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Cranial nerves are the nerves that emerge directly from the brain, of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck, including the special senses of vision, taste, smell, and hearing.
The inner ear is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the temporal bone of the skull with a system of passages comprising two main functional parts:
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the organ of Corti, the sensory organ of hearing, which is distributed along the partition separating the fluid chambers in the coiled tapered tube of the cochlea.
The facial nerve, also known as the seventh cranial nerve, cranial nerve VII, or simply CN VII, is a cranial nerve that emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue. The nerve typically travels from the pons through the facial canal in the temporal bone and exits the skull at the stylomastoid foramen. It arises from the brainstem from an area posterior to the cranial nerve VI and anterior to cranial nerve VIII.
The vestibulocochlear nerve or auditory vestibular nerve, also known as the eighth cranial nerve, cranial nerve VIII, or simply CN VIII, is a cranial nerve that transmits sound and equilibrium (balance) information from the inner ear to the brain. Through olivocochlear fibers, it also transmits motor and modulatory information from the superior olivary complex in the brainstem to the cochlea.
The basilar membrane is a stiff structural element within the cochlea of the inner ear which separates two liquid-filled tubes that run along the coil of the cochlea, the scala media and the scala tympani. The basilar membrane moves up and down in response to incoming sound waves, which are converted to traveling waves on the basilar membrane.
The saccule is a bed of sensory cells in the inner ear. It translates head movements into neural impulses for the brain to interpret. The saccule detects linear accelerations and head tilts in the vertical plane. When the head moves vertically, the sensory cells of the saccule are disturbed and the neurons connected to them begin transmitting impulses to the brain. These impulses travel along the vestibular portion of the eighth cranial nerve to the vestibular nuclei in the brainstem.
The organ of Corti, or spiral organ, is the receptor organ for hearing and is located in the mammalian cochlea. This highly varied strip of epithelial cells allows for transduction of auditory signals into nerve impulses' action potential. Transduction occurs through vibrations of structures in the inner ear causing displacement of cochlear fluid and movement of hair cells at the organ of Corti to produce electrochemical signals.
The auditory system is the sensory system for the sense of hearing. It includes both the sensory organs and the auditory parts of the sensory system.
Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment.
A pseudounipolar neuron is a type of neuron which has one extension from its cell body. This type of neuron contains an axon that has split into two branches. A single process arises from the cell body and then divides into an axon and a dendrite. They develop embryologically as bipolar in shape, and are thus termed pseudounipolar instead of unipolar.
The cochlear nerve is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information from the cochlea of the inner ear directly to the brain. The other portion of the vestibulocochlear nerve is the vestibular nerve, which carries spatial orientation information to the brain from the semicircular canals, also known as semicircular ducts.
The internal auditory meatus is a canal within the petrous part of the temporal bone of the skull between the posterior cranial fossa and the inner ear.
The cochlear nuclear (CN) complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN). The ventral cochlear nucleus is unlayered whereas the dorsal cochlear nucleus is layered. Auditory nerve fibers, fibers that travel through the auditory nerve carry information from the inner ear, the cochlea, on the same side of the head, to the nerve root in the ventral cochlear nucleus. At the nerve root the fibers branch to innervate the ventral cochlear nucleus and the deep layer of the dorsal cochlear nucleus. All acoustic information thus enters the brain through the cochlear nuclei, where the processing of acoustic information begins. The outputs from the cochlear nuclei are received in higher regions of the auditory brainstem.
The superior ganglion of the vagus nerve is a sensory ganglion of the peripheral nervous system. It is located within the jugular foramen, where the vagus nerve exits the skull. It is smaller than and proximal to the inferior ganglion of the vagus nerve.
The spiral (cochlear) ganglion is a group of neuron cell bodies in the modiolus, the conical central axis of the cochlea. These bipolar neurons innervate the hair cells of the organ of Corti. They project their axons to the ventral and dorsal cochlear nuclei as the cochlear nerve, a branch of the vestibulocochlear nerve.
The crista ampullaris is the sensory organ of rotation. They are found in the ampullae of each of the semicircular canals of the inner ear, meaning that there are three pairs in total. The function of the crista ampullaris is to sense angular acceleration and deceleration.
Otic vesicle, or auditory vesicle, consists of either of the two sac-like invaginations formed and subsequently closed off during embryonic development. It is part of the neural ectoderm, which will develop into the membranous labyrinth of the inner ear. This labyrinth is a continuous epithelium, giving rise to the vestibular system and auditory components of the inner ear. During the earlier stages of embryogenesis, the otic placode invaginates to produce the otic cup. Thereafter, the otic cup closes off, creating the otic vesicle. Once formed, the otic vesicle will reside next to the neural tube medially, and on the lateral side will be paraxial mesoderm. Neural crest cells will migrate rostral and caudal to the placode.
A bipolar neuron, or bipolar cell, is a type of neuron characterized by having both an axon and a dendrite extending from the soma in opposite directions. These neurons are predominantly found in the retina and olfactory system. The embryological period encompassing weeks seven through eight marks the commencement of bipolar neuron development. Many bipolar cells are specialized sensory neurons for the transmission of sense. As such, they are part of the sensory pathways for smell, sight, taste, hearing, touch, balance and proprioception. The other shape classifications of neurons include unipolar, pseudounipolar and multipolar. During embryonic development, pseudounipolar neurons begin as bipolar in shape but become pseudounipolar as they mature.
Bernd Fritzsch is a German–American distinguished neurobiologist, professor emeritus, and was the chair of the department of Biology at University of Iowa. He is a fellow of the American Association for the Advancement of Science. He is known for his expertise in comparative molecular neuroembryology, particularly in discovering the molecular evolution and development of sensory cells within the inner ear, including auditory hair cells and neurons. His research identifies critical developmental stages that could facilitate the restoration of hearing abilities.
References
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- ↑ Vasković J (3 November 2023). Grujičić R (ed.). "Vestibular system". Kenhub. Retrieved 2023-11-13.
- ↑ Pavlinkova G (December 2020). "Molecular Aspects of the Development and Function of Auditory Neurons". International Journal of Molecular Sciences. 22 (1): 131. doi: 10.3390/ijms22010131 . PMC 7796308 . PMID 33374462.
- ↑ Sato H, Sando I, Takahashi H (September 1992). "Three-dimensional anatomy of human Scarpa's ganglion". The Laryngoscope. 102 (9): 1056–1063. doi: 10.1288/00005537-199209000-00018 . PMID 1518353.
- ↑ Jucker-Kupper P. "Antonio Scarpa". whonamedit.com.
- ↑ Scarpa A (1785). Anatomicarum Annotationum. 2 volumes (2nd ed.). Milano.
