Lecture Notes on Muscle Physiology and Digestion

Vocabulary flashcards based on lecture notes about neuromuscular junctions, muscle fiber banding, sliding filament theory, muscle proteins, muscle contraction and regulation, digestion, and related processes.

Neuromuscular junction

Site where a motor neuron stimulates a muscle fiber.

Motor end plate

Area of the muscle fiber sarcolemma where a motor neuron stimulates it using the neurotransmitter, acetylcholine.

Motor unit

A single motor neuron (with its collateral branches) and all the muscle fibers it innervates.

Graded contractions

Varied contraction strength due to different numbers of motor units being activated.

Motor unit recruitment

The process by which contraction strength comes from motor unit recruitment.

Myofibrils

Densely packed subunits within muscle fibers that run the length of the fiber.

I bands

Bands in muscle fibers that contain only thin filaments, primarily of the protein actin.

A bands

Bands in muscle fibers that contain all of the thick filaments with some thin filaments overlap; the thick filament is the protein, myosin.

H bands

The center of the A band with no thin filament overlap.

Z discs (lines)

Found in the center of each I band.

Sarcomere

The basic subunit of striated muscle contraction that runs from one Z disc to the next.

Titin

Protein that runs from the Z disc to the M line, that contributes to the force of contraction due to elastic recoil.

M lines

Found in the center of each A band and help hold down thick filaments.

Sliding Filament Theory

Theory explaining muscle contraction where actin filaments slide over myosin filaments, shortening the sarcomere.

Sarcomere

The repeating functional unit of muscle fibers, responsible for muscle contraction; contains actin, myosin, Z-disc, and M-line.

Actin

Thin filament with binding sites for myosin that gets pulled during muscle contraction.

Myosin

Thick filament that pulls actin to cause contraction.

Titin

Elastic protein that stabilizes and recoils the sarcomere.

Cross-bridges

Formed when the myosin head of a thick filament binds to a specific site on a thin filament (actin).

Tropomyosin

Protein that wraps around actin and prevents myosin from grabbing it, preventing muscle contractions until the proper signal arrives.

Troponin C (TnC)

Subunit of troponin that binds to calcium ions, which is essential for initiating muscle contraction; present in both skeletal and cardiac muscles.

Troponin I (TnI)

Subunit of troponin that inhibits the interaction between actin and myosin, preventing muscle contraction; specific to cardiac muscle and often measured to detect heart damage.

Troponin T (TnT)

Subunit of troponin that binds to tropomyosin, which is a protein that plays a role in muscle contraction; present in cardiac muscle and is also measured to detect heart damage.

Acetylcholine

Released from the motor neuron to stimulate a muscle fiber.

Action potentials

Generated along the sarcolemma and down the T tubules as part of muscle fiber stimulation.

Nicotinic ACh receptors

Receptors on the muscle cell membrane (sarcolemma) that bind acetylcholine (ACh), triggering sodium channels to open and causing an action potential.

Twitch

Muscle quickly contracts and relaxes after a single electrical shock of sufficient voltage.

Summation

When a second twitch will partially piggyback the first because a second shock is applied immediately after the first.

Latent period

Time between the stimulus and the contraction.

Tetanus

Refers to a sustained muscle contraction.

Complete tetanus (or fused tetanus)

Occurs when muscle fibers are stimulated so rapidly that they don't have time to relax between stimuli, resulting in a smooth, sustained contraction.

Incomplete tetanus (or unfused tetanus)

Occurs when the muscle fibers are stimulated at a less rapid rate, leading to a series of contractions with brief periods of relaxation between them, resulting in a less smooth, more jerky contraction.

Treppe

When a muscle is stimulated repeatedly right after it relaxes, and each contraction gets stronger than the last up to a point.

Slow-twitch (type I) fibers

Muscle fiber with slower contraction speed, sustained contraction for long periods without fatigue, rich capillary supply, more mitochondria, more respiratory enzymes, and more myoglobin.

Fast-twitch (type IIX) fibers

Muscle fiber with faster contraction speed, fatigue fast, thicker, fewer capillaries, mitochondria, respiratory enzymes, and less myoglobin.

Muscle Fatigue

A reversible, exercised-induced reduction in the ability to generate force.

Atrophy

The wasting away or decrease in size of muscle tissue.

Smooth muscle contraction

Complex process regulated by calcium and myosin light chain kinase (MLCK), leading to sustained contractions in organs and vessels.

Cardiac muscle contraction

Essential for heart function, involves excitation-contraction coupling, converting an electrical signal into a mechanical response.

Ingestion

Taking food into the mouth (eating).

Propulsion

Moving food through the digestive tract, includes swallowing and peristalsis.

Mechanical digestion

Physically breaking food into smaller pieces.

Chemical digestion

Breaking down food molecules using enzymes.

Secretion

Release of enzymes, bile, and acid to aid digestion.

Absorption

Moving nutrients into the blood or lymph.

Defecation

Removal of indigestible waste as feces.

Peristalsis

A wave-like muscle contraction that propels food and fluids through the digestive tract.

Segmentation

Churning and mixing while moving forward.

Deglutition

Swallowing.

Esophageal peristalsis

Wave-like muscle contractions that propel food down the esophagus and into the stomach.

Mucosa

Inner secretory and absorptive layer of the alimentary tract; may be folded to increase surface area; contains goblet cells.

Submucosa

Vascular layer of the alimentary tract, picks up nutrients; also has some glands and nerve plexuses.

Muscularis

Smooth muscle layer of the alimentary tract; responsible for peristalsis and segmentation; myenteric plexus for control by the ANS.

Serosa

Outer binding and protective layer of the alimentary tract; visceral peritoneum covers the organs, and the parietal peritoneum lines the abdominal cavity.

Parasympathetic division (extrinsic regulation)

Stimulates esophagus, stomach, small intestine, pancreas, gallbladder, and proximal part of large intestine via vagus nerve.

Sympathetic division (extrinsic regulation)

Reduces peristalsis and secretion; stimulates contraction of sphincters.

Barrett’s esophagus

Acid damage causes cell type change in the esophagus, which raises cancer risk

Villi

Finger-like projections in the small intestine that help increase surface area for absorption

Lacteals

Absorb fats (lipids).

Microvilli (brush border)

folding of the apical surface of each epithelial cell of villus

Enterokinase (enteropeptidase)

Important brush border enzyme that activates trypsin (from the pancreas).

Lactose Intolerance

It means the body can’t digest lactose, a sugar found in milk

Diarrhea

There is too much fluid in the stool.

Defecation

Increases, the internal anal sphincter relaxes, producing the need to defecate

Liver

The largest organ in the abdomen

Hepatocytes

liver cells arranged in rows called hepatic plates

Sinusoids

special capillaries: They have fenestrae (tiny holes), with no basement membrane

Hepatic Portal System

The nutrients (like sugars and amino acids) are absorbed in the intestines and sent to the liver first

Bile

a digestive fluid produced by the liver that helps break down fats and is then stored in the gallbladder

Bilirubin

Produced in spleen, liver, and bone marrow

Gallstones

Hard lumps that form in the gallbladder, mostly made of cholesterol

Islets of Langerhans cells

Make insulin and glucagon

Acinar cells

Make pancreatic juice

Pancreatic Juice

amalyase, trypsin, lipase

Bicarbonate formation

Made by cells lining the ductules, bicarbonate dissociates to form H+ and bicarbonate

Cystic fibrosis

Blocks bicarbonate and enzyme flow, causing thick mucus that can damage the pancreas

Enterokinase

Enzyme that activates trypsinogen trypsin.

Cephalic phase

control by brain via vagus nerves

Gastric phase

triggered by arrival of food into stomach

Intestinal phase

inhibition of gastric activity when chyme enters the small intestine

Gastrin

Stimulates HCl (parietal cells) and pepsinogen (chief cells) secretion

Ghrelin

Increases appetite and food intake

Secretin

Stimulates bicarbonate & water from pancreas/liver; helps CCK action

Cholecystokinin (CCK)

Stimulates gallbladder contraction and pancreatic enzymes

GIP (or GLIP)

Inhibits gastric movement & secretion; stimulates insulin release

PYY (Polypeptide YY)

Reduces hunger and food intake

Motilin

Stimulates stomach & small intestine contractions

GLP-1 (Glucagon-like peptide-1)

Inhibits gastric motility and secretion; boosts insulin release

Guanylin

Increases Cl⁻ secretion, leading to NaCl & water loss in feces

Somatostatin

reduces gastric activity

Salivary amylase

breaks starch into shorter chains

Pancreatic amylase

breaks starch into maltose, maltotriose, and small sugars

Glucose & galactose

cotransported with Na+

Fructose

Uses facilitated diffusion

pepsin and hydrochloric acid

produces short-chain polypeptides

brush border enzyme

aminopeptidase

Free amino acids

cotransported with Na+

Dipeptides and tripeptides

cross via secondary active transport using a H+ gradient

Fats

mix with bile to form micelles

Fats are reassembled into triglycerides, cholesterol, and phospholipids

They are packed with proteins to form chylomicrons