muscular system chapter 6 muscles l are muscles organs? l yes, each individual muscle is an organ...

MUSCULAR SYSTEMMUSCULAR SYSTEM

CHAPTER 6

MusclesMuscles Are muscles organs? Yes, each individual muscle is an organ

– Why? Muscles are composed of more than just muscle

tissue.– Example: Skeletal muscles are composed of skeletal

muscle tissue, nervous tissue, blood, and connective tissue.

3 types:– Skeletal (We will concentrate mostly on this one.)– Smooth– Cardiac

Skeletal muscleSkeletal muscle

Cells - long, cylindrical, multinucleated, striated

Voluntary - controlled by the nervous system– When are they not voluntary?

Speed of contraction varies (fast to slow) Location?

Cardiac muscleCardiac muscle

Cells - branching, intercalated discs, striated Involuntary - controlled by its own

pacemaker and hormones Slow rhythmic contractions

Smooth muscleSmooth muscle

Cells - no striations, appear smooth and “stringy” in microscope

Involuntary Very slow contraction

– Can be rhythmic Location

Functions of Skeletal MusclesFunctions of Skeletal Muscles Produce Movement

– Muscles cross joints and work in opposite forces to move the bones

Maintain Posture– Always working against gravity

Stabilize joints– Many joints don’t fit together well, muscles help reinforce

those (ex. shoulder) Generate Heat

– Heat is a by product of muscle activity

Rules of Skeletal MusclesRules of Skeletal Muscles

1. All muscles cross at least one joint A few minor exceptions

2. Typically, the bulk of the muscle is going to be proximal to the joint crossed

3. All muscles have at least two attachment points: the origin and insertion

4. Muscles can only pull, they cannot push 5. During contraction, the insertion point moves

toward the origin.

Structure of Skeletal MusclesStructure of Skeletal Muscles Muscles have multiple layers

– Cylinders within cylinders Levels of muscles, largest to smallest: Muscle fascicle muscle fiber myofibril

filaments Each level of a muscle is enclosed in a layer of

connective tissue Fascia - connective tissue that covers and is

intertwined with the muscles. Has three purposes

– DQ – What do you think those might be?

Structure of Skeletal MusclesStructure of Skeletal Muscles

1. Separate Muscle levels from one another Endomysium – separates each muscle fiber Perimysium – separates each fascicles

2. Separate muscles from one another Epimysium – holds the group of fascicles together

3. Hold muscles in place– 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.

Skeletal Muscle FibersSkeletal Muscle Fibers

DQ - What is the purpose of all the levels? Help with contraction, all of them can work

together– Muscle contracts at the smallest level, many small

contractions add up to one big one Muscle fiber – individual muscle cell that

contracts in response to stimulation. These have different names for the cell

membrane and cytoplasm– Cell membrane = sarcolemma– Cytoplasm = sarcoplasm


Each muscle fiber contains numerous threadlike myofibrils that lie parallel to one another.

Myofibrils contain two kinds of protein filaments.– Myosin – “Thick” filaments

Cross bridges for contraction

– Actin - “Thin” filaments Active sites for contraction

These filaments overlap, what type of appearance does this create in skeletal muscle?– Striated


Striation pattern parts: I band - (light bands) where only actin

filaments are. Z – line – where actin filaments meet A bands - (dark bands) where both myosin

and actin are overlapping H zone – where only myosin is From Z line to Z line is one sarcomere.

Structure of Skeletal MuscleStructure of Skeletal Muscle Within the muscle fiber there are networks of

tubes:– Sarcoplasmic reticulum - run parallel to each

myofibril

– Transverse tubules – perpendicular to SR and run in between myofibrils (a.k.a. T tubules)

DQ - What would these structures be used for?

Activate entire fiber when contraction occurs

Muscle activationMuscle activation DQ - How are muscles activated to

contract? The neuromuscular junction (pg 185) Each skeletal muscle fiber is connected to an

extension of a motor neuron. The nerve fiber and the muscle meet at the

neuromuscular junction.– The two do not actually touch – Synaptic cleft

– Then how does the nerve stimulate the fiber? Neurotransmitters (Acetylcholine)

Motor UnitsMotor Units

DQ - Does each nerve only control one muscle fiber?

NO, we have motor units. (pg 184)– consists of a motor neuron and all of the muscle

fibers it controls. The number of muscle fibers in a motor unit

varies considerably. DQ - Why would that be? The fewer muscle fibers in the motor units, the

finer the movement.– Examples – hands vs legs

Muscle ResponseMuscle Response When the nerve receives an impulse, the neuron

releases the neurotransmitter into the synaptic cleft. DQ - What will this cause? All muscle fibers in the motor unit contract

– All or none response DQ - Does this mean entire muscle

contracts? NO, just the motor unit

– DQ - How do we control force of muscles? More motor units = More force

Skeletal Muscle ContractionSkeletal 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

- Shortening ofa sarcomere


All of this binding and changing of shape will take energy, where will it come from?

ATP DQ – How is ATP made? (3 ways) (pg 190) Direct Phosphorylation

– Fastest, Lasts ~15 sec No O2 needed

Anaerobic glycolysis– Lasts ~ 30 – 60 sec Causes lactic acid, No O2

Aerobic respiration– Slowest, Lasts hours, Requires O2


Two types of contractions Isotonic contractions

– Contractions that shorten the muscle.– This is the more common one

Isometric contractions – Tension in the muscle but no change in

length.

Steps to muscle contractionSteps 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 be permeable to Ca2+

4. Ca2+ binds to troponin

Steps to muscle contractionSteps 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 linkages8. Myosin cross-bridges pull actin filaments

inward9. Muscle fiber shortens and contracts

Muscle RelaxingMuscle Relaxing DQ - What happens when a muscle

does not need to contract anymore? – Think about how it all started

The acetylcholine needs to be removed the Ach is rapidly decomposed by action of an

enzyme called acetylcholinesterase. (Ach-ase)– This enzyme is present at the neuromuscular junction

The Ach-ase stops a single nerve impulse from continuously stimulating the muscle fiber.

Muscle RelaxingMuscle Relaxing As Ach is broken down, the calcium pump

quickly moves Ca2+ back into the sarcoplasmic reticulum.

DQ - 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 FatigueMuscle Fatigue Muscle fatigue most often occurs because of oxygen

debt.– Causing?

Anaerobic respiration – accumulation of lactic acid. Oxygen debt is “paid back” after activity

– Heavy breathing DQ - How are people able to “train” to

prolong activity before muscle fatigue? Exercise stimulates new capillaries to grow within the

muscles and it also causes an increase in the number of mitochondria

Rigor MortisRigor Mortis What is it? Stiffening of joints and locking into place after

death– Why does it happen?

As the cells begin to shut down, the sarcoplasmic reticulum becomes permeable to Ca2+

– Causing what? Sets in place within a few hours of death

– Lasts ~72 hrs

Fast and Slow MusclesFast and Slow Muscles

There are two types of muscle fibers 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 contract

ten times faster than the muscles involved in posture.

Fast and Slow MusclesFast and Slow Muscles Slow-contracting (slow twitch) fibers Often called red muscles

– Why?

The fibers contain red, oxygen-storing myoglobin.

Well supplied with blood. What would these be useful for?


Fast-contracting (fast twitch) muscle fibers

Also called white muscles. – Why?

They contain less myoglobin and have a poorer blood supply than red muscles.

What would these be used for? Shown to have a very well developed

sarcoplasmic reticulum?– Why is that important?


Research has discovered an intermediate fiber.

How would you classify these?

Fast and Slow twitchFast and Slow twitch

How would you describe the ratio of fast twitch to slow twitch for Usain Bolt? Ryan Hall?

Muscle HypertrophyMuscle Hypertrophy

Hypertrophy – increasing in size Occurs when more proteins are produced, not

more muscle fibers/muscle cells. Occurs from working muscles hard or isometric

exercises When worked hard the muscle becomes

damaged, which causes an increased number of nuclei.– DQ - Why would increased nuclei cause increase

in muscle mass (think transcription and translation)?

Muscle AtrophyMuscle Atrophy

Atrophy – decrease in size Caused by lack of use

– Proteins broken down

Muscle interactionsMuscle interactions

Prime movers – responsible for major movements Antagonists – muscles that oppose or reverse a

movement (examples?) Synergists - help prime movers by helping with

movement or stabilizing unwanted movement. (Examples?)

Fixators – specialized synergists, stabilize bones (Ex. rhomboideus major)

DIDN’T USEDIDN’T USE

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


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.


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.


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.


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


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.


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

Stimulus for ContractionStimulus 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.


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.


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.


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.


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 Energy Sources of ContractionContraction

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 Oxygen Supply and Cellular RespirationRespiration

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 Oxygen Supply and Cellular RespirationRespiration

Myoglobin, in the muscle cells, can store oxygen temporarily.

This reduces a muscle’s need for a continuous blood supply during a contraction.

Oxygen DebtOxygen 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.

Muscle CrampsMuscle 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

How would a lack of ATP be a problem? ATP required to release cross bridges Exercise stimulates new capillaries to grow

within the muscles and it also causes an increase in the number of mitochondria.

Heat ProductionHeat 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 ProductionHeat Production

Active muscles release large amounts of heat.

Blood transports this heat to other tissues to help maintain body temperature.

Smooth MuscleSmooth 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.


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.


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.


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 ContractionSmooth 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 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 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 MuscleCardiac 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.


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.


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.


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 ActionMuscle 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.

muscular system chapter 6 muscles l are muscles organs? l yes, each individual muscle is an organ...

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