Cranial Nerves

Nervous system anatomy - cranial nerves, brain, CSF

Cranial nerves

Trochlear Nerve

- Supplies the superior oblique muscle of the eye. The nucleus for this nerve lies in the midbrain, and passes through the cavernous sinus to enter the superior orbital fissure of the orbit.

Oculomotor Nerve

- The nucleus for the occulomotor nerve (cranial nerve III) lies in the upper portion of the brainstem.

- Controls certain extraoccular muscles and movements. The Edinger Westphal nucleus is part of the third nerve nucleus, and sends parasympathetic fibers to cause constriction of the pupils. - Through a sophisticated connection of fibers in the brainstem, the occulomotor nerve (cranial nerve III) interacts with the abducens nerve (cranial nerve VI), causing the eyes to move in unison, allowing stereo vision.

- Note that the parasympathetic fibers cause constriction of the pupils, while the sympathetic fibers cause dilation of the pupils.

Questions

Optic nerve

-Vision

- Each of the two optic nerves leave the orbit through the optic foramen and unites with the other optic nerve at the optic chiasm.

- In the chiasm, some of the nerve fibers cross to the other side.

- The optic tract continues and sends signals to the lateral geniculate body, then through the optic radiations to the calcarine cortex of the occipital lobe (where vision is perceived).

Olfactory nerve

- Sense of smell

- Within the mucosa of the nasal cavity exist fine nerves, which ascend through the cribriform plate (part of the skull base which contains fine perforations through which nerves may travel) and then travel in the olfactory tract to enter the undersurface of the brain.

Trigeminal nerve

Abducens nerve

- Both sensation to the face as well as the motor muscles of mastication.

- Both the motor and sensory portions of the trigeminal nerve emerge from the lateral aspect of the pons. The nerve travels out to the Gasserian ganglion, from where three branches or divisions of the nerve are given off.

• Ophthalmic division supplies sensation to the forehead and cornea. This division is responsible for the corneal reflex (when something touches the cornea, the eye reflexively blinks shut).
• Maxillary division supplies the cheek
• Mandibular division supplies the jaw (has anterior and posterior). The anterior branch supplies the muscles of mastication (masseter, pterygoids and temporalis), and also supplies sensation to the cheek and gums through the buccal nerve. The lingual branch (posterior) helps in supplying taste to the anterior two thirds of the tongue.

Facial nerve

-Responsible for abducting the eye by causing constriction of the lateral rectus muscle.

-The nerve leaves the front of the brainstem between the pons and the medulla. It then travels through a dural canal known as Dorello's canal, before it passes through the cavernous sinus, to enter the orbit.

Vestibulocochlear nerve

- Carries mainly motor fibers to the muscles of facial expression, also carries fibers which relay taste, as well as fibers carrying parasympathetic information.

-The motor nucleus lies in the lower pons, and the facial nerve exits the lateral aspect of the brainstem, to enter the internal auditory canal.

-The parasympathetic component (goes to the lacrimal gland to produce tears) and the taste fibers (receive taste from the anterior two thirds of the tongue, through the chorda tympani nerve) both run in a small nerve adjacent to the facial nerve, known as the nervus intermedius.

- Consists of the vestibular portion and the cochlear portion.

-The vestibular portion is responsible for responding to rotational and linear acceleration, which helps maintain equilibrium and body orientation in space. Semicircular canals contain endolymph, which during movement, displaces small hair cells, which in turn send a signal through the vestibular nerve.

-The cochlea is responsible for hearing, as it converts sound waves into signals in the cochlear neurons.

Glossopharyngeal nerve

- Combination of motor, sensory and parasympathetic function. The stylopharyngeus muscle and the parotid gland is supplied by it.

- Supplies taste from the posterior third of the tongue.

-The nerve leaves the medulla portion of the brainstem, as 5 to 6 small rootlets.

Hypoglossal nerve

Accessory nerve

Supplies the intrinsic muscles of the tongue.

The nerve originates in the hypoglossal nucleus in the medulla of the brainstem, and leaves the skull through the hypoglossal canal.

Vagus

-These nerves supply motor function to the sternocleidomastoid and trapezius muscles.

-Originates in the medulla or the brainstem, as well as the upper to mid cervical spinal cord. The spinal portion arises to enter the skull through the foramen magnum, and it joins with the cranial portion to exit the skull through the jugular foramen.

- Provides motor, sensory and parasympathetic function.

-The motor function supplies the pharynx, soft palate and larynx, and originates in a nucleus in the brainstem known as the nucleus ambiguus.

-The parasympathetic function supplies the thoracic and abdominal viscera.

-Sensory fibers arise in the pharynx, larynx and external auditory meatus.

The vagus nerve leaves the brainstem as a number of converging rootlets, and leaves the skull through the jugular foramen.

Brain anatomy

Cranial Nerves

CSF

12 Cranial nerves

- I (olfactory) and II (optic) attached to forebrain

- The rest all come from the brainstem

- All distribute head and neck aside from the vagus nerve (cranial nerve X). Vagus also supplies abdominal and thorax structures

• Cerebrospinal Fluid (CSF) is a clear colourless fluid contained in the subarachnoid space and ventricular system of the brain and spinal cord.
• Protects brain and spinal cord
• .Important component of the blood brain barrier which controls the brain’s internal environment.
• Nutritional and immunological (protection against infection, disease, etc.) functions.
• CSF is produced and reabsorbed at 450 ml/24 h. The volume of CSF within the brain and spinal cord is 150 ml-75 ml in the brain, 75 ml in the spinal cord.
• Secreted by the choroid plexus in the ventricles and by the ependymal lining of the ventricles
• Flow is from lateral ventrical - foramen monro into the third ventricle - to the fourth ventricle (via aqueduct of Sylvius) - to lateral foramina of Luschka (via posteror foramen of Magendie) to the cisterna magna
• Absorbed by subarachnoid granulations (circulates round spinal cord/subarachnoid space first)

Cranial nerve Function

I: Olfactory nerve Smell

II: Optic nerve Vision

III: Oculomotor nerve Pupil constriction, accommodation, moves eye up, down and medially, opens eyelids

IV: Trochlear nerve Supplies superior oblique muscle: moves eye down and inwards

V: Trigeminal nerve Sensation to face, muscles of mastication

VI: Abducens nerve Supplies lateral rectus, moves eye laterally

VII: Facial nerve Supplies muscles of facial expression, taste (anterior two-thirds tongue), closes eyelids,

lacrimal, nose, palate glands, submandibular and sublingual salivary glands

VIII: Vestibulocochlear nerve Hearing, regulates balance

IX: Glossopharyngeal nerve Sensation/taste to posterior one-third tongue, posterior pharynx, stylopharyngeus - swallowing, parotid gland - salivation

X: Vagus nerve Parasympathetic supply to eye, heart, gut, lungs, larynx (sensation to airway, motor to vocal cords)

XI: Accessory nerve Supplies sternocleidomastoid (rotates head) and trapezius (lifts shoulders)

XII: Hypoglossal nerve Supplies tongue muscles