Skeletal Muscles

Antagonistic Pairs

The first key concept to understand is that muscles work in antagonistic pairs. The word antagonist means ‘a person who opposes the other’, and muscle pairs work in this way, as they oppose each other.

We are going to look at a bicep curl to see how this works. Here, the group of antagonistic muscle pairs includes the biceps & triceps. Other structures to be aware of:

• Tendons — connect muscles to bones, allowing movement of the skeleton (e.g. lifting the arm). You may have heard of tendonitis, a common sports injury (e.g. Achilles tendonitis). The suffix ‘itis’ means inflammation, so tendonitis means inflammation of a tendon.
• Ligaments — connect bone to bone, providing stability at joints. A common sports injury is tearing the anterior cruciate ligament (ACL) in the knee.

Bicep Curl Example

• Pulling the lower arm upwards:
  • When the bicep contracts (this is known as the agonist), the tricep does the opposite and relaxes (this is called the antagonist), so the lower arm is pulled upwards.
• Pulling the lower arm downwards:
  • When the tricep contracts (this time it’s the agonist), the bicep relaxes (the antagonist) and the lower arm is pulled downwards.

Does this work for other muscle pairs? Absolutely! There are many examples: quadriceps and hamstrings can pull the leg upwards or downwards.

Structure of Muscles

Let’s continue with the bicep curl as an example, and try to understand what is happening at a structural level that is causing the muscle to contract.

High Level Organisation

• Skeletal muscle is a highly organized tissue.
• Muscles are made of bundles, called fascicles (which are encased and separated by layers of connective tissue)
• These fascicles are made of muscle fibres (again muscle fibres are separated from each other by connective tissue)
• These muscle fibres are made of myofibrils

A single muscle fibre contains hundreds to thousands of myofibrils. Interestingly, if you are more trained (i.e. you have done more bicep curls) you can have more myofibrils than someone who hasn’t hit the gym in a while.

Myofibrils are made of sarcomeres which are repeating units and the basis of muscle contraction. When these sarcomeres contract, the entire muscle contracts.

Muscle Fascicle -> Muscle Fibre	Myofibril

Images adapted from OpenStax College, via Wikimedia Commons (CC BY 4.0).

Muscle Fibre Structures

• Sarcolemma: the membrane surrounding the entire muscle fibre (not individual myofibrils).
• Sarcoplasm: cytoplasm of muscle cells, which contains other structures (e.g. sarcoplasmic reticulum, nuclei)
• Nuclei: the muscle fibre contains many nuclei, allowing high rates of protein synthesis for actin and myosin.
• Mitochondria: required for ATP production, which is especially important for muscle contraction.
• T-tubules: deep infoldings of the sarcolemma that transmit action potentials to the sarcoplasmic reticulum.
• Sarcoplasmic reticulum: controls calcium release, and calcium triggers muscle contraction.

You’ll see how the sarcolemma, T-tubules and sarcoplasmic reticulum work together at the neuromuscular junction below, and on the Synaptic Transmission page.

Ultrastructure of Myofibril

Our understanding of muscles came from observing myofibrils under a microscope.

Scientists observed 2 main protein filaments actin (thin filament) and myosin (thick filament).

The animation below shows you an example of 1 sarcomere unit, but in reality there are many repeating sarcomeres. You need to be aware of the following lines/bands (Note: these can be quite challenging to remember!)

• M lines — denote the middle (M for middle) of the sarcomere and provide attachment points for myosin filaments
• Z lines — outline the boundaries of the sarcomere
• I bands — contain actin only. Because there is a single filament present, this region appears lighter under a microscope
• A band — the length of the myosin filaments. It includes the regions where actin and myosin overlap, which appear darker because two filaments are present
• H zone — contains myosin only. This region appears lighter under a microscope because only one filament is present

Neuromuscular Junction

What stimulates the muscles to contract?

Let’s look at the neuromuscular junction to show how the release of calcium controls this and how muscle contraction is brought about.

Note: There is a separate topic, Synaptic Transmission, which goes into more detail on synapses. I’m outlining it here as it relates to muscle contraction.

Sliding Filament Theory

Now let’s look in more detail as to how this sarcomere contracts, specifically the sliding filament mechanism.

Steps in Sliding Filament Theory

1. Actin-myosin binding sites are blocked: When muscle is resting the tropomyosin has blocked actin-myosin binding sites, which means that myosin can’t bind to actin and muscle contraction can’t happen.
2. But, when the calcium is released from sarcoplasmic reticulum (what we just looked at in neuromuscular junctions) it binds to troponin. This causes the tropomyosin to change positions exposing the actin-myosin binding sites.
3. The myosin heads can now bind to actin, and a cross bridge is formed.
4. Next, the myosin heads perform a power stroke which pulls the actin filaments towards the centre of the sarcomere, releasing ADP and Pi (you can see on the animation that the sarcomere has shortened)
5. Next, ATP binds to the myosin head and causes it to detach from actin.
6. ATP hydrolysis then occurs releasing energy to re-cock the myosin head back to original position, so that the cycle can continue.

Role of ATP & Phosphocreatine

Role of ATP in Muscle Contraction

1. The hydrolysis of ATP provides energy to re-cock the myosin head back into the high-energy position, which then drives the power stroke, pulling the actin filaments towards the centre of the sarcomere

2. ATP is needed to detach the actinomyosin bridge and re-set the myosin head back to its starting position.

3. Finally, it’s responsible for active transport of calcium ions to return to sarcoplasmic reticulum, when the muscle is relaxed.

(Note: ‘Actinomyosin bridge’ is the term used directly from the AQA specification. It means the link between myosin and actin and is often referred to as ‘cross-bridge’.)

What are the sources of ATP?

We’ve just seen how important ATP is for muscle contraction, but how is it made?

There are 3 main sources of ATP:

• Aerobic respiration
• Anaerobic respiration (glycolysis)
• Phosphocreatine

Aerobic respiration requires oxygen and takes a lot longer to produce ATP, making it unsuitable for quick short bursts of high intensity exercise (e.g. sprinting from a lion, because that is a regular occurrence!).

High intensity exercise is fuelled partially by glycolysis and by phosphocreatine which is stored in muscles.

Phosphocreatine

• Is a phosphorylated form of creatine, stored in the muscles.
• It is used to rapidly regenerate ATP for high intensity exercise (e.g. short sprints, 100m)

• It’s a reversible reaction, and the stores of phosphocreatine become replenished during rest.
• Interestingly
  • Some people take creatine supplements to help increase stores of phosphocreatine, to improve their performance at very high-intensity exercise.
  • If you wanted to train your phosphocreatine system (i.e. high-intensity/short sprints) it’s important to take long enough rests in between sprints to ensure you can replenish your phosphocreatine stores

Slow & Fast Twitch Muscle Fibres

There are two main types of muscle fibres: Slow & Fast Twitch.

These exist in different proportions across individuals and across different types of muscles.

For example, sprinters often have a higher proportion of fast twitch muscle fibres than ultramarathon runners. The relative proportions also vary depending on the location of the muscle in the body (e.g. calf muscles often have a higher proportion of slow twitch muscles as they need to be able to withstand continuous muscle contraction).

Slow Twitch

Function

• Contract more slowly than fast twitch
• Less powerful contractions over a long period of time
• Used for longer sustained endurance activities

Structural adaptations

• More myoglobin (protein that acts as an oxygen store, appears bright red)
• More mitochondria for aerobic respiration to produce more ATP
• Rich supply of blood vessels to deliver oxygen and glucose for aerobic respiration
• High resistance to fatigue (relies on aerobic respiration, generates large amounts of ATP)

Fast Twitch

Function

• Contract quickly
• More powerful contractions over a short period of time
• Used for shorter, higher intensity activities

Structural adaptations

• Larger (thicker) fibres, so they are able to generate powerful contractions
• High stores of phosphocreatine
• High stores of glycogen
• Fatigues quickly (relies on anaerobic respiration, lactate is produced)

Exam Question Practice

More questions on this topic in Exam Practice →