Cranial Nerves Lab Report
Sihan (Angela) Liu
Biology 203 Lab
November 17, 2018
Abstract
This study is the observation of a five-year-old girl named Zoe who had been experiencing vision problems. An evaluation performed by an ophthalmologist concluded that her left eye had a tendency to rotate medially. Zoe’s parents noticed that when she attempts to focus on an object that is directly in front of her, she turns her head slightly to the left. This behavior caused her doctor to believe that the misalignment of her eyes is due to a developmental problem with one of her cranial nerves. (BIOL 203 Lab Report Assignment, 2018)
Introduction
This study is an examination of 5-year-old Zoe, whom was brought in for an evaluation pertaining to her vision problems. Zoe’s parents observed her rotating her head slightly to her left when focusing on an object that was directly in front of her. Her left eye had a tendency to rotate medially. The ophthalmologist examined Zoe moving a penlight, directed towards her eyes, in front of her face. During the observation, Zoe was able to follow the penlight with both her eyes when it was moved above her head and then moved down below her chin. She was also able to follow the light with both eyes when it was moved to the patient-right side of her head. When the penlight was moved to the patient-left side, her left eye was able to track the light while her right eye remained facing forward. The doctor concluded that Zoe’s strabismus is making it difficult for her to focus both her eyes on a single object
Purpose
The purpose of this observation is to determine which one of Zoe’s cranial nerves experienced the developmental problems.
Background
There are twelve cranial nerves that are associated with the brain. These twelve nerves include optic, olfactory, trochlear, oculomotor, trigeminal, facial, abducens, glossopharyngeal, vestibulocochlear, accessory, vagus, and hypoglossal. Cranial nerves are categorized as nerves that have motor functions, sensory functions, and nerves that encompass both (Marieb, 2012).
The following cranial nerves that perform only motor functions include oculomotor (CN III), trochlear (CN IV), abducens (CN VI), accessory (CN XI), and hypoglossal (CN XII). The extraocular muscles innervated by CN III include the inferior, medial, and superior recti, the levator palpebrae muscles, and the inferior oblique. Its function is mostly to provide motor capabilities to the eye. CN IV innervates the superior oblique muscle. This nerve is responsible for supplying somatic motor fibers and carry proprioceptor fibers from the superior oblique muscle (Marieb, 2012). CN VI innervates the lateral rectus muscle (Cohen, Fadul, Jenkyn, & Ward, 2008). The responsibility of the abducens nerve is to supply somatic fibers to the lateral rectus muscle and convey proprioceptor impulses from the lateral rectus muscle to the brain (Marieb, 2012). CN XI innervates the sternocleidomastoid and trapezius. When combined, the sternocleidomastoid and trapezius is responsible for the movement of head and neck, which motor capabilities are provided to by the CN XI (Marieb, 2012). CN XII innervates the genioglossus, styloglossus, and hyoglossus; these would be three of the four extrinsic muscles of the tongue (Jaffe & Stewart, 2016). Since the CN XII innervates three of the four extrinsic muslces of the tongue, its primary job is to provide motor function to the tongue.
Cranial nerves that are associated with only sensory functions include olfactory (CN I) and optic (CN II). CN I innervates the superior orbital fissure and the function of CN I is to carry signals from smells to the brain. CN II innervates the optic canal and the primary function of this nerve is to transmit visual information to the brain (Marieb, 2012).
The cranial nerves that are responsible for a mix of sensory and motor functions are trigeminal (CN V), facial (CN VII), vestibulocochlear (CN VIII), glossopharyngeal (CN IX), and vagus (CN X) (Marieb, 2012). CN V innervates the muscles of mastication and the tensor muscle. The motor function of CN V involves controlling the muscles required for chewing while the sensory function of this nerve is responsible for transferring feelings of pain, touch, temperature, and pressure. CN VII innervates the muscles of facial expression. The motor function of CN VII is transmitting motor impulses to the muscles of facial expression (but does not include chewing muscles), while the sensory responsibility of CN VII is to transmit autonomic motor impulses to the lacrimal, nasal, palatine, submandibular, and sublingual salivary glands. Additionally, CN VII is also responsible for conveying sensory impulses from the taste buds of the anterior two-thirds of the tongue to the brain. CN VIII innervates the vestibular system of the inner ear. CN VIII made up of a vestibular component and cochlear component. Although the function of CN VIII is mostly sensory because it transmits impulses for the sense of sound, it also has a motor function that is responsible for adjusting the sensitivity of the sensory receptors. CN IX innervates the oropharynx, posterior one third of the tongue, middle ear cavity, eustachian tube, carotid body and sinus, and stylopharyngeus muscle of the pharynx. This nerve provides motor function to the skeletal muscles in the larynx, soft palate, and pharynx. The sensory responsibilities of CN IX include taste and conducting general sensory impulses. Lastly, the CN X innervates the skin of the external acoustic meatus, the jugular foramen, and descends through the neck region into the abdomen and thorax. CN X provides motor function for the muscles of the larynx and pharynx and it provides sensory function to the taste buds on the epiglottis (Marieb, 2012).
Standard eye movement is controlled by the muscles that are innervated by CN III, CN IV, and CN VI. Eye movement is broken down into two categories: 1) extraocular muscle function and 2) intrinsic ocular muscle function. Extraocular muscle function is responsible for the movement of the eye while the intrinsic ocular muscle function is responsible for lens and pupil. To specify, extraocular muscles include the following muscles: “medial, inferior, and superior recti, the inferior oblique, and levator palpebrae muscles.” (Cohen, Fadul, Jenkyn, & Ward, 2008) These muscles are innervated by CN III, CN IV, and CN VI. The muscles that are responsible include the iris sphincter, the ciliary muscle, and the radial pupillodilator muscles. These muscles are innervated by the parasympathetic component of CN III. For the purpose of this observation, the focus for standard ocular movement is on the extraocular muscles and their functionality. All of the extraocular muscles have a baseline muscle tone that is meant to stabilize the position of the eye. When the eye is triggered to move, different muscles will increase or decrease activity as needed. For medial eye movement, the medial rectus increases activity. Conversely, lateral eye movement is an increase in activity of the lateral rectus. For eye elevation and depression (above or below the horizontal plane), pairs of muscles will increase activity (Cohen , Fadul, Jenkyn, & Ward, 2008).
Hypothesis
The hypothesis is that Zoe’s misalignment of the eyes is caused by a developmental problem in her cranial nerves, causing her left eye to rotate medially.
Procedure
In order to establish a baseline control for eye movement to compare to Zoe, several tests were performed. The test procedures and goals are listed below.
Optic Nerve: Visual Acuity (Snellen Test)
The intent of this test is to test the functionality of the optic nerve by conducting a visual acuity (sharpness of visual image) test. (Howard, 2018)
Steps:
1. Have subject stand 20 ft away from and facing a Snellen eye chart on the wall.
2. Have subject cover one eye and read letters on the chart as the experimenter points to them. Begin with the top line and work to the bottom.
3. Take note of the lowest row of letters that the subject can read accurately.
4. Repeat for other eye.
Optic Nerve: Peripheral Vision
The intent of this test is to test the functionality of the optic nerve by conducting a peripheral (side vision) test. (Howard, 2018)
Steps:
1. Have the subject sit directly in front of the experimenter at the same level eye height and a distance of two feet.
2. Have subject cover right eye and look directly at experimenter with their left eye. The subject hold this position for the duration of the test.
3. The experimenter will hold a pencil to the subject’s left side until they can no longer see the pencil. Then, the experimenter will move the pencil in an arch towards the subject’s nose.
4. The subject will report when they first see the pencil, while continuing to stare forward directly at the experimenter.
5. Take note of when the subject first sees the pencil and record an estimated angle from the center of the subject’s pupil.
6. Repeat with right eye.
Oculomotor Nerve: Motor Function
The intent of this test is to test for the functionality the oculomotor nerve by testing the muscles that control eye movement (Howard, 2018).
1. Have the experimenter stand directly in front of the subject.
2. Have the subject stare directly ahead.
3. Instruct the experimenter to trace the letter “H” in space in the subject’s field of vision and look for smooth coordinated movement.
4. Have the experimenter take note of the subject’s eye movement.
Oculomotor Nerve: Eyelid Position
The intent of this test is to check for functionality of the oculomotor nerve by testing the levator palpebrae superioris muscle that is responsible for raising of the upper eyelid. (Howard, 2018)
1. Have the experimenter stand directly in front of subject.
2. The experimenter will look at the subject and record whether or not the eyelid is covering an estimated one-third of the iris.
Oculomotor Nerve: Pupil Size and Shape
The intent of this test is to check the functionality the oculomotor nerve by testing the parasympathetic fibers that have control over the sphincter pupillae (Howard, 2018).
1. Have the experimenter stand directly in front of subject.
2. The experimenter will look at the subject and record whether or not the subject’s pupils are equally dilated.
Oculomotor Nerve: Pupillary Constriction
The intent of this test is to determine whether or not the oculomotor nerve functions properly by testing the sphincter pupillae. The activation of the sphincter pupillae would cause pupil constriction (Howard, 2018).
1. Have the subject stand and focus on a far away object.
2. The experimenter will stand to the side and then hold a penlight about 20 centimeters to the lateral side of the subject’s eye and direct the light towards the subject’s eye.
3. Record the reaction of the subject’s pupil(s).
Oculomotor Nerve: Accommodation
The intent of this test is to determine whether or not the oculomotor nerves are functioning properly to allow a subject’s eyes to accommodate distance of object (Howard, 2018).
1. Have the subject stare straight ahead at a distant object.
2. Have the experimenter hold a pencil about 1 meter in front of the subject’s face.
3. Instruct the subject to stare and follow the pencil as the experimenter moves the pencil in a straight line towards the subject’s nose.
4. Observe and record the reaction of the subject’s eyes.
Trochlear Nerve: Motor Function
The intent of this test is to determine whether or not the trochlear nerve is functioning properly by testing the superior oblique (Howard, 2018).
1. Instruct the subject look down at the tip of their nose.
2. Observe and record whether or not the subject’s eye movement is symmetrical and smooth.
Abducens Nerve: Motor Function
The intent of this test is to determine whether or not the abducens nerve is functioning properly by testing the lateral rectus (Howard, 2018).
1. Have the subject stare straight ahead and then look to the left and then to the right.
2. Observe and record whether or not the movement was symmetrical and smooth.
Results
Optic Nerve: Visual Acuity
Control Subject Right Eye Left Eye
With glasses 20/20 20/20
Zoe Right Eye Left Eye
With glasses 20/20 20/20
Optic Nerve: Peripheral Vision
Control Subject Upper Lower Left Right
Left Eye 45° 45° 90° 90°
Right Eye 45° 45° 90° 90°
Zoe Upper Lower Left Right
Left Eye 45° 45° 90° 90°
Right Eye 45° 45° 90° 90°
Oculomotor Nerve: Motor Function
Control Subject Yes No
Able to track "H" with both eyes X
Zoe Yes No
Able to track "H" with both eyes X
Oculomotor Nerve: Eyelid Position
Control Subject Normal Drooping
Right Eye X
Left Eye X
Zoe Normal Drooping
Right Eye X
Left Eye X
Oculomotor Nerve: Pupil Size and Shape
Control Subject Yes/ Round No/Oval
Pupil equal in size X
Left Pupil X
Right Pupil X
Zoe Yes/ Round No/Oval
Pupil equal in size X
Left Pupil X
Right Pupil X
Oculomotor Nerve: Pupillary Constriction
Control Subject Yes/Present No/Absent
Pupillary Reflex X
Consensual X
Zoe Yes No
Pupillary Reflex X
Consensual X
Oculomotor Nerve: Accommodation
Control Subject Yes No
Accommodation X
Zoe Yes No
Accommodation X
Trochlear Nerve: Motor Function – Ability to look at tip of nose
Control Subject Normal Abnormal
Right Eye X
Left Eye X
Zoe Normal Abnormal
Right Eye X
Left Eye X
Abducens Nerve: Motor Function – Ability to turn eyes laterally
Control Subject Normal Abnormal
Right Eye X
Left Eye X
Zoe Normal Abnormal
Right Eye X
Left Eye X
Discussion
The participant in this experiment has been identified as Control Subject. After all the cranial nerve tests related to eye functionality tested normal, the results are deemed as the control baseline. Comparatively, Zoe tested normal in all areas except the abducens nerve test. The abducens nerve the motor nerve that carries commands from brain to eye and it innervates the lateral rectus. When the lateral rectus muscle receives impulses and increases in activity, the eye is turned laterally. Currently, Zoe’s left eye rotates medially; which shows an active medial rectus. This goes to indicate that there is no activity in the lateral rectus. If Zoe’s abducens nerve was developed normally, the lateral rectus would receive impulses for maintaining a specific muscle tone in order to maintain stability of Zoe’s left eye.
In the beginning, it was hypothesized that one of Zoe’s cranial nerves may have had a developmental problem and therefore Zoe currently suffers from strabismus (misalignment of eyes). The results of this experiment do support the original hypothesis. After testing, the cause of Zoe’s vision problem has been isolated to either the abducens nerve or her lateral rectus muscle. However, additional testing is required to determine if either her abducens nerve or lateral rectus muscle is responsible for her strabismus or if both are responsible.
References
BIOL 203 Lab Report Assignment [Word document]. (2018). n/a. Retrieved from https://howardcc.instructure.com/courses/1082435/files/64223952?module_item_id=11300689
Cohen, J., MD, Fadul, C., MD, Jenkyn, L., MD, & Ward, T., MD. (2008). Chapter 4 –
Extraocular movement. Retrieved November 16, 2018, from
https://www.dartmouth.edu/~dons/part_1/chapter_4.html
Howard Community College. (2016). Human anatomy and physiology 1 Laboratory manual. 3rd ed. Minneapolis, MN: bluedoor, LLC.
Jaffe, C., MD, & Stewart, W., PhD. (2016, March 25). Cranial Nerves. Retrieved November 18,
2018, from https://medicine.yale.edu/cranialnerves/nerves/hypoglossalnerve.aspx
Marieb, E. N., Hoehn, K. (2016). Human anatomy & physiology 10th ed. London, England: Pearson Education, Inc.