Skeletal muscles and muscle contraction

In a nutshell

Muscle contraction requires the coordination of many different effectors and mechanisms. The initiation of muscle contraction begins with very small changes which eventually cause the movement of entire limbs.

Skeletal muscles

Definition

Skeletal muscles are the muscles that enable movement. They can also be called striated, striped and voluntary muscles. Tendons are used to attach skeletal muscles to bones.

Muscles work in pairs

Muscles must work in pairs because they can only contract in one direction. Muscles can create a pulling force when contracting.

Example

Two muscles which work in a pair are the biceps and triceps. As they work in opposite directions muscle pairs are termed antagonistic pairs.

To generate movement in the lower arm tendons connect the biceps and triceps muscles to the bones in the lower arm.

Biology; Nervous coordination; KS5 Year 12; Skeletal muscles and muscle contraction

1.	*Bending arm*	To bend the arm the triceps muscle must relax and the biceps muscle must relax.
2.	*Straightening arm*	To straighten the arm the biceps muscle must relax and the triceps muscle must contract.

Note. The muscle that is contracting in the antagonistic pair is known as the agonist, whereas the relaxing muscle is known as the the antagonist.

Muscle fibres

Muscle fibres have defining features which you need to be aware of.

Biology; Nervous coordination; KS5 Year 12; Skeletal muscles and muscle contraction — 1. Muscle, 2. Muscle fibre, 3. Sarcolemma, 4. Transverse (T) tubule, 5. Myofibril

Features of muscle fibres	Description
Sarcolemma	The cell membrane of muscle fibre cells is termed the sarcolemma and it has many mitochondria as it requires a lot of ATP. The sarcolemma is described as multinucleate which means there are lots of nuclei in each muscle fibre cell.
Transverse (T) tubules	Folds in the sarcolemma along the length of the muscle fibre enable action potentials to disperse across all of the muscle fibre. The folds are termed T tubules and protrude into the sarcoplasm. The sarcoplasm is the name given to the muscle fibres cytoplasm.
Sarcoplasmic reticulum	Many membranes inside the muscle fibre make up a structure called the sarcoplasmic reticulum which contains lots of calcium ions. It controls the release of $Ca^{2+}$ which are necessary for stimulating muscle contraction.
Myofibrils	Muscle fibres have organelles called myofibrils which have a cylindrical shape. Myofibrils are made up of proteins which are responsible for contraction.

Sarcomere

Sarcomeres are short repeating sections which altogether make up a myofibril. In each sarcomere segment there are thick and thin myofilaments which slide in opposite directions to trigger contraction. The myofilaments contain proteins called myosin (thick myofilaments) and actin (thin myofilaments).

Biology; Nervous coordination; KS5 Year 12; Skeletal muscles and muscle contraction — 1. Sarcomere, 2. I-band, 3. A-band, 4. H-zone, 5. Z-line, 6. M-line, 7. Myosin, 8. Actin, 9. Cross section through the myofibril.

z-line	At either side of each sarcomere unit is a Z-line.
m-line	The M-line marks the middle of the sarcomere unit and also marks the middle of the myosin filaments.
H-zone	The H-zone represents the area of the sarcomere with only myosin filaments.
I-Band	The I-band represents the area of the sarcomere with only actin filaments.
A-band	The A-band represents the length of the myosin filaments.

Sliding filament

To trigger contraction myosin and actin filaments slide over one another reducing the length of the overall sarcomere. The I-band and the H-zones also get smaller. When all of the sarcomeres contract it leads to contraction of myofibrils and muscle fibres.

Biology; Nervous coordination; KS5 Year 12; Skeletal muscles and muscle contraction

1.	Relaxed sarcomeres
2.	Contracted sarcomeres
A.	A-bands stay the same length
B.	I-band gets shorter
C.	H-zones get shorter
D.	The sarcomeres get shorter

Contraction

Myosin and actin filaments

To enable contraction, the sarcomeres have machinery to generate sliding. Globular heads on myosin filaments are hinged so they can move in both directions. Globular heads also have an ATP binding site. Actin-myosin binding sites on actin filaments allow binding of globular heads to actin filaments.

Tropomyosin

Tropomyosin is a protein located on actin filaments. Tropomyosin enables better movement of myofilaments whilst also blocking actin-myosin binding sites at rest to prevent unwanted contraction.

Biology; Nervous coordination; KS5 Year 12; Skeletal muscles and muscle contraction — 1. Actin filament, 2. Tropomyosin, 3. Myosin filament, 4. Myosin globular head

calcium ion influx	An action potential travels down a motor neurone and reaches a muscle cell. The sarcolemma is depolarised and the action potential travels through the sarcolemma and the T tubules. The sarcoplasmic reticulum is now depolarised triggering the release of calcium ions into the sarcoplasm.
Tropomyosin	$Ca^{2+}$ interact with tropomyosin altering its shape removing it from blocking the actin-myosin binding sites. Now the globular heads and the actin-myosin binding site can interact with each other forming the actin-myosin cross bridge.
globular head	ATP hydrolase is an enzyme which facilitates the break down of ATP into ADP and an inorganic phosphate. This reaction supplies energy for the globular heads to swing which pulls the actin filament along.
atp hydrolase	ATP hydrolase catalyses another ATP hydrolysis reaction providing energy to break the actin-myosin cross bridge. The globular head now swings back and attaches to an actin-myosin binding site further along the actin myofilament. This process repeats many times to fully contract the sarcomere.

Sarcomere contraction is stopped when $Ca^{2+}$ is no longer present. When there are no longer calcium ions in the environment the tropomyosin beings to block the actin-myosin binding sites and the actin filaments return to their normal position.

Fast and slow twitch skeletal muscle fibres

There are two different types of muscle fibres in skeletal muscles. They have different features which are important for you to know.

Slow twitch	fast twitch
Slow contraction.	Fast contraction.
Long lasting contraction. Example Back muscles which help maintain posture.	Quick contraction. Example Leg muscles which allow you to run.
Endurance for slow twitch skeletal muscle is important and these muscles can remain contracted for long periods of time without getting tired.	Quick period of contraction in fast twitch skeletal muscles is important, however, they get tired after short periods of time.
Energy is provided by aerobic respiration. They contain lots of mitochondria to provide this energy.	Energy is provided by anaerobic respiration where glycogen is converted into glucose. There are less mitochondria when compared to slow twitch muscle fibres.
Red colour due to lots of myoglobin protein which holds oxygen for aerobic respiration.	White colour due to lower levels of myoglobin. Myoglobin is not required in fast twitch muscle fibres and anaerobic respiration.