PHYSIOLOGY TEST 1

Study of the normal functioning of a living organism and its component parts (includes all chemiscal and physical processes)

Heart, blood vessels, and blood; Transport of materials b/t all cells of the body

Stomach, intestine, liver, and pancreas; conversion of food into particles that can be transported into the body (elimination of some wastes)

Throid gland, adrenal gland; Coordination of body function through synthesis and release of regulatory molecules.

Thumus, spleen, lymph nodes; Defense against foreign invaders.

Skin; protection from external enviroment.

Skeletal muscles and bones; Support and movement.

Brain and spinal cord; Coordination of bodu functions through electrical signals and release of regulatory molecules.

Ovaries, uterus, and testes; Perpetuation of the species

Lungs and airways; Echange of oxygen and carbon dioxide b/t internal and external enviroments.

Kidneys and bladder; Maintance of water and solutes in the internal enviroment/waste removal.

Regulation of the body’s internal environment (maintaing a stable internal enviroment/state of balance)

Outside world, extracellular fluid, and intraceullular fluid.

Focuses on the purpose (the "why"). EXAMPLE: Why do red blood cells transport oxygen?

Focuses on the process (the "how"). EXAMPLE: How do red blood cells transport oxygen?

body's ability to maintain stability while constantly making small adjustments to respond to changes in the environment. It keeps conditions within a normal range but is always active and adjusting.

Intracellular Fluid and Inside cells (about ⅔ of total body water)

Extracellular Fluid and Outside cells (about ⅓ of total body water), which includes: intersitial fluid and blood plasma.

Low in ICF and high in ECF

High in ICCF and low in ECF

Low in ICF and High in ECF

Very low in ICF and higher than ICF in ECF

Yes; the body constantly exchanges energy and matter with its surroundings. It is not a closed or isolated system. (Energy exchange through food or movement, matter exhange through oxygen and CO2 and information exchange through external stimuli like temp, light, or sounds).

A regulated variable is a physical condition (like body temperature, blood glucose, or blood pressure) that the body keeps within a normal range using control mechanisms.

The body keeps regulated variables near their set point or optimum value to maintain homeostasis.

Local and reflex control

Local control is when regulation occurs within a small area of the body, such as blood vessels dilating near active muscles to increase blood flow.

Reflex control involves long-distance regulation, often involving the nervous or endocrine systems, like the brain signaling the heart to beat faster when blood pressure drops.

Input signal, integrating center, output signal, and response.

Control systems maintain homeostasis by regulating key variables like body temperature, blood pressure, and hormone levels.

Local control happens in a small area of the body without involving the brain.
Reflex control involves long-distance communication, usually through the nervous or endocrine systems.

Stimulus, Sensor, Input signal, Integrating center, Output signal, Target, Response

A positive feedback loop amplifies the stimulus to push it further, like increasing contractions during childbirth.

A negative feedback loop stabilizes a variable and brings it back to normal, like cooling down when body temperature rises.

Feedforward control anticipates a change and prepares the body to respond, like salivating when you smell food.

Small fluctuations around the body’s normal setpoint due to external or internal changes.

When blood sugar rises, insulin lowers it. When blood sugar normalizes, insulin stops.

Childbirth—contractions cause more contractions until delivery.

Repeating patterns in the body, such as circadian rhythms (24-hour cycles).

Sleep-wake cycle, hormone release, and body temperature changes throughout the day.

Hollow organs with a lumen, such as the heart, lungs, blood vessels, and intestines.

The inside space of a hollow organ (e.g., the inside of the stomach or intestines).

Internal Environment = The fluids and spaces inside the body where cells live.
External Environment = Everything outside the body OR inside hollow organs open to the outside (e.g., air in the lungs, food in the intestines).

Epithelium, a protective tissue that controls what enters and exits.

Fluid outside of cells. It includes plasma (blood fluid) and interstitial fluid (fluid between cells).

Fluid inside cells where most cell functions take place.

Cranial cavity (brain), thoracic cavity (heart & lungs), and abdominopelvic cavity (digestive & reproductive organs).

Body fluid compartments: Intracellular Fluid (ICF) inside cells and Extracellular Fluid (ECF) outside cells (plasma & interstitial fluid).

A model of biological membranes showing how phospholipids, proteins, cholesterol, and carbohydrates work together to create a flexible, selective barrier.

They form tiny channels between cells to allow molecules and signals to pass through (e.g., in the heart). Commmunication and connexin membrane protein!

They seal cells together to prevent leakage (e.g., in the intestines to keep bacteria out). Occluding, claudin/occludin membrane protein, and actin cytoskeleton fibers!

They hold cells together like strong anchors to resist stretching (e.g., in the skin). Anchoring, cadherin membrane protein, and intermediate filaments cytoskeleton fibers.

They help cells stick together and communicate (e.g., in tissues that stretch and contract). Anchoring, Cadherin membrane proteins, and actin cytoskleton fibers)

Cell-matrix junctions that anchor cells to the basement membrane using integrins. Found in skin cells.. Anchoring, intergrin membrane proteins, actin cytoskeleton fibers, and fibronetin matrix protein.

Junctions that connect the actin cytoskeleton of a cell to the extracellular matrix, important for movement and contraction. Anchoring, intergrin membrane proteins, Keratin (intermediate filaments) cytoskeleton fibers, and Laminin matrix proteins.

A tightly packed sheet of cells that covers external surfaces and lines internal surfaces, attached to a basement membrane.

Pseudostratified Ciliated Columnar (trachea, mucus movement)
Stratified Squamous (skin, protection)
Simple Cuboidal (kidneys, secretion/absorption)
Simple Squamous (lungs, rapid exchange)
Stratified Columnar (rare, male urethra)
Simple Columnar (intestines, absorption/secretion)

Exchange – Thin, allows rapid movement (lungs, blood vessels).
Transporting – Absorbs & secretes (intestines, kidney).
Ciliated – Moves fluids (trachea, reproductive tract).
Protective – Many layers, strong (skin, mouth).
Secretory – Releases substances (glands, pancreas, thyroid).

Single layer of flattened cells with pores for easy molecule passage. Found in lungs and blood vessels.

Single layer of cuboidal/columnar cells with tight junctions to prevent leakage and microvilli to increase surface area. Found in intestines, kidney, and some glands.

Single layer of columnar/cuboidal cells with cilia to move fluids. Found in nose, trachea, upper airways, and female reproductive tract.

Many layers of flattened surface cells, connected by desmosomes for strength. Found in skin and cavities open to the environment.

Columnar or polygonal cells that produce proteins (with lots of rough ER) or steroids (with lipid droplets & smooth ER). Found in glands (pancreas, sweat, salivary, thyroid, gonads).

A thin layer of extracellular material that lies between the epithelium and connective tissue. It provides support, transport of nutrients/waste, and filtration (especially in the kidneys).

Structural support – Anchors epithelial cells.
Transport – Allows nutrients and waste to pass.
Filtration – Acts as a barrier for large molecules (kidneys).

They form when epithelial cells grow inward, creating secretory structures. Some remain connected (exocrine), while others detach (endocrine).

Exocrine glands – Have ducts, release secretions onto surfaces (sweat, saliva).
Endocrine glands – No ducts, release hormones into the blood (thyroid, adrenal glands).

Sweat glands, salivary glands, pancreas (digestive enzymes).

Thyroid, adrenal glands, pancreas (insulin production).

Ground substance (matrix) – Hydrated gel-like material that provides structural support and facilitates molecular diffusion.
Connective tissue cells – Fixed (maintenance, repair, energy storage) & Mobile (immune defense).
Matrix fibers – Collagen (tensile strength), elastin (elasticity), fibrillin (scaffolding), fibronectin (adhesion & cell migration).

Provides structural support for cells.
Facilitates cell signaling and interactions.
Regulates cellular functions (adhesion, migration, differentiation).
Acts as a biochemical barrier controlling molecule diffusion.

Fibroblasts (-blasts) – Synthesize collagen, elastin, and ECM components.
Fibrocytes (-cytes) – Maintain connective tissue.
Osteoblasts & Osteocytes – Bone formation and maintenance.
Chondroblasts & Chondrocytes – Cartilage formation.
Macrophages – Phagocytosis & immune response.
Mast cells – Release histamine (inflammation).
Adipocytes – Fat storage & insulation.

*Glycosaminoglycans (GAGs) – Highly charged polysaccharides involved in hydration and elasticity. Examples:
- Hyaluronan (hydration & joint lubrication)
- Aggrecan (cartilage structure)
- Chondroitin sulfate, Heparan sulfate, Keratan sulfate.
*Fibrous Proteins – Provide tensile strength & elasticity.
- Collagen (structural integrity).
- Elastin (recoil ability).
- Fibrillin & Fibronectin (scaffolding & adhesion).
* Glycoproteins (non-collagen proteins) – Cell-ECM interactions.
- Fibronectin (cell adhesion, migration, wound healing).
- Laminin (found in basal lamina, supports epithelial cells).

Provides tensile strength to tissues.
Forms fibrils, networks, and anchoring structures.
Key component in skin, bones, tendons, ligaments, and cartilage.
Defects in collagen cause osteogenesis imperfecta, Ehlers-Danlos syndrome, and glomerulonephritis.

Type I – Found in bones, tendons, and skin (90% of body collagen). Defect: Osteogenesis imperfecta (brittle bone disease).
Type II – Found in cartilage and intervertebral discs. Defect: Osteoarthritis, chondrodysplasia.
Type III – Found in skin, blood vessels, internal organs. Defect: Ehlers-Danlos syndrome (fragile skin & loose joints).
Type IV – Forms the basal lamina. Defect: Kidney disease (glomerulonephritis).
Type VII – Anchors epithelial layers to connective tissue. Defect: Skin blistering disorders.

Epidermis – Outer protective layer, keratinized epithelium.
Dermis – Connective tissue with collagen, elastin, and fibroblasts.
Hypodermis (subcutaneous layer) – Fat storage & insulation.

Keratinocytes – Produce keratin for waterproofing & protection.
Melanocytes – Produce melanin (pigmentation & UV protection).
Langerhans cells – Immune response (antigen-presenting).
Merkel cells – Sensory receptors (touch & pressure).

Forms a barrier against water loss.
Prevents entry of microbes & harmful substances.
Essential for maintaining homeostasis in dry environments.

Desmosomes – Strong cell-to-cell junctions that hold keratinocytes together in the epidermis.
Hemidesmosomes – Anchor epithelial cells to the basal lamina using integrins and type IV collagen.
Defects in these structures can cause blistering disorders (e.g., epidermolysis bullosa).

Structural support for epithelial cells.
Selective filtration barrier (especially in kidneys).
Cell signaling and differentiation.
Major components: Laminin & Type IV Collagen.

Osteogenesis Imperfecta (Type I Collagen defect) → Fragile bones, fractures.
Ehlers-Danlos Syndrome (Type III defect) → Hypermobile joints, fragile skin, vascular rupture.
Marfan Syndrome (Fibrillin-1 mutation) → Tall stature, cardiovascular issues, lens dislocation.
Epidermolysis Bullosa (Desmosome/Hemidesmosome defect) → Severe skin blistering.

Fibrosis – Excess collagen deposition (e.g., pulmonary fibrosis, liver cirrhosis).
Cancer metastasis – Altered ECM allows cancer cells to invade and spread.
Arthritis – Breakdown of cartilage ECM (Type II collagen degradation).

Simple diffusion and facilitated diffusion

Chemically gated, voltage-gated, and mechanically gated channels

Non-random movement of water across a membrane in response to a solute concentration gradient, occurring through lipid bilayers, aquaporins and water-filled ion channels

280-296 mOsM

ECF: 135-147 mEq/L; ICF: 10-15 mEq/L

ECF: 3.5-5.0 mEq/L; ICF: 120-150 mEq/L

High glucose levels increase metabolism and ATP, closing KATP channels, which leads to cell depolarization, calcium channel opening, and insulin release

It helps maintain electrochemical gradientesting membrane potential, with cells being about 40x more permeable to K+ than Na+

The movement of molecules from an area of higher concentration to an area of lower concentration, using their own kinetic energy, without needing an external energy source.

Diffusion uses the kinetic energy of molecules.
Molecules move from high to low concentration.
Diffusion continues until equilibrium is reached.
After equilibrium, molecular movement still continues, but concentrations stay the same.

Larger concentration gradients.
Shorter distances.
Higher temperatures.
Smaller molecules.

Diffusion can happen in open systems or across a partition that separates two systems.

Larger surface area.
Thinner membrane.
Larger concentration gradient.
More permeable membrane to the molecule.

Lipid solubility of the molecule.
Size of the molecule.
The lipid composition of the membrane.

The rate of diffusion is directly proportional to:
Surface area.
Concentration gradient.
Membrane permeability.

Lipid solubility.
Molecular size.
Concentration gradient.
Membrane surface area.
Composition of the lipid layer.

It depends on the molecule's lipid solubility and size, as well as the lipid composition of the membrane.

The diffusion coefficient, based on temperature, molecule size (radius), and viscosity of the medium

Higher temperature increases molecular movement, which speeds up diffusion.

The rate of diffusion is proportional to surface area, concentration gradient, and membrane permeability.

Smaller molecules diffuse faster than larger molecules.

It is a physical constant that relates temperature and molecular movement in diffusion.

Surface area of the membrane.
Thickness of the membrane.
Concentration gradient.
Membrane permeability (based on lipid solubility and molecular size).

Extracellular fluid (ECF) and intracellular fluid (ICF).

Intracellular fluid (ICF) is 2/3 of total body water.
Extracellular fluid (ECF) is 1/3 of total body water.

Interstitial fluid (3/4 of ECF) – lies between cells and the circulatory system.
Plasma (1/4 of ECF) – the liquid matrix of blood.

TBW = 0.6 × body weight.
For a person weighing 70 kg, TBW = 42 L.

ECF = 0.2 × body weight (14 L).
ICF = 0.4 × body weight (28 L).

Infants: 65% of body weight.
Men: Decreases from 65% in infants to 52% by age 60+.
Women: Decreases from 65% in infants to 46% by age 60+.

The concentrations of ions and molecules are different in the ECF and ICF. This disequilibrium is maintained by selective barriers like the cell membrane.

The cell membrane is a selectively permeable barrier that separates the ECF and ICF, regulating the movement of substances between them.

Substances move across the leaky exchange epithelium of the capillary wall.

Na+/K+ ATPase: Decreases intracellular Na+ and increases K+.
K+ channels: Some K+ exits the cell, generating a negative membrane potential (Vm).
Other transport mechanisms: Drive Cl- and Ca++ out of the cell to maintain low intracellular Ca++ levels.

Intracellular fluid: High in K+, phosphate (Pi), and proteins.
Extracellular fluid: High in Na+, Cl-, and Ca++.

Proteins are bound to ions like Ca2+ and Pi, especially in the intracellular fluid, and help maintain the chemical composition of the compartments.

Osmosis is the non-random movement of water across a membrane in response to a solute concentration gradient.

Water moves through lipid bilayers, aquaporins, and water-filled ion channels.

Osmotic pressure is the pressure exerted by water moving across a membrane due to a solute concentration gradient.

Osmolarity (OsM): Moles of solute per liter of solvent.
Osmolality (Osm/kg): Moles of solute per kilogram of solvent.

Osmolarity is a colligative property because it depends on the total solute concentration, not the type of solute.

The normal range is 280-296 mOsM.

Oncotic pressure is associated with plasma proteins and is greater than predicted by van't Hoff's law for osmotic pressure.

Membrane permeability affects how easily solutes can cross the membrane, influencing concentration gradients and osmosis.

Penetrating solutes can move across the membrane and reach equilibrium between compartments.
Non-penetrating solutes remain in disequilibrium between compartments.

Isosmotic: Solutions with equal osmolarity.
Hyperosmotic: Solutions with higher osmolarity than the reference solution.
Hyposmotic: Solutions with lower osmolarity than the reference solution.

Isotonic: No change in cell volume (osmolarity of solution equals the cell’s).
Hypertonic: Cell shrinks (osmolarity of solution is higher than the cell’s).
Hypotonic: Cell swells (osmolarity of solution is lower than the cell’s).

Van't Hoff's law calculates the osmotic pressure (Π = nCRT), where:
Π = osmotic pressure,
n = number of dissociable particles,
C = concentration of solute,
R = gas constant,
T = temperature in Kelvin.

Osmolarity helps predict tonicity, but tonicity specifically describes the volume change of cells when exposed to a solution, based on nonpenetrating solute concentrations.

0.9% saline: Isosmotic, isotonic.
5% dextrose in 0.9% saline: Hyperosmotic, isotonic.
5% dextrose in water: Isosmotic, hypotonic.
0.45% saline: Hyposmotic, hypotonic.
5% dextrose in 0.45% saline: Hyperosmotic, hypotonic.

Assume all intracellular solutes are nonpenetrating.
Compare osmolarities before the cell is exposed to the solution.
Tonicity describes the volume change of a cell.
Compare nonpenetrating solute concentrations to determine tonicity.
Hyposmotic solutions are always hypotonic.

The Gibbs-Donnan effect occurs when penetrating ions diffuse across a membrane, resulting in electrical equilibrium at the expense of chemical equilibrium. This imbalance drives osmosis and influences osmotic pressure.

The electrical charge of penetrating and non-penetrating solutes affects chemical disequilibrium and osmotic pressure by influencing ion distribution and membrane voltage, which can drive water movement.

In the Gibbs-Donnan effect, a membrane is permeable to ions like Na+ and Cl-, but not proteins. When Cl- diffuses, it causes an electrical imbalance that drives Na+ to diffuse and increases osmolality, causing water to flow into the compartment.

Cells swell in hypotonic media and undergo a regulatory volume decrease (RVD), where they lose KCl and organic osmolytes, reducing osmotic pressure and causing water to leave the cell.

Cells shrink in hypertonic media and undergo a regulatory volume increase (RVI), where NaCl and organic osmolytes enter the cell, increasing intracellular osmotic pressure and bringing water back into the cell.

Passive Diffusion: Movement through the lipid bilayer.
Facilitated Diffusion: Movement through channels or carriers.
Active Transport: Requires ATP to move molecules against concentration gradients.

Channel-mediated transport involves ion channels (e.g., water channels, ion channels) that allow passive diffusion of molecules across the membrane. Channels can be open or gated (chemically, voltage, or mechanically).

Carrier-mediated transport involves proteins that facilitate the movement of solutes across the membrane. It can be passive or active, depending on whether energy is required.

Electrolyte selectivity is influenced by the charge of the channel, the hydration of ions, and the size of the ions. For example, K+ interacts more easily with channels than Na+ because K+ is larger and dissociates from water more readily.

Symport carriers: Transport two different molecules in the same direction, such as Na+-glucose or Na+-amino acids.
Antiport carriers: Transport two molecules in opposite directions, such as Na+-H+ or Na+-Ca2+.

Symport: Na+-glucose (SGLT), Na+-amino acids.
Antiport: Na+-H+ (NHE), Na+-Ca2+ (NCX).

Passive Diffusion: Uses molecular motion, no ATP required.
Active Transport: Requires ATP to move molecules against concentration gradients.

GLUT transporters facilitate glucose entry into cells down its concentration gradient. There are different GLUT transporters with tissue-specific expression, such as GLUT1 (blood), GLUT2 (liver), GLUT3 (brain), and GLUT4 (muscle and adipose tissue).

Facilitated diffusion via GLUT transporters brings glucose into the cell by utilizing a concentration gradient. Glucose is converted into glucose-6-phosphate to keep intracellular glucose low and prevent equilibrium.

In the presence of other sugars, such as galactose, competition for the transporter slows down glucose transport, as the transport rate depends on the concentration of glucose and other competing substrates.

SGLT uses the energy from the sodium gradient (created by the Na+/K+ pump) to bring glucose into the cell. Sodium binding increases the glucose binding affinity, and glucose enters the cell as sodium is released.

Na+-K+-ATPase: Pumps sodium out and potassium into the cell, maintaining the electrochemical gradient.
Ca2+-ATPase: Pumps calcium out of the cell.
H+-ATPase: Pumps protons (H+) across membranes.
H+-K+-ATPase: Involved in acid secretion in the stomach.

The Na+-K+-ATPase exchanges 3 Na+ out of the cell for 2 K+ into the cell, utilizing ATP. This process helps maintain the resting membrane potential and the electrochemical gradient.

Membrane potential is measured using two electrodes: one inside the cell and the other in the extracellular bath. The difference in electrical charge between the inside and outside of the cell gives the membrane potential (Vm).

Depolarization: The membrane potential becomes less negative.
Repolarization: The membrane potential returns to resting state.
Hyperpolarization: The membrane potential becomes more negative.

Paracellular transport: Movement of substances between adjacent cells.
Transcellular transport: Movement of substances through the cells themselves.

Glucose is absorbed from the lumen through the Na+-glucose symporter (SGLT) against its gradient. Then, GLUT transporters move glucose across the basolateral membrane into the extracellular fluid.

Transcytosis is the process of vesicular transport across endothelial cells, where substances like plasma proteins are endocytosed, transported across the cell, and exocytosed into the interstitial fluid.

In the presence of high glucose, glucose is taken into the beta cells via GLUT transporters. This increases ATP, which closes KATP channels, depolarizes the cell, and opens voltage-gated Ca2+ channels. Calcium influx triggers exocytosis of insulin from secretory vesicles.

Skeletal muscle fibers are large, multinucleate, and striated. They are made up of muscle fascicles, which are bundles of muscle fibers, and are connected to bones via tendons.

Cardiac muscle fibers are smaller, branched, and uninucleate. They are also striated and are connected by intercalated discs, which facilitate synchronized contractions.

Smooth muscle fibers are small, lack striations, and are uninucleate. They are found in the walls of hollow organs such as the intestines and blood vessels.

Flexion: Moves bones closer together.
Extension: Moves bones away from each other.
Flexors bring bones together, while extensors move them apart. These movements typically occur in antagonistic muscle pairs.

A sarcomere consists of the following key regions:
A band: Contains myosin filaments and overlapping actin.
I band: Contains only actin filaments.
H zone: Contains only myosin filaments.
Z disk: Anchors actin filaments.
M line: Holds myosin filaments in place.

During contraction, the sarcomere shortens. The I band and H zone shorten, but the A band remains constant. Actin and myosin slide past each other without changing length.

Titin: Provides elasticity and stabilizes myosin filaments.
Nebulin: Aligns actin filaments at the Z disk and helps maintain their structure.

Skeletal muscles are organized into:
Muscle fibers (cells) grouped into muscle fascicles.
Connective tissue (epimysium, perimysium, and endomysium) surrounds and binds these structures.
Blood vessels and nerves support muscle function.

Skeletal muscle fibers: Multinucleate.
Cardiac muscle fibers: Uninucleate.
Smooth muscle fibers: Uninucleate.

Antagonistic muscle pairs are muscles that work in opposition to each other, such as the biceps (flexor) and triceps (extensor). One muscle contracts while the other relaxes to produce movement.

Desmin connects the sarcomeres and the sarcolemmal cytoskeleton, maintaining the structural integrity of cardiac muscle cells and facilitating contractile force transmission.

Proteins like α-actinin anchor actin filaments, while others such as T-cap and filamin are involved in structural regulation and force transmission. Mutations in these proteins are associated with cardiomyopathies.

The myotendinous junction connects the end of muscle fibers to tendon collagen fibers. It has folded sarcolemma to increase surface area, allowing more efficient force transfer between muscle fibers and tendons.

Costameres facilitate the lateral transmission of force to the extracellular matrix, enhancing muscle efficiency, maintaining integrity, distributing force, and protecting the fiber from injury.

T-tubules are extensions of the sarcolemma that bring action potentials into the interior of muscle fibers. They are functionally linked to the sarcoplasmic reticulum, which stores calcium ions (Ca²⁺) for contraction.

A somatic motor neuron releases acetylcholine (ACh) at the neuromuscular junction.
The muscle fiber generates an action potential that travels down the T-tubules.
The DHP receptor in the T-tubule alters its conformation and opens the RyR calcium release channels in the sarcoplasmic reticulum.
Ca²⁺ is released into the cytoplasm and binds to troponin, allowing actin-myosin binding for contraction.

Once Ca²⁺ binds to troponin, it shifts tropomyosin, allowing myosin heads to bind to actin. The myosin heads then execute a power stroke, sliding actin filaments toward the center of the sarcomere, resulting in contraction.

The sarcoplasmic Ca²⁺-ATPase (SERCA) pumps Ca²⁺ back into the sarcoplasmic reticulum.
As Ca²⁺ levels decrease, Ca²⁺ unbinds from troponin, causing tropomyosin to cover the actin binding sites.
Myosin heads release, and the muscle returns to its relaxed state.

The sarcoplasmic reticulum (SR) stores Ca²⁺ in its terminal cisternae. During contraction, Ca²⁺ is released into the cytoplasm through RyR channels. During relaxation, Ca²⁺ is pumped back into the SR by SERCA pumps, facilitated by proteins like calsequestrin and sarcalumenin.

In healthy skeletal muscle, dystrophin is involved in connecting the cytoskeleton to the extracellular matrix. In DMD, mutations in dystrophin cause its absence, leading to muscle fiber damage and progressive muscle weakness.

Lateral force transfer: Increases muscle efficiency, maintains integrity, prevents injury, and distributes force across the myofiber.
Longitudinal force transfer: Directly transfers force to tendons for muscle contraction.

Skeletal muscle is surrounded by connective tissues:
Epimysium: Surrounds the entire muscle.
Perimysium: Surrounds muscle fascicles.
Endomysium: Surrounds individual muscle fibers.
Blood vessels and nerves supply nutrients and signals to muscle fibers.

In the rigor state, myosin is tightly bound to actin without ATP. The myosin head is locked in place, and the actin-myosin bond is in its most stable form, preventing further contraction.

The power stroke occurs when myosin binds strongly to actin, and the myosin head swivels, pulling the actin filament toward the M line, shortening the muscle fiber.

ATP binds to myosin, causing it to release actin. Myosin then hydrolyzes ATP, which provides energy to rotate the myosin head into the cocked position, ready for the next contraction.

Ca²⁺ binds to troponin, causing tropomyosin to move away from the myosin-binding sites on actin, allowing myosin to bind strongly to actin and initiate the power stroke.

Action potentials from the CNS arrive at the neuromuscular junction.
Acetylcholine (ACh) is released and binds to receptors on the muscle fiber, generating an action potential.
The action potential travels down the T-tubules, causing Ca²⁺ release from the sarcoplasmic reticulum (SR).
Ca²⁺ binds to troponin, initiating the actin-myosin interaction and contraction.

Latent Period: The time between stimulation and the beginning of contraction.
Contraction Phase: The muscle generates tension and contracts.
Relaxation Phase: The muscle relaxes as Ca²⁺ is pumped back into the SR.

Resting muscle stores energy in phosphocreatine. When the muscle contracts, phosphocreatine is broken down to regenerate ATP, providing energy for contraction, calcium pumping, and ion restoration.

Central Fatigue: Psychological effects and protective reflexes.
Peripheral Fatigue: Includes Ca²⁺ leakage from the SR, depletion of energy stores (glycogen, phosphocreatine), and accumulation of metabolic byproducts like inorganic phosphate (Pi) and lactate.

Slow-Twitch Fibers: Use oxidative metabolism, fatigue-resistant, smaller in diameter, and used for posture and endurance activities.
Fast-Twitch Fibers: Use anaerobic metabolism, fatigue easily, larger in diameter, and used for quick, powerful movements.

Slow-Twitch Oxidative (Red): High myoglobin, numerous mitochondria, fatigue-resistant, used for posture.
Fast-Twitch Glycolytic (White): Larger diameter, fewer mitochondria, fatigue quickly, used for rapid, fine movements.

Summation: Occurs when stimuli are close enough that the muscle doesn’t fully relax between contractions.
Unfused Tetanus: Partial relaxation between stimuli, leading to increased tension.
Complete Tetanus: No relaxation between stimuli, producing maximum tension until fatigue sets in.

The optimal length of a muscle is when thick and thin filaments overlap at an ideal point, generating the maximum tension. Too much or too little overlap results in decreased tension.

Normal muscle contractions are graded responses involving:
Frequency Summation: Increased stimulus frequency leads to more forceful contractions.
Tetany: A sustained contraction resulting from rapid stimulus frequency.
Motor Unit Recruitment: More motor units are recruited to increase contraction force.
Asynchronous Recruitment: Different motor units take turns maintaining tension, helping to avoid fatigue.

A motor unit consists of one motor neuron and all the muscle fibers it innervates. A muscle can have many motor units of different fiber types.

Isotonic Contractions: The muscle generates force and shortens to move a load (concentric or eccentric).
Isometric Contractions: The muscle generates force without shortening or moving a load.

Elastic components (like tendons) allow isometric contractions by stretching while sarcomeres shorten. In isotonic contractions, sarcomeres shorten more, but the elastic components are already stretched, allowing muscle shortening.

The bones act as levers, and the joints act as fulcrums. In the forearm, the elbow is the fulcrum, the load is the weight of the arm, and force is applied by muscles to move the load.

To hold the arm stationary at 90 degrees, the rotational force produced by the biceps must equal the rotational force produced by the weight of the forearm and hand.
Example: Biceps force = (Rotational force down / distance from fulcrum) = 2 kg x 15 cm / 5 cm = 6 kg of force.

For the biceps to hold the additional load:
Rotational force (upward) = 7 kg x 25 cm = 175 kg·cm.
The total biceps force needed = 175 kg·cm / 5 cm = 35 kg of force.

The insertion of the biceps close to the fulcrum results in a small movement of the biceps generating a much larger movement of the hand. For example, if the biceps shortens 1 cm, the hand moves 5 cm.

Vascular
Gastrointestinal
Urinary
Respiratory
Reproductive
Ocular

Phasic smooth muscles: Cycle between contraction and relaxation (e.g., intestine).
Tonic smooth muscles: Can maintain prolonged contraction (e.g., sphincters, vascular smooth muscle).

Single-unit smooth muscle (visceral): Cells are connected by gap junctions and contract as a single unit.
Multi-unit smooth muscle: Cells are not electrically linked, requiring independent stimulation.

Vascular smooth muscle is located in blood vessels, controlling blood flow and pressure. It is involved in both phasic and tonic contraction.

Phasic smooth muscle: Cycles between contraction and relaxation (e.g., intestine).
Tonic smooth muscle: Maintains contraction (e.g., sphincters, blood vessels) or adjusts contraction force as needed.

Activation occurs through voltage-gated Ca²⁺ channels, ligand-gated Ca²⁺ channels, and stretch-activated calcium channels, along with release from the sarcoplasmic reticulum.

Ca²⁺ binds to calmodulin (CaM).
Ca²⁺-CaM activates myosin light chain kinase (MLCK).
MLCK phosphorylates myosin light chains, activating myosin ATPase.
Myosin crossbridges slide along actin, generating tension.

Ca²⁺ is removed from the cytosol.
Ca²⁺ unbinds from calmodulin, reducing MLCK activity.
Myosin light chain phosphatase dephosphorylates myosin, reducing ATPase activity.
Less myosin ATPase activity decreases muscle tension.

Autonomic neurons (sympathetic and parasympathetic)
Hormones and paracrine signals
Examples:
Histamine (constricts airways)
Nitric oxide (relaxes blood vessels)

Sympathetic control: Uses norepinephrine to cause contraction (e.g., blood vessels).
Parasympathetic control: Uses acetylcholine (ACh) to cause relaxation (e.g., stomach, small intestine).

Smooth muscle: Uninucleate, no sarcomeres, slower contraction, graded contraction, and influenced by autonomic control.
Skeletal muscle: Multinucleate, striated, rapid contraction, requires ACh from motor neurons.
Cardiac muscle: Striated, uninucleate, autorhythmic, controlled by autonomic neurons.

It refers to muscle contraction or relaxation caused by chemical signals without significant change in membrane potential.

Muscle cramps: Caused by overuse or fatigue.
Duchenne muscular dystrophy: Absence of dystrophin leading to muscle weakness.
McArdle's disease: Deficiency in myophosphorylase, impairing glycogen breakdown.