Introduction
There are 12 pairs of cranial nerves originating from nuclei in the inferior surface of the brain, some sensory, some motor, and some mixed. Their names suggest their distribution or function, which is generally related to the head and neck. They are numbered using Roman numerals according to the order they connect to the brain, starting anteriorly.
List of Cranial Nerves
- Olfactory : sensory
- Optic : sensory
- Oculomotor : motor
- Trochlear : motor
- Trigeminal : mixed
- Abducens : motor
- Facial : mixed
- Vestibulocochlear (auditory) : sensory
- Glossopharyngeal : mixed
- Vagus : mixed
- Accessory : motor
- Hypoglossal : motor
I. Olfactory Nerves (Sensory)
These are the nerves of the sense of smell. Their sensory receptors and nerve fibres originate in the upper part of the mucous membrane of the nasal cavity, pass upwards through the cribriform plate of the ethmoid bone and then pass to the olfactory bulb. The nerves then proceed backwards as the olfactory tract, to the area for the perception of smell in the temporal lobe of the cerebrum.
II. Optic Nerves (Sensory)
These are the nerves of the sense of sight. Their fibres originate in the retinae of the eyes and combine to form the optic nerves. They are directed backwards and medially through the posterior part of the orbital cavity and then pass through the optic foramina of the sphenoid bone into the cranial cavity and join at the optic chiasma. The nerves proceed backwards as the optic tracts to the lateral geniculate bodies of the thalamus. Impulses pass from there to the visual areas in the occipital lobes of the cerebrum and to the cerebellum. In the occipital lobe sight is perceived, and in the cerebellum the impulses from the eyes contribute to balance, posture, and orientation of the head in space.
III. Oculomotor Nerves (Motor)
These nerves arise from nuclei near the cerebral aqueduct. They supply:
- Four of the six extrinsic muscles, which move the eyeball (superior, medial, inferior recti, and inferior oblique).
- The intrinsic (intraocular) muscles: ciliary muscles (change lens shape) and circular muscles of the iris (constrict pupil).
- The levator palpebrae muscles, which raise the upper eyelids.
IV. Trochlear Nerves (Motor)
These nerves arise from nuclei near the cerebral aqueduct and supply the superior oblique muscles of the eyes.
V. Trigeminal Nerves (Mixed)
These contain motor and sensory fibres and are among the largest of the cranial nerves. They are the chief sensory nerves for the face and head, transmitting impulses for pain, temperature, and touch. The motor fibres stimulate the muscles for chewing (mastication).
Branches of Trigeminal Nerve
- Ophthalmic: Sensory only, supplies lacrimal glands, conjunctiva, forehead, eyelids, scalp, and nasal mucosa.
- Maxillary: Sensory only, supplies cheeks, upper gums, upper teeth, and lower eyelids.
- Mandibular: Mixed; supplies teeth, gums of the lower jaw, ears, lips, tongue (sensory), and muscles for chewing (motor).
VI. Abducens Nerves (Motor)
These arise from nuclei lying under the floor of the fourth ventricle. They supply the lateral rectus muscles of the eyeballs causing abduction.
VII. Facial Nerves (Mixed)
Composed of both motor and sensory fibres, arising from nuclei in the lower part of the pons. The motor fibres supply the muscles of facial expression. The sensory fibres convey impulses from taste buds in the anterior two-thirds of the tongue.
VIII. Vestibulocochlear (Auditory) Nerves (Sensory)
These nerves have two divisions:
- Vestibular Nerves: Arise from semicircular canals of the inner ear, conveying impulses to the cerebellum, maintaining posture and balance.
- Cochlear Nerves: Originate in the spiral organ (of Corti) and convey hearing impulses to the cerebral cortex.
IX. Glossopharyngeal Nerves (Mixed)
The motor fibres arise from nuclei in the medulla oblongata and stimulate the muscles of the tongue, pharynx, and the parotid glands. The sensory fibres convey impulses from the posterior third of the tongue, tonsils, and pharynx. These nerves are essential for swallowing, gag reflexes, and blood pressure regulation via the carotid sinus.
X. Vagus Nerves (Mixed)
These nerves have the most extensive distribution among cranial nerves. They pass through the neck into the thorax and abdomen and form an important part of the parasympathetic nervous system.
- Motor fibres: Supply smooth muscles and glands of the pharynx, larynx, heart, lungs, stomach, intestines, pancreas, gall bladder, spleen, kidneys, and blood vessels.
- Sensory fibres: Convey impulses from the membranes lining the same structures to the brain.
XI. Accessory Nerves (Motor)
These arise from nuclei in the medulla oblongata and spinal cord. They supply the sternocleidomastoid and trapezius muscles. Branches also join the vagus nerves and supply the pharyngeal and laryngeal muscles.
XII. Hypoglossal Nerves (Motor)
These arise from nuclei in the medulla oblongata. They supply the muscles of the tongue and muscles surrounding the hyoid bone, aiding in swallowing and speech.
Detailed Notes:
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