digestion - lecture 1 Flashcards
what is the gastrointestinal tract
alimentary canal
continuous tube from mouth to anus
what makes up digestive system
gastrointestinal tract and accessory digestive organs
what are accessory digestive organs
salivary glands
pancreas
liver
gallbladder
secretions into tract to help with digestion
what are git functions
conveys food along git
allows it be broken down into small molecules which can be absorbed into circulation
while moving down tract - used for energy needed by all cells of body
name the 6 activities of digestive system
- ingestion - chew
- secretion
- motility
- digestion - muscular and secretory
- absorption
- defecation
describe secretion
glandular activity - exocrine and endocrine
leads to chemical breakdown
describe motility
propulsion and physical breakdown - muscular activity of git
moves through
describe absorption
transfer to blood circulation and to all cells of body
what are most important git functions
secretion
motility
absorption
describe homeostasis
git is an important contributor
fundamental principle in physiology
at all levels of organization, body activities are directed at maintaining optimal and relatively constant internal conditions - in nutrient concentrations in body
what is gits role in homeostasis
provide nutrients to body
from external environment = food, git breaks food down into absorbable molecules and then processed to all cells = energy and raw materials for growth and repair, function and regulation
describe git capacity
digestive and absorptive capacity
git is exquisitely efficient = digest and absorb almost all the food we eat
carbs = 99%
fat = 95%
protein = 92%
name activities for git and integration
propulsive, secretory and absorptive activities highly coordinated and integrated for high functional efficiency by neural and hormonal mechanisms
the efficiency of git is due to this
describe git structure
tubular structure
communicates with external environment at both ends = lumen contacts, central lumenal cavity = considered as extracellular space
good for function - if toxins or bacteria = wont get into blood stream, but also must be able to absorb nutrients and molecules we need
name parts of git
mouth –> stomach –> si –> li –> anus
what is git length
4.5 m in living adult = longer than human (1.5 meters)
in cadaver = 10 m since humans have lots of muscle tone = always contracts while alive
describe growth of git - sa
increase in total internal sa
adult = 200-250 m^2
600x larger than external sa due to = invaginations and fingerlike projections - in and out pushings
FORM FOLLOWS FUNCTION
describe differentiation of git
tube into alimentary canal and all accessory organs near canal
describe wall of gi tract
4 main layers =
mucosa (secretory and absorptive cells)
submucosa (lymphatics and circulation = absorb nutrients and provide nutrients to cells in git)
muscularis externa
serosa
describe mucosa of git
3 layers =
epithelial layer = epithelial and glandular cells - exocrine and endocrine secretory cells and absorptive cells
lamina propria = loose ct
Muscularis mucosae = smooth muscle
describe submucosa of git
loose ct with lymphatics and bvs and neuronal networks
describe muscularis externa of git
2 diff layers of muscle =
Inner layer circular fibers - decrease aperture of lumen when contracts
outer layer longitudinal fibers - along lenght of git = if contract it shortens
striated muscle at beginning of git (oral cavity, pharynx and upper 1/3 eso and external anal sphincter) then majority smooth (involuntary) and then striated at anus again
describe serosa of git
thin tough layer of ct
in some places = serosa acc continuous with peritoneum (abdominal mesentery) (lining of abdominal cavity) (holds git in place)
what is ens
Enteric nervous system
controls secretion and motility
neurons reside within wall of git
Billions of neuron’s
describe ens
independent (of cns/other nervous systems) and integrative nervous system
can extend processes and control diff parts of git, integrate functions over long distances
initiates, programs, regulates and coordinates activities of muscular and secretory and absorptive elements of git
describe git wall innervation
submucosal plexus = between muscularis mucosae and circular muscle
myenteric plexus = between circular and longitudinal muscle of muscularis externa
though anatomicall distinct - the 2 plexuses behave as one functional unit
define plexus
collection of nerve cell bodies
contain all elements needed for reflex arcs
describe sensory neurons
sense parameters
with receptors in mucosa or muscle
respond to stretch receptors (moving of gut), chemoreceptors and osmoreceptors (sample contents = ph, concentrations or osmolarities)
describe motor/effector neurons
activate secretory and muscle cells
secretory = synapse on muscles and leads to contraction or relaxation result
describe interneurons
expand responses to stimuli in git and allow integration over longer distances = connect sensory and interneurons
describe ex of git wall innervation
spring like ending = type of neuron that responds to stretch or distension
sensory neuron in mucosa
signals to interneurons to effector neurons
effector neurons may lead ti excitation of secretory elements = increase secretion
could also activate inhibitory neurons and relax musculature
precise and graded activity of git due to circuitry = many types of neurons and connections
describe enteric neurons
excitatory = release ach on muscarinic receptors, blocked by atropine
inhibitory = release non adrenergic and non cholinergic = transmitter like nitric oxide
ultimate activity = algebraic sum of influences (five excite and one inhibits = effect excitatory)
describe intramural reflexes
short enteric
all things happens in gut gall
stimulus sensed by chemo, osmo and mechano receptors and sends to nerve plexus then effect at smooth muscle or gland cell and leads to motility and secretion and response
Independent and integrative innervation = regulates git
what does ens consist of - plexuses
myenteric plexus = between longitudinal and circular muscle layers
and submucosal plexus
each = Anatomically distinct but behave as one functional unit = all elements required for reflex arcs (sensory, motor and inter neurons)
what does ens consist of - cells
ganglion cells and their processes that synapse with sm cells, endocrine and exocrine cells and other ganglion cells
describe enteric neurons - specifics
may be excitatory = release ach on muscarinic receptors
or
inhibitory = release nanc (non adrenergic non cholinergic transmitters like NO)
describe sensory fibers ens
have cell bodies in plexuses
get info from gut chemo and mechano receptors
describe ans innervation of git enteric neurons
cns –> parasympathetic preganglionic nicotinic excitatory release ach and synapse on ens
cns –> excitatory nicotinic ach receptor activation to sympathetic chain of ganglia, sympathetic = postganglionic noradrenaline inhibitory - synapse on ens neurons and inhibit
ans gives modulatory input, never synapses directly on muscles or glandular cells = always impinges on enteric neurons
describe parasympathetic innervation of git
preganglionic
excitatory
vagus X - most of input
pelvic nerves - mostly for distal part of git - colon
describe sympathetic innervation of git
postganglionic
inhibitory
celiac ganglion
superior mesenteric ganglion
inferior mesenteric ganglion
how can ans influence git
via the ens only
parasymp = excitatory = release ach
symp = inhibitory = release noradrenaline
long reflexes from ans to modulate git
what does ans do
modulates ens
via long extrinsic reflexes = synaptic connections with ens neurons
describe ans parasympa
neurons reach wall of git as preganlionic fibers, synapse via nicotinic ach receptors, with enteric neurons (excitatory and inhibitory), exert excitatory effect
Parasympathetic input → dilation of blood vessels
describe ans sympa
reach wall of git as postganglionic fibers, synapse via NA receptors with enteric neurons (excitatory and inhibitory) exert inhibitory effect
neurons also innervate smooth muscle in bvs =
Sympathetic input → constriction of blood vessels
describe overview of git - gen
sensory neurons allow for long reflexes
v specialized and highly developed alimentary canal
at each level of git = diff specializations - motility, absorption or both
all secretory and muscular functions controlled by enteric nervous system and ans impinging on ens neurons = motility and secretion
