lecture 2 muscle tissue cont
TRANSCRIPT
Principles of Human Anatomy and Physiology, 11e 1
Chapter 10
Muscle Tissue
Lecture Outline
Principles of Human Anatomy and Physiology, 11e 2
Overview: How Does Contraction Begin?• 1. Nerve impulse reaches an axon terminal
• 2.synaptic vesicles release acetylcholine (ACh)
• 3. ACh diffuses to receptors on the sarcolemma
• 4. stimulus provided by binding of ACh to the
• sarcolemma;
• 5. Na+ channels open and Na+ rushes into the cell
• 6. resulting action potential travels along sarcolemma and into T tubules, triggering release of calcium ions from SR;
• 7. calcium ions bind to troponin; resulting shape change causes myosin binding site to be exposed;
• 8. myosin heads bind to actin, and swivel (power stroke), pulling Z discs closer together, shortening myofiber.-the contraction cycle begins
Principles of Human Anatomy and Physiology, 11e 3
Contraction Cycle
• Repeating sequence of events that cause the thick & thin filaments to move past each other.
• 4 steps to contraction cycle
– ATP hydrolysis
– attachment of myosin to actin to form crossbridges
– power stroke
– detachment of myosin from actin
• Cycle keeps repeating as long as there is ATP available & there is a high Ca+2 level near the filaments.
Principles of Human Anatomy and Physiology, 11e 4
Steps in the Contraction Cycle
• Notice how the myosin head attaches and pulls on the thin filament with the energy released from ATP
Principles of Human Anatomy and Physiology, 11e 5
ATP and Myosin
• Myosin heads are activated by ATP
• Activated heads attach to actin & pull (power stroke)
• ADP is released. (ATP released P & ADP & energy)
• Thin filaments slide past the thick filaments
• ATP binds to myosin head & detaches it from actin
• All of these steps repeat over and over
– if ATP is available &
– Ca+ level near the troponin-tropomyosin complex is high
Principles of Human Anatomy and Physiology, 11e 6
Excitation - Contraction Coupling
• All the steps that occur from the muscle action potential reaching the T tubule to contraction of the muscle fiber.
Principles of Human Anatomy and Physiology, 11e 7
Relaxation• Acetylcholinesterase (AChE) breaks down ACh within the
synaptic cleft
• Muscle action potential ceases
• Ca+2 release channels close
• Active transport pumps Ca2+ back into storage in the sarcoplasmic reticulum
• Calcium-binding protein (calsequestrin) helps hold Ca+2 in SR (Ca+2 concentration 10,000 times higher than in cytosol)
• Tropomyosin-troponin complex recovers binding site on the actin
Principles of Human Anatomy and Physiology, 11e 8
Overview: From Start to Finish
• Nerve ending
• Neurotransmittor
• Muscle membrane
• Stored Ca+2
• ATP
• Muscle proteins
Principles of Human Anatomy and Physiology, 11e 9
Neuromuscular Junction (NMJ) or Synapse
• NMJ = myoneural junction
– end of axon nears the surface of a muscle fiber at its motor end plate region (remain separated by synaptic cleft or gap)
Principles of Human Anatomy and Physiology, 11e 10
Anatomy of Cardiac Muscle
• Striated , short, quadrangular-shaped, branching fibers • Single centrally located nucleus• Cells connected by intercalated discs with gap junctions• Same arrangement of thick & thin filaments as skeletal
Principles of Human Anatomy and Physiology, 11e 11
CARDIAC MUSCLE TISSUE - Overview• Cardiac muscle tissue is found only in the heart wall Its
fibers are arranged similarly to skeletal muscle fibers.
– Cardiac muscle fibers connect to adjacent fibers by intercalated discs which contain desmosomes and gap junctions
– Cardiac muscle contractions last longer than the skeletal muscle twitch due to the prolonged delivery of calcium ions from the sarcoplasmic reticulum and the extracellular fluid.
– Cardiac muscle fibers contract when stimulated by their own autorhythmic fibers.
• This continuous, rhythmic activity is a major physiological difference between cardiac and skeletal muscle tissue.
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Appearance of Cardiac Muscle
• Striated muscle containing thick & thin filaments
• T tubules located at Z discs & less SR
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Physiology of Cardiac Muscle
• Autorhythmic cells
– contract without stimulation
• Contracts 75 times per min & needs lots of O2
• Larger mitochondria generate ATP aerobically
• Extended contraction is possible due to slow Ca+2 delivery
– Ca+2 channels to the extracellular fluid stay open
Principles of Human Anatomy and Physiology, 11e 14
SMOOTH MUSCLE
• Smooth muscle tissue is nonstriated and involuntary and is classified into two types: visceral (single unit) smooth muscle and multiunit smooth muscle
– Visceral (single unit) smooth muscle is found in the walls of hollow viscera and small blood vessels; the fibers are arranged in a network and function as a “single unit.”
– Multiunit smooth muscle is found in large blood vessels, large airways, arrector pili muscles, and the iris of the eye. The fibers operate singly rather than as a unit.
Principles of Human Anatomy and Physiology, 11e 15
Two Types of Smooth Muscle
• Visceral (single-unit)
– in the walls of hollow viscera & small BV
– autorhythmic
• Multiunit
– individual fibers with own motor neuron ending
– found in large arteries, large airways, arrector pili muscles,iris & ciliary body
Principles of Human Anatomy and Physiology, 11e 16
Microscopic Anatomy of Smooth Muscle
• Sarcoplasm of smooth muscle fibers contains both thick and thin filaments which are not organized into sarcomeres.
• Smooth muscle fibers contain intermediate filaments which are attached to dense bodies.
• Small, involuntary muscle cell -- tapering at ends• Single, oval, centrally located nucleus• Lack T tubules & have little SR for Ca+2 storage
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Microscopic Anatomy of Smooth Muscle
• Thick & thin myofilaments not orderly arranged so lacks sarcomeres
• Sliding of thick & thin filaments generates tension
• Transferred to intermediate filaments & dense bodies attached to sarcolemma
• Muscle fiber contracts and twists into a helix as it shortens -- relaxes by untwisting
Principles of Human Anatomy and Physiology, 11e 18
Physiology of Smooth Muscle
• Contraction starts slowly & lasts longer
– no transverse tubules & very little SR
– Ca+2 must flows in from outside
• In smooth muscle, the regulator protein that binds calcium ions in the cytosol is calmodulin (in place of the role of troponin in striated muscle);
– calmodulin activates the enzyme myosin light chain kinase, which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate.