Pupillary reflex
Overview
 
The pupillary light reflex is a reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 that controls the diameter of the pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...

, in response to the intensity (luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...

) of light that falls on the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

, thereby assisting in adaptation
Adaptation (eye)
In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.-Efficacy:The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the...

 to various levels of darkness and light, in addition to retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

l sensitivity. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in).
Encyclopedia
The pupillary light reflex is a reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...

 that controls the diameter of the pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...

, in response to the intensity (luminance
Luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...

) of light that falls on the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

 of the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

, thereby assisting in adaptation
Adaptation (eye)
In ocular physiology, adaptation is the ability of the eye to adjust to various levels of darkness and light.-Efficacy:The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the...

 to various levels of darkness and light, in addition to retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

l sensitivity. Greater intensity light causes the pupil to become smaller (allowing less light in), whereas lower intensity light causes the pupil to become larger (allowing more light in). Thus, the pupillary light reflex regulates the intensity of light entering the eye.

Mechanism

The optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

, or more precisely, the photosensitive ganglion cell
Photosensitive ganglion cell
Photosensitive ganglion cells, also called photosensitive Retinal Ganglion Cells , intrinsically photosensitive Retinal Ganglion Cells or melanopsin-containing ganglion cells, are a type of neuron in the retina of the mammalian eye.They were discovered in the early 1990sand are, unlike other...

s through the retinohypothalamic tract
Retinohypothalamic tract
The retinohypothalamic tract is a photic input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive retinal ganglion cells , which contain the photopigment melanopsin...

, is responsible for the afferent
Afferent
Afferent is an anatomical term with the following meanings:*Conveying towards a center, for example the afferent arterioles conveying blood towards the Bowman's capsule in the Kidney. Opposite to Efferent.*Something that so conducts, see Afferent nerve fiber...

 limb of the pupillary reflex - it senses the incoming light. The oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

 is responsible for the efferent
Efferent
Efferent is an anatomical term with the following meanings:*Conveying away from a center, for example the efferent arterioles conveying blood away from the Bowman's capsule in the kidney. Opposite to afferent....

 limb of the pupillary reflex - it drives the muscles that constrict the pupil.

Neuron 1

The pupillary reflex pathway begins with the photosensitive retinal ganglion cell
Ganglion cell
A retinal ganglion cell is a type of neuron located near the inner surface of the retina of the eye. It receives visual information from photoreceptors via two intermediate neuron types: bipolar cells and amacrine cells...

s, which convey information to the optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

 (via the optic disc
Optic disc
The optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot"...

). The optic nerve connects to the pretectal nucleus
Pretectum
The pretectum, also known as the pretectal area, is a region of neurons found between the thalamus and midbrain. It receives binocular sensory input from retinal ganglion cells of the eyes, and is the region responsible for maintaining the pupillary light reflex.-Outputs:The pretectum, after...

 of the upper midbrain, bypassing the lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....

 and the primary visual cortex.

These "intrinsic photosensitive ganglion cells" are also referred to as "melanopsin
Melanopsin
Melanopsin is a photopigment found in specialized photosensitive ganglion cells of the retina that are involved in the regulation of circadian rhythms, pupillary light reflex, and other non-visual responses to light. In structure, melanopsin is an opsin, a retinylidene protein variety of...

-containing" cells, and they influence the circadian rhythms and the pupillary light reflex.

Neuron 2

From the pretectal nucleus, axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

s connect to neurons in the Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

, whose axons run along both the left and right oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

s.

Neuron 4

Neuron #4 innervates the constrictor muscle
Iris sphincter muscle
The iris sphincter muscle is a muscle in the part of the eye called the iris...

 of the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

.

A Mathematical Description

Pupillary Light Reflex is modeled as a physiologically-based non-linear delay differential equation that describes the changes in the pupil diameter as a function of the environment lighting:





where is the pupil diameter measured in millimeters and is the luminous intensity reaching the retina in a time , which can be described as : luminance reaching the eye in lumens/mm2 times the pupil area in mm2. is the pupillary latency, a time delay between the instant in which the light pulse reaches the retina and the beginning of iridal reaction due nerve transmission, neuro-muscular excitation and activation delays. , and are the derivatives for the function, pupil diameter and time .

Since the pupil constriction velocity is approximately 3 times faster than (re)dilation velocity, different step sizes in the numerical solver simulation must be used:





where and are respectively the for constriction and dilation measured in milliseconds, and are respectively the current and previous simulation times (times since the simulation started) measured in milliseconds, is a constant that affects the constriction/dilation velocity and varies among individuals.
The higher the value, the smaller the time step used in the simulation and, consequently, the smaller the pupil constriction/dilation velocity.

In order to improve the realism of the resulting simulations, the hippus effect can be approximated by adding small random variations to the environment light (in the range of 0.05 Hz to 0.3 Hz) as proposed by.

Clinical significance

In addition to controlling the amount of light that enters the eye, the pupillary light reflex provides a useful diagnostic tool. It allows for testing the integrity of the sensory
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...

 and motor
Motor system
The motor system is the part of the central nervous system that is involved with movement. It consists of the pyramidal and extrapyramidal system....

 functions of the eye.

Under normal conditions, the pupils of both eyes respond identically to a light stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

, regardless of which eye is being stimulated. Light entering one eye produces a constriction of the pupil of that eye, the direct response, as well as a constriction of the pupil of the unstimulated eye, the consensual response
Consensual response
A Consensual response is any reflex observed on one side of the body when the other side has been stimulated.For example, if an individual's right eye is shielded and light shines into the left eye, constriction of the right pupil will occur, as well as the left...

. Comparing these two responses in both eyes is helpful in locating a lesion
Lesion
A lesion is any abnormality in the tissue of an organism , usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury.- Types :...

.

For example, a direct response in the right pupil without a consensual response in the left pupil suggests a problem with the motor connection to the left pupil (perhaps as a result of damage to the oculomotor nerve
Oculomotor nerve
The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

 or Edinger-Westphal nucleus
Edinger-Westphal nucleus
The Edinger-Westphal nucleus is the accessory parasympathetic cranial nerve nucleus of the oculomotor nerve , supplying the constricting muscles of the iris...

 of the brainstem). Lack of response to light stimulation of the right eye if both eyes respond normally to stimulation of the left eye indicates damage to the sensory input from the right eye (perhaps to the right retina or optic nerve
Optic nerve
The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

).

Emergency room physicians routinely assess the pupillary reflex because it is useful for gauging brain stem
Brain stem
In vertebrate anatomy the brainstem is the posterior part of the brain, adjoining and structurally continuous with the spinal cord. The brain stem provides the main motor and sensory innervation to the face and neck via the cranial nerves...

 function. Normally, pupils react (i.e. constrict) equally. Lack of the pupillary reflex or an abnormal pupillary reflex can be caused by optic nerve damage, oculomotor nerve damage, brain stem death
Brain stem death
Brain Stem Death is a clinical concept, implying an irreversibly unconscious patient, with irreversible apnea and irreversible loss of brain stem reflexes. The concept defines the core physiological basis for neurological diagnosis of death in the United Kingdom and elsewhere...

 and depressant drugs, such as barbiturates.

Normally, both pupils should constrict with light shone into either eye alone. On testing each reflex for each eye, several patterns are possible.
  • Optic nerve
    Optic nerve
    The optic nerve, also called cranial nerve 2, transmits visual information from the retina to the brain. Derived from the embryonic retinal ganglion cell, a diverticulum located in the diencephalon, the optic nerve doesn't regenerate after transection.-Anatomy:The optic nerve is the second of...

     damage on one side: (Example in parens.: Left optic nerve lesion)
    • The ipsilateral direct reflex is lost (Example: when the left eye is stimulated, neither pupil constricts, as no signals reach the brain from the left eye due to its damaged optic nerve)
    • The ipsilateral consensual reflex is INTACT (because light shone into the right eye can signal to the brain, causing constriction of both pupils via the normal oculomotor nerves)
    • The contralateral direct reflex is intact (because light shone into the right eye can signal to the brain, causing constriction of both pupils via the normal oculomotor nerves)
    • The contralateral consensual reflex is lost (because light shone into the eye on the damaged side cannot signal to the brain; therefore, despite the right eye's motor pathway (oculomotor nerve) being intact, no signals from the left eye are able to stimulate it due to the damage to the sensory pathway (optic nerve) of the left eye)

  • Oculomotor nerve
    Oculomotor nerve
    The oculomotor nerve is the 3rd of 12 paired cranial nerves. It enters the orbit via the superior orbital fissure and controls most of the eye's movements, including constriction of the pupil and maintaining an open eyelid by innervating the Levator palpebrae superiors muscle. The optic nerve is...

    damage on one side: (Example in parens: Left oculomotor lesion)
    • The ipsilateral direct reflex is lost (Example: when the left eye is stimulated, only the right pupil constricts)
    • The ipsilateral consensual reflex is lost (Example: when the right eye is stimulated, only the right pupil constricts)
    • The contralateral direct reflex is intact (because light shone into both eyes can still signal to the brain, and the pupil on the undamaged side will still be able to constrict via its normal oculomotor nerve)
    • The contralateral consensual reflex is intact (because light shone into the left eye can still signal to the brain via the normal optic nerve, causing attempted constriction of both pupils; the contralateral pupil constricts via its normal oculomotor nerve, but the ipsilateral pupil is unable to constrict due to its damaged oculomotor nerve)

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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