Special Senses HISTO PDF

**[Special senses ]** Our rich environment is perceived via receptors that generate nerve impulses in response to their specific stimuli. These afferents are then relayed via the respective sensory nerve to the brain. The section on the special senses include our sense vision, hearing, balance, tas...

**[Special senses ]** Our rich environment is perceived via receptors that generate nerve impulses in response to their specific stimuli. These afferents are then relayed via the respective sensory nerve to the brain. The section on the special senses include our sense vision, hearing, balance, taste, and olfaction (smell). This practical looks at the histological features of these receptors and will hopefully highlight how they are able to carry out their specialized function. **1. The Eye** The eye is a specialized structure that is receptive to light and thus gives rise to our sense of vision.  It is a slightly elongated hollow sphere that has a three-layer wall.  The three layers/ tunics of the eye are: a\) the outermost **fibrous tunic** that comprises the sclera and cornea b\) the middle **vascular tunic** that forms the iris, ciliary body, and choroid c\) the innermost **nervous tunic**, which is also known as the retina https://uta.pressbooks.pub/app/uploads/sites/36/2019/02/MCP27\_20x-50x-labeledcrop-300x159.png Fig 1. The layers of the eyeball The crystalline lens with variable focal length is suspended and held in place by a suspensory ligament called the **zonular fibres** and this combined structure divides the interior of the eye into two regions/cavities viz the **anterior** and **posterior cavities**.  The anterior cavity is further divided into anterior and posterior chambers by the iris. This creates three distinct fluid filled compartments within the eye that are communicating. We need to briefly mention of these: 1\) the **anterior chamber** extends from the posterior surface of the cornea to the iris 2\) the** posterior chamber** extends from the iris to the front/anterior surface of the lens 3\) the **vitreous chamber** that extends from the posterior surface of the lens to the retina is the largest component of the eye. Note that the anterior and posterior chambers are filled with the **aqueous humor which is** a watery fluid that has a similar consistency to plasma **with a volume of approximately 0**,3 ml. It plays a significant role in maintaining the form of the cornea as well as regulating the intraocular pressure. The vitreous chamber on the other hand is filled with the **vitreous body**, a translucent, gel-like substance that is also called **vitreous humor**. Many age related defects in vision can be attributed to molecular changes of this gel particularly in its hyaluronic acid and collagen components.  Fig 2. Cross section of the eyeball and optic nerve Let us look at details of each of the 3 layers of the eye: a\) The fibrous tunic The anterior part of this layer is translucent and forms the **cornea** and the remainder forms the opaque **sclera** which is white. Interestingly, these two areas despite being transparent and opaque have very similar compositions but differ largely in the arrangement of the collagen fibres. The transparent cornea refracts light and therefore focuses incoming light onto the retina. It also serves to protect the eye.  The microscopic layers of the cornea that can be identified during light microscopy with H&E staining are: stratified squamous epithelium (outermost, non-keratinized), Bowman's membrane, Stromal layer, Descemet's membrane and simple squamous epithelium Fig 3. Cross section of cornea On its external, convex surface, the cornea is covered by stratified squamous epithelium.  The innermost layer of the epithelium is columnar and lies on the **Bowman's membrane**.  In histological slides stained with H/E, the Bowman's membrane is a solid pink layer directly under the epithelium.  The **stroma** which forms the bulk of the cornea is made up of many parallel arrays/layers of collagen fibres that are produced by resident fibroblasts. They characteristically stain light pink with H&E and there is often delamination upon slide preparation. Interspersed in this layer are dark staining purple nuclei of the fibroblasts which are the cells that are for responsible the synthesis of the collagen.  The internal, concave surface of the cornea is covered by simple squamous epithelium resting on **Descemet's membrane**. Careful observation under light microscopy does show the clear absence of blood vessels this layer does contain nerve fibres and many of them have their origins as pain receptors. The entire cornea is also loosely covered by a membrane, the **conjunctiva** which also extends to the internal surface of the eyelids.  The conjunctiva lubricates the eye and keeps it moist hence preventing it from drying/dehydration.  The space between the conjunctiva and cornea is filled with tears, which are produced by the **lacrimal apparatus**.  Fig 4. Conjunctiva and cornea The sclera is the white part of the fibrous tunic and covers the majority of the eyeball.  The point at which the cornea and the sclera join/ meet is called the** limbus**.  The sclera is made of connective tissue composed of bundles of collagen and elastic fibres.  These fibres are less organized than in the cornea which results in the opacity of the sclera.  The major cell type that is visible in the scleral stroma under light microscopy are fibroblasts. In contrast to the cornea, the sclera has blood vessels as well as sensory nerve fibres.  The visible part of the sclera, in the front of the eye, is covered by the conjunctiva.  Posteriorly, the sclera is penetrated by the optic nerve exiting the eyeball. b\) The vascular tunic (**uvea)** This layer of the eye is situated between the outer fibrous tunic and the inner nervous tunic.  The vascular tunic is comprised of three parts viz the iris, the ciliary body and the choroid The **iris** which gives our eyes their colour is part of the anterior portion of the vascular tunic.  The iris also divides the anterior cavity of the eye into the anterior and posterior chambers. The size or diameter of central pupil/aperture of the iris can be altered by the muscles that are located in its stroma. This effectively varies the amount of light that enters the eye which projects onto the retina. The **ciliary body** is the part of the vascular tunic that is adjacent to the lens.  It is basically made up of a of a ring of **ciliary processes** which are finger-like projections of vascular tissue covered by the **ciliary epithelium. This structure** secretes the aqueous humor which fills the anterior and posterior chambers of the eye.  The ciliary body also has a layer of muscle that controls the shape of the lens.  The **choroid** is the largest part of the vascular tunic, extending from the ciliary body to the optic disc where the optic nerve leaves the eyeball.  The choroid is made of capillaries and is responsible for the nutrition of the outer parts of the retina.  As such, it is tightly attached to the underlying retinal pigment epithelium, but loosely attached to the overlying sclera. detail of the choroid and the optic nerve c\) The nervous tunic The **retina** is essentially neuronal tissue that contains the rods and cones which are the photoreceptors that respond when light in the visible range enters the eye.  In response to the presence of light the photoreceptors of the retina, produce nerve impulses that project to the visual cortex of the brain where it is interpreted. The cells in the retina are organized into five layers from the outer choroid towards the vitreous body as follows: rods and cones → horizontal cells → bipolar cells → amacrine cells → ganglion cells Note that once the light enters the eye, it has to travel through the ganglion cells, amacrine cells, bipolar cells and horizontal cells to strike the rods and cones The retinal circuit is complex and only three of the layers listed above are in the direct pathway of the signal transduction.  They are the rods and cones, **bipolar cells** and the ganglion cells.  The **horizontal cells** synapse with the rods and cones, while **amacrine cells** synapse with ganglion cells and are necessary for visual processing of the signal eg increasing contrast, etc. The axons of all the **ganglion cells** of the retina converge to the **optic disk** and project as the **optic nerve** to the brain When studying the histological features of the retina using light microscopy using standard H&E staining, as many as ten-layers can be identified by morphological characteristics such as the presence or absence of pink-stained fibres or of dark staining (purple) nuclei. The following layers are often identified in the retina under the light microscope (from the chorion to the vitreous body): The basic retinal structure. Histological appearance of \... Fig 5. Layers of the retina Adjacent to the choroid, the outermost layer of the retina is the **retinal pigment epithelium**, a cuboidal epithelium that contains melanin and provides nourishment to the retina.  Projections of these cells, especially rich in melanin granules, extend between the outer segments of rods. **Rods** and **cones** are the photoreceptive cells of our eyes and are located in the deepest layer of the retina.  Both rods and cones have two segments viz the outer segment, where the photoreceptors are located, and the inner segment, which has the cell body with its short axon.  Rod cells are highly sensitive to light, but provide minimal detail and contribute little to colour vision.  Rods are found in large numbers in the outer areas of the retina. They are therefore responsible for peripheral vision and night vision.  The cones in contract function best in bright light and provide detailed colour vision.  Cones are found primarily in the **fovea**, which is also a rod free area and provides acute vision for activities where detail is essential.  The outer segments of rods and cones form the **photoreceptor outer segments layer**.  The inner segments of the rods and cones, containing the cell bodies (and thus the nuclei), form the** outer nuclear layer**. The **outer plexiform layer** is formed by the axons of rods and cones synapsing with the dendrites of the bipolar cells, and by horizontal cells that interact with the surrounding photoreceptive and bipolar cells The** inner nuclear layer** consists of nuclei of bipolar, horizontal, and amacrine cells. In the **inner plexiform layer**, axons of bipolar cells synapse with dendrites of ganglion cells, and with amacrine cells that modulate the surrounding ganglion cells. The **ganglion cell layer** contains the cell bodies of ganglion cells and their surrounding neuroglia and axons of ganglion cells extend toward the optic disk to form the optic nerve. **2) The Ear** This organ is specialized for both hearing and balance.  The ear has three parts/regions viz, the external ear, the middle ear, and the inner ear. The **external** **ear** is made of the elastic **pinna** and the **external auditory canal**, which extends to the **tympanic membrane**/ **eardrum**.  The pinna directs sound waves into the auditory canal towards the ear drum and is essentially hyaline cartilage covered with fat and skin. You have studied this type of cartilage when you looked at the primary tissues types. The **middle** **ear** is located within a cavity in the temporal bone and extends from the tympanic membrane to the **oval window** in the bony labyrinth.  The middle ear has three auditory ossicles, the malleus, incus, and stapes.  These bones are arranged in such a manner that they amplify incoming sound waves and transmit them to the inner ear. The **inner** **ear** is located within the temporal bone and consists of interconnected canals that is filled with a **endolymph**.  The **cochlea** on the other hand is a spiral canal/tube that winds itself around a central bony column called the **modiolus**.  This system is partitioned by two membranes, Reissner's membrane and the basilar membrane into three separate chambers, the **scala vestibuli**, the **scala tympani**, and the **scala media**/**cochlear** **duct**.  The scala vestibuli and scala tympani are continuous via the oval and round windows. The organ of Corti which is responsible for converting sound waves into nerve impulses is located within the cochlear duct and sits on the basilar membrane. Sound waves that travel through the endolymph vibrate the **hair cells (mechanoreceptors)** of the organ of Corti. The hair cells are tonotopically arranged ie cells at the basal end respond best to higher frequencies and cells at the apical end respond best to low frequencies. These specialized receptor cells translate their movement by generating nerve impulses that relayed via the **vestibulocochlear nerve** to the brain, where they the sound is interpreted.  Fig 6. Schematic diagram showing cross section of cochlear +-----------------------------------+-----------------------------------+ | corti8.jpg (55270 bytes) | | | | | | Fig 7. Cross section through | | | organ of Corti. (H&E) | | +-----------------------------------+-----------------------------------+ This typical section showing components of organ of Corti including the auditory nerve towards the middle on left. The **vestibular system**, which provides our sense of balance is also located bilaterally in canals of bony labyrinth of inner ear.  This system is comprised of three fluid filled **semicircular canals that are strategically orientated in the 3 planes of space** so that they optimally indicate/report changes in body orientation. Basically, as the head/body rotates in various directions, the resultant movement of the fluid within the canals pushes on the **cupula **within each loop.  The cupula is a specialized structure that contains hair cells which transduce our rotational movement into neural signals. Located in the vestibule (point where the semi-circular canals communicate) are two otolith organs, the **utricle** and the **saccule**.  Both these organs contain a **macula**, which is sensitive to linear acceleration. They are 3 three layered structures and the bottom layer has the sensory hair cells, each with 40-70 stereocilia surrounding a large **kinocilium**.  The ends of these cilia are embedded in the **otolithic membrane**, which is weighed down by heavy calcium carbonate crystals, the **otoliths**.  When the head is upright, the otolithic membrane pushes perpendicularly down on the hair cells thus stimulating the hair cells minimally. This signals the brain that the head is stabilized.  However, when the head is tipped, gravity influences the otoliths embedded within the otolithic membrane, thus bending the cilia and stimulating the hair cells.  Visual cues are thereafter combined/ compared to the input from the otolith organs and the brain distinguishes whether the head alone or the entire body is being tilted. **3) Taste** The surface of tongue has a large number of prominent rough areas/ bumps called **lingual papillae**.  They serve to increase the surface area of the tongue as well as to increase the friction between the tongue and food, allowing for a better taste sensation.  There are four different types of lingual papillae viz **folate**, **circumvallate**, **fungiform**, and **filiform**. Taste buds are found all except the filiform papillae. They are located primarily in deep crevices called **taste** **pores**, which surround lingual papillae. Taste buds are also be found on the soft palate, the pharynx, and on the epiglottis.  In the slide below, taste buds are indicated by arrows.  At a magnification of x400, details of taste buds are clear they appear as finger-like projections within the epithelial layer.  Located in the taste buds are the receptor cells which respond to the chemical properties of food particles that are in dissolved in saliva. When stimulated, they depolarize leading to formation of nerve impulses that are relayed to the brain which in turn interprets the sensation of taste. There are five basic taste sensations: bitter, salty, sour, sweet, and the more recently added umami ("savoury," and is due to the presence of glutamates and nucleotides in food).  The various taste sensations do not arise from significant physiological differences in taste buds, and specific tastes are not localized to particular areas of the tongue.  Fig 8. Cross section of a lingual papillae showing taste buds **4) Olfaction** Olfaction, our sense of smell, occurs when odorants bind to olfactory receptors located in the nasal cavity.  There are many types of odour receptors and they are from the family of G-protein coupled receptors. They characteristically respond to a number of similarly structured odorants rather than a single type of odorant.  This diversity allows us to distinguish a large number of different smells including those that have not been previously encountered. After transduction at these receptors, nerve impulses are relayed to the olfactory bulb in the brain, via the **olfactory nerve** where they are interpreted as smell. Our sense of smell, in addition to having strong links to our memory and emotion also pairs with our sense of taste to form the sense of flavour in foods. The olfactory epithelium is kept moist by secretions from the Bowmans glands. Odorants also dissolve in this mucus based secretion to ensure efficient binding with the receptors. The image below shows a typical section of the olfactory epithelium with associated cells stained in H&E \*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\*\* This diagram gives us an idea of the transduction process in hair cells of inner ear.  Hair cell transduction: 1\) Sound wave deflects hair cell is deflected against tectorial membrane and 'tip-links' open K Channels, K^+^ enters the cell leading to depolarization (∆ nerve!) 2\) Depolarization: Ca^2+^ enters the cell through voltage-dependant channels (inward current) 3\) Higher Ca concentration: K extrusion toward basolateral portion and Glutamate release in synaptic cleft 4\) Action potential propagated along acoustic nerve (VIII) **\*\*\*\* End\*\*\*\***