Human Anatomy & Physiology II - Dr. Sullivan Unit III The Sense Organs Chapter 16

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1 Human Anatomy & Physiology II - Dr. Sullivan Unit III The Sense Organs Chapter 16 I. Sensation a) Definition: the conscious or subconscious awareness of external or internal stimuli. b) The type of reaction generated by the sensation is determined by the nerve impulse s ultimate CNS destination. i) i.e. Spinal cord: spinal reflex ii) Lower Brain Stem: heart rate, breathing rate iii) Thalamus of brain: touch, pain, hearing, taste, crude awareness of location iv) Cerebral Cortex: Precise locations v) Perception: the conscious awareness and interpretation of meaning of sensations. c) There is no perception of a sensation unless it reaches the Thalamus and the cerebral cortex i) i.e. blood pressure is constantly monitored by the body, but you are not aware of it because it s sensation impulse stops in the medulla oblongata of the brainstem. d) Sensory Modalities i) Defintion: a type of sensation (1) I.e. hearing, touch, pain, vision, etc. ii) Selectivity: Each sensory neuron carries information for one modality only iii) Two classes of sensory modalities: (1) General Senses iv) Somatic Senses: tactile (touch, pressure, & vibration), thermal, pain, and proprioception. v) Visceral Senses: monitor conditions within the internal organs. (1) Special Senses: Smell, taste, vision, hearing, equilibrium (balance). e) Process of Sensation i) Sensory receptor: a specialized cell or dendrite of sensory neuron monitors a particular condition in the internal or external environment. f) Receptive field: an area of skin or tissue innervated by one sensory receptor. i) a stimulus must stimulate the tissue within its receptive field to activate the (1) receptor (2) Each receptor responds vigorously to only one type of stimulus. (3) Four Steps of Sensation: (a) Stimulation of sensory receptor: must occur within the receptive field (portion of the receptor capable of responding to stimulus). (b) Transduction of the stimulus: the sensory receptor converts the stimulus to a graded potential that can vary in amplitude. (c) Generation of Impulses: Graded potential created an impulse that travels toward the CNS. (4) First Order Neurons: sensory neurons that propagate nerve impulses from the PNS to the CNS (a) Integration of Sensor Input: A specific region of the CNS receives and interprets the impulse depending on what type of sensation it is. II. The Nature of Sensory Receptors a) Types of Sensory Receptors: structure i) Free Nerve Endings: bare dendrites with no structural specialization. (1) pain, thermal, tickle, itch, and some touch sensations ii) Encapsulated Nerve Endings: dendrites are enclosed in a connective tissue capsule such as Pacinian Corpuscles iii) Separate Cells: synapse with first order neurons (1) Special senses b) Types of potentials i) Generator potentials: produced by free nerve endings, encapsulated nerve endings, and olfactory receptors

2 (1) generates one or more impulses that propagates toward the CNS. ii) Receptor potentials: produced by the specialized cells responsible for vision, hearing, taste and balance. (1) causes exocytosis of synaptic vesicles and result in a PSP of the first-order-neuron. This could result in a nerve impulse that propagates toward the CNS. c) Other ways to group sensory receptors: Location in the body i) Exteroreceptors: at or near the body surface and responsible for stimuli originating outside the body and provide information about the outside the environment. ii) Interoreceptors: Located in blood vessels, visceral organs, and nervous system. Provide information about internal environment and conditions. Not consciously perceived. iii) Proprioceptors: Located in muscles, tendons, joints, and inner ear. Responsible for knowing body position and movement as well as muscle length and tension. d) 3 rd way to group sensory receptors: Types of stimulus they respond to i) Mechanoreceptors: detect mechanical pressure and stretching (1) touch, pressure, vibration, proprioception, hearing, and balance. (2) Thermoreceptors: detect changes in temperature (3) Nociceptors: detect physical or chemical damage (resulting in perception of pain) (4) Photoreceptors: detect light striking the retina (5) Chemoreceptors: detect chemicals in the mouth, nose, and body fluids. III. Adaptation in Sensory Receptors a) Adaptation: generator or receptor potentials decrease in amplitude during a maintained, constant stimulus. i) Also, the frequency of nerve impulses in a first order neuron also decreases during a prolonged stimulus. ii) The receptor loses its ability to respond after time. (1) i.e.: when you first put your shirt on, you feel it on your body, but eventually, you don t even notice it s there. iii) Rapidly Adapting Receptors (Phasic): Adapt very quickly and specialized for detecting changes in a particular stimulus. (1) Pressure, touch, and smell iv) Slowly Adapting Receptors (Tonic): adapt slowly and continue to trigger nerve impulses throughout a stimulus. (1) Pain, body position, chemical composition of the blood. IV. Somatic Sensations a) Stimulation to the sensory receptors of the skin and subcutaneous layer, mucous membranes of the mouth, vagina, and anus, in the muscles, tendons, and joints, and inner ear. b) Some parts of the body have a higher concentration of somatic sensory receptors than others. c) Modalities: tactile, thermal, nociception, proprioception i) Tactile Sensation (1) Touch, pressure, vibration, itch, tickle. (2) Touch (a) Crude Touch: something has contacted the skin (b) Discriminative touch: specific information such as exact point touched, shape, size, and texture. (c) Rapidly Adapting Touch Receptors (3) Meissner s Corpuscles: receptors of discriminative touch located in the dermal papillae of hairless skin, such as the finger tips and palms of the hand (also located in eyelids, tip of the tongue, lips, nipples, soles, clitoris, and tip of the penis). 40% of tactile receptors in hands. (4) Hair Root Plexuses: found in hairy skin wrapped around the hair follicles. Movement of hair stimulates free nerve endings. (a) Slowly Adapting Touch Receptors (5) Merkel Discs: free nerve ending, discriminative touch mechanoreceptors found in the fingers, lips, and external genitalia. 25% of tactile receptors in hands.

3 (6) Ruffini Corpuscles: encapsulated receptors located deep in the dermis, in the ligaments and tendons. 20% of sensory receptors in the hands. Detect stretching in the moving digits or limbs. (7) Pressure and Vibration (a) Pressure: received by Meissner corpuscles and Pacinian Corpuscles. Pacinian Corpuscles are rapidly adapting receptors found in the dermis underlying mucous membranes, around joints, tendons, and muscles, the periosteum, mammary glands, external genitalia, pancreas, and urinary bladder. (8) Vibration: received as rapidly repetitive sensory signals from tactile receptors. Use Meissner (low frequency) and Pacinian (high frequency) Corpuscles. (9) Itch and Tickle (a) Itch: stimulation of free nerve endings by certain chemicals such as bradykinin. Often result of inflammatory response. (b) Tickle: The only sensation you can not elicit yourself (mystery). Mediated by free nerve endings and Pacinian Corpuscles. ii) Thermal Sensations (1) Thermoreceptors: rapidly adapting, free nerve endings with a 1mm receptive field on the skin. (a) Cold receptors: activated by temperatures from 10 to 40 degrees centigrade. (i) Attached mostly to medium-diameter, myelinated A fibers and some to C fibers. (b) Warm receptors: activated by temperatures between 32 and 48 degrees centigrade. (i) Attached C fibers (ii) Below 10 degrees centigrade, temperature is received mostly by nociceptors resulting in pain. iii) Pain Perception (1) Protects us by signaling the presence of noxious, tissue-damaging conditions. (2) Nociceptors do not sense pain, they sense tissue damage or noxious stimuli. Pain is perceived when the impulse reaches the cerebral cortex of the brain. (3) Nociceptors: free nerve endings in every tissue of the body except the brain. (a) Intense thermal, mechanical, or chemical stimuli can activate nociceptors. (b) Tissue irritation or injury releases chemicals (such as prostaglandins, kinins, or K+) that stimulate nociceptors. (c) Nociceptors are very slow adapting and the chemicals that stimulate them linger, therefore pain can continue after the stimulus is removed. (4) Pain is caused by excessive distention or dilation of a structure, prolonged muscular contraction, or ischemia (among other things). (5) Types of Pain (a) Fast pain: felt very quickly and carried on A-delta fibers (medium-diameter, myelinated axons). (i) Acute pain, sharp pain, or pricking pain felt by a pin prick or knife cut. (ii) Not felt in deeper tissues of the body. (b) Slow Pain: Begins slowly after a stimulus and gradually increases in intensity over time. (i) Carried over C fibers. (ii) Chronic, burning, aching, or throbbing pain 1. Can occur in skin and in deeper tissues or internal organs (such as a toothache or kidney stone). (6) Superficial Somatic Pain: arises from nociceptors in the skin (7) Deep Somatic Pain: arises from nociceptors in the skeletal muscles, joints, tendons, and fascia. (8) Visceral Pain: arises from nociceptors in the visceral organs. (9) Localization of Pain (a) Referred Pain: Pain that is not precisely localized to the area of injury or stimulus. Usually the site of stimulus and the area where referred pain is felt are innervated by the same segments of the spinal cord (i) i.e.: the Heart muscle and the skin along the left arm

4 (ii) Phantom Limb sensation: 1. A person can perceive touch, itch, vibration, or pain as coming from an amputated limb because the cerebral cortex interprets stimulus from the proximal axon of a neuron that used to go to that limb as still coming from that limb. (iii) Also, neurons in the brain that previously received sensory impulses from the limb are still active. iv) Proprioceptive Sensations (1) Tell us about our body s position and movement in space (a) Degree of muscle contraction, position of joints, tension of tendons and ligaments, and orientation of the head relative to the ground during movement. (b) Kinesthesia: the perception of body movements (c) Proprioceptors: receive proprioceptive information and adapt slowly and only slightly. (d) Examples: hair cells in inner ear for balance, muscle spindles, joint kinesthesthetic receptors, and tendon organs. (2) Muscle Spindles (a) Proprioceptors within the skeletal muscle that detect sudden or prolonged stretch. The stretch receptor sends the message to the spinal cord, the spinal cord interprets it as possibly damaging to the muscle and sends back a motor response that involuntarily contracts the muscle to keep it from stretching anymore. (3) Tendon Organs (a) Proprioceptors found at the musculotendinous junction that protect against excessive tension. The tension stimulus is received and the response is to involuntarily relax the muscle to prevent damage to the tendon. (4) Joint Kinesthetic receptors (a) Proprioceptors found in ligaments and joint capsules that adjust reflex inhibition of the adjacent muscles when excessive strain is placed on a joint. V. Somatic Sensory Pathways a) Relay information from the somatic sensory receptors to the primary somatosensory area in the cerebral cortex and/or the cerebellum i) Primary somatosensory area of the brain is known as the post-central gyrus. b) First-order Neurons: conduct impulses from the somatic sensory receptors to the spinal cord or the brainstem. i) Sensory from face, mouth, teeth, and eyes propagate along the cranial nerves ii) Sensory from the neck, body, and back of the head propagate along spinal nerves c) Second-order neurons: Conduct impulses from the spinal cord or the brainstem to the thalamus. i) Second-order neurons decussate (or cross-over to the opposite side) in the cord or brainstem. Therefore, all sensory information is perceived by the contralateral side of the thalamus. d) Third-order neurons: conduct impulses from the thalamus to the primary somatosensory area of the cerebral cortex where conscious perception of the sensation results. SPECIAL SENSES: Smell, Taste, Vision, Hearing, Equilibrium VI. Olfaction: Smell a) Smell and taste are chemical senses. Both sensations arise from chemical reactions of molecules with smell and taste receptors. b) Smell and taste propagate to the limbic system in the brain (as well as other cortical areas), and therefore certain tastes and smells evoke emotional responses and/or memories. c) Anatomy of olfaction i) The nose contains million olfactory receptors over a 5cm 2 ii) Three types of cells in the nose for smell iii) Olfactory receptors: the first order neurons of the olfactory pathway (1) The tip of these receptors is exposed as a knob-shaped dendrite (2) Olfactory hairs: cilia on the end of the dendrite

5 iv) Supporting cells: cells of the mucous membrane of the nose that provide physical support and nourishment to the olfactory receptors and help detoxify chemicals that come into the nose. v) Basal Stem Cells: Precursors to olfactory receptors (1) One of the few areas where neurons are replaced vi) Olfactory (Bowman s) Glands: produce mucous that covers the olfactory epithelium. d) Physiology of Olfaction i) An odorant molecule enters the nose and is dissolved within the mucous ii) An olfactory hair reacts to the chemical stimulation produced by the dissolved odorant molecule and transduces it into a generator potential that stimulates the olfactory receptor s dendrite (first-order neuron). iii) First-order neuron: The dendrite propagates an impulse along the receptor s axon (CN I or Olfactory Nerve), which synapses with a dendrite of an olfactory bulb neuron. iv) Second-order neuron: The impulse is propagated to the axon of the olfactory bulb neuron, which makes up the olfactory tract. v) The Olfactory tract carries the impulse to the lateral olfactory area of the temporal lobe of the brain. (1) This is where conscious awareness of smell begins (2) This is part of the limbic system and is linked to the hypothalamus. (a) This is why certain smells are linked to emotional or memory-evoked responses. (b) I.e.: sexual excitement from perfume, nausea from smelling something that made you sick, or remembering something from childhood from a familiar smell. vi) Third-order neuron: From the lateral olfactory area, pathways of olfaction also extend to the frontal lobe via the thalamus to the orbitofrontal or Brodmann s are 11. (1) This is an important region for odor identification. Injury or other lesion to this are can affect a person s ability to discriminate between smells. vii) Other important notes on olfaction: (1) Olfactory receptors are rapidly adapting (2) Olfaction has a low-threshold and only very small amounts of chemicals are needed to stimulate an odor. VII. Gustation: Taste a) Also a chemical sense b) Only five classes of taste can be distinguished: i) Sour, Sweet, Bitter, Salty, & Umami (1) Umami, meaning meaty or savory, is a fairly new discovery from Japan (2) All other tastes are combinations of those 5 plus the accompanying olfactory sense that comes with it. (3) A food s smell travels to the nose through the mouth and stimulates olfactory receptors. (4) Olfaction is much more sensitive than taste and therefore contributes a lot more to the flavor of something than does taste. (5) Therefore, when people are stuffed up from a cold, they complain of not being able to taste their food. c) Anatomy of Gustation: mouth i) Taste buds: contain taste receptors (1) Located mostly on the tongue (about 10,000) and decrease in number with age. (2) Also found on the soft palate, pharynx, and larynx (3) Taste Pore: a hole in the top of the taste bud (4) Three types of cells within a taste bud: (a) Supporting cells: provide physical support to the gustatory receptors, eventually become receptor cells (b) Gustatory receptors: taste receptors (c) Last for 10 days (d) Gustatory hair: part of the receptor that projects from the receptor out of the taste pore (e) Basal Cells: found on the periphery of the taste bud. Produce supporting cells.

6 (f) Papillae: elevations on the tongue which sometimes contain taste buds and give the tongue its rough appearance. (i) Three types of papillae 1. Circumvillate (vallate) Papillae: largest type 2. Circular and form the inverted V-shaped row at the back of the tongue. 3. Always contain taste buds 4. Fungiform Papillae: mushroom-shaped elevations over the entire tongue. 5. Most of them contain taste buds 6. Foliate (filiform) Papillae: over entire surface of tongue, pointed and thread like 7. Rarely contain taste buds d) Physiology of Gustation i) A chemical is dissolved in saliva and makes contact with the gustatory hairs ii) Gustatory hairs transduce the contact to the receptors and produce a receptor potential. iii) The receptor potential stimulates an impulse in the first-order neurons that synapse with the gustatory receptors. iv) First-order neurons of gustation can be carried on three different cranial nerves: (1) Facial (CN VII): anterior 2/3 of tongue (2) Glossopharyngeal (CN IX): posterior 1/3 of tongue (3) Vagus (CN X): throat and epiglottis (4) First-order neurons: The cranial nerves carry the taste impulse to the medulla oblongata & pons (5) Second-order neurons: From the medulla oblongata & pons, some taste fibers project to the limbic system and the hypothalamus. Some travel to the thalamus. (6) Third-order neurons: from the thalamus or limbic system/hypothalamus, the impulse travels to the primary gustatory area in the parietal lobe where conscious perception of taste takes place. v) Important difference between taste and smell: In gustation, the receptor is not the firstorder neuron as it is in olfaction. The receptor synapses with one of the cranial nerves mentioned. In olfaction, the axons of the receptors make up the olfactory (CN I) nerve and are therefore the first-order neuron. vi) Threshold levels in taste are different based on the taste that is being present: (1) Taste is most sensitive to bitter to protect us from poisonous substances that mostly have a bitter taste. (2) Second is Sour (3) Least sensitive is salty and sweet, which are about equal. vii) Taste is also rapidly adapting and can complete adaptation to a taste can occur within 1-5 minutes of continuous stimulation. (1) Each receptor may respond to any of the four primary tastes, however, specific regions of the tongue are more sensitive to certain tastes than others. (a) Tip of the tongue: sweet and salty (b) Posterior of the tongue: bitter (c) Lateral aspects of the tongue: sour VIII. Vision a) Anatomy of Vision i) Eyelids (upper and lower) (1) also called palpebrae (2) shade during sleep (3) protect from foreign objects (4) lubricate the eyes (5) The upper eyelid is the more moveable of the two (6) Moves via the levator palpebrae superioris muscle b) Palpebral Fissure: the space between the lids that reveals the eyeball c) Lateral Commissure: the angle produced by the lateral meeting of the upper and lower eyelid d) Medial Commissure: the angle produced by the medial meeting of the upper and lower eyelids. e) Conjuctiva: thin layer of protective connective tissue

7 i) Palpebral Conjunctiva: inner aspect of the eyelids ii) Bulbar conjunctiva: anterior surface f the eyeball lateral to the cornea iii) Conjunctivitis: infection of the conjunctiva (also known as Pink Eye) f) Eyelashes & Eyebrows: protect eye from direct sunlight, sweat, and foreign objects. g) Sebaceous ciliary glands at the base of the eyelash hair follicle secrete lubricating fluids. i) infection of such is called a sty h) Lacrimal Glands secrete tears through the lacrimal duct. i) Tears (aka lacrimal fluid) protect, clean, lubricate, and moisten the eyeball (1) anti-bacterial properties in lysozyme (2) Crying: Expressed with both happiness and sadness in humans (3) Parasympathetic stimulation to the lacrimal glands produces excessive lacrimal fluid production, which spills over the eyelids and fills the nasal cavity. i) 6 Extrinsic Muscles move the eye i) Superior rectus: moves eye superiorly ii) Inferior rectus: moves eye inferiorly iii) Lateral Rectus: lateral iv) Medial Rectus: medial v) Superior oblique: inferior and toward the nose vi) Inferior Oblique: superior and toward the temple j) Eyeball Anatomy: i) Fibrous Tunic: avascular, superficial coating of the eyeball (1) consists of cornea anteriorly, and sclera posteriorly ii) Vascular Tunic (uvea): middle layer of the eyeball (1) consists of the choroid, the ciliary body, and the iris iii) Ciliary body (1) made up of ciliary muscles that change the shape of the lens for focusing and ciliary processes that connect the muscle to the suspensory ligaments of the lens. iv) Iris: colored portion of the eyeball v) Pupil: Hole in center of Iris preventing light rays from entering throught the periphery of the eye. (1) light rays entering at the periphery would not be clearly focused on the retina and cause blurry vision. vi) Cornea: transparent covering of the iris vii) Lens: suspended by suspensory ligaments posterior to the iris and pupil (1) focuses light onto the retina viii) Anterior Cavity: contains the following (1) Filled with Aqueous Humor: a watery substance that nourishes the cornea and lens and contributes to the maintenance of the pressure in the eye (intraocular pressure). (2) The pressure prevents the eyeball from collapsing (3) Anterior chamber: space between cornea and iris (4) Posterior chamber: space between iris and suspensory ligaments/lens ix) Posterior cavity (aka vitreous chamber): from just posterior to the lens back to the retina. (1) contains the vitreous body, which is filled with a jelly-like substance that also contributes to the maintenance of the intraocular pressure) and the hyaloid canal, which is a narrow channel that runs through the vitreous body from the optic disc to the posterior aspect of the lens. (2) Three layers covering the eyeball except the cornea x) Sclera: outermost layer (whites of the eyes) xi) Choroid: middle layer xii) Retina: interior layer (posterior ¾) (1) The beginning of the visual pathway (2) Wall contains fibers of the optic nerve (II) (3) Blood vessels course across the retina s anterior surface (4) Only place in the body where blood vessels can be viewed directly (5) Pathological changes in the blood vessels can be seen as a result of diabetes mellitus and hypertension

8 (6) Two types of photoreceptors on the retina: (a) Rods: low-light vision, shades of gray (no color) 1. average person: 120 million rods (b) Cones: Bright-light, color (i) avg. person: 6 Million cones xiii) Optic Disc: the site where the optic nerve exits the eyeball (alo called the blind spot b/c it contains no rods or cones) xiv) Central Fovea: spot in the retina where visual acuity is the highest (a) The reason you more your eyes around to see something is to focus the most direct light from an object onto the central fovea. xv) Macula Lutea: the exact center portion of the retina (a) Aka: the visual axis of the eye k) Image Formation i) Refraction of Light Rays (1) Refraction: the bending of light rays when passing through transparent substances of different densities (a) i.e. a pencil sticking half way up in a glass of water looks bent. (2) Light is refracted by the posterior and anterior surfaces of the cornea when it enters the eye (3) The Lens refracts light even more (fine-tuning it) so that it comes in contact with the retina in exact focus (4) 75% of total refraction happens at the cornea (5) The shape of the lens is changed by contracting the ciliary muscles therfore reducing the tension on the suspensory ligaments, allowing the lens to revert back to its rounder, less taut shape for maximum focusing power. (6) When focusing on an object from more than 6 meters, the lens is usually taut and flat by relaxing the ciliary muscles. (7) Images focused on the retina are inverted and mirrored (a) However, the brain learns early in life that this happens, so it interprets the objects we see in their actual orientation (8) Accomodation: refracting rays coming from nearer than 6 meters to achieve focus on the retina. (a) achieved by contraction of the ciliary muscles and allowing the lens to take a rounder shape for maximum focusing power. (b) Near Point Vision: the minimum distance from the eye that an object can be clearly focused with maximum effort. (i) about 10 cm in a young adult. (ii) The lens loses elasticity with age and therefore you lose accommodation and cannot read small print from a close distance. (iii) This is called presbyopia (9) Emmetropic Eye: Normal Eye (a) the emmetropic eye can sufficiently refract light rays from an object 6 meters away so that a clear image is focused on the retina. (b) Abnormalities: (i) Myopia: inability to clearly focus distant objects (aka nearsightedness) (ii) Hyperopia: inability to clearly focus nearby objects (aka farsightedness) (iii) Asigmatism: irregular curvature of the cornea or lens causing parts of an image to be out of focus. l) Binocular Vision: i) Most animals have their eyes situated on their head so that the left eye sees the left side and the right sees the right. (1) Humans use both eyes to focus on one object, giving us depth perception and the ability to appreciate the three-dimensional properties of objects. ii) In order to maintain binocular vision, light rays from an object must be striking similar areas on each retina

9 (1) Convergence: as an object gets closer to our eyes, in order to keep them focused on the proper spot on the retina, the eyes must both rotate medially (toward the nose) to maintain the binocular vision. This is called convergence. (2) Coordination of the extrinsic eye muscles is required to achieve convergence. m) Physiology of Vision i) Photoreceptors: Rods and Cones (Named for their shape) (1) Light is absorbed by a Photopigment (2) Photopigment: colored protein that undergoes changes when struck by light. Initiates events that lead to a receptor potential (a) Rhodopsin: photopigment in rods (3) There are three different photopigments for cones (resulting in color vision) (a) different colored light selectively strikes different cones (b) deficiency of one of these cone types results in color blindness (c) Each photopigment has two components (i) a glycoprotein called opsin and a derivative of vitamin A called Retinal 1. Vitamin A deficiencies can cause night blindness or nyctalopia (4) After a series of structural changes to the pigments, a receptor potential is generated in the rod or cone. (5) The photoreceptor potential synapses with the optic nerve (II) fibers in the wall of the retina and propagate toward the optic disc. n) Neural Pathways of Vision i) Nerve impulses leave the retina on axons at the optic disc, which become the optic nerve. ii) At the optic chiasm, nerve fibers from the temporal half of each retina do not cross, but travel to the ipsilateral thalamus. iii) Nerve fibers from the nasal half of each retina cross and continue to the lateral geniculate nucleus in the contralateral thalamus. iv) Axon collaterals posterior to the optic disc travel to the midbrain and coordinate pupil constriction, sleep patterns, and the movements of your eyes and head. v) The axons in the lateral geniculate nucleus leave the thalamus and form the optic radiation, which travels to the primary visual area in the ipsilateral occipital lobe. vi) The optic pathway is unique because for each eye, half of the neurons from the optic nerve travel through the optic chiasm and travel to the contralateral optic tract on their way to the cerebral cortex. However, the other half of the neurons do not enter the optic chiasm rather they travel to the ipsilateral optic tract on their way to the cerebral cortex. vii) So now let s consider the divisions of the retina. The retina consists of nerve fibers that lead to the optic nerve (CN II). The lateral half of the retina (near the temporal bone) is called the temporal retina. The medial half (near the nose) is called the nasal retina. There are also two fields of view for each eye (field of view consist of the space in front of the eye from where the light rays are coming). Since light travels in straight lines through the pupil toward the retina, the light rays reflecting off of objects from the temporal field of view strike the nasal retina. Conversely, light reflecting off of objects from the nasal field of view strike the temporal retina. (1) The optic nerve fibers from temporal half of the retina DO NOT ENTER THE OPTIC CHIASM, rather travel lateral to the chiasm and toward the ipsilateral optic tract. Therefore, images that are in the nasal field of view are perceived by the ipsilateral cerebral cortex. (2) The optic nerve fibers from the nasal half of the retina CROSS THE OPTIC CHIASM and travel to the contralateral optic tract. Therefore the images in the temporal field of view are perceived by the contralateral cerebral cortex. (3) To sum this up, if there is a lesion in one place of the optic pathway, it can be traced back to the retina and each retina s field of view. For example, if there is a lesion in the optic chiasm, that means that impulses from the bilateral nasal retinas are blocked and therefore the bilateral temporal fields of view become blind spots. IX. Hearing and Equilibrium a) Anatomy of the Ear: There are three principal regions of the ear i) External Ear (outer ear): collects sound waves and channels them to the inside

10 ii) Middle Ear: conveys sound vibrations to the oval window iii) Internal Ear (inner ear): houses the receptors for hearing and equilibrium iv) External Ear (outer ear) (1) Consists of the auricle, the external auditory canal and the eardrum (2) Auricle (aka Pinna): a flap of elastic cartilage shaped like the end of a trumpet and covered with skin. (a) The rim of the auricle is called the Helix (b) The interior portion of the auricle is called the lobule (3) External Auditory Canal: a 2.5cm, curved tube within the temporal bone that runs from the auricle to the eardrum. (4) Ceruminous glands: specialized sebaceous glands that secrete cerumen (aka earwax). (a) A combination of hairs and cerumen within the external auditory canal prevent dust and foreign objects from entering the ear. (b) Cerumen also has insect-repellent qualities that prevent most insects from entering the ear. (c) Cerumen usually dries up and falls out of the ear, however some people produce excess cerumen, which impacts on the eardrum and muffles sounds. v) Middle Ear (1) A small, air-filled cavity within the temporal bone, lined with epithelium. (2) Eardrum (aka tympanic membrane): a thin, translucent partition between the external auditory canal and the middle ear. (3) The middle ear is separated from the external ear by the eardrum and from the internal ear by two membrane-covered openings: the round window and oval window. (4) Auditory Ossicles: the three smallest bones in the body (a) Extend across the middle ear and are attached to the middle ear by ligaments (b) The auditory ossicles are attached to each other via synovial joints. (c) Auditory ossdicles distal to proximal: (i) The Malleus (aka hammer): attached distally to the interior surface of the eardrum and articulates proximally with the body of the incus. (ii) The Incus (aka anvil): articulates proximally with the head of the stapes (iii) The Stapes (aka stirrup): proximally fits into the oval window. (5) Round window: inferior to the oval window and enclosed by a membrane called the secondary tympanic membrane (a) Two tiny muscles attach to the ossicles (i) Tensor Tympani Muscle: limits movement and increases tension on the eardrum to prevent inner damage due to loud noises. Innervated by the mandibular branch of the Trigeminal Nerve (V) (ii) Stapedius Muscle: dampens large vibrations of the stapes, due to loud noises, protecting the oval window. Also decreases the sensitivity of hearing. 1. Therefore, paralysis of the stapedius muscle causes hyperacusia, or abnormally sensitive hearing. (iii) These muscles take a fraction of a second to contract and can therefore protect from prolonged loud noises, but not brief loud noises, such as a gunshot. (6) Auditory Tube (aka eustachian tube): connects the middle ear with the nasopharynx (upper portion of the throat). (a) normally closed at the pharyngeal end, except during yawning and swallowing. (b) Yawning or swallowing opens the tube, allowing air into the middle ear until the pressure inside is equal to the pressure outside (c) When pressure is equal, the eardrum vibrates in response to sound waves so we can hear, when pressure is not equal, you can experience pain, hearing impairment, ringing in the ears (tinnitus), and/or vertigo. (d) Pathogens, such as bacteria, can also travel from the throat to the middle ear. vi) Internal Ear (aka inner ear): also called the Labyrinth. (1) Two Main Divisions: (a) Bony Labyrinth: series of cavities in the temporal bone (i) Divided into three areas

11 1. The semicircular canals: contain receptors for equilibrium 2. The vestibule: contain receptors for equilibrium 3. Cochlea: contain receptor s for hearing (ii) The bony labyrinth contains perilymph, a fluid similar to CSF. (b) Membranous Labyrinth: a series of sacs and tubes containing a fluid called endolymph. (i) Endolymph is rich in potassium and plays a role in the generation of auditory signals. (c) The Vestibule: the oval, central portion of the labyrinth. (i) Within the vestibule are two sacs called the utricle and the saccule. 1. They are connected by a small duct (d) Semicircular Canals: superior and posterior to the vestibule. (i) There are three semicircular canals 1. Anterior, posterior, and lateral (e) Cochlea: a bony, spiral canal anterior to the vestibule, resembling a snail s shell encircling a central, bony core called the modiolus. (i) The cochlea is divided into three channels by a bony partition forming the letter Y. 1. Scala Vestibuli: is above the bony partition and connects to the middle ear at the oval window. 2. Scala Tympani: is below the bony partition and connects to the middle ear at the round window. 3. Cochlear duct (aka scala media): in between the wings of Y shape. Contains endolymph. a. The scala vestibuli and the scala tympani are both filled with perilymph and connected at a hole called the helicotrema. b. Vestibular Membrane: separates the cochlear duct from the scala vestibuli c. Basilar Membrane: separates the cochlear duct from the scala tympani d. Organ of Corti: a coiled sheet of epithelial cells containing hair cells, which serve as the receptors for hearing. e. Tectorial Membrane: a flexible, gelatinous membrane projecting over and in contact with the hair cells. b) Physiology of Hearing: Sound wave Transmission i) The auricle directs sound waves into the external auditory canal ii) Sound waves strike the eardrum (tympanic membrane) causing it to vibrate back and forth. (1) The eardrum vibrates slowly in response to low-frequency (low-pitched) sounds and quickly in response to high frequency (high-pitched) sounds. iii) The eardrum vibration transmits to the malleus, causing it to vibrate. In turn, the vibrating malleus causes the incus to vibrate, and so on with the stapes. iv) The stapes vibration causes the oval window to vibrate. v) The oval window vibration causes waves in the perilymph of the scala vestibuli, which travel to the helicotrema and transmit the waves to the scala tympani and eventually to the round window. vi) The waves deform the walls of the scala tympani and scala vestibuli causing pressure on the vestibular membrane, producing waves in the endolymph of the cochlear duct. vii) The waves in the endolymph cause the basilar membrane to vibrate, moving the hair cells of the Organ of Corti against the tectorial membrane. These hairs bend, opening up mechanically-gated potassium ion channels (potassium is rich in endolymph and therefore diffuses into the receptor cell) and produce receptor potentials that eventually lead to the generation of nerve impulses (action potentials) along the cochlear branch of the vestibulocochlear nerve (VIII).

12 c) Auditory Neural Pathway (one ear) i) First, the nerve impulses generated by auditory receptor potentials travel to the ipsilateral cochlear nuclei (origin of the cochlear nerve) of the pons and synapse with two different sets of neurons. ii) Second, one of those sets of neurons will travel to the ipsilateral superior olivary nucleus in the pons and synapse with neurons that will then travel to the ipsilateral inferior colliculus in the midbrain. The other set will cross the midlinein the medulla and travel to the contralateral inferior colliculus (through the contralateral superior olivary nucleus). (1) Sound waves from a single sound enter both ears. Since the ear closer to the sound gets the waves first, there is a slight time difference (microseconds) in when the nerve impulses reach each superior olivary nucleus. The difference in this time allows our brain to perceive from where a sound is coming. (2) Due to the crossing of nerve fibers from each ear at the superior olivary nucleus, from this point on, each part of the brain receives impulses from both ears. iii) Third, the impulse travels to the inferior colliculus of the midbrain, helping to determine where in space a sound is coming from, evaluating pitch in a person s voice, and mediate rapid head movement, reflexive to hearing a loud sound. iv) Fourth, to the medial geniculate body of the thalamus. v) And last to the primary auditory area in the superior temporal gyrus of the cerebral cortex where we can identify a sound. d) Physiology of Equlibrium i) There are two types of equlibrium ii) Static Equilibrium: maintaining the position of the body relative to the force of gravity. (1) The saccule and the utricle contain hairs covered by a gelatinous layer called the otolithic membrane. When the head moves, the otolithic membrane responds to gravity, bending the hairs and triggering receptor potentials. iii) Dynamic Equilibrium: maintaining body position in response to sudden movements, such as rotation, acceleration, and deceleration. (1) The membranous semi-circular canals along with the saccule and the utricle function in dynamic equilibrium. (2) An elevation in the semicircular canals, called the crista, contains hairs and is covered by a gelatinous material called the cupula. (3) As the head moves, the hairs bend as the cupula and endolymph lag behind. (4) This stimulates a receptor potential in the ampullary nerve to the Vestibular branch of the vestibulocochlear nerve (VII). (5) Many sections of the brainstem, cerebellum, and brain work together to coordinate movement and balance. (6) The receptor organs for equilibrium (saccule, utricle, and semi-membranous membranes) are collectively known as the Vestibular Apparatus.