This response involves the relaxation of the iris sphincter and contraction of the iris dilator. The iris dilator is controlled by the sympathetic nervous system. The pupillary dark reflex neural circuit : The pathway controlling pupil dilation involves the. Axons from the superior cervical ganglion also innervate the face vasculature, sweat and lachrymal glands and the eyelid tarsal muscles.
When the superior cervical ganglion or its axons are damaged, a constellation of symptoms, known as Horner's syndrome , result. This syndrome is characterized by miosis pupil constriction , anhidrosis loss of sweating , pseudoptosis mild eyelid droop , enopthalmosis sunken eye and flushing of the face.
The accommodation response is elicited when the viewer directs his eyes from a distant greater than 30 ft. The accommodation near point response is consensual i. The accommodation response involves three actions:. Pupil accommodation: The action of the iris sphincter was covered in the section on the pupillary light reflex. During accommodation, pupil constriction utilizes the "pin-hole" effect and increases the depth of focus of the eye by blocking the light scattered by the periphery of the cornea Nolte, Figure , Pg.
The iris sphincter is innervated by the postganglionic parasympathetic axons short ciliary nerve fibers of the ciliary ganglion Figure 7. Lens accommodation: Lens accommodation increases the curvature of the lens, which increases its refractive focusing power. The ciliary muscles are responsible for the lens accommodation response.
They control the tension on the zonules, which are attached to the elastic lens capsule at one end and anchored to the ciliary body at the other end Figure 7.
The ciliary muscles function as a sphincter and when contracted pull the ciliary body toward the lens to decrease tension on the zonules see Figure 7. The decreased tension allows the lens to increase its curvature and refractive focusing power. When the ciliary muscle is relaxed, the ciliary body is not pulled toward the lens, and the tension on the zonules is higher. High tension on the zonules pulls radially on the lens capsule and flattens the lens for distance vision.
The ciliary muscles are innervated by the postganglionic parasympathetic axons short ciliary nerve fibers of the ciliary ganglion. Convergence in accommodation: When shifting one's view from a distant object to a nearby object, the eyes converge are directed nasally to keep the object's image focused on the foveae of the two eyes. This action involves the contraction of the medial rectus muscles of the two eyes and relaxation of the lateral rectus muscles.
The medial rectus attaches to the medial aspect of the eye and its contraction directs the eye nasally adducts the eye. The medial rectus is innervated by motor neurons in the oculomotor nucleus and nerve. The accommodation neural circuit : The circuitry of the accommodation response is more complex than that of the pupillary light reflex Figure 7.
Ocular motor control neurons are interposed between the afferent and efferent limbs of this circuit and include the. During accommodation three motor responses occur: convergence medial rectus contracts to direct the eye nasally , pupil constriction iris sphincter contracts to decrease the iris aperture and lens accommodation ciliary muscles contract to decrease tension on the zonules.
An excellent way to test your knowledge of the material presented thus far is by examining the effects of damage to structures within the ocular motor pathways. The observed motor loss s provide clues to the pathway s affected; and the muscle s and eye affected provide clues to the level of the damage.
Cranial nerve damage : Damage to cranial nerves may result in sensory and motor symptoms. The sensory losses would involve those sensations the cranial nerve normally conveys e.
The motor losses may be severe i. The cranial nerves involved in the eye blink response and pupillary response are the optic, oculomotor, trigeminal and facial nerves.
The patient, who appears with a bloodshot left eye, complains of an inability to close his left eye. When asked to rise his eyebrows, he can only elevate the right eyebrow.
When asked to close both eyes, the right eyelid closes but the left eyelid is only partially closed. Touching the right or left cornea with a wisp of cotton elicits the eye blink reflex in the right eye, but not the left eye Figure 7. However, the patient reports he can feel the cotton when it touches either eye.
He can smile, whistle and show his teeth, which indicates his lower facial muscles are functioning normally. Physical examination determines that touch, vibration, position and pain sensations are normal over the entire the body and face. There are no other motor symptoms. Pathway s affected : You conclude that structures in the following motor pathway have been affected.
When lower motor neurons are damaged , there is a flaccid paralysis of the muscle normally innervated. The action of the muscle will be weakened or lost depending on the extent of the damage.
There will be a weakened or no reflex response and the muscle will be flaccid and may atrophy with time. The Facial Nerve. Section of the facial nerve on one side will result in paralysis of the muscles of facial expression on the ipsilesional side of the face. There will be an inability to close the denervated eyelid voluntarily and reflexively. The eyelids may have some mobility if the oculomotor innervation to the levator is unaffected.
The patient complains of a badly infected left eye. When he is asked to close both eyes, both eyelids close. Touching the right cornea with a wisp of cotton elicits the eye blink reflex in the both eyes Figure 7. However, touching the left cornea with a wisp of cotton does not elicit the eye blink reflex in the either eye Figure 7.
The patient cannot detect pinpricks to his left forehead. However, he reports that pinpricks to rest of his face are painful. Difference in the light response of the eyes could help identify lesions. An abnormal pupillary reflex may indicate nerve lesion e. Plants, like animals, produce hormones to regulate plant activities, including growth. They need these hormones to respo.. Neurons generate electric signals that they pass along to the other neurons or target tissues.
In this tutorial, you wil.. Human Neurology deals essentially with the nervous system of humans. It also features the various theories put forward b.. Hormones are produced in the endocrine glands of animals. The pituitary gland and hypothalamus are the most impor.. Hormones are essential in the regulation of the activity of the various biological systems of the human body. Dilation lag detection using infrared videography is the most sensitive diagnostic test for Horner syndrome [4].
The reflex describes the finding of pupillary constriction in darkness or as part of closing eyelids when going to sleep. It is hypothesized that it is due to oculomotor disinhibition. The ciliospinal reflex is pupillary dilation in response to noxious stimuli, such as pinching, to the face, neck, or upper trunk.
Pathway: The trigeminal nerve or cervical pain fibers, which are part of the lateral spinothalamic tract, carry the afferent inputs of the ciliospinal reflex. If the pupillary dilation is due to the ciliospinal reflex, prolonged pupillary light stimulation should constrict the pupils [8] However, prolonged light stimulation cannot overcome pupillary dilation caused by bilateral third nerve palsies and midbrain dysfunction [8].
It consists of a pupillary accommodation reflex, lens accommodation reflex, and convergence reflex. Afferent pathway for pupillary constriction, lens accommodation, and convergence: Afferent input from the retina is sent to the lateral geniculate nucleus via the optic tract [2]. Fibers from the LGN then project to the visual cortex. Efferent pathway for pupillary constriction: Efferent parasympathetic fibers from the E-W nucleus project via the oculomotor nerve to the ciliary ganglion and then short ciliary nerves to innervate the iris sphincter muscle to cause pupillary constriction [2].
Efferent pathway for lens accommodation: Efferent parasympathetic fibers from the E-W nucleus project via the oculomotor nerve to the ciliary ganglion and then short ciliary nerves to innervate the ciliary muscle to cause contraction [2]. Contraction of the ciliary muscle allows the lens zonular fibers to relax and the lens to become more round, increasing its refractive power. Efferent pathway for convergence: Efferent fibers from the medial rectus subnucleus of the oculomotor complex in the midbrain innervate the bilateral medial rectus muscles to cause convergence [2].
Ophthalmologic considerations: Deficits in accommodation are usually acquired due to aging and presbyopia [4].
Isolated accommodation deficits can occur in healthy persons or in patients with neurological or systemic conditions such as in children after a viral illness and in women before or after childbirth. Accommodation insufficiency is also less commonly associated with primary ocular disorders e. Light-near dissociation describes constriction of the pupils during the accommodative response that is stronger than the light response, and it is the primary feature of Argyll Robertson pupils in patients with neurosyphilis [4].
Immediately following denervation injury, there is a dilated pupil that is unresponsive to light or near stimulation. Ciliary muscle dysfunction gradually improves over several months as injured axons regenerate and reinnervate the ciliary muscle, and the pupil becomes smaller over time. While the near response of the pupil begins to improve, the light response remains impaired, causing light-near dissociation.
The corneal reflex causes both eyes to blink in response to tactile stimulation of the cornea [2]. Pathway: Inputs are first detected by trigeminal primary afferent fibers i. These primary afferent fibers synapse on secondary afferent fibers in the spinal trigeminal nucleus, which send axons to reticular formation interneurons, which travel to the bilateral facial nuclei. Fibers from the facial nuclei motor neurons send axons through the facial nerve to the orbicularis oculi muscle, which lowers the eyelid.
Ophthalmologic considerations: The corneal reflex can be utilized as a test of corneal sensation in patients who are obtunded or semicomatose [4]. However, an abnormal corneal reflex does not necessarily indicate a trigeminal nerve lesion, as unilateral ocular disease or weakness of the orbicularis oculi muscle can also be responsible for a decreased corneal response [4].
An abnormal blink reflex may be present in patients with various posterior fossa disorders, including acoustic neuroma, multiple sclerosis, Parkinson disease, trigeminal nerve lesions, and brainstem strokes, tumors, or syrinxes [4]. The vestibulo-ocular reflex VOR allows for eye movements in the opposite direction of head movement to maintain steady gaze and prevent retinal image slip [4].
Horizontal VOR involves coordination of the abducens and oculomotor nuclei via the medial longitudinal fasciculus. VOR can be assessed in several ways. VOR can be evaluated using an ophthalmoscope to view the optic disc while the patient rotates his or her head; if the VOR is abnormal, catch-up saccades will manifest as jerkiness of the optic disc.
Caloric stimulation can also be used to examine the VOR [4]. Irrigation of the external auditory meatus with ice water causes convection currents of the vestibular endolymph that displace the cupula in the semicircular canal, which induces tonic deviation of the eyes toward the stimulated ear [4]. Examination of the VOR via head rotation or caloric stimulation can be useful in the evaluation of unconscious patients, as tonic eye deviation indicates preserved pontine function [4].
Pathway: Afferent fibers are carried by facial nerve. Efferent fibers travel in the oculomotor nerve to the superior rectus muscle to cause an upward deviation of the eyes. This reflex is especially visible in patients with Bell palsy, an acute disorder of the facial nerve, due to failure of adequate eyelid closure [10]. An absent reflex may be the only neurological abnormality in patients with idiopathic epilepsy, Sturge-Weber syndrome, and tuberous sclerosis. The lacrimatory reflex causes tear secretion in response to various stimuli: 1.
Pathway: Afferent signals are from the ophthalmic branch of the trigeminal nerve [1]. The superior salivatory nucleus in the pons gives off parasympathetic fibers that join other parasympathetic efferents from the salivatory nucleus [1]. These fibers run with gustatory afferents parallel to the facial nerve as the nervus intermedius and exit at the geniculate ganglion [12] [13].
The parasympathetic fibers then leave CNVII as the greater superficial petrosal nerve and synapse in the sphenopalatine ganglion. Postganglionic fibers travel with the lacrimal nerve to reach the lacrimal gland and cause reflex tearing. Ophthalmologic considerations: Abnormalities in this pathway may cause hypolacrimation, hyperlacrimation, or inappropriate lacrimation [4].
Hypolacrimation may be secondary to deafferentation of the tear reflex on one side, which can be due to severe trigeminal neuropathy, or damage to the parasympathetic lacrimal fibers in the efferent limb of the reflex [4]. Lesions may affect the nervus intermedius, greater superficial petrosal nerve, sphenopalatine ganglion, or zygomaticotemporal nerve. Hyperlacrimation may be due to excessive triggers of the tear reflex arc or from efferent parasympathetic fiber overstimulation.
Inappropriate lacrimation can occur with the gustolacrimal reflex, described below. The reflex describes unilateral lacrimation when a person eats or drinks [14]. The nerves may redirect themselves through the greater superficial petrosal nerve to reach the lacrimal gland, causing ipsilateral tearing when the patient eats.
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