Bony and membranous labyrinths (draw cross section of bony cochlea and cochlear duct), vestibulocochlear nerve, nystagmus

Bony and membranous labyrinths:

The inner ear is the innermost part of the ear, located in the petrous part of the temporal bone, in between the middle ear and the internal acoustic meatus. It houses the vestibulocochlear organs, and is consequently divided into 2 portions; the BONY labyrinth and the MEMBRANOUS labyrinth.

BONY LABYRINTH- consists of a series of bony cavities within the petrous part of the temporal bone. It is composed of the cochlea, vestibule and three semi-circular canals. All these structures are lined internally with periosteum and contain a fluid called perilymph.

MEMBRANOUS LABYRINTH- lies within the bony labyrinth. It consists of the cochlear duct, semi-circular ducts, utricle and the saccule. The membranous labyrinth is filled with fluid called endolymph.

Vestibulocochlear nerve:

Vestibulocochlear nerve (eighth cranial nerve, CN VIII) is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the brain. It is divided into 2 separate nerve which then travel together; the vestibular nerve and the cochlear nerve.

Cochlear nerve: It transmits auditory information from the cochlea to the central nervous system (specifically the cochlear nuclei, with which it synapses with after entering the brainstem at the level of the medulla). The nerve fibers are bipolar, and originate from the Organ of Corti. The cochlear nerve is responsible for hearing, frequency/pitch of sound, sound intensity and localization. It is possible to see the spiral ganglion, where fibers of the cochlear nerve can be seen "leaving" from, in the picture above.

How the signal is created and transmitted to the cochlear nuclei in the brainstem:

• The signal activates. When sound waves reach your cochlea, they vibrate a membrane inside called the Basilar membrane. The vibrations cause sensory hair cells on the membrane to shift. The shifting creates a signal about the sound quality. This includes the frequency of the sound waves (how high or low the pitch is) and loudness.
• Nerve cells carry the signal along the auditory nerve. The neurons carry the signal until the message arrives at the cochlear nuclei in your brainstem. The cochlear nuclei have three divisions. Each division receives different types of information about the sound. For example, some receive information about lower sounds (low frequencies). Others receive information about higher sounds (high frequencies).
• Your brain processes sound information. The cochlear nuclei send sensory signals about the sound to the auditory cortex in your temporal lobe. This is the part of your brain that processes sound information.

Vestibular nerve: The vestibular nerve sends information from your vestibular system organs to your brain. The vestibular organs detect information about your head position and movement to help you maintain your sense of balance. Your vestibular organs include utricle and saccule and three semicircular canals. They are filled with fluid lined with sensory cells that shift in response to head movements.

How the signal is created and transmitted to the vestibular nuclei in the brainstem:

• The signal activates. When your head moves, fluid inside your vestibular organs shift, moving the sensory cells inside. The shift creates a signal about your body’s position and movement.
• Nerve cells carry the signal along the vestibular nerve. The nerve cells (neurons) carry the signal until the message arrives at a structure called the vestibular nuclei complex in your brainstem. The complex contains four nuclei. Each nucleus (singular “nuclei”) in your brain is responsible for directing the signal to various parts of your body so you can maintain your balance.
• Your brain processes balance information. Some nuclei send sensory signals about your body’s position to parts of your brain, like your cerebral cortex and cerebellum. Other nuclei send sensory signals to your neck or legs to help you adjust your position. Some nuclei send sensory signals to your eye muscles to help them remain focused even when your body’s in motion. This is called the vestibulo-ocular reflex (VOR).

Nystagmus:

a condition of involuntary (or voluntary, in some cases) eye movement. There are multiple forms of nystagmus, and some can even be physiological. However, we will talk about the pathological manifestations of nystagmus:

Vestibular Nystagmus: This type is caused by a problem in the inner ear or the vestibular nerve itself. Nystagmus in peripheral vestibular disorders is typically unidirectional, and the fast phase (jerk) of the nystagmus usually beats away from the affected ear. Causes of this can include inflammation in the labyrinth (labyrinthitis) or inflammation of the vestibular nerve itself (vestibular neuritis).

Central Nystagmus: This type originates from issues in the brainstem or cerebellum, which receive and process the vestibular signals. So it is unrelated to the signals being picked up by the vestibular nerve in the inner ear and subsequent transmission to the vestibular nuclei in the brainstem. The issue comes after that, from the pathways spreading out from the vestibular nerve towards the rest of the CNS. Causes of this include brain tumors, brain stroke, etc.