‘which caused spikes only while it was , Olmstead, J. M. D., and M. W. Morgan, Jr. Am. J.
agplied.or-released,.in short, while the a iene wwe aM D. Olmstead, and W. 6
‘ A any } Morgan, M. W., Jr., J. M. D. stead, an . G.
masclé was moving. a ar owe experiments, Waltous. Am. 4. Physel. 128: 588, To40.

elliary she cnrle di not Doddety —% Olmstead, JM. D., and M. W. Morgan Jr. Am. J.

‘ Physiol. '133: 720, 1941.
d ‘produce depolarization. Jacob- 19, stobney, J. B, M. W. Morgan, Jr., J. M. D, Olmstead,
{not record spikes when recordings and J. H. Wagman. Am. J, Physiol. 135: 759, 1942.
from the intact eye when the muscle 11, Olmstead, J. M. D. J. Nerv. Ment, Dis. 99: 794,
ted via its nerve supply. The dis- 1944,
bility exists that this multiunit smooth 12, Fleming, D. G., and J. M. D. Olmstead. Am. J.
- muscle does not normally show spike-like dis- Physiol. 181; 664, 1985.

: but rather shows endplate-type sus- 15. Fleming, D. G., and James T. Hall. Am. J. Ophthal.

* " 43; 287, 1059.
gation associated with ‘tension ,

Soe ‘ 14, Stebeck, Robert. Gr. Arch. f. Ophthal. 153: 425,
maintained for long periods. 1952, Ph

has not yet been thoroughly

a 15. Henson, V., and C. Voelckers. Experimentalunter-
‘and: may represent a fruitful area of suchung uber den Mechanismus der accommodation,
‘Kiel, Schwartz, 1868, Quoted from Helmholtz, Hand-

that th . buch der Physiologischen Optik, p. 139, Verl

: at the radial m wale layer of the Lenpold Voss Hamburg and aa 139.
eiliary muscle is the primary effector o an tho 16. Romer, P., and 0, Dufour. Arch. f. Ophthal. 54: 419,

modative change for both far and near in the 1902. f

by the observation of Walls” Hess ine. Arch. f. 46: 243,
uscle layer (Muller's muscle) " a Ge aad Ie Holos. nck, f. Ophthal. 40: 2

-Giy in primates, seals, some toothed — 18. Meesman, A. Gr. Arch. f, Ophthal, 152: 188, 1952.
aud some ungulates, 19. Meesman, A. Gr. Arch. {. Ophthal. 157: 188, 1955.
vasqilar 20. Wolter, J. R. Presentation at the 58th meeting of the
German Ophthalmological Society in Heidelberg, 1953,
|. Matteuci, P. Am. J. Ophthal. 31: 1025, 1948.
. Sachs, E. Am. J. Ophthal, ZS: 1277, 1952.
Christenson, K. J. Anat. 70: 225, 1938.

21.
22,
23.
2A. Warwick, R. J. Anat. 88: 71, 1954.
25.
28.

Ophthal. 1: 1 (Past 2), 1855.

, Schubert, G. Gr. Arch. f. Ophthal. 157: 116, 1955.
P, and M. Doyon.| Ann. d’ 106: 28,

mM Jacobson, Hart, Hunter H. Romaine, G. Pet
“180. f ‘ Ex gi ‘Stephens. Am. J. Ophthal. 46:
‘3. Henderson, T. Tr. Ophth. Soc. JJ. K. 46; 300, 1926. 231, 1958.
a J.G, Studies on the Physiology of the Eye, H. 97, Walls, G. L. The Vertebrate Eye and Its Adaptive
i 4 Lewis and Co., Ltd., London, 1988. Radiation, p. 285-286, Cranbrook Institute of Sckence,

‘Klin. Monatsbl. f. Augenh. 80: 749, A928. Bull #19, Bloomfield Hills, Michigan, 1942.

G.- Arch, Ophthal, 18: 739. 2), Melton, C. E., E. W. Purnell and G. A. Brecher. Am.
4 1. Ophthal. 40: 155, 1955.

8

@ US GOVERNMENT PRINTING OFFICE :1963 0 - 692439.

-h-


approximation distance is called the punctum
proximum or near point.

Rays of light emanating from an object lo-
cated at an infinite distance are parallel, In

| the normal eye, incident parallel rays of light

come to a focus on the retina when the lens
has minimal dioptric strength. The lens be-
comes minimally refractive when zonular ten-
sion is greatest; i.e., the elasticity of the lens
capsule is maximally opposed and the central
anterior face of the lens is flattened. In the
normal eye, therefore, the far point (punctum
remotum) is located at infinity,

The most widely held theory of visual accom-
modation is the Young-Helmholtz theory which
states that contraction of the circular layer of
the ciliary muscle causes the lens to increase
in dioptric strength and that the eye may be
adjusted for distant vision solely by relaxation
of accommodation, In other words, that rays
of light emanating from objects located at any
distance between the near point and the far
point may be sharply focused on the retina by
contraction or relaxatién of the ciliary muscle.
The functions of the radial and meridional
layers have not been so clearly assigned as has
the function of the circular layer.

Evidence for relaxation of accommodation
being the mechanism for “distance accommo-
dation” is derived principally from pharma-
cological and neurological experiments, Cyclo-
plegic drugs cause the éye to gp into a state
of minimal dioptric strength. Section of the
parasympathetic Third Nerve. (Oculomotor)

causes the éye to become minimally

refractive. ' :
From time to time over the past ceiltury

the suggestion has been made that distant

. Gaze is mediated at least in part by the sym-

pathgtic division of the autonomic nervous
system. Participatign of the sympathetics is
often invoked to explain the decrease in accom-
modation necessary to change the focus of the
eye from the “rest-point of accommodation”
to infinity, The “rest-point of accommodation”
as determined with the eye looking into an
empty field of uniform brightness like the sky
on a clear day or in darkness is reported to
correspond to an object distance of about 80 cm

rather than infinity. This phenomenon is re-
ferred to as “space myopia” or “night myopia.”

Over a century ago Helmholtz’ inferred from
anatomical studies that there was a possibility
of antagonistic muscle fibers in the ciliary
muscle, one set actively flattening the lens
while the other caused the lens to become
thicker; F~vever, he rejected the idea in favor

.Of passive distance accommedation. Others,**
also, theorized that fibers of the ciliary muscle,
innervated by the sympathetics, could cause
a decrease in dioptric strength of the lens.
Byme* first made measurements of changes in
curvature of the lens upon sympathetic and
parasympathetic stimulation in animals and
demonstrated an active process causing lens
flattening. Poos’ showed that epinephrine in-
stilled into the eye caused a decrease in accom-
modation if a lesion had previously been made
in the cervical sympathetic trunk,

Cogan* reviewed the literature pertaining to
the subject and cited his own clinical’ observa-
tions on six patients with Homer's Syndrome
to support the idea of active distance accom-
modation. Cogan showed that the amplitude
of accommodation was greater on the affected
than on the normal side. As to the mechanism,
Cogan states, “It will be remembered that the
bulk of the radial fibers is placed anteriorly.
The anterior displacement would in itself tend
to release the tension on the zonule were it not
compensated, and. in my opinion overbalanced
by the outward] movement which increases the
zonular tension and flattens the lens. This an-
terior displacement may have the effect of
decreasing the angle between the radial fibers
and the zonular "Fibers and thereby rendering
resultarit outward forces more nearly tangential
to the anterior surface of the lens, The anterior
surface is, of course, the place where the main
accommodative changes take place. It appears
to me, therefore, that the radial fibers com-
prise that part of the ciliary muscle that accom-
modates the lens for distance.” .

The opinions set forth by Cogan in his
review gave impetus to other work, principally
that of Olmstead and Morgan with their col-
laborators."" These workers by a variety of
techniques demonstrated that stimulation of
the cervical sympathetics caused a decrease in


stimulus oo”

LONS CILIARY
NERVE
(SYMPATHETIC)

CILIARY
GANGLION
( PARASYMPATHETIC }

NEAR ACCOMMODATICN

LENS MORE SPHERICAL— Pell.

|. CONSTRICTED

PARASYMPATHETIC
VIA CILIARY GANSLION

Ficons 3. Diagrammatle some

of the response of the citi.ry muscle to stimulation of the

ganglion. Hesv Heavy arrow indicates the main outward movement; tension
- sade Teed ranaing angentially forward acreases as a vector of this outward
movement alone or by sumaing with the acti:e backward movement.

recorded with the thread at a right angle to
the surface of the muscle (Table I, Fig. 4A).

- Likewise in these cases, an increase in tension
was recorded with the thread tangential to the
surface of the muscle: both in ‘front of and
behind the eye (Figs, 4B’ and 4C), Obviously,
the same point on the nluscle could not move
inward, forward, and backward simultaneously.
Since the mai movement was radially inward,
the increases in tension on the tpi in
front of and behind the eye regarded as
vectors of the main inward movement.

In those animals in which the same muscle
was excited by each set of nerves, the responses
were directionally opposite in half of the cases.
When the responses were identical, they were
always of the parasympathetic type; pupillary
constriction together with radially inward move-
ment of the ciliary muscle never occurred, In
those eyes in which the movements cf the
ciliary muscle were directionally opposite, si-
multaneous stimulation of both sets of nerves
yielded no response.

The inwardly directed movement in response

‘

to sympathetic stimulation is diagrammaticall}
summarized in Fig. 5.

c. Microelectrode experiments: Impalement
of ciliary muscle cells with ultramicroelectrode:
were eatied out in the hope that delineation o
sympathet ‘cally and parasympathetically in
nervated fibers could be accomplished.

The ~esults of this phase of the investigatior
were estentially negative. One hundred anc

.sixty-three successful punctures of the ciliar

muscles of sixteen cats revealed that the trans
membrane potential was low compared witl
other smooth muscles, averaging 26.8 millivolts
Since any damage to a cell during the impale
ment of it results in a decrease of the mem
brane potential, greater significance is assignec
to the higher potentials, The average of thi
maximum potentials was 51.0 millivolts,

Only rarely were phasic electrical discharge.
seen, and these did not accomplish complet
depolarization as they were only about 10 milli
volts in amplitude, It was not possible to deter
mine whether or not these “spikes” were ever

-7~


an

1

circulatory changes or extraocular muscle move-
ments when each of the divisions of the ocular
autonomic nerve supply was stimulated.

METHODS

Cats were considered to be the animals of
choice for this study because (1) the cat's eye
is large in proportion to body size, (2) cats
tend to uniformity in body structure, (3) sym-
pathetic fibers do not usually pass through the
ciliary ganglion” and (4) the ciliary muscle
of this animal is known to develop measureable
tension.”

The animals were deeply anesthetized with
sodium pentobarbital administered intraperi-
toneally, The contents of the orbit were
removed as completely as possible in a single
mass and nerve dissection was carried out in
vitro. Adherent extraocular tissue was cut and
reflected from the field of dissection to reveal
the point of entry of the optic nerve into the
eyeball, The intertwined short and long ciliary
nerves could be clearly seen surrounding the
optic nerve. Further dissection usually revealed

ciliary ganglion embedded in the retro-
bulbar fat. Commonly, five short ciliary nerves
were given off from this ganglion. Other fine
nerves which entered the orbit were variable
in number and electrical stimulation revealed
which of them were sympathetic by their dilat-
ing effect on the pupil, Stimulation’ of the
ciliary ganglion or the short ciliary nerves
produced pupillary constriction. The pupil of
the enucleated cat eye is intermediate in size
when neither of the sets of nerves is being
stimulated. When the ciliary ganglion ¢ould
not be found or when it had inadvertently been
severed from the short ciliary nerves, discrete
points could be found about the point of entry
of the optic nerve into the eyeball where stim-
yielded either pupillary constriction or
pupillary dilatatio®.

Pupillary constriction in response to ciliary
ganglion stimulation was symmetrical, Pupillary

* dilatation in response to long ciliary nerve
stimulation was assymetrical in the meridian
of the stimulated nerve.

The ciliary muscle was revealed through a

scleral window measuring about 2 X 9 milli-

meters with its long dimension directed antero-
posteriorly, The front edge of the window
was cut 2 millimeters back of the limbus. A
loop of 6-0 silk thread was passed through the
superficial layer of the ciliary muscle and was
attached to the plate pin of a vacuum tube
transducer (RCA 5734) incorporated into ap-
propriate circuitry for tube operation and bal-
ancing. The output of the tube was amplified
by a Brush dc. amplifier and recorded on a
paper chart strip by a single channel Brush ink-
writing oscillograph. The system was cali-
brated by weights hung on the pin of the
transducer.

Records were taken with the thread running
in each of three directions from the same point
on the muscle, vertically above the eye, tan-
gentially in front of the eye, and tangentially
behind the eye (Fig. 1).

Electrical sliocks were delivered either by a
Harvard inductorium or an American Elec-
tronics Laboratories Model 104 electronic stim-
ulator. By means of switches shocks could be
directed to either set of nerves or to both
simultaneously. Two sets of stimulating elec-
trodes composed of silver wires mounted on
separate manipulator assemblies were used.

The preparation was bathed constantly in a
stream of warm (37C) Ringer solution through-
out the experiment. When nerves were to be
stimulated they were lifted clear of the Ringer
solution and adherent drops were gently blotted
off of the stimulating electrodes,

‘the recording arrangement was such that an
increase in tension on the transducer pin pro-
duced an upward deflection of the ink-writer
pen and a decrease in tension was registered
as a downward deflection. The upward de-

‘flections are designated as plus (+) responses;

the downward deflections indicate release of
resting tension on the transducer and are des-
ignated by a minus (—) sign.
Ultramicroelectrodes were pulled from 1 mm
stock capillary tubing by hand or by an Indus-
trial Science Associates microelectrode puller.
When filled with 3M KC1 such electrodes had
resistances of 5 to 25 megohms and their tips
could not be seen microscopically. The micro-
electrodes were connected through a 3M KCl

-4-


Pou .

as

refractive power of the eyes of rabbits, dogs,
cats, and monkeys, The opposite effect was
obtained upon stimulation of the parasympa-
thetic Third Nerve, The sympathetic effect
was present when the Third Nerve was cut,
so the effect could not have been due to cen-
tral inhibition of the parasympathetics. These
workers also demonstrated by skiascopy and by
photographs of the Purkinje-Sanson images in
humans and animals that startling stimuli
caused a transient decrease in accommodation
coincident with a decrease in skin resistance,
decrease in foot volume, pupillary dilation, and
increase in heart rate. They reasoned that the
dioptric change was a result of the generalized
sympathetic discharge. Olmstead et al believed
initially that the sympathetic effect was medi-
ated through contraction of ciliary muscle
fibers. Later, Fleming and Olmstead” modified
their original view by ascribing the sympathetic
effect to vascular changes in the ciliary body.
Sympathetic discharge was viewed as causing
vasoconstriction, thereby decreasing the volume
of the ciliary body and tightening the zonular
fibers; parasympathetic excitation ‘was believed
to have the opposite effect. The evidence for
this point of view was taken from experiments
showing that the time course for subsidence of
myopia following cervical sympathetic gangli-
onectomy in the cat was the same as that for
the subsidence of vascular dilatation in other
organs following sympathectomy, [ater Flem-
ing and Hall” offered histologic evidence in
favor of this idea. Re of how the effect
was mediated, Olmstead et al found that the
sympathetic fibers causing loss of accommo-
dation emerged from C8,through T3 segments
of the spinal cord, synapsed in thg superior
cervical ganglion and éntered the eye by way
of the long ciliary nerves. Olmstead et al
found that 0.5 to 20 diopters of “negative
accommodation” was mediated by the sympa-
thetic system %n dogs, cats, rabbits, monkeys
anid humans,
. Siebeck" used objective refracting techniques
to show that the far point and the rest point
of accommodation were not identical and that
sympathomimetic drugs instilled into the eye
shifted the rest point toward the far point.
Siebeck determined that an average of 0.75

diopers of negative ac.ommodation was sym-
pathetically mediated.

Numerous attempts have been made to record
movements of the ciliary muscle directly, The
first of these efforts was made by Henson and
Voelckers* in 1868, who demonstrated by the
movement of needles thrust through the sclera
that the ciliary muscle moved when the ciliary
ganglion was stimulated and that the choroid
was pulled forward by such contraction, Simul-
taneously, needles positioned to rest on the
anterior and posterior faces of the lens were
shown to move in a direction consistent with
thickening of the lens, Romer and Dufour"
in 1902 recorded movements of needles that
were thrust through the sclera into the ciliary
muscle when the cervical sympathetics were
stimulated in animals. Hess and Heine” could
not demonstate muscle movements though they
did show that stimulation of the cervical sym-
pathetics in the dog caused a decrease in
accommodation amounting to 1.5 diopters.
Cogan* demonstrated a loss of accommodation
of 0.5 to 0.75 diopters in a dog upon cervical
sympathetic stimulation. Meesman*” photo-
graphed movements of bits of casein placed
on the exposed ciliary muscle of enucleated
eyes of guinea pigs, cattle, pigs, dogs, cats
and humans in response to electrical and
pharmacological stimulation, The contractions
induced by parasympathomimetic agents could
be partially or entirely counteract@l by sym-
pathomimetic agents. Meesman concluded that

‘the ciliary muscle was dually innervated.

Wolter” and MatteuciTM presented histological
evidence 6£:sympathetic fibers in the ciliary
muscle, Sachs" recorded movements of- the
exposed ciliary muscle upon direct electrical
stimulation in excised eyes of dogs and cats.
Sachs described the muscle as moving back-
ward at the anterior coronal region and forward
in the posterior choroidal region, the two being
separated by a central stationary zone, Neither
parasympathomimetic nor sympathomimetic
drugs had any effect on the movement of the
muscle in Sachs’ experiments,

Our own observations were directed toward
contributing direct evidence regarding move-
ments of the ciliary muscle in the absence of

- -~3-


»

= =
=
=====
=? SSS — H
= = |
=
SS ===" i
H
=p
2
_ a
Sj
=
——— a
e
a

Excure 2. Response of ciliary muscle to stimulation of

ciliary ganglion:

A. Vertical recording. Decrease in static load
on the transducer equals 45 mg.

B, Transducer in front of the eye. Increase in
tension equals 25 mg.

C. Transduces in back of the eye. Decrease in
Static tension on transducer equals 10 mg.

a. Parasympathetic stimulation: The main di-
rection of movement of the ciliary muscle in
response to stimulation of the ciliary ganglion
or the short ciliary nerves was radially outward
as shown in the tables and illustrated in Fig. 2A.
The outwardly directed, vertically recorded
movement was the invariable result of parasym-
‘pathetic stimulation. Table I shows that on
parasympathetic stimulation 32 of the respon-
sive animals showed outward movement (ver-
tical —), but none showed inward movement
(vertical +).

In some of the experiments the radially out-
ward movement was accompanied by a back-
wardly directed movement which produced an
increase in tension on the transducer pin when .
the connecting thread was tangential to the
muscle in front of the eye (Fig. 2B). This
movement was visible under low (15x) magni-
fication, A contributing factor, however, was
a vector of the main outward movement.

Recordings taken tangentially to the muscle
in back of the eye sometimes showed a decrease
and sometimes an increase in tension on the
transducer pin (Table I, Fig. 2C), The down-
ward deflections were caused by the backward
movement whereas the upward deflections were
caused by a vector of the radially outward
movement. Forward movements of the muscle
were never visually observed and those record-
ings indicating forward movement are regarded
as artifacts.

In summary, the ciliary muscle moved in
response to parasympathetic stimulation prin-
cipally in a radially outward direction. In about
half of the cases the outward movement was
accompanied by a backward movement. This
response is “diagrammatically summarized in
Fig. 3. ‘

b. Sympathetic stimulation: The responses to
stimuli applied to the long ciliary nerves were
not as consistent as the responses to para-
sympathetic stimulation (Table I), In half
of the cases stimulation of those retrobulbar
nerves causing pupillary dilatation caused cil-
iary muscle movements identical with those
resulting from ciliary ganglion stimulation, In
the other half of the. cases sympathetic stimula-
tion produced movements directionally opposite
to those resulting from parasympathetic stim-
ulation, ie. radially inward movements as

-6-—


tT | ADY 3B BIZ £ART 63-5
WASA NES- RY 06

FEDERAL AVIATION AGENCY
| AVIATION MEDICAL SERVICE.
AEROMEDICAL RESEARCH DIVISION
"CIVIL AEROMEDICAL RESEARCH INSTITUTE ~~
/OXLAHOMA CITY, OKLAHOMA a

/ -

MARCH 1963



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*

Ficune 4.
&

Response of the ciliary’ muscle to stimulation of

the long ciliary nerves

A. Vertical recording. Increase in tensien
equals 25 mg. :

B. Transducer in front of the eye. Increase in
tension equals 20 mg.

C. Transducer in back of the eye. Increase in
tension equals 10 mg.

generated in the impaled cell, and their signifi- ° 7

cance must be doubted,

Stimulation of the retrobulbar nerves pro-
duced sudden depolarizations of the muscle but
did not give rise to spike potentials, Pilocarpine
in concentrations ranging from 10TM* gms/cc to
10°* gms/cc likewise produced depolarization
but no spikes,

Attempts to differentiate muscle cells on the
basis of their innervation were uniformly
unsuccessful. -

DISCUSSION

These experiments demonstrate that stimula-
tion of ocular sympathetic nerves can produce
active contractions of fibers within the ciliary
muscle. The direction of the movement pro-
duced by contraction of these fibers is direction-
ally opposite to the muscle movement produced
by parasympathetic stimulation and is con-
sistent with the concept that the radial fibers
are responsible,

The inward and outward movements as re-
corded in these experiments are interpreted
as those which would produce changes in
zonular tension in the intact eye. In these
experiments the outer surface of the exposed
muscle was free to move; in the intact eye the
outer surface of the muscle is fixed. Therefore,
the inward movement as recorded in these
experiments would be translated in the intact
eye into an outward pull, or increased tension,
on the zonule* The outward movement of the
musale as recorded by the transducer would,
in the intact eye, allow tensjon on the zonular
fibers to relax,

The movementé of the ciliary muscle in re-
sponse ‘to parasympathetic nerve impulses are
interpreted as a relaxation of autonomous tone
of the radial fibers. Such relaxation in the in-
tact eye would cause slackening of the zonular
fibers thus allowing the lens to thicken. The —
backward movement may represent participa-
tion by the meridional fibers in effecting near
accommodation by decreasing tension on the
forward-directed zonular fibers.

The inconsistency of the sympathetic re-
sponse may be due to unequal distribution of
autonomic fibers to the iris and ciliary body.
Warwick" has shown that only 3% of the para-
sympathetic fibers whose cell bodies lie in the

-8-


if

TRANSDUCER

A. Vertical recording

“thd

Ficcnz 1. Diagram of recording arrangement:

a. RCA 5734 mechano-electronic transducer

b. 60 silk thread

ce Ciliary muscle expgoed through scleral window.

B. Front recording ’
C. Rear recording

s
bridge to a chlorided silver ‘wire, A short
shielded cable connegted the electrode assembly
to the input of a Keithley Model 303 indicgti
dc. amplifier. An inditf ferent wick eles ode
made contact with the posterior part
exposed ciliary muscle. The amplifier had an
input resistance in excess of 10° ohms. The
output of the amplifier drove a Sanborn single
channel recorder. The electrode assembly was
mounted on a mitromanipulator (Brinkmann
RP IV) and was guided to the desired punc-
ture site while being observed under a micro-
scope (AO Cycloptic), The ciliary muscle was
revealed, as previously described, through a
scleral window.
Microelectrode punctures were made on cat
ciliary muscles both in vivo and in vitro.

‘ RESULTS

Stimulation of either set of autonomic nerves
to the eye catised the ciliary muscle to move.
The direction of movement in the three record-
ing directions in response to stimulation of the
parasympathetics and the sympathetics is shown
vin Table I. The numbers in this table include
all animals, regardless of whether or not re-
sponses were obtained from both sets of nerves
in each animal, In some animals it was possible
only to get one type of response, In other
experiments it was possible to get responses
to stimulation of the sympathetics and parasym-
pathetics in the same animals. These data are
shown in Table II and are grouped into the
four possible combinations of (+) and (-)

responses in'the three recording directions,

—5-


NEURAL CONTROL OF THE CILIARY MUSCLE®

CARLTON E. MELTON, JR, Ph. D.

ABSTRACT
The cat

muscle was shown to respond to both parasympathetic and

sympathetic
nerve impulses in citro. Sym responses were x opposite to parasympathetic
responses and are i as active contractions of the radial muscle layer. Such con-
tractions in the intact eye should result in accommodation of the eye for distant vision.

The normal mammalian eye is so constructed
that incident rays of light, provided they do
not diverge too greatly, can be focused on the
retina. The principal refracting surface is at
the cornea-air interface; however, the dioptric
strength of the eye can be adjusted by a change
in the radius of curvature of the crystalline
Tens so that rays of light emanating from
objects located at various distances may be
sharply focused on the retina. For example, the
normal (emmetropic) eye gazing into the dis-
tance has a strength of about 67 diopiters. When
an object held 7 cm in‘front of the face is
seen clearly, the eye has 2 strength of about
82 diopters, when it is said to be fully accom-
modated for near vision. .

The intraocular structures participjting in
accommodation are (1) the lens ich is
suspended by (2) a fibrillar membrane, the
zonule of Zinn, from (3) the ciliary body. The
lens is enclosed within an elastic capsule that
exerts forces on the lens tending to cause it

o decrease its radius of curvature and increase

its dioptric strength, The pull of the zonular
fibers counteracts this elasticity; thus, when
zonular tension is lessened, the lens increases
in dioptric strength and when zonular tension
is increased, the lens decreases in dioptric

strength,

The ciliary muscle by contracting to various
degrees exerts a variable amount of tension on
the zonule and thereby adjusts the dioptric
strength of the eye.

The human muscle is a smooth muscle
organ composed of three layers. The outer-
most layer lies just beneath the sclera and its
fibers run meridionally or antero-posteriorly.
This outer layer is called the meridional layer
or Brucke’s muscle. It originates at the anterior
border of the choroid and inserts on the scleral
spur. The middle layer is composed of a felt-
work of fibers running principally in a radial
direction: The innermost layer is the circular
smuscle layer (Muller's muscle) which runs
sphincter-bke parallel to the equatorial plane

ey

The cil: muscle is innervated by the Ocu-
lomotor Nerve. In primates, at least, contrac-
tion of the sphincter-like circular muscle layer
causes the diameter of the ciliary ring to de-
crease, thus releasing tension on the zonular
fibers and allowing the lens to become thicker.
The thickening takes place mainly at the central
anterior face. of the lens which bulges during
accommodation. The amount of thickening of
which ‘the lens is capable determines how
closely an object can be held in front of the
unaided eye and still be seen clearly. This

*Part of the work reported here has previously been published with Edward W._Pumell and

G. A. Brecher as co-authors.TM

-1-


ege

FOREWORD

This study is of particular relevance to the matter of “space myopia” and “night
myopia”, These’ two conditions will be encountered by civil aircraft pilots with increas-
ing frequency as the newer and faster aircraft (for example the supersonic transport)
become operational, An understanding of the mechanisms of these conditions is highly

significant to this new era of civil avi

A

ation activities.



_Z

NEURAL CONTROL OF THE CILIARY MUSCLE
a

oO

CARLTON E. MELTON, JR., Ph. D.
Chief, Electrophysiology Section

Nevropuysiotocy Brancy

FEDERAL AVIATION ACENCY
AVIATION MEDICAL SERVICE
AEROMEDICAL RESEARCH DIVISION
CIVIL AEROMEDICAL RESEARCH INSTITUTE
OKLAHOMA CITY, OKLAHOMA
MARCH 1963



- pupillary movements were used as a

a

STIMULUS

LONG CILIARY
NERVE
{SYM PATHETIC)

CILIARY
GANGLION
(PARASYMPATHETIC )

FAR ACCOMMODATION

LENS FLATTENED — PUPIL DILATED

SYMPATHETIC
VIA LONG CILIARY NERVE

- Ficung 5. Diagrammatic

summary of the response of the ciliary muscle to thetic
nerve stimulation. The heavy arrow indicates the main radially inward

move-

ment, The recordings made in the tangential directions show increased tension

as vectors of this main movement.

ciliary ganglion go to, the iris; the rest are
presumed to go to the ciliary muscle. Since
ériterion
in identification of nerves, it,is possible that

sympathetic nerves to the irs and parasym- -

pathetic nerves to the cilidry muscle were
stimulated, It is also possible that the sym-
tic nerves were unresponsive becaygse of
ypoxia or trauma and in increasing th¢ stim-
ulating voltage in attempts to attain thfeshold,
the current spread to the parasympathetic
nerves. In view of this possibility it would be
interesting to see if the outward movement
upon sympathetic stimulation could be blocked
by atropine. HdXvever, the reason for the in-
consistency of the response is not yet fully
understood nor can it be explained,
The literature pertaining to electrical activity

of the ciliary muscle is sparse. SchubertTM has

reported that a slow potential was recorded
from the comea during near accommodation
but that the potential could not be recorded
when the iris alone was stimulated to contract

by the consensual light reflex, He ascribed
the potential to the ciliary muscle. Jacobson,
et al.” reported that they recorded with micro-
electrodes trains of small spike potentials in
response to stretch of excised strips of ciliary
mtscle. The spikes were abolished by topical
procaine. These workers further reported slow
potential changes recorded with external elec-
trodes placed ‘on the sclera upon accommoda-
tive ‘effort in‘ humans. Jacobson et al, also
recorded slow electrical waves directly from
the exposed ciliary muscles of animals paralyzed
with succinylcholine when objects or bits of
food were brought close to the animals’ faces.
The same slow change took place regardless
of whether the human subjects or paralyzed
animals were changing gaze from far to near
or vice versa.

Our own failure to record spikes may repre-
sent a truer picture of the state of affairs than
the spike potentials recorded by Jacobson and
co-workers. It is probauly significant that the
only effective stimulus reported by them was

-10-