The cranial nerves...

CN 0... CN I... CN II... CN III... CN IV... CN V... CN VI... CN VII... CN VIII... CN IX... CN X... CN XI... CN XII...

CN 0 - Terminal CN I - Olfactory CN II - Optic CN III - Oculomotor CN IV - Trochlear CN V - Trigeminal CN VI - Abducens CN VII - Facial CN VIII - Vestibulocochlear CN IX - Glossopharyngeal CN X - Vagus CN XI - Accessory CN XII - Hypoglossal

Shoulder elevation and head-turning (XI)...

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.[1] Additionally, if the nerve is damaged, weakness or an inability to elevate the scapula may be present because the levator scapulae muscle is now solely able to provide this function.[14] Depending on the location of the lesion there may also be weakness present in the sternocleidomastoid muscle, which acts to turn the head so that the face points to the opposite side.

General overview of cranial nerves...

Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), in contrast to spinal nerves (which emerge from segments of the spinal cord). 10 of 12 of the cranial nerves originate in the brainstem. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck. Depending on definition in humans there are twelve or thirteen cranial nerves pairs, which are assigned Roman numerals I-XII, sometimes also including cranial nerve zero. The numbering of the cranial nerves is based on the order in which they emerge from the brain, front to back (brainstem). The terminal nerves, olfactory nerves (I) and optic nerves (II) emerge from the cerebrum or forebrain, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain. The cranial nerves are considered components of the peripheral nervous system (PNS), although on a structural level the olfactory, optic and trigeminal nerves are more accurately considered part of the central nervous system (CNS).

Facial expression (VII)...

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. A very common and generally temporary facial palsy is known as Bell's palsy. 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.[1] Bell's palsy occurs very rarely, affecting around 40,000 Americans annually. 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.[15] Bell's Palsy is a temporary condition usually lasting 2-6 months, but can have life-changing effects and can reoccur. 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.

Vagus nerve (X)...

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 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

How the facial nerve (VII) exits the skull and an extracranial course...

The facial nerve (VII) and vestibulocochlear nerve (VIII) both enter the internal auditory canal in the temporal bone. The facial nerve then reaches the side of the face by using the stylomastoid foramen, also in the temporal bone. Its fibers then spread out to reach and control all of the muscles of facial expression. The vestibulocochlear nerve reaches the organs that control balance and hearing in the temporal bone, and therefore does not reach the external surface of the skull.

How the glossopharyngeal (IX), vagus (X) and accessory nerves (XI) exit the skull and an extracranial course...

The glossopharyngeal (IX), vagus (X) and accessory nerve (XI) all leave the skull via the jugular foramen to enter the neck. The glossopharyngeal nerve provides innervation to the upper throat and the back of the tongue, the vagus provides innervation to the muscles in the voicebox, and continues downward to supply parasympathetic innervation to the chest and abdomen. The accessory nerve controls the trapezius and sternocleidomastoid muscles in the neck and shoulder.

Oral sensation, taste, and salivation (IX)...

The glossopharyngeal nerve (IX) innervates the stylopharyngeus muscle and provides sensory innervation to the oropharynx and back of the tongue.[1][16] The glossopharyngeal nerve also provides parasympathetic innervation to the parotid gland.[1] Unilateral absence of a gag reflex suggests a lesion of the glossopharyngeal nerve (IX), and perhaps the vagus nerve (X).

How the hypoglossal nerve (XII) exits the skull and an extracranial course...

The hypoglossal nerve (XII) exits the skull using the hypoglossal canal in the occipital bone and reaches the tongue to control almost all of the muscles involved in movements of this organ.

Tongue movement (XII)...

The hypoglossal nerve 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 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.

How the mandibular division of the trigeminal nerve (V3) exits the skull and an extracranial course...

The mandibular division of the trigeminal nerve (V3) passes through foramen ovale of the sphenoid bone to supply the lower face with sensory innervation. This nerve also sends branches to almost all of the muscles that control chewing.

How the maxillary division of the trigeminal nerve (V2) exits the skull and an extracranial course...

The maxillary division of the trigeminal nerve (V2) passes through foramen rotundum in the sphenoid bone to supply the skin of the middle of the face.

Eye movement (III, IV, VI)...

The oculomotor nerve (III), trochlear nerve (IV) and abducens nerve (VI) coordinate eye movement. Damage to nerves III, IV, or VI may affect the movement of the eyeball (globe). 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. Nerves III, IV and VI are tested by observing how the eye follows an object in different directions. This object may be a finger or a pin, and may be moved at different directions to test for pursuit velocity.[11] If the eyes do not work together, the most likely cause is damage to a specific cranial nerve or its nuclei.[11] 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).[12][12] Lesions may also lead to inability to open the eye due to paralysis of the levator palpebrae muscle. 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.[11] Damage to the trochlear nerve (IV) can also cause diplopia with the eye adducted and elevated.[12] The result will be an eye which can not move downwards properly (especially downwards when in an inward position). This is due to impairment in the superior oblique muscle, which is innervated by the trochlear nerve.[11] Damage to the abducens nerve (VI) can also result in diplopia.[12] This is due to impairment in the lateral rectus muscle, which is innervated by the abducens nerve.[11]

How the oculomotor nerve (III) exits the skull and an extracranial course...

The oculomotor nerve (III), trochlear nerve (IV), abducens nerve (VI) and the ophthalmic branch of the trigeminal nerve (V1) travel through the cavernous sinus into the superior orbital fissure, passing out of the skull into the orbit. These nerves control the small muscles that move the eye and also provide sensory innervation to the eye and orbit.

Smell (I)...

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.

How the olfactory nerve (I) exits the skull and an extracranial course...

The olfactory nerve (I), actually composed of many small separate nerve fibers, passes through perforations in the cribiform plate part of the ethmoid bone. These fibers terminate in the upper part of the nasal cavity and function to convey impulses containing information about odors to the brain.

How the optic nerve (II) exits the skull and an extracranial course...

The optic nerve (II) passes through the optic foramen in the sphenoid bone as it travels to the eye. It conveys visual information to the brain.

Vision (II)...

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.[12] Vision may be tested by examining the visual field, or by examining the retina with an ophthalmoscope, using a process known as funduscopy. Visual field testing may be used to pin-point structural lesions in the optic nerve, or further along the visual pathways.[11]

Trigeminal nerve (V)...

The trigeminal nerve (V) comprises three distinct parts: The Ophthalmic (V1), the Maxillary (V2), and the Mandibular (V3) nerves. Combined, these nerves provide sensation to the skin of the face and also controls the muscles of mastication (chewing).[1] Conditions affecting the trigeminal nerve (V) include trigeminal neuralgia,[1] cluster headache,[13] and trigeminal zoster.[1] Trigeminal neuralgia occurs later in life, from middle age onwards, most often after age 60, and is a condition typically associated with very strong pain distributed over the area innervated by the maxillary or mandibular nerve divisions of the trigeminal nerve (V2 and V3).

Hearing and balance (VIII)...

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. The cochlear nerve transmits information from the cochlea, allowing sound to be heard.[3] 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.[1][11] Damage to the vestibulocochlear nerve can also present as repetitive and involuntary eye movements (nystagmus), particularly when looking in a horizontal plane.[11] Damage to the cochlear nerve will cause partial or complete deafness in the affected ear.