Peripheral Nervous System Theory Course PDF

0  Ear & Auditory Pathway - The ear consists of the external ear, the middle ear (tympanic cavity), and the internal ear (labyrinth). o The internal ear is called the statoacoustic organ because it’s responsible for both hearing and balance. Externa...

0  Ear & Auditory Pathway - The ear consists of the external ear, the middle ear (tympanic cavity), and the internal ear (labyrinth). o The internal ear is called the statoacoustic organ because it’s responsible for both hearing and balance. External Ear The external ear has an auricle and an external auditory meatus. 1. Auricle (ear pinna) It’s made of elastic cartilage covered by skin. It collects air vibrations (sound waves). It possesses both extrinsic and intrinsic muscles o Supplied by the facial (VII) nerve. 2. External Acoustic [Auditory] Meatus (EAM) A tube that leads from the auricle to the tympanic membrane o It conducts sound waves from the auricle to the tympanic membrane. Osteocartilaginous tube: o Outer (lateral) 1/3: elastic cartilage. o Inner (medial) 2/3: Bone, by tympanic part of temporal bone: The walls of the EAM are Lined with skin. o This outer 1/3 of skin has hair, sebaceous & ceruminous glands. ▪ Ceruminous glands secrets cerumen (earwax). ▪ Cerumen provides a sticky barrier to prevent foreign bodies trying to enter the EAM. Innervation of External Ear The sensory nerve supply of the lining skin is derived from: o The auriculotemporal nerve. ▪ It’s a branch of the mandibular division (V3) of trigeminal nerve. o The auricular branch. ▪ Branch of the vagus nerve (X). o Great auricular nerve ▪ Spinal nerve branching from cervical plexus. ➔ The lymph drainage is to the superficial parotid, mastoid, and superficial cervical L.N.. Middle Ear (Tympanic Cavity) Narrow air filled chamber cavity within petrous part of temporal bone o Lined with mucous membrane. Middle ear communicates: o Anteriorly, with nasopharynx through pharyngotympanic tube. o Posteriorly, it has aditus )‫ (مدخل‬to mastoid antrum. Boundaries (walls) of Tympanic Cavity 1) Roof (R): Formed by the tegmen tympani. o It’s a thin plate of bone. o Part of the petrous temporal bone. Separates the tympanic cavity from middle cranial fossa o Which contains the meninges and the temporal lobe of the brain. 1  2) Floor (F): Formed by a thin bony plate. o Can be partly replaced by fibrous tissue. Separates tympanic cavity from superior bulb of internal jugular vein. 3) Anterior Wall (A): Formed by a thin bony plate Separates tympanic cavity from internal carotid artery. Contains 2 canals: o Auditory tube opening: lower and larger. ▪ Auditory Tube is also called pharyngotympanic Tube (Eustachian Tube). o Tensor tympani muscle: upper and smaller. Auditory Tube It connects the anterior wall of the tympanic cavity to the nasal pharynx. Its posterior 1/3 is bony, and anterior two-thirds is cartilaginous. It serves to equalize air pressures in the tympanic cavity and the nasal pharynx. 4) Posterior Wall (P): In its upper part, it has a large irregular opening. o This is the aditus to mastoid antrum. o Separates tympanic cavity from mastoid air cells. In its lower part, it has the pyramid. o From the apex of pyramid, the tendon of the stapedius muscle emerges. 5) Medial wall (M): Formed by lateral wall of inner ear. o Separates middle ear from inner ear (labyrinthine). Contains: o Promontory ▪ It’s a rounded projection. ▪ Results from the underlying first turn of the cochlea. o Fenestra vestibuli (Oval window) ▪ is located above and behind the promontory. ▪ It is closed by base of stapes. o Fenestra cochleae (Round window) ▪ is located below the posterior end of promontory. ▪ It is closed by secondary tympanic membrane. o Prominence (bulge) for facial nerve canal. ▪ A rounded ridge runs horizontally backward above the promontory and the fenestra vestibuli. ▪ On reaching the posterior wall, it curves downward behind the pyramid. 6) Lateral wall (membranous wall): Formed by the tympanic membrane Separates middle ear from EAM. 2 The Tympanic Membrane (eardrum) Thin, circular, fibrous membrane. o 1 cm in diameter. o Obliquely placed, facing downward, forward, and laterally. Concave laterally. o At the depth of the concavity is a small depression, the umbo. ▪ Produced by the tip of the handle of the malleus. The circumference is thickened and slotted )‫ (مثبت‬into a groove in the bone. o The groove is called tympanic sulcus. o Tympanic sulcus is deficient superiorly, which forms a notch. ▪ From the sides of the notch, two bands appears: The anterior and posterior malleolar folds. Separates between external ear and middle ear. - It has 2 parts: 1. Pars Flaccida o Small triangular flaccid (lax) part o Located between anterior and posterior malleolar folds. 2. Pars tensa o The rest of tympanic membrane is tense. Nerve Supply (tympanic membrane is extremely sensitive to pain): o Outer surface is supplied by auriculotemporal nerve. o Inner surface is supplied by tympanic branch of the glossopharyngeal nerve (CN IX). Mastoid Antrum It lies behind the middle ear. o In the petrous part of the temporal bone It communicates with the middle ear by the aditus - Relations of the Mastoid Antrum: o Anterior wall: related to the middle ear. ▪ Contains the aditus to the mastoid antrum. o Posterior wall: ▪ Separates the antrum from the sigmoid venous sinus and cerebellum o Lateral wall: forms the floor of the suprameatal triangle. ▪ Mastoid antrum is located medial to suprameatal triangle. o Medial wall: is related to the posterior semicircular canal o Superior wall: is the thin plate of bone, the tegmen tympani, ▪ Above it, the meninges of the middle cranial fossa and the temporal lobe of the brain o Inferior wall: is perforated with holes. ▪ The antrum communicates with the mastoid air cells Auditory Ossicles 3 bones that transmits vibrations of the tympanic membrane to the perilymph of the internal ear. o Smallest bones in the body. 3  1. Malleus Largest ossicle. ➔ Consists of: Head o It articulates posteriorly with body of incus. Handle o It’s attached to medial surface of tympanic membrane. o It’s attached to tensor tympani muscle. Anterior process and lateral process. 2. Incus ➔ Consists of: Body o It articulates with head of malleus Long process o It articulates with head of stapes Short process o It attaches to posterior wall of middle ear 3. Stapes ➔ Consists of: Base o It’s attached to oval window (fenestra vestibuli). Neck o It’s attached to stapedius muscle. Head o It articulates with long process of incus. Anterior and posterior limbs. Muscles of the ossicles 1) Stapedius Muscle Origin: pyramid (bony projection). Insertion: neck of stapes. Innervation by facial nerve (VII). Action: reduce (dampen down) the vibrations of stapes 2) Tensor Tympani Muscle Origin: from a bony canal above auditory tube. Insertion: handle of malleus. Innervation by mandibular division of trigeminal nerve (V3). Action: reduce (dampen down) the vibrations of tympanic membrane. The Internal Ear (Labyrinth) The labyrinth is situated in the petrous part of the temporal bone. - Consists of: o Bony labyrinth: cavities within bone. o Membranous labyrinth: membranous sacs & ducts within the bony cavities. ▪ Membranous labyrinth is inside the bony labyrinth. 4 Bony Labyrinth Lined by endosteum Contain a clear fluid, called perilymph. - It’s made of 3 parts: 1. Vestibule The central part of bony labyrinth. o Posterior to cochlea. o Anterior to semicircular canals. In its lateral wall: o Fenestra vestibuli (oval window). o Fenestra cochleae (round window). Lodged Inside the vestibule are the membranous labyrinth: o Saccule and Utricle. 2. Semicircular canals They are arranged in 3 different planes (at right angles to each other). o Superior (anterior) semicircular canal [in vertical plane]. o Posterior semicircular canal [in vertical plane]. o Lateral semicircular canal [in transverse plane]. Open into the posterior part of vestibule. Each canal has a swelling at one end, called ampulla. Lodged inside the canals are the semicircular ducts. o Part of the membranous labyrinth. 3. Cochlea Open into the anterior part of vestibule. It’s a snail shell-shaped structure. It consists of a central modiolus. o Around which a hollow bony tube makes two and 1/2 spiral turns. The apex faces anterolaterally The base faces posteromedially. o The first basal turn of the cochlea is responsible for the promontory seen on the medial wall of the middle ear. The modiolus: o Perforated by branches of the cochlear nerve. o It has a broad base at the bottom of the internal acoustic meatus. The cochlea is divided into 3 channels: o Scala vestibuli is a bony channel above the cochlear duct. ▪ Ends at the fenestra vestibuli (oval window). o Cochlear duct (scala media): part of membranous labyrinth. o Scala tympani is a bony channel below the cochlear duct. ▪ Ends at the fenestra cochlea (round window). The scala vestibuli and scala tympani are completely separated by the cochlear duct o Except for an opening at the helicotrema. ▪ Helicotrema is the apex of the cochlea. 5  Vestibular membrane separates the cochlear duct from the scala vestibuli. Basilar membrane separates the cochlear duct from the scala tympani. o Resting on the basilar membrane is the spiral organ of Corti. ▪ Organ for hearing. Membranous Labyrinth Lodged within the bony labyrinth. It is filled with a fluid, called endolymph. o Surrounded by perilymph. All of membranous labyrinth structures freely communicate with one another 1. Utricle & Saccule They’re sacs within vestibule. Utricle is the larger of the two vestibular sacs. Saccule is globular. Located on the walls of the utricle and saccule: o Specialized sensory receptors for balance. 2. Semicircular ducts They’re ducts within semicircular canals. There are sensory receptors for balance in the ampullae. 3. Cochlear duct It’s a duct within cochlea. Contains the spiral organ of Corti. o Contains sensory receptors for hearing. NOTE: The utricle is indirectly connected to the saccule and the ductus endolymphaticus by the ductus utriculosaccularis. o The ductus endolymphaticus, after being joined by the ductus utriculosaccularis, passes on to end in a small blind pouch, the saccus endolymphaticus. Cochlear duct is connected to the saccule by the ductus reuniens. 6 The Auditory Pathway 1st order neurons are bipolar neurons in the cochlear (spiral) ganglion. o They send their peripheral processes into the spiral organ of Corti. ▪ To conduct nerve impulses concerned with sound. o They project their central processes to form the cochlear nerve. Cochlear nerve enter the cranial cavity through the internal acoustic meatus to reach pons. o The cochlear nerve enter the anterior surface of the brainstem at the lower border of the pons. ▪ On the lateral side of the facial nerve. ▪ Separated from the facial nerve by the vestibular nerve. o Cochlear nerve divides into 2 branches to synapse with anterior and posterior cochlear nuclei. ▪ Anterior and posterior cochlear nuclei are situated on the surface of the inferior cerebellar peduncle. 2nd order neurons are neurons in the cochlear nuclei. o They send axons that synapse with: the posterior nucleus of the trapezoid body and the superior olivary nucleus. ▪ Bilaterally on the same or the opposite side. Most axons cross to the opposite side. 3rd order neurons are the posterior nucleus of the trapezoid body and the superior olivary nucleus. o The axons (from 3rd order neurons) ascend through the posterior part of the pons and midbrain to form a tract. o This tract is called the lateral lemniscus. ▪ Each lateral lemniscus consists of fibers of third order neurons from both sides. ▪ Some fibers relay in nucleus of the lateral lemniscus. On the midbrain, the fibers of the lateral lemniscus: o Some fibers synapse directly with the medial geniculate body (4th order neurons). ▪ Medial geniculate body (MGB) is part of the thalamus. ▪ Axons of MGB run through the acoustic radiation of the internal capsule. ▪ To reach the auditory cortex in the temporal lobe of the cerebral hemisphere. o Some fibers terminate in the nucleus of the inferior colliculus. ▪ Fibers coming from inferior colliculus then go to the medial geniculate body. Here, the inferior colliculus is the fourth order neuron Medial geniculate body is considered the fifth order neuron. ➔ The primary auditory cortex (areas 41 and 42) includes the gyrus of Heschl. o On the upper surface of the superior temporal gyrus. ➔ The recognition and interpretation of sounds on the basis of past experience take place in the secondary auditory area. ➔ Nerve impulses from the ear are transmitted along auditory pathways on both sides of the brainstem, more projected along the contralateral pathway. o Hearing from left ear is transmitted in auditory pathway of right and left sides ▪ But more in the auditory pathway of right side. 7