Dysfunction of one or more cranial nerves indicates compression or stimulation by some lesion. For example an acoustic schwanoma may initially cause disturbance in hearing but with further growth of tumor it may involve other cranial nerves and the patient may present with pain resembling trigeminal neuralgia when the tumor involves trigeminal nerve or diplopia due to abducent nerve involvement similarly facial palsy with facial nerve compression.
A patient presenting with ptosis may have a posterior communicating artery aneurysm compressing the oculomotor nerve during its intracranial course.
Facial pain in the distribution of any one or all divisions of trigeminal nerve suggests stimulation of trigeminal nerve roots by a near by vessel. The cell bodies of many of the neurons of most of the cranial nerves are contained in one or more nuclei in the brainstem. These nuclei are important relative to cranial nerve dysfunction because damage to these nuclei such as from a stroke or trauma can mimic damage to one or more branches of a cranial nerve.
The fibers of these cranial nerves exit the brainstem from these nuclei. Some of the cranial nerves have sensory or parasympathetic ganglia collections of cell bodies of neurons, which are located outside the brain but can be inside or outside the skull. The sensory ganglia are directly correspondent to dorsal root ganglia of spinal nerves and are known as cranial sensory ganglia. After emerging from the brain, the cranial nerves travel within the skull , and some must leave this bony compartment in order to reach their destinations.
Often the nerves pass through holes in the skull, called foramina , as they travel to their destinations. Other nerves pass through bony canals, longer pathways enclosed by bone. These foramina and canals may contain more than one cranial nerve and may also contain blood vessels. The cranial nerves provide motor and sensory innervation mainly to the structures within the head and neck.
The sensory innervation includes both "general" sensation such as temperature and touch, and "special" innervation such as taste , vision , smell , balance and hearing [1] [11]. The vagus nerve X provides sensory and autonomic parasympathetic motor innervation to structures in the neck and also to most of the organs in the chest and abdomen. The terminal nerve 0 is involved in hormonal responses to smell, and has been implicated in sexual response and mate selection.
The olfactory nerve I conveys the sense of smell. Damage to the olfactory nerve I can cause an inability to smell anosmia , a distortion in the sense of smell parosmia , or a distortion or lack of taste. If there is suspicion of a change in the sense of smell, each nostril is tested with substances of known odors such as coffee or soap.
Intensely smelling substances, for example ammonia, may lead to the activation of pain receptors nociceptors of the trigeminal nerve that are located in the nasal cavity and this can confound olfactory testing. The optic nerve II transmits visual information.
Damage to the optic nerve II affects specific aspects of vision that depend on the location of the lesion. A person may not be able to see objects on their left or right sides homonymous hemianopsia , or may have difficulty seeing objects on their outer visual fields bitemporal hemianopsia if the optic chiasm is involved.
Visual field testing may be used to pin-point structural lesions in the optic nerve, or further along the visual pathways. Both or one eye may be affected; in either case double vision diplopia will likely occur because the movements of the eyes are no longer synchronized. This object may be a finger or a pin, and may be moved at different directions to test for pursuit velocity.
Damage to the oculomotor nerve III can cause double vision diplopia and inability to coordinate the movements of both eyes strabismus , also eyelid drooping ptosis and pupil dilation mydriasis. Individuals suffering from a lesion to the oculomotor nerve may compensate by tilting their heads to alleviate symptoms due to paralysis of one or more of the eye muscles it controls. Damage to the trochlear nerve IV can also cause diplopia with the eye adducted and elevated. This is due to impairment in the superior oblique muscle , which is innervated by the trochlear nerve.
Damage to the abducens nerve VI can also result in diplopia. The trigeminal nerve V comprises three distinct parts: Combined, these nerves provide sensation to the skin of the face and also controls the muscles of mastication chewing. Lesions of the facial nerve VII may manifest as facial palsy. This is where a person is unable to move the muscles on one or both sides of their face. In blunt trauma , the facial nerve is the most commonly injured cranial nerve.
Bell's Palsy is the result of an idiopathic unknown cause , unilateral lower motor neuron lesion of the facial nerve and is characterized by an inability to move the ipsilateral muscles of facial expression , including elevation of the eyebrow and furrowing of the forehead. Patients with Bell's palsy often have a drooping mouth on the affected side and often have trouble chewing because the buccinator muscle is affected.
The Cranial Nerves
There are studies in mice and humans suggesting members of the family Herpesviridae are capable of producing Bell's palsy. Facial paralysis may be caused by other conditions including stroke, and similar conditions to Bell's Palsy are occasionally misdiagnosed as Bell's Palsy. Strokes typically also affect the seventh cranial nerve by cutting off blood supply to nerves in the brain that signal this nerve and so can present with similar symptoms. The vestibulocochlear nerve VIII splits into the vestibular and cochlear nerve. The vestibular part is responsible for innervating the vestibules and semicircular canal of the inner ear ; this structure transmits information about balance , and is an important component of the vestibuloocular reflex , which keeps the head stable and allows the eyes to track moving objects.
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The cochlear nerve transmits information from the cochlea , allowing sound to be heard. When damaged, the vestibular nerve may give rise to the sensation of spinning and dizziness. Function of the vestibular nerve may be tested by putting cold and warm water in the ears and watching eye movements caloric stimulation. The glossopharyngeal nerve IX innervates the stylopharyngeus muscle and provides sensory innervation to the oropharynx and back of the tongue.
Loss of function of the vagus nerve X will lead to a loss of parasympathetic innervation to a very large number of structures. Major effects of damage to the vagus nerve may include a rise in blood pressure and heart rate. Isolated dysfunction of only the vagus nerve is rare, but - if the conflict or lesion is located above the point at which the vagus first branches off - can be diagnosed by a hoarse voice, due to dysfunction of one of its branches, the recurrent laryngeal nerve. Damage to this nerve may result in difficulties swallowing.
Damage to the accessory nerve XI will lead to ipsilateral weakness in the trapezius muscle. This can be tested by asking the subject to raise their shoulders or shrug, upon which the shoulder blade scapula will protrude into a winged position. The hypoglossal nerve XII is unique in that it is innervated from the motor cortices of both hemispheres of the brain.
Damage to the nerve at lower motor neuron level may lead to fasciculations or atrophy of the muscles of the tongue.
The Brain and Cranial Nerves - Chapter Summary and Learning Objectives
The fasciculations of the tongue are sometimes said to look like a "bag of worms". Upper motor neuron damage will not lead to atrophy or fasciculations, but only weakness of the innervated muscles. When the nerve is damaged, it will lead to weakness of tongue movement on one side. When damaged and extended, the tongue will move towards the weaker or damaged side, as shown in the image. Physicians, neurologists , and other medical professionals may conduct a cranial nerve examination as part of a neurological examination to examine the functionality of the cranial nerves.
This is a highly formalized series of tests that assess the status of each nerve. The visual fields are tested for nerve lesions or nystagmus via an analysis of specific eye movements. The sensation of the face is tested, and patients are asked to perform different facial movements, such as puffing out of the cheeks. Hearing is checked by voice and tuning forks. The position of the patient's uvula is examined because asymmetry in the position could indicate a lesion of the glossopharyngeal nerve. After the ability of the patient to use their shoulder to assess the accessory nerve XI , and the patient's tongue function is assessed by observing various tongue movements.
Nerves may be compressed because of increased intracranial pressure , a mass effect of an intracerebral haemorrhage , or tumour that presses against the nerves and interferes with the transmission of impulses along the nerve. An increase in intracranial pressure may lead to impairment of the optic nerves II due to compression of the surrounding veins and capillaries, causing swelling of the eyeball papilloedema. A pituitary tumour may compress the optic tracts or the optic chiasm of the optic nerve II , leading to visual field loss.
A pituitary tumour may also extend into the cavernous sinus, compressing the oculuomotor nerve III , trochlear nerve IV and abducens nerve VI , leading to double-vision and strabismus. These nerves may also be affected by herniation of the temporal lobes of the brain through the falx cerebri.
The cause of trigeminal neuralgia , in which one side of the face is exquisitely painful, is thought to be compression of the nerve by an artery as the nerve emerges from the brain stem. Occlusion of blood vessels that supply the nerves or their nuclei, an ischemic stroke , may cause specific signs and symptoms that can localise where the occlusion occurred.
A clot in a blood vessel draining the cavernous sinus cavernous sinus thrombosis affects the oculomotor III , trochlear IV , opthalamic branch of the trigeminal nerve V1 and the abducens nerve VI. Inflammation resulting from infection may impair the function of any of the cranial nerves. Inflammation of the facial nerve VII may result in Bell's palsy. Multiple sclerosis , an inflammatory process that may produce a loss of the myelin sheathes which surround the cranial nerves, may cause a variety of shifting symptoms affecting multiple cranial nerves.
Trauma to the skull, disease of bone such as Paget's disease , and injury to nerves during neurosurgery such as tumor removal are other possible causes of cranial nerve damage. Finally, in , German anatomist Samuel Soemmering named the 12 pairs of nerves that are generally accepted today. In other animals, it appears to be important to sexual receptivity based on perceptions of phermones [1] [28].
Cranial nerves are also present in other vertebrates. Other amniotes non-amphibian tetrapods have cranial nerves similar to those of humans. In anamniotes fishes and amphibians , the accessory nerve XI and hypoglossal nerve XII do not exist, with the accessory nerve XI being an integral part of the vagus nerve X ; the hypoglossal nerve XII is represented by a variable number of spinal nerves emerging from vertebral segments fused into the occiput.
These two nerves only became discrete nerves in the ancestors of amniotes non-amphibian tetrapods. Ventral view of the sheep 's brain. The exits of the various cranial nerves are marked with red. Gross Anatomy of the Human Brain: Major Anatomical Structures and Terminology Get familiar with the brain's anatomy and learn common vocabulary associated with the brain. Cerebrospinal Fluid in the Brain: Cranial Nerves of the Face and Mouth: Motion and Sensation Functionality Investigate the nerves that help our tongues move.
The Vagus Nerve and its Functionality Determine the role that the vagus nerve plays in the body. In this lesson, we take a look at your brain when you were the size of a peanut and your brain the size of a pea. What major structures did your brain have back then? What were they called?
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And most interestingly, how do these structures affect your life today? Your brain is in charge of telling the rest of your body what to do. Learn about the parts of your brain including the cerebrum, the cerebellum, the brain stem and the pituitary gland and how they help you live, play and grow. The brain's complexity can often be difficult to understand because it serves so many important functions. In this lesson, you'll get a little help from Nrr, the alien biologist, who will guide you through understanding the structure and functions of the human brain.
Check out this video lesson to learn about the four lobes of the human brain - the frontal, parietal, occipital and temporal. You'll learn about the functions and processes of each region. In this lesson, we'll learn how you are able to breathe without even thinking about it. You'll also find out why you breathe faster when you run. Finally, we'll learn where things as diverse as sneezing and blood pressure are controlled. The limbic system is composed of structures in the brain that deal with emotions such as anger, happiness and fear as well as memories.
This article will address the limbic system, its parts and their functions in the human body. In this video, you'll find out how and why we divide our brain into major sections. It's not just random names and games. You'll learn about where these divisions occur, what we call them, and what they separate. Ever heard of a split personality? How about a split brain? Well, we can't be sure about the personality one, but brains can be split in two!
Neuroscience for Kids - Cranial Nerves
In this lesson, you'll learn that and much more on how the brain communicates within itself. You'll never look at a shopping mall the same way again once you learn in this lesson how similar it is to your brain. We'll also find out why connections between blood vessels are important and how your brain protects itself against toxic substances in your blood. In this lesson, you'll learn that you have holes in your brain and that it's actually a good thing!
You'll also find out how your brain uses a water cushion for protection and where this water cushion is made. We wouldn't be able to talk, taste, chew, or swallow without the cranial nerves of our face and mouth. Find out how these nerves help us move our tongue and enjoy everything from kissing to food to conversation. Find out what the vagus nerve cranial nerve X does in your body.
While perhaps not as impressive, you'll also find out what the accessory nerve cranial nerve XI does and why it is important to the vagus nerve. Did you know… We have over college courses that prepare you to earn credit by exam that is accepted by over 1, colleges and universities. You can test out of the first two years of college and save thousands off your degree.
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