Cell Physiology: Structure, Function, and Environment

• Cell physiology covers introduction, parts of the cell, cell division, functional systems, cellular transport, and cell death.

Cell

• Cell is the functional unit of the body.
• The human body contains about 100 trillion cells.
• 25 trillion cells are red blood cells (RBCs).
• 75 trillion cells are other types of cells.
• Cells are formed into tissues, which form organs, then organ systems, and finally an organism.

Cellular Environment

• Body environment consists of 60% cell fluids.
• Intracellular fluid makes up 40% : Magnesium (Mg), Phosphate Molecule (PO4), Potassium (K), Sulfate (SO4), Hydrogen Carbonate (HCO3), Proteins, Lipids, and Carbohydrates.
• Extracellular fluid (internal environment) makes up 20% : Oxygen (O2), Hydrogen Carbonate ( ), Sodium (Na), Chlorine (Cl), Glucose, Fatty/Amino Acids, and Carbon Dioxide (CO2).

Parts of the Cell - Cell Membrane

• Cell membrane covers the cell and is fluid.
• It is 7.5 - 10nm thick.
• Also known as the "Phospholipid Bilayer".
• Consists of phosphate heads (hydrophilic) and lipid tails (hydrophobic).
• It is semi-permeable.
• Permeable to lipid-soluble substances like Oxygen (O2), Carbon Dioxide (CO2), and Alcohol.
• Impermeable to water-soluble substances like ions, glucose, and urea.

Composition of the Cell Membrane

• Proteins make up 55% of the cell membrane.
• Glycoproteins are included. -Integral proteins (span both sides) act as channels, carriers, and enzymes.
• Peripheral proteins (on one side only) act as enzymes and controllers of cellular functions.
• Phospholipids make up 25% of the cell membrane.
• Major membrane lipids consist of lipid bilayers.
• Structure acts as a barrier to protect the cell from environmental factors.
• Enables multiple cellular processes to occur in subcellular compartments.
• Cholesterol makes up 13% of the cell membrane.
• Affects rate of permeability.
• Affects quality/fluidity of the cell membrane.
• Phospholipids and cholesterol are insoluble in water.
• Forms the cell membrane and intracellular membrane barriers. High-Density Lipoproteins are considered "Good cholesterol" (60 mg/dL).
• Low-Density Lipoproteins are considered “Bad cholesterol” (100 mg/dL).
• Triglycerides/Neutral Fats form 95% of fat cells as the main storehouse.
• Total Cholesterol should be approximately 200 mg/dL for cardiovascular health.
• Other lipids make up 4% of the cell membrane.
• Carbohydrates make up 3%.
• They coat the cell with "Glycocalyx".
• Glycocalyx aids cell identification and adhesion between cells.

Cytoplasm

• The cytoplasm contains solid parts (organelles and particulates) and a liquid part (cytosol).
• The cytoplasm has two zones:
• Ectoplasm (cortex), which is the outer zone.
• Ectoplasm has semi-solid, gel-like support.
• Endoplasm is liquified and contains organelles.

Organelles

• Endoplasmic Reticulum (ER) synthesizes substances and transports products via ER vesicles.
• Rough ER is granular with ribosomes and performs protein synthesis.
• Smooth ER is agranular without ribosomes and performs lipid synthesis.
• Golgi Apparatus is the "Packaging Center" that receives raw materials from the ER for more synthesis.
• Lysosomes are known as "suicide bags."
• They destroy worn-out cells and foreign bodies using hydrolase.
• Lysosomes originate from the Golgi Apparatus.
• Peroxisomes perform detoxification and contain oxidase.
• They come from the smooth ER.
• Vesicles provide temporary storage.
• Mitochondria are the "powerhouse of the cell" and produce ATP.
• Microfilaments and Microtubules are polymerized fibrillar proteins/microfilaments, providing structural support and acting as a cytoskeleton.
• Mostly seen in cytoplasm.
• Polymerized tubulin/microtubules facilitate cellular locomotion and form cilia and structure.
• Nucleus is the command center of the cell, containing genetic materials and codes. It is enclosed by the nuclear membrane/envelope.
• Nuclear pores allow the passage of genetic materials.
• The outer layer is continuous with the rough ER.
• Nucleolus is rich in RNA.
  • It lacks a limiting membrane.

Cell Division

• DNA (deoxyribonucleic acid) has a double helix strand, it transports genetic material into the cytoplasm.
• RNA (ribonucleic acid) is a half-strand of the double helix.
• Messenger RNA transports genetic materials to the cytoplasm.
• Transfer RNA transports activated amino acids to ribosomes.
• Ribosomal RNA is involved in protein synthesis and ribosome production.

Formation of DNA and RNA

• DNA consists of phosphoric acid, sugar base (deoxyribose), and nitrogenous bases (Guanine, Adenine, Thymine, Cytosine).
• RNA consists of a single strand of the double helix, sugar base (ribose), and nitrogenous bases (Guanine, Adenine, Uracil, Cytosine).
• Mitosis is somatic cell division, resulting in diploid cells with 46 chromosomes.
• Meiosis is reproductive cell division, producing gametes (sperm/ovum).
• Results in haploid cells with 23 chromosomes.

Cell Division Stages (IPMAT)

• Interphase involves no cell division but includes DNA replication.
• Prophase (AFAAMCP): includes aster formation, fragmentation of the nuclear membrane, attachment of spindles to kinetochore, migration of centrioles to opposite poles, condensation of chromatid pairs, and pairing of chromatids.
• Metaphase: chromosomes align at the center (metaphase/equatorial plate).
• Anaphase: Chromatid pairs split and move towards opposite poles.
• Telophase: It includes cleavage furrow formation, resulting in two daughter cells with a reformed nuclear membrane.

Functional Systems of the Cell

• Endocytosis (ingestion) involves:
  • Pinocytosis (small particles)
  • Phagocytosis (large particles)
• Exocytosis (expulsion)
  • process of expelling cellular contents.

Cellular Transport

• Passive Transport moves substances from a high to a low concentration (downhill), requires no ATP, faces no resistance, and uses normal kinetic energy.
• Active Transport moves substances from a low to a high concentration (uphill), requires ATP, faces resistance, and works against a concentration gradient.

Passive Transport

• Simple Diffusion occurs through the membrane for lipid-soluble substances (O2, CO2, Alcohol) or through gated channels for H2O-soluble substances (ions, glucose, urea). Gated channels are triggered by voltage or chemicals (ligands).
• Facilitated Diffusion uses "carrier proteins"
  • Ex: GLUT 1 transports glucose.

Factors Affecting the Rate of Diffusion

• Distance and Size/Mass have an inverse relationship with the rate of diffusion.
• Temperature, Surface Area, Steepness of diffusion, Concentration gradient, and Partition coefficient have a direct relationship with the rate of diffusion.

Active Transport

• Primary Active Transport uses pumps (Na-K, Mg, Ca).
• Secondary Active Transport utilizes energy from primary active transport. Types include:
  • Co-Transport (Symport): Substances move in the same direction (e.g., Na-glucose linked transporter).
  • Counter-Transport (Antiport): Substances move in opposite directions (e.g., Na-Ca exchanger).
• Osmosis is a simple diffusion of H2O.

Cell Death

• Apoptosis is programmed, natural cell death.
• Necrosis is cell death due to injury and is premature.

Nerve Physiology

• Nerve, the functional unit of the nervous system.

Parts of a Neuron

• Soma: "Body" and Cellular Process.
• Dendrites: Receiving terminals that transfer information TOWARDS the soma.
• Axon: Transfers impulses AWAY from the soma.
• Axon Hillock: Origin of the axon.
• Initial Segment: Where the action potential is generated.
• Myelin Sheath: Insulated layers.
• Nodes of Ranvier: Uninsulated layers that allow "saltatory conduction".
• Axon Terminals: The branched out ends of the axon.

Glial Cells

• Two main types are microglia and macroglia.
• Glial cell is greek for "glue".
• Microglia: Act as scavenger cells and as macrophages in the CNS.
• Macroglia: Includes astrocytes and oligodendrocyte and Schwann cells.
  • Astrocytes are found in the CNS and formation of the brain.
  • Astrocytes maintain blood brain barriers and secrete/absorb neurotransmitters.
  • Oligodendrocytes (CNS) and Schwann Cells (PNS) produce myelin sheaths ("COPS").

Axonal Transport

• This process occurs via "KARD" (Anterograde/Orthograde and Retrograde).
• Anterograde/Orthograde: Transports from soma to axon terminals.
• Retrograde: Transports from axon terminals to soma, using kinesin and dynein.

Types of Fibers

• Alpha: Large, myelinated fibers (12-12 micrometers) involved in proprioception and somatic motor functions.
• Beta: Large, myelinated fibers (5-12 micrometers) related to touch and pressure.
• Gamma: Large, myelinated fibers (3-6 micrometers) that motor to ms. spindles.
• Delta: Small, myelinated fibers (2-5 micrometers) for acute pain and temperature ("cold").
• B: Small, myelinated fibers (<3 micrometers) that are preganglionic autonomic.
• C (Dorsal Root): Small, non-myelinated fibers (0.4-1.2 micrometers) for chronic pain and "temp hot".
• C (Sympathetic): Small, non-myelinated fibers (0.3-1.3 micrometers) that are postganglionic sympathetic.

According to Number

• Ia: Ms spindles, amulo-spiral, A. Alpha.
• Ib: Golgi Tendon, Organ, Alpha. A.
• II: ms spindle, flower spray, Beta. A.
• III: Acute Pain and Cold Temp, Delta. A.
• IV: Chronic Pain and Hot, C. Temp.

Dermal Receptors

• Free Nerve Endings respond to Pain.
• Merkel's respond to touch.
• Meissner's respond to touch.
• Ruffini's respond to hot stimuli.
• Krause's respond to cold stimuli.
• Pacinian respond to pressure.

Membrane Potential

• Conditions for membrane potential: Concentration Gradient and Ion Permeability.
• The difference in charges between what's inside and outside of the cell.
• The ability of certain IONS to penetrate the cell membrane.

Resting Membrane Potential

• RMP:
  • Nerve -70mV
  • Heart - 88mV
  • Smth ms. - 40 - 60mV
  • GI -56 mV
  • Skeletal ms - 90mV
• Controlling Mechanism:
  • Na-K ATPase (Pump)
  • 2 K+ ions in
  • 3 Na+ ions out
  • Two Functions:
  • Maintenance of the equilibrium
  • Maintain of the cell volume

Action Potential

• Process:
  • RMP
  • "ALL OR NONE" principle (threshold -55mV)
  • very few Na gates open
  • all remaining Na+ gates open
  • Positive feedback
• Overshoot
  • rapid closing of Na+
  • slow opening of K+ gates
• K + gates is complete open
• Slow closing of k+ gates
• Negative feedback
• Hyperpolarization
• RMP

Refractory Periods

• • No other stimulus can excite the nerve.
• Threshold -> 1/3 repolarization
• • A stronger than N stimulus can excite the nerve.
• 1/3 repolarization -> after depolarization

Summation

• Temporal Summation: One presynaptic neuron stimulates the neuron in succession.
• Spatial Summation: Many presynaptic neurons stimulate the neuron at the same time.

Muscle Physiology

• Focuses on muscle, organization, contractile filaments, events, and type of contraction.

Muscle

• Generates force and facilitates movement (locomotion) of organs.
• Types:
  • Skeletal Ms: Striated, voluntary, and multinucleated.
  • Cardiac Ms: Striated, involuntary, and uninucleated.
  • Smooth Ms: Non-striated, involuntary, and uninucleated.
    • Multi-Unit: trachea, iris, large vessels.
    • Unitary: blood vessels, digestive tract, and urinary tract.

Organization of Muscle

• Sarcomere: Functional contractile unit of the muscle, found between 2 z-discs.
• Sarcolemma: Cell membrane of muscle cells.
• Sarcoplasm: Cytoplasm of the muscle cells.
• Muscle Fibers (Myocyte): Long, red-like structures (myofibril/myosin) thick bands (myofilaments/actin complex).
• Epimysium: Outermost covering of the muscle.
• Perimysium: Covers the fascicle.
• Endomysium: Covers the muscle fibers.

Muscle Fiber Types

• Type I: Slow contraction, oxidative energy utilization, red color, resistant to fatigue, and suitable for long-distance endurance.
• Type IIa: Fast contraction, oxidative energy utilization, red color, moderate fatigability, and suitable for force and power exercises.
• Type IIb: Fast contraction, glycolytic energy utilization, white color, easily fatigued, and suitable for force and power with short distances.

Bands

• Z-discs: Mark the sarcomere's boundaries, approximates and anchors actin fibers, located between2 z-discs.
• H-zone: Contains Myosin, also known as the Hanap, obliterates the light.
• A-band: Contains Myosin and actin (- change is the absence of change in light)
• I-band: Contains Actin is also the "ipit" shortens.
• M-Line: Located in middle line, anchors myosin, shows (-) change (absence of color change).

Contractile Filaments

• Myosin: Thick filament anchored at the M-line.
• (+) has Myosin Heads.
• Actin: Thin filaments anchored at the z-disc.
  • Tropomyosin covers the actin active sites.
• Troponin Complex ICT (Troponin I, C, T).
  • I binds to actin.
  • C binds to calcium.
  • T binds to tropomyosin.

Events in Muscle Contraction

• At Rest:
  • Calcium is stored in the sarcoplasmic reticulum.
  • Tropomyosin covers the AAS.
  • Myosin heads cannot connect with the AAS.
• During Contraction:
• Calcium is released from the sarcoplasmic reticulum.
• Ca binds with Troponin C
  • Unbinds AAS.
• Myosin heads connect with AAS
  • Produces contractions.

Sequence of Contraction

• Discharge at presynaptic neurons.
• Release of Acetylcholine (Ach).
• Attachment of Ach to Ach Receptors.
• Increase conductance of Na & K ions.
• Action Potential.
• Inward spread of action potential through the T-Tubules.
• Ca++ release from the sarcoplasmic reticulum.
• Tropomyosin uncovers the AAS.
• Formation of cross-bridges between Actin & Myosin. = Power Stroke

Relaxation

• Calcium is pumped back to the sarcoplasmic reticulum.
• Unbinding of Calcium from troponin C.
• Recovering AAS by tropomyosin.
• The release of Calcium from the sarcoplasmic reticulum to the cytoplasm initiates muscle contraction.

Types of Contraction

• Isometric: Same length of muscle; no work is done.
• Isotonic: Same tension.
• Concentric means shortening.
• Eccentric means lengthening.

Muscle Twitch vs. Spasm

• Muscle Twitch: Invisible muscle contraction, lasts 1 min visible, typically has no pain.
• Spasm: Prolonged muscle contraction, invisible muscle contraction, and has pain.