PHGY 215 Final
The Sliding Filament Mechanism - ✔️✔️- A sarcomere goes from z-line to z-line and
has thick filaments (myosin) in the middle within the thin filaments (actin) on the sides
- During contraction, the thin filaments move inwards over the thick filaments
- When this happens, the z-lines move closer together
- This occurs simultaneously along the entire fibre and all sarcomeres shorten to the
same degree
- Neither the length of the thin filaments or thick filaments themselves changes, just the
degree of overlap
- As a result, the whole muscle shortens in what is called a concentric contraction
Myofibrils - ✔️✔️- Along the length of a muscle fibre, the cell is divided into discrete
contractile elements called myofibrils
- A myofibril displays a pattern of light and dark bands that give the muscle fibre a
striated pattern
- There is a highly organized cytoskeletal pattern of thick and thin filaments, which are
myosin and actin, respectively
- Myofibrils are the contractile elements in muscle fibres
- They display a pattern of light and dark bands, which we call I bands and A bands,
respectively
Striated - ✔️✔️Marked with a linear ridge or groove, often as one of a number of similar
parallel features
Muscle - ✔️✔️A whole skeletal muscle is made up of individual muscle fibres, each of
which runs the entire length of the muscle
Muscle Fibre - ✔️✔️- The muscle fibres run parallel to each other and are surrounded
by connective tissues
- A muscle fibre is actually a single muscle cell
- These cells are multinucleated and have a very large number of mitochondria
A Band - ✔️✔️- The A bands, also called the dark bands, are made up of stacked thick
and thin filaments that are aligned parallel to each other and its borders are defined by
the length of the thick filaments
- The middle of the A band is slightly lighter as the thin filament do not reach this far
from the ends
- This lighter portion is also called the H zone
,I Band - ✔️✔️- The I bands, also called the light bands, are made up of the portion of
the thin filaments that do not extend into the A band
- In the middle of the I band is a vertical line called the Z line
H Zone - ✔️✔️- The slightly lighter portion of the A band is called the H zone
- It only contains proteins that hold the thick filaments, myosin, together in a stack
- Myosin is composed of 2 heavy chains and 2 light chains
- The H zone only contains the heavy chains
- These proteins are seen as the M line running down the centre of the H zone
M Line - ✔️✔️- The proteins that hold the thick filaments together in a stack are seen
as the M line
- The M line runs down the centre of the H zone
Z Line - ✔️✔️- In the middle of the I band is a vertical line called the Z line
- The distance from one Z line to the next Z line is called a sarcomere and is what we
consider to be the functional unit of skeletal muscle
Sarcomere - ✔️✔️- When relaxed, a sarcomere is about 2.5 x 10^-6 m in width
- When muscles are growing, they extend the length of the muscle fibre by adding new
sarcomeres onto the ends
- The area in the A band where the thick and thin filaments overlap contains cross-
bridges, that extend from the thick filaments and from when thin filaments bind
Cross-bridges - ✔️✔️Connection formed when mobile myosin heads bind to actin
molecules in muscles
Skeletal Muscle - ✔️✔️- One of three major muscle types, the others being cardiac and
smooth
- Skeletal muscle is striated and under voluntary control
- Made up of several levels of structural organization: muscle, muscle fibre, myofibril,
sarcomere, thick and thin filaments
Thick Filament - ✔️✔️- The thick filament is composed of the protein myosin
- Myosin is a motor protein, in that it uses ATP to mov along actin filaments
- Each molecule of myosin is a dimer of two subunits
- Each subunit looks like a golf club, in that it has a long shaft with a globular head
- When the dimers come together, the "shaft" or tail portions wrap around each other
- Within a thick filament, two dimers come together in a tail-to-tail formation and then
these stack up with other myosin molecules
- The heads stick out and contain two important sites, an actin binding site, and a
myosin ATPase site
Thin Filament - ✔️✔️- The thin filament is made up of the proteins actin, tropomyosin,
and troponin
,- The main structural component is two actin filaments
Actin Filaments - ✔️✔️Made up of individual spheric actin molecules that come
together to form a double helix structure
Tropomyosin - ✔️✔️- A thin, double helix protein that lies end to end along the actin
helix structure
- It is a regulatory protein that covers the active binding sites, preventing the interaction
of actin and myosin
Troponin - ✔️✔️- A regulatory protein complex made of three polypeptides
- One binds to tropomyosin, one binds to actin, and one binds to calcium
Muscular Contraction - ✔️✔️- Cross-bridges form between the myosin head and actin
- This cross-bridge formation forms the basis of the sliding filament mechanism which
brings about muscular contraction
Contraction - ✔️✔️- The activation of tension-generating sites within a muscle fibre
- When a muscle contracts, it shortens
The Power Stroke - ✔️✔️- The power stroke refers to the interaction between myosin
and actin that leads to a shortening of the sarcomere
- The power stroke occurs when the cross-bridge bends, pulling the thin myofilament
inwards towards the centre of the thick filament
1. Binding: Myosin cross-bridges bind to actin molecule
2. Power stroke: the myosin head bends, pulling thin myofilament inward
3. Detachment: cross-bridge detaches at the end of power stroke and returns to original
conformation
4. Binding: cross-bridge binds to more distal actin molecule; cycle repeats
Actin and Myosin in the Power Stroke - ✔️✔️- The result of a power stroke is the actin
molecules being pulled closer to the centre of myosin molecules
- On each successive cross-bridge cycle, the actin is pulled even more
- Each myosin molecule is surrounded by six actin molecules on each end, all of which
are pulled inward simultaneously in muscle contraction
At any given time, not all cross-bridges are actively pulling actin; some are just holding
the actin in position while others prepare for the next power stroke
- As well, each myosin has two heads, which act independently in that only one of them
may be attached to actin at any given time
Excitation-Contraction Coupling - ✔️✔️- The energy that is required for the power
stroke comes from excitation-contraction coupling
- Excitation-contraction coupling refers to the process of converting an electrical signal
into an actual contraction
, - Acetylcholine (Ach) is released into the neuromuscular junction where it causes
permeability changes and initiates an action potential that is conducted across the entire
muscle membrane
- Skeletal muscles have two membrane structures that help to transmit this signal to the
muscle fibres: the sarcoplasmic reticulum, T-tubules
Sarcoplasmic Reticulum (SR) - ✔️✔️- The sarcoplasmic reticulum is also a
membranous structure an is called the endoplasmic reticulum in non-muscle cells
- In general, the SR runs parallel to the fibres
- At its end, the lateral sacs of the SR are in close proximity to the T-tubules
- The SR is a storage site for calcium
T-tubules - ✔️✔️- Transverse tubules (T-tubules) are invaginations of the the plasma
membrane
- At the junction of the A and I bands, T-tubules dip in to the fibre and run perpendicular
to the fibres
Relationship Between T-Tubules and SR - ✔️✔️- When the plasma membrane
depolarizes, this wave of depolarization spreads no only across the plasma membrane
but also goes deeper into the cells by spreading down the T-tubules
- Because they are close in proximity, this electrical signal is also transmitted from the
T-tubule to the SR
1. The SR runs longitudinally with segments that expand to form the lateral sacs lying
adjacent to the T-tubules. The T-tubules dip deep into the muscle fibre at junctions
between A and I bands of myofibrils
2. On the surface of the T-tubules are dihydropyridine receptors which are essentially
voltage-sensors, which sense the wave of depolarization as it makes its way down the
T-tubules
3. Immediately opposite to these to these on the SR are ryanodine receptors
4. When the wave of excitation enters the T-tubules, it is sensed by the dihydropyridine
receptors which influence the ryanodine receptors of the SR to undergo a
conformational change. The ryanodine receptors are a form of calcium channel and
when they are activated, they open and calcium enters the cytoplasm
Calcium Release - ✔️✔️- Membrane depolarization in T-tubules results in the release
of calcium from the SR
- The release of calcium is important as calcium is the primary trigger to allow skeletal
muscles to contract
- In a relaxed muscle, contraction cannot take place because tropomyosin and troponin
are positioned in such a way as to prevent cross-bridge formation by blocking the
myosin binding site on the actin molecules
- Calcium is released from the SR and leads to the exposure of the actin binding sites
- The exposure of the actin binding sites allows for ATP-powered cross-bridge cycling
Relaxation - ✔️✔️- The cause of muscle relaxation is decreased nerve activity at the
neuromuscular junction
