MUSCULAR SYSTEM CHAPTER 8

Muscles 3 types: Skeletal Smooth Cardiac We will concentrate mostly on this one. Smooth Cardiac All three are composed of more than just muscle tissue. Example: Skeletal muscles are composed of skeletal muscle tissue, nervous tissue, blood, and connective tissue.

Structure of Skeletal Muscles Why has it been hard to see some of the muscles within the cats? They are covered with connective tissue. There is loose connective tissue and the other type. FASCIA - connective tissue that covers muscles. Has two purposes: separates individual muscles from adjacent muscles hold muscles in position. How does it hold muscles in position? 2 ways

Structure of Skeletal Muscles TENDONS - connective tissue that connects muscles to the bone. It is an area beyond the end of the muscle where the fascia gets very dense and compact. APONEUROSES - associated with sheet-like muscles, connects muscle to muscle.

Structure of Skeletal Muscles Muscles have multiple layers Cylinders within cylinders Levels of muscles, big to small: Muscle  fascicle  muscle fiber  myofibril  filaments (page 178) Each level of a muscle is enclosed in a layer of connective tissue Epimysium – covers entire muscle Perimysium – separates muscle into fascicles Endomysium – separates fascicles into muscle fibers

Skeletal Muscle Fibers What is the purpose of all the levels? Help with contraction, all of them can work together Skeletal muscle fiber – individual cell that contracts in response to stimulation. Each one is a thin, elongated cylinder with rounded ends These have different names for the cell membrane and cytoplasm Cell membrane = sarcolemma Cytoplasm = sarcoplasm

Skeletal Muscle Fibers The sarcoplasm contains numerous threadlike myofibrils that lie parallel to one another. Myofibrils are involved in muscle contraction. Myofibrils contain two kinds of protein filaments.

Skeletal Muscle Fibers Myosin Filaments - The “thick” filaments that are composed of protein. Actin Filaments – The “thin” filaments that are composed of protein What effect do these have on the appereance of muscle tissue? The arrangement of these filaments is the reason for the striations that we see with skeletal muscles.

Skeletal Muscle Fibers Striation pattern parts: I band (light bands) – where only actin filaments are. Where they meet is known as the Z line. A bands (dark bands) – where the thick myosin filaments are overlapping with thin actin filaments. Usually thicker bands H zone is in the middle From Z line to Z line is one sarcomere.

Structure of Skeletal Muscle Sarcoplasmic reticulum - network channels that run parallel to each myofibril Transverse tubules – tubes that extend inward and pass through the fiber. What would these be used for? Activate muscle contraction when the fiber is stimulated.

Muscle activation How are muscles activated to contract? The neuromuscular junction Each skeletal muscle fiber is connected to an extension of a motor neuron. The nerve fiber and the muscle meet at the neuromuscular junction.

Neuromuscular Junction There is a tiny gap between the neuron and the muscle called the synaptic cleft. How does the nerve actually stimulate the muscle with a gap there? The neuron contains many tiny vesicles (synaptic vesicles) that store chemicals called neurotransmitters.

Neuromuscular Junction When the nerve receives and impulse, the neuron releases the neurotransmitter into the synaptic cleft. What will this cause? Binds with a channel protein, allowing Ca+ to pass through, it is then able to enter the muscle fiber This action stimulates the muscle fiber to contract.

Motor Units Does each nerve only control one muscle fiber? NO, we have motor units. A motor unit consists of a motor neuron and all of the muscle fibers it controls. The number of muscle fibers in a motor unit varies considerably. Why would that be? The fewer muscle fibers in the motor units, the finer the movement.

Skeletal Muscle Contraction Myosin has protein strands with globular parts called cross-bridges projecting outward along their length. Think about your arm while flexing your bicep Actin molecules, arranged together in a double twisted strand, form an actin filament. Tropomyosin & troponin are the two proteins associated with actin filaments.

Skeletal Muscle Contraction The sliding filament theory is the most widely accepted theory on how muscle contraction works. It says that the head of a myosin cross-bridge can attach to an actin binding site and bend slightly What will this cause? The actin to move (or slide) with it The head can then release, straighten itself, and combine with another binding site farther down the actin filament. Example, arms with a meter stick This can happen several times causing the muscle fiber to shorten dramatically

Skeletal Muscle Contraction All of this binding and changing of shape will take energy, where will it come from? ATP The enzyme ATPase causes the breakdown of ATP to supply energy for these actions. Acetylcholine (ACh) is a neurotransmitter that is synthesized in the cytoplasm of the motor neuron and is stored in vesicles.

Muscular Responses How does a muscle respond to activation? The stimulus increases until it reaches the threshold stimulus Threshold stimulus is the minimum strength of stimulus required to cause a contraction. Once stimulated the muscle will respond completely. There is no partial contraction. This is called an All-or-None Response.

Steps to muscle contraction 1. Nerve impulse arrives at the neuromuscular junction 2. Acetylcholine diffuses across the gap at the neuromuscular junction 3. T Tubules carry acetlycholine to sarcoplasmic reticulum, which causes cell membrane to permeable to Ca2+ 4. Ca2+ binds to troponin

Steps to muscle contraction 5. Conformation shift - troponin pulls tropomyosin off active binding sites 6. One ATP per myosin Head releases and activates myosin 7. Actin and myosin filaments form linkages 8. Myosin cross-bridges pull actin filaments inward 9. Muscle fiber shortens and contracts

Isotonic and Isometric Isotonic contractions Contractions that shorten the muscle. Isometric contractions Tension in the muscle but no change in length.

Muscle Relaxing What happens when a muscle needs to relax? When a muscle relaxes, the Ach is rapidly decomposed by action of an enzyme called acetylcholinesterase. (Ach-ase) What was Ach used for in the first place? This enzyme is present at the neuromuscular junction in the membranes of the motor end plate.

Muscle Relaxing The Ach-ase stops a single nerve impulse from continuously stimulating the muscle fiber. As this ceases, the calcium pump quickly moves calcium ions back into the sarcoplasmic reticulum. What does this do? The linkages will break the troponin and tropomyosin return to the normal conformation, blocking the binding sites. The muscle fiber relaxes.

Muscle Fatigue Muscle fatigue most often occurs from an accumulation of lactic acid. What is this caused by? Anaerobic respiration An interruption of the blood supply or the lack of ACh can also cause fatigue. Muscle fatigue can cause the muscle to cramp.

Muscle Cramps Cramps occur when the muscle contracts spasmodically, but does not relax completely. The condition is due to a lack of ATP needed to move calcium or other ions, can also be caused by a lack of those ions Why would there be a lack of ATP? Anaerobic respiration does not produce much ATP. Exercise stimulates new capillaries to grow within the muscles and it also causes an increase in the number of mitochondria.

Fast and Slow Muscles There are two types of muscles relating to the speed of contraction. Fast and Slow The speed of contraction is related to the specialized function of a muscle. Example: Eye muscles that blink are ten times faster than the muscles involved in posture.

Fast and Slow Muscles Slow-contracting (slow twitch) muscles Often called red muscles Why? Most of their fibers contain red, oxygen-storing myoglobin. Well supplied with blood. Have a high respiratory capacity. They generate enough ATP to keep up with the activities. (which means they have a lot of what?)

Fast and Slow Muscles Fast-contracting (fast twitch) muscles Also called white muscles. Why? They contain less myoglobin and have a poorer blood supply than red muscles. White muscles contains fewer mitochondria which reduces the respiratory capacity.

Fast and Slow Muscles What makes white fibers able to contract quicker? White Fibers have more extensive sarcoplasmic reticulum. Why does this help? Most skeletal muscles contain both types of fibers. Research has discovered an intermediate fiber.

Assignment Due Friday Pg. 210 Review Ex. #2,8,12,15, Critical thinking #3

DIDN’T USE ANY SLIDES AFTER THIS POINT, I DID NOT USE, BUT KEPT IN CASE I WANTED TO.

Skeletal Muscle Contraction In the presence of calcium ions, the myosin cross-bridges react with actin filaments and form linkages with them. This reaction between the myosin and actin filaments provides the force that shortens myofibrils during muscle contraction.

Skeletal Muscle Contraction Actin accounts for about 1/4 of the total protein in skeletal muscle. Actin molecules, arranged together in a double twisted strand, form an actin filament. Tropomyosin & troponin are two proteins associated with actin filaments.

Skeletal Muscle Contraction The tropomyosin-troponin complex blocks the binding sites on the actin molecules when the muscle is at rest. If a high concentration of calcium ions is present, the calcium ions bind to the troponin, and this modifies the position of the tropomyosin.

Skeletal Muscle Contraction The tropomyosin molecules move, exposing the binding sites on the actin filaments, and the linkages form between the actin and myosin filaments.

Skeletal Muscle Contraction The sliding filament theory of muscle contraction suggests that the head of a myosin cross-bridge can attach to an actin binding site and bend slightly, pulling the actin filament with it. Then the head can release, straighten itself, and combine with another binding site farther down the actin filament.

Skeletal Muscle Contraction The enzyme ATPase causes the breakdown of ATP to supply energy for these actions.

Stimulus for Contraction Acetylcholine (ACh) is a neurotransmitter that is synthesized in the cytoplasm of the motor neuron and is stored in vesicles. A nerve impulse reaches the end of the axon, some of these vesicles release ACh into the gap between the nerve and the motor end plate.

Stimulus for Contraction The ACh diffuses rapidly across the gap, combines with certain protein molecules in the sarcolemma, and thus stimulates the muscle fiber membrane. This stimulus causes a muscle impulse that passes in all directions over the surface of the sarcolemma.

Stimulus for Contraction It also travels through the sarcoplasmic reticulum and the transverse tubules. The sarcoplasmic reticulum contains a high concentration of calcium ions compared to the sarcoplasm.

Stimulus for Contraction In response to a muscle impulse, the membranes of the cisternae become more permeable to these ions and the calcium ions diffuse into the sarcoplasm of the muscle fiber.

Stimulus for Contraction When a relatively high concentration of calcium ions is present in the sarcoplasm, linkages form between the actin and myosin filaments, and a muscle contracts.

Energy Sources of Contraction The energy for muscle contractions comes from ATP. The muscle has enough ATP to contract briefly. Therefore, when a fiber is active, ATP must be regenerated. Creatine phosphate supplies the energy to change ADP back to energy rich ATP

Oxygen Supply and Cellular Respiration Oxygen is transported by the red blood cells. It is loosely bound to molecules of hemoglobin. The hemoglobin releases the oxygen in areas of the body that are low in oxygen content

Oxygen Supply and Cellular Respiration Myoglobin, in the muscle cells, can store oxygen temporarily. This reduces a muscle’s need for a continuous blood supply during a contraction.

Oxygen Debt During strenuous exercise, the available oxygen supply may be used up. The body then relies on anaerobic respiration creating an oxygen debt. Anaerobic respiration builds up lactic acid. The liver converts lactic acid back to glucose, but it takes several hours to complete the conversion.

Heat Production Since muscle tissue represents a large proportion of the total body mass, it is a major source of heat. About 25% of the energy released in cellular respiration is available for use in metabolic processes.

Heat Production Active muscles release large amounts of heat. Blood transports this heat to other tissues to help maintain body temperature.

Smooth Muscle Smooth muscle characteristics: Shorter than the fibers of skeletal muscle. Single, central nucleus. Cells are elongated with tapering ends. Filaments are more randomly arranged than skeletal muscle.

Smooth Muscle The two types of smooth muscle are multiunit and visceral. Multiunit muscles are less organized and occur as separate fibers. Multiunit are found in the irises of the eyes and walls of blood vessels. They contract through motor nerve impulse or hormone action.

Smooth Muscle Visceral smooth muscle is composed of sheets of spindle shaped cells held together by gap junctions. Visceral smooth muscles are found in walls of hollow organs (ex. Stomach, intestines) There usually will be two muscle layers, a longitudinal and a circular layer.

Smooth Muscle When one fiber is stimulated, the impulse will also excite adjacent cells causing a rhythmic contraction. Peristalsis - wavelike contraction that occurs in tubular organs.

Smooth Muscle Contraction Smooth muscle contraction is similar to skeletal muscle contraction. Smooth muscle lacks troponin. Instead it uses a protein called calmodulin to bind calcium ions. Calcium diffuses into the cell from the extracellular fluid.

Smooth Muscle Contraction Smooth muscle reacts to the neurotransmitter ACh and norepinephrine. Some hormones cause smooth muscle contraction. (Ex. Childbirth) Stretching smooth muscle can cause contractions. (Ex. Digesting food in the stomach)

Smooth Muscle Contraction Smooth muscle is slower to contract and slower to relax. Smooth muscle can contract for a longer time with the same amount of ATP. Smooth muscles stretch without changing tautness while a hollow organ fills.

Cardiac Muscle Found only in the heart. Striated cells joined end to end forming interconnecting branched three-dimensional network. Each cell contains a single nucleus. Well developed sarcoplasmic reticulum and transverse tubules.

Cardiac Muscle Sarcoplasmic reticulum stores less calcium but the enlarged transverse tubules store extra calcium. The extra calcium allows cardiac muscle to maintain a contraction longer than skeletal muscles.

Cardiac Muscle Intercalated disks separate opposing ends of cardiac cells. The disks help hold adjacent cells together and transmit the force of contraction from cell to cell.

Cardiac Muscle When one portion of the cardiac network is stimulated, the impulse passes throughout the network causing the whole structure to contract as a unit.

Muscle Action The main muscle is the prime mover. Muscles that contract along with the prime mover are called synergists. Muscles opposing the prime mover are called antagonists.