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Excitable cells - Lecture 15 (20/03/2018)
Classification of Muscle
Many, but not all skeletal muscle is attached to bone via tendons.
Skeletal muscle has two main functions: movement and the generation of heat.
You may not have thought about this, but shivering actually does warm you up - it is simply a series of muscle contractions and relaxations and muscle contraction is an exothermic (heat generating) process.
Skeletal Muscle Structure
Skeletal muscle has a distinct structural organisation.
Individual muscle cells are termed myocytes (also known as muscle fibres). These are covered with a layer of connective tissue called the endomysium.
Each muscle fibre is multinucleated and is formed by the fusion of cells during development.
Muscle fibres are grouped into bundles termed fascicles
(related to the word fascist - it means bundle of sticks) which in turn are covered with another layer of connective tissue called the perimysium.
The fascicles are then grouped together by a sheath called the epimysium to form the muscle.
Skeletal Muscle Fibre
If you look at the structure of an individual muscle fibre, you will see it is highly organised.
These contains bundles of protein filaments known as myofibrils.
Myofibrils are in turn composed of bundles of protein filaments called myofilaments.
There are "tunnels" called T tubules leading off from the sarcolemma (cell membrane). These tunnels lead into the interior of the muscle fibre and mean that the membrane has a very high surface area.
The second thing to notice is that the endoplasmic reticulum (called the sarcoplasmic reticulum in muscle cells) wraps around the myofibrils.
Specialised parts of the sarcoplasmic reticulum, called the terminal cisternae, interact with the T-tubules to form a structure called a triad.
This structure is key to coupling excitation of the muscle membrane to contraction.
When we look at myofibrils in more detail, it becomes clear that they are a long string of repeating structures, termed Sarcomeres.
It is the sarcomeres that give striated muscle its name because they have a characteristic series of bands and lines that give a striped appearance (striated = striped). The bands and lines arise due to the arrangement of proteins in the sarcomere.
H-Zone: Thick filaments not overlapping thin.
A-Band: The length of an entire thick filament.
I-Band: where the thin filaments do not overlap thick.
Looking at the sarcomere in more detail it reveals that there are two major structural features: thick filaments (composed of myosin and titin) and thin filaments composed of Actin and
Both filaments proteins are connected to the Z disk (alpha actinin) and the thick filaments are also connected to the proteins of the M-line in the centre of the sarcomere.
Titin and Nebulin have important structural roles in the sarcomere.
Note that the pattern of stripes seen in skeletal muscle depends on how much the muscle is contracted. This is because the mechanism of contraction involves the filaments sliding over one-another and thus the degree of overlap changes.
The sliding filament model also has some important consequences for the relationship between the amount a muscle is stretched (eg: its length) and the force it can produce.
There is obviously going to be a point at which the overlap between the actin and myosin is optimal: this is when the maximum force is produced.
If the muscle is stretched past this point, the force decreases.
Similarly, if it is compressed, the overlap is again less than optimal and less force is produced.
Myosin is a highly diverse family of motor proteins.
Skeletal muscle myosin is myosin type II and there are different kinds of myosin II in cardiac and smooth muscle.
The myosin head has ATPase activity.
Its motor is powered by hydrolysis of ATP.
The thick filaments are composed of myosin and the structural protein titin.
Each myosin is a hexamer of two heavy chains whose tails are coiled round one another to form an alpha helix, and four light chains.
The heavy chain heads, which have ATPase activity, form interactions with the actin in the thin filaments.
The conformation of myosin can be changed by binding and hydrolysis of ATP, this feature is what allows myosin to act as a motor.
The light chains are bound to the neck region of the myosin. The regulatory light chains modulate the activity of the head group.
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