Exam 1 Material *REVIEW CRANIAL NERVES
Chapter 16 – Sense Organs
Properties of Receptors
 Receptor – any structure specialized to detect a stimulus (simple nerve ending or complex
sense organ)
 Sensory receptors are transducers converting stimulus energy into electrochemical energy
= sensory transduction
 Information about stimulus that can be conveyed:
o Modality or type of stimulus or sensation (vision, hearing, taste)
Transduce is o Location of stimuli
 Each sensory receptor receives input from its receptive field
to change
 Brain identifies site of stimulation = sensory projection
 Post central gyrus of parietal lobe
o Intensity (frequency, numbers of fiber & which fibers – low threshold vs. high)
o Duration = change in firing frequency over time
 Phasic receptor – burst of activity initially & quickly adapt and stop
sending signals (smell & hair receptors)
 i.e. getting used to a bad smell
 Tonic receptor – adapt slowly, generate impulses continually
(proprioceptor)
Classification of Receptors
 By modality: chemoreceptors (receiving signals from chemicals), thermoreceptors
(receptors for heat), nociceptors (receptors for pain), mechanoreceptors (stimulate nerves
by mechanical ways – like sitting down) and photoreceptors (visual receptors)
 By origin of stimuli:
o Interceptors = detect internal stimuli
o Proprioceptors = sense position & movements of body
o Exteroceptors = sense stimuli external to body
 By distribution:
o General (somesthetic) sense – widely distributed
o Special senses – limited to head (hearing, taste, smell)
Unencapsulated Nerve
 Free nerve endings include
o Warm, cold, pain
 Tactile (Merkel) discs are associated with cells at base epidermis - tonic
 Hair receptors monitor movement of hairs - phasic
Encapsulated Nerve
 Dendrites wrapped by glial cells or connective tissue, more sensitive
o Tactile (Meissner) corpuscles
 Light touch & texture - phasic
o Krause End Bulb
 Tactile corpuscles in mucous membranes – phasic
o Lamellated (Pacinian) corpuscles
 Deep pressure, stretch, tickle, and vibration – phasic
o Ruffini corpuscles
 Heavy touch, pressure, joint movements & skin stretching – tonic
Somesthetic Projection Pathways
 First-order neuron or afferent neuron
o from below head, enter the dorsal horn of spinal cord via spinal nerves
o from head, enter pons and medulla from cranial nerve
o touch, pressure & proprioception are carried on large, fast, myelinated axons
o heat & cold are carried on small, unmyelinated, slow fibers
 Second-order neuron
o decussation (crossing of fibers) of signals to opposite side in spinal cord or
medulla
o end in thalamus, except for proprioception (cerebellum)
 Third-order neuron
o extend from thalamus to primary somesthetic cortex of cerebrum on contralateral
side
Pain
 Found in all tissues except the brain
 Somatic pain arises from skin, muscles & joints
 Visceral pain from stretch, chemical irritants or ischemia (lack of blood flow to tissues)
of viscera (poorly localized)
 Referred pain is misinterpreted pain
o brain “assumes” pain is coming from skin or superficial sites
o heart pain felt in left shoulder or arm (or even the thumb!) because both send pain
input to the interneurons of spinal cord segments T1 to T5
CNS Modulation of Pain
 Intensity of pain is affected by state of mind
 Endogenous opioids (enkephalins, endorphins, dynorphins)
o far more potent that opium, morphine, or heroin
o produced by CNS and other organs under stress
o found especially in dorsal horn of spinal cord, explaining the spinal gating of pain
o act as neuromodulators blocking the transmission of pain
 Spinal gating stops pain signals at dorsal horn
o descending analgesic fibers from reticular formation travel down reticulospinal
tract to dorsal horn
Cephalgia –
 secrete inhibitory substances, enkephalins and dynorphins, that block pain
headache (head
fibers from secreting substance P
pain)
 pain signals never ascend
o dorsal horn fibers inhibited by input from mechanoreceptors
 i.e. rubbing a sore arm reduces pain because you’re inhibiting pain
receptors
The Chemical Sense – Taste
 Gustation is the sensation of taste resulting from the action of chemicals on the taste buds
 Lingual papillae (4 types)
o filiform (no taste buds)
 important for texture
o foliate (no taste buds)
o fungiform
 at tip & sides of tongue
o vallate (circumvallate)
 7-12 at rear of tongue
 contains 1/2 of taste buds
Physiology of Taste
 To be tasted, molecules must dissolve in saliva
 5 primary sensations: salty, sweet, sour, bitter & umami (meaty taste of amino acids such
as MSG)
o taste is also influenced by food texture, aroma, temperature, and appearance
 mouthfeel (texture) is detected by lingual nerve branches of Cranial Nerve
V in papillae
o hot pepper stimulates free nerve endings (pain)
 Sweet tastes concentrated on tip of tongue, salty & sour on lateral margins of tongue &
bitter (alkaloid toxins) at rear
o all tastes can be detected throughout the tongue surface
o the “bitter” taste can often be from poisonous aspects of plants – defense
mechanism
 Mechanisms of action
o sugars, alkaloids & glutamates bind to receptors & activate 2nd messenger
systems (i.e. cyclic AMP)
o sodium & acids penetrate cells & depolarize them directly
 Which taste sensation would be most impaired by:
o Damage to the facial nerve? Sweet (facial nerve is front 2/3 of tongue
o Damage to the glossopharyngeal nerve? Bitter (glossopharyngeal nerve is back
1/3 of the tongue)
 Taste on the palate, pharynx, and epiglottis is carried by the vagus nerve
The Chemical Sense – Smell
 Receptor cells for olfaction form olfactory mucosa
o smell is highly sensitive (more so in women than men)
 dormitory effect – study done that women who are often close to each
other tend to have menstrual cycles sync up due to sense of pheromones
2
 Covers 5cm of superior concha & nasal septum
Cells of the Olfactory Mucosa and Bulb
 Olfactory cells
 10 to 20 million cells
o neurons with 20 cilia called olfactory hairs
 binding sites for odor molecules in thin layer of mucus, immobile cilia
o axons pass through cribriform plate and are known collectively as Cranial Nerve
1?
o Olfactory cells survive 60 days
o These are the only neurons in body exposed to external environment
 Basal cells divide & differentiate into new olfactory cells (unusual for neurons)
 A fracture of which cranial bone cause anosmia? Anosmia is without smell so ethmoid
bone
Pitch and Loudness
 Frequency at which party of the ear vibrate gives us sense of pitch (low- or high- pitched
sounds)
o Hearing range is 20-20,000 Hz (cycles/sec)
 Human speech tends to fall within 1500-4000 Hz where hearing is most
sensitive
 Loudness if perception of intensity of sound energy
Outer Ear
 Fleshy auricle (pinna) directing air vibrations down auditory canal (external auditory
meatus)
Middle Ear
 Air-filled cavity in temporal bone separated from air outside the head by tympanic
membrane (tympanum, eardrum)
o 1 cm in diameter, slightly concave, freely vibrating membrane
 Tympanic cavity filled with air by auditory tube (Eustachian tube) connected to
nasopharynx
o opens during swallowing or yawning to equalize air pressure on both sides of
eardrum
o offers a path for upper respiratory infections to spread from the throat to the
middle ear, resulting in otitis media (inflammation of middle ear)
 Auditory ossicles span tympanic cavity
o malleus attached to eardrum, incus, stapes attached to membranous oval window
of inner ear
 Auditory ossicles concentrate sound waves and conduct them to oval window
Inner Ear
 Passageways in temporal bone = bony labyrinth
 Fleshy tubes lining bony tunnels = membranous labyrinth
o filled with endolymph (similar to intracellular fluid)
o floating in perilymph (similar to cerebrospinal fluid)
Anatomy of the Cochlea
 Stereocilia of hair cells embedded in gelatinous tectorial membrane.
 Hearing comes from inner hair cells -- outer ones adjust cochlear responses to different
frequencies increasing precision— not neurons
Physiology of Hearing -- Middle Ear
 Eardrum vibrates quite easily
o 18 times the area of the oval window
 creates enough force/unit area (conducted through auditory ossicles to the
stapes’ connection to the oval window) in order to vibrate the endolymph
in the scala vestibuli
 The auditory ossicles do NOT amplify the vibration, they concentrate it
 Protection of cochlea by muscle contraction in response to loud noises (tympanic reflex)
o tensor tympani pulls eardrum inward, tightening it
o stapedius reduces mobility of stapes
o 40 msec latency
o designed for slowly building noises like thunder not gunshots (irreversible
damage by breaking stereocilia)
 does not protect us from sustained loud noises such as music, either
o muscles also contract while speaking – so we can hear others
Potassium Gates of Cochlear Hair Cells
 Stereocilia bathed in high K+ concentration (endolymph) creating electrochemical
gradient from tip to base
 Stereocilia of OHCs have tip embedded in tectorial membrane which is anchored to
bony core of cochlea called the modiolus
 Movement of basilar membrane bends the cell’s many stereocilia
 Bending of taller stereocilia pulls on protein tip links of shorter neighboring ones and
opens ion channels
 K+ flows in -- depolarizing it & causing release of neurotransmitter stimulating sensory
dendrites at its base
Sensory Coding
 Loudness produces more vigorous vibrations & excites more hair cells over a larger area
o triggers higher frequency of action potentials from a given location on the basilar
membrane
o brain interprets this as louder
 Determination of pitch depends on which part of basilar membrane is vibrated at peak
amplitude of standing wave
o membrane is narrow & stiffer at basal end (collagen)
 brain interprets signals from IHC at basal end as high-pitched
o at distal end is 5 times wider & more flexible
 brain interprets signals from IHC at distal end as low-pitched
Auditory Projection Pathway
 Cochlear nerve joins vestibular nerve to form vestibulocochlear nerve (cranial nerve #8)
 Lead to cochlear nuclei in pons and synapse with second-order neurons there or in the
inferior colliculus of the midbrain
 Third-order neurons in the inferior colliculus ascend to the thalamus
o Any sensation you are conscious of must go through the thalamus on the way to
the cerebral cortex.
 Fourth-order neurons (unusual for afferent pathways to have 4) finish the pathway to the
primary auditory cortex in the temporal lobes
Deafness
 Term encompasses any hearing loss: mild and temporary to complete and irreversible
 Conduction deafness
o due to interference of conduction of vibrations to inner ear
o damaged eardrum, otitis media, blockage of auditory canal with cerumen
(cerumen), otosclerosis (fusion of ossicles)
 Sensorineural (nerve) deafness
o due to death of hair cells or any nervous element involved in hearing
o occupational hazard for musicians, factory and construction workers
Equilibrium
 Control of coordination and balance
 Receptors in vestibular apparatus
o semicircular ducts contain cristae
o saccule & utricle contain maculae
 Static equilibrium is perception of head orientation
o perceived by macula

Dynamic equilibrium is perception of motion or acceleration
o linear acceleration perceived by macula
o angular (rotational) acceleration perceived by crista
The Saccule and Utricle
 Saccule & utricle chambers contain maculae
o patch of hair cells with their stereocilia & one kinocilium buried in a gelatinous
otolithic membrane weighted with granules (calcium carbonate) called otoliths
o otoliths add to the density & inertia and enhance the sense of gravity and motion
Macula Sacculi and Macula Utriculi
 With the head erect, stimulation is minimal, but when the head is tilted, weight of
membrane bends the stereocilia (static equilibrium)
 When car begins to move at green light, linear acceleration is detected since heavy
otoliths of the utricle lag behind (one type of dynamic equilibrium) (horiz. oriented)
 When ascending in an elevator, the otoliths of the saccule lag behind. (vert. oriented)
Crista Ampullaris of Semicircular Ducts
 Crista ampullaris consists of hair cells buried in a mound of gelatinous membrane, the
cupula (one in each duct)
Crista Ampullaris & Head Rotation
 As head turns, the endolymph lags behind pushing the cupula and stimulating its hair
cells
 The macula and the crista CANNOT detect constant, steady motion—they only detect
changes in motion. Why? constant rotation will be very smooth so you can’t tell you’
Equilibrium Projection Pathways
 Hair cells of macula sacculi, macula utriculi & semicircular ducts synapse on vestibular
nerve
Vision and Light
 Vision (sight) is perception of light emitted or reflected from objects in the environment
 Visible light is electromagnetic radiation with wavelengths from 400 to 750 nm
 Light must cause a photochemical reaction
to produce a nerve signal our brain can notice
o radiation with wavelengths below 400 nm (ultraviolet, etc.) has so much energy it
kills cells
o radiation with wavelengths above 750 nm (infrared, etc.) has too little energy to
cause photochemical reaction (it only warms the tissue)
 Note: shorter wavelength correlates with more energy
Eyebrows and Eyelids
 Eyebrows provide facial expression, protection from glare & perspiration
 Eyelids (palpebrae)
o block foreign objects, help with sleep, blink to moisten
o meet at corners (commissures/canthi)
o consist of orbicularis oculi muscle & tarsal plate covered with skin outside &
conjunctiva inside
 conjunctiva is transparent mucous membrane that lines the eyelids and
covers anterior surface of eyeball except cornea
 Richly innervated & vascular (heals quickly)
 Conjunctivitis is pink eye

What mechanism involving the conjunctiva would produce a
blood-shot eye? vasodilation
o tarsal glands secrete oil that reduces tear evaporation
o eyelashes help keep debris from the eye
Innervation of Extrinsic Eye Muscles
 Superior oblique muscle – Trochlear Nerve (IV)
 Lateral rectus muscle – Abducens Nerve (VI)
 Levator palpebra superioris muscle (elevate eyelid), superior rectus muscle, medial rectus
muscle, inferior rectus muscle, inferior oblique muscle – Oculomotor Nerve (III)
The Tunics (Layers) of the Eyeball
 Fibrous layer (tunica fibrosa) = sclera and cornea
 Vascular layer (tunica vasculosa) = choroid, ciliary body & iris - also called the uvea
 Internal layer (tunica interna) = retina and optic nerve
The Optical Components
 Series of transparent structures that bend or refract light rays to focus them on the retina
o cornea is transparent covering of anterior surface of eyeball (which does most of
the refracting!)
Detached retina
o
aqueous humor is clear serous fluid filling area in front of lens (between lens and
is when the
cornea)
retina pulls away
o lens is suspended by ring of suspensory ligaments
 capable of changing shape to help focus light rays
Glaucoma is
 more rounded when no tension on it (ciliary muscle contracted,
increased intraocular
near vision)
pressure- smashes

somewhat flattened normally due to pull of suspensory ligaments
retina against
(ciliary muscle relaxed, far vision)
eyeball and can lead
o vitreous humor (vitreous body) is jelly filling the space between the lens and
to blindness
retina
Aqueous Humor
 Serous fluid produced by ciliary body flows from posterior chamber through pupil to
anterior chamber -- reabsorbed into canal of Schlemm (scleral venous sinus)
The Neural Components
 Neural apparatus includes the retina & optic nerve
 Retina forms as an outgrowth of the diencephalon
o attached only at optic disc where optic nerve begins and at ora serrata (its anterior
margin) Retina is actually part of the brain. Only part seen without dissection.
o pressed against rear of eyeball by vitreous body
 Detached retina
o blow to head or lack of sufficient vitreous body
o blurry areas in field of vision
o leads to blindness if uncorrected due to disruption of blood supply
Ophthalmoscopic Examination of Eye
 Cells on visual axis of eye = macula lutea (3 mm area)
o fovea centralis is the center of macula where most finely detailed images are seen
Lutea - yellow
due to packed receptor cells
 Eye exam provides direct evaluation of blood vessels
Test for Blind Spot
 Optic disk or blind spot is where optic nerve exits the posterior surface of the eyeball
o no receptor cells are found in optic disk
 Blind spot can be seen using the above illustration
o with right eye closed, stare at X and move toward screen until red dot disappears
 Visual filling is the brain filling in the green bar across the blind spot area (better than a
hole in visual field)
Formation of an Image
 Light must pass through the lens to form tiny inverted (upside down) image on retina
 Pupillary constrictor is smooth muscle cells encircling the pupil
  know the difference
o parasympathetic stimulation narrows the pupil
between constrictor and dilator
 Pupillary dilator is spokelike myoepithelial cells
o sympathetic stimulation widens the pupil to admit more light
 Remember this using fight-or-flight mnemonic
 Active when light intensity changes (photopupillary reflex) or when shift gaze from
distant object to nearby object (to decrease spherical aberration due to refraction)
o both pupils constrict if one eye is illuminated (consensual light reflex)
Refraction
 Bending of light rays occurs when light passes through substance with different refractive
index at any angle other than 90 degrees
o refractive index of air is arbitrarily set to n = 1
o refractive index of cornea is n = 1.38
o refractive index of lens is n = 1.40
 Cornea refracts light more than lens does
o lens fine-tunes the image as shift focus between near and distant objects
The Near Response
 Emmetropia = eyes focused on distant object (more than 6 m away) receive parallel light
waves & focus without effort
 Near response occurs if focus on object closer (light waves are NOT parallel and must be
bent/refracted more to compensate)
o convergence of eyes
 eyes orient their visual axis toward the object
o constriction of pupil (pupillary miosis)
 does not admit peripheral light rays & reduces spherical aberration (blurry
edges)
o accommodation of lens
 contraction of ciliary muscle relaxes suspensory ligaments which allows
lens to relax to a more convex shape
 light is refracted more strongly & focused onto retina
Hyperopia and Myopia
 Hyperopia is farsightedness (eye is too short front to back)
o eyeball is too short, focused image would fall behind the retina
o CAN see far objects because the ciliary muscle contracts to fatten the lens and
bend the parallel rays enough to make the image fall on the retina. A normal
eye’s ciliary muscle does not have to do this.
Convex lens – fat in the
middle skinny on the edges
Concave lens – thin in the
middle thicker on the edges
o Close objects can’t be focused because the lens is as fat as it can get and it’s still
not enough to force the focused image onto the retina
o correct with convex lens which helps bend the light waves before they reach the
eye so the image falls on the retina properly
 Myopia is nearsightedness (eye is too long front to back)
o eyeball is too long, focused image would fall in front of retina
o CAN see near objects (even nearer than a normal eye) because the ciliary muscle
doesn’t have to work as much to refract the light waves by fattening the lens. (A
-opia means eye
thinner lens than normal can get the image onto the retina.)
o Far objects are a problem to see because the lens is as thin as it can get and the
light waves are still being bent too much. (Remember: the cornea cannot be
adjusted to stop bending light waves.) The focused image falls in front of the
retina and starts to diverge again before it reaches the retina, producing a blurry
image.
o correct with concave lens which diverges the light waves before they reach the
eye to compensate for the “excessive” refracting produced by the cornea.
Effects of Corrective Lenses
 Presbyopia = “old eyes”
o Old lens can’t see near objects due to increasing stiffness of the lens. Corrected
with bifocals.
 Astigmatism (cornea is funny shaped)
o Warped cornea makes focusing all parts of the image difficult. Corrected with
cylindrical lenses.
Retinal Cells
 Posterior layer of retina is pigment epithelium
o purpose is to absorb stray light & prevent reflections
o unlike tapetum lucidum of cow eyeball which
 Acts like a mirror to improve night vision
 Photoreceptors cells are next layer
o derived from stem cells that produced ependymal cells
 Rod cells (night vision)
o outer segment is stack of coinlike membranous discs studded with rhodopsin
pigment molecules
 Cone cells (color vision in bright light)
o outer segment tapers to a point
 Notice that light must travel through a bunch of neurons before ever reaching the
photoreceptors!
Nonreceptor Retinal Cells
 Bipolar cells (1st order neurons)
Moiety - portion of a molecule that
o synapse on ganglion cells
refers to a certain part/portion.
o large amount of convergence
 Ganglion cells (2nd order neurons)
o axons of these form optic nerve
o more convergence occurs (114 receptors to one optic nerve fiber)
 A lot of processing done before even leaving the eye!
 Horizontal & amacrine (anaxonic neurons) cells form connections between other cells
Thalamus is where 3rd order
neurons are found (later
geniculate nuclei specifically)
o enhance perception of contrast, edges of objects & changes in light intensity
Visual Pigments
 Visual pigment of the rod cells is called rhodopsin (visual purple)
 2 major parts to the molecule
o protein called opsin
o vitamin A derivative called retinal
 Rod cells contain single kind of rhodopsin with an absorption peak at wavelength of 500
nm
 Cones contain photopsin (iodopsin)
o opsin moieties contain different amino acids that determine which wavelengths of
light are absorbed
o 3 kinds of cones (red, green, blue) absorbing different wavelengths of light
produce color vision
The Photochemical Reaction in Rod Cells
 When rhodopsin absorbs light, it is converted from the bent shape (cis-retinal) [crooked =
cis] to the straight (trans-retinal) which then dissociates from the opsin (bleaching)
 Takes 5 minutes to regenerate 50% of rhodopsin
o trans-retinal converted back to cis-form in the pigment epithelium & then returned
to the rod to be reunited with opsin there
Light and Dark Adaptation
 Light adaptation (wake up in middle of night and turn on bright light)
o pupil constriction and pain from over stimulated retinas
o color vision & acuity not optimal for 5 to 10 minutes
 Dark adaptation (sitting in a bright room at night and power failure occurs)
o dilation of pupils occurs
o 20 to 30 minutes required for bleached rhodopsin to return to maximal possible
sensitivity in the dark
Scotopic System (Night Vision)
 Sensitivity of rods in dim light
o extensive neuronal convergence
o 600 rods converge on 1 bipolar cell
o many bipolar cells converge on each ganglion cell
o high degree of spatial summation (very weak light can stimulate ganglion cell) but
no ability to resolve detail
 one ganglion cell receives information from huge 1 mm2 of retina
producing only a coarse image
 Edges of retina with widely spaced receptor cells is low-resolution system only alerting
us to motion
Photopic System (Day Vision)
 Fovea contains only 4000 tiny cone cells and no rods
o no neuronal convergence
o each foveal cone cell has “private line to the brain”
Color Vision
 Primates have well developed color vision
o nocturnal vertebrates have only rods
 Cones are named for absorption peaks of photopsins
o blue cones peak sensitivity at 420 nm
o green cones peak at 531 nm
o red cones peak at 558 nm (orange-yellow)
 Perception of color is based on mixture of nerve signals
 Color blindness is hereditary lack of one photopsin
o red-green is common (lack either red or green cones)
 incapable of distinguishing red from green
 sex-linked recessive (8% of males, 0.5% of females)
Stereoscopic Vision (Stereopsis)
 Depth perception is the ability to judge how far away objects are
 Requires 2 eyes with overlapping
visual fields
o panoramic vision has eyes on sides of head (horse)
 Fixation point is spot on which eyes are focused
o objects farther away require image focus medial to the fovea
o objects closer result in image focus lateral to fovea
o the location of the image of an unfocused object on the retina relative to the fovea
tells the brain if it is nearer or farther away than the object that the eye is focused
on
Visual Projection Pathway KNOW THE PATHWAY DIAGRAM FOR THE EXAM!
 Bipolar & ganglion cells in retina are 1st & 2nd order neurons (axons of ganglion cells
form CN II)
 Hemidecussation occurs in optic chiasm
o 1/2 of fibers decussate so that images of all objects in the left visual field (which
fall on right half of each retina) go to the right visual cortex
o each side of brain sees what is on the side where it has motor control over limbs
 3rd order neurons in lateral geniculate nucleus of thalamus form optic radiation to 1
visual cortex in the occipital lobe where conscious visual sensation occurs
Hemianopsia – “half no
o Few fibers project to superior colliculi & midbrain for visual reflexes
sees”
(photopupillary & accommodation)
Visual Information Processing
Decussation – to cross
 Some processing occurs in the retina
o adjustments for contrast, brightness, motion & stereopsis
 Primary visual cortex is in the occipital lobe
 Visual association areas in parietal & temporal lobes process visual data
o object location, motion, color, shape, boundaries
o store visual memories (recognize printed words)
 Read about the history of anesthesia in Clinical Insight 16.5
Chapter 25 – Digestive System
Digestive Functions
 Ingestion = intake of food
 Digestion = breakdown of molecules
 Absorption = uptake nutrients into blood/lymph
 Defecation = elimination of undigested material
Stages of Digestion
 Mechanical digestion is physical breakdown of food into smaller particles
o teeth & churning action of stomach & intestines
 Chemical digestion is series of hydrolysis reactions that break macromolecules into their
monomers
 The process of hydrolysis requires the use of what molecule? Water
o enzymes from saliva, stomach, pancreas & intestines
 results
o polysaccharides into monosaccharides
o proteins into amino acids
o fats into glycerol and fatty acids
Digestive Processes
 Motility = muscular contractions that break up food, mix it with enzymes & move it
along
 Secretion = digestive enzymes & hormones will be secreted to coordinate the actions of
digestive system
 Membrane transport = absorption of nutrients
Subdivisions of the Digestive System
 Digestive tract (alimentary canal)
o 30-foot-long tube extending from mouth to anus
o gastrointestinal (GI) tract includes only the stomach and intestines
 Accessory organs
o teeth, tongue, liver, gallbladder, pancreas, salivary glands
 Material inside the digestive tract us not considered to be inside the body. Why? You
have to absorb it into the wall of the GI tract for body to properly absorb the material.
Tissue Layers of the GI Tract
 Mucosa (actually touches the food)
o epithelium - next to lumen (center opening of tube)
o lamina propria - connective tissue
o muscularis mucosae - thin layer of smooth muscle
 Submucosa- blood/lymph vessels, nerve plexus
 Muscularis externa
o inner circular layer
o nerve plexus between
o outer longitudinal layer
 Serosa or Adventitia
o serosa is areolar CT topped by mesothelium found from the infradiaphragmatic
esophagus through the sigmoid colon
o adventitia is fibrous CT found elsewhere on digestive tract (oral cavity, pharynx,
portion of the esophagus above the diaphragm, and the rectum)
Enteric usually refers to the
intestines
Enteric Nervous Control
 Enteric nervous system – esophagus, stomach, and intestines have their own nervous
network:
o controls motility & secretion in response to stimuli to the mucosa
o part of the parasympathetic nervous system
o Two subdivisions:
 submucosal (Meissner) plexus
 located between submucosa and inner layer of muscularis externa
 controls glandular secretions and movements of muscularis
mucosae
 myenteric (Auerbach) plexus
 located between the inner and outer layers of the muscularis
Peristalsis – movement
externa
in the tube
 controls peristalsis and other movements of muscularis externa
-ante in this
Relationship to the Peritoneum
case means
 Most of the gastrointestinal tract is anteperitoneal (within the peritoneal sac)
o Only duodenum, pancreas, & parts of large intestine are retroperitoneal
inside, usually
 Dorsal mesentery suspends GI tract & forms serosa (visceral peritoneum) of stomach & means in
intestines
front of
 Ventral mesentery forms lesser omentum & greater omentum
o greater omentum hangs like an apron over the viscera
 lacy layer of connective tissue contains lymph nodes, lymphatic vessels
and blood vessels
 lesser omentum runs from lesser curvature of stomach to liver and may also be called the
gastrohepatic ligament (gastro - stomach; hepatic - liver)
 the omenta adhere to perforations or inflamed areas of stomach or intestines, contribute
immune cells to the site, and isolate infections that might otherwise cause peritonitis
Mesentery and Mesocolon
 Mesentery of small intestines holds many blood vessels
 Mesocolon anchors the colon to the posterior body wall
Regulation of Digestive Tract
 Neural control
o short myenteric reflexes (i.e., swallowing)
 act through myenteric nerve plexus to stimulate nearby areas of muscularis
externa
 reflex arc travels a short distance
o long vagovagal reflexes
 parasympathetic stimulation of digestive motility and secretion by
autonomic nerve fibers of the vagus nerves
 impulses travels from gut to head and back to gut (a long distance)
 Hormones like gastrin and secretin
o long-range chemical messenger molecules that diffuse into the bloodstream and
stimulate distant target cells
 Paracrine secretions
o short-range chemical messenger molecules that diffuse through extracellular
tissue fluid (NOT the bloodstream) to nearby target cells
o histamine and prostaglandins stimulate digestive function
Features of the Oral Cavity
 Cheeks and lips keep food between teeth for chewing, are essential for speech & suckling
in infants
o vestibule is space between teeth & cheeks
o cutaneous area versus red or vermilion area
 Tongue is sensitive, muscular manipulator of food
o papillae & taste buds on dorsal surface
o lingual glands secrete saliva, tonsils in root
 How does the tongue avoid being bitten? Proprioception
 Hard & soft palate
o allow breathing & chewing at same time
o palatoglossal (anterior) & palatopharyngeal (posterior) arches
 palatine tonsils found in space between known as fauces
 Friction ridges (palatal rugae) of the hard palate help the tongue in manipulating food
Teeth
 The teeth are collectively called the dentition.
 The meeting of the teeth when the mouth closes is called occlusion. (Poor meeting =
malocclusion)
 Types of teeth:
o 8 incisors  chisel-like for cutting and biting off pieces of food
o 4 canines  pointed for puncturing and shredding (reduced in humans)
o 8 premolars 2 cusps (bumps) each, so also called bicuspids
Broad occlusal surfaces for grinding and crushing.
Not present in children.
o 12 molars  4-5 cusps each, for grinding and crushing

Only 8 in children. Third molars may not erupt in
small-jawed adults, becoming impacted.
Permanent & Baby Teeth
 Baby (deciduous) teeth (20) erupt by 2 years of age; Adult teeth (32) erupt between 6 and
25
Deciduous – they fall out
Tooth Structure
 Periodontal ligament is modified periosteum (covering on outside of bone made of Dense
Irr. CT)
o anchors into alveolus
 Cementum & dentin are living tissue
 Enamel is noncellular secretion formed during development
 Root canal leads into pulp cavity
o nerves & blood vessels
 Gingiva or gums
o Gingivitis – inflammation of gums
Mastication or Chewing
 Breaks food into smaller pieces to be swallowed
o  surface area exposed to digestive enzymes
 Contact of food with sensory receptors triggers chewing reflex
o tongue, buccinator & orbicularis oris manipulate food
o muscles of mastication move the mandible
Saliva
 Functions of saliva
o moisten, begin starch & fat digestion, cleanse teeth, inhibit bacteria, bind food
together into bolus
 pH of 6.8 to 7.0 (neutral)
Hypotonic – less solutes in it
 Hypotonic solutions of 99.5% water and solutes:
than in the blood
o salivary amylase = begins starch digestion immediately
 pH optimum of 7 (neutral pH)
Salivary amylase – made of
o lingual lipase = digests fat after reaches the stomach
 pH of 2 in stomach is optimal for lingual lipase activity protein; enzyme
o mucus = aids in swallowing
o lysozyme = enzyme that kills bacteria
o immunoglobulin A = inhibits bacterial growth
o electrolytes = Na+, K+, Cl-, phosphate & bicarbonate
Salivary Glands
 Small intrinsic glands found under mucous membrane of mouth, lips, cheeks and tongue - secrete at constant rate
 3 pairs extrinsic glands connected to oral cavity by ducts
 parotid, submandibular and sublingual
Histology of Salivary Glands
 Compound tubuloacinar glands
 Mucous cells secrete mucus
 Serous cells secrete thin fluid rich in amylase
 Mixed acinus is possible
 What does “acinus” mean? Berry “Demilune”? Half moon, technically
Salivation
 Total of 1 to 1.5 L of saliva per day
 Cells filter water from blood & add other substances
 Food stimulates receptors that signal salivatory nuclei in the medulla & pons via C.N.s
CN 7,9, & 10
VII & IX
carry taste
o parasympathetic stimulation  salivary glands produce thin saliva, rich in
enzymes
CN 3,7,9, & 10
o sympathetic stimulation  produce less abundant, thicker saliva, with more
carry
mucus
parasympathetic
 Higher brain centers stimulate salivatory nuclei so sight, smell & thought of food cause
fibers
salivation
Pharynx
 Skeletal muscle
o deep, inner layer – longitudinal orientation
o superficial, outer layer – circular orientation
 circular muscle divided into superior, middle and inferior pharyngeal
constrictors
 Inferior pharyngeal constrictor remains constricted when food is
not being swallowed to exclude air from the esophagus.

Note that the muscular layers of the pharynx are “backwards” from the normal
orientation (circular inner, longitudinal outer)
The Esophagus
 Straight muscular tube 25-30 cm long
o nonkeratinized stratified squamous epithelium
o esophageal glands in submucosa
 Extends from pharynx to cardiac stomach passing through esophageal hiatus in the
diaphragm
 Lower esophageal sphincter closes cardiac orifice to prevent gastroesophageal reflux
(GERD, “heartburn”)
 Superior one-third is skeletal muscle, middle one-third is mixture of skeletal and smooth
muscle, and lower one-third is smooth muscle
Barrett’s
o corresponds to shift from voluntary to involuntary control
esophagus –
Swallowing or Deglutition
severe GERD
 Series of muscular contractions coordinated by swallowing center in medulla & pons
o motor signals from cranial nerves V, VII, IX and XII
 Buccal phase
o tongue collects food & pushes it back into oropharynx
 Pharyngeal-esophageal phase
o root of tongue blocks oral cavity
o soft palate rises & blocks nasopharynx
o infrahyoid muscles lift larynx & epiglottis is folded back
o pharyngeal constrictors push bolus down esophagus
 called peristalsis— sequential pushing of food material through alimentary
canal
 liquids reach stomach in 2 seconds -- food bolus may take 8 seconds
 lower esophageal sphincter relaxes
 can swallow even if standing on your head!
Introduction to the Stomach
 Mechanically breaks up food particles, liquefies the food & begins chemical digestion of
protein & fat
o resulting soupy mixture is called chyme
 Stomach does not absorb any significant amount of nutrients
o does absorb aspirin & some lipid-soluble drugs
Gross Anatomy of the Stomach
 Muscular sac (internal volume from 50ml to 4L, FULL)
o J-shaped organ with lesser & greater curvatures
o regional differences
 cardiac region just inside cardiac orifice
 fundus is domed portion superior to esophageal opening
 ‘fundus is fun – you burp out of it’
 body is main portion of organ
 pyloric region is narrow inferior end (like a funnel)
 2 parts: antrum & pyloric canal
 Pylorus is opening to duodenum
o thick ring of smooth muscle forms a pyloric sphincter
Innervation and Circulation
 Innervation by parasympathetic fibers from vagus & sympathetic fibers from the celiac
plexus
 All blood drained from stomach is filtered through the liver (hepatic portal circulation)
before returning to heart
Unique Features of Stomach Wall
 Mucosa
o simple columnar glandular epithelium
o lamina propria is filled with tubular glands (gastric pits)
 Muscularis externa has 3 layers
o outer longitudinal, middle circular & inner oblique layers
Cells of the Gastric Glands and Their Secretions
 Mucous cells secrete mucus
 Regenerative cells divide rapidly to produce new cells that migrate upwards towards
surface
 Chief cells secrete pepsinogen throughout life
o Pepsinogen is a zymogen (inactive enzyme precursor)
 HCl converts it to pepsin (active form, digests protein)
 pepsin then activates more pepsinogen by “digesting” it to form
more active pepsin (autocatalytic effect—positive feedback loop –
self-speeding up)
 During infancy only, chief cells also secrete chymosin (curdles milk by coagulating its
proteins) & lipase (digests butterfat in milk)
Cells of the Gastric Glands and Their Secretions
 Parietal cells secrete HCl acid & intrinsic factor
o Parietal cells also secrete a hunger-producing hormone called ghrelin
o HCl activates pepsinogen to pepsin
o Intrinsic factor is needed to absorb Vitamin B12 (cobalamin)
 Vitamin B12 is required for hemoglobin synthesis
 Deficiency of Vitamin B12 results in what disease? Pernicious Anemia –
can’t produce hemoglobin
 Enteroendocrine cells secrete hormones & paracrine messengers
Gastric Acid Secretions
 2 to 3 L of gastric juice/day (H2O, HCl & pepsin)
 Parietal cells contain carbonic anhydrase (CAH)
o CO2 + H2O  H2CO3  HCO3- + H+
o H+ is pumped into stomach lumen by H+-K+ATPase (active transport)
 antiporter uses ATP to pump H+ out & K+ in
o HCO3- exchanged for Cl- (chloride shift)
 Cl- pumped out to join H+ forming HCl
  HCO3- in blood causes alkaline tide (blood pH ) when food is being
digested
Functions of Hydrochloric Acid
Lingual lipase – secreted in
 Activates enzymes pepsin & lingual lipase
mouth, breaks up fats
 Breaks up connective tissues & plant cell walls
o liquefying food to form chyme
Chemical
digestion in
mouth and
stomach

Converts ingested ferric ions (Fe+3) to ferrous ions (Fe+2) that can be absorbed & utilized
for hemoglobin synthesis
 Destroys ingested bacteria & pathogens
Oligo – few
Enzymes versus Hormones
 Enzymes are released by glandular cells into the lumen of the alimentary canal to work
on the food particles there.
o Examples include:
 pepsin(ogen)
 -ogen means inactive precursor
 Chymosin
 lipase
 amylase (salivary or pancreatic)
 carboxypeptidase
 watch for the ending -ase
 Trypsin
 Chymotrypsin
 Hormones are released by enteroendocrine cells into the bloodstream to tell other cells
elsewhere what to do.
o Examples include:
 gastrin
 ghrelin
 cholecystokinin (CCK)
 vasoactive intestinal peptide (VIP)
 gastric inhibitory peptide
 secretin
Gastric Motility
 Swallowing center signals stomach to relax
 Arriving food stretches the stomach activating a receptive-relaxation response
o resists stretching briefly, but relaxes to hold more food
 Rhythm of peristalsis controlled by pacemaker cells in longitudinal muscle layer
o gentle ripple of contraction every 20 seconds churns & mixes food with gastric
juice
o stronger as reaches pyloric region squirting out 3 mL
 duodenum neutralizes acids and digests nutrients little at time
 typical meal is emptied from stomach in 4 hours
Vomiting (Emesis)
 Induced by excessive stretching of stomach, psychological stimuli or chemical irritants
(bacterial toxins)
 Emetic center in medulla causes lower esophageal sphincter to relax as diaphragm &
abdominal muscles contract
 Retching involves thoracic expansion and abdominal contractions to dilate esophagus
o lower esophageal sphincter (located at opening into the cardiac stomach) relaxes
as stomach and duodenum contract spasmodically
o pushes chyme up esophagus
o upper esophageal sphincter (located posterior to larynx) remains contracted,
preventing ejection of chyme

Vomiting occurs when upper esophageal sphincter opens
o esophagus and body of stomach relax
o chyme driven out by strong abdominal contraction and reverse peristalsis of
gastric antrum and duodenum
o may even expel some contents of small intestine
Protection of Stomach and Peptic Ulcer
 Three mechanisms to protect stomach tissue from digesting itself:
o mucous coat: thick, highly alkaline mucus
o epithelial cell replacement: cells live only 3-6 days and are then replaced by
regenerative cells in gastric pits
o tight junctions: stop HCl from trickling between epithelial cells to digest lamina
propria underneath
 Peptic ulcer is erosion of the gastric wall accompanied by gastritis (stomach
inflammation)
o Caused by psychological stress?
NO!
o Caused by excess acid secretion?
NO!
 Used to prescribe cimetidine (Tagamet) to block histamine from binding
to and stimulating H2 receptors on parietal cells, but this doesn’t treat the
real cause…
o Helicobacter pylori! (A bacterial infection!)
 Antibiotics have a much better cure rate (90% versus 20%-30%) in less
time and at lower cost than cimetidine.
Regulation of Gastric Function
 Cephalic Phase (Cephalic refers to “towards the brain”)
o vagus nerve stimulates gastric secretion & motility just with sight, smell, taste or
thought of food
 Gastric phase
o activated by presence of food or semidigested protein
 by stretch or  in pH
o secretion is stimulated by
 ACh (from parasympathetic fibers), histamine (from gastric
enteroendocrine cells) and gastrin (from pyloric G cells)
 receptors for all 3 substances on parietal cells
 receptors for ACh and gastrin (not histamine) on chief cells
 As dietary protein is digested, smaller peptides and amino acids stimulate
G cells to produce more gastrin (+ feedback).
 amino acids and peptides buffer pH so it does not fall too low
 but when chyme leaves stomach, peptides also leave, so pH drops
below 2.0
 low pH inhibits parietal and G cells (- feedback) which winds
down gastric phase
 Intestinal phase - duodenum regulates gastric activity through hormones & nervous
reflexes
o at first gastric activity increases (if duodenum is stretched or amino acids in
chyme cause gastrin release)
o enterogastric reflex = duodenum inhibiting stomach


caused by acid and semidigested fats in duodenum
inhibition of vagal nuclei, stimulation of sympathetic neurons which
inhibit action of stomach.
 Like saying, “Stop! Wait until I finish with what you just gave me!”
o chyme stimulates duodenal enteroendocrine cells to release secretin,
cholecystokinin (CCK) & gastric inhibitory peptide
 all 3 suppress gastric secretion & motility to give the duodenum time to
work on the chyme it already has in it
 Look at Figure 25.18 in your text book.
Liver, Gallbladder and Pancreas
 All release important secretions into small intestine to continue digestion
Gross Anatomy of Liver
 3 lb. organ (body’s largest gland) located inferior to the diaphragm
 4 lobes -- right, left, quadrate & caudate
o falciform ligament separates left and right
o round ligament (aka ligamentum teres) is remnant of umbilical vein
 Gallbladder adheres to inferior surface between right and quadrate lobes
Inferior Surface of Liver
 Porta hepatis is irregular opening between quadrate and caudate lobes containing hepatic
portal vein, proper hepatic artery, and bile passages.
Porta Hepatis – liver door
Microscopic Anatomy of Liver
 Tiny cylinders called hepatic lobules (2mm by 1mm)
 Central vein surrounded by sheets of hepatocyte cells separated by sinusoids lined with
fenestrated epithelium
 Blood filtered by hepatocytes on way to central vein
o Liver removes nutrients including glucose, amino acids, iron, & vitamins from
blood.
o Also removes and degrades hormones, toxins, bile pigments, drugs, bacteria &
debris
o Secretes albumin, lipoproteins, clotting factors, angiotensinogen, etc. into blood
Histology of Liver -- Hepatic Triad
 3 structures found in corner between lobules
o hepatic portal vein and hepatic artery bring blood to the liver
 provide nutrient-laden, oxygen-poor blood & oxygen-rich blood,
respectively
o bile ductules collect bile from bile canaliculi between sheets of hepatocytes to be
secreted from liver via right and left hepatic ducts
Ducts of Gallbladder, Liver & Pancreas
 Bile passes from bile canaliculi between cells to bile ductules to right & left hepatic ducts
 Right & left ducts join outside the liver to form common hepatic duct
 Cystic duct from gallbladder joins common hepatic duct to form the bile duct
 Pancreatic duct and bile duct combine to form hepatopancreatic ampulla emptying into
the duodenum at the major duodenal papilla
o sphincter of Oddi (hepatopancreatic sphincter) regulates release of bile &
pancreatic juice
Cholecystectomy – removal of
your gallbladder
Gallbladder and Bile
 Gallbladder is a sac on underside of liver -- 10 cm long
 500 to 1000 mL bile are secreted daily from liver
 Gallbladder stores & concentrates bile
o bile backs up into gallbladder from a filled bile duct
o between meals, bile is concentrated by factor of 20
 Yellow-green fluid containing minerals, bile acids, cholesterol, bile pigments &
phospholipids
o bile acid (salts) emulsify fats & aid in their digestion
 enterohepatic circulation - recycling of bile salts (80%) from ileum
 cholesterol used to synthesize more bile acids to replace the 20% of those
secreted that are lost in feces
 this is the only way the body eliminates excess cholesterol (in the
bile)
 What effect would a drug that blocks bile acid reabsorption have
and why? Excretion of bile acids which would lower your
cholesterol
o all other components of bile are destined for excretion
 bilirubin pigment from hemoglobin breakdown
 intestinal bacteria convert to urobilinogen = brown color
Gross Anatomy of Pancreas
 Retroperitoneal gland posterior to stomach
o head, body and tail
 Endocrine and exocrine gland
o secretes insulin & glucagon into the blood
o secretes 1500 mL pancreatic juice into duodenum
 water, enzymes, zymogens, and sodium bicarbonate
 other pancreatic enzymes are activated by exposure to bile and ions
in the intestine
 Pancreatic duct runs length of gland to open at sphincter of Oddi (pancreatic juice mixes
with bile)
o accessory duct opens independently on duodenum
o Why would pancreatic juice need to contain sodium bicarbonate? It helps to
neutralize the acid; from the pH of 2 to 8 or 9
Pancreatic Acinar Cells
 Zymogens = proteases
o Trypsinogen (Enterokinase converts this to trypsin which converts the following:)
o chymotrypsinogen
o procarboxypeptidase
 Other enzymes (not zymogens)
o pancreatic amylase – What molecule does it break down? Starch
o pancreatic lipase
o ribonuclease and deoxyribonuclease
o These are activated when exposed to bile and ions in intestinal lumen.
Hormonal Control of Secretion
 Cholecystokinin released from duodenum in response to arrival of acid and fat which
causes:
o contraction of gallbladder
o secretion of pancreatic enzymes
o relaxation of hepatopancreatic sphincter
 The ultimate goal being… getting it into the duodenum
 Secretin released from duodenum in response to acidic chyme
o stimulates all ducts to secrete more bicarbonate ion
 Gastrin from stomach & duodenum weakly stimulates gallbladder contraction &
pancreatic enzyme secretion
Small Intestine
 Much of the body’s chemical digestion occurs in the small intestine. Also the mouth and
stomach.
 Practically all of the body’s nutrient absorption occurs in the small intestine.
Gross Anatomy of Small Intestine
 Duodenum curves around head of pancreas (10 in.)
o retroperitoneal along with pancreas
o receives stomach contents, pancreatic juice & bile
o neutralizes stomach acids, emulsifies fats, pepsin inactivated by pH increase,
pancreatic enzymes
Pepsin works at a pH of 2
 Jejunum is next 8 ft. (in upper abdomen)
o covered with serosa and suspended by mesentery
 Ileum is last 12 ft. (in lower abdomen)
o covered with serosa and suspended by mesentery
o ends at ileocecal junction with large intestine
Large Surface Area of Small Intestine
 Circular folds (plicae circularis) up to 10 mm tall
o involve only mucosa and submucosa
o chyme flows in spiral path causing more contact
 Villi are fingerlike projections 1 mm tall
o contain blood vessels & lymphatics (lacteal)
 nutrient absorption
 fat absorption by lacteals
 Microvilli 1 micron tall
o brush border on cells
o brush border enzymes are integral proteins embedded in the cells’ plasma
membranes
 for contact digestion -- final stages of digestion, necessitates spiral path
Intestinal Cryptus
 Pores opening between villi lead to intestinal crypts
o absorptive cells, goblet cells & at base, rapidly dividing cells
 life span of 3-6 days as migrate up to surface, then sloughed off &
digested
o paneth cells – antibacterial secretions
 Brunner’s glands (in duodenum’s submucosa) secrete bicarbonate mucus
Goblet cells make mucus


Peyer patches (in ileum) are populations of lymphocytes (WBC) to fight pathogens
Secrete 1-2 L of intestinal juice/day
o water & mucus, pH 7.4-7.8
Intestinal Motility
 Mixes chyme with intestinal juice, bile & pancreatic juice
 Churns chyme to increase contact with mucosa for absorption & digestion
 Moves residue toward large intestine
o Segmentation (kneading the materials inside the tube/mixing it in place)
 random ringlike constrictions mix & churn contents
 12 times per minute in duodenum
o Peristaltic waves begin in duodenum but each one moves further down (termed a
migrating motor complex)
 push remaining undigested residue along for 2 hours toward cecum
 refilling of stomach at the next meal suppresses peristalsis and reactivates
segmentation
 Food in stomach also causes gastroileal reflex (relaxing of ileocecal valve & filling of
cecum)
Segmentation in the Small Intestine
 Purpose of segmentation is to mix & churn not to move material along as in peristalsis
Peristalsis
 Gradual movement of contents towards the colon
 Begins after absorption occurs
 Migrating motor complex controls waves of contraction
o second wave begins distal to where first wave began
Carbohydrate Digestion in Small Intestine
 Salivary amylase stops working in acidic stomach (if < 4.5)
o 50% of dietary starch digested before it reaches small intestine
 Pancreatic amylase completes first step in 10 minutes (at pH of 8)
 Brush border enzymes act upon oligosaccharides, maltose, sucrose, lactose & fructose
o dextrinase, glucoamylase, maltase, sucrase, and lactase
o lactose (milk sugar) is indigestible after age 4 in most humans (lack of lactase)
Carbohydrate Absorption
 Sodium-glucose transport proteins (SGLT) in membrane help absorb glucose & galactose
 Fructose absorbed by facilitated diffusion then mostly converted to glucose inside the cell
 Some glucose is absorbed by paracellular route (solvent drag)
Protein Digestion & Absorption
 Pepsin has optimal pH of 1.5 to 3.5 -- inactivated when passes into duodenum & mixes
with alkaline pancreatic juice (pH 8)
 Pancreatic enzymes take over protein digestion by hydrolyzing polypeptides into shorter
oligopeptides…chains shorter than 10 or 15 amino acids
 These include trypsin, chymotrypsin, and carboxypeptidase
 Brush border enzymes (carboxypeptidase again, aminopeptidase, dipeptidase) finish the
task producing amino acids that are absorbed into the intestinal epithelial cells
o amino acid cotransporters move amino acids into epithelial cells & facilitated
diffusion moves amino acids out into the blood stream
 Infants absorb proteins by pinocytosis (maternal IgA)
o This ability still exists in the adult small intestine as well.
Fat Digestion & Absorption
 Fat globule  emulsification droplets  micelles  into cell  chylomicrons  into
lacteal
Nucleic Acids, Vitamins, and Minerals
 Nucleases hydrolyze DNA & RNA to nucleotides
o nucleosidases & phosphatases of the brush border split them into phosphate ions,
ribose or deoxyribose sugar, & nitrogenous bases
 Vitamins are absorbed unchanged
o Fat-soluble vitamins are A, D, E & K and must be absorbed with other lipids
o B complex vitamins & vitamin C absorbed by simple diffusion
o B12 absorbed only if bound to intrinsic factor
o Which cells made this? Parietal cells of gastric mucosa
 Minerals are absorbed all along small intestine
o Na+ cotransported with sugars & amino acids
o Cl- exchanged for bicarbonate reversing stomach’s chloride shift
o Iron (ferrous ions (Fe2+) only) & calcium absorbed as needed
 other minerals are absorbed at constant rates and the kidney is left to
excrete any excess
 Why would women have four times as many iron transport proteins as
men? Because of their menstrual cycle
Water Balance
 Digestive tract receives about 9 L of water/day
o .7 L in food, 1.6 L in drink, 6.7 L in secretions
o 8 L is absorbed by the small intestine & .8 L by the large intestine
 Water is absorbed by osmosis following the absorption of salts & organic nutrients
 Diarrhea occurs when too little water is absorbed
o feces pass through too quickly if irritated by bacteria
o feces contains high concentrations of a solute (lactose)
 Constipation occurs when too much water absorbed
o feces pass too slowly, feces become hardened
o caused by lack of dietary fiber or exercise, laxative abuse, emotional upset
 also caused by chiropractic misalignment
Gross Anatomy of Large Intestine
 5 feet long and 2.5 inches in diameter in cadaver
 Begins as cecum & vermiform appendix in lower right corner
Vermis – “worm”
 Ascending, transverse and descending colon frame the small intestine
 Sigmoid colon is S-shaped portion leading down into pelvis
 Rectum is straight portion ending at anal canal
Microscopic Anatomy
 Mucosa is simple columnar epithelium
o anal canal is stratified squamous epithelium
 No circular folds or villi to increase surface area
 Intestinal crypts (glands sunken into lamina propria) produce mucus only
More Anatomy of Large Intestine
 Teniae coli are three ribbon-like strips of longitudinal fibers of the muscularis externa.
o This is different than anywhere else in the digestive tract. Usually the
longitudinal layer covers the entire surface.
 Haustra (-um) are pouches in the large intestine caused by the teniae coli
 Epiploic (omental) appendages are clublike fatty pouches of peritoneum of unknown
function.
Bacterial Flora & Intestinal Gas
 Bacterial flora populate large intestine
o called the gut microbiome
o ferment cellulose & other undigested carbohydrates
o synthesize vitamins B and K
 diet alone does not usually provide enough vitamin K to ensure adequate
clotting
 Flatus (gas)
o average person produces 500 mL per day
o Next question: How many “emissions” per day? 14
o most is swallowed air but it can contain hydrogen, nitrogen, carbon dioxide,
methane, hydrogen sulfide, indole & skatole
 the last three cause the odor of feces and flatus
 hydrogen gas is highly flammable – flatus has been known to explode
during surgeries that use electrical cauterization!
Absorption and Motility
 Transit time of large intestine is 12 to 24 hours
o reabsorbs water and electrolytes
 Feces consist of water & solids (bacteria, mucus, undigested fiber, fat & sloughed
epithelial cells
 Haustral contractions occur every 30 minutes
o distension of a haustrum stimulates it to contract
 Mass movements occur 1 to 3 times a day
o triggered by gastrocolic and duodenocolic reflexes
 filling of the stomach & duodenum stimulates motility
 sort of like “Oh, there’s more coming. I can get rid of this batch.”
 moves residue for several centimeters with each contraction
Anatomy of Anal Canal
 Anal canal is 3 cm total length
 Anal columns are longitudinal ridges separated by mucus secreting anal sinuses
 Rectal valves (3) are internal transverse folds of rectal tissue that enable the rectum to
retain feces while passing gas
 Hemorrhoids are permanently distended hemorrhoidal veins that protrude into the anal
canal or form bulges distal to the anus
Defecation
 Stretching of the rectum stimulates defecation
o intrinsic defecation reflex via the myenteric plexus
 causes muscularis to contract & internal sphincter to relax
 relatively weak contractions
 defecation occurs only if external anal sphincter is voluntarily relaxed
o parasympathetic defecation reflex involves spinal cord
 stretching of rectum sends sensory signals to spinal cord
 pelvic splanchnic nerves return signals intensifying peristalsis
 Abdominal contractions increase abdominal pressure on the rectum (Valsalva “manure”
?)
 Levator ani lifts anal canal upwards
 Read Insight 25.5: The Man with a Hole in His Stomach
Neural Control of Defecation
 1. Filling of the rectum
 2. Reflex contraction of rectum & relaxation of internal anal sphincter
 3. Voluntary relaxation of external sphincter
Cranial Nerves
I.
Olfactory Nerve
II. Optic Nerve
III. Oculomotor Nerve
IV. Trochlear Nerve
V. Trigeminal Nerve
VI. Abducens Nerve
VII. Facial Nerve
VIII. Vestibulocochlear Nerve
IX.
Glossopharyngeal Nerve
X.
Vagus Nerve
XI.
Accessory Nerve
XII.
Hypoglossal Nerve