1. MUSCLE Mania!!
Sarah Bartley
Lauren Thames
Annie Lee

2. 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….

3. Vertebrate Skeletal Muscle
Bundle of muscle fibers
Single muscle fiber
(cell)
Plasma membrane
Myofibril
Light band
Dark band
Z line
Sarcomere
TEM
0.5 m
I band
A band
M line
Thick filaments (myosin)
Thin filaments (actin)
H zone
Nuclei
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

4. 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

5. Sliding-Filament Model of 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

6. Myosin-actin interaction
Thick filament
Thin filaments
Thin filament
ATP
ADP
P i
Cross-bridge
Myosin head (low- energy configuration)
Myosin head (high- 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.
5 Binding of a new mole-
cule of ATP releases the
myosin head from actin,
and a new cycle begins. 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 to actin, forming a cross- bridge. 3 4
Releasing ADP and ( i), myosin relaxes to its low-energy configuration, sliding the thin filament. P

7. 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
Actin
Tropomyosin
Ca2+-binding sites
Troponin complex
(a) Myosin-binding sites blocked

8. (b) Myosin-binding sites exposed
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. The
Synaptic 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.

9. 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.

10. 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)

11. Glycolitic fibers rely on glycolysis, all fast
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.

12. 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.

13. 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.