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Table of Contents
Table of Contents
5.1 Introduction
5.1 Introduction
The neuron is the basic constituent of all nervous tissue. The neuron is unique in that it possesses the specialized properties of irritability and conductibility. For the most part, these properties reside in the neuron's plasma membrane, since that is where the electrical excitation is initiated and conduction of the resulting impulse occurs.
Neurons come in a variety of forms: (1) sensory, which react to energy in the environment and generate nerve impulses, (2) motor, which send signals to muscles telling them to move, and (3) association, which interconnect other neurons and allow logic and arithmetic functions to occur in the nervous system.
Neurons are separated by a specialized intercellular synaptic gap over which impulses are transmitted from one neuron to the next. The transmission of these impulses occurs by the release of specific chemicals (called neurotransmitters). An electrical impulse propagated to the end of a neuronal process triggers the release of the chemicals into the synaptic gap by exocytosis. The chemical neurotransmitter then diffuses across the intercellular space and, upon reaching the membrane of the adjacent neuronal process, elicits or inhibits an electrical potential.
The data processing-logic and arithmetic computation-occurs primarily at these synapses and involves the production of generator potentials. If the generator potentials are large enough, the neurons fire all-or-none electric signals, called nerve impulses. The nerve impulses, then, are coded transmission signals sent to other places in the nervous system where they can be decoded. In this program you will observe different forms of neurons, their most important parts, and concentrate on a special kind of neuron-the sensory one.
Sensory neurons put you in touch with the world. They "transduce" environmental energy into coded nerve impulses and tell your nervous system what is going on-both out in the world and in your own body. The heart of the sensory neuron is the transducer which is called a receptor. In this program you will observe touch receptors, pressure receptors, light receptors, movement detectors, and a variety of other important transducers. Study them carefully, for their aggregated input is the basis for your daily experience of consciousness-the knowledge of who you are, who others are, and what is going on in your environment.
5.2 Neurons
5.2 Neurons
There are three principal components to a neuron: (1) a cell body, or perikaryon; (2) dendrites; and (3) an axon. Dendrites function to receive a stimulus and conduct the impulse to the cell body. Note in image 5.2a that the arrow points to a dendrite which connects to the large bulbous cell body. This neuron is a Purkinje cell which is found in the cerebellum of your brain. It has numerous dendrites (allowing for as many as 200,000 synaptic inputs). The dendrites summarize incoming information and then the neuron computes an output signal (which leaves via a large axon) that helps control your movements. For example, some of these Purkinje cells compute your "approach distance" to an object and would stop your hand movement when you reached the appropriate distance to the object. The cell body, or perikaryon, contains the nucleus, specialized organelles and microtubules (two blue cell bodies and their attached fibers- the dendrites and axons are pictured above). The axons and dendrites in the picture are indistinguishable by anatomy alone-you must know if the nerve impulses are coming or going to the cell body to distinguish these two types of fibers.
Image 5.2b show the pyramidal cells of the hippocampus, an ancient part of the cerebrum located in the floor of the lateral ventricle. They get their name from their triangular shape. These hippocampal neurons are believed to be involved in processing short-term memories and sending signals to long-term memory banks and they are also involved in processing spatial information.
The arrow in image 5.3c indicates another dendrite, with the axon being the single, large, unbranched process extending upward from the cell body. This axon is considered the major output fiber of the neuron and it can go on to ultimately branch extensively, innervating many other neurons. The swollen portion of the axon, located immediately next to the cell body, is the axon hillock. The axon hillock is the site which generates the all-or-none nerve impulses.
Image 5.2a: Dendrite (arrow) of a neuron.
Image 5.2b: Pyramidal cells of the hippocampus.
Image 5.2c: Dendrite (arrow) with axon extending to cell body.
5.3 Neurolemma
5.3 Neurolemma
Pictured in image 5.3a are six parallel axons wrapped in a special membrane called the neurolemma. The neurolemma is a "jelly-roll" structure of Schwann cells with a myelin sheath sandwiched in between the rolls. The Swann cells butt against one another producing indentations so that the axons are interrupted at regular intervals, these points of interruption are called the nodes of Ranvier (seen as dark lines cutting across the axons transversely). The myelin sheath, which is a white fatty insulator, breaks at these nodes of Ranvier, and it is believed that the nodes allow for the fast transmission of nerve signals along the nerve fiber.
In image 5.3b, the arrow indicates the nucleus of a Schwann cell. The Schwann cell wraps itself around the axon many times, the innermost wrappings form the myelin sheath, while the Schwann cells form the outer neurolemma. Schwann cells are restricted to the PNS (peripheral nervous system) while special glia cells called oligodendrocytes form the myelin sheaths on neurons in the CNS (central nervous system).
Image 5.3c is a cross-section of a peripheral nerve, showing the cut ends of many neurons. The arrows in image 5.3c indicate the outer, darker Schwann cells.
Image 5.3d depicts two axons of a teased nerve preparation. The arrow in image 5.3d indicates a node of Ranvier. It is here that the myelin sheath breaks and the Schwann cells contact one another.
Image 5.3a: Axons wrapped in neurolemma.
Image 5.3b: Nucleus of a Schwann cell (arrow).
Image 5.3c: Cross-section of peripheral nerve with Schwann cells (arrows).
Image 5.3d: Axons with node of Ranvier (arrow).
5.4 Glial Cells
5.4 Glial Cells
The colored bodies in image 5.4a are drawings of three types of glial cells (called neuroglia), supportive cells which are distributed abundantly among neurons. Glia cells outnumber neurons in the brain 10 to 1 and the glia constitute 50% of all the brain's volume. Ignored for a long time, they are now are know to be vitally important to neuron survival and stability. The larger glial cell at the upper right of image 5.4a is an astrocyte, the most numerous of the glial cells. The two small purple glial cells in image 5.4a are oligodendrocytes. They have few fiber processes, and seem to resemble small spiders. Some of these processes wrap around axons within the CNS, forming a myelin sheath as Schwann cells do in the peripheral nervous system. The highly branched glial cell in image 5.4a on the extreme left is a microglial cell. They are phagocytic, removing dead tissue and foreign matter from the CNS.
Image 5.4b is an actual picture of an astrocyte with its cell body (labeled "a" in image 5.4b), end-foot process (labeled "b" in image 5.4b), and a capillary (arrow in image 5.4b). Astrocytes provide structural support for the neurons, they transport substances between capillaries and neurons, providing an important protective function for neurons. Neurons, because they are so delicate, cannot be exposed directly to many of the chemicals found in the blood (such as ammonia) and the astrocytes provide a barrier to these substances. Astrocytes also help regulate ion concentrations around nerve fibers, for example, by removing excess potassium, and they also help regenerate neurotransmitters, such as glutamate, so that synaptic trans-mission can occur.
Image 5.4c is an oligodendrocyte in the white matter of the brain. The arrow in image 5.4c indicates the cell body.
Image 5.4d is a microglial cell in the brain. Recent research has discovered that the AIDS virus can infect these cells and thereby move through the CNS. This complicates the treatment of this lethal disease, since the virus, which debilitates human lymphocytic T cells, can also hide and spread in the nervous system.
Image 5.4a: Three types of glial cells- astrocyte, microglia, and oligodendrocyte.
Image 5.4b: Astrocyte with its cell body (labeled "a"), end-foot process (labeled "b"), and a capillary (arrow).
Image 5.4c: Oligodendrocyte in the brain and its cell body (arrow).
Image 5.4d: Microglial cell in the brain.
5.5 General Senses
5.5 General Senses
General senses include pain, touch, pressure, heat, and cold, among others. Within the skin, there are different types of receptors for different sensations. Free nerve endings are unmodified bare dendrites important as pain receptors, but there are also found sensors for touch, pressure, and temperature. The "deep touch" or pressure receptors are called corpuscles which are surrounded by connective tissue capsules. Pictured in the diagram of image 5.5a and the light micrograph of image 5.5b is a Pacinian corpuscle, a pressure receptor located deep in the skin. If you press the palm of your hand hard with your thumb, you will feel a deep inner pressure sensation that is primarily due to the firing of Pacinian corpuscles. These miniature "little onions" also line blood vessels in your body, reporting when they swell up with blood and they are also distributed throughout your internal organs reporting stretch and distortion of these structures.
In image 5.5c, the arrow points to Meissner's corpuscle, a light pressure receptor located close to the surface of the skin. These "light touch" receptors are located in the dermal papillae, little finger-like projections of dermis which fold into the epidermis of the skin. The corpuscles are so close to the surface of the skin that very light touch can trigger these receptors. Rub your index finger very lightly over the palm of your hand and feel the light touch (almost a tickling) sensation that comes from the Meissner's corpuscles. Anatomically, they are simply a twirling of free nerve endings which respond to non-damaging deformation of the skin.
In Image 5.5d the dark central body is a Golgi tendon organ, also called a neurotendinous organ, composed of highly branched dendrites within a tendon. Strong muscle contraction activates the Golgi tendon organ by stretching the tendon and causing the tendon organ to fire impulses to the spinal cord, thereby inhibiting further contractions and preventing damage to the tendon itself. Tendon organs also give your brain continuously updated information on the position and movement in your joints. Along with muscle spindles and "joint receptors", they provide the knowledge needed to compute coordination for your movements.
Image 5.5a: Pacinian corpuscle illustration.
Image 5.5b: Light micrograph of a pacinian corpuscle.
Image 5.5c: Meissner's corpuscle (arrow).
Image 5.5d: Golgi tendon organ (dark body in the center).
5.6 Olfactory Epithelium
5.6 Olfactory Epithelium
The olfactory mucosa consists of a pseudostratified columnar epithelium. Image 5.6a is a side-view of olfactory epithelium. This view shows the multilayered appearance of this epithelium and the cilia which sense the smell molecules, but this light-microscope view does not show the functional detail seen on a surface view with the scanning electron microscope (image 5.6b). In image 5.6b, you can see the dendrites of the olfactory cells extending out of the epithelial surface, where they expand into olfactory vesicles (labeled "OV" in image 5.6b) which radiate numerous snake-like cilia in all directions. Some cilia are very long and thin and are called "modified cilia" (labeled "MC" in image 5.6b). Tall, columnar sustentacular cells are closely packed around the olfactory neurons and possess apical microvilli (labeled "Mv" in image 5.6b). An odorous substance must dissolve in the mucus layer covering the receptor cells in order to stimulate olfactory receptor cells. There is evidence that the olfactory cells can move up and down in the epithelium becuase when they withdraw downwards, they leave an opening (labeled "Op" in image 5.6b) and can extend out into the nasal cavity to sample chemicals passing by.
Image 5.6c is a close-up view of the olfactory epithelium, clearly showing the cilia at the free ends of the columnar cells.
Image 5.6a: Side-view of olfactory epithelium.
Image 5.6b: Electron microscope scan of olfactory epithelium with olfactory vessicles (OV), modified cilia (MC), microvilli (Mv), and openings from the downward shift of the olfactory cells (Op) identified.
Image 5.6c: Close-up view of olfactory epithelium.
5.7 Taste Buds
5.7 Taste Buds
Most taste buds are located on the surface and the papillae of the tongue, but some are found in the oral cavity and pharynx. Pictured in image 5.7a is a typical taste bud. Note the resemblance to a "rose bud" with the petal-like neuroepithelial cells (labeled "NE" in image 5.7a) arranged circularly within the bud. Image 5.7b shows a low-powered section of a papilla of the tongue, showing several taste buds (one of is indicated by the arrow in image 5.7b) aligned in a row.
Image 5.7c is a high-powered close-up of a taste bud. The arrow points to a taste pore which allows fluids in the mouth to contact the receptor cells within the taste bud. Sites on the receptor cell membrane form loose combinations with molecules in food and drink, depolarizing the receptor cell and generating an impulse to the brain that is interpreted as sweet, salty, bitter, sour, or some other combination thereof. The molecules must be dissolved in water to gain access to the receptor membranes. A perfectly dry tongue will not taste.
Image 5.7a: Taste bud with neuroepithelial cells (NE)
Image 5.7b: Section of tongue papillae with taste buds (arrow).
Image 5.7c: Close up of taste bud with taste pore (arrow).
5.8 Eye
5.8 Eye
Pictured in image 5.8a is a labeled cross-section illustration of the eye whereas image 5.8b is a vertically oriented eye specimen. A majority of parts labeled in image 5.8a can be matched with those in image 5.8b but not all structures, since image 5.8b's view of the eye is not a true median section.
Image 5.8c is a low-powered view of a sectioned whole eye. In this image, a sectioned optic nerve (labeled "a" in image 5.8c) and the retina (labeled "b" in image 5.8c)-the photosensitive layer that is directly responsible for light reception, can be identified. The lens (labeled "c" in image 5.8c) shape is altered by ciliary muscle for focusing; the iris (labeled "d" in image 5.8c) is a circular curtain of tissue that regulates the amount of light entering the eye; and the cornea (labeled "e" in image 5.8c) admits light and contributes to light refraction (the bending of incoming light rays).
Image 5.8d shows a rare genetic condition called Cyclopsis. Some scientists believe that cyclopsis represents a genetic deformity that is associated with mis- regulating "homeobox" genes, very primitive blueprints for positioning body parts in their proper place and for determining their number. In this individual only a single eye is centrally located (which represents the mythical cyclops from which this disease gets its name). Above the eye is another strange structure-an ancient "proboscus" nose.
Image 5.8a: Cross-section illustration of the eye.
Image 5.8b: Eye specimen.
Image 5.8c: Eye with optic nerve (labeled "a"), retina (labeled "b"), lens (labeled "c"), iris (labeled "d"), and cornea (labeled "d") identified.
Image 5.8d: Cyclopsis.
5.9 Choroid/Sclera
5.9 Choroid/Sclera
The picture in image 5.9a is a scanning electron micrograph of the choroid and the sclera layers of the eye. The sclera is seen at the bottom as a thick layer of closely packed collagenous fibers (labeled "CF" in image 5.9a) all running parallel left to right. Above the sclera is the choroid (labeled "Ch" in image 5.9a) layer which is subdivided into three layers: (1) the outer vessel layer (labeled "VL" in image 5.9a), (2) the inner capillary layer (labeled "CL" in image 5.9a), and (3) an innermost Bruch's membrane (labeled "BM" in image 5.9a) which is a thin glassy membrane that contacts the pigmented epithelium (labeled "PE" in image 5.9a) of the retina. Just above the PE is shown the outer broken segments of the rods (labeled "RC" in image 5.9a).
The external coat of the eye consists of a sclera that is continuous anteriorly with the transparent cornea. The dense connective tissue of the sclera serves to protect the eye, to maintain turgor pressure and shape of the eye, and is the attachment site for extraocular muscles used in moving the eye. In image 5.9b, the arrows bracket the sclera, with the mottled black layer being the choroid coat lying immediately above, and the retina being the stratified purplish layers on top. The choroid is part of the vascular tunic, or uvea. The choroid is a thin, highly vascular layer that lines most of the internal surface of the sclera. The choroid contains numerous pigment-producing melanocytes, which give it a dark brownish color to prevent light waves from being reflected out of the eyeball.
The purple vertical stripe on the bottom of the image 5.9c stretches across the thickness of the choroid. The choroid is darkly pigmented and it contains many blood vessels. The purplish layers of the retina lie above the choroid in image 5.9c.
Image 5.9a: Sclera and choroid layers of the eye with identified collagenous fibers (CF), choroid (Ch),
Image 5.9b: Sclera (between arrows) and choroid (above sclera).
Image 5.9c: Choroid (thickness indicated by vertical purple stripe).
5.10 Retina
5.10 Retina
The innermost layer of the eye is the internal tunic, or retina. The stratified nature of the retina can be seen in the diagram of image 5.10a and also in the photographic representation of image 5.10b. Note the "stacking" arrangement of the layers in both images. The upper right corner of image 5.10b shows a dark, thick line which is the pigment layer of the retina that lies tightly adhered to the choroid. The layer adjacent to the pigment layer of the retina consists of columnar receptors for sensing light: the layer of rods and the cones. The layers below the rods and cones consist of two other principal nerve cell types: the bipolar cells, which transfer the nerve signals to the ganglion cell layer, and ganglion neurons themselves, which summarize all the inputs from bipolar calls and translate them into coded nerve impulses. The ganglion neurons contain axons that leave the eye and comprise the nerve fibers of the optic nerves, optic chiasma, and optic tracts. Not shown in the diagram of image 5.10a are the amacrine and horizontal cells which conduct nerve signals laterally in the retina and help sharpen the edges of images that we see.
Image 5.10c depicts a special area of the retina, the area of greatest visual acuity called the fovea. Here the layer of ganglion cells has been pushed to the side as well as the bipolar cells. Light can then fall directly upon the rod and cone layer, which only contains cones so that maximum clarity can be obtained. The fovea centralis (labeled "a" in image 5.10c) lies within a concavity at the posterior pole of the eye. Only cones, densely arrayed, are found in the area labeled "b" in image 5.10c, providing for very high visual acuity.
Image 5.10d shows the overall retina structure: the area labeled "a" in image 5.10d is the choroid coat, with the pigmented layer of the retina running vertically just to the right; the area labeled "b" in image 5.10d contains a portion of the rods and cones called the outer segment; the area labeled "c" in image 5.10d contains the cell bodies of the rods and cones; the area labeled "d" in image 5.10d contains synaptic connections between the rod and cones and the bipolar cells; the area labeled "e" in image 5.10d contains the cell bodies of the bipolar, horizontal and amacrine cells; the area labeled "f" in image 5.10d contains the axons of the bipolar cells and dendrites of the ganglion cells; and the area labeled "g" in image 5.10d contains cell bodies of ganglion cells.
Image 5.10e is a view of the back of the retina as a doctor would view it through an opthalmoscope. The center of the image is dominated by the optic disc (the blind spot), from which blood vessels radiate to the entire eye. Lateral to the optic disc is the macula lutea (yellow spot), on the left edge of the circular, illuminated field. In the center of the macula is a depression, the fovea centralis.
Image 5.10a: Diagram of the retina, from top of the image (layer attached to the choroid) to bottom: pigment layer, rod/cone layer, outer nuclear layer, outer plexiform, inner nuclear, inner plexiform, and ganglion cell layer.
Image 5.10b: Photographic representation of the layers of the retina.
image 5.10c: Fovea of the retina showing the fovea centralis (labeled "a").
Image 5.10d: Retina structure with labeled choroid coat (labeled "a"); outer segment (labeled "b"); area containing cell bodies of rods/cones (labeled "c"); area containing synaptic connections between rods/cones with bipolar cells (labeled "d"); area containing cell bodies of the bipolar, horizontal, and amacrine cells (labeled "e"); area contining axons of the bipolar cells and dendrites of the ganglion cells (labeled "f"); and area contining cell bodies of ganglion cells (labeled "g").
Image 5.10e: View of back of retina with optic disc, blood vessels, and fovea centrali visible.
5.11 Rods/Cones
5.11 Rods/Cones
Image 5.11a is a diagram of a rod (top of image 5.11a) and a cone (bottom of image 5.11a). Both have a dendrite on the left, cell body in the middle, and an axon to the right. The outer segment (far left) of the rod contains some 2000 membrane discs (oval structures) containing molecules of rhodopsin (dots). The inner segment contains mitochondria and Golgi bodies. The axon contains numerous, tiny synaptic vesicles which contain transmitter substance. Cones are responsible for color vision. Note the more tapered outer segment (on the far left of the cone; containing discs) and the more bulbous inner segment of the dendrite of the cone.
The retina is shown in image 5.11b. The area labeled "a" in image 5.11b contains the cell bodies of the rods and cones. The area labeled "c" in image 5.11b marks the pigmented layer of the retina and the area labeled "b" in image 5.11b is the stratum filled with the outer segments of the rods and cones. The cones constitute the system responsible for daytime vision, while the rods allow nightime vision. The rod system is highly sensitive to light and movement, but has a low visual acuity, which makes it difficult to see objects with sharp borders. Rods produce a retinal images in black and white with shades of gray. The cone system is not as sensitive to light; it is coded for color perception and requires good illumination. Cones are concentrated at the posterior retina, especially in the fovea, whereas the rods are more numerous in the peripheral portions of the retina.
Image 5.11a: Rod (top structure) and cone (bottom structure) diagram.
Image 5.11b: Retina with an area contining cell bodies of rods and cones (labeled "a"), pigmented layer of the retina (labeled "c"), and the stratum contining the outer segments of the rods and cones (labeled "b") identifed.
5.12 Fovea
5.12 Fovea
Only cones are located at the fovea, the central "V" shaped gap illustrated in image 5.12a and image 5.12b. Each cone has a more or less direct axonal pathway to the brain, which results in high resolution images. In the fovea, the overlying neuronal processes are generally displaced to one side, thereby allowing light to pass relatively unimpeded to the cones. This makes the fovea the area into which any image must be focused in order to see it clearly. Failure of the eye to properly place any image in this fovea results in diseases of accommodation, such as myopia (nearsightedness) or hyperopia (farsightedness).
Image 5.12a: Iillustration containing the fovea.
image 5.12b: Retina with fovea (arrow).
5.13 Ciliary Process/Iris
5.13 Ciliary Process/Iris
Image 5.13a is a scanning micrograph looking at the backside of the pupil, iris, and ciliary body as if you were inside the eye looking out forward. The lens has been removed so that you can see the structures more clearly. The pupil (labeled "Pu" in image 5.13a) and greatly dialated iris (labeled "Ir" in image 5.13a) are identified. The ciliary process (labeled "CP" in image 5.13a) is the part of the ciliary body which secretes the aqueous humor (that is why it is so heavily folded-to provide a large surface area for secretion).
In image 5.13b, the muscular layer of the iris (labeled "a" in image 5.13b), the pigmented layer (labeled "b" in image 5.13b, and the lens proper (labeled "d" in image 5.13b) can be seen. The lens epithelium (labeled "c" in image 5.13b) is where new cells are constantly added to the lens cortex. In image 5.13c, the arrow indicates the muscular layer of the iris. The radial muscles of the iris cause dilation in dim light, while the contraction of the circular muscles produces constriction in light of excessive brightness. In image 5.13d, the arrow indicates the ciliary body, which contains smooth muscles that alter the shape of the lens by their contraction and which also secretes aqueous humor from its folded border, the ciliary process.
Image 5.13a: Scanning micrograph of the backside of the pupil (Pu), iris (Ir), and ciliary body with ciliary processes (CP).
Image 5.13b: Muscular layer of the iris (labeled "a"), pigmented layer (labeeld "b"), lens epithelium (labeled "c"), and lens proper (labeled "d").
Image 5.13c: Muscular layer of the iris (arrow).
Image 5.13d: Ciliary body (arrow).
5.14 Ear
5.14 Ear
The ear is a very complex sense organ that contains receptors for hearing and receptors that detect head position and movement. It has three main sections: (1) the outer ear, with its pinna and external auditory meatus, (2) the middle ear, an air-filled cavity containing the three little ossicle bones, and (3) the snail-shaped inner ear, a fluid-filled cavity in which fluid vibrations are translated into coded nerve signals and head motion and position are detected.
Image 5.14a is an overview of the external (outer) ear within the skull. Note the outermost funnel-shaped pinna (or auricle) which captures sound waves and passes them inwardly through the tube, the external auditory meatus, to eventually bounce against the eardrum. Image 5.14b shows the middle ear (arrow in image 5.14b). The middle ear contains the tympanic membrane, the ossicles, and the tube called the pharyngotympanic (auditory) tube which connects the middle ear to the pharynx. Image 5.14c depicts the inner ear. The arrow in image 5.14c is on the vestibule (body) of the inner ear. Note the looping semicircular canals directly above the arrow and the coiled snail's tail (the cochlea) just below it.
The scanning micrograph in image 5.14d is a ripped open view of the middle ear. The three little bony ossicles are clearly visible: the malleus (labeled "Ma" in image 5.14d), the incus (labeled "In" in image 5.14d), and the stapes (labeled "St" in image 5.14d). Also visible is the rounded lenticular process (labeled "LP" in image 5.14d) of the incus and the large footplate (labeled "FP" in image 5.14d) of the stapes. These bones sit in a hollow cavity called the tympanic cavity (labeled "TC" in image 5.14d) which is surrounded by temporal bone (labeled "TB" in image 5.14d). To the right of the footplate you can see the inner ear cavity, the vestibule (labeled "Ve" in image 5.14d) which is normally filled with fluid.
Image 5.14a: Overview of the ear.
Image 5.14b: Middle ear (arrow).
Image 5.14c: Vestibule of the inner ear (arrow).
Image 5.14d: Scanning micrograph of middle ear visibly showing the malleus (Ma), incus (In), stapes (St), lenicular process of the incus (LP), footplate of the stapes (FP), tympanic cavity (TC), temproal bone (TB), and vestibule (Ve).
5.15 Ossicles
5.15 Ossicles
The three middle ear bones (ossicles), as diagramed in image 5.15a, form a flexible bridge across the middle-ear chamber, transmitting and amplifying sound waves from the tympanic membrane to the oval window. The positions of the ossicles within the middle ear cavity can be noted in image 5.15b. Note in particular the footplate of the stapes pushing on the oval window of the inner ear and the attachment of the green maleus to the cone-shaped eardrum. Image 5.15c shows the several types of synovial joints between the ossicles: (1) the saddle joint between the malleus and the incus, (2) the ball-and-socket joint between the incus and the stapes, and (3) the hinge joint of the stapes. Image 5.15d identifies the footplate of the stapes (arrow in image 5.15d). Vibrations on the stapes causes the membrane of the oval window to flex in and out, generating pulses of compression waves into the fluid of the inner ear. These waves are then detected by receptors in the cochlea (spiral organ).
Image 5.15a: Diagram of the middle ear ossicles.
Image 5.15b: Ossicles of the middle ear.
Image 5.15c: Types of synovial joints between the ossicles: 1) the saddle joint between the malleus and the incus; 2) the ball-and-socket joint between the incus and the stapes; and 3) the hinge joint of the stapes.
Image 5.15d: Footplate of the stapes (arrow).
5.16 Inner Ear
5.16 Inner Ear
The inner ear is located medial to the middle ear in the petrous portion of the temporal bone. The inner ear is region directly responsible for both sound detection and equilibrium. The drawing in image 5.16a identifies the major structures of the inner ear. In image 5.16b, the markers/stars indicate the ampullae of the three semicircular canals. Each ampulla contains a group of hair cells (crista) embedded in a gelatinous mass (cupula). In general, the "hairs" of the crista are flexed by the ebb and flow of fluid within the semicircular canals during rotational movements. Such flexing of the hairs generates impulses to the brain that are interpreted as turning movements.
In image 5.16c, the marker/star indicates the vestibule, which contains groups of hair cell receptors collectively referred to as the macula. Macular "hairs" are embedded in a gel cap containing particles of calcium carbonate called otoliths. Moving the head out of the vertical position causes the gel cap to sag, which flexes hairs and generates impulses to the brain that are interpreted as positional change. The macula also detects linear acceleration of the entire body in any direction and also provides the brain with information about head position.
In image 5.16d, the cochlea is identified at the tip of the arrow. The cochlea contains hair cells (the spiral organ, or organ of Corti). The hair cells are anchored in a basilar membrane and the "hairs" that project upward from the tops of the hair cells contact a tectorial membrane that lies above. Pressure waves in the fluid of the inner ear pass up the cochlea and generate vibrations in the basilar membrane. This vibration causes the attached hair cells to flex their hairs, and impulses are sent to the brain, which are then interpreted as sound.
Image 5.16a: Illustration of inner ear structures.
Image 5.16b: Ampullae of semicircular canals (stars/markers).
Image 5.16c: Vestibule (star/marker).
Image 5.16d: Cochlea (tip of arrow).
5.17 Membranous Labyrinth
5.17 Membranous Labyrinth
In image 5.17a, the membranous labyrinth is seen on the inside of the inner ear structure. Inside this membrane structure is found a fluid called endolymph. Surrounding the membranous labyrinth is a fluid called perilymph. The outer outline follows the margin of the inner ear: this is the osseous labyrinth. The diagram in image 5.17b is a dissected version of the membranous labyrinth. It shows the saccule and utricle of the vestibule (for sensing position and linear acceleration), the semicircular ducts and ampullae (for sensing rotation), and the cochlear duct which contains the Organ of Corti (for sensing sound). Image 5.17c is a close-up of the utricle, saccule, and cochlear duct.
Image 5.17a: Membranous labyrinth.
image 5.17b: Diagram of dissected membranous labyrinth.
Image 5.17c: Close-up of the utricle, saccule, and cochlear duct.
5.18 Cochlea
5.18 Cochlea
Image 5.18a is a diagram of a frontal section cut down through the cochlea of the human ear. The labeled parts correspond with structures seen in image 5.18b. Image 5.18b is a similar section through the turnings of the snail's shell, where the cochlear branch of Cranial Nerve VIII (labeled "a" in image 5.18b), runs through the central bony core (the modiolus). The scala vestibuli (labeled "b" in image 5.18b), cochlear duct (labeled "c" in image 5.18b), and scala tympani (labeled "d" in image 5.18b) are also seen. Image 5.18c is a closer view of the cochlea. The vestibular membrane (labeled "a" in image 5.18b) has the scala vestibuli immediately above. The basilar membrane (labeled "b" in image 5.18b) has hair cells of the Organ of Corti (spiral organ) lying directly superior and "c" in image 5.18c marks the cochlear branch of the vestibulocochlear nerve.
Image 5.18a: Diagram of cochlea.
Image 5.18b: Section through cochlea with cochlear branch of cranial nerve VIII (labeled "a"), scala vesibuli (labeled "b"), coclear duct (labeled "c"), and scala tympani (labeled "d") identified.
Image 5.18c: Closer view of cochlea with structures identified: vestibular membrane (labeled "a"), basilar membrane (labeled "b"), and cochlear branch of cranial nerve VIII (labeled "c").
5.19 Organ of Corti
5.19 Organ of Corti
Image 5.19a is a cross section of the Organ of Corti. Note the overhanging tectorial membrane and the rows of outer hair cells to the left. Image 5.19b shows the tectorial membrane (labeled "a" in image 5.19b) and the basilar membrane (labeled "b" in image 5.19b). Image 5.19c is a close up of the tectorial membrane at the tip of the arrow; image 5.19d shows the outer hair cells of spiral organ; image 5.19e show the inner hair cell of spiral organ; and image 5.19f shows the supporting cells of spiral organ.
Image 5.19a: Cross section of organ of corti illustration.
Image 5.19b: Tectorial membrane (labeled "a") and basilar membrane (labeled "b").
Image 5.19c: Close up of the tectorial membrane (tip of arrow).
Image 5.19d: Outer hair cells of spiral organ (tip of arrow).
Image 5.19e: Inner hair cells of spiral organ (tip of arrow).
Image 5.19f: Supporting cells of spiral organ (tip of arrow).
5.20 Utricle/Saccule
5.20 Utricle/Saccule
The utricle is the larger membranous sac shown in the center of the diagram in image 5.20a and the saccule is the smaller sac to the right of it. Both contain a common fluid called endolymph and both contain a sensory detector called the macula on the right side of the sacs) which functions in the detection of position and linear motion. Image 5.20b shows the saccule (star/marker in image 5.20b) and image 5.20c shows the utricle (star/marker in image 5.20c). Note also how the footplate of the stapes pushes into the oval window beneath these two sac-like structures.
Image 5.20a: Diagram of inner ear structures.
Image 5.20b: Saccule (star/marker).
Image 5.20c: Utricle (star/marker).
5.21 Macula
5.21 Macula
Image 5.21a shows a dissected view of the maculae found in the vestibule of the inner ear. The saccule is dissected at the bottom right and the utricle above it on the left in an attempt to reveal the maculae inside. On the surface of the macula structure itself are found clusters of hair cells (as seen in image 5.21b). Each hair cluster contains a kinocilium (labeled "Ki" in image 5.21b) and a number of stereocilia (labeled "St" in image 5.21b). When these cilia are bent, specifically when the bending occurs in the direction of the kinocilium, the nerve cell membrane becomes depolarized favoring the output of nerve impulses. Movement of the cilia away from the kinocilium hyperpolarizes the membrane and disfavors impulse output. Image 5.21c is an illustration of the workings of the macula. The arrow indicates the direction of shift taking place in the gel cap, causing the hairs to flex. The "sticks of chalk" in the upper left corner are otoliths, and when they shift the ciliary hairs bend, causing the brown-colored neurons to fire.
Image 5.21a: Dissected view of maculae found in vestibule of the inner ear.
Image 5.21b: Hair cells on the macula containing kinocilium (Ki) and sterocilia (St).
Image 5.21c: Illustration of the macula with an arrow indicating the directional shift in the gel gap that causes the hairs to flex.
5.22 Crista Ampullaris
5.22 Crista Ampullaris
The crista ampullaris is the hair cell sensory organ of the semicircular canals. Image 5.22a shows the major parts of the crista ampullaris: the elevated epithelium that contains the purple hair cells mixed with white supporting cells; the long black hairs projecting upwards from the hair cells; and the brown cupula (white in cross section view). The cupula is a jelly-like organ which projects into the lumen of the ampulla and is pushed by endolymph during rotational movements. Image 5.22b illustrates the bending action of the cupula, which in turn bends the hairs and stimulates the hair cells to fire nerve impulses. Image 5.22c is an actual photograph of a crista ampullaris. The cupula (labeled "Cu in image 5.22c) which radiates gelatinous strands (labeled "St" in image 5.22c) outwardly to contact the longest cilia of the hair cells (labeled "HC" in image 5.22c) which are held up by a layer of polygonal supportive cells (labeled "SC" in image 5.22c). The snowflake shape is the tip of the cupula.
Image 5.22a: Crista ampularis.
Image 5.22b: Bending action of cupula.
Image 5.22c: Photograph of crista ampullaris with the cupula (Cu), strands (St), hair cells (HC), and supportive cells (SC) labeled.
Created by Marissa Fukunaga, 2022Images by Bob Moeng, 2018Descriptions adapted from Bob Moeng, 2018
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