The All-or-None Principle Motor units also comply to a rule known as the all-ornone principle (or law).

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1 The All-or-None Principle Motor units also comply to a rule known as the all-ornone principle (or law). This principle stipulates that, when a motor unit is stimulated to contract, it will do so to its fullest potential. In other words, if a motor unit consists of 10 muscle fibres (or 800 muscle fibres) and they are turned on, either all fibres will contract or none will contract Thompson Educational Publishing, Inc. 1

2 Tennis Elbow & Golfer s Elbow Injuries often occur due to the enormous forces generated where muscle attaches to bone Thompson Educational Publishing, Inc. 2

3 Muscle Atrophy Decrease in size of muscle Use it or lose it Can occur following malnutrition, disease, abandoned longterm resistance training May cause permanent shortening of muscle fibers Flaccid-tone is lost and muscle becomes limp This happens to astronauts in space in no gravity atmosphere

4 Hypertrophy Increase in muscle size Increased capillary density Increased muscle proteins (actin and myosin) Increased storage capacity for glucose, glycogen, ATP, and CP Due to forceful contractions over long periods of time

5 Transient Hypertrophy the pump Increase in fluid accumulation to the specific muscle or muscles exercised A sudden rush of blood to muscles making them look bigger Short-term

6 Hyperplasia Fibre-splitting Theory-not proven in humans One a fibre has reached its maximal hypertrophy, any further size and strength will come through the formation of two daughter cells, created by fibre splitting

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8 The Motor Unit The motor neuron, its axon (pathway), and the muscle fibres it stimulates are together referred to as the motor unit. Nerves transmit impulses in waves that ensure smooth movements. A single nervous impulse and the resulting contraction is called a muscle twitch. One neuron or nerve (called the motor neuron ) may be responsible for stimulating a number of muscle fibres Thompson Educational Publishing, Inc. 8

9 Motor Neuron, Axon, and Muscle Fibres 2015 Thompson Educational Publishing, Inc. 40

10 The Anatomy of the Skeletal Muscle (deep to superficial) Skeletal muscle fibers (cells) are surrounded by a layer of connective tissue called endomysium. (separates each muscle fibre) Muscle fibers are grouped in bundles called fascicles. These fascicles are covered in a connective tissue sheath called perimysium. (surrounds groups of ten to one hundred or more individual muscle fibres)

11 The Anatomy of the Skeletal Muscle The muscle as a whole are surrounded by epimysium. It extends beyond the muscle where it s properties change and then it becomes one with the tendon followed by the tendon connecting to the bone.

12 The Anatomy of Skeletal Muscle Sarcolemma. A plasma membrane that lies beneath the endomysium, a sheath of connective tissue that surrounds a muscle fibre. Sarcoplasm. The muscle cell s cytoplasm, which is contained by the sarcolemma. Sarcomeres. The units of skeletal muscle containing the cellular proteins myosin and actin. Sarcoplasmic reticulum. A network of channels in each muscle fibre that transport the electrochemical substances involved in muscle activation Thompson Educational Publishing, Inc. 12

13 The Anatomy of the Skeletal Muscle The plasma membrane is called the sarcolemma. The sarcolemma encloses the muscle fibre. The sarcolemma contains the muscle cell s cytoplasm, which is called sarcoplasm.

14 Microscopic Anatomy of Skeletal Muscle Transverse tubules (TT) are tunnels that carry electrical current from surface of cell to inside Myofibrils are small bundles of protein that run along the muscle fibre length. There are two types of myofibrils; thick and thin. Myoglobin is the pigment that makes the red colour of muscle

15 More Structures In A Cell Sarcoplasmic reticulum (SR)- smooth endoplasmic reticulum in a cell SR is a reservoir for calcium storage (very important)

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17 The Neuromuscular Junction 2015 Thompson Educational Publishing, Inc. 17

18 The Sliding Filament Theory The muscle as a whole contracts, but the mechanism by which this is achieved is not through a shortening, but rather an overlapping of actin and myosin filaments (in a rachet-like manner). This overlapping causes the sarcomere (and thus the whole muscle fibre) to contract (i.e., to shorten). This is known as the sliding filament theory of muscle contraction. The sliding filament theory is accepted as a description of how the process of muscular contraction occurs. Its discovery dates back to the 1950s and accurately describes what happens during contraction Thompson Educational Publishing, Inc. 18

19 Myofilaments-Thick Filament These are the key to muscle contraction Each myofibril contains bundles of parallel protein microfilaments called myofilaments There are 2 kinds of myofilaments 1. Thick filaments- made from a protein called myosin, shaped like a golf club Dark part of the striations Binding site for actin on the head

20 Thin Filaments 2. Thin filaments- made from actin protein containing an active site that fits the head of the myosin perfectly during contraction. Light part of the striations Two proteins that make up actin: troponin which has a binding site for calcium and tropomyosin which is a cord-like structure that blocks the bonding site on the actin for the myosin

21 Striations Organization of actin and myosin cause the striped appearance in cardiac and skeletal muscle A bands alternate with I bands to get the striped appearance A bands are thick myofilaments, there is some overlap of thin and thick, creating a really dark area. I bands are composed of ONLY thin myofilament I bands: Isotropic A bands: Aniosotropic

22 Striations continued In the middle of the A band, there is a space where there is ONLY thick filament. This is called an H zone. Each I band is bisected by a dark, narrow Z line which anchors the thin filaments (and the elastic filaments) The area between Z lines is called a sarcomere (the functional unit of a muscle) Z line- Zwischen - German word for between

23 Organization of Muscle Fiber-Summary I bands attach to Z lines at the end of the sarcomere I bands are thin myofilaments A bands are thick myofilaments and overlap the thin myofilaments H zone is an area where only thick myofilament occurs M line helps hold myosin in place; at the centre of the H zone

24 I band A band H zone Z line M line SARCOMERE

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27 Sliding Filaments At very high levels of magnification, it is possible to detect small bridges on the thick filaments that extend to the thin filaments. These myosin crossbridges attach, rotate, detach, and re-attach in a ratchet- like fashion. This process results in a shortening of the sarcomere ( muscle contraction ) Thompson Educational Publishing, Inc. 27

28 SLIDING FILAMENT THEORY A message is created by the brain that is sent to the CNS. The message travels down the spinal cord and branches off at the appropriate level and sends a message to the peripheral nervous system. It then travels from the axon branch to the axon terminal to recruit muscle fibres. It then is carried through the axon terminal via Ach to the sarcolemma of each muscle fibre involved. Ach causes the sarcoplasmic reticulum to release Ca ions from the T-Tubules (terminal cisternae).

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30 Sliding Filament Theory Tropomyosin obscures binding sites on the actin Calcium binds with troponin located on the actin s tropomyosin. The troposmyosin swivels allowing the binding site on the actin for myosin to be exposed. ATP cleaves into ADP and Pi The myosin head attaches to the binding site on the actin forming a cross-bridge.

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32 SLIDING FILAMENT THEORY Once it binds with the actin, the myosin head tilts, pulling the actin filament so that the two slide across each other. (power stroke) During the power stroke ADP +Pi on the myosin head falls out of the binding site. More ATP is formed from the synthesis of ADP+Pi, which will bind to the myosin head. When the ATP binds, the myosin releases the actin.

33 SLIDING FILAMENT THEORY The ATP molecule then splits into ADP and Pi again which resets the myosin to its original position Contraction of filaments will continue as long as Ca is present. Muscle action ends when the calcium is actively pumped out of the sarcoplasm back into the sarcoplasmic reticulum for storage. Troponin and tropomyosin will cover the binding sites again Muscle returns back to rest

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35 Excitation-Contraction Coupling The process of muscle contraction is often referred to as excitation-contraction coupling. The electrical signal that begins the process originates in the spinal cord and moves along the nerve axon to the neuromuscular junction. The signal is then transmitted down into the muscle fibre through tubular membranes. Calcium ions are released into the sarcoplasm. The interaction of calcium with the proteins troponin and tropomyosin removes the obstacles to actin-myosin interaction. The coupling effect is then allowed to unfold, and muscle contraction is the result 2015 Thompson Educational Publishing, Inc. 35

36 Rigor Mortis Usually begins 3-4 hours after death (depending on temperature and environment) After death there is a lack of ATP, so the myosin cross bridge can t detach from actin (the release of calcium from sarcoplasmic reticulum allows more contractions temporarily Bacteria decompose tissue and decay will make rigor mortis soften..eventually..