muscle physiology muscle proteins and mechanisms

Physiology of muscles and muscle contraction

1. MUSCLE PHYSIOLOGY

2. Functions of Muscle Tissue sustained contraction or alternating contraction and relaxation, muscular tissue has four key functions: Produces body movements. Stabilizes body positions. Stores and moves substances within the body. Generates heat.

3. Properties of Muscle Tissue Muscular tissue has four special properties that enable it to function and contribute to homeostasis: Electrical excitability Contractility Extensibility Elasticity

4. Electrical excitability : a property of both muscle and nerve cells to respond to certain stimuli by producing electrical signals called action potentials Contractility ability of muscular tissue to contract forcefully when stimulated by an action potential. When a muscle contracts, it generates tension (force of contraction) while pulling on its attachment points.

5. Extensibility ability of muscular tissue to stretch without being damaged. Extensibility allows a muscle to contract forcefully even if it is already stretched. Elasticity ability of muscular tissue to return to its original length and shape after contraction or extension

6. Classification of Muscles Muscles are classified by three different methods, based on different factors: I. Depending upon the presence or absence of striations II. Depending upon the control III. Depending upon the situation.

7. DEPENDING UPON STRIATIONS Depending upon the presence or absence of cross striations, the muscles are divided into two groups: 1. Striated muscle 2. Non-striated muscle.

8. Striated Muscle Striated muscle is the muscle which has a large number of cross-striations (transverse lines). Examples are skeletal muscle and cardiac muscles.

9. Non-striated Muscle Muscle which does not have cross-striations is called non-striated muscle. It is also called plain muscle or smooth muscle. It is found in the wall of the visceral organs.

10. II. DEPENDING UPON CONTROL Depending upon control, the muscles are classified into two types: 1. Voluntary muscle 2. Involuntary muscle.

11. 1. Voluntary Muscle Voluntary muscle is the muscle that is controlled by our own will. Skeletal muscles are the voluntary muscles. These muscles are innervated by somatic nerves. 2. Involuntary Muscle Muscle that cannot be controlled by the will is called involuntary muscle. Cardiac muscle and smooth muscle are involuntary muscles

12. DEPENDING UPON LOCATION Depending upon location, the muscles are classified into three types: 1. Skeletal muscle 2. Cardiac muscle 3. Smooth muscle.

13. 1. Skeletal Muscle Skeletal muscle is situated in association with bones forming the skeletal system. form 40% to 50% of body mass and are voluntary and striated. Supplied by somatic nerves. Fibers of the skeletal muscles are arranged in parallel. Muscle fibers are attached to tendons on either end. Skeletal muscles are anchored to the bones by the tendons.

14. Skeletal Muscle Description: Long, cylindrical, striated fibers with many peripherally located nuclei; voluntary control. Location: Usually attached to bones by tendons. Function: Motion, posture, heat production, and protection.

15. Structure of Skeletal Muscle

16. Skeletal muscle Longitudinal section of skeletal muscle tissue Skeletal muscle fiber nucleus striations Skeletal muscle fiber

17. MUSCLE MASS OR TISSUE Muscle mass or muscle tissue is made up of a large number of individual muscle cells or myocytes . The muscle cells are commonly called muscle fibers because these cells are long and slender in appearance. Skeletal muscle fibers are multinucleated and are arranged parallel to one another with some connective tissue in between.

18. Muscle mass is separated from the neighboring tissues by a thick fibrous tissue layer known as fascia. Beneath the fascia, muscle is covered by a connective tissue sheath called epimysium . In the muscle , the muscle fibers are arranged in various groups called bundles or fasciculi . Connective tissue sheath that covers each fasciculus is called perimysium . Each muscle fiber is covered by a connective tissue layer called the endomysium

19. A. Skeletal muscle mass; B. Cross-section of muscle; C. One muscle fasciculus .

20. MUSCLE FIBER Each muscle cell or muscle fiber is cylindrical in shape. Average length of the fiber is 3 cm (1-4 cm, depending upon the length of the muscle). The diameter of the muscle fiber varies from 10 μ to 100 μ and it varies in a single muscle. Muscle fibers are attached to a tough cord of connective tissue called tendon. Tendon is in turn attached to the bone. Tendon of some muscles is thin, flat and stretched but tough. Such type of tendon is called aponeurosis . Each muscle fiber is enclosed by a cell membrane called plasma membrane, that lies beneath the endomysium . It is also called sarcolemma . Cytoplasm of the muscle is known as sarcoplasm .

21. Structures embedded within the sarcoplasm are: 1. Nuclei 2. Myofibril 3. Golgi apparatus 4. Mitochondria 5. Sarcoplasmic reticulum 6. Ribosomes 7. Glycogen droplets 8. Occasional lipid droplets.

22. Sarcolemma It is the cell membrane of the muscle fiber. The sarcolemma consists of a true cell membrane, called the plasma membrane, and an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer of the sarcolemma fuses with a tendon fiber. Tendon fibers in turn collect into bundles to form the muscle tendons that then insert into the bones.

23. MYOFIBRIL Each muscle fiber contains several hundred to several thousand myofibrils. Myofibrils or myofibrillae are the fine parallel filaments present in sarcoplasm of the muscle cell. some of the myofibrils are arranged in groups called Cohnheim’s areas or fields. Each myofibril is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules responsible for the actual muscle contraction.

24. MYOFIBRIL The myofibrils contain 2 kinds of protein filaments . Thick filaments – composed of myosin . Thin filaments – composed of Actin , troponin and tropomyosin . c. striations are produced by alternating light and dark filaments .

25. 2. Cardiac Muscle Cardiac muscle forms the musculature of the heart. These muscles are striated and involuntary. Cardiac muscles are supplied by autonomic nerve fibers.

26. CARDIAC MUSCLE Heart Longitudinal section of cardiac muscle tissue Cardiac muscle fibers Nucleus Cardiac muscle fiber (cell) Striations

27. CARDIAC MUSCLE Description: Branched striated fibers with one or two centrally located nuclei; contains intercalated discs; involuntary control. Location: Heart wall. Function: Pumps blood to all parts of the body.

28. SMOOTH MUSCLE Description: Spindle-shaped (thickest in middle and tapering at both ends), nonstriated fibers with one centrally located nucleus; involuntary control. Location: Iris of the eyes, walls of hollow internal structures such as blood vessels, airways to the lungs, stomach, intestines, gallbladder, urinary bladder, and uterus. Function: Motion (constriction of blood vessels and airways, propulsion of foods through gastrointestinal tract, contraction of urinary bladder and gallbladder).

29. 3. Smooth Muscle Smooth muscle is situated in association with viscera. It is also called visceral muscle. It is different from skeletal and cardiac muscles because of the absence of cross striations. Smooth muscle is supplied by autonomic nerve fibers. Smooth muscles form the main contractile units of wall of the various visceral organs E.g. stomach.

30. Artery Smooth muscle Longitudinal section of smooth muscle tissue Nucleus of smooth muscle fiber Smooth muscle fiber (cell) SMOOTH MUSCLE

31. Organization of Skeletal Muscle Fibers Figure 10–3

32. The Sarcolemma The cell membrane of a muscle cell Surrounds the sarcoplasm (cytoplasm of muscle fiber) A change in transmembrane potential begins contractions All regions of the cell must contract simultaneously

33. Transverse Tubules ( T tubules ) Transmit action potential – impulses through cell Allow entire muscle fiber to contract simultaneously Have same properties as sarcolemma Filled with extracellular fluid

34. Myofibrils- 1-2um in diameter Lengthwise subdivisions within muscle fiber Made up of bundles of protein filaments ( myofilaments ) Myofilaments - are responsible for muscle contraction 2 Types of Myofilaments Thin filaments : made of the protein actin Thick filaments : made of the protein myosin

35. Sarcoplasmic Reticulum (SR) A membranous structure surrounding each myofibril Helps transmit action potential to myofibril Similar in structure to smooth endoplasmic reticulum Forms chambers ( terminal cisternae ) attached to T tubules

36. A Triad Is formed by 1 T tubule and 2 terminal cisterna Cisternae Concentrate Ca 2+ ( via ion pumps) Release Ca 2+ into sarcomeres to begin muscle contraction

37. Structural components of the Sarcomeres Figure 10–4 -The contractile units of muscle -Structural units of myofibrils -Form visible patterns within myofibrils

38. Muscle Striations A striped or striated pattern within myofibrils: alternating dark, thick filaments (A bands) and light, thin filaments (I bands)

39. M Lines and Z Lines M line : the center of the A band at midline of sarcomere Z lines : the centers of the I bands at 2 ends of sarcomere Zone of Overlap The densest, darkest area on a light micrograph Where thick and thin filaments overlap

40. The H Zone The area around the M line Has thick filaments but no thin filaments Titin Are strands of protein Reach from tips of thick filaments to the Z line Stabilize the filaments

41. Sarcomere Structure Figure 10–5

42. Sarcomere Function Transverse tubules encircle the sarcomere near zones of overlap Ca 2+ released by SR causes thin and thick filaments to interact

43. Figure 10–6 (1 of 5) Level 1: Skeletal Muscle Level 2: Muscle Fascicle

44. Level 3: Muscle Fiber Figure 10–6 (3 of 5) Level 4: Myofibril

45. Level 5: Sarcomere Figure 10–6 (5 of 5)

46. Muscle Contraction Is caused by interactions of thick and thin filaments Structures of protein molecules detemine interactions

47. A Thin Filament Figure 10–7a

48. 4 Thin Filament Proteins F actin : is 2 twisted rows of globular G actin the active sites on G actin strands bind to myosin Nebulin : holds F actin strands together Tropomyosin : is a double strand prevents actin–myosin interaction Troponin : - a globular protein binds tropomyosin to G actin controlled by Ca 2+

49. Troponin and Tropomyosin Figure 10–7b Ca2+ binds to receptor on troponin molecule Troponin–tropomyosin complex changes Exposes active site of F actin Initiating Contraction

50. A Thick Filament Contain twisted myosin subunits Contain titin strands that recoil after stretching

51. The Mysosin Molecule Tail : binds to other myosin molecules Head : made of 2 globular protein subunits reaches the nearest thin filament

52. Mysosin Action During contraction, myosin heads : interact with actin filaments, forming cross-bridges pivot, producing motion