Muscles and Movement – Part 2
Skeletal Muscle Fibers
Assessment Statement
11.2.5
11.2.6
11.2.7
11.2.8
Describe the structure of striated muscle fibres, including the myfibrils
with light and dark bands, mitochondria, the sarcoplasmic reticulum,
nuclei and the sarcolemma
Draw and label a diagram to show the structure of a sarcomere, including
Z lines, actin filaments, myosin filaments with heads, and the resultant
light and dark bands
Explain how skeletal muscle contracts, including the release of calcium
ions from the sarcoplamic reticulum, the formation of cross bridges, the
sliding of actin and myosin filaments and the use of ATP to break cross
bridges and reset the myosin heads
Analyse electron micrographs to find the state of contraction of muscle
fibers
A skeletal muscle is made up of bundles of thousands of muscle fibers, or muscle cells.
The muscle fibers lie parallel to one another and range from 10 to 100 um in diameter.
The typical length is 100 um, but some can be as long as 30 cm.
Each muscle fiber is surrounded by a sarcolemma (sarco – flesh, lemma – sheath). This
is the muscle fiber’s cell membrane.
The muscle fibers are developed from the fusion of smaller cells during development, and
therefore, have many nuclei and mitochondria.
Each muscle fibers contains many myofibrils. The cytoplasm, called sarcoplasm,
contains mitochondria packed between the myofibrils. In between the myofibrils is a
transverse tubular endoplasmic reticulum, called the sarcoplasmic reticulum.
See diagram 1 and 2 on handout.
Diagram of a Skeletal Muscle Fiber
Structure and Contraction of Striated Muscle
In the muscle cell, there are thin myofibrils. The fibers cause the typical striated pattern
of skeletal muscles.
In the myofibrils, there are two protein myofilaments called myosin and actin.
The myosin is the thick filament, that appears darker.
The actin is the thin filament, that appears light.
The stripes are caused by the alternating light and dark bands. (Diagram of the electron
micrograph)
The actin filaments are about 7 nm in diameter and are held together by transverse
bands called Z line. The actin partially overlaps the myosin filaments, which gives
the section a dark appearance. This is called the A band. The area where only myosin
is seen is called the H band. This is between the two Z lines. The entire unit between
the two Z lines is called the sarcomere.
As mentioned before, across the fibers there are transverse tubules, or T – tubules. The
tubules touch the sarcolemma and associated with vesicles which are part of the
sarcoplasmic reticulum. A T-tubule with a pair of vesicles is called a triad.
The vesicles are important. They regulate the movement of calcium ions to and from the
sarcoplasm. The Ca+2 concentration determines the activity of ATPase (which
hydrolyses ATP, releasing energy). This determines the activity of the muscle.
Diagram of a Sarcomere
Contraction of Striated Muscle
When muscles contract, it was discovered that the actin and myosin filaments slide past
one another. The A band, myosin, is the same length in contracted and relaxed
muscles. This lead to the sliding filament theory.
The contraction of muscles occurs in a series of steps, sometimes described as a ratchet
mechanism. Lots of ATP is used in the process.
Along the actin filament are binding sites for the heads of the myosin filament. Actin
filaments contain actin as well as tropomyosin and troponin. Tropomyosin forms two
strands which wind around the actin filament, covering the binding site. The
tropomyosin is held in place by the troponin. This is how the actin filament remains at
rest.
The thick filaments are composed of myosin molecules, each with a bulbous head. The
head protrudes from the length of the myosin filament. The head, when the muscle cell
contracts, binds to the site on the actin filament. This is what causes movement of the
filaments, and eventually the movement of the whole muscle.
The contraction of the sarcomere is best described in four steps:
1.
The myofibril is stimulated to contract by the arrival of an action potential. This
triggers the release of calcium ions from the sarcoplasmic reticulum, to surround
the actin molecules. The calcium ions react with the troponin. When the
troponin is activated, it triggers the removal of the blocking molecule,
tropomyosin. The binding sites are now exposed.
2.
Each bulbous head has an ADP and Phosphate group attached to it (called a
charged bulbous head). It reacts with a binding site on the actin molecule beside
it. The phosphate group detaches.
3.
The ADP molecule is then released from the head, and this is a trigger for the
rowing movement of the head. The head tilts 45o and pushes the actin filament
along. This step is called the power stroke and the myofibril contracts.
4.
A molecule of ATP binds to the head. The protein, ATPase, catalyses the
hydrolysis of ATP. The result is ADP and Phosphate is attached to the bulbous
head, and it is said to be charged again. The charged head detaches from the
binding site and straightens.
The cycle of movement repeats itself many times per second, with thousands of heads
working along each myofibril. ATP is used up and the muscle may shorten by about
50% of its relaxed length.
When no more impulses arrive, calcium ions are moved back into the vesicles of the
sarcoplasmic reticulum by active transport. The binding sites are covered by the
tropomysin and the muscle relaxes.
(SEE ATTACHED DIAGRAMS)
Controlled Movements and Posture
Muscles involved in maintaining body posture, like the muscles of the back, use subtle,
delicate movements. They can also move in vigorous actions as well.
Nervous control of muscle contraction may cause the muscle to contract moderately, or
fully, depending on the movement required. The overall length of the sarcomere is
changed according to the movement required.
Extension – Neuro / Muscular Diseases
Multiple Sclerosis
An autoimmune disease, which the myelin sheath is attacked and destroyed. The scars
(scleroses) are what is left. As a result, the nerves do not conduct and connect to the
skeletal muscles properly, causing jerky movements or no movement at all.
Muscular Dystrophy
A degeneration of muscle fibers. As a result, the muscle atrophies and weakens. In one
type of MD, Duchenne MD, there is a protein called dystrophin not present in the
sarcolemma of the cell. This may cause calcium to leak into the sarcoplasm, that may
activate and enzyme that causes the muscle fibers to degenerate.
Assignment 3 – What is the clinical definition of Rigor Mortis?