Cranial Nerves Flashcards
I. Olfactory Nerve
The first cranial nerve or olfactory nerve is a sensory nerve consisting of thin fibres running from the olfactory mucosa (a patch of odor-sensitive tissue high up in the nasal sinuses) to the olfactory bulb. These fine fibres are always torn off when the brain is removed from the skull. Destruction of the olfactory mucosa, the nerve fibres or the olfactory bulbs results in anosmia, an inability to detect odors.
II. Optic Nerve
the optic nerve is a sensory nerve which carries information about the visual world from the eye to the brain. The retina in the eye contains specialized receptor cells which react to light. These cells act on other cells through several synapses, finally exciting the retinal ganglion cells whose axons leave the eye in the optic nerve. The optic nerves form a chiasma (Greek: chiasma=cross) at the base of the brain. In the optic chiasma some of the fibres from the left eye cross to the right side of the brain and some of the fibres do not cross in this way. Similarly, some of the fibres from the right eye cross to the left side of the brain. As a result of this incomplete crossing, the optic nerve caudal to the chiasma (it is called the optic tract caudal to the chiasma), contains fibres from both eyes. Damage to the optic tract, therefore, will produce partial blindness in both eyes while damage to the optic nerve fibres between the retina an the optic chiasma will produce blindness in one eye only.
III. Oculomotor Nerve
The oculomotor nerve is a motor nerve some of whose fibres control the activity of muscles that turn the eyeball in its socket (extraocular muscles). Action potentials passing outward in others of the fibres of the third nerve cause) the pupil of the eye to constrict. This happens whenever the eye is exposed to a bright light. Fibres of the oculomotor nerve also control a tiny muscle which changes the shape of the lens in order to focus sharply the images of objects located at various distances from the eye.
IV. Trochlear Nerve
The trochlear nerve shares with the oculomotor and abducens nerves the function of controlling the extraocular muscles. It is not visible on the ventral surface of the brain but can be found emerging just behind the inferior colliculus.
V. Trigeminal nerve
The trigeminal nerve is a mixed nerve which means that it contains both sensory and motor fibres. The main portion, which is split into three divisions, carries sensory from the skin, muscles and other structures of the head such as the bones and teeth. A dentist can reduce or abolish the pain produced by drilling in teeth by injecting a local anesthetic into the vicinity of branches of the Vth nerve. Local anesthetics are drugs that block the passage of nerve impulses. After injection, drilling can still set up nerve impulses at the ends of he nerve fibres but because these impulses cannot reach the brain, no pain is felt. In addition to the sensory branches the trigeminal nerve includes a motor nerve which runs to the jaw muscles to control biting and chewing.
VI. Abducens
The abducens is a pure motor nerve which together with the oculomotor and trochlear nerves, controls the movements of the eye.
VII. Facial
The facial nerve is a mixed nerve, carrying sensory fibres for taste from the tongue as well as motor fibres to the muscles of the face. Movements such as winking or smiling depend upon an intact facial nerve. Other motor fibres run to the salivary glands and tear glands to produce, respectively, salivation and the secretion of tears
VIII. Vestibulo-cochlear Nerve
The VIIIth nerve is a sensory nerve consisting of a cochlear division and a vestibular division, which run from the cochlea and the vestibular apparatus, respectively. The cochlea is a coiled spiral structure (Latin: cochlea=snail shell) which contains receptor cells that are sensitive to vibrations set up by sounds acting on the eardrum. Thus, the cochlear nerve is concerned with hearing.
The vestibular apparatus is a complex system of liquid filled sacs and tubes which contain receptors sensitive to gravity and acceleration. These receptors, via the cells of the vestibular nerve, inform the brain of body position and movement. If the vestibular nerve (or the vestibular apparatus) is destroyed, the sense of balance is impaired. It becomes difficult to maintain an erect posture, especially in the dark or when the eyes are closed.
IX. Glossopharyngeal Nerve
the glossopharyngeal nerve (Greek: glossa=tongue+pharynx) is a mixture nerve carrying taste-sensitive fibres from the tongue, other sensory fibres from the pharynx, plus motor fibres to a muscle in the throat and to the salivary glands.
X. Vagus
The vagus nerve (Latin: vagus=wandering) is a mixed nerve which innervates many of the viscera or internal organs such as the heart, lungs, and stomach. By means of sensory vagal fibres the brain is kept informed of the state of these organs and by means of motor vagal fibres it can control their activity. The vagus also contains sensory and motor fibres that run to the pharynx, palate and nearby structures.
XI. Spinal Accessory Nerve
The accessory nerve is a motor nerve carrying fibres that originate in the medulla and others that originate in the spinal cord. These fibres control muscles in the neck and shoulder.
XII. Hypoglossal
The hypoglossal is a motor nerve controlling the muscles of the tongue.
