Digestive System (Midterm 2)

Midterm 2

Most organisms have similar digestive structures, but emphasize different sections depending on:

• Diet

• Evolutionary History

Alimentary Canal

muscular, one-way tube running from the mouth to the anus, responsible for ingesting, digesting, absorbing nutrients, and eliminating waste

The alimentary canal has ________ ______ that help supplement digestion

Accessory Glands

What are the four steps of digestion

1. Ingestion

2. Breakdown

3. Absorption

4. Elimination

what’s the first step of digestion

Ingestion

• capture and intake

what is the second step of digestion

Breakdown

• mechanical - teeth, stomach, and tongue

• chemical - stomach, saliva, pancreas, gall bladder

Mechanical Breakdown

• mechanical - teeth, stomach, and tongue

Chemical Breakdown

• chemical - stomach, saliva, pancreas, gall bladder

What is the major disadvantage of porifera’s digestive system?

• If your mouth is you anus, you can’t sequentially consume and digest

• you eat, digest, excrete. Only then can you eat again

A complete digestive system allows

Sequential Consumption

• one section eats

• one section digests

• one section absorbs

Herbivores trends

long AC focused on extraction of poor-quality food

Carnivore Trends

Short AC because food is easily digestible

The goal of digestion is to…

break apart large, complex, molecules for both energy and to make our own complex molecules from Catabolic digestion and anabolic synthesis

Catabolic digestion

the metabolic process of breaking down complex food molecules into smaller, simpler units to release energy and provide raw materials for cellular function

Anabolic Synthesis

the metabolic process of building complex molecules from simpler ones

Eating is the act of…

stealing the storage molecules from something else and releasing the components

Complex Carbohydrates are?

long chains of glucose

Complex Carbohydrates are broken down into?

Simple sugars (single molecules of glucose) because of enzymes

Digestive enzymes are one method of…

chemical digestion

most enzymes are very specific about the…

actions they can do

Major action by the tongue, teeth, and stomach, is an example of…

mechanical digestion

when plants first evolved the cellulose-like lignin, couldn’t be broken down , because…

there was no enzyme able to break the carbohydrate links

No break down of plants (~360mya) meant…

no decomposition and plant material built up for 40million years, which lead to carbon fuels

The four major macromolecules for life

• Carbohydrates

• Proteins

• Lipids (fats)

• Nucleic Acids

The primary law of digestion

• More Surface Area = More Digestion

• Smaller objects have a larger surface area to volume ratio

Similarities of Peristalsis Vs Segmentation

Both are caused by the smooth muscle that surrounds the canal, but have different purposes

Peristalsis vs Segmentation

• Peristalsis moves food down the canal (unidirectional)

• Segmentation sloshes food back-and-fourth and divides the food into smaller chunks

Peristalsis

Peristalsis consists of rhythmic, wavelike contractions that push food forward (unidirectional).

Segmentation

Segmentation uses localized, back-and-forth contractions of circular muscles, primarily in the small intestine, to mix food with digestive juices and enhance absorption.

The Alimentary Canal is…

• “The tube that has to do with food”

• It is technically outside the body

Peritoneal Cavity

• Posterior to thoracic cavity (heart/lungs)

• Enlarged area to horse gut

The alimentary canal is wrapped in a ….

thin sheet of endothelial cells called Peritoneum

The peritoneum is similar to what other body parts?

• Pericardium (surrounding the heart)

• Pleura (Surrounding the lungs)

peritoneum

the serous membrane lining the cavity of the abdomen and covering the abdominal organs.

The (peritoneum) sheet is continuous, but it split into two functional types:

• Parietal

• Visceral

Parietal Peritoneum

lines the outer abdominal wall

Visceral Peritoneum

lines the organs themselves

Peritoneum originates from

mesoderm that wraps around the endoderm gut

Mesentery

The peritoneum that connects organs to the abdominal wall

the Mesentery serves to

• Hold organs mostly in place (limited sloshing)

• Act as a bridge to connect sensitive blood and lymph vessels and nerves

Not all organs float freely in the abdomen, some are…

partially enveloped into the dorsal wall

retroperitoneal organs

abdominal organs located behind the parietal peritoneum and outside the peritoneal cavity, positioned against the posterior abdominal wall

retroperitoneal organs include…

the kidneys, esophagus, rectum, and others

The alimentary canal has four main tunics surrounding the lumen:

1. Mucosa

2. Submucosa

3. Muscularis externa

4. Serosa

Mucosa

Directly in contact with lumen

Three main functions of the Mucosa:

• Protect against pathogens

• Absorb broken-down digestive products

• Secrete mucous / enzymes / hormones that aid in digestion

Submucosa

Connective tissue housing large blood and lymphatic vessels as well as nerves

Submucosa Functions

• Supportive to the mucosa

Muscularis Externa (muscularis)

• Two perpendicular bands of muscle

  • Inner circular layer

  • Outer longitudinal layer

• A thickened region of circular muscle is called a sphincter

Sphincters help…

halt the flow of food material down the digestive tract

Failure of the esophageal sphincter is a common cause for

Acid Reflux Disease

Serosa consists of

Consists of a thin connective layer surrounded by visceral peritoneum

The Serosa forms

This forms the outer structure of the mesentery

Portions of AC that are attached to the posterior wall have

adventitia instead of serosa

Serosa does what?

This holds the tube firmly to surrounding tissues

The esophagus is the only portion…

entirely surrounded by adventitia

A

The secretory cells, exocrine pancreatic cells, acid generating stomach cells, and the many other accessory glands need to be…

turned on and off

• the enteric NS controls this

Food is coming down the pipe —>

Start releasing enzymes

The stomach is empty —>

Slow acid production

Enteric Nervous System

complex, network millions of nerves lining the gastrointestinal tract from the esophagus to the rectum

Who has more nerve cells, the spinal cord of the enteric NS?

The Enteric NS

Submucosal Nerve Plexus

controls the glands and smooth muscle of the mucosa

Myenteric Nerve Plexus

controls circular and longitudinal smooth muscles

A reflex arc is…

the response to a stimulus from sensation → integration (sometimes brain) → action

In the ‘gut brain’ there are

long and short reflexes

Short reflexes are handled…

entirely within the digestive system

Short Reflex

When the signals and reactions are entirely controlled by the ENS

Long reflex

The ‘normal’ fxn is modified by parasympathetic (stimulates) and sympathetic (inhibits/halts) nerves

Mechano-osmo-chemo-receptors

detect the signal

Neurons

regulate activity by integrating multiple signals, PNS/SNS, endocrines

Smooth muscle and glands

initiates change

Enteric Nervous System Parts

• Mechano/osmo/chemo receptors

• Neurons

• Smooth muscle and glands

Hirschsprung’s is an example of

myenteric plexus function loss

The microbiome

• the assemblage of bacteria and fungi that reside in the
  stomach and intestine

• is often telling of a species diet

Your microbiome is more determined by

diet than species relationships

Carnivore Guts

• ‘Feast and Famine’ means you might go long periods without food, then acquire large amounts

• Simpler guts and expandable stomachs

Herbivore Guts

• More difficult to digest food requires bacterial help

• Foregut vs Hindgut is simply where are housing the fermenting bacteria

The Mouth’s majors components

• lips

• tongue

• salivary glands

• teeth

Where is the initial site of physical and chemical breakdown

The mouth

The mouth is made out of…

Highly keratinized stratified squamous skin to survive abrasion

The lips (labia) and tongue do what?

manipulate food, arranging it for mastication by the teeth

ostracoderms + placoderms had…

• The dermal plates of ostracoderms + placoderms provide protection from predators

• These large sheets of enamel were inflexible, limiting movement

odontodes

Large armor plates break up into many smaller enamel flakes

surface odontode scales teeth were exapted to

odontode teeth in the newly evolved jaws

• odontode theory

odontode theory

vertebrate teeth evolved from dermal denticles—small, armored, tooth-like skin structures found on ancient armored fish (odontodes)—which migrated into the mouth

All vert teeth share:

• Enamel (or enamel-like)
  outer layer

• Dentin

• Pulp

All the layers of the teeth are derived from

neural crest cell mesoderm + oral region ectoderm

The tooth is mostly

Dentin

dentin

a firm but slightly pliable protein-rich calcium matrix

Enamel

is extremely hard but brittle and surrounds the crown portion of dentin

Dental Formula

expressed as I:C:P:M (Incisors, Canines, Premolars, Molars)

• times this by two to get the top and bottom

diastema

a space separating teeth of different functions, especially that between the biting teeth (incisors and canines) and grinding teeth (premolars and molars) in rodents and ungulates.

why might there be diastema before the premolars

Probably to be able to chew fibrous food, as it’s much more common in herbivores

The Earliest Dental Formula

• 0000

• Pangolins, Anteaters, and some baleen whales have the easiest dental formula

Haplodont

• Haplodont teeth are the simplest form of tooth, characterized by a single, conical crown lacking ridges, cusps, or tubercles.

• Found commonly in reptiles, amphibians, and some marine mammals, these teeth are designed for prehension (grasping) or combat rather than chewing

Triconodont

• Triconodont teeth are characteristic molars of early, largely carnivorous Mesozoic mammals featuring three, usually cone-shaped, cusps arranged in a linear or slightly curved row (a-b-c).

• They are known for providing a, self-sharpening, shear-based cutting mechanism

Tribosphenic

Tribosphenic refers to a specialized, three-cusped mammalian molar tooth design that combines shearing and grinding, essential for the evolutionary success of marsupials and placentals

The interactive effect of tribosphenic molars acts as a

motar-and-pestle to efficiently grind food particles

“The most important mammalian innovation”

The interactive effect of tribosphenic molars