Smooth and Cardiac Muscle Physiology Lecture Notes PDF

Muscle Physiology Part II: Smooth Muscle 1) Found in internal organs, blood vessels, iris of eye, other locations 2) Under involuntary control by autonomic nervous system  Smooth Muscle Anatomy Usually arranged in sheets; often in multiple layers Figure 13.27b Properties o...

Muscle Physiology Part II: Smooth Muscle 1) Found in internal organs, blood vessels, iris of eye, other locations 2) Under involuntary control by autonomic nervous system  Smooth Muscle Anatomy Usually arranged in sheets; often in multiple layers Figure 13.27b Properties of Smooth Muscle 1) Spindle-shaped 2) One nucleus 3) Small, approximately 1/10 size of skeletal muscle 4) Have thick and thin myofilaments a) No sarcomeres; myofilaments not arranged into sarcomeres b) Appears “smooth” with microscope c) Thick and thin myofilaments arranged diagonally Properties of Smooth Muscle 1) Thin myofilaments contain tropomyosin, but no troponin 2) Dense bodies analogous to Z disks (thin myofilaments attach to dense bodies) 3) Slow myosin ATPase 4) Myosin ATPase 10-100 times slower in smooth muscle compared to skeletal muscle- slow contraction time 5) Little sarcoplasmic reticulum (SR)  Smooth Muscle Contraction Diagonal organization of actin/myosin (gets wider and shorter) Figure 13.28 Sliding Filament Mechanism of Contraction 1) Actin and myosin are longer in smooth than skeletal muscle (slide over greater area) 2) Myosin heads along whole length of thick filaments 3) Longer range of contraction 4) Calcium is trigger for contraction a) No troponin b) Use different regulatory protein than skeletal muscle c) Smooth muscle- Calcium first binds to a cytoplasmic protein called calmodulin  Source of Calcium 1) Most calcium required for contraction comes from the extracellular fluid (ECF) 2) Large concentration gradient for calcium (high ECF; low ICF) 3) Smooth muscle cells do not have an extensive SR (different than skeletal muscle) a) Some (little) calcium stored in caveolae (small, membrane bound vesicles; usually found beneath sarcolemma)  Steps of Excitation- Contraction Coupling in Smooth Muscle 1. Opening of calcium channels in plasma membrane a) Voltage gated b) Ligand (chemical) gated c) Mechanically-gated 2. Calcium influx into cell (a little calcium is released from caveolae and SR) 3. Calcium binds to calmodulin  Steps of Excitation- Contraction Coupling 4. Ca2+ -Calmodulin complex activates an enzyme called myosin light-chain kinase 5. Myosin light-chain kinase phosphorylates myosin (adds a phosphate)- myosin heads can now bind to actin on thin filaments 6. Cross-bridge cycling Relaxation of Smooth Muscle 1. Phosphatase removes phosphate from myosin a) No longer forms crossbridges with actin 2. Calcium removed from cytoplasm a) Ca2+ -ATPase pumped out of cell b) Ca2+ -Na+ counter transport -- Ca2+ transported out of the muscle cell; Na+ into cell  Regulation of Myosin Unphosphorylatedmyosin kinase Phosphorylated Myosin Myosin phosphatase No ATPase activity ATPase activity No Cross-bridges Cross-bridges Contraction  Neural Regulation of Smooth Muscle Contraction 1) Innervated by autonomic nervous system a) Sympathetic and/or Parasympathetic 2) May be excitatory or inhibitory, depending on receptor type 3) Target cell response depends on receptor type 4) Neurotransmitter released from varicosities 5) Diffuse binding of neurotransmitter to receptors Non-Neural Regulation of Contraction  Intracellular [Ca2+] determines tension  Intracellular [Ca2+] influenced by: 1. Hormonal control- many different hormones a. Many open ligand gated calcium channels (stimulate contraction; others close calcium channels (inhibit contraction) Example: certain hormones produced in gastrointestinal tract influence contraction of intestinal smooth muscle cells 2. Paracrines (chemicals produced by other cell types near smooth muscle cells)- open calcium channels 3. Physical stretch– some smooth muscle respond to being stretched by contracting (opens Ca2+ channels)  Classification of Smooth Muscle 1.Single-Unit Smooth Muscle 2.Multi-Unit Smooth Muscle  Single-Unit Smooth Muscle 1) Most common type 2) Location a) Intestinal tract b) Urinary Bladder c) Uterus 3) Muscle cells activated synchronously a) Cells connected by gap junctions b) Contract together as a single unit -- Stimulate one cell to contract, all cells in unit will contract Properties of Single-Unit Smooth Muscle 1) Gap junctions 2) Pacemaker cells with spontaneous depolarizations 3) Innervation to few cells; others stimulated via gap junctions 4) Tone = level of contraction without stimulation 5) Contract for long periods  Pacemaker Activity Single-unit smooth muscle 1)Pacemaker potentials a) Spontaneous depolarizations to threshold b) Lead to opening of voltage-gated calcium channels c) Depolarizations spread to neighboring smooth muscle cells via gap junctions, leading to contraction of these cells as well Properties of Multi-Unit Smooth Muscle 1) Located in large airways, eye (ciliary muscle and iris), blood vessels 2) Few if any gap junctions 3) Each muscle cell acts individually a) Receives own innervation b) No tone Cardiac Muscle Cardiac Muscle  Cardiac muscle cells are only found in the heart wall Two specialized types of cardiac muscle cells 1) Contractile cells (99%) 2) Auto-rhythmic cells (1%) - both types connected by gap junctions Properties of Contractile Cells 1) Smaller than skeletal muscle; cells often branch 2) Contain thick and thin myofilaments arranged into sarcomeres (similar to skeletal muscle) a) Cardiac muscle is also striated muscle 3) Contractile mechanism is similar to skeletal muscle cells with one exception: a) Action potentials traveling across surface membrane and down T tubules cause voltage gated calcium channels in these membranes to open b) Calcium enters the cells from the extracellular fluid 1) The calcium from the ECF, however, is not enough to bring about complete contraction of the cell 2) Calcium entering from ECF causes calcium channels on SR to open known as Ca2+ - induced, calcium release from SR 3) Calcium from SR plus calcium from ECF binds to troponin; contraction just like in skeletal muscle 4) Also, Ca2+ released from SR by action potentials in T tubules (just like in skeletal muscle) Excitation-Contraction Coupling in Cardiac Muscle Voltage-gated Ca2+ channel Figure 14.13 Relaxation of Cardiac Muscle Remove calcium from cytosol 1) Ca2+ ATPase in sarcoplasmic reticulum a) Pump Ca2+ back into SR 2) Ca2+ ATPase in plasma membrane a) Pump back into extracellular fluid 3) Na+-Ca2+ exchanger in plasma membrane a) Pump back into ECF Calcium leaves troponin tropomyosin return to position covering myosin binding sites on actin  Digitalis: drug that increases heart contraction strength (increases intracellular calcium) Foxglove Decreases Na+/K+ pump activity – decreases Na+ concentration gradient reduces Na+ - Ca2+ exchange so more calcium remains in cells and increases strength of heart contractions Properties of Contractile  cells-cont. All contractile cells are connected to one another by gap junctions 1)Intercalated disks = areas of contact between cells with a high concentration of gap junctions 2)Allows spread of action potentials between cells 3)Contract as unit: depolarize one contractile cell, all contractile cells in heart will also be depolarized and contract  Large number of desmosomes: resist stress of beating