Peripheral Nervous System, Autonomic NS, and Special Senses (Stanbridge University)

9/14/2023 Peripheral Nervous System and Autonomic Nervous System Fox Chapter 9 © Stanbridge University 2023 1  9/14/2023 Peripheral Nervous System (PNS) Objectives: List the different...

9/14/2023 Peripheral Nervous System and Autonomic Nervous System Fox Chapter 9 © Stanbridge University 2023 1  9/14/2023 Peripheral Nervous System (PNS) Objectives: List the different components of the PNS List the different types of receptors for the PNS Understand the regeneration of nerve axons in the PNS Define Wallerian degeneration List the cranial nerves and the main functions of each Describe how peripheral nerves come out of the spinal cord from rootlet to rami or nerve Understand the reflex arc © Stanbridge University 2023 2  9/14/2023 Peripheral Nervous System Marieb, 2019 © Stanbridge University 2023 3  9/14/2023 Classification of sensory receptors by stimulus type Mechanoreceptors: respond to mechanical force like touch, pressure, stretch, and vibration (includes sensing blood pressure) Thermoreceptors: respond to temperature changes Photoreceptors: respond to light, ex. in eyes Chemoreceptors: respond to chemicals in solution, ex. smell, taste, changes in blood chemistry Nociceptors: respond to potentially damaging stimuli that results in pain, ex. extreme heat and cold, excessive pressure, inflammatory chemicals © Stanbridge University 2023 4  9/14/2023 Classification of sensory receptors by location Exteroceptors: respond to stimuli outside the body located on or close to body surfaces; include touch, pressure, pain, temperature receptors from the skin and most of the receptors of the special senses (sight, hearing, equilibrium, smell, and taste) Interoceptors (visceroceptors): respond to stimuli from within the body ex. from internal viscera and blood vessels; monitor chemical changes, stretch, temperature, as well as other Proprioceptors: respond to internal stimuli; located in tendons, joints, skeletal muscles, ligaments, connective tissue coverings of bones and muscles (sometimes equilibrium is grouped in this category); respond to stretch © Stanbridge University 2023 5  9/14/2023 Classification of Sensory Receptors by Structure General senses: modified dendritic endings of sensory nerves; most common and largest in number Encapsulated and non-encapsulated (free nerve endings) Special senses: located in special sensory organs © Stanbridge University 2023 6  9/14/2023 Simple Receptors of the General Senses: Non-encapsulated/ Free Nerve Endings Located almost everywhere in the body Mostly located in epithelium and connective tissue Mostly respond to painful stimuli, changes in temperature, some respond to tissue movements by pressure, and itch Hot or cold outside range of thermoreceptors activates nociceptors and are perceived as painful Epithelial tactile complexes (Merkel cells and discs): where epidermis meets the dermis, respond to light pressure Hair follicle receptors: wrapped around hair cell follicles, sense bend of hair © Stanbridge University 2023 7  9/14/2023 Simple Receptors of the General Senses: Encapsulated Nerve Endings One or more sensory neuron fiber terminals enclosed in a connective tissue capsule Almost all are mechanoreceptors © Stanbridge University 2023 8  9/14/2023 Simple Receptors of the General Senses: Encapsulated Nerve Endings: Types Tactile corpusles or Meissner’s corpuscles: few sensory terminals surrounded by Schwann cells and connective tissue located in hairless skin areas like the palms of the hand (like the hair follicle version but in hairless skin) Lamellar corpuscles or Pacinian corpuscles: sense first deep pressure best at vibration or on/off stimulus Bulbous corpuscles or Ruffini endings: located in dermis, subcutaneous, and joint capsules look like tendon organs respond to deep continuous pressure Marieb, 2019 © Stanbridge University 2023 9  9/14/2023 Simple Receptors of the General Senses: Encapsulated Nerve Endings: Types Muscle spindles: proprioceptors in perimysium (located around muscle fascicles) contains a bundle of modified skeletal muscle fibers enclosed in connective tissue detect muscle stretch initiate a reflex that causes the muscle to contract Marieb, 2019 Tendon organs: proprioceptors in tendons close to the skeletal muscle/tendon area small bundles of tendon enclosed in a layered capsule with sensory terminals coiled around and between the fibers Triggers reflex Compression of Activates tendon causing Tension on tendon fibers nerve fibers organs contracting muscle to relax © Stanbridge University 2023 10  9/14/2023 Simple Receptors of the General Senses: Encapsulated Nerve Endings: Types Joint kinesthetic receptors: proprioceptors monitor stretch in capsules around synovial joints contains several receptor types lamellar and bulbous corpuscles free nerve endings similar receptors to tendon organs provide information on joint position and motion © Stanbridge University 2023 11  9/14/2023 Neural Integration in Sensory Systems: Sensation and Perception Marieb, 2019 © Stanbridge University 2023 12  9/14/2023 Pain Perception All people have the same pain threshold: start to feel pain at the same stimulus intensity Pain tolerance varies between individuals Visceral pain travels along the same pathways as somatic pain Referred pain occurs when pain stimuli is in one location and felt in another Brain interprets pain coming from the same area when innervated by sensory nerves from the same spinal cord segments © Stanbridge University 2023 13  9/14/2023 Structure of a Nerve (Marieb, 2019) © Stanbridge University 2023 14  9/14/2023 Peripheral Nerves Bundles of axons and connective tissue wrappings, myelin, blood vessels, and lymphatic vessels Carry somatic and visceral information Mixed nerves: contain sensory and motor fibers relaying information to and from the central nervous system(CNS); most nerves Sensory nerves: only contain sensory information going towards the CNS Motor nerves: only contain motor information going away from the CNS © Stanbridge University 2023 15  9/14/2023 Ganglia versus Nuclei Nuclei: collection of neuron cell bodies of the CNS Ganglia: collection of neuron cell bodies associated with the PNS Afferent nerve fiber ganglia: sensory information Efferent nerve fiber ganglia: motor information, usually from the autonomic nervous system © Stanbridge University 2023 16  9/14/2023 Regeneration of Peripheral Nerves Damage to neurons: Mature neurons do not divide If damage causes cell to die, other neurons stimulated by this neuron may die as well If cell body is intact, axons of peripheral nerves can regenerate (only in the PNS, not in the CNS) © Stanbridge University 2023 17  9/14/2023 Why can’t CNS axons regenerate? Most CNS axons never regenerate after injury Oligodendrocytes in the CNS have growth-inhibiting proteins that suppress axon growth Astrocytes at injury form scar tissue that blocks growth © Stanbridge University 2023 18  9/14/2023 Regeneration of axons in the PNS Neuronal cell body has rough endoplasmic reticulum break apart and the cell body swells due to increased protein synthesis Axons regenerate at 1.0mm per day Regrowth does not match the original The longer the distance between the injury, less likely to regenerate Other tissues block the growth © Stanbridge University 2023 19  9/14/2023 Regeneration of a nerve fiber in a peripheral nerve (Marieb, 2019) © Stanbridge University 2023 20  9/14/2023 Spinal Nerves 31 pairs of spinal nerves: C1-8, T1-12, L1-5, S1-5, Co1 Branches that exit the spinal cord and innervate the body Make up the PNS Send sensory information to the CNS Send motor information from the CNS Some form plexi: cervical, brachial, lumbar, sacral © Stanbridge University 2023 21  9/14/2023 Spinal Nerves C1-7: exit above the vertebrae bone they are named for C8: exits below C7 vertebrae (between C7 and T1 bones) T1 and below: exit inferior to the vertebrae they are named for © Stanbridge University 2023 22  9/14/2023 Nerves from the spinal cord out (Marieb, 2019) © Stanbridge University 2023 23  9/14/2023 Nerves from the Spinal Cord out Ventral and dorsal rootlets attach along the length of the corresponding spinal cord segment, then form dorsal and ventral roots: Ventral roots→ motor/efferent fibers from the ventral horn motor neurons to skeletal muscle Dorsal roots → sensory/afferent fibers from the dorsal root ganglia form the peripheral receptors to the spinal cord Roots pass laterally from the spinal cord and unite just distal to the dorsal root ganglion to form a spinal nerve before leaving the vertebral column through the intervertebral foramina Dorsal and ventral rami (each both motor and sensory) are after the spinal nerve: dorsal rami supply posterior body and trunk; ventral rami supply rest of the trunk and limbs © Stanbridge University 2023 24  9/14/2023 Nerves from the spinal cord out forming a plexus Dorsal (sensory) and ventral (motor) rootlets of spinal nerve Dorsal (sensory) and ventral (motor) roots (respectively) Spinal nerve (both motor and sensory) Dorsal and ventral rami (each both motor and sensory) © Stanbridge University 2023 25  9/14/2023 Tiny meningeal branch enters the vertebral canal to innervate the meninges and blood vessels of the canal -Ventral rami (not from T2-T12) form cross-over in plexuses © Stanbridge University 2023 26  9/14/2023 Stretch Reflex (Fox, 2021) © Stanbridge University 2023 27  9/14/2023 Golgi Tendon Organ (Fox, 2021) © Stanbridge University 2023 28  9/14/2023 The Tendon Reflex (Marieb, 2019) © Stanbridge University 2023 29  9/14/2023 The Crossed- Extensor Reflex Marieb, 2019 © Stanbridge University 2023 30  9/14/2023 Cranial Nerves All serve only the head and neck, except the vagus nerves that goes into the abdomen 12 pairs of cranial nerves Some are sensory, Some motor, and some Both motor and sensory first two pairs attach to the forebrain, Rest attach to the brainstem Marieb, 2019 © Stanbridge University 2023 31  9/14/2023 Cranial Nerves (Marieb, 2019) © Stanbridge University 2023 32  9/14/2023 Cranial Nerves basic functions CN I: olfactory nerve: sense of smell CN II: optic nerve: visual acuity CN III: oculomotor nerve: motor fibers to 4 extrinsic eye muscles, opens the eye lid, pupil constriction, eye lens shape/focus CN IV: trochlear (means pulley): superior oblique eye muscle CN V: trigeminal nerve: mainly sensory, has 3 branches: 1.Ophthalmic: cornea, nose, scalp 2.Maxillary: teeth, cheek, scalp, chin 3.Mandibular: sensory: anterior tongue (not taste), lower teeth; motor: chewing © Stanbridge University 2023 33  9/14/2023 Cranial Nerves basic functions (continued) CN VI: abducens nerve: motor to the lateral rectus eye muscle CN VII: facial nerve: muscles of the face, upper eye lid closes, salivary glands, taste buds on anterior tongue CN VIII: vestibulocochlear nerve: sensory mainly: balance and hearing CN IX: glossopharyngeal nerve: sensory and taste tongue: swallow (elevates the pharynx), salivary glands, receptor for oxygen, carbon dioxide, and blood pressure (BP) © Stanbridge University 2023 34  9/14/2023 Cranial Nerves basic functions (continued) CN X: vagus nerve: heart, lungs, abdominal viscera; heart rate, breathing, gastrointestinal tract; swallow, voicebox (proprioceptive and motor); receptors for BP and respiration CN XI: accessory nerve: innervates and provides motor function to the sternocleidomastoid muscle and the trapezius muscle CN XII: hypoglossal nerve: muscles of the tongue (“glossal” means tongue) © Stanbridge University 2023 35  9/14/2023 Autonomic Nervous System (ANS) Objectives: Understand the ANS versus the somatic system Explain the differences between the sympathetic and parasympathetic nervous systems in function and pathway © Stanbridge University 2023 36  9/14/2023 Peripheral Nervous System (Marieb, 2019) © Stanbridge University 2023 37  9/14/2023 Autonomic Nervous System Regulates the action of glands, smooth muscles, vessels, and heart muscle Preganglionic neuron connects spinal cord or brainstem to ganglion Postganglionic neuron connects ganglion to effector © Stanbridge University 2023 38  9/14/2023 Effectors of the Somatic and Autonomic Pathways Effectors Somatic Motor pathways Skeletal muscles Autonomic pathways Parasympathetic and Sympathetic Cardiac muscle, smooth muscle, and glands © Stanbridge University 2023 39  9/14/2023 Sympathetic versus Parasympathetic Nervous Sympathetic Parasympathetic Involves E activities – exercise, Involves the D activities – digestion, excitement, emergency, embarrassment defecation, diuresis Fight-or-flight response Rest and Digest Increase blood pressure Decrease blood pressure Dilates pupils Constricts pupils Stimulates sweating Leaves sweating alone Decreases digestion Increase digestion Increase Heart Rate Decrease Heart Rate Dilates Lungs Constricts Lungs © Stanbridge University 2023 40  9/14/2023 Somatic and ANS Comparison (Marieb, 2019) © Stanbridge University 2023 41  9/14/2023 Differences between the Sympathetic and Parasympathetic Systems Sites of origin: parasympathetic fibers originate in the brain and sacral spinal cord; sympathetic fibers originate in the thoracic and lumbar regions of the spinal cord (in the lateral horn and leave through the ventral root) Length of fibers: parasympathetic has long preganglionic and short postganglionic fibers; sympathetic has short preganglionic and long postganglionic Location of their ganglia: most parasympathetic ganglia are located in or near the visceral effector organs; sympathetic ganglia are located close to the spinal cord © Stanbridge University 2023 42  9/14/2023 Parasympathetic © Stanbridge University 2023 Nervous System (Marieb, 2019) 43  9/14/2023 Sympathetic © Stanbridge University 2023 Nervous System (Marieb, 2019) 44  9/14/2023 Parasympathetic Nervous System (Marieb, 2019) © Stanbridge University 2023 45  9/14/2023 Sympathetic NS: Three pathways of innervation 1. Synapse at same level in trunk ganglion 2. Synapse in trunk ganglion at a higher or lower level 3. Pass through the sympathetic trunk to synapse in a collateral ganglion anterior to the vertebral column (Marieb, 2019) © Stanbridge University 2023 46  9/14/2023 Sympathetic Nervous system (not shown genitalia) Marieb, 2019 © Stanbridge University 2023 47  9/14/2023 Autonomic Nervous System Hypothalamus is the main integration center of the ANS Brain stem exerts the most direct control over the ANS There is a functional overlap between the somatic and ANS: They differ in their effectors, efferent pathway including ganglia, and target organ response to the neurotransmitter There is rarely an all-or-none function of the sympathetic and parasympathetic nervous systems, they usually in homeostasis and have only fine adjustments © Stanbridge University 2023 48  9/14/2023 Special Senses Fox, Chapter 10 © Stanbridge University 2023 49  9/14/2023 Objectives: Identify the main special senses Be able to name the various main components of each special sense and follow its pathway from reception to interpretation Identify the role of various cranial nerves specific to each special sense © Stanbridge University 2023 50  9/14/2023 Special Senses The eye and vision The ear: hearing and balance Chemical senses: taste and smell © Stanbridge University 2023 51  9/14/2023 The Eye and Visual Sense Extrinsic Eye Muscles: Superior, inferior, and medial rectus muscles: innervated by the oculomotor nerve (CN III) Lateral rectus muscle: innervated by the abducens nerve (CN VI) Inferior oblique muscle: innervated by the oculomotor nerve (CN III) Superior oblique muscle: innervated by the trochlear nerve (CN IV) © Stanbridge University 2023 52  9/14/2023 Extrinsic Eye Muscles: Location and Action Marieb, 2019 © Stanbridge University 2023 53  9/14/2023 Structure of the Eyeball 1. Fibrous outer layer: Sclera “white of the eye”: protects and shapes the eye ball; provides an anchor for extrinsic eye muscles Cornea: transparent or a window: major part of the light bending of the eye; well supplied with nerve ending especially for pain but no blood vessels Marieb, 2019 © Stanbridge University 2023 54  9/14/2023 Structure of the Eyeball 2. Vascular layer: Choroid: full of blood vessels and nourishes all eye layers; brown pigment helps to absorb light (preventing scattering and reflecting of light in the eye) Ciliary body: intrinsic eye muscle; anteriorly the choroid becomes this thickening ring of tissue that encircles the lens; mainly smooth muscle that controls the shape of the lens; also secretes fluid into the anterior eye Iris: intrinsic eye muscle: visible colored part of the eye, most anterior part of this layer; pupil which is an opening, allows light to enter the eye; includes two smooth muscle layers Dilate the pupil for distant vision or in dim light Constrict the pupil for close vision or in bright light © Stanbridge University 2023 55  9/14/2023 Vascular Layer of the Eye: Pupil Dilation and Constriction Marieb, 2019 © Stanbridge University 2023 56  9/14/2023 Function of the lens Convex on both sides Changes shape to precisely focus light on the retina Enclosed in a thin, elastic capsule and held in place just behind the iris by the ciliary zonule Avascular The lens and the ciliary zonule divide the anterior and posterior segments of the eye The iris divides the anterior segment into the anterior and posterior chambers, both are filled with aqueous humor, however the posterior chamber is between the iris and the lens and is smaller © Stanbridge University 2023 57  9/14/2023 Structure of the Eyeball 3. Inner Layer/Retina: Location of neurons processing the light response Glia Pigmented and neural layer (directly involved with vision) ¼ billion photoreceptors that transduce/ convert light energy: Rods: more sensitive to light so respond best in dim light and in peripheral vision, Not sharp images and no color vision Cones: function best in bright light, have high resolution and color vision © Stanbridge University 2023 58  9/14/2023 Inner Layer of the Eye/ Retina Optic disc: where the optic nerve exits the eye Called the “blind spot” because there are no photo receptors located there 2/3rs of blood supply to the neural layer is from the central artery and vein that enter and leave in the center of the optic nerve Rest of the blood supply is from the choroid supply © Stanbridge University 2023 Marieb, 2019 59  9/14/2023 The process of vision Light comes into the eye The medial part of the visual field from each eye crosses in the optic chiasma The lateral part of each eye’s visual field travels together on the right and left sides in optic tracts Most neurons synapse in the thalamus (lateral geniculate nuclei) Neurons travel to the primary visual cortex in the occipital lobe of the brain ending in an optic radiation There is other visual processing that occurs in surrounding areas and other lobes of the brain and the hypothalamus © Stanbridge University 2023 60  9/14/2023 Inferior view of the brain, right side dissected out to see internal structures (Marieb, 2019) © Stanbridge University 2023 61  9/14/2023 Hearing and Ear Anatomy Outer or external ear: hearing only Middle ear: hearing only Inner ear: hearing and equilibrium Marieb, 2019 © Stanbridge University 2023 62  9/14/2023 Hearing and Balance Ear Anatomy: External ear: only involved with hearing Includes: 1. pinna or auricle (ear itself) includes 2. helix (rim) 3. lobule (lobe) Funnels sound into the external acoustic meatus (auditory canal) extends from auricle to ear drum (Fox, 2021) © Stanbridge University 2023 63  9/14/2023 Hearing and Balance Ear Anatomy: External ear: only involved with hearing Ear drum or tympanic membrane divides the outer and middle ear transfers energy waves from sound, by vibrating, to the bones of the middle ear (Fox, 2021) © Stanbridge University 2023 64  9/14/2023 Hearing and Balance Ear Anatomy: Middle ear: only involved with hearing Medial part of each is a bony wall with two openings (oval and round windows) The anterior portion of the wall contains the opening to the pharyngotympanic/auditory tube (Eustachian tube) links the middle ear to the nasopharynx (Fox, 2021) © Stanbridge University 2023 65  9/14/2023 Hearing and Balance Ear Anatomy: Middle ear: only involved with hearing Pharyngotympanic tube is flat normally, but yawning and swallowing opens it briefly to equalize the pressure in the middle ear cavity with the external air pressure important because the ear drum only vibrates freely if the pressure on each side is equal (Fox, 2021) © Stanbridge University 2023 66  9/14/2023 Middle Ear Mucosa of middle ear is continuous with mucosa of the throat (pharynx) Bones in the middle ear are called auditory ossicles: malleus (hammer), incus (anvil), and stapes (stirrup) Handle of the malleus is secured to the ear drum and the stapes fits into the oval window → translate vibration of sound to the oval window (Marieb, 2019) © Stanbridge University 2023 67  9/14/2023 Hearing and Balance Ear Anatomy: Internal ear (labyrinth): hearing and equilibrium Contains fluid (perilymph) which moves with the vibration from the bones of the middle ear Hearing receptors become excited (Fox, 2021) © Stanbridge University 2023 68  9/14/2023 Hearing and Balance Ear Anatomy: Internal ear (labyrinth): hearing and equilibrium Bony labyrinth: channels that turn and move through the bone, hallow space inside Vestibule, semicircular canals, cochlea Membranous labyrinth continuous sacs of membrane and ducts within the bony labyrinth, outside is perilymph, and inside is endolymph (Fox, 2021) © Stanbridge University 2023 69  9/14/2023 Hearing Cochlea: Bony chamber contains the cochlear duct and the Organ of Corti (receptor organ of hearing) Cochlea divided into 3 chambers (Fox, 2021) © Stanbridge University 2023 70  9/14/2023 The Process of Hearing Transmits Vibrates the Moves to Interpreted Sound Vibrates the through the basilar the auditory in the waves tympanic CN VIII oval window membrane reflex center primary funneled membrane projects to to the inner which of the auditory into the ear and 3 bones the medulla ear and activates midbrain, cortex in the via the of the and pons perilymph the hair then to temporal outer ear middle ear vibrates cells thalamus lobe © Stanbridge University 2023 71  9/14/2023 Marieb, 2019 © Stanbridge University 2023 72  9/14/2023 Balance (Inner Ear) Vestibule: Two membranous labyrinth sacs 1. saccule (vertical movement) 2. utricle (horizontal movement and head tilt) Respond to the pull of gravity (linear acceleration) and changes in head position (key in controlling posture) Contain hair cells with microvilli Hair cells synapse with the Fox, 2021 vestibular nerve (CN VIII) © Stanbridge University 2023 73  9/14/2023 Balance (Inner Ear) Semicircular Canals: Posterior and lateral to the vestibule; each contain an ampulla (equilibrium receptor region); respond to rotational movements of the head Anterior: right angle to the posterior canal in vertical plane; detect head up or down Posterior: right angle to the anterior canal in the vertical plane; detect head side tilt right and left (Fox, 2021) © Stanbridge University 2023 74  9/14/2023 Balance (Inner Ear) Semicircular Canals: Lateral: horizontally oriented; detect rotation of head Endolymph flows and bends the cupula (full of hair cells) with movement triggering the vestibular nerve (wrapped around hair cell base) (Fox, 2021) © Stanbridge University 2023 75  9/14/2023 Balance and Equilibrium Transmission to the Brain and Reflexes (Marieb, 2019) © Stanbridge University 2023 76  9/14/2023 Chemical Senses Smell (olfaction) Taste (Gustatory Sense) © Stanbridge University 2023 77  9/14/2023 Smell/olfaction Air enters through the nasal cavity but must make a sharp turn to hit the olfactory receptors before going to the respiratory pathway Olfactory sensory neurons are located on the superior part of the nasal conchae Olfactory cilia (don’t move much) increase the surface area on the receptors and are covered by thin mucus Mucus traps the odorants The receptors form the olfactory nerve (CN I) It projects through the ethmoid bone and synapses in the olfactory bulbs © Stanbridge University 2023 78  9/14/2023 (Fox, 2021) © Stanbridge University 2023 79  9/14/2023 Pathway of Smell The nerves that synapse in the olfactory bulbs, synapse with mitral cells (refine signal and amplify it before relaying it) contained in glomeruli Glomeruli have neurons from the same kind of receptor (like a single part of the odor) Signal flows from the bulbs in olfactory tracts to the olfactory cortex (in temporal lobe) Two possible pathways after that: 1. Frontal lobe for interpretation and identification 2. Hypothalamus, and other parts of the limbic system for emotional responses to the odor could be protective causing sneeze or choking (unpleasant odors) trigger the gastrointestinal tract causing digestion (appetizing odors) fight or flight trigger (smells associated with danger, i.e. smoke, gas) © Stanbridge University 2023 80  9/14/2023 Taste or the Gustatory Sense We have about 10,000 taste buds in our mouth, the most on our tongue Located on papillae projections on the tongue Each taste bud has gustatory epithelial cells the receptor cells, long hairs project from them and extend through a taste pore to the surface of epithelium, saliva covers epithelium Sensory dendrites coil around the cells, each fiber receives input from several gustatory cells in a taste bud Some release neurotransmitter (NT) and some release ATP that acts as a NT © Stanbridge University 2023 81  9/14/2023 Papillae on the Tongue and Taste Bud Anatomy (Marieb, 2019) © Stanbridge University 2023 82  9/14/2023 5 Basic Tastes Sweet: sugars, alcohols, saccharin, some amino acids, some lead salts Sour: acids and their hydrogen ions in solution Salty: metal ions (inorganic salts), ex. table salt Bitter: alkaloids (quinine, nicotine, caffeine, morphine, strychnine) and non- alkaloid (aspirin) Umami: amino acids – glutamate and aspartate “beef taste”, aged cheese and MSG (monosodium glutamate) We may be able to taste long-chain fatty acids from lipids We respond quicker to protective tastes © Stanbridge University 2023 83  9/14/2023 Taste process Must be dissolved in saliva, diffuse into a taste pore, and contact gustatory hairs to be sensed Taste touches gustatory cells → graded depolarizing potential → NT release → triggers action potential in neurons Depolarization mechanism depends on the taste: Salty: sodium ions move through channels Sour: hydrogen ion binding allows other cations to move through as well Bitter, sweet, and umami activation leads to calcium ion release from intracellular stores → cation channels in the cell membrane to open → depolarize cell → release ATP as a NT © Stanbridge University 2023 84  9/14/2023 Taste Process continued Anterior 2/3rds of the tongue → branch of the facial nerve (CN VII) Posterior tongue and pharynx → glossopharyngeal nerve (CN IX) Epiglottis and lower pharynx (few taste buds) → vagus nerve (CN X) All these synapse in the medulla trigger increased saliva secretion in the mouth, and gastric juice into the stomach Then travel to the thalamus, then to the gustatory cortex in the insula lobe They also project to the hypothalamus and the limbic system © Stanbridge University 2023 85  9/14/2023 Taste Pathway Marieb, 2019 © Stanbridge University 2023 86  9/14/2023 Taste affected by: Smell (80% of our taste is smell) Thermoreceptors, mechanoreceptors, and nociceptors in the mouth also affect taste by enhancing or distracting it © Stanbridge University 2023 87