Chapter 47 Lecture 16 How do muscles contract? Dr. Alan McElligott

How Do Muscles Contract? Aims: To examine the structure and function of the three types of muscle tissue To take a closer look at muscle contraction and describe the roles of actin and myosin

How Do Muscles Contract? Aims: To examine the structure and function of the three types of muscle tissue To take a closer look at muscle contraction and describe the roles of actin and myosin These lecture aims form part of the knowledge required for learning outcome 3: Describe mechanisms for life processes (LOC3)

Essential reading Pages How Do Muscles Contract? 47.1 How Do Muscles Contract?

Muscles and skeletons: the musculoskeletal system. Muscles and skeletons are the effectors that produce movement.

47.1 How Do Muscles Contract? Three types of vertebrate muscle: Skeletal: voluntary movement, breathing Cardiac: beating of heart Smooth: involuntary, movement of internal organs

47.1 How Do Muscles Contract? Skeletal muscle (striated): Cells are called muscle fibres: multinucleate Form from fusion of embryonic myoblasts One muscle consists of many muscle fibres bundled together by connective tissue

Figure 47.1 The Structure of Skeletal Muscle

47.1 How Do Muscles Contract? Contractile proteins: Actin: thin filaments Myosin: thick filaments Each muscle fibre has several myofibrils: bundles of actin and myosin filament.

47.1 How Do Muscles Contract? Each myofibril consists of repeating units: sarcomeres. Sarcomere: overlapping actin and myosin filaments. Bundles of myosin filaments are held in place by the protein titin, the largest protein in the body.

Figure 47.1 The Structure of Skeletal Muscle (Part 2)

Figure 47.1 The Structure of Skeletal Muscle (Part 3)

47.1 How Do Muscles Contract? The sliding filament theory of muscle contraction: Depends on structure of actin and myosin molecules. Myosin heads can bind specific sites on actin molecules to form cross bridges. Myosin changes conformation, causes actin filament to slide 5–10 nm.

47.1 How Do Muscles Contract? Contraction of skeletal muscle

Figure 47.2 Sliding Filaments

47.1 How Do Muscles Contract? Muscle contraction is initiated by action potentials from a motor neuron at the neuromuscular junction. A motor unit: all the muscle fibres activated by one motor neuron.

Figure 47.3 Actin and Myosin Filaments Overlap to Form Myofibrils

47.1 How Do Muscles Contract? One muscle may have many motor units. To increase strength of muscle contraction: increase rate of firing of motor neuron or recruit more motor neurons to fire (more motor units activated).

47.1 How Do Muscles Contract? Muscle cells are excitable: the plasma membrane can conduct action potentials. Acetylcholine is released by the motor neuron at the neuromuscular junction and opens ion channels in the motor end plate.

47.1 How Do Muscles Contract? Action potentials also travel deep within muscle fibre via T tubules. T tubules (transverse tubules) descend into the sarcoplasm (muscle fibre cytoplasm). T tubules run close to the sarcoplasmic reticulum (ER): a closed compartment that surrounds every myofibril.

Figure 47.5 T Tubules in Action

47.1 How Do Muscles Contract? Sarcoplasmic reticulum has Ca 2+ pumps. At rest there is high concentration of Ca 2+ in the sarcoplasmic reticulum. Action potential reaches receptor proteins and opens the Ca 2+ channels, Ca 2+ flows out of sarcoplasmic reticulum and triggers interaction of actin and myosin.

47.1 How Do Muscles Contract? Actin filaments also include tropomyosin and troponin. Troponin has three subunits: one binds actin, one binds myosin, and one binds Ca 2+. At rest, tropomyosin blocks the binding sites on actin.

47.1 How Do Muscles Contract? When Ca 2+ is released, it binds to troponin, which changes conformation. Troponin is bound to tropomyosin— twisting of tropomyosin exposes binding sites on actin. When Ca 2+ pumps remove Ca 2+ from sarcoplasm, contraction stops.

Figure 47.6 The Release of Ca 2+ from the Sarcoplasmic Reticulum Triggers Muscle Contraction

47.1 How Do Muscles Contract? Cardiac muscle is also striated; cells are smaller than skeletal and have one nucleus. Cardiac muscle cells also branch and interdigitate: can withstand high pressures. Intercalated discs provide mechanical adhesions between cells.

Figure 47.7 There are Three Kinds of Muscle

47.1 How Do Muscles Contract? Pacemaker and conducting cells initiate and coordinate heart contractions. Heartbeat is myogenic—generated by the heart muscle itself. Autonomic nervous system modifies the rate of pacemaker cells, but is not necessary for their function.

47.1 How Do Muscles Contract? Contraction of cardiac muscle: DHP proteins in T tubules are Ca channels; ryanodine receptors are ion- gated Ca 2+ channels, sensitive to Ca 2+. Action potential causes Ca 2+ to flow into sarcoplasm from T tubules; increase in Ca 2+ opens the Ca 2+ channels in sarcoplasmic reticulum—large increase in Ca 2+ in sarcoplasm—initiates contraction. Ca 2+ -induced Ca 2+ release

47.1 How Do Muscles Contract? Contraction of cardiac muscle: Cardiac muscle cell beating

47.1 How Do Muscles Contract? Smooth muscle: in most internal organs; under autonomic nervous system control. Smooth muscle cells are arranged in sheets; have electrical contact via gap junctions. Action potential in one cell can spread to all others in the sheet.

47.1 How Do Muscles Contract? Plasma membrane of smooth muscle cells sensitive to stretch. Stretched cells depolarize and fire action potentials which starts contraction.

47.1 How Do Muscles Contract? Smooth muscle contraction: Ca 2+ influx to sarcoplasm stimulated by stretching, action potentials, or hormones. Ca 2+ binds with calmodulin: activates myosin kinase which phosphorylates myosin heads—can then bind and release actin.

Figure 47.8 Mechanisms of Smooth Muscle Activation

Figure 47.8 Mechanisms of Smooth Muscle Activation (Part 1)

Figure 47.8 Mechanisms of Smooth Muscle Activation (Part 2)

47.1 How Do Muscles Contract? Skeletal muscle: minimum unit of contraction = a twitch. Twitch measured in terms of tension, or force it generates. A single action potential generates a single twitch. Force generated depends on how many fibres are in the motor unit.

47.1 How Do Muscles Contract? Tension generated by entire muscle depends on: Number of motor units activated Frequency at which motor units are firing

47.1 How Do Muscles Contract? Single twitch: if action potentials are close together in time, the twitches are summed, tension increases. Twitches sum because Ca 2+ pumps can not clear Ca 2+ from sarcoplasm before the next action potential arrives. Tetanus: when action potentials are so frequent there is always Ca 2+ in the sarcoplasm.

Figure 47.9 Twitches and Tetanus

47.1 How Do Muscles Contract? How long muscle fibre can sustain tetanic contraction depends on ATP supply. ATP is needed to break the myosin-actin bonds, and “re-cock” the myosin heads. To maintain contraction, actin–myosin bonds have to keep cycling.

47.1 How Do Muscles Contract? Muscle tone: a small but changing number of motor units are contracting. Muscle tone is constantly being adjusted by the nervous system.

Check out 47.1 Recap, page 1031, questions 2 and CHAPTER SUMMARY, page 1022, See WEB/CD Activity 47.1 Self Quiz page : Chapter 47, questions 1 and 2 For Discussion page 1023: Chapter 47, question 1 How Do Muscles Contract?

Key terms: actin, autonomic nervous system, calmodulin, cardiac muscle, fibres, intercalated discs, musculoskeletal, myogenic, myoblasts, myosin, myosin kinase, myosin phosphotase, pacemaker, rigor mortis, sacromere, sacroplasm, sarcoplasmic reticulum, skeletal (striated) muscle, smooth muscle, T tubule, titin, tropomyosin, troponin How Do Muscles Contract?

Figure The Human Endoskeleton

Figure Types of Joints

Figure Joints, Ligaments, and Tendons