5 ink-writing
polygraph.
The superior rectus of one eye was separated
from the globe and a suture placed through the
tendon, the thread then being connected to a
Grass force displacement transducer (FT-03).
Contractions of the ipsilateral nictitating membrane were recorded by a similar force displacement transducer. The animal's head was immobilized in a stereotaxic apparatus. The cornea of
the test eye was incised and the eyeball eviscerated and ligatured. Intra-arterial injections were
made via a polyethylene cannula inserted into the
ipsilateral common carotid artery. Intravenous injections were made via a cannula inserted into the
left femoral vein.
Nerve stimulation. For preganglionic stimulation, the sympathetic trunk was cut just above the
sternum and laid on a pair of silver electrodes.
For postganglionic stimulation, the superior cervical ganglion was exposed, and postganglionic
nerve isolated and laid on a pair of silver electrodes. The preparation was prevented from drying with warm liquid paraffin. Rectangular pulses
with parameters determined in each experiment
were delivered by a Grass S4 stimulator in conjunction with a stimulus isolation unit.
Drugs used in this study were 1-epinephrine
chloride (Parke, Davis & Co.), phenoxybenzamine hydrochloride (Dibenzyline, Smith, Kline &
French Labs.), phentolamine (Regitine, Ciba
Pharmaceutical Products, Inc.), pronethalol (Alderlin, Ayerst Labs.), propanolol (Inderal, Ayerst
Labs.), atropine sulfate (Burroughs Wellcome &
Co.), and cocaine hydrochloride. The drugs were
diluted in 0.9 per cent weight per volume saline.
All doses refer to the. salts.
EPINEPHRINE
t — 16
C
D
t
t8
kg
S.R.
2OO r
B.P.
mm Hg
tl
t 16
Fig. 1. Cat anesthetized with pentobarbital. Effect of increasing intravenous doses of epinephrine on the superior rectus muscle (S.R.). Calibrations 2 g tension and 1 minute.
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Volume 6
Number 3
Role of autonomic nervous system
Results and discussion
Intravenous injections of epinephrine (1
to 16 /xg per kilogram) were repeatedly
found to produce an increase in the resting tension of the superior rectus muscle.
A typical result is illustrated in Fig. 1. In
these experiments, injections were made
every 5 to 10 minutes; with more frequent
administration a marked decrease in the
response was observed. The response of the
extraocular muscle to epinephrine did not
appear to be associated with cardiovascular
changes, since an increase in muscle tension was always noted, irrespective of
whether the dose of epinephrine resulted
in a rise or fall in the arterial blood pressure. Fig. 2 illustrates the voltage-dependent increase in tension of the superior
rectus muscle to electrical stimulation of
the ascending cervical sympathetic nerve
carried out at a frequency of 20 cycles per
second. It was of importance to determine
whether these responses of the superior
rectus muscle were the result of a direct
action of sympathetic stimulation and
epinephrine on this striated muscle or if the
changes in extraocular muscle tension were
secondary to an effect on intraorbital
smooth muscle. Consequently, we compared the responses of both the superior
rectus muscle and the nictitating membrane to these procedures in the presence
of various compounds known to modify the
response of smooth muscle. First of all, we
studied the sympathetic a- and /^-receptor
blocking agents. The ^-receptor blocking
agents, pronethalol (5 mg. per kilogram
intravenously) and propanolol (1 mg. per
kilogram intravenously), did not antagonize
the response of either muscle to sympathetic stimulation or epinephrine. Examples of these results are seen in Figs. 3
and 4. The doses of the blocking agents
used in these experiments are well known
to produce substantial sympathetic /3-receptor blockade elsewhere. The increased
pressor response to epinephrine after the
dose of propanolol in Fig. 4 indicates a high
degree of /^-receptor blockade in the animal.
Substantial antagonism of the responses
of both muscles to epinephrine and sympathetic stimulation was observed after
treatment with the sympathetic a-receptor
blocking agents, phenoxybenzamine (1 to
2 mg. per kilogram intravenously) and
phentolamine (2 to 3 mg. per kilogram intravenously). Fig. 5 illustrates the effect of
an intravenous injection of 1 mg. per kilogram of phenoxybenzamine on the response
of the nictitating membrane and superior
rectus muscle to postganglionic sympathetic stimulation and intra-arterial injection of 3 [xg of epinephrine. It can be seen
that all the responses were markedly depressed by this sympathetic a-receptor
blocking agent.
Epinephrine is well known to modify
skeletal neuromuscular transmission in the
cat (see Bowman and Rapers for references); it has not previously been demonstrated to alter resting tension in normally
innervated mammalian striated muscles,
•although chronically denervated mammali-
S.R.
2V
2.5 V
255
3V
4V
Fig. 2. Cat anesthetized with pentobarbital. Effect of electrical stimulation of the cervical
sympathetic nerve on the superior rectus muscle (S.R.) 20 per second, 3 msec, duration,
Calibrations 2 g tension and 1 minute .
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256 Eakins and Katz
Investigative Ophthalmology
June 1967
10 g
N.M.
I min
ig
S.R.
Imin
t
15V
15V
Pronethalol
5mg/kg
Fig. 3. Cat anesthetized with pentobarbital. Lack
of eflect of pronethalol on the response of the
superior rectus (S.R.) and nictitating membrane
(N.M.) to sympathetic stimulation. Calibrations in
g tension and minutes as indicated.
A
an striated muscle will respond to epinephrine with a slowly developing reversible
increase in resting tension."19> 10~13 It has
been reported that the response of the
chronically denervated tibialis anterior
and soleus muscles corresponds on Ahlquist's7 classification of catecholamine receptive mechanisms to a sympathetic /?receptor effect, since the response was most
effectively produced by Levisoprenaline
and was selectively blocked by the sympathetic /3-receptor blocking agents, dichlorisoproterenol and pronethalol.0 The results
obtained after systemic administration of
epinephrine in these experiments indicate
that the response of the striated superior
rectus muscle to the catecholamine differs
from the response obtained from chronically denervated mammalian skeletal muscles, since the response was unaffected by
the sympathetic /3-receptor blocking agents,
but was abolished by the sympathetic areceptor blocking agents.
B
N.M.
S.R.
200r
BP
mm Hg
100
0L
tAd
t Ad
Propanalol
I mg/kg
Fig. 4. Cat anesthetized with pentobarbital. Response of the superior rectus (S.R.) and nictitating membrane (N.M.) to epinephrine (Ad) before (A) and after (B) 1 mg. per kilogram of
propanolol. All drugs given intravenously. Note increase in effect of epinephrine on arterial
blood pressure (B.P.) after the propanolol. Calibrations in g tension and minutes as indicated.
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Role of autonomic nervous system 257
Volume 6
Number 3
S.R.
I min
N.M.
Imin
•
Postgang.stim.
Postgong.stim.
Ad
•
Ad
Phenoxybenzamine
Img/kg
Fig. 5. Cat anesthetized with pentobarbitone. Inhibition of the responses of both muscles to
sympathetic stimulation and intravenous epinephrine (Ad) by phenoxybenzamine. (A) Control
responses of superior rectus (S.R.) and nictitating membrane (N.M.); (B) responses after phenoxybenzamine. Calibrations in g tension and minutes as indicated.
A
D
B
N.M.
S.R.
Pregang
stim.
t
Ad
t
t
Pregang
stim.
COCAINE
5mg/kg
Ad
Fig. 6. Cat anesthetized with pentobarbital. Effect of cocaine on the responses of the superior
rectus (S.R.) and nictitating membrane (N.M.) to preganglionic' sympathetic stimulation and
epinephrine (Ad) (A) and (B) control responses; (C) and (D) responses after cocaine. All drugs
injected intravenously. Calibrations in g tension and minutes as indicated.
The effect of cocaine (5 mg. per kilogram intravenously) on these responses is
seen in Fig. 6. Notice that the responses of
both muscles to preganglionic sympathetic
stimulation were slightly depressed, but
that the epinephrine responses were definitely enhanced. Further studies showed
that these doses of cocaine always potentiated the effects of postganglionic sympathetic stimulation on both muscle systems.
This difference between the effect of
cocaine on the responses to pre- and postganglionic sympathetic stimulation has
been reported by other workers, the im-
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258 Eakins and Katz
Investigative Ophthalmology
June 1967
portant point here being that both muscles
responded in the same way. Lastly, we
examined the effect of atropine (1 to 2
mg. per kilogram intravenously) on these
responses. A typical result is seen in Fig.
7. It can be observed that the injection of
1 mg. per kilogram of atropine differentiated the responses to some extent. The
response of the superior rectus muscle and
nictitating membrane to sympathetic stimulation and the response of the nictitating
membrane to epinephrine were depressed
by atropine, whereas the response of the
superior rectus muscle to epinephrine was
unaffected.
The identical behavior of the nictitating
membrane and superior rectus muscle to
sympathetic stimulation through all the
procedures reported here raises the possibility that a part of the response of the
superior rectus muscle produced by stimulation may be a pseudocontracture secondary to contraction of intraorbital smooth
muscle. However, it is possible that the
sympathetic nervous system may be involved with something other than the control of the vasculature in the extraocular
muscles. We hope that studies now in
progress (in collaboration with Dr. R.
Barrett of Columbia University), utilizing
the fluorescence technique developed by
Hillarp and Falck for the histological localization of catecholamines in tissues, will
yield more precise information on this
problem. In addition, we also have to rule
out the possibility that the extraocular muscles, already rather unusual in terms of
their structure as compared with other
skeletal muscles,14'15 may possess smooth
muscle elements which may be responsible
for the present observations.
It is also difficult to explain the response
of the superior rectus muscle to epinephrine entirely in terms of an effect on orbital
smooth muscle in view of the differential
action of atropine. It has been demonstrated before that atropine is capable of
antagonizing the response of the smooth
muscle cells of the nictitating membrane
to both epinephrine and sympathetic stimulation.10' 17 Thus, the observation that
atropine did not reduce the response of
the superior rectus muscle to epinephrine
argues against the total extraocular muscle
response being secondary to changes in
tone of the intraorbital smooth muscle.
This effect of epinephrine on the superior
rectus muscle may be related to its antiB
S.R.
5g
N.M.
Imin
•
Postgang stim.
tAd
Ad
Postgang stim.
Atropine
I mg/kg
Fig. 7. Cat anesthetized with pentobarbital. Differential action of atropine. (A) Control responses of the superior rectus (S.R.) and nictitating membrane (N.M.) to sympathetic stimulation and intra-arterial epinephrine (1 Hg); (B) responses after intravenous atropine. Note all
responses depressed except the response of S.R. to epinephrine. Calibrations in g tension and
minutes as indicated.
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Volume 6
Number 3
curare action in partially curarized striated
muscles.18'21 In common with the present
response of the superior rectus muscle, the
anticurare action is blocked by sympathetic
a-receptor blocking agents.21"23 This anticurare effect of epinephrine is thought to
be due to an increase in acetylcholine
release resulting from a hyperpolarizing
action of epinephrine on motor nerve endings.-'1- 25 Thus, mobilization of acetylcholine from prejunctional storage sites in
the superior rectus muscle by epinephrine
would result in an increase in tension in
the muscle, the acetylcholine most probably affecting the "slow" or multiply innervated muscle fibers. Since the contracture
of the superior rectus muscle produced by
acetylcholine is unaffected by atropine this
would also explain the lack of effect of
atropine on the response to epinephrine
described in this paper. However, these
experiments do not rule out two further
possibilities which could also explain the
response of the extraocular muscle to
epinephrine: (1) the presence of smooth
muscle elements within the extraocular
muscle, as suggested above, and (2) that
epinephrine may be acting directly on the
neuromuscular junctions, most probably
the multiple endplates of the "slow" fibers.
We hope that work currently in progress
in our laboratory will yield answers to some
of these problems.
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Role of autonomic nervous system 259
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Investigative Ophthalmology
June 1967
260 Eakins and Katz
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