Histology CNS and Special Senses Theoretical PDF

Histology CNS and Special Senses Theoretical By Dr. Mohamed Salah Elgendy Professor and Head of Histology Department & Department Members Faculty of Medicine - Fayoum University 2024-2025...

Histology CNS and Special Senses Theoretical By Dr. Mohamed Salah Elgendy Professor and Head of Histology Department & Department Members Faculty of Medicine - Fayoum University 2024-2025 1  CNS The central nervous system (CNS) is a division of the nervous system whose function is to analyze and integrate various intra- and extra personal information, as well as to generate a coordinated response to these stimuli. Put simply, the CNS is the supreme command center of the body. The CNS consists of two organs which are continuous with each other; the brain and spinal cord. They are enveloped and protected by three layers of meninges, and encased within two bony structures; the skull and vertebral column, respectively. The brain consists of:  Cerebrum  Cerebellum  Brain stem Midbrain Pons Medulla  Diencephalon (Regions of Thalamus)  Thalamus  Metathalamus  Subthalamus  Hypothalumus 2   Epithalamus The spinal cord continues inferiorly from the brainstem and extends through the vertebral canal.  45 cm & invested by meninges  Continuous above with medulla  Conical lower end (conus medullaris)  Shorter than vertebrae  31 pairs of spinal nerves (8 cervical , 12 thoracic , 5 lumbar , 5 sacral & 1 coccygeal)  Each nerve is attached to the corresponding segment by two roots (ventral & dorsal)  It shows fissures & sulci on its external surface  1- Ventral (anterior) Median fissure  2- Posterior Median Sulcus from which extends the posterior median septum  3- 2 dorsal Intermediate Sulci (In cervical & thoracic segments)  4- 2 Dorsolateral Sulci (along which the sensory root enter the spinal cord ) 3  5- 2 Ventrolateral Sulci (along which the motor roots are attached to the spinal cord) 4  Grey and white matter The chief cells of the brain and spinal cord are the neurons, which receive and transmit neural impulses. Each neuron has a body which is its micro-command center, and it has a grey color when observed microscopically. The neurons possess two or more neural processes that stem from the body and relay the neural information. The neural processes are divided into short (dendrites) and long (axons). Most of the axons are ensheathed by a substance called myelin, which gives them a distinguishable white color. The parts of neurons comprise what we call grey and white matter. The gray matter is made up of clusters of neuronal bodies, while the white matter is composed of their myelinated axons. The axons are not just randomly interwoven through the neural tissue, but rather are organized into bundles that connect certain parts of gray matter and carry the relevant impulses. In the CNS, these bundles are called pathways and tracts, while in the PNS they comprise nerves. The distribution of gray and white matter is highly specific throughout the brain and spinal cord;  In the brain, the majority of gray matter is found superficially comprising the cerebral cortex, while the white matter composes its inner part. Smaller clusters of gray matter are found deep within the white matter comprising the subcortical structures, such as basal ganglia and diencephalon. Every unit of gray matter in the brain which is outside of the cortex is called the nucleus (plural - nuclei).  In the spinal cord, the gray matter comprises its inner part and has a characteristic butterfly shape when observed on cross-section. The white matter, consisting of spinal cord pathways, is located externally surrounding the grey matter 5  1. Structure and Shape of the Spinal Cord: Cervical Thoracic Lumbar Shape Oval Rounded Oval Position of Ventral Slightly Central central ventral canal White Large Less Little matter amount amount Grey Little Less Large matter amount amount Posterior Thin & Thin & // Thick & not horns diverging & reaching reaching the surface the surface Anterior Thick Thin & // Thick horns Lateral Absent Present Absent horns (constriction) 6  1. Motor fibers exit the spinal cord through the ventral (anterior) root. Summary of Fiber Types:  Dorsal Root (Posterior): Sensory (afferent) fibers only. 7   Ventral Root (Anterior): Motor (efferent) fibers only.  Spinal Nerve (after merging): A mixed nerve carrying both sensory and motor fibers. Once the spinal nerve exits the intervertebral foramen, it branches into:  Dorsal Ramus: Innervates the muscles and skin of the back.  Ventral Ramus: Innervates the anterior and lateral parts of the trunk and the limbs. This arrangement allows the spinal nerves to transmit sensory input to the central nervous system (CNS) and send motor commands from the CNS to the peripheral muscles and organs. 3. Grey Matter of the Spinal Cord:  Shape: The gray matter of the spinal cord has an "H" or butterfly shape in cross- section.  Horns: o Dorsal Horn: Receives sensory input from the body (afferent signals). o Ventral Horn: Contains motor neurons that project to skeletal muscles (efferent signals). 8  o Lateral Horn: Found in the thoracic and upper lumbar regions; contains neurons of the sympathetic nervous system. Key Nuclei in the Grey Matter: Sensory nuclei:  Substantia Gelatinosa (Dorsal Horn): Processes pain and temperature sensations.  Nucleus Proprius: Transmits touch and pressure sensations.  Clarke’s Nucleus: Found in the thoracic segments, relays proprioceptive information to the cerebellum. Lateral horn nuclei & interneurons:  Intermediolateral Cell Column: Located in the lateral horn, it's involved in autonomic (sympathetic) functions.  Commissural nuclei: Around central canal, Essential for reflex arc & visceral autonomic functions, Anterior, posterior 9  Motor nuclei:  Multipolar neurons (motor nuclei): Cells are arranged into 5 groups in cervical & lumbar segments : DM , VM , DL , VL & central motor nuclei. In thoracic segments they are reduced into two groups only : DM & DL 4. White Matter of the Spinal Cord: The white matter surrounds the grey matter and is composed of myelinated and unmyelinated nerve fibers. These fibers are organized into tracts (bundles of axons) that carry signals up and down the spinal cord. Major Tracts: group of fibers having the same function, origin, termination  Long Ascending (Sensory) Tracts: o Dorsal Column-Medial Lemniscus Pathway: Transmits fine touch, vibration, and proprioception. 10  o Spinothalamic Tract: Carries pain, temperature, and crude touch information. o Spinocerebellar Tract: Conveys proprioceptive information to the cerebellum for balance and posture control.  Long Descending (Motor) Tracts: o Corticospinal Tract: Controls voluntary motor movements (e.g., fine movements of limbs). o Rubrospinal and Reticulospinal Tracts: Influence motor control, postural adjustments, and automatic movements.  Short ascending and descending tracts  Fasciculi proprii tract: all levels (connect sensory and motor nuclei)  Lissauer’s tract: all levels (from spinal ganglia)  Comma shaped tract: cervical and upper thoracic (fibers from cuneate tract) 11   Septomarginal tract: lower thoracic and lumbar and Sacral (fibers from gracil tract) 5. Spinal Cord Nuclei (group of cells having the same function) and Fibers:  Motor Neurons: o Found in the ventral horn, these neurons send axons out through the ventral roots to innervate skeletal muscles. o Alpha motor neurons control muscle contractions, while gamma motor neurons regulate muscle tone through the muscle spindle.  Sensory Neurons: o First-order neurons from the peripheral sensory receptors enter through the dorsal roots and synapse in the dorsal horn. o Sensory pathways ascend to the brain through the posterior columns and anterolateral system.  Interneurons: o These are located in the intermediate zone of the grey matter and form circuits that modulate reflexes and coordinate complex movements. 6. Reflex Arcs: The spinal cord also plays a crucial role in mediating reflexes, which are rapid, involuntary responses to stimuli. Reflex arcs involve:  Sensory neurons: Detect stimuli and send afferent signals to the spinal cord.  Interneurons: Process the information within the spinal cord.  Motor neurons: Send efferent signals to muscles to produce a response (e.g., withdrawal reflex). 7. Blood Supply: The spinal cord’s blood supply is crucial for its function:  Anterior Spinal Artery: Supplies the anterior two-thirds of the spinal cord. 12   Posterior Spinal Arteries: Supply the posterior third of the spinal cord.  Radicular Arteries: Branches of segmental arteries that support the longitudinal supply. Ischemia in these arteries can lead to significant deficits such as anterior spinal artery syndrome, where motor function is lost but sensation is preserved. General Rules for sensory pathways 13 General rules in sensory pathways First order neuron Pseudounipolar nerve cells with long axons (dendrites) (peripheral sensory fibers) which carry peripheral sensory impulses from terminal receptors They send fibers via dorsal root to terminate in the 2nd order neuron of the same side Second Order neuron Multipolar nerve cells in posterior horns of the grey matter of the same side This 2nd order neuron might be in spinal cord or medulla The axons cross in front of the central canal & ascend as a tract (in SC) or lemnisci (in medulla) to terminate in the 3rd order neuron Third order neuron Cell bodies ar located in PLVNT (posero-lateral ventral nucleus of thalamus) Their axons are called sensory radiations (Thalamo-cortical fibers) They pass in the posterior half of the posterior limb of the internal capsule to the sensory area in the posterior central gyrus (area 3 , 1 & 2)  Pathway of Proprioception & Fine Touch from the body 14 Receptor for proprioception 1. Muscle spindle in the muscle. 2. Tendon spindle in the tendon. 3. Pacinian corpuscle in the connective tissue capsule of joints. Receptors for fine touch 1- Meissner’s corpuscle. 2- Ruffini’s corpuscle. 3- Merkel’s disc Proprioceptive & fine Touch pat 15   Receptors:????? st  1 order neuron : -Large cells in spinal ganglia receive impulse by -Thick myelinated nerve fibers -Impulses enter spinal cord via Medial division of post root. -Sacral, lumbar & lower thoracic segments ↑ as gracile tract on same side. -Upper thoracic & cervical ↑ as cuneate tract on same side. nd  2 order neuron : -Gracile & Cuneate nuclei in closed medulla on the same side. -axons from both nuclei divided into external arcuate fibers end into arcuate nucleus in open medulla then enter cerebellum therough inferior cerebellar peduncle, or internal arcuate fibers that cross to the opposite side (Sensory decussation) Then ascend in the brain stem as medial lemniscus ening in:. rd  3 order neuron : -PLVNT posterolateral ventral nucleus of Thalamus, their axons ascend in internal capsule forming -Sensory radiation (axons ascend to sensory area 3 , 1 , 2 in Cortex) 16 17 lemniscus:Ascending Bundles of sensory fibers within the brainstem Medial lemniscus Def:ascending sensory bundle in the brainstem. Origin:gracile&cuneate nuclei in closed medulla of opposite side that give rise to internal arcuate fibers.The fibers cross to opposite side in upper half of closed medulla (sensorydecussation). Course:ascendinbrainsteminamedialposition. Termination:P.L.V.N in thalamus. Function:carries proprioception & fine touch from opposite side of the body. 18 Clinical significance: Dorsal Column Injuries (Tabes dorsalis) : Loss of fine touch and proprioception on the same side (ipsilateral) below the injury. Pain&Temperature pathway: Receptors: free nerve endings, Ruffnii organ: heat, Krause organ: cold Pain and temperature are carried from receptors by thin myelinated nerve fibers to 1st order neuron:Small cells in spinal ganglia. Central branches (axons of cells) enter the spinal cord through Lateral division of post root and reach the Lissaurer’s tract where they ascend or descend for 1 or 2 segments then end on: 2nd order neuron : cells of Substantia Gelatinosa of Rolandi(SGR). Fibers cross in front of the central canal to the opposite side and ascend upwards as Lateral spinothalamic tract in lateral column Join ventral spinothalamic tract at the pons to form spinal lemniscus, rely with cells of : 3rd order neuron: PLVNT , axons of these cells ascend in internal capsule forming Sensory radiation, ending in Cortex area 3 , 1 , 2 19 20  Clinical significance:  Spinothalamic Tract Injuries: Loss of pain and temperature sensation on the opposite side (contralateral) below the injury. Crude touch pathway: Receptors: 1-Free nerve endings. 2-Peritrichial Nerve Endings. 3- Merkel’s Disc. 4- Meissner’s corpuscle. 1st order neuron : medium sized cells in spinal ganglia. Peripheral branches: Medium sized myelinated. Central branches: Medial division. 2nd order neurone : nucleus proprius (MSN) main sensory nucleus. Fibers cross in front of the central canal Form ventral spinothalamic tract Join lateral spinothalamic tract at the pons to form spinal lemniscus 3rd order neurone : PLVNT Sensory radiation-----Cortex area 3 , 1 , 2 21 22  Lateral spinothalamic tract Ventral spinothalamic tract Def Long ascending sensory tract in same spinal cord Origin Cells of SGR of opposite side Cells of MSN of opposite side Course Crossed Crossed Lat column (all levels). Vent column (all levels). Joint ventral spinothalamic tract at Joins lateral spinothalamic tract at the pons to form spinal lemniscus the pons to form spinal lemniscus Termination PLVNT PLVNT Function Pain & temp Crude touch From the opposite side of the From the opposite side of the body. body. 23   Spinocerebellar Injuries: Ataxia or loss of coordination, usually on the same side (ipsilateral). Understanding these long ascending tracts is vital for diagnosing and localizing spinal cord injuries or lesions based on the types of sensory deficits observed. Different types of sensations are transmitted to the brain via sensory pathways in the nervous system. These sensations allow the body to interact with the external environment and maintain internal homeostasis. Sensory receptors detect changes and Spinal lemniscus Def: ascending sensory bundle in the brain stem formed by lateral & ventral spinothalamic tracts Course: in pons → mid brain. Termination: PLVNT. Function: pain, temperature & crude touch from the opposite side of body.  Clinical relevance: Syringomyelia: Congenital dilatation of central canal. In lower cervical & upper thoracic region. Pressure on lateral spinothalamic tract. 24  Bilateral loss of pain & temperature in upper limbs & chest. Lat spinothalamic is intact so sensation above & below are normal. Touch is felt in areas of skin with lost pain & temp (dissociated sensory loss).  Pathway of sensory impulses to subcortical levels 1- Dorsal spinocerebellar tract. 2- Ventral spinocerebellar tract. 3- Spino olivary tract. 4- Spino tectal. Tract. 25  Spino- olivary tract Spino-tectal tract Origin Nucleus proprius on same the opposite side Course Ascends in lateral white column Same of spinal cord in all levels & in brain stem Termination Inferior olivary nucleus in Superior colliculus in mid medulla that sends olivocerebellar fibers to brain cerebellum through inferior cerebellar peduncle. Function Carry proprioceptive impulses Carry sensory impulses to the to tectum, where it drives spino visual reflexes cerebellum. 26 27  Short tracts Fasciculi proprii Lissauer’s tract Comma shaped Septomarginal Origin Associative nuclei Small cells of sp Collateral from Collateral from ganglia Cuneate tract Gracil tract Termination Ant, post horns Post horn Ant horns Ant horns (SGR) Coarse Ascending or Axons of small Descending for Descending for descending cells enter few segments few segments spinal cord through lateral division, ascend or descend 1 or 2 segments Function Coordination of Pain temp of Stretch reflex Stretch reflex different regions same side of spinal cord Descending tracts The nerve cells present in cerebral cortex or subcortical level & terminate at motor nuclei of cranial nerves or AHCs of spinal cord …….. Upper motor neuron The motor nuclei of cranial nerves & the AHCs give axons that terminate at the muscles……. Lower motor neuron:  The upper motor neuron are classified into pyramidal & extrapyramidal tracts.  Pyramidal tracts: descending motor tracts from cerebral cortex to anterior horn cells of spinal cord. They form the pyramid in medulla & include:  Corticospinal tract. 28   Corticobulbar tract. A- Corticospinal Tract  Origin :Betz cells & others in upper 2/3 of area 4 in cerebral cortex.  Course: white matter: Corona radiata. Internal capsule : genu & anterior 2/3 of posterior limb Midbrain: Basis pedunculi (intermediate part). 29  Pons: Basis pontis (separated by transeverse pontine fibres). Closed medulla : pyramidal decussation 85 % of fibers form crossed pyramidal tract & descend in lateral column of spinal cord (lateral corticospinal tract) 15 % descend as direct pyramidal tract in ventral column of spinal cord (ventral corticospinal tract) then they cross in the lower thoracic region  Termination: AHCs of the opposite side & few in the same side  Function: control voluntary movements Fine motor functions as typing B- Corticobulbar tract: Descending motor tracts from cerebral cortex to motor nuclei of cranial nerves in brain stem 1- Medial corticobulbar 30  Origin: area 8 (motor) in cerebral cortex.  White matter: Corona radiate  Internal capsule: in the genu Midbrain: Descend in basis pedunculi very close to midle line. Ends: midbrain & pons in motor nuclei of cranial nerves 3 , 4 , 6 (oculomotor nerves) on both sides Function: regulate the movement of both eyes 2- Lateral corticobulbar  Origin : lower 1/3 of area 4 in cerebral cortex.  Course:  White matter: Corona radiata  Internal capsule: Genu  Midbrain: basis pedunculi (lat to corticospinal)  Ends: pons & medulla in crainial nerve nuclei 5 , 7 , 9 , 10 , 11 & 12 mostly on both side  Function: movement of muscles of face, mastication & pharynx 31  Extrapyramidal tracts  Sulcomarginal  Olivospinal  Rubrospinal  Lateral tectospinal  Ventral tectospinal  Lateral vestibulospinal  Ventral vestibulospinal  Lateral reticulospinal  Ventral reticulospinal 32 33 34 1. Medulla Oblongata Structure:  The medulla oblongata is the lowest part of the brainstem, directly continuous with the spinal cord.  Contains several important structures, including:  Grey matter: o Pyramids: Contain the corticospinal tracts, which are responsible for voluntary motor control. o Olives: Lateral to the pyramids, these structures are involved in motor coordination and learning. o Reticular formation: A network of neurons involved in autonomic control. o Cranial Nerve Nuclei: The medulla contains nuclei for cranial nerves IX (Glossopharyngeal), X (Vagus), XI (Accessory), and XII (Hypoglossal), which control functions like swallowing, heart rate, and tongue movements.  White matter:  Descending Pathways: The corticospinal tracts decussate (cross) in the pyramids, allowing for contralateral control of voluntary motor actions.  Ascending Pathways: The dorsal columns (gracile and cuneate fasciculi) transmit sensory information like proprioception and fine touch to the brain.  Other Pathways: The spinothalamic tract carries pain and temperature information, while the spinocerebellar tracts transmit proprioceptive data for motor coordination. 35 36 2. Pons Structure:  The pons is the middle section of the brainstem, situated between the medulla and the midbrain.  Key structures include:  Grey matter: o Pontine Nuclei: These relay signals from the cerebral cortex to the cerebellum, playing a crucial role in motor control. o Cranial Nerve Nuclei: The pons houses nuclei for cranial nerves V (Trigeminal), VI (Abducens), VII (Facial), and VIII (Vestibulocochlear). These nuclei control facial sensation, motor functions of the face, and eye movements. o Reticular Formation: A network involved in regulating sleep and arousal, found throughout the pons.  White matter: 37  Corticopontine Tracts (transverse): Carry motor information from the cerebral cortex to the pons.  Corticospinal Tracts (longitudinal): Continue through the pons and into the medulla, transmitting motor commands.  Medial Longitudinal Fasciculus (MLF): Connects the vestibular system to eye movements, coordinating head and eye movements. Basis Tegmentum pontis Inf M S Nuclei Pontine 5 sp, 5 Ms,Mo 5 Mn 6, 7 Mn 8: coch: D, V Vest: M, L, S Fibers T Tracts, Tracts, Tracts, pontine MLB MLB MLB L Pyram Lemnisci:1 Lemnisci:4 Lemnisci: 4 Peduncles Middle I S S ----- ++++ ++++ ++++ 38 39 3. Midbrain (Mesencephalon) Grey matter:  Cranial nerve nuclei: Includes nuclei for cranial nerves III (oculomotor) and IV (trochlear).  Periaqueductal gray matter: Surrounds the cerebral aqueduct and plays a role in pain modulation.  Substantia nigra: Part of the basal ganglia involved in motor control, and its degeneration is associated with Parkinson's disease.  Red nucleus: Involved in motor coordination, especially of limb movements.  Tectum: Houses the superior colliculi (visual reflexes) and inferior colliculi (auditory reflexes). 40  White matter:  Cerebral peduncles: Contain descending motor tracts from the cerebral cortex (corticospinal and corticobulbar tracts).  Medial lemniscus and spinothalamic tract: Carry sensory information to the thalamus. Summary of midbrain Tectum Tegmentum Basis pedunculi (Crus cerebri) Dorsal to an In between Anterior part rd th immaginary Contains 3 & 4 Contains : horizontal line crainial nerves nuclei + 1- Lateral & medial Corticobulbar passing through th 2- Middle 3/5 Corticospinal the aqueduct of 5 (mesencephalic) 3- Lateral & medial Corticopontine Sylvius Decussations of bundles Contains : 1- RS Descending fibers of cortical origin 1- Inferior 2- SCP colliculus 3-TS 2- Superior colliculus Level inf Level sup colliculus colliculus Ext pyr Substantia Substantia nigra nigra Red nucleus Decussation Superior Rubrospinal cerebellar Tectospinal Cranial N 4 + 5 Ms 3 + 5 Ms 41  Aqueduct Quadrilateral Pear shaped Lemnisci 4 3 Colliculi Inferior Superior One oval mass (layers) ZGOLP Red nucleus: Extra pyramidal nucleus in sup level of midbrain Red in fresh state (vascularity, iron) Receives Affarent Gives Efferent to from Cerebrum, Reticular cerebellum, formation (rubro- (diencephalon) reticular) subthalamus, Motor cranial globus pallidus nerves (rubro- bulbar) Inferior olivary (rubro-olivary) AHCs of spinal cord (rubrospinal) reticular, olivary 42 43 Pathways from the head Proprioception Pain, temp Crude touch Receptors Muscle, tendon, Free nerve Free, peri, pacinian endings 1 st Mesencephalic N Trigeminal Trigeminal of trigeminal Ganglion Ganglion nerve inside CNS 2 nd MSN Spinal N MSN pathway Axons cross, ascend in trigeminal lemniscus 3 rd PMVNT Auditory pathway: Receptors: hair cells in organ of corti 1 st: spiral ganglion 2 nd: dorsal, ventral cochlear N Course: axons, trapezoidd body, lateral lemniscus, end in inferior colliculus or medial geniculate body 3 rd: from medial geniculate body to area 41 Hearing is bilaterally represented: Vent cochlear N give fibers to: Ascend in the same side lateral lemniscus, Relay in nuclei in superior olivary N, trapezoid body and lateral lemniscus 44 Vestibular pathway Receptors (5): macula utericuli, saculi, cristae ampularis in 3 semicircular canals 1st vest ganglia: impulses from receptors to bipolar nerve cells in vestibular ganglia. Axons of these cells (vestibular nerve) enter pons at pontomedullary junction to end in 2nd vest nuclei: superior, medial, lateral nuclei in pons, inferior nucleus in pontomedullary junction, directly to cerebellum Connections(5): vest ocular tracts: 3, 4, 6 fix eye Vest reticular: RF vomiting Vest spinal: AHC extensors 45  Vest bulbar Vest cerebellar: cerebellum equlibrium Medial Longitudinal Bundle The medial longitudinal bundle (MLB), also known as the medial longitudinal fasciculus (MLF), is a white matter tract in the brainstem that coordinates eye movements, head movements, and gaze stabilization. It plays an important role in coordinating movements between the eyes and the neck. Key components and functions: 1. Ascending Fibers: o Connect the vestibular nuclei (involved in balance) with the oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nuclei. o Responsible for coordinating conjugate eye movements (e.g., horizontal gaze). 46 2. Descending Fibers: o Involved in transmitting information between the vestibular system and spinal cord to control head and neck movements, contributing to postural control. 3. Cranial Nerve Connections: o Oculomotor Nerve (CN III): Controls most eye muscles, including those that move the eye up, down, and medially. o Trochlear Nerve (CN IV): Controls the superior oblique muscle, which helps rotate and depress the eye. o Abducens Nerve (CN VI): Controls the lateral rectus muscle, responsible for moving the eye laterally (abduction). 4. Vestibular Nuclei: o Receive input from the vestibular apparatus (inner ear) about head movements and position in space, playing a key role in maintaining gaze stability during head movements. 47 Visual pathway 1. First-Order Neurons (Photoreceptors to Bipolar Cells):  Location: Retina  Function: The first-order neurons are the photoreceptors (rods and cones) in the retina that detect light and convert it into electrical signals.  These signals are passed on to bipolar cells, which are the true first-order neurons in the visual pathway. Bipolar cells connect the photoreceptors to the second-order neurons. 2. Second-Order Neurons (Bipolar Cells to Retinal Ganglion Cells):  Location: Retina  Function: The second-order neurons are the retinal ganglion cells. The bipolar cells transmit the electrical signals to these ganglion cells.  The axons of the retinal ganglion cells form the optic nerve. As the optic nerves leave the retina, they carry the information from each eye toward the brain, crossing at the optic chiasm. 3. Third-Order Neurons (LGN to Visual Cortex):  Location: Lateral Geniculate Nucleus (LGN) of the Thalamus to the Primary Visual Cortex (V1)  Function: The third-order neurons are located in the lateral geniculate nucleus (LGN) of the thalamus. The optic tracts carry the visual information from the retinal ganglion cells to the LGN.  From the LGN, third-order neurons project via the optic radiations to the primary visual cortex (V1) in the occipital lobe. Here, the brain processes and interprets the visual information to form conscious vision 48 Histology of the Cerebellum: The cerebellum is responsible for coordinating voluntary movements, balance, and posture. Its histological structure consists of two main layers: 1. Cerebellar Cortex: The cerebellar cortex has three distinct layers: Layers of the Cerebellar Cortex: 1. Molecular Layer (Outer Layer): o Contains dendrites of Purkinje cells, axons of granule cells, and stellate and basket cells (inhibitory interneurons). o Mostly composed of fibers and synapses, with relatively few cell bodies. 2. Purkinje Cell Layer (Middle Layer): 49  o A single layer of large, flask-shaped Purkinje cells, which are the only output neurons of the cerebellar cortex. o Purkinje cells send inhibitory signals to the deep cerebellar nuclei and play a crucial role in motor coordination. 3. Granular Layer (Inner Layer): o Contains densely packed granule cells, which are the most numerous neurons in the brain. o Golgi cells are also present here. Granule cells send axons up to the molecular layer, where they synapse with Purkinje cells. 2. White Matter: Afferent: climbing from inf olive to Purkinje and mossy (from pons (pontine nuclei to granular cells) Efferents: from Purkinje Some nuclei embedded in it: – Fastigeal, globose, emboliform, dentate serve as the main output of the cerebellum, receiving inhibitory input from Purkinje cells and excitatory input from other sources. Afferents and efferents form (superior, middle and inferior cerebellar peduncles) 50 51 Histology of the Cerebrum: The cerebrum is the largest part of the brain, responsible for higher brain functions such as thinking, memory, and sensory processing. The histological structure of the cerebrum consists of the following: 1. Cerebral Cortex:  The cerebral cortex is the outermost layer of the cerebrum, composed of gray matter (neuronal cell bodies, dendrites, and glial cells).  The cortex is divided into six layers (in the neocortex), each with distinct types of neurons and functions. Layers of the Cerebral Cortex (Neocortex): 1. Molecular Layer (Layer I): o Contains few neurons and mostly dendrites and axons from deeper layers, as well as synapses between them. o Dominated by glial cells and horizontal fibers. 2. External Granular Layer (Layer II): o Small granule cells (small neurons) dominate, along with some pyramidal neurons. o Responsible for receiving inputs from other cortical areas. 3. External Pyramidal Layer (Layer III): o Contains small and medium-sized pyramidal cells (neurons with a triangular cell body). o Involved in sending signals to other cortical areas and distant brain regions. 4. Internal Granular Layer (Layer IV): o Densely packed with small granule cells. o Main recipient of sensory input from the thalamus. 5. Internal Pyramidal Layer (Layer V): o Large pyramidal cells called Betz cells are found here, especially in the motor cortex. o Sends outputs to subcortical structures such as the spinal cord and brainstem. 6. Multiform Layer (Layer VI): o Mixed population of cells, including spindle-shaped neurons. o Sends output to the thalamus and other brain areas. 2. White Matter:  Located beneath the gray matter of the cortex, consisting of myelinated axons. 52   Connects different regions of the brain and transmits information between them. 3. Glial Cells:  Astrocytes, oligodendrocytes, and microglia are present throughout the cerebrum.  Astrocytes support and nourish neurons, oligodendrocytes myelinate axons, and microglia act as immune cells. Meninges: The meninges are the three protective connective tissue membranes that cover the brain and spinal cord. These layers are known as the dura mater, arachnoid mater, and pia mater. Each of these layers has a distinct structure and function, and together, they protect the central nervous system (CNS), provide structural support, and allow for the circulation of cerebrospinal fluid (CSF). 1. Dura Mater (Outer Layer): 53   Tough , thick , collagenous sheath  Outer endosteal layer :  Attached firmly to periosteum  Supplied with nerves & vessels  Inner meningeal layer :  Fibrous layer covered internally by endothelium  These two layers separate from each other at certain locations to form venous dural sinuses  Between dura & arachnoid there is a potential subdural space 2. Arachnoid Mater (Middle Layer):  It is made of :  Thin membranous roof close to the dura  Irregular system of trabeculae that hold up the roof like pillars  Both of these components are made of C.T. (collagen & elastic fibers)  The space between arachnoid roof and pia is called subarachnoid space which contains : 1- CSF 2- BVs of the brain  Pia & arachnoid are regarded as a combined membrane = pia arachnoid or leptomeninges 3. Pia Mater (Inner Layer):  Delicate inner thin vascular membrane covered with simple squamous epithelium  Adheres closely to the brain gyri (convolutions) & spinal cord  It extends to the depth of the sulci  It conveys small BVs down into the substance of CNS  Connected to the arachnoid by trabeculae Summary of the Meninges:  Dura Mater: Tough, outer layer made of dense connective tissue; provides mechanical protection and forms dural sinuses. 54   Arachnoid Mater: Web-like, middle layer that creates the subarachnoid space, through which CSF circulates; absorbs CSF via arachnoid villi.  Pia Mater: Delicate, inner layer closely adhering to the brain and spinal cord; highly vascularized and supplies nutrients to the CNS. Brain Barriers: The brain is protected by several barriers that regulate the movement of substances between the blood, brain, and cerebrospinal fluid. These barriers are crucial for maintaining the brain’s microenvironment and protecting it from toxins and pathogens. 1. Blood-Brain Barrier (BBB):  It separates blood in the blood capillaries from the brain tissue Components :  Non-fenestrated endothelium (with occludens junction)  Continuous basement membrane  End feet processes of astrocytes 55 2. Blood-CSF Barrier:  It separates the blood from CSF Components :  Non-fenestrated endothelium  BM of blood capillaries (endothelial cells)  BM of ependymal cells  Tight junction between ependymal cells 56  2. Brain CSF barrier It separates the brain tissue from the CSF Components  Simple cuboidal ependymal cells connected by tight junctions  BM of ependymal cells  Subependymal neuroglia processes connected by tight junctions 4. Arachnoid Barrier:  Between extracerebral capillaries & subarachnoid space  Cells joined by tight junction  Block substances leaving extracerebral capillaries from reaching subarachnoid space , brain tissue & ventricles 57 Histology of the Choroid Plexus: The choroid plexus is a specialized structure in the brain that produces cerebrospinal fluid (CSF). It is located in the ventricles of the brain (lateral, third, and fourth ventricles) and plays a critical role in the homeostasis of the brain's internal environment. Structure of the Choroid Plexus: 1. Epithelium: o The choroid plexus is covered by a single layer of cuboidal to columnar epithelial cells known as ependymal cells (specifically, choroid plexus epithelial cells). o These cells have tight junctions that create the blood-CSF barrier and prevent the free passage of substances from the blood to the CSF. o Apical surfaces of these cells are covered with microvilli, which increase the surface area for the secretion of CSF. o Basal surfaces are in contact with the underlying capillaries, which provide the blood supply for the production of CSF. 2. Vascularized Connective Tissue: 58  oBeneath the epithelium is a core of highly vascularized connective tissue (loose connective tissue) containing numerous fenestrated capillaries. o The fenestrations (small pores) in the capillaries allow plasma components to pass into the connective tissue, which are then processed by the epithelial cells to form CSF. 3. Pia Mater: o The choroid plexus is continuous with the pia mater, one of the meninges that cover the brain. Function of the Choroid Plexus:  The choroid plexus produces CSF, which circulates through the ventricles and the subarachnoid space, surrounding the brain and spinal cord.  The CSF helps to cushion the brain, remove waste products, and provide a stable environment for neuronal function. 59  THE EYE The eye is a photoreceptor organ. It is found in a bony cavity in the skull which is called the orbit. The wall of the eye ball is formed of three layers: I) The outer layer: It is a fibrous coat, formed of cornea anteriorly and sclera posteriorly. II) The middle layer: It is the vascular coat, formed of three parts 1-The iris. 2-The ciliary body. 3-The choroid. III) The Inner layer: It is the nervous coat, formed of the retina. The eye contains three compartments: 1-The anterior chamber between the cornea and the iris. 2-The posterior chamber between the iris, ciliary body and the lens. 3- The vitreous space behind the lens. The anterior and the posterior chambers are connected by small opening in the iris called the pupil. Both chambers contain protein-poor fluid called aqueous humor, while the vitreous space is filled with a gelatinous material called vitreous body. The lens of the eye is a biconvex transparent structure, attached to the ciliary body by a ligament called zonule. 60 The outer fibrous coat I) The cornea: The cornea represents the anterior part of the outer coat of the eye. It is the transparent rounded disk. 1-The epithelium It is formed of stratified squamous non- keratinized epithelium, formed of the following layers: a-The basal cell layer: It is formed of one layer of cubical cells rest on a straight basement membrane. b-The intermediate layers: It is formed of polygonal cells, which are connected by desmosomes. C-The top layer: It is formed of flat squamous epithelium. Their free borders contain microvilli which retain a film of tears over the corneal surface to keep it wet. The corneal epithelium is very sensitive because it is richly supplied by free nerve endings.The epithelium showed also high rate of regeneration from the basal cell layer. 2- Bowman’s membrane: It is non cellular transparent homogenous membrane. It is formed of a network of collagen and intercellular substances. It acts as a protective layer against any trauma or bacterial invasion. Injury to this membrane results in scar formation and corneal opacity. 3-Substantia propria It forms the main thickness of the cornea (about 90%) It is formed of about 200 lamellae of collagen type I.The collagen fibrils are arranged parallel to the corneal surface. They are embedded in ground substance 61 highly rich in sulphated glycosaminoglycan. In-between these fibrils, fibroblasts (Corneal corpuscles) are located. 4-Descemet’s membrane It is non cellular homogenous membrane. It is formed of two layers: a- An outer layer of irregularly arranged collagen fibrils. b- An inner layer of basal lamina. 5-Descemet's endothelium It is formed of one layer of flat squamous cells that lines the posterior surface of the cornea. These cells are highly rich in mitochondria and microvesicles. Function: They actively absorb water from the substantia propria to the aqueous humor. Why the cornea is transparent? 1- The cornea is avascular structure ( not penetrated by blood vessels). So the cornea takes its nutrition from the aqueous humor and from the scleral capillaries in the corneo-scleral junction. 2- Regular arrangement of collagen lamellae. 3-The same refractive index of the collagen fibrils and the ground substance. 4-Continuous withdrawal of water from the cornea by Descemet’s endothelial cells. 2) The sclera: The sclera is the opaque white posterior part of the outer fibrous coat of the eye ball. Its opacity is due to a- irregular arrangement of its collagen fibers 62  b- its relative high content of water. The sclera is formed of three layers: 1- Episclera: formed of fibroelastic connective tissue. 2- Sclera proper: formed of dense irregular connective tissue. (white fibrous C.T.) 3- Lamina fusca: formed of loose connective tissue. The posterior part of the sclera which is perforated by the optic nerve is called lamina cribrosa. Changes at the corneo-scleral junction (The limbus) 1-The corneal epithelium (Stratified squamous epithelium) forms the bulbar conjunctiva that covers the anterior part of the sclera. 2-Bowman’s membrane stops and it is replaced by subconjunctival connective tissue. 3-Substantia propria of the cornea forms the sclera proper, in which the collagen fibrils are irregularly arranged. A circular canal called canal of Schlemm surrounds the limbus. The canal is connected to the anterior chamber by minute spaces called spaces of Fontana. 4-Descmet’s membrane splits into fine trabeculae which form a pictinate ligament. The trabeculae enclose spaces of Fontana. 5-Corneal endothelium forms the lining of canal of Schlemm and spaces of Fontana. II) The middle vascular coat The middle coat is also called uveal tract. It is formed of three parts, the choroid, the ciliary body and the iris. 1-The choroid: It is the posterior part of the vascular coat, separated from the sclera by the perichoroidal space. It is formed of the following layers 63  1) Suprachoroid layer: It is formed of collagen, elastic fibers and connective tissue cells as fibroblasts, macrophages, lymphocytes, melanocytes and mast cells. 2)The vessel layer: This contains loose connective tissue with large and medium sized blood vessels. 3)The chorio-capillary layer It is highly rich with wide fenestrated blood capillaries. It gives the nutrition for the retina. 4)Bruch’s membrane It is non-cellular layer, represents the innermost part of the choroid. It is formed of five layers. a) The basement membrane of the capillaries in the chorio-capillary layer. b) Layer of collagen fibers. c) Layer of elastic fibers. 64  d) Layer of collagen fibers. e) The basement membrane of the pigmented epithelium of the retina. 2-The ciliary body: It represents the middle part of the vascular coat. In section, it appears triangular in shape. Its apex is continuous posteriorly with the choroid and its base faces the iris. The ciliary processes project from its medial border. It is covered by two layers of cubical cells, the inner layer is pigmented. It is formed of 1)Loose connective tissuehighly rich in elastic fibers and melanocytes. 2)Ciliary muscles which is formed of smooth muscle fibers arranged in two bundles redial and circular fibers. It is attached anteriorly to the sclera and posteriorly extends to different regions in the ciliary body. Contraction and relaxation of these muscles can change in the shape of the lens which is important in visual accommodation. 65 3) Ciliary processes These are projection from the medial side of the ciliary body. They are formed of loose connective tissue and fenestrated blood capillaries, covered by two layers of cubical cells as the ciliary body. Function: 1-Secretion of the aqueous humor 2-They give the attachment to the Suspensory ligament of the lens (Zonule) 3-The iris It is the anterior part of the middle coat of the eye. 1) Anterior stromal layer of pigment cells and fibroblasts 2) Posterior vessel layer, which is more vascular 3) Loose connective tissue, which contains the muscle of the iris a)The sphincter pupillae muscle that forms a circular band around the pupil. Its contraction causes constriction of the pupil. b)The dilator pupillae muscle which lies near the posterior border of the iris. Its contraction causes dilatation of the pupil. 4) Posterior surface is covered by two layers of epithelial cells as that cover the ciliary body. The inner one is pigmented. 66 Function of the iris 1-It gives the colour of the eye. 2-The pupillae muscle controls the size of the pupil. 3-Heavy pigmentation of the epithelium prevents passage of light into the interior of the eye except through the pupil. The lens The lens is a transparent biconvex structure.It has three main components 1-Lens capsule which is rich in collagen and glycoprotein. 2-Subcapsular epithelium formed of single layer of cubical cells on the anterior surface of the lens. 3-The lens fibers,which are originated from the epithelial cells that lose their nuclei and organoids. They are elongated fibers and contain crystalline protein, some microtubules and microfilaments. The lateral borders of the lens fibers are interlocked to prevent sliding of the fibers. The vitreous body It is a gel non vascular mass that contains water, collagen and hyaluronic acid. It occupies the interior of the eye ball between the lens and retina. It is traversed by the hyaloid canal. Function 1-Transmission of light to the retina. 2-Supports the lens and the retina. 3-Allows transfer of materials from and to the retina. 67  Aqueous humor It is a thin watery protein -poor fluid found in both the anterior and the posterior chambers of the eye. Formation It is secreted from the non-pigmented epithelium of the ciliary body. Circulation The aqueous humor is secreted in the posterior chamber, and then it passes through the pupil to the anterior chamber to the spaces of Fontana and canal of Schlemm to the aqueous veins and the venous circulation. Refractive media of the eye There are four transparent components through which the light passes till it reaches the retina. 1-The cornea. The most effective one. 2-The aqueous humor. 3-The lens. 4-The vitreous body. III) The inner nervous coat (The retina) The retina is formed of ten layers 1)The pigmented epithelium It is formed of single layer of columnar epithelium with basal nuclei comes in contact with the Bruch’s membrane of the choroid. The cell membrane of these cells shows 1-The lateral surface is connected to adjacent cell by junctional complex and gap junction. 2-The apical border shows microvilli that envelop the tips of the photoreceptors. 3-The basal border shows basal infoldings with numerous mitochondria. The cytoplasm of these cells contains 1-Abundant smooth endoplasmic reticulum. 2-Numerous mitochondria. 3-Numerous lysosomes. 4-Rough endoplasmic reticulum and Golgi apparatus. 5-Melanin pigments. Function 1-Absorption of extra light to prevent blurring of vision. 68  2-Phagocytosis of the old disks detached from the tips of rods and cones. 3-Formation of vitamin A which passes to the rods and cones. 4-The junctional complex between the cells protects retina from any undesirable materials that may found in the choroid forming the blood retinal barrier. 2) The layer of rods and cones It is formed of the outer photosensitive segments of both rods and cones. 3)The outer limiting membrane. It is formed by the junctional complex between the outer processes of the Muller’s cells and the adjoining rods and cones. 4) The outer nuclear layer Consists of cell bodies of both rods and cones, the nuclei of cones are located near the outer limiting membrane, while the nuclei of the rods are more numerous and found in different levels. 5) The outer plexiform layer It is formed by synaptic connection between the synaptic bodies of both rods and cones with the dendrites of bipolar nerve cells and the processes of the horizontal cells. 69 6) The inner nuclear layer: It contains the nuclei of a- The bipolar nerve cells:Two types of bipolar cells 1-Diffuse bipolar cells, that synapses with two or more of rods. 2-Monosynaptic bipolar cells (Midget cells) that synapse with a single cone. b-Horizontal cells They lie in the outer region of the inner nuclear membrane. They form synapse with the rods and cones. c-Amacrine cells They form synapse with the axons of the bipolar cells and the dendrites of the ganglion cells. d-Muller cells They are supporting cells with long cytoplasmic processes The outer processes extend externally to form the outer limiting membrane, while the inner processes extend internally to form the inner limiting membrane. 70 7) The inner plexiform layer It is formed of synaptic connection between the axons of the bipolar nerve cells and the dendrites of the ganglion cells. The processes of the Amacrine cells synapse with these two cells. 8) The ganglion cell layer It is formed of large nerve cells with vesicular rounded, central nuclei. Their dendrites synapse with the axons of bipolar cells in the inner plexiform layer, while their axons form the optic nerve. There are two types: a-Diffuse ganglion cells that synapse with rod bipolar cells. b-Midget ganglion cells that synapse with one midget cell. 9) The optic nerve layer It is formed of axons of the ganglion cells that pass at the right angle to form the optic nerve. 10) The inner limiting membrane It is formed of the inner processes of the Muller cells lying on the basement membrane. Rod and cone cells These are photoreceptors. Both have the same structure with some differences. Rods mediate dim light vision. There are about 120 million of rods in each human retina. The rod cell is formed of two segments, an outer and an inner segments connected by a narrow region called the connecting stalk. 1) The outer segment: Its terminal ends come in contact with the pigmented epithelium. It is formed of transversally arranged parallel membranous disks. These disks are not continuous with the surrounding cell membrane. The disks contain visual pigment (Rhodesian). The connecting stalk contains a modified cilium showing nine peripheral doublets of microtubules with no central pair of microtubules , arising from a basal body in the inner segment 2) The inner segment: It is the protein forming part.  The outer part of the inner segment contains: The basal body of the cilium, rough endoplasmic reticulum, Golgi apparatus, many ribosomes, numerous mitochondria and microtubules.  The inner part of the inner segment contains: 71  The nucleus appears small rounded and dark.  Synaptic region which is in the form of spherule that contains synaptic vesicles and mitochondria. Function of the rod cells: 1-The inner segment synthesizes the protein opsin which passes to the outer segment to be linked with retinene to form rhodopsin (visual pigments) in the disks. 2-The rods are stimulated by dim light which breaks down the rhodopsin into opsin and retinene causing hyperpolarization of their cell membrane that initiates photic impulses. 72 The cone cells: They mediate day light vision. Each human retina contains about 6 million cone cells. Each cell has the same structure of the rod cell except the following differences: 1)The outer segment of cone is conical in shape with wide base and tapering ends. 2)The membranous disks are continuous with the cell membrane. 3)The membranous disks contain iodopsin pigments which are sensitive to colour vision blue, green and red light. 4)The cone nuclei are large and lie near to the outer limiting membrane. 5)Synaptic region is called pedicle that synapse with a single bipolar nerve cell in the outer plexiform layer. Fovea centralis: It is a rounded depression in the posterior pole of the eye, which is characterized by: 1-The layers from the fourth layer inwards displaced laterally. 2-Photoreceptors are of the cones only that are thinner and close to each others. 3-It has a high visual acuity. The blood- retinal barriers: 1) The outermost barrier is formed of tight junction between the pigmented epithelium of the retina. It prevents the escape of the foreign bodies from the capillaries of the choroid to the outer part of the retina. 2) The innermost barrier is formed by the tight junction between the endothelial cells of the retinal blood capillaries. Accessory structures of the eye 1-The conjunctiva: It is a thin transparent membrane that covers the anterior part of the eye. It is reflected at the fornix of the eye to line the inner surface of the eye lid. 1)The bulbar conjunctiva: That covers the anterior part of the sclera up to the cornea. It is formed of stratified squamous non- keratinized epithelium and underlying loose connective tissue. 2)The palpebral conjunctiva 73  It covers the inner surface of the eye lid. It is formed of stratified columnar epithelium with goblet cells. Function: Lubrication and protection of the eye by the mucous secretion of the goblet cells. 2) The eye lids: The eye lid is a movable skin fold that protects the eye from any injury. It is formed by the following structures: 1)Thin skin that covers the anterior surface of the eye lid, with underlying loose connective tissue. No subcutaneous fat. 2)Skeletal muscle called orbicularis oculi muscle that closes the eye. 3)The tarsal plate which is formed of dense fibrous connective tissue containing modified sebaceous gland called Meibomian gland that opens in the free margin of the lid. 4)The palperal conjunctiva. The free margin of the eye lid contains 3-4 rows of stiff hairs called eye lashes. These lashes are provided with modified sebaceous glands (glands of Zeis) and sweat glands (glands of Moll). 3) The lacrimal gland: They are two compound tubule-alveolar glands present in the superiolateral part of the orbit. Each gland is formed of serous secretory acini that are lined with pyramidal cells, supported by myoepithelial cells. They secrete tears by small ducts into the superior fornix of the eye. 74 The tears are water secretion that moistens the cornea and the conjunctiva. Drainage of the tears: The tears are drained by two canliculi into the lacrimal sac, which is drained by a nasolacrimal tube into the nasal cavity. Both the lacrimal sac and the nasolacrimal tube are lined by two layers of columnar cells with goblet cells. 75  The Ear The ear is formed of three parts: 1) The external ear. 2) The middle ear. 3) The inner ear. I) The external ear is formed of: -The auricle (Ear pinna): elastic cartilage covered by skin. It has sebaceous and sweat glands. -The external auditory meatus: outer part is formed of elastic cartilage, inner part is formed of bone. It is lined with hairy skin rich in ceruminous glands (waxy secretion) - The tympanic membrane (Ear drum)  It covers the medial side of the meatus.  Outer layer: Continuous with the skin of the external auditory canal.  Middle fibrous layer: Contains collagen fibers.  Inner layer: Lined by simple cuboidal epithelium continuous with the mucosa of the middle ear cavity.  Function: It transmits sound vibration to the ossicles of the middle ear. 76 II) The middle ear The middle ear lies in a cavity in the temporal bone between the tympanic membrane and the bony wall of the inner ear. It is communicated Anteriorly with the nasopharynx by the Eustachian tube. Posteriorly, it is continuous with the mastoid air sinus It is lined by simple cuboidal epithelium. The middle ear contains: Three bony ossicles, malleus, incus and stapes, two muscles: tensor tympani and stapedius, one nerve: chorda tympani. They transmit the sound vibrations from the tympanic membrane to the lateral wall of the inner ear. Two skeletal muscles: 1- Tensor tympani muscle, its contraction makes the Ear drum tense. 2- Stapedius muscle, its contraction pulls the stapes out to protect the organ of Corti from high vibrations. The medial wall of the middle ear has two openings: 1-The oval window, which is closed by the footplate of stapes. It transmits the vibration to the inner ear. 2- The round window, which is closed by elastic membrane (Secondary tympanic membrane) The Eustachian tube ( Auditory tube ) It connects the middle ear with the nasopharynx. It is formed of two parts: 1- The bony posterior part, which is lined by columnar ciliated epithelium. 77  2- The anterior cartilagenous part, which is lined by respiratory epithelium, pseudostratified columnar ciliated cells with goblet cells. Function: It equalizes pressure in the middle ear with the atmospheric pressure in the nasopharynx. III) The inner ear I) The bony labyrinth: It lies in the petrous part of temporal bone. It consists of: - The cochlea. - The vestibule. - The semicircular canals. The bony labyrinth is filled with the perilymph which is similar to the CSF and contains membranous tubes (Membranous labyrinth). II) The membranous labyrinth: It is found inside the bony labyrinth, formed of - The cochlear duct in the bony cochlea. - The utricle and saccule in the vestibule. - The membranous semicircular canals in the bony semicircular canals. The membranous labyrinth is filled with the endolymph and contains sensory organs for hearing and equilibrium and lined by simple squamous epithelium. 78 1) The cochlea:  It is a bony tube which is turned spirally 2 ¾ turns around a central bony axis called the modiolus which contains spiral ganglia and cochlear nerve.  The lumen of the bony cochlea appears circular, and contains the membranous cochlear duct which is triangular in shape.  At the lateral sides of modiolus, there are 5 bony cavities  Each cavity is divided by 2 membranes into 3 compartments: 1- Scala media or the membranous cochlear duct in the middle and is filled with endolymph. 2- Scala vestibuli above the cochlear duct, communicate with vestibule and filled with the perilymph. 1- Scala tympani below the cochlear duct, communicates with tympanic cavity through round window and filled with the perilymph.  There are two membranes: 1- Vestibular membrane (consists of two layers of the simple squamous cells): roof of the cochlear duct 2- Basilar membrane (which consists of collagenous and elastic Fibers): floor of the cochlear duct  Lateral wall of cochlear duct: stria vascularis which secrete the endolymph and lined by cuboidal epithelium resting on vascular connective tissue. 79  Organ of Corti It is the sensory organ in the cochlear duct. It lies on the basilar membrane. It is formed of hair cells and supporting cells. 1) The supporting cells  The outer and inner pillar cells: These are tall columnar cells rest on the basilar membrane. Their bases are connected to each other enclosing a space called tunnel of Corti. Their cytoplasm contains microfilaments and microtubules that extend from the base of the cells to their apex.  The outer phalangeal cell: These are tall columnar cells, rest on the basilar membrane, and lateral to the tunnel of Corti. Their apical parts show a concavity to enclose the outer hair cells.  The inner phalangeal cells: They form one row of columnar cells that rest on the basilar membrane, medial to the tunnel of Corti. Their apical parts enclose the inner hair cells.  Border cells: These are a row of columnar cells that support the inner side of the organ of Corti.  Hensens cells: These are columnar cells lateral to the outer phalangeal cells.  Claudius cells: low columnar cells lateral to Hensens cells. 2- Hair cells  two types of hair cells: 1) Inner hair cells (Type I): These are flask-shaped cells which are arranged in one row surrounded by the inner phalangeal cells. Each one has a basal rounded nucleus and numerous mitochondria near its apex.Their free borders bear stereocilia (highly specialized microvilli) in linear pattern. 2) Outer hair cells ( Type II ): columnar cells arranged in three rows. surrounded by the outer phalangeal cells.Their free surface bears stereocilia arranged in W- shaped manner. Both types of hair cells receive afferent nerve fibers for hearing, which are dendrites of bipolar cells in the spiral ganglion. 3- Tectorial membrane: It is a homogenous glycoprotein membrane that forms the roof for the hair cells, in contact with the steriocilia. 80. 81  2) The vestibule:  It is part of the bony labyrinth that contains membranous labyrinth utricle and saccule filled with endolymph.  The utricle and saccule are two membranous sacs, lined by simple squamous epithelium with underlying thin connective tissue.  They contain localized sensory areas called maculae ureteculi and sacculi  Structure of the macula: It is a neuroepithelial cells, formed of: 1- Hair cells both type I and type II, their free borders have a single cilium (Kinocilium) and stereocilia, which are arranged in rows with the longest are located adjacent to the kinocilium. 2- Supporting cells, which are columnar cells found in-between the hair cells. They give the nutrition and the support for the hair cells.They also share in the secretion of the endolymph. 3- Otolithic membrane, which is formed of protein and calcium carbonate crystals called (otoliths). The cilia and the stereocilia of the hair cells are embedded in this membrane. Function: Sense of gravity and linear movement (horizontal: utricle, vertical: saccule) 82 3) Semicircular canal: 1- They are three thin semicircular canals,. 2- They contain membranous semicircular ducts that are lined by simple squamous cells. 3- Each membranous duct is provided with an expanded region called the ampulla, which contains a sensory organ called crista ampullaris. Structure of the crista ampullaris:  It is a neuroepithelial cells in the membranous duct. Each crista is formed of a central core of connective tissue covered by: 1- Hair cells both type I and type II as in maculae. 2- Supporting cells as in maculae 3- The cells are covered by thick gelatinous membrane called cupula in which the cilia and stereocilia are embedded.  Function: sense of angular (rotational) movement of the head Function of the maculae and crista ampullaris: 1- The maculae maintain the body equilibrium in relation to the linear acceleration of the head. 2- The crista controls the body equilibrium in relation to the angular acceleration of the head. 83  Deflection of the kinocilium – stereocilia complex in a direction towards the kinocilium produces depolarization of the hair cells that leads to generation of the afferent impulses to the vestibular nerve. 84