Cell Biology: Signal Transduction and Muscle Function

Signal Transduction

• Transmission of info from one side of the cell membrane to the other, using membrane proteins

Lipophilic Signal Molecules

• Diffuse through cell membranes
• Bind to cytosolic receptors or nuclear receptors
• Slow process

Lipophobic Signal Molecules

• Bind to receptors on the cell membrane
• Rapid process

Membrane Receptors

• 4 categories of membrane receptors
  • Receptor Channels
  • G-protein Coupled
  • Receptor-Enzymes
  • Integrin Receptors

Receptor Channels

• Ligand binding opens or closes the channel

G-protein Coupled Receptors

• Ligand binding opens an Ion channel or alters enzyme activity.

Receptor Enzymes

• Ligand binding activates an intracellular enzyme.

Integrin Receptors

• Ligand binding alters enzymes or the cytoskeleton.

Muscle Tension

• Force created by muscle

Load

• Weight or force opposing contraction

Contraction

• Creation of tension in muscle

Relaxation

• Release of tension

Major Steps Leading Up To Skeletal Muscle Contraction

• Events at the neuromuscular junction
• Excitation-Contraction (E-C) Coupling
• Contraction-Relaxation Cycle

Events at the Neuromuscular Junction

• Converts an ACh signal from a somatic motor neuron into an electrical signal in the muscle fiber.
• Steps:
  • Somatic motor neuron releases Ach at the neuromuscular junction
  • Net entry of sodium through the Ach receptor channel initiates a muscle action potential
  • Action potential in t-tubule alters the sarcoplasmic reticulum

Excitation Contraction Coupling

• Muscle action potentials are translated into calcium signals
• Calcium signals initiate a contraction-relaxation cycle
• Explained by the sliding filament theory of contraction.

Contraction-Relaxation Cycle

• The SR pumps calcium back into its lumen.
• Decrease in free cytosolic calcium causes calcium to unbind from troponin
• Tropomyosin re-covers the binding site
• When myosin heads release, elastic elements pull filaments back to their relaxed position.

Sliding Filament Theory of Contraction

• Overlapping actin and myosin myofibrils.
• Thin filaments slide past thick filaments in an energy-dependent process
• Results in muscle contraction.

Troponin

• Controls the position of tropomyosin.
• Binds reversibly to calcium.

Tropomyosin

• Prevents myosin and actin from interacting by covering the myosin-binding site.

Contraction Steps

• Calcium release, binds to troponin, troponin pulls tropomyosin from myosin-binding sites on actin.
• Myosin binds tightly to and moves actin
• Repeats as long as binding sites are uncovered and ATP and calcium are available.

Structure of the Kidney

• Divided into an outer cortex (80% nephrons) and an inner medulla (20% nephrons)

Bowman's Capsule

• Site of plasma filtration with the glomerulus.
• Bowman's capsule combined with glomerulus is referred to as the renal corpuscle.

Proximal (Closeby) Tubule

• Filtered fluid flows here.

Loop of Henle

• Segment that dips toward the medulla and then back up.
• "U" shaped.

Distal (Far Away) Tubule

• Fluid passes, which drains into a single larger tube called the collecting duct

Collecting Ducts

• Single larger tube towards the end after the distal tube.
• Converges and drains into the renal pelvis.
• Filtered and modified fluid (urine) flows into the ureter on its way to excretion.

Juxtaglomerular Apparatus

• Area where the nephrons twist and fold back on itself.
• Final part of the ascending limb of the loop passes between afferent/efferent arterioles.

Cortex (Upper Half)

• All of Bowman's capsule, proximal and distal tubes.

Medulla (Lower Half)

• Loop of Henle and collecting ducts.

Nephron

• Two arterioles and two sets of capillaries that form a portal system.

Lumen

• Inside space of a tubular structure, such as an artery or intestine.

Clearance

• Movement from blood to lumen.
• How much of a plasma that flowed through the glomerulus per unit of time was CLEARED of that substance.

Inulin and Creatinine

• Inulin
  • Neither reabsorbed or secreted, ONLY EXCRETED.
  • Filter freely in the nephron, when no inulin is present it's considered cleared from inulin.
• Creatinine
  • Breakdown product of phosphocreatine.
  • Small amount secreted, so it can be used to measure creatinine clearance.

Micturition (Urine)

• Removal of urine from the body; voiding of the bladder.
• Two sphincters control urine flow from the bladder (internal/smooth and external/skeletal sphincter).

This quiz covers key concepts in signal transduction and muscle physiology. It explores the mechanisms of lipophilic and lipophobic signal molecules, types of membrane receptors, and the basics of muscle tension and contraction. Test your understanding of these essential biological processes.