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Muscle Tissue

Muscle Tissue. Muscle Structure and Function. Types of Muscle Tissue. Skeletal Muscle Tissue – moves the body by pulling on bones of the skeleton Allows us to walk, move, pick up and throw objects Voluntary – we can control Cardiac Muscle Tissue – pumps blood through the circulatory system

rhonda-holden
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Muscle Tissue

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  1. Muscle Tissue Muscle Structure and Function

  2. Types of Muscle Tissue • Skeletal Muscle Tissue – moves the body by pulling on bones of the skeleton • Allows us to walk, move, pick up and throw objects • Voluntary – we can control • Cardiac Muscle Tissue – pumps blood through the circulatory system • Involuntary – we can’t control • Smooth Muscle Tissue – pushes material through the digestive tract and controls the diameter of small arteries. • Involuntary – we can’t control

  3. Functions of Skeletal Muscle • Produce skeletal movement • Maintain posture and body positioning • Support of soft tissues • Guard entrances and exits • Maintain body temperature • Store nutrient reserves

  4. Organization of Muscle Tissue • From smallest structure to largest structure: • Muscle fiber (cell) Muscle fascicle (bundle of cells) Skeletal Muscle (organ)

  5. Anatomy of Skeletal Muscle • 3 layers of connective tissue • Epimysium • Surrounds entire muscle • Perimysium • Divides skeletal muscle into compartments called fascicles • Contains blood vessels and nerve fibers • Endomysium • Surrounds each individual muscle fiber • Contains capillaries that supply blood to fiber, satellite cells (stem cells that repair muscle cells), nerve fibers that control the muscle.

  6. Anatomy of Skeletal Muscle • The fibers of the epimysium, endomysium and perimysium are interwoven to form either a bundle (tendon) or a broad sheet (aponeurosis)

  7. Skeletal Muscle Fibers (cells) • Different than typical cells • Very large • Can run the length of your thigh (30cm) • Multi-nucleated • Contain hundreds of nuclei • Control production of enzymes and proteins necessary for muscle function

  8. Skeletal Muscle Fibers (cells) • Formed during development from the fusing of multiple embryonic cells (myoblasts) • Some myoblasts don’t fuse • Called satellite cells in adult muscles • Repair damaged muscle

  9. Skeletal Muscle Fibers (cells) • Parts of a muscle fiber • Sarcolemma – cell membrane of muscle cells • Sarcoplasm – cytoplasm of muscle cells • Transverse tubules (T-tubules) – narrow tubes that carry the electric signal for contraction deeper into the cell

  10. Skeletal Muscle Fibers (cells) • Parts of a muscle fiber • Myofibrils • Consist of bundles of protein filaments • Thin filaments – composed of actin • Thick filaments - composed of myosin • Actively shortening component of muscle • Responsible for muscle contractions

  11. Skeletal Muscle Fibers (cells) • Sarcoplasmic reticulum (SR) • Similar to endoplasmic reticulum in regular cells • Tightly bound to the T-tubules • Forms a network around each myofibril • Stores Ca++ ions for muscle contractions • Up to 40,000 time the amount found in the sarcoplasm • A contraction begins when Ca++ ions are released into the sarcoplasm • Video Clip

  12. Skeletal Muscle Fibers (cells) • Sarcomeres • Functional unit of skeletal muscle • Actual contracting unit • About 10,000 sarcomeres, end-to-end, make up a myofibril

  13. Skeletal Muscle Fibers (cells) • Each sarcomere has dark bands (A bands) and light bands (I) • The A band • Made up of thick filaments (myosin) • 3 subdivisions • M line – dark, central line where the thick filaments are connected to their neighbors • H zone – light area around the M line. Has thick filaments but no thin filaments • Zone of overlap – thin and thick filaments overlap

  14. Skeletal Muscle Fibers (cells) • The I band • Contains just the thin (actin) filaments • Extends from the A band from one sarcomere to the A band of the next • Z line – marks the boundary between adjacent sarcomeres • The A and I bands are visible with a light microscope and are called striations, thus skeletal muscle is also known as striated muscle

  15. Skeletal Muscle Fibers (cells) • Thin Filaments • Contains strands of proteins (actin) • Has active sites that are used during muscle contraction

  16. Skeletal Muscle Fibers (cells) • Thick Filaments • Contains roughly 300 myosin molecules • The myosin tail is long and is bound to other myosin molecules in the thick filament • The free head projects out toward the nearest thin filament • When the head interacts with the thin filament during a contraction, it is called a cross-bridge

  17. Skeletal Muscle Fibers (cells) • Thick Filaments • Myosin molecules are arranged with their tails towards the M line • The heads are arranged in a spiral • H zone contains no myosin heads

  18. Sliding Filament Theory • When a muscle contracts: • H zones and I bands get smaller • Zones of overlap get larger • Z lines move closer together • Width of the A band remains constant • This only makes sense if the thin filaments slide alongside the thick filaments toward the center of the sarcomere (M line) • This is known as the sliding filament theory

  19. Muscle Tissue Muscle Contraction

  20. Muscle Contraction • When muscle fibers contract, they actively pull on the tendon fiber the way people pull on a rope. • This pull is called tension • It is an active force, so it requires energy • For movement to occur, the tension must overcome the resistance of the object. • Resistance depends on the weight, shape, friction, and other factors. • MUSCLES PULL…THEY DO NOT PUSH!!!!

  21. Overview of Skeletal Muscle Contraction • Skeletal muscle fibers are activated by neurons (nerve cells) • Activated by stimulation of the sarcolemma • Excitation-contraction coupling occurs next • Calcium ions are released from sarcoplasmic reticulum • Calcium ions trigger interactions between thick and thin filaments, resulting in fiber contraction and the consumption of energy in the form of ATP • Tension is produced.

  22. Control of Skeletal Muscle • Skeletal muscle only contracts under control of the nervous system • Communication that occurs between muscle and nerve takes place at what is known as a neuromuscular junction (NMJ) • Each muscle fiber is controlled by a neuron at a single NMJ midway along its length.

  23. Control of Skeletal Muscle • The neuron branches when it reaches the muscle • At the end of each branch, there is a synaptic terminal • Contains the neurotransmitter Acetylcholine (Ach) • The synaptic cleft is the narrow space between the synaptic terminal and the sarcolemma. • The sarcolemma surface of the synaptic cleft is known as the motor end plate • The synaptic terminal and the sarcolemma contain acetylcholinesterase (AChE) • Breaks down ACh

  24. Control of Skeletal Muscle • Stimulation of the muscle occurs through 5 steps. 1. Arrival of action potential • Electrical impulse arrives at synaptic terminal 2. Release of ACh • The action potential triggers the release of ACh into the synaptic cleft 3. ACh binds at the Motor End Plate • ACh molecules diffuse across cleft and bind to receptors on Motor End Plate • Increases the sarcolemma’s permeability of sodium ions, and sodium ions rush into the sarcolemma

  25. Control of Skeletal Muscle 4. Appearance of action potential in the sarcolemma • The rush of sodium ions causes an action potential in the sarcolemma • Travels inward via the T-tubules 5. Return to initial state • ACh is broken down by AChE. • ACh is recycled

  26. Excitation-Contraction Coupling • The step between the generation of the action potential in the sarcolemma and the start of a muscle contraction is called excitation-contraction coupling. • The action potential in sarcolemma triggers the release of calcium ions(Ca++) from the sarcoplasmic reticulum.

  27. Excitation-Contraction Coupling • Remember that the thin filament has active sites on it. • At rest, these are covered • After the Ca++ is released from the sarcoplasmic reticulum, the active site is uncovered, allowing the myosin head to bind. • Muscle contraction now begins

  28. The Contraction Cycle • The myosin head is already energized, ready to act. • Step 1: Exposure of Active Sites • Ca++ binds to troponin, exposing the active sites • Step 2: Formation of Cross-Bridges • The myosin heads bind to the exposed active sites • Step 3: Pivoting of myosin heads • When at rest, the myosin head points away from the M line. Myosin head is “cocked” • After cross-bridge formation, the head pivots toward the M line as energy is released (called the power stroke)

  29. The Contraction Cycle • Step 4: Detachment of Cross-Bridges • When another ATP binds to the myosin head, it detaches from the active site • Active site can now form another cross-bridge • Step 5: Reactivation of Myosin • Occurs when myosin head splits the ATP • This energy is used to re-cock the myosin head • This cycle can be repeated several time each second

  30. The Contraction Cycle • Each power stroke shortens the sarcomere by 1 percent • Because all sarcomeres contract together, the entire muscle shortens at the same rate. • To better understand how tension is produced in a muscle fiber, think of a tug-of-war.

  31. Relaxation • Duration of contraction depends on: • Duration of stimulation at NMJ • Presence of free Ca++ in the sarcoplasm • Availability of ATP

  32. Relaxation • If one action potential arrives at the NMJ, Ca++ levels in the sarcoplasm will quickly return to normal. • Two mechanisms are involved in this process: • Active transport of Ca++ across the cell membrane into the extracellular fluid • Active transport of Ca++ into the SR • This one is much more important

  33. Relaxation • As the Ca++ levels in the sarcoplasm fall, • Active sites are re-covered. • The contraction ends

  34. Return to Resting Length • Since muscle can’t actively lengthen, outside forces must lengthen the muscle • Opposing muscle contractions • Gravity

  35. Rigor Mortis • Within a few hours after death, muscle fibers run out of ATP • The SR can not pump Ca++ out of the sarcoplasm, triggering a sustained contraction • Myosin heads don’t detach from active sites. • Rigor mortis lasts until the Z-lines are broken down 15-25 hours after rigor mortis sets in.

  36. Muscle Tissue Tension Production

  37. Tension Production • Tension depends on the amount of pivoting cross-bridges. • There is no mechanism to regulate the amount of tension by changing the number of contracting sarcomeres • When Ca++ is released, it is released from all SR in the muscle fiber • Muscle fiber is either “on” or “off”

  38. Tension Production • Tension at the muscle fiber level does vary. It depends on: • The fiber’s resting length at the time of stimulation • The frequency of stimulation

  39. Length-Tension Relationship • The amount of tension depends on the number of cross-bridges along the length of the fiber. • Depends on the degree of overlap between thick and thin fibers. • When a fiber is stimulated to contract, only the myosin heads in the zone of overlap can bind to the active sites. • The more myosin heads in the zone, the more tension…To a point

  40. Length-Tension Relationship • A sarcomere works most efficiently within an optimal range • If the sarcomere is stretched, there will be less myosin heads in the zone of overlap. • If the sarcomere is compressed/shortened, it has less room to shorten because the actin filaments are already close to the M line.

  41. Length-Tension Relationship

  42. Frequency of Stimulation • Twitch • A single stimulation producing a single contraction • Lasts 7-100 milliseconds depending on the muscle • Can be divided into 3 phases • Latent period – begins at stimulation, lasts 2 msec • Action potential sweeps across sarcolemma, and Ca++ is released from SR • NO TENSION IS PRODUCED YET

  43. Frequency of Stimulation • Contraction phase – tension rises to a peak • Cross-bridges are forming • Ends about 15msec after stimulation • Relaxation phase – lasts about 25msec • Ca++ levels fall • Active sites are being covered • Decrease in cross-bridges • Tension falls back to resting levels

  44. Frequency of Stimulation

  45. Frequency of Stimulation • Treppe • When the muscle is stimulated immediately after the relaxation phase has ended. • The resulting contraction will produce slightly more tension than the first • This will continue for the first 30-50 contractions, then tension levels off • Tension rises because there is extra Ca++ left over from previous stimulus. SR doesn’t have enough time to pump all Ca++ back in.

  46. Frequency of Stimulation • Wave Summation • If a second stimulus arrives before the relaxation phase has ended, a second, more powerful contraction occurs. • A stimulus of greater of 50 per second will produce wave summation • Incomplete Tetanus • If stimulation continues and the muscle is never allowed to relax completely, tension will rise until it reaches a peak 4X that of treppe. • This is incomplete tetanus

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