In humans, the spinal cord begins at the occipital bone, passing through the foramen magnum and entering the spinal canal at the beginning of the cervical vertebrae. The spinal cord extends down to between the first and second lumbar vertebrae, where it ends. The enclosing bony vertebral column protects the relatively shorter spinal cord. It is around 45 cm (18 in) in men and around 43 cm (17 in) long in women. The diameter of the spinal cord ranges from 13 mm (1⁄2 in) in the cervical and lumbar regions to 6.4 mm (1⁄4 in) in the thoracic area.

The spinal cord functions primarily in the transmission of nerve signals from the motor cortex to the body, and from the afferent fibers of the sensory neurons to the sensory cortex. It is also a center for coordinating many reflexes and contains reflex arcs that can independently control reflexes.

It is also the location of groups of spinal interneurons that make up the neural circuits known as central pattern generators. These circuits are responsible for controlling motor instructions for rhythmic movements such as walking.

Anatomical Position and Structure

The spinal cord is a cylindrical structure, greyish-white in colour. It has a relatively simple anatomical course:

• The spinal cord arises cranially as a continuation of the medulla oblongata (part of the brainstem).
• It then travels inferiorly within the vertebral canal, surrounded by the spinal meninges containing cerebrospinal fluid.
• At the L2 vertebral level the spinal cord tapers off, forming the conus medullaris.

As a result of the termination of the spinal cord at L2, it occupies around two thirds of the vertebral canal. The spinal nerves that arise from the end of the spinal cord are bundled together, forming a structure known as the cauda equina.

During the course of the spinal cord, there are two points of enlargement. The cervical enlargement is located proximally, at the C4-T1 level. It represents the origin of the brachial plexus. Between T11 and L1 is the lumbar enlargement, representing the origin of the lumbar and sacral plexi.

The spinal cord is marked by two depressions on its surface. The anterior median fissure is a deep groove extending the length of the anterior surface of the spinal cord. On the posterior aspect there is a slightly shallower depression – the posterior median sulcus.

Spinal Meninges

The spinal meninges are three membranes that surround the spinal cord – the dura mater, arachnoid mater, and pia mater. They contain cerebrospinal fluid, acting to support and protect the spinal cord. They are analogous with the cranial meninges.

Distally, the meninges form a strand of fibrous tissue, the filum terminale, which attaches to the vertebral bodies of the coccyx. It acts as an anchor for the spinal cord and meninges.

Dura Mater

The spinal dura mater is the most external of the meninges. It extends from the foramen magnum to the filum terminale, separated from the walls of the vertebral canal by the epidural space. This space contains some loose connective tissue, and the internal vertebral venous plexus.

As the spinal nerves exit the vertebral canal, they pierce the dura mater, temporarily passing in the epidural space. In doing so, the dura mater surrounds the nerve root, and fuses with the outer connective tissue covering of the nerve, the epineurium.

The spinal arachnoid mater is a delicate membrane, located between the dura mater and the pia mater. It is separated from the latter by the subarachnoid space, which contains cerebrospinal fluid.

Distal to the conus medullaris, the subarachnoid space expands, forming the lumbar cistern. This space accessed during a lumbar puncture (to obtain CSF fluid) and spinal anaesthesia.

Pia Mater

The spinal pia mater is the innermost of the meninges. It is a thin membrane that covers the spinal cord, nerve roots and their blood vessels. Inferiorly, the spinal pia mater fuses with the filum terminale.

Between the nerve roots, the pia mater thickens to form the denticulate ligaments. These ligaments attach to the dura mater – suspending the spinal cord in the vertebral canal.

Formation of the Spinal Nerves

The spinal nerves are mixed nerves that originate from the spinal cord, forming the peripheral nervous system.

Each spinal nerve begins as an anterior (motor) and a posterior (sensory) nerve root. These roots arise from the spinal cord, and unite at the intervertebral foramina, forming a single spinal nerve.

The spinal nerve then leaves the vertebral canal via the intervertebral foramina, and then divides into two:

• Posterior rami – supplies nerve fibres to the synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.
• Anterior rami – supplies nerve fibres to much of the remaining area of the body, both motor and sensory.

The nerve roots L2-S5 arise from the distal end of the spinal cord, forming a bundle of nerves known as the cauda equina.

Vasculature

The arterial supply to the spinal cord is via three longitudinal arteries – the anterior spinal artery and the paired posterior spinal arteries.

• Anterior spinal artery – formed from branches of the vertebral arteries. They travel in the anterior median fissure.
• Posterior spinal arteries – originate from the vertebral artery or the posteroinferior cerebellar artery. They anastamose with one another in the pia mater.

Additional arterial supply is via the anterior and posterior segmental medullary arteries – small vessels which enter via the nerve roots. The largest anterior segmental medullary artery is the artery of Adamkiewicz. It arises from the inferior intercostal or upper lumbar arteries, and supplies the inferior 2/3 of the spinal cord.

Venous drainage is via three anterior and three posterior spinal veins. These veins are valveless, and form an anastamosing network along the surface of the spinal cord. They also receive venous blood from the radicular veins. The spinal veins drain into the internal and external vertebral plexuses, which in turn empty into the systemic segmental veins. The internal vertebral plexus also empties into the dural venous sinuses superiorly.

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The peripheral nervous system refers to parts of the nervous system outside the brain and spinal cord. It includes the cranial nerves, spinal nerves and their roots and branches, peripheral nerves, and neuromuscular junctions.

Structure

Each nerve is covered on the outside by a dense sheath of connective tissue, the epineurium. Beneath this is a layer of fat cells, the perineurium, which forms a complete sleeve around a bundle of axons. Perineurial septae extend into the nerve and subdivide it into several bundles of fibres. Surrounding each such fibre is the endoneurium. This forms an unbroken tube from the surface of the spinal cord to the level where the axon synapses with its muscle fibres, or ends in sensory receptors. The endoneurium consists of an inner sleeve of material called the glycocalyx and an outer, delicate, meshwork of collagen fibres. Nerves are bundled and often travel along with blood vessels, since the neurons of a nerve have fairly high energy requirements.

Within the endoneurium, the individual nerve fibres are surrounded by a low-protein liquid called endoneurial fluid. This acts in a similar way to the cerebrospinal fluid in the central nervous system and constitutes a blood-nerve barrier similar to the blood-brain barrier. Molecules are thereby prevented from crossing the blood into the endoneurial fluid. During the development of nerve edema from nerve irritation (or injury), the amount of endoneurial fluid may increase at the site of irritation. This increase in fluid can be visualized using magnetic resonance neurography, and thus MR neurography can identify nerve irritation and/or injury.

Afferent, Efferent, and Mixed Nerves

Some of the nerves in the body are specialized for carrying information in only one direction, similar to a one-way street. Nerves that carry information from sensory receptors to the central nervous system only are called afferent nerves. Other neurons, known as efferent nerves, carry signals only from the central nervous system to effectors such as muscles and glands. Finally, some nerves are mixed nerves that contain both afferent and efferent axons. Mixed nerves function like 2-way streets where afferent axons act as lanes heading toward the central nervous system and efferent axons act as lanes heading away from the central nervous system.

Cranial Nerves

Extending from the inferior side of the brain are 12 pairs of cranial nerves. Each cranial nerve pair is identified by a Roman numeral 1 to 12 based upon its location along the anterior-posterior axis of the brain. Each nerve also has a descriptive name (e.g. olfactory, optic, etc.) that identifies its function or location. The cranial nerves provide a direct connection to the brain for the special sense organs, muscles of the head, neck, and shoulders, the heart, and the GI tract.

Spinal Nerves

Extending from the left and right sides of the spinal cord are 31 pairs of spinal nerves. The spinal nerves are mixed nerves that carry both sensory and motor signals between the spinal cord and specific regions of the body. The 31 spinal nerves are split into 5 groups named for the 5 regions of the vertebral column. Thus, there are 8 pairs of cervical nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs of sacral nerves, and 1 pair of coccygeal nerves. Each spinal nerve exits from the spinal cord through the intervertebral foramen between a pair of vertebrae or between the C1 vertebra and the occipital bone of the skull.

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Similar to the eyes of other mammals, the human eye’s non-image-forming photosensitive ganglion cells in the retina receive light signals which affect adjustment of the size of the pupil, regulation and suppression of the hormone melatonin and entrainment of the body clock.

External (Extraocular) Anatomy

Extraocular Muscles

There are six muscles that are present in the orbit (eye socket) that attach to the eye to move it. These muscles work to move the eye up, down, side to side, and rotate the eye.

The superior rectus is an extraocular muscle that attaches to the top of the eye. It moves the eye upward. The inferior rectus is an extraocular muscle that attaches to the bottom of the eye. It moves the eye downward. The medial rectus is an extraocular muscle that attaches to the side of the eye near the nose. It moves the eye inward toward the nose. The lateral rectus is an extraocular muscle that attaches to the side of the eye near the temple. It moves the eye outward.

The superior oblique is an extraocular muscle that comes from the back of the orbit. It travels through a small pulley (the trochlea) in the orbit near the nose and then attaches to the top of the eye. The superior oblique rotates the eye inward around the long axis of the eye (front to back). The superior oblique also moves the eye downward.

The inferior oblique is an extraocular muscle that arises in the front of the orbit near the nose. It then travels outward and backward in the orbit before attaching to the bottom part of the eyeball. It rotates the eye outward along the long axis of the eye (front to back). The inferior oblique also moves the eye upward.

Conjunctiva

The conjunctiva is a transparent mucous membrane that covers the inner surface of the eyelids and the surface of the eye. When it is inflamed or infected it becomes red or pink. This is called conjunctivitis or “pink eye”.

Lacrimal gland

The lacrimal gland produces tears that lubricate the eye. It is located under the lateral edge of the eyebrow in the orbit.

Tenon’s Cpsule

Tenon’s capsule is a layer of tissue that lies between the conjunctiva and the surface of the eye.

Sclera

The sclera is the white outer wall of the eye. It covers nearly the entire surface of the eyeball. It is a strong layer made of collagen fibers. The tendons of the six extraocular muscles attach to the sclera.

Cornea

The cornea occupies the front center part of the outer wall of the eye. It is made of collagen fibers in a very special arrangement so that the cornea is clear. One looks through the cornea to see the iris and pupil. The cornea bends light coming into the eye so that it is focused on the retina. The cornea is the part of the eye on which contact lenses are placed.

Internal (Intraocular)Anatomy

Anterior Chamber

The anterior chamber is a fluid (aqueous humor) filled space inside the eye. The cornea lies in front of the anterior chamber, and the iris and the pupil are behind it.

Iris/Pupil

The iris is the colored part of the eye. It is disc shaped with a hole in the middle (the pupil). Muscles in the iris cause the pupil to constrict in bright light and to dilate in dim light. The change in pupil size regulates the amount of light that reaches the posterior (back) part of the eye.

Lens

The lens of the eye is located directly behind the pupil. The lens bends light coming into the eye to help focus it on the retina. It changes shape to help the eye focus to see objects clearly at near. The lens is suspended from the wall of the eye by many small fibers (zonules) that attach to its capsule.

Ciliary Body

The ciliary body is attached to the outer edge of the iris near the wall of the eye. The ciliary body produces the fluid (aqueous humor) that fills the eye and nourishes its structures. It also helps to change the shape of the lens when focusing occurs.

Vitreous Cavity

The vitreous cavity lies between the lens and the retina and fills 4/5 of the space inside the back part of the eye. A gelatinous substance known as the vitreous humor fills the cavity. This plays an important role in nourishing the inner structures of the eye. Light comes into the eye through the pupil and passes through the vitreous to be projected on the retina.

Retina

The retina is a thin, transparent structure that covers the inner wall of the eye. The eye works like a camera, and the retina is similar to the film in the camera. It is where images are first projected before they are transmitted through the optic nerve to the brain. It is a very complex structure with 10 layers of specialized cells including the photoreceptor cells (rods and cones).

Photoreceptors

Photoreceptors are highly specialized cells of the retina that receive light impulses and change them into chemical energy that can be transmitted by nerve cells to the brain. The two types of photoreceptors are rods and cones. Rods perceive black and white and serve night vision primarily. Cones are responsible for color perception and central vision.

Macula

The macula is a small, specialized area of the retina that has very high sensitivity and is responsible for central vision.

Retinal Pigment Epithelium (RPE)

The retinal pigment epithelium is a layer of cells deep in the retina. This single layer of cells helps maintain the function of the photoreceptor cells in the retina by processing vitamin A products, turning over used photoreceptor segments, absorbing light, and transporting nutrients in and out of the photoreceptor cells.

Choroid

The choroid is a tissue layer that lies between the retina and the sclera. The choroid has a rich supply of blood vessels that nourish the retina.

Uveal tract

The uveal tract is a pigmented component of the eye that is comprised of 1) the iris, 2) the ciliary body, and 3) the choroid.

Optic nerve

The optic nerve connects each eye to the brain. It is a structure that sends the picture seen by the eye to the brain so that they can be processed. The optic nerves end in a structure called the optic chiasm. In an adult, the optic nerve is about the diameter of a pencil. There are over 1 million individual nerve cells in the optic nerve.

Optic Chiasm

The optic chiasm is the place in the brain where the two optic nerves meet. The individual nerve fibers from each nerve are sorted in the chiasm. The sorting occurs in such a way that the right side of the brain controls the view of objects in left visual space and the left side of the brain controls the view of objects in right visual space [See figure 3].

Visual Cortex

This is an area of the brain in the posterior occipital lobe to which the neurons in the retina ultimately give visual information. The visual cortex helps to process information regarding the image such as its color, composition, and relation in space to other objects. This information is then sent to other parts of the brain that serve higher visual functions.

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The mastoid bone surrounds the middle ear. The external ear collects sound waves. The ear canal carries sound waves to the eardrum. The eardrum vibrates from sound waves, setting the middle ear bones in motion. The middle ear bones (ossicles) vibrate, transmitting sound waves to the inner ear. When the ear is healthy, air pressure remains balanced in the middle ear. The eustachian tube helps control air pressure in the middle ear. The semicircular canals help maintain balance. The vestibular nerve carries balance signals to the brain. The auditory nerve carries sound signals to the brain. The cochlea picks up sound waves and makes nerve signals. 

Structure

Parts of the External Ear

The external ear can be divided functionally and structurally into two parts; the auricle (or pinna), and the external acoustic meatus – which ends at the tympanic membrane.

Auricle

The auricle is a paired structure found on either side of the head. It functions to capture and direct sound waves towards the external acoustic meatus.

It is a mostly cartilaginous structure, with the lobule being the only part not supported by cartilage. The cartilaginous part of the auricle forms an outer curvature, known as the helix. A second innermost curvature runs in parallel with the helix – the antihelix. The antihelix divides into two cura; the inferoanterior crus, and the superoposterior crus.

In the middle of the auricle is a hollow depression, called the concha. It continues into the skull as the external acoustic meatus. The concha acts to direct sound into the external acoustic meatus. Immediately anterior to the beginning of the external acoustic meatus is an elevation of cartilaginous tissue – the tragus. Opposite the tragus is the antitragus.

External Acoustic Meatus

The external acoustic meatus is a sigmoid shaped tube that extends from the deep part of the concha to the tympanic membrane. The walls of the external 1/3 are formed by cartilage, whereas the inner 2/3 are formed by the temporal bone.

The external acoustic meatus does not have a straight path, and instead travels in an S-shaped curve as follows:

• Initially it travels in a superoanterior direction.
• In then turns slightly to move superoposteriorly.
• It ends by running in an inferoanterior direction.

Tympanic Membrane

The tympanic membrane lies at the distal end of the external acoustic meatus. It is a connective tissue structure, covered with skin on the outside and a mucous membrane on the inside. The membrane is connected to the surrounding temporal bone by a fibrocartilaginous ring.

The translucency of the tympanic membrane allows the structures within the middle ear to be observed during otoscopy. On the inner surface of the membrane, the handle of malleus attaches to the tympanic membrane, at a point called the umbo of tympanic membrane.

The handle of malleus continues superiorly, and at its highest point, a small projection called the lateral process of the malleus can be seen. The parts of the tympanic membrane moving away from the lateral process are called the anterior and posterior malleolar folds.

Vasculature

The external ear is supplied by branches of the external carotid artery:

• Posterior auricular artery
• Superficial temporal artery
• Occipital artery
• Maxillary artery (deep auricular branch) – supplies the deep aspect of the external acoustic meatus and tympanic membrane only.

Venous drainage is via veins following the arteries listed above.

Innervation

The sensory innervation to the skin of the auricle comes from numerous nerves:

• Greater auricular nerve (branch of the cervical plexus) – innervates the skin of the auricle
• Lesser occipital nerve (branch of the cervical plexus) – innervates the skin of the auricle
• Auriculotemporal nerve (branch of the mandibular nerve) – innervates the skin of the auricle and external auditory meatus.
• Branches of the facial and vagus nerves – innervates the deeper aspect of the auricle and external auditory meatus

Some individuals can complain of an involuntary cough when cleaning their ears – this is due to stimulation of the auricular branch of the vagus nerve (the vagus nerve is also responsible for the cough reflex).

Parts of the Middle Ear

The middle ear can be divided into two parts:

• Tympanic cavity – located medially to the tympanic membrane. It contains three small bones known as the auditory ossicles: the malleus, incus and stapes. They transmit sound vibrations through the middle ear.
• Epitympanic recess – a space superior to the tympanic cavity, which lies next to the mastoid air cells. The malleus and incus partially extend upwards into the epitympanic recess.

Borders

The middle ear can be visualised as a rectangular box, with a roof and floor, medial and lateral walls and anterior and posterior walls.

• Roof – formed by a thin bone from the petrous part of the temporal bone. It separates the middle ear from the middle cranial fossa.
• Floor – known as the jugular wall, it consists of a thin layer of bone, which separates the middle ear from the internal jugular vein
• Lateral wall – made up of the tympanic membrane and the lateral wall of the epitympanic recess.
• Medial wall – formed by the lateral wall of the internal ear. It contains a prominent bulge, produced by the facial nerve as it travels nearby.
• Anterior wall – a thin bony plate with two openings; for the auditory tube and the tensor tympani muscle. It separates the middle ear from the internal carotid artery.
• Posterior wall (mastoid wall) – it consists of a bony partition between the tympanic cavity and the mastoid air cells.
  • Superiorly, there is a hole in this partition, allowing the two areas to communicate. This hole is known as the aditus to the mastoid antrum.

Bones

The bones of the middle ear are the auditory ossicles – the malleus, incus and stapes. They are connected in a chain-like manner, linking the tympanic membrane to the oval window of the internal ear.

Sound vibrations cause a movement in the tympanic membrane which then creates movement, or oscillation, in the auditory ossicles. This movement helps to transmit the sound waves from the tympanic membrane of external ear to the oval window of the internal ear.

The malleus is the largest and most lateral of the ear bones, attaching to the tympanic membrane, via the handle of malleus. The head of the malleus lies in the epitympanic recess, where it articulates with the next auditory ossicle, the incus.

The next bone – the incus – consists of a body and two limbs. The body articulates with the malleus, the short limb attaches to the posterior wall of the middle, and the long limb joins the last of the ossicles; the stapes.

The stapes is the smallest bone in the human body. It joins the incus to the oval window of the inner ear. It is stirrup-shaped, with a head, two limbs, and a base. The head articulates with the incus, and the base joins the oval window.

Mastoid Air Cells

The mastoid air cells are located posterior to epitympanic recess. They are a collection of air-filled spaces in the mastoid process of the temporal bone. The air cells are contained within a cavity called the mastoid antrum. The mastoid antrum communicates with the middle ear via the aditus to mastoid antrum.

The mastoid air cells act as a ‘buffer system‘ of air –  releasing air into the tympanic cavity when the pressure is too low.

Muscles

There are two muscles which serve a protective function in the middle ear; the tensor tympani and stapedius. They contract in response to loud noise, inhibiting the vibrations of the malleus, incus and stapes, and reducing the transmission of sound to the inner ear. This action is known as the acoustic reflex.

The tensor tympani originates from the auditory tube and attaches to the handle of malleus, pulling it medially when contracting. It is innervated by the tensor tympani nerve, a branch of the mandibular nerve. The stapedius muscle attaches to the stapes, and is innervated by the facial nerve.

Auditory Tube

The auditory tube (eustachian tube) is a cartilaginous and bony tube that connects the middle ear to the nasopharynx. It acts to equalise the pressure of the middle ear to that of the external auditory meatus.

It extends from the anterior wall of the middle ear, in an anterior, medioinferior direction, opening onto the lateral wall of the nasopharynx. In joining the two structures, it is a pathway by which an upper respiratory infection can spread into the middle ear.

The tube is shorter and straighter in children, therefore middle ear infections tend to be more common in children than adults.

Structure of the Inner Ear

The inner ear is located within the petrous part of the temporal bone. It lies between the middle ear and the internal acoustic meatus, which lie laterally and medially respectively. The inner ear has two main components – the bony labyrinth and 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.

The inner ear has two openings into the middle ear, both covered by membranes. The oval window lies between the middle ear and the vestibule, whilst the round window separates the middle ear from the scala tympani (part of the cochlear duct).

Bony Labyrinth

The bony labyrinth is a series of bony cavities within the petrous part of the temporal bone. It consists of three parts – the cochlea, vestibule and the three semi-circular canals.

Vestibule

The vestibule is the central part of the bony labyrinth. It is separated from the middle ear by the oval window, and communicates anteriorly with the cochlea and posterioly with the semi-circular canals. Two parts of the membranous labyrinth; the saccule and utricle, are located within the vestibule.

Cochlea

The cochlea houses the cochlea duct of the membranous labyrinth – the auditory part of the inner ear. It twists upon itself around a central portion of bone called the modiolus, producing a cone shape which points in an anterolateral direction. Branches from the cochlear portion of the vestibulocochlear (VIII) nerve are found at the base of the modiolus.

Extending outwards from the modiolus is a ledge of bone known as spiral lamina, which attaches to the cochlear duct, holding it in position. The presence of the cochlear duct creates two perilymph-filled chambers above and below:

• Scala vestibuli: Located superiorly to the cochlear duct. As its name suggests, it is continuous with the vestibule.
• Scala tympani: Located inferiorly to the cochlear duct. It terminates at the round window.

Semi-circular Canals

There are three semi-circular canals; anterior, lateral and posterior. They contain the semi-circular ducts, which are responsible for balance (along with the utricle and saccule).

The canals are situated superoposterior to the vestibule, at right angles to each other. They have a swelling at one end, known as the ampulla.

Membranous Labyrinth

The membranous labyrinth is a continuous system of ducts filled with endolymph. It lies within the bony labyrinth, surrounded by perilymph. It is composed of the cochlear duct, three semi-circular ducts, saccule and the utricle.

The cochlear duct is situated within the cochlea and is the organ of hearing. The semi-circular ducts, saccule and utricle are the organs of balance (also known as the vestibular apparatus).

Cochlear Duct

The cochlear duct is located within the bony scaffolding of the cochlea. It is held in place by the spiral lamina. The presence of the duct creates two canals above and below it –  the scala vestibuli and scala tympani respectively. The cochlear duct can be described as having a triangular shape:

• Lateral wall – Formed by thickened periosteum, known as the spiral ligament.
• Roof – Formed by a membrane which separates the cochlear duct from the scala vestibuli, known as the Reissner’s membrane.
• Floor – Formed by a membrane which separates the cochlear duct from the scala tympani, known as the basilar membrane.

The basilar membrane houses the epithelial cells of hearing – the Organ of Corti. A more detailed description of the Organ of Corti is beyond the scope of this article.

Saccule and Utricle

The saccule and utricle are two membranous sacs located in the vestibule. They are organs of balance which detect movement or acceleration of the head in the vertical and horizontal planes, respectively.

The utricle is the larger of the two, receiving the three semi-circular ducts. The saccule is globular in shape and receives the cochlear duct.

Endolymph drains from the saccule and utricle into the endolymphatic duct. The duct travels through the vestibular aqueduct to the posterior aspect of the petrous part of the temporal bone. Here, the duct expands to a sac where endolymph can be secreted and absorbed.

Semi-circular Ducts

The semi-circular ducts are located within the semi-circular canals, and share their orientation. Upon movement of the head, the flow of endolymph within the ducts changes speed and/or direction. Sensory receptors in the ampullae of the semi-circular canals detect this change, and send signals to the brain, allowing for the processing of balance.

Vasculature

The bony labyrinth and membranous labyrinth have different arterial supplies. The bony labyrinth receives its blood supply from three arteries, which also supply the surrounding temporal bone:

• Anterior tympanic branch (from maxillary artery).
• Petrosal branch (from middle meningeal artery).
• Stylomastoid branch (from posterior auricular artery).

The membranous labyrinth is supplied by the labyrinthine artery, a branch of the inferior cerebellar artery (or, occasionally, the basilar artery). It divides into three branches:

• Cochlear branch – supplies the cochlear duct.
• Vestibular branches (x2) – supply the vestibular apparatus.

Venous drainage of the inner ear is through the labyrinthine vein, which empties into the sigmoid sinus or inferior petrosal sinus.

Innervation

The inner ear is innervated by the vestibulocochlear nerve (CN VIII). It enters the inner ear via the internal acoustic meatus, where it divides into the vestibular nerve (responsible for balance) and the cochlear nerve (responsible for hearing):

• Vestibular nerve – enlarges to form the vestibular ganglion, which then splits into superior and inferior parts to supply the utricle, saccule and three semi-circular ducts.
• Cochlear nerve – enters at the base of the modiolus and its branches pass through the lamina to supply the receptors of the Organ of Corti.

The facial nerve, CN VII, also passes through the inner ear, but does not innervate any of the structures present.

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