An Overview of CNS Histology and Sensory Pathways

Histology CNS  CNS 1. Compare between the grey and white matter Grey matter Formed of: - Unmyelinated nerve fibers and neuroglial cells - 2 dorsal horns containing sensory nuclei - 2 ventral horns containing motor nuclei - The grey commissure contains commissural nuclei White matter Formed of: Myelinated nerve fibers and neuroglial cells It’s divided into three columns in each half of the spinal cord: Posterior horn: contains ascending tracts only Lateral column: contains both ascending and descending fibers Anterior column: contains both ascending and descending fibers 2. Discuss in brief the pathway of conscious proprioception and fine touch from the body Proprioception Fine touch Definition The sense of position, movement and Sense of tactile localization, discrimination vibration and stereognosis Receptors - Muscle spindle - Meissner’s corpuscle - Tendon spindle - Merkel’s disc - Pacinian corpuscle - Ruffini’s corpuscle st 1 order - Large cells in spinal ganglia neurons - Receive impulses by peripheral thick myelinated nerve fibers - Send central branches that enter the spinal cord through the medial division of the posterior roots - Fibers ascend in the posterior column of the white matter as gracile tract (sacral, lumbar and lower segments of the body on the same side) also as cuneate tract (upper thoracic segments on the same side) - Gracile and cuneate tracts give descending short tracts called septomarginal and comma shaped nd 2 order - Gracile and cuneate nuclei in the medulla neurons - They also give internal arcuate fibers (cross to the opposite side at the upper half of the closed medulla to form medial lemniscus rd 3 order - V.P.L.N of the thalamus neurons - Their axons ascend in the posterior limb of the internal capsule and corona radiata to reach sensory area on post sensory gyrus of cerebral cortex area 3,1,2 3. Give a short account on the gracile and cuneate tracts Gracile tract Cuneate tract Definition Long ascending sensory tract origin Large cells in the spinal ganglia receive impulses by peripheral thick myelinated fibers and send central branches to enter the spinal cord form the medial division of the posterior roots Course - Posterior column of the white - Posterior column of the white matter matter - Cervical and upper thoracic levels - In all levels of the spinal cord Termination Gracile nucleus in the closed Cuneate nucleus in the closed medulla on medulla on the same side the same side Function Proprioception and fine touch from Proprioception and fine touch from the the lower half of the body on the upper half of the body on the same side same side 4. Name and trace out the tract carrying proprioception and fine touch from the lower half of the body.  Name of the tract: Gracile Tract  Origin: Large cells in spinal ganglia at sacral, lumbar and lower thoracic segments that have thick myelinated peripheral branches and central branches entering the spinal cord in medial divisions of posterior roots.  Course: In posterior column of white matter. In all levels of spinal cord  Termination: In gracile nucleus in closed medulla on the same side. 5. Name and trace out the bundle carrying proprioception and fine touch from the opposite side of the body  Name of the bundle: medial lemniscus  Definition: ascending sensory bundle in the brain stem  Origin: gracile and cuneate nuclei in closed medulla. The fibers cross to the opposite side in the upper half of the closed medulla  Course: ascends in the brain stem in a medial position  Termination: V.P.L.N of thalamus 6. Describe the pathway of crude touch from the body (receptors, 1 st,2nd, 3rd order neurons) Receptors: - free nerve endings - Peritracheal nerve endings - Merkel’s disc - Meissner’s corpuscle 1st order neurons Medium sized cells in spinal ganglia Receive impulses by peripheral medium sized myelinated fibers Central branches enter the spinal cord through the medial division of the posterior root The fibers ascend or descend 1 or 2 segments 2nd order neurons Main sensory nucleus (MSN) Axons cross to the opposite side Infront of the central canal to form ventral spinothalamic tract in ventral column of the white matter In pons it joins the lateral spinothalamic tract forming spinal lemniscus 3rd order neurons V.P.L.N of thalamus Their axons (sensory radiation) ascend in the posterior limb of internal capsule and corona radiata to reach area 3, 2, 1 in the CCx. 7. Describe the pathway of pain and temperature from the body (receptors, 1 st, 2nd,3rd order neurons) Receptors: - free nerve endings 1st order neurons Small sized cells in spinal ganglia Receive impulses by peripheral thin myelinated fibers Central branches enter the spinal cord through the lateral division of the posterior root The fibers ascend or descend 1 or 2 segments as Lissauer’s tract 2nd order neurons Substantia gelatinosa of Rolandi (SGR) Axons cross to the opposite side Infront of the central canal to form lateral spinothalamic tract in ventral column of the white matter In pons it joins the ventral spinothalamic tract forming spinal lemniscus 3rd order neurons V.P.L.N of thalamus Their axons (sensory radiation) ascend in the posterior limb of internal capsule and corona radiata to reach area 3, 2, 1 in the CCx. 8. Name and trace out the tract carrying crude touch from the opposite side of the body  Name: ventral spinothalamic tract  Definition: long ascending sensory tract in the spinal cord  Origin: cells of MSN of the opposite side  Course: o crossed Infront the central canal o ventral column of the spinal cord all levels o joins the lateral spinothalamic tract to form the spinal lemniscus  termination: V.P.L.N of the thalamus 9. Name and trace out the tract carrying crude pain and temperature from the opposite side of the body  Name: lateral spinothalamic tract  Definition: long ascending sensory tract in the spinal cord  Origin: cells of SGR of the opposite side  Course: o Crossed Infront and closer to the central canal o Lateral column of the spina; cord (all levels) o Joins the ventral spinothalamic tract at pons to form the spinal lemniscus  Termination: V.P.L.N of thalamus 10. Give a short account on the spinal lemniscus  Definition: ascending sensory bundle in the brain stem formed by lateral and ventral spinothalamic tracts  Origin: formed by union of ventral and lateral spinothalamic tracts  Course: starts at pons >> mid brain  Termination: V.P.L.N of thalamus  Function: carries pain, temperature and crude touch from the opposite side of the body 11. Describe the subcortical level sensory impulses pathway of the dorsal spinocerebellar tract Origin course Termination Function o Clark’s nucleus (C8-C9) o Spinal cord Enters Carries proprioceptive o On the same side (Lateral column of the cerebellum impulses from trunk and o Receives proprioceptive white matter at upper through ICP lower limb to cerebellum to impulses from large cells lumbar, thoracic, and be used in coordination of in the spinal ganglia cervical segments) individual limb muscles and o Closed medulla maintenance of posture 12. Describe the subcortical level sensory impulses pathway of the ventral spinocerebellar tract Origin course Termination Function o Cells in the posterior o Spinal cord Enters Carries proprioceptive horn of the grey matter (Lateral column of the cerebellum impulses from lower limb to in lumbosacral region white matter at all through SCP cerebellum to be used in (laminae 5-7) in lumbar, levels) coordination of individual limb sacral, and coccygeal o Brain stem muscles and maintenance of spinal; segments posture o On the same and opposite side o Receives proprioceptive impulses from large cells in the spinal ganglia 13. Compare between spino-olivary and spinotectal tracts (regarding any point from the following) Spino-olivary tract Spinotectal tract Origin Nerve cells on the posterior horn Nerve cells on the posterior horn (Laminae 4 and 5) on the opposite side (Laminae 1 and 4) on the opposite side that that receives proprioceptive impulses receives proprioceptive impulses from the from large cells in the dorsal root dorsal root ganglia ganglia Course Ascends in all levels of the spinal cord Ascends in all levels of the spinal cord and the brain stem Termination Inferior olivary nucleus in the medulla Superior colliculus in the midbrain that sends olivocerebellar fibers to cerebellum through ICP Function Carry proprioceptive impulses to It transmits sensory impulses to the tectum, cerebellum where it drives spinovisual reflexes and brings about movements of the eyes and head towards the source of stimulation 14. Enumerate the sensory structures that carry unconscious proprioception to the cerebellum o External arcuate fibers o Dorsal spinocerebellar tract o Ventral spinocerebellar tract o Cuneo-cerebellar tract o Spino-olivary tract 15. Compare between corticobulbar and corticospinal tracts regarding (any point of the following might be a Q) Corticobulbar Corticospinal Definition Fibers that arise in the CCx and terminate in the motor The fibers arise from the CCx and nuclei of cranial nerves in the brainstem terminate in the AHCS of the spinal cord Origin Course Medial Motor area 8 in the CCx The fibers descend in corona radiata and then lie in the genu of internal capsule. In the midbrain they are also present in the basis pedunculi very close to the middle line Lateral Lower 1/3 of motor area 4 IN the CCx The fibers descend in corona radiata and then lie in the genu of internal capsule. In the midbrain they are also present in the basis pedunculi lateral to the corticospinal fibers Betz cells and others in upper 2/3 of motor area 4 in the CCx o Corona radiata in the white matter o Genu and ant. 2/3 of post. Limb of internal capsule o Intermediate part of basis pedunculi in midbrain o Basis pontis as bundles separated by transverse pontine fibers and pontine nuclei o Pyramid in upper medulla o Pyramidal decussation in lower part of closed medulla (motor decussation) o In the spinal cord the crossed pyramidal tract is present in the ant. Column of the white matter in cervical and upper thoracic levels Termination Motor nuclei of CN: Motor nuclei of CN: 3,4 5,7,9,10,11,12 on both sides and 6 on both sides except: o Portion of the tongue that provide innervation for the tongue protrusion o The part of the facial nerve that innervates the muscles of the lower face o They only receive contralateral innervation from the pyramidal tract AHCs of the opposite side. Some to the same side 16. Describe the pathway of extrapyramidal tracts of the spinal cord (any of the following) Origin Type Site Rubrospinal Red nucleus Olivospinal Inferior olivary nucleus Crossed Ventral tegmental decussation Cervical and thoracic Direct Cervical Sulcomarginal Continuation of medial longitudinal bundle All levels Reticulospinal Reticular formation Tectospinal Superior colliculus in tectum Lateral: crossed Ventral; direct Crossed Dorsal tegmental decussation Lateral: cervical Ventral: cervical and thoracic All levels Vestibulospinal Lateral: lateral vestibular nucleus Ventral: lateral, medial and inf. Nuclei Direct Lateral: All levels Ventral: Cervical 17. Describe sensory pathway of crude touch from the face (1st, 2nd, 3rd) order neurons 1st order neuron Trigeminal ganglion 2nd order neuron Cells in the main sensory nucleus. Their axons cross and enter trigeminal lemniscus 3rd order neuron Cells of VPMN of thalamus. Their axons end in the sensory area in the CCx 18. Describe the sensory pathway of proprioception and fine touch from the face (1st, 2nd, and 3rd order neurons) 1st order neuron Mesencephalic nucleus (inside the CNS) 2nd order neuron The axons of the mesencephalic nucleus descend to synapse with the cells in the main sensory nucleus. Their axons cross and enter the trigeminal lemniscus 3rd order neuron Cells of VPMN of thalamus. Their axons end in the sensory area in the CCx 19. Describe the sensory pathway of pain and temperature from the face (1st, 2nd, and 3rd order neurons) 1st order neuron Trigeminal ganglion 2nd order neuron Cells in the spinal nucleus. Their axons cross and enter the trigeminal lemniscus 3rd order neuron Cells of VPMN of thalamus. Their axons end in the sensory area in the CCx 20. Give a short account on blood brain barrier regarding def., structure and function o Definition: the barrier between blood and nerve cells o Structure: it’s formed of: - tight junction between non fenestrated endothelium lining capillaries - thick continuous basement membrane (basal lamina) - end feet astrocytes firmly applied on capillaries o Function: - Prevents harmful materials from reaching the nerve cells - Allows nutrients and precursors of neurotransmitters to pass by facilitated diffusion/ active transport 21. Give a short account on the blood CSF barrier regarding def., structure and function o Definition: the barrier that separates blood from CSF o Structure: it’s formed of: - Endothelium of choroidal capillary - Basement membrane of choroidal capillary - Basement membrane of ependymal cells forming the choroid plexus - Tight junctions between ependymal cells forming the choroid plexus o Function: - Protection - Selection of substances to pass from blood to CSF - Maintenance and constancy of CSF 22. Give a short account on the blood CSF barrier regarding def., structure and function o Definition: the barrier between extracerebral capillaries and subarachnoid space o Structure: it’s formed of: - Arachnoid barrier layer formed by cells joined by tight junctions and desmosomes o Function: - Block substances leaving extracerebral capillaries from reaching subarachnoid space, brain tissue and ventricles.  EYE 23. Compare between the 2 membranous layers of the cornea regarding their histological structure Bowman’s membrane - the straight basement membrane of the stratified epithelium - non-cellular - formed by collagen fibrils - very thick to provide stability and strength - protects against infection Descemet’s membrane - the basement membrane of the corneal epithelium - non-cellular - formed of fine interwoven collagen fibers 24. describe the structure of the nonkeratinized stratified squamous epithelium of the cornea and mention the function of each component Basal layer - one layer - cubical or columnar cells resting on straight basement membrane - numerous mitotic figures Numerous mitotic figures indicate high regenerative power as long as the basement membrane is intact Intermediate layer - 3-4 layers - Polyhedral cells - Numerous free unmyelinated nerve endings Top layer - squamous cells - apical microvilli Protection by the free unmyelinated nerve endings Apical microvilli retain a protective tear film 25. Describe the histological structure of substantia propria and mention its function o it forms the main thickness of the cornea o consists of about 60 layers of parallel collagen bundles with fibroblast -like cells called keratocytes in between o the collagen fibrils and the keratocytes are embedded in a ground substance called lumican (proteoglycans that maintain organization 26. Enumerate the factors that are responsible for the corneal transparency o o o o o o Non-keratinized surface epithelium Unmyelinated free nerve endings Regular arrangement of the lamellae of the substantia propria Same refractive index (of the cells, lamellae, and matrix) Avascularity of the cornea Minimal hydration of the cornea 27. Enumerate the histological changes occurring at the corneoscleral junction Non-keratinized stratified squamous epithelium Bowman’s space Collagen bundles Canal encircling the limbus Descemet’s membrane and endothelium Microvasculature >>> Stratified columnar with goblet cells of the bulbar conjunctiva >>> Subconjunctival C.T >>> Irregular >>> Canal of Schlemm >>> Trabecular meshwork >>> For metabolite distribution to the cornea exits at the limbus 28. Give a short account on the refractive media of the eye o These are the transparent media through which the light passes until it reaches the retina o They include - Cornea - Aqueous humor - Vitreous body 29. Describe the histological structure of the eye lens and mention 2 factors responsible for the lens transparency  Structure: Lens capsule Subcapsular epithelium Lens fibers -  Thick, homogenous Rich in proteoglycan and type 4 collagen Covers the whole lens Supports the epithelium ant. One layer of cuboidal epithelium Present only in the ant. Surface of the lens Cells near the equator divide allowing growth of the lens (stem cells) Elongated flattened cells Developed from stem cells Lost their organelles and become filled with group of proteins called crystallin’s Highly specialized for light refraction Factors responsible for transparency - Avascularity of the lens - Lens has no nerve supply 30. Mention the functions of the iris and the ciliary body - - Iris Pupillary muscles control the size of the pupil and the amount of light entering the eye Pigmented epithelium blocks the passage of light except to the pupil Pigment cells of the stroma are responsible for the color of the eye - Ciliary body Formation of aqueous humor by nonpigmented epithelium Accommodation for near vision by ciliary muscles 31. Give a short account on the structure of the choroid - - o Suprachoroidal lamina (SCL): formed of CT rich in elastic fibers, pigment cells, fibroblasts and macrophages o Choriocapillary lamina (CCL): formed of fenestrated capillaries to nourish the outer part of the retina o Bruch’s membrane: Acellular formed of 5 layers 1. Outermost > basement membrane of the choriocapillaris 2. Innermost > basement membrane of the retinal pigmented epithelium 3. Middle 3 layers > 2 collagen layers 1 elastic fibrous layer in between It acts as retinal barrier as it limits the entrance of macro-molecules from choriocapillaris to the outer part of the retina 32. Compare between the outer and inner segments of the Photoreceptors. Rods Outer segment - Rod shaped. Contains flattened discs containing visual pigment = rhodopsin. These discs are continuously shed-off and phagocytosed by the pigmented cells. - Inner segment - - Cons - Cone-shaped. Contains flattened discs containing the visual pigment = iodopsin. - They are infrequently shed-off and phagocytosed by the pigmented cells. Connected by a short cilium to the outer segment. Site of protein formation so it is rich in rER, Golgi apparatus & mitochondria. 33. Give reason: fovea centralis is the area of the best visual acuity Due to: - Cones are the only photoreceptors within it - Layers of the retina are markedly reduced in this area - It contains no blood vessels so light falls directly on cones 34. Give a short account on the structure of the conjunctiva and mention its function - It’s a transparent mucus membrane that covers the anterior surface of sclera and continues to line the inner surface of the eyelid - It consists of stratified columnar epithelium that contains goblet cells and rest on a thin lamina propria of loose C.T. at the corneoscleral junction it is continuous with the stratified squamous epithelium covering the cornea Function: goblet cell secretion forms a part of the tear film that protects the epithelium of the eye 35. Name the major layers that form the tear film 1. Superficial thin lipid layer: produced by meibomian glands Function: reduces tear evaporation and act as lubricant 2. Middle thick aqueous layer: produced by lacrimal glands 3. Innermost hydrophilic mucin layer: produced by conjunctiva goblet cells Function: allows the aqueous layer to spread over the corneal epithelium  EAR 36. describe the histological structure of ear drum layers, and name the defect results from its rupture it consists of 3 layers: 1. outer epidermal layer: thin layer of keratinized stratified squamous epithelium continuous with that of the external auditory meatus 2. middle fibrous layer: dense C.T with fibroblasts, elastic fibers, radial collagen fibers on the outer surface and circular collagen fibers on the inner surface 3. inner mucosal layer: simple cuboidal epithelium continuous with the lining of the middle ear in case of rupture: hearing loss 37. name the 2 windows of the middle ear and mention their function - the oval window: closed by footplate of stapes function: transmission of ear drum vibrations to perilymph of the inner ear - the round window: closed by elastic membrane function: dissipates sound waves after they stimulate the receptors of the inner ear 38. describe the auditory tube regarding site, lining epithelium and function - it connects the middle ear cavity with the nasopharynx - its usually collapsed, but opens during swallowing and yawning to equalize the pressure on both sides of the drum - it’s lined with pseudostratified columnar ciliated epithelium only 1/5th of which contains goblet cells - function: it amplifies mechanical vibrations generated in the external ear and conducts them to the inner ear 39. describe the histological structure of the inner ear Bony labyrinth Membranous labyrinth - it is a system of canals and - it is a system of tubules and sacs cavities within the petrous part inside the bony labyrinth of temporal bone - it includes: cochlear duct, utricle - it includes: cochlea, vestibule and saccule and membranous and 3 semicircular canals semicircular canal it’s filled with perilymph - it is filled with endolymph