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13: The Somatic Nervous System (Special Senses)

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    12533
  • Gustation (Taste)

    Gustation is the special sense associated with the tongue. The surface of the tongue, along with the rest of the oral cavity, is lined by a stratified squamous epithelium. Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction. There are four types of papillae, based on their appearance (Figure 13.1): circumvallate, foliate, filiform, and fungiform. Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells for the transduction of taste stimuli. These receptor cells are sensitive to the chemicals contained within foods that are ingested, and they release neurotransmitters based on the amount of the chemical in the food.

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    Figure 13.1 The Tongue The tongue is covered with small bumps, called papillae, which contain taste buds that are sensitive to chemicals in ingested food or drink. Different types of papillae are found in different regions of the tongue. The taste buds contain specialized gustatory receptor cells that respond to chemical stimuli dissolved in the saliva. These receptor cells activate sensory neurons that are part of the facial and glossopharyngeal nerves. LM × 1600. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    Olfaction (Smell)

    Like taste, the sense of smell, or olfaction, is also responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity (Figure 13.2). This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.

    The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

    The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.

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    Figure 13.2 The Olfactory System (a) The olfactory system begins in the peripheral structures of the nasal cavity. (b) The olfactory receptor neurons are within the olfactory epithelium. (c) Axons of the olfactory receptor neurons project through the cribriform plate of the ethmoid bone and synapse with the neurons of the olfactory bulb (tissue source: simian). LM × 812. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    Audition (Hearing)

    Hearing, or audition, is the transduction of sound waves into a neural signal that is made possible by the structures of the ear (Figure 13.3). The large, fleshy structure on the lateral aspect of the head is known as the auricle. Some sources will also refer to this structure as the pinna, though that term is more appropriate for a structure that can be moved, such as the external ear of a cat. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves. The auricle, ear canal, and tympanic membrane are often referred to as the external ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane. The tube is normally closed but will pop open when the muscles of the pharynx contract during swallowing or yawning.

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    Figure 13.3 Structures of the Ear The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively.

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    A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct (Figure 13.4). The cochlear duct contains several organs of Corti, which tranduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea. Lower frequency waves move the region of the basilar membrane that is near the tip of the cochlea.

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    Figure 13.4 Cochlea and Organ of Corti LM × 412. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    Vision

    Vision is the special sense of sight that is based on the transduction of light stimuli received through the eyes. The eyes are located within either orbit in the skull. The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye (Figure 13.5). The eyelids, with lashes at their leading edges, help to protect the eye from abrasions by blocking particles that may land on the surface of the eye. The inner surface of each lid is a thin membrane known as the palpebral conjunctiva. The conjunctiva extends over the white areas of the eye (the sclera), connecting the eyelids to the eyeball. Tears are produced by the lacrimal gland, located beneath the lateral edges of the nose. Tears produced by this gland flow through the lacrimal duct to the medial corner of the eye, where the tears flow over the conjunctiva, washing away foreign particles.

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    Figure 13.5 The Eye in the Orbit The eye is located within the orbit and surrounded by soft tissues that protect and support its function. The orbit is surrounded by cranial bones of the skull.

    Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball (Figure 13.6). Four of the muscles are arranged at the cardinal points around the eye and are named for those locations. They are the superior rectus, medial rectus, inferior rectus, and lateral rectus. When each of these muscles contract, the eye to moves toward the contracting muscle. For example, when the superior rectus contracts, the eye rotates to look up. The superior oblique originates at the posterior orbit, near the origin of the four rectus muscles. However, the tendon of the oblique muscles threads through a pulley-like piece of cartilage known as the trochlea. The tendon inserts obliquely into the superior surface of the eye. The angle of the tendon through the trochlea means that contraction of the superior oblique rotates the eye medially. The inferior oblique muscle originates from the floor of the orbit and inserts into the inferolateral surface of the eye. When it contracts, it laterally rotates the eye, in opposition to the superior oblique. Rotation of the eye by the two oblique muscles is necessary because the eye is not perfectly

    aligned on the sagittal plane. When the eye looks up or down, the eye must also rotate slightly to compensate for the superior rectus pulling at approximately a 20-degree angle, rather than straight up. The same is true for the inferior rectus, which is compensated by contraction of the inferior oblique. A seventh muscle in the orbit is the levator palpebrae superioris, which is responsible for elevating and retracting the upper eyelid, a movement that usually occurs in concert with elevation of the eye by the superior rectus (see Figure 13.5).

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    Figure 13.6 Extraocular Muscles The extraocular muscles move the eye within the orbit.

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    Figure 13.7 Structure of the Eye The sphere of the eye can be divided into anterior and posterior chambers. The wall of the eye is composed of three layers: the fibrous tunic, vascular tunic, and neural tunic. Within the neural tunic is the retina, with three layers of cells and two synaptic layers in between. The center of the retina has a small indentation known as the fovea.

    Light falling on the retina causes chemical changes to pigment molecules in the photoreceptors, ultimately leading to a change in the activity of the RGCs. Photoreceptor cells have two parts, the inner segment and the outer segment (Figure 13.8). The inner segment contains the nucleus and other common organelles of a cell, whereas the outer segment is a specialized region in which photoreception takes place. There are two types of photoreceptors—rods and cones—which differ in the shape of their outer segment. The rod-shaped outer segments of the rod photoreceptor contain a stack of membrane-bound discs that contain the photosensitive pigment rhodopsin. The cone-shaped outer segments of the cone photoreceptor contain their photosensitive pigments in infoldings of the cell membrane. There are three cone photopigments, called opsins, which are each sensitive to a particular wavelength of light. The wavelength of visible light determines its color. The pigments in human eyes are specialized in perceiving three different primary colors: red, green, and blue.

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    Figure 13.8 Photoreceptor (a) All photoreceptors have inner segments containing the nucleus and other important organelles and outer segments with membrane arrays containing the photosensitive opsin molecules. Rod outer segments are long columnar shapes with stacks of membrane-bound discs that contain the rhodopsin pigment. Cone outer segments are short, tapered shapes with folds of membrane in place of the discs in the rods. (b) Tissue of the retina shows a dense layer of nuclei of the rods and cones. LM × 800. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

    LAB 13 EXERCISES 13-1

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    B

    A

    Draw the following: Olfactory bulb, Olfactory receptor dendrites

    Label the following: Cribiform plate, Olfactory mucosa.

    Label the following: Circumvallate papillae, Fungiform papillae, Filiform papillae, sulcus terminalis, Median lingual sulcus, Lingual tonsil, Palatine tonsil

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    LAB 13 EXERCISES 13-2

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    BASIC EAR ANATOMY

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    Match the following items to their proper location: Auricle, Auditory canal, Auditory tube, Auditory ossicles, Cochlea, Semicircular Canals, Tensor Tympani muscle., Vestibule, Vestibulocochlear nerve.

    Label the following on this stylized

    diagram: Auricle, Auditory canal, Pharyngotympanic tube, Stapes, Incus, Malleus, Tympanic membrane, Cochlea, Outer ear, Middle ear, Inner ear.

    Outer ear

    Middle ear

    Inner ear

    LAB 13 EXERCISES 13-3

    INNER EAR ANATOMY

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    Label the following: Auricle, External Acoustic meatus, Pharyngotympanic tube, Cochlea, Vestibule, Semicircular canals, Tympanic membrane, Middle ear

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    LAB 13 EXERCISES 13-4

    INNER EAR ANATOMY

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    Label the following: Cochlea (uncoiled), Round window, Scala vestibuli, Scala tympani, Scala media, Spiral ganglion, Cochlear nerve (part of the vestibulocochlear nerve), Vestibular membrane, Basilar membrane, Tectorial membrane, Organ of Corti.

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    LAB 13 EXERCISES 13-5

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    INNER EAR ANATOMY

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    Label the following: Utricle, Saccule,

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    Semicircular canals, Ampulla, Cochlea, Cranial nerve VIII, Malleus, Incus, Stapes

    Label the following: Lens, Suspensory ligaments, Ciliary body, Cornea, Iris, Choroid, Vitreous humor

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    LAB 13 EXERCISES 13-6

    IDENTIFY INTERNAL REGIONS AND LAYERS OF THE EYE: FIBROUS TUNIC, VASCULAR TUNIC, NEURAL TUNIC, IRIS, LENS, CORNEA, SCLERA, OPTIC NERVE, VITREOUS HUMOR, AQUEOUS

    HUMOR, CILIARY BODY, SUSPENSORY LIGAMENTS.

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    LAB 13 EXERCISES 13-7

    EYE ANATOMY

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    Label the following: Levator palpebrae m., Tarsal plate, Lacrimal gland, Lacrimal ducts, Lacrimal Sac, Lacrimal caruncle, Medial canthus, Lateral canthus,

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    LAB 13 EXERCISES 13-8

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    EXTRINSIC EYE MUSCLES

    Identify the extrinsic eye muscles

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    (4 are clearly visible)

    Identify the extrinsic eye muscles

    (4 are clearly visible)

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    LAB 13 EXERCISES 13-9

    OPTIC CHIASM

    Label the following:

    1. Conscious visual sensation;

    2. Beginning of axons of retinal ganglion cells;

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    3. 3rd order neuron;

    4. synapse between 2nd & 3rd order neuron;

    5. Half of nerve fibers decussate here;

    6. First region with axons from only the left side of both eyes

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    LAB 13 EXERCISES 13-10

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    RETINA HISTOLOGY

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    Label the following: Rod, Cone, Horizontal Cell, Bipolar Cell, Ganglion cell, Amacrine cell, Pigmented epithelium

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    Label the following: Rods and cones, Bipolar cells, Ganglion cells, Optic nerve axons, Pigmented epithelium, Vitreous humor.

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    MODELS:

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    ye, E

    d-Sa

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    so, Mi

    ar, T

    E 2

    or gittal H 7

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    ead

    and Cochlea

    Models

    Olfaction (sagittal head model)

    • Nare

    • Vestibule

    • Nasal conchae (superior, middle, inferior)

    • Nasal meatuses

    • Nasal cavity

      Eye (various models)

       External Structures

    • Palpebrae

    • Tarsal plates

    • Levator Palpebrae m.

    • Orbicularis Oculi m.

    • Medial and lateral canthus

    • Lacrimal caruncle

    • Lacrimal gland

    • Lacrimal ducts

    • Lacrimal sac

       Extra-ocular muscles:

    • Lateral rectus m.

    • Medial rectus m.

    • Superior rectus m.

    • Inferior rectus m.

    • Superior oblique m.

    • Inferior oblique m.

       Internal structures

    • Fibrous tunic:

       Sclera

       Cornea

    • Eye, cont.

    • Vascular tunic:

       Choroid

       Ciliary body / muscle

       Suspensory ligaments

       Iris

    • Sensory (neural) tunic:

       Retina

       Macula Lutea

       Fovea centralis

       Optic Nerve

       Optic disc

    • Lens

       Posterior cavity/Vitreous Humor

       Anterior cavity/ Aqueous Humor

      Structures of the ear (various models)

       Outer ear

    • Auricle

    • External auditory canal

       Middle ear

    • Tympanic membrane

    • Auditory ossicles:

       Malleus

       Incus

       Stapes

    • Pharyngotympanic tube (Eustachian, auditory)

      Lab list continued on next page! 

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      Ear, cont.

       Inner ear

      • Bony labyrinth

      • Membranous labyrinth

      • Vestibule

         Utricle

         Saccule

      • Semicircular canals

         Semicircular ducts

         Ampulla

      • Cochlea

      • Oval window

      • Round window

      • Vestibulocochlear nerve

         Cochlea (cross-sectional model)

      • Vestibular duct (scala vestibuli)

      • Cochlear duct (scala media)

      • Tympanic duct (scala tympani)

      • Vestibular membrane

      • Basilar membrane

      • Organ of Corti (& hair cells)

      • Tectorial membrane

      • Spiral ganglion

    • Tongue (tongue model)

      • Papillae:

        • Filiform

        • Fungiform

        • Foliate

        • Circumvallate

    • Tongue, cont.

      • Sulcus terminalis

      • Median sulcus

      • Taste buds

        • Taste pore

        • Support & taste cells

    • Histology:

    • Eye – retina:

      • Rods & Cones

      • Bipolar cell layer

      • Ganglion cell layer

      • Axons of ganglion cells

      • Retinal pigment epithelium

      • Choroid layer

      • Sclera

    • Ear

      • Basilar membrane

      • Scala media (cochlear duct)

      • Scala tympani (tympanic duct)

      • Scala vestibuli (vestibular duct)

      • Spiral ganglion

      • Spiral organ (Organ of Corti)

      • Tectorial membrane

      • Vestibular membrane

    • Tongue

      • Taste bud

      • Taste pore

      • Support and Taste cells

    COW EYE DISSECTION

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    1. Examine the outside of the eye. You should be able to find the sclera, or the whites of the eye. This tough, outer covering of the eyeball has fat and muscle attached to it 

    2. Locate the covering over the front of the eye, the cornea. When the cow was alive, the cornea was clear. In your cow’s eye, the cornea may be cloudy or blue in color. 

    3. Cut away the fat and muscle, this may only be necessary if fat is covering the cornea of the eye and is in your way. Fat around the backside of the eye can be left alone. Flip the eye over to find the optic nerve where it exits out the back of the eye. It will be stronger and more rope-like than the surrounding fat tissue. 

    4. Use a scalpel or scissors to make an incision in the cornea. The cornea is tougher than it appears and may require some force to puncture, be careful when using the scalpel. Once the cornea is broken, clear liquid will leak (or squirt) out – this liquid is the aqueous humor

    5. Quick Check: Outer Tunic

      1. The white of the eye is the

      2. The front surface of the eye,continuous with (A) is the

      3. The liquid found in the front of the eye is the

      4. What is the name of the nerve found on the back of the eye?

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    6. Use a scalpel or scissors to make an incision in the sclera so that you can cut around the outside of the eye. Your goal is to separate the eye into a front and a back half. 

    7. Separate the inner parts of the eye.

      The gelatinous liquid in the middle of the eye is the vitreous humor image, which will also contain a hard, sphere-shaped lens. image

      Find the cornea (which you punctured in step 1) and then the disk-shaped iris behind it. image The iris will be dark in color and contain a center opening, the pupil

    8. The image below shows how each part of the eye appears when it has been separated. Take a photo of your own eye and share it on social media. #coweye (optional)Image_333.jpg

    9. The back of the eye has two layers, a very thin layer of cells that is easy to scrape off (and may fall off on its own), which is the retina image. Behind, the retina is a blue, reflective layer known as the tapetum. image

    10. The retina will converge at a point on the eye where it connects with the optic nerve. This is the optic disk. It may be easiest to find by scraping off the retina and locating the spot where it remains closely attached. imageFlipping the eye over will also show how that spot is directly in front of the optic nerve.

    LAB 13 EXERCISES 13-10

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    Label the Cow Eye (use your book or other resources)

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    Cow Eye Dissection maybe Downloaded at: https://www.biologycorner.com/worksheets/cow_eye_dissection.html

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