Muscles II: Microscopic Anatomy and Contraction October 24 - 25 2017

Fascicle (bundle of fibers) Muscle Structure Muscle Fascicle (bundle of fibers) Muscle Fiber (single cell) Myofibril (organelle) Sarcomere (unit of contraction) Single muscle cell up to 30 cm -- relate back to cell theory Light / dark regions are ?? Small gray things between myofibrils are … ?

Microscopic Anatomy of Skeletal Muscle Large, cylindrical, multinucleate cells Contain many mitochondria; nearly filled with myofibrils Some organelles have unique vocabulary: Sarcolemma: cell membrane Sarcoplasm: cytoplasm Sarcoplasmic reticulum: modified ER, surrounds each myofibril; store Ca2+ Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Microscopic Anatomy of Skeletal Muscle Each myofibril can be divided into contractile units called sarcomeres. Sarcomeres consist of overlapping protein filaments of actin and myosin. Regular arrangement of dark and light bands. Dark bands occur where myosin is present. Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Microscopic Anatomy of Skeletal Muscle The M line is where the myosin attaches Z discs (a membrane) mark the edge of each sarcomere; serve as attachment site for actin Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Microscopic Anatomy of Skeletal Muscle Use the picture to come up with a definition of the following: I band A band H zone Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Microscopic Anatomy of Skeletal Muscle Use the picture to come up with a definition of the following: I band – area without myosin fibers; aka light band A band – area with myosin fibers; aka dark band H zone – area without actin fibers Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Turn & Talk First match the words … actin cell myofibril group of cells sarcomere cell membrane fascicle protein muscle fiber organelle sarcolemma contractile unit Then, write a paragraph that uses all the words in both columns above and explains that structure of the muscle. Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Contraction Overview Globular heads of myosin filaments attach to actin filaments. Myosin pulls actin filaments : “Sliding filament theory” Causes sarcomere to shorten, particularly the light bands Sarco = greek root for “flesh” or “muscle” T-tubules spread action potential to sarcoplasmic reticulum -- sarcoplasmic reticulum open to allow flood of Ca2+ which initiates contraction

Contraction Overview Globular heads of myosin filaments attach to actin filaments. Myosin pulls actin filaments : “Sliding filament theory” Causes sarcomere to shorten, particularly the light bands light dark light On the HW Question 7 turned today, I will be looking for diagrams that clearly show appropriate lengths … change if its not there. If you have already done it correctly then quiz a partner on vocabulary – 2 min. light dark light

Which shows contracted muscle fibers? Contraction Overview Can have degrees of contraction, even within sarcomere Which shows contracted muscle fibers? How can you tell?

Contraction Overview Relaxed muscle Contracted muscle Can have degrees of contraction, even within sarcomere Relaxed muscle Contracted muscle (large light bands) (small light bands)

Contraction Overview See animation! What are these? Where are the z disks? The m line? A single sarcomere What are these? Are the myofibrils organized left to right or top to bottom? Mitochondria Left to right See animation!

Contraction Details A motor neuron stimulates the muscle cell by releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell. Note: A motor unit is a single motor neuron and all the muscle fibers it activates Motor neurons can activate a few cells or 100s of cells

Contraction Details A motor neuron stimulates the muscle cell by releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell. ACh causes an electric current called an action potential to move through the muscle cell. Motor neurons can activate a few cells or 100s of cells

Contraction Details A motor neuron stimulates the muscle cell by releasing the neurotransmitter acetylcholine ACh into the synaptic cleft between the neuron and muscle cell. ACh causes an electric current called an action potential to move through the muscle cell. The action potential causes the release of Ca2+ from the sarcoplasmic reticulum. Can have degrees of contraction, even within sarcomere

Contraction Details Ca2+ exposes myosin-binding sites on actin filaments. Myosin heads (& ADP) attach to actin binding sites, forming cross-bridges. actin ADP + P myosin head NOT necessary to know that tropomysin blocks binding sites, and troponin exposes them (by changing position on tropomysin) when Ca2+ is bound to them. myosin Muscle relaxed. Ca2+ present. No Ca2+ present. Cross-bridge formed.

Contraction Details Myosin heads release ADP, move the actin filament in “power stroke” actin Can have degrees of contraction, even within sarcomere myosin Power stroke, ADP + P released

Contraction Details Myosin heads release ADP, move the actin filament in “power stroke” ATP binds to myosin head. The crosslink between actin and myosin breaks. ATP becomes ADP + P, readying the myosin head to reattach to actin. actin Can have degrees of contraction, even within sarcomere myosin Power stroke, ATP binds, ADP + P released cross-links break

Contraction Details Once the action potential is over, the If Ca2+ is still present, cycle will repeat, with myosin heads reattaching and contracting the muscle even more. Once the action potential is over, the Ca2+ is reabsorbed into the sarcoplasmic reticulum. Without Ca2+, myosin cannot attach to actin. Can have degrees of contraction, even within sarcomere Watch me!

Contraction Details NOTE: ATP is required to break cross-links, not to form them. Explains rigor mortis Why then do muscles need ATP? To reset head so it can contract further -- contraction is a series of sliding motions. Can have degrees of contraction, even within sarcomere

Turn & Talk Describe the role of each of the following in muscle contraction Scholar with more siblings…. ACh Ca2+ Scholar with less siblings … ATP Action potential Sequence these! Stimulation of contraction is different – have self-excitory cells. Cardiac fairly similar. Also, in smooth muscle Ca2+ comes from outside cell, process slightly different but still involves sliding of actin past myosin. Less organized – no striations – means shortening occurs in all directions, not just length-wise (like tightening a drawstring bag). Challenge question: We discussed the process of contraction in skeletal muscles. How do you expect it differs for cardiac and smooth muscles?

Exit Ticket In comparing electron micrographs of a relaxed skeletal muscle fiber and a fully contracted muscle fiber, which would be seen only in the relaxed fiber? Z discs Triads I bands A bands H zones Can have degrees of contraction, even within sarcomere MORE TIME? Have them draw contracted and relaxed sarcomeres, labeling zones.

Exit Ticket 2. Which word describes the unit of contraction of a muscle? Myofibril Sarcomere A band H band Can have degrees of contraction, even within sarcomere

Exit Ticket 3. Which of the following correctly lists the order of structure of the muscle from largest to smallest? fascicle, myofibril, sarcomere, muscle fiber myofibril, fascicle, sarcomere, muscle fiber fascicle, muscle fiber, myofibril, sarcomere muscle fiber, fascicle, myofibril, sarcomere Can have degrees of contraction, even within sarcomere

Exit Ticket Which of these stores calcium ion? Sarcoplasmic reticulum Sarcomere Sarcolemma mitochondria Can have degrees of contraction, even within sarcomere

Exit Ticket Which of these best describe the process of muscle contraction? The actin filaments shorten The myosin filaments shorten The light bands shorten The dark bands shorten Can have degrees of contraction, even within sarcomere

Exit Ticket 6. Which of these best describe the process of muscle contraction? Myosin heads attach to actin filaments that are exposed by the presence of ATP Myosin heads attach to actin filaments that are exposed by the presence of Ca2+ Actin heads attach to myosin filaments that are exposed by the presence of ATP Actin heads attach to myosin filaments that are exposed by the presence of Ca2+ Can have degrees of contraction, even within sarcomere