BIOL1XX8_2024_L12_Muscles.pdf
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Movement Muscle structure and contraction MEDS1001 _2024 BIOL1008_2024 Associate Professor Michelle McDonald School of MedicalSciences [email protected] I acknowledge and pay my respects to the Gadigal people of the Eora Nation, whose land I walk, work, and gather on every day. The Uni...
Movement Muscle structure and contraction MEDS1001 _2024 BIOL1008_2024 Associate Professor Michelle McDonald School of MedicalSciences [email protected] I acknowledge and pay my respects to the Gadigal people of the Eora Nation, whose land I walk, work, and gather on every day. The University of Sydney Page 1 Our Skeleton Needs Muscles to Move! Approximately 640 muscles in adult body Strongest muscle Masseter (jaw) Largest muscle – gluteus maximus Smallest stapedius muscle The University of Sydney Page 2 Key Learning Outcomes Understand basic properties and functions of muscles Knowledge of muscle structure Be able to describe how muscle fibers contract Explore the different muscle types Fiber types Draw comparisons between muscle type and mechanisms of contraction Describe regulation of muscle contraction/tension The University of Sydney Page 3 Muscle Structure and Function The many roles of muscle Movement of: bones food (GIT) blood (vessels) fluids (excretory system) Thermoregulation – creates body heat Energy metabolism and storage (glycolysis) Appetite regulation (myokines and satiety) Endocrine functions The University of Sydney Page 5 Properties of Muscle Men (40% of body mass) have more skeletal muscle than women (30% of body mass) The gender differences are greater in the upper than lower body. Muscle mass declines rapidly with age – leading to sarcopenia and reduced physical capacity The University of Sydney Page 6 Properties of Muscle The University of Sydney https://youtu.be/DuDdgeQwFRI Page 7 Properties of Muscle Contractility: Ability of muscle to shorten with a force, requires energy, relaxes passively Excitability: Capacity of muscle to respond to stimulation (nerves) Electrical stimulation Extensibility: Muscle can be stretched to its normal resting length and beyond to a limited degree Stomach/bladder expand as fill Elastic: Our Muscles return or recoil to resting position when relaxed The University of Sydney Page 8 Types of Muscles Skeletal Attached to the skeleton – responsible for movement Voluntary movements Cardiac (myocardium) Forms the heart – responsible for pumping blood Involuntary movement Smooth Located in the tissues – responsible for controlling diameter of structures and peristalsis Involuntary movement The University of Sydney Page 9 Types of Muscles Skeletal Attached to the skeleton – responsible for movement Voluntary movements Cardiac (myocardium) Forms the heart – responsible for pumping blood Involuntary movement Smooth Located in the tissues – responsible for controlling diameter of structures and peristalsis Involuntary movement The University of Sydney Page 10 Skeletal Muscle Cells cell muscle fascicle Multiple peripheral nuceli Blood vessels run between fibers Skeletal muscle fascicles Transverse section Parallel cells with connective tissue between to form fascicles Cytoplasmic striations Longitudinal section Muscle Fiber Structure Actin = thin myofilament 6nm diameter, 1m long Myosin = thick myofilament 15nm diameter, 1.5ųm long Sarcomere made up of actin and myosin filaments which provide striations on muscle cells in longitudinal direction The University of Sydney Baron R Anatomy and Ultrastructure of Bone in Favus MJ Ed. Page 12 Muscle Contraction Cell structure is essential to muscle Contraction The University of Sydney Baron R Anatomy and Ultrastructure of Bone in Favus MJ Ed. Page 14 Muscle Contraction The University of Sydney https://youtu.be/99R-XCGme8Q Page 15 Muscle Contraction 1. ATP bound myosin is in the relaxed position – 2. When ATP is dephosphorylated to ADP+Pi it is positioned to form a cross bridge. Tropomyosin inhibits/blocks binding sites on actin and prevents cross-bridge formation 3. Electrical excitation of the muscle cell releases calcium from the Sarcoplasmic reticulum. Calcium binds to troponin and moves the tropomyosin out of the way to allow cross bridge formation 4. Power stroke: release of phosphate bound to myosin head – myosin head moves along the actin filament The speed of ATP use and replacement determines how quickly muscle cells contract. The University of Sydney 5. At end of excitation, calcium pumps back into SR allowing tropomyosin to block myosin binding again and ATP is bound again Page 16 Skeletal Muscle Contraction The University of Sydney https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5226009/ Page 17 Review The University of Sydney Term Meaning Sarco- & Myo- Relating to muscle striated Parallel lines skeletal muscle lots of bundles of muscle cells held together by connective tissue fascicle bundle of muscle cells myofibril located inside cytoplasm of muscle cell myofilament actin & myosin and arranged to form fibrils sarcomere the repeating, overlapping arrangement of myofilaments that create the myofibril sarcoplasmic reticulum smooth endoplasmic reticulum organelle involved in muscle contraction Page 18 Types of Muscles Skeletal Attached to the skeleton – responsible for movement Voluntary movements Cardiac (myocardium) Forms the heart – responsible for pumping blood Involuntary movement Smooth Located in the tissues – responsible for controlling diameter of structures and peristalsis Involuntary movement The University of Sydney Page 19 Cardiac vs Skeletal Muscle The University of Sydney Page 20 Skeletal Muscle Fiber Types Type I /slow twitch /red: Contracts slowly, but slow to fatigue Supplied by nerves that activate to contract Use aerobic (slower) metabolism to generate large amounts of ATP – adapted to deliver O2 to mitochondria Fibers red dye to myoglobin content which transports O2 Type II /fast twitch /white The University of Sydney Contract quickly, and fast to fatigue Use anaerobic (faster) metabolism to generate less ATP – not adapted to deliver O2 to mitochondria Page 21 Function of Skeletal Muscle Fiber Types The University of Sydney Page 22 Types of Muscles Skeletal Attached to the skeleton – responsible for movement Voluntary movements Cardiac (myocardium) Forms the heart – responsible for pumping blood Involuntary movement Smooth Located in the tissues – responsible for controlling diameter of structures and peristalsis Involuntary movement The University of Sydney Page 23 Smooth Muscle Spindle shaped cells without striations Single central nucleus Contract more slowly with less power than skeletal muscle. Two types of contraction: Phasic – rapid Tonic – slower maintain tension longer Responsive to hormones, neural stimulation, stretch Found in: Walls of organs, vessels Respiratory tract Eye – dilation constriction of pupil Skin Kidneys The University of Sydney Page 24 Smooth Muscle Contraction 1. 2. 3. 4. Ca2+ influx into cell from SR and extracellular sources via L type Ca2+ channels Ca2+ binds calmodulin in sarcoplasm Ca2+-/calmodulin activates MLCK which phosphorylates head of myosin filament (ATP) – cross bridge formation. NO influx - relaxation The Ca2+ concentration determines the force of contraction: The higher the Ca2+ influx, the more force is generated. Smooth Muscle vs Skeletal Muscle Contraction Calmodulin instead of tromopmyosin MLCK phosphorylation of myosin cross bridge Smooth muscle contraction 100-1000 times slower than skeletal - allows smooth muscle to maintain prolonged tonic contraction while consuming little ATP and O2 Stimulus for smooth muscle contraction can be varied – stretch, neural (autonomic NS), hormonal. Skeletal -innervation (somatic NS) Control of Muscle Contraction Controlling Muscle Tension: Motor Units A motor unit is one motor neuron which supplies a group of muscle cells Small motor units Innervate less muscle fibers Generate less tension Enable fine control Fatigue resistant Large motor units Generate more tension Fatigue more quickly Slow motor units – type I fibers – sustained contraction/maintenance Fast motor units – type II fibers -large rapid forces Controlling Muscle Tension: Motor Units Increase in force generated by activating more motor units simultaneously Branch within muscles to synapse on many different fibers across muscle to ensure force is spread evenly The tension produced in a skeletal muscle is a function of: the frequency of neural stimulation; and the number of motor units involved Smooth muscle cells can be: Single-unit (visceral) : All the cells function collectively and simultaneously as a single unit (unitary). Multiunit : all the cells cannot function collectively and work independently Muscle Tension Latent period - action potential is being propagated along the membrane and Ca2+ ions are released from the sarcoplasmic reticulum. Contraction phase – period of cross-bridge formation. Relaxation phase - Ca is pumped back into the SR, and cross-bridge cycling stops. One action potential in a motor neuron produces one contraction. This contraction is called a twitch. Muscle Tension: Summation If a muscle cell is stimulated while a previous twitch is still occurring, the second twitch will be stronger. The second stimulus releases more Ca2+ ions, and allows more crossbridge formation. A Tetanic contraction is a sustained muscle contraction evoked a motor nerve emits action potentials at a very high rate https://opentextbc.ca/anatomyandphysiologyopenstax/ Skeletal muscle fiber types Fast twitch fibers possess smaller neuro muscular junctions (NMJs), which may facilitate efficiency and speed of neurotransmission. Slow twitch fibers possess NMJs with larger surface area which may facilitate sustained stimulation The University of Sydney https://www.biorxiv.org/content/10.1101/2020.04.04.025106v1.full Page 32 Key Learning Outcomes Understand basic properties and functions of muscles – movement, thermoregulation, energy metabolism, 18% protein, 75% water, muscle mass declines with age, Contraction, excitation, extend, elastic Knowledge of muscle structure Bundle, fascicle, fiber, myofibril, sarcomere Be able to describe how muscle fibers contract Ca2+, troponin, tropomyosin, cross bridge, ATP-ADP, power stroke, contract, relax Explore the different muscle types – skeletal, cardiac, smooth muscle Fiber types – fast and slow twitch Draw comparisons between muscle type and mechanisms of contraction Smooth vs skeletal – Calmodulin, MLCK phos of ATP, slower prolonged, autonomic vs somatic NS Describe regulation of muscle contraction/tension motor units, tension, action potential – twitch, summation, tetany. The University of Sydney Page 33
