2Function of Muscles Produce movement Maintain posture Stabilize joints Generate heat
3Characteristics of Muscles Muscle cells are elongatedmuscle cell = muscle fiberContraction of muscles is due to the movement of microfilamentsAll muscles share some terminologyPrefix myo refers to musclePrefix mys refers to musclePrefix sarco refers to flesh
4Characteristics of Muscle Tissue ExcitabilityMuscle tissue (and nervous cells) receive and respond to stimuli by producing electrical signalsContractabilityAbility to shorten and thicken when stimulatedExtensibilityAbility to stretch without damagedElasticityAbility to return to its original shape
5Types of Muscle TissueCardiac muscleSkeletal muscleSmooth muscle
6Cardiac Muscle Has striations Usually has a single nucleus Joined to another muscle cell at an intercalated discInvoluntaryFound only in the heartFigure 6.2b
7Smooth Muscle No striations Spindle-shaped cells Single nucleus Involuntary – no conscious controlFound mainly in the walls of hollow organs
8Skeletal Muscle Most are attached by tendons to bones Cells are multinucleateStriated – have visible bandingVoluntary – subject to conscious controlCells are surrounded and bundled by connective tissue
9Connective Tissue Wrappings of Skeletal Muscle Tendon – cord-like structure, attaches muscle to boneEndomysium – around single muscle fiberPerimysium – around a fascicle (bundle) of fibersEpimysium – covers the entire skeletal muscleFigure 6.1
10Connective Tissue Wrappings of Skeletal Muscle Fascia – on the outside of the epimysiumSuperficial: subcutaneous tissue; made of areolar connective tissue and adiposeDeep: holds muscles together and separates them into functional groups; made of dense irregular connective tissueFigure 6.1
11Skeletal Muscle Tissue Each skeletal muscle is a separate organ composed of hundred to thousands of skeletal muscle cells called muscle fibers because of their elongated shapes. Connective tissue surround muscle fibers and whole muscles, blood vessels, and nerves penetrate muscle.
12Microscopic Anatomy of Skeletal Muscle Muscle fibers(cells) are arranged parallel to one another.Sarcomeres are the basic functional unit of striated muscle fibers; occurs at the overlap of filaments.Sarcolemma is the plasma membrane that covers each muscle fiberSarcoplasm is the muscle fiber’s cytoplasm.Tranverse (T) tubules are tunnel-like extensions of the sarcolemma that pass through muscle fiber from side to sideSarcoplasmic reticulum is a network of membrane –enclosed tubules that stores Ca2+ ions required for muscle contractions.Myoglobin is found in the sarcoplasm; reddish pigment; stores oxygen until needed by mitochondria for ATP production.Myofibrils extend along the entire length of the muscle fiber; are cylindrical; consists of two types of protein filaments: light & dark
14Organization of the Scarcomere Z-discs (lines) are zigzagging zones that separates sarcomeresA bands (dark bands) extend the entire length of thick filaments; at end thick & thin filaments overlapI bands (light bands) are composed of thin filaments onlyH zone is found at the center of each A band; contains only thick filaments
15Organization of the Sarcomere Thick filaments are composed of the protein myosin. Shaped like 2 golf clubs twisted togetherThin filaments are composed of the protein actin. Twisted into helix.Thin filaments also contains the proteins tropomyosin & troponin
16Sliding Filament Theory of Muscle Contraction Overall summary of what happens in a sarcomere when a muscle contracts:Myosin heads of thick filaments pull on thin filamentsThin filaments slide toward center of sarcomereI bands and H zone becomes narrowerI band and H zone maximum contraction
17Sliding Filament Theory of Muscle Contraction Figure 6.7
18Contraction Terminology Neuromuscular Junction (NMJ)- area of contact between axon terminal & portion of sarcolemmaAxon terminal-branches of motor neuron that approaches, but not touch the sarcolemmaAcetylcholine (Ach)- neurotransmitterSynaptic cleft- narrow gap that separates axon terminal of one neuron from muscle fiberMotor end plate- part of sarcolemma that receives the neurotransmitterAcetylcholinesterase (AChE)- enzyme that breaks down acetylcholine
19Muscle ContractionNerve impulse arrives at axon terminal of motor neuron and triggers release of acetylcholine (ACh)…ACh binds to its receptors and is activated, this causes Na/K ions to flow across membrane...Inflow of Na ions generates a muscle action potential, which travels down sarcolemma & through T-tubules…As the impulse moves down SR, Ca2+ is released from the SR to the thick and thin myofilaments…Ca2+ binds to troponin molecules in thin filaments, causing the troponin-tropomyosin complex to change shape….
20Muscle ContractionThis change in shape of the troponin/tropomysosin complex causes movement of the attached tropomyosin molecule….Allowing the myosin head to contact/bind actin, causing the myosin head to swivel (this requires ATP!)During the swivel, the myosin head is firmly attached to the actin, so when the head moves, it pulls the actin filament forward.This is called the ”power stroke” of contractionMany myosin heads are swiveling simultaneously and their collective efforts are enough to pull the entire actin filament past the myosin filament into the H zone and causes shortening (contraction) of the muscle fiber
21Muscle RelaxationNeurotransmitter Ach (acetylcholine) is broken down by AChE.Muscle action potentials stopCa2+ release channels in the SR closeAs levels of calcium in sarcoplasm falls, troponin releases calcium and slides back into original position where it covers the myosin binding sitesThin filaments slips back into their relaxed positions.