MUSCLE Mania!!

Sarah BartleyLauren ThamesAnnie Lee

Contracting Muscles (49.6 in text)

• All animal movement is based on one of two basic contractile systems, both of which consume energy in moving protein strands against one another– Microtubules: beating of cilia and undulations of flagella– Microfilaments: amoeboid movement, contractile elements of

muscle cells

• Muscles always contract (extend only passively)– The ability to move body parts in opposite directions requires the

muscles to be attached to the skeleton in antagonistic pairs– Each member of the pair is working against the other – Example: Flexing and extending your arm

• To understand how a muscle contracts, we need to understand its structure….

Vertebrate Skeletal Muscle• Skeletal muscle is attached to the bones and

responsible for their movement

– Consists of a bundle of long fibers running parallel to the length of the muscle

– Each fiber is a single cell with multiple nuclei (formed by the fusion of many embryonic cells)

– A fiber itself is a bundle of myofibrils arranged longitudinally

• These are composed of myofilaments:

– Thin filaments- two stands of actin, one strand of regulatory protein coiled around one another

– Thick filaments- staggered arrays of myosin molecules

Muscle

Bundle ofmuscle fibers

Single muscle fiber(cell)

Plasma membrane

Myofibril

Lightband Dark band

Z line

Sarcomere

TEM 0.5 mI band A band I band

M line

Thickfilaments(myosin)

Thinfilaments(actin)

H zoneSarcomere

Z lineZ line

Nuclei

Contd…

• Skeletal muscle is also called striated muscle because the regular arrangement of the myofilaments creates a pattern of light and dark bands– Each repeating unit is a sarcomere, the basic contractile unit of

the muscle– Z lines border the sarcomere– Area near the edge of the sarcomere where there are only thin

filaments is called the I band– A Band- the broad region that corresponds to the length of the

thick filaments• H Zone is the center

– This arrangement of thick and thin filaments is the key to how the sarcomere, and hence the whole muscle, contracts

Sliding-Filament Modelof Muscle Contraction

• According to the model, neither the thin filaments nor the thick filaments change in length when the sarcomere shortens

• The filaments slide past each other longitudinally, producing more overlap between filaments

• As a result, both the region occupied by thin filaments (the I Band) and the region occupied by thick filaments (the H zone) will shrink.

• The sliding is based on the interaction between the actin and myosin molecules that make up the filaments– Myosin molecules have a tail region that adheres to the tails of other

myosin molecules that form the thick filaments

– The head it the center of bioenergetic reactions that power muscle contractions

• It can bind ATP into ADP and inorganic phosphate• See model on next slide

Myosin-actin interactionThick filament

Thin filaments

Thin filament

ATP

ATP

ADPADP

ADP

P i P i

P i

Cross-bridge

Myosin head (low-energy configuration)

Myosin head (high-energy configuration)

+

Myosin head (low-energy configuration)

Thin filament moves toward center of sarcomere.

Thick filament

Actin

Cross-bridge binding site

1 Starting here, the myosin head is bound to ATP and is in its low-energy confinguration.

2 The myosin head hydrolyzes ATP to ADP and inorganic phosphate ( I ) and is in its high-energy configuration.

P

1 The myosin head binds toactin, forming a cross-bridge.

3

4 Releasing ADP and ( i), myosinrelaxes to its low-energy configuration, sliding the thin filament.

P

5 Binding of a new mole-cule of ATP releases the myosin head from actin,and a new cycle begins.

Role of Calcium and Regulatory Proteins

• A skeletal muscle fiber contracts only when stimulated by a motor neuron

• When a muscle is at rest, the mysosin binding sites on the thin filament are blocked by the regulatory protein tropomyosin

ActinTropomyosin Ca2+-binding sites

Troponin complex

(a) Myosin-binding sites blocked

• For a muscle fiber to contract, those binding sites must be uncovered

• This can happen when calcium ions bind to another set of regulatory proteins (troponin complex) which controls the position of the tropomyosin on the thin filamen Ca2+

Myosin-binding site

(b) Myosin-binding sites exposed

The stimulus leading to the contraction of a skeletal muscle fiber is an action potential in a motor neuron that makes a synapse with the muscle fiber. TheSynaptic terminal of a motor neuron releases acetylcholine, which depolarizes the plasma membrane of the muscle fiber. The depolarization causes action potentials to sweep across the fiber and trigger the release of calcium from the plasmic reticulum into the cytosol. Calcium is what initiates the sliding of filaments through the binding of myosin to actin.

Neural Control of Muscle Tension

• When action potential in a motor neuron releases acetyl-choline on a skeletal muscle fiber, the muscle fiber respons by producing a brief contraction called a twitch

• Contraction of a whole muscle, however, is graded• 2 basic mechanisms:

– Varying the number of muscle fibers that contract– Varying the rate at which muscle fibers are stimulated

Motor units- Each muscle fiber has a single synapse with one motor neuron, but each motor neuron typically synapses with several or many muscle fibers. A motor neuron and all the muscles fibers it make up a motor unit.

Types of Muscle Fibers

• All skeletal muscle fibers contract when stimulated by an action potential in a motor neuron, but the speed at which they contract differs among muscle fibers

• Mainly due to the rate at which the myosin heads hydrolyze ATP

• Based on speed of contraction, we can classify muscles as fast or slow– Fast- brief, rapid, powerful contractions– Slow- long contractions (less sarcoplasmic reticulum and slower

calcium pumps..more calcium in cytosol longer)

• Can also be classified by the major metabolic pathway they use for producing ATP

• Oxidative fibers rely on aerobic respiration. They are specialized to make use of a steady supply of energy.– Myoglobin is an oxygen- storing protein that is the brownish pigment in

the dark meat of poultry and fish that binds oxygen more tightly than does hemoglobin, so it an effectively extract oxygen from the blood

– Can either be fast or slow

• Glycolitic fibers rely on glycolysis, all fast• Therefore, considering both contraction speed and ATP synthesis,

we can identify three main types of skeletal muscle fibers:– Slow oxidative– Fast oxidative– Fast glycolitic

Most human skeletal muscles contain all three fiber types, but the muscles of the eye and hand lack slow oxidative fibers. If a muscle is used repeatedly for activities requiring high endurance, some fast glycolitic fibers can develop into fast oxidative fibers. Since fast oxidative fibers fatigue more slowly than fast glycoli- tic fibers, the muscle as a whole will become more resistant to fatigue.

Other Types of Muscle• The vertebrate cardiac muscle is found in the heart. While skeletal muscle

fibers will not produce action potentials unless stimulated by a motor neuron, cardiac muscle cells have ion channels in their plasma membrane that cause rhythmic depolarizations, triggering action potentials without input from the nervous system.– Action potential is up to 20 times longer than skeletal muscle fibers– Play a key role in controlling the duration of contraction– Plasma membranes of adjacent cardiac muscle cells interlock at

specialized regions called intercalated disks, where gap junctions provide a direct electrical coupling, generating an action potential in one part of the heart that will spread to all other cardiac muscle cells, and the whole heart will contract.

Contd.

• Smooth muscle is found in the walls of hollow organs like blood vessels. Instead of regularly arrayed filaments, the thick filaments are scattered throughout the cytoplasm, and thin filaments are attached to structures called dense bodies, some tethered to the p. membrane.

• Contract relatively slowly but over a much greater range of length than striated muscle. Some only contract when stimulated by neurons, but others can generate action potentials without neural input and are electrically coupled to one another.