The Muscular System 6 1 The Muscular System

The Muscular System 6 -1

The Muscular System – an overview n There are three types of muscular tissue: q q q n n Skeletal muscle Smooth muscle Cardiac muscle Muscular tissue is contractive tissue. What is the function of the muscular system? 6 -2

The Muscular System Has Many Functions n n n Allows movement through the environment Stabilizes movement at the joints Aids in the flow of lymph and blood through the body Protects our internal organs Maintains homeostasis by producing heat 6 -3

Skeletal Muscles are Contractile Organs n n All human skeletal muscles have a similar function and structure. Function: q q n They contract to produce movement. They relax to their original length. Structure: q q The origin is the end that remains stationary when the organ shortens. The insertion is the end that moves during contraction. 6 -4

Figure 6. 1 Muscle origin and insertion 6 -5

Body movements n To coordinate and control body movements, most human skeletal muscles function as a member of an antagonistic or synergistic pair. q Antagonistic (synergistic) – muscles with opposing actions working together to provide smooth and controlled movements. n Example: moving your hand to your shoulder requires: 1. simultaneous contraction of the prime movers (the brachialis and biceps brachii muscles), and 2. relaxation of the antagonist (the triceps brachii). 6 -6

Figure 6. 2 Anterior view of the superficial muscles of the body 6 -7

Figure 6. 3 Posterior view of the superficial muscles of the body 6 -8

Anatomy of a muscle n n n Skeletal muscle is composed of numerous elongated structures, one nested inside the other. Individual muscle cells are long (sometimes 30 cm), slender and fragile. There are essentially 3 layers of the muscle: q q q Epimysium – separates muscles Perimysium –covers and supports muscle cells Endomysium – covers muscle cells on top of membrane 6 -9

Figure 6. 4 Anatomy of a muscle 6 -10

Organization of skeletal muscles n n n Myofiber – a single muscle cell Sarcolemma – cell membrane covering myofiber T tubules – specialized areas in the sarcolemma that conduct the contraction messages Myofibrils – linearly arranged groups of the contractile proteins actin and myosin Sarcomeres - contractile units which hold the proteins in regular arrangements q Sarcomeres are banded (striated) Copyright 2008 John Wiley & Sons, Inc. 6 -11

Figure 6. 5 Organization of skeletal muscles from gross to molecular Copyright 2008 John Wiley & Sons, Inc. 6 -12

Figure 6. 5 Organization of skeletal muscles from gross to molecular 6 -13

Figure 6. 5 Organization of skeletal muscles from gross to molecular 6 -14

n Actin and myosin are microscopic proteins that interact and cause the entire muscle tissue to shorten and, thus, move the skeletal tissue. q q Actin is a thin, globular protein Myosin is a larger, heavier protein Figure 6. 6 Structure of thick and thin filaments Figure 6. 7 Myosin filament 6 -15

Muscle Contraction Occurs as Filaments Slide Past One Another The contraction of a skeletal muscle begins when an impulse reaches the neuromuscular junction. At this junction, the motor neuron ends very close to muscle cells separated by the synapse. Nerves send a contraction impulse across the synapse with neurotransmitters. n n n q Acetylcholine (ACh) is the most common. 6 -16

Muscle Contraction Occurs as Filaments Slide Past One Another n n n ACh is released from the axon terminal and diffuses across the synaptic cleft and binds to receptors on the muscle cell membrane. The skeletal muscle will contract in response to ACh binding. The impulse to contract is then passed through the entire muscle cell via T tubules. 6 -17

Figure 6. 8 Neuromuscular junction 6 -18

More details… n n n Inside the muscle cell, sarcoplasmic reticulum (SR) stores calcium ions and releases them when the ACh binds to the surface of the cell. Calcium is held in the SR by the enzyme calcium sequestrin. This enzyme on the surface of the SR stores and releases calcium from the cytoplasm into the SR. q n n n The enzyme works by converting ATP to ADP. Since free calcium in the cell is toxic, this ensures the survival of the cell. Neither actin nor myosin undergo chemical transformations. Actin merely slides over the myosin filament. 6 -19

Figure 6. 9 Contraction cycle 6 -20

Figure 6. 10 Summary of events in contraction and relaxation of skeletal muscle 6 -21

Whole-Muscle Contractions Emerge from Tiny Impulses n How does an entire large muscle like that of your thigh contract and generate movement? q Muscle cells are grouped in motor units n q q 1 motor neuron and the set of muscle cells it controls The entire motor unit contracts when it receives a signal from the motor nerve. This causes calcium ions to be released which triggers the sliding action. 6 -22

Whole-Muscle Contractions Emerge from Tiny Impulses Figure 6. 12 Motor unit 6 -23

Muscle cells contract on an all-ornothing basis n n If the nerve stimulus is too weak, nothing happens. When the threshold stimulus is reached, calcium is released and the entire muscle cell contracts. Single twitches are not effective in producing body movement, because they last a fraction of a second. The motor unit requires multiple stimuli. 6 -24

n n n Each contraction builds on the last until the muscle cell is continuously contracted. The buildup of contractions is called summation. Increased strength for similar stimuli is known as treppe. q n Example – warming up for an athletic event takes advantage of treppe Once continuous contraction is achieved, the muscle is in tetanus. q Example – the neck muscles of an adult are in tetanus most of the day 6 -25

Muscles Require Energy to Work Smoothly and Powerfully n What is the general source of energy inside cells? q n ATP The body can make ATP for muscular contractions through either aerobic or anaerobic pathways. 6 -26

Figure 6. 16 Krebs cycle 6 -27

ATP production n Aerobic pathway (highly efficient) q q n Burns glucose, forming water, carbon dioxide and ATP Aerobic ATP is generated in the mitochondria Produces the largest amount of ATP Is the dominant method of energy production During heavy muscle activity, oxygen cannot keep up with the energy demands. ATP production shifts to anaerobic pathways. 6 -28

ATP production n Anaerobic pathway (less efficient) q q Produce fewer ATP molecules per glucose molecule Produces lactic acid which is eventually removed from the tissue by conversion to pyruvic acid n n This requires oxygen (which is one reason we breathe heavily after exertion). Oxygen is carried through the bloodstream to the lactic acid-laden tissue, reacts with the acid converting it to pyruvic acid and then to coenzyme A. 6 -29

Figure 6. 17 Conversion of lactic acid to pyruvate and then on to ATP 6 -30

n Creatine phosphate is important in the anaerobic phase of muscle energy production because it stores energy much like ATP, in a phosphate bond. Figure 6. 18 Creatine phosphate reaction 6 -31

Muscle twitches can be fast or slow n There are two types of muscle cells: q Slow twitch (slow oxidative; aerobic) n n q Appear red, have a large blood supply, have many mitochondria, and store myoglobin. Purpose is to supply oxygen to the mitochondria of the cells and sustain the supply of ATP within sarcomeres. Fast twitch (fast oxidative; anaerobic) n n Provide short bursts of incredible energy and contraction power, but fatigue quickly. These cells are thicker and usually contain larger glycogen reserves and less developed blood supply. 6 -32

Figure 6. 19 Fast twitch and slow twitch cells 6 -33

Toned muscles work better, look better n n Muscle tone is the constant partial contraction of muscle when the body is “in shape”. Toned muscles are more effective at burning energy (using more ATP per gram). q Therefore, people who are in shape can eat more without gaining weight because the low-level contractions of their toned muscles constantly burn ATP. 6 -34

Exercise or chemical compounds can change the size of a muscle. n n Scientists think that the total number of muscle fibers is inherited; therefore, we alter the muscle by enlarging cells (hypertrophy). Hypertrophy is caused by the addition of new myofibrils within the endomysium of muscle cells, which thickens the myofibers. Hypertrophic muscles should have thicker muscle cells, packed with more sarcomeres. Exercise that requires muscle to contract to at least 75% of maximum tension will cause hypertrophy. 6 -35

Summary n Our muscles are designed to: q q q n Provide movement Manipulate the environment Maintain homeostasis Protect our organs Maintain our upright position All movement requires the production of ATP, either stored in the cell or produced via metabolic pathways. 6 -36