What is muscle contraction?
Muscle contraction occurs when tension-generating sites within muscle cells are activated. Muscle contraction does not always imply muscle shortening since muscle tension can be produced without changes in muscle length. For example, when holding something heavy in the same position.
Muscle relaxation, or the return of muscle fibres to their low tension-generating state, occurs after muscle contraction.
Types of muscle contractions
Several types of muscle contractions occur, which are distinguished by changes in muscle length during contraction.
An isometric contraction is a type of work that involves the static contraction of a muscle with no visible movement in the joints.
This is typical of hand and forearm muscles, whose muscles do not change length and the joints do not move, so force for grip is sufficient.
When the muscles of the forearm and hand grip an object, the joints of the hand fail to move, but the muscles generate enough force to keep the object from falling.
Isotonic contractions maintain constant muscle tension as the muscle length changes. This can happen only when the maximal force of contraction of a muscle exceeds the total load on the muscle.
Isotonic muscle contractions can be concentric (muscle shortens) or eccentric (muscle lengthens).
An eccentric contraction causes a muscle to elongate while still generating force where the resistance exceeds the force generated.
Eccentric contractions can be voluntary or involuntary in nature. A voluntary eccentric contraction, for example, would be the controlled lowering of the heavyweight raised preceding concentric contraction.
When a certain weight is too heavy for a muscle to bear, it is gradually lowered while under tension, resulting in an involuntary eccentric contraction.
A concentric contraction is a muscle contraction that occurs when the muscles shorten while producing force and overcoming resistance.
A concentric contraction of the biceps makes the arm bend at the elbow and lift the weight towards the shoulder when lifting a heavy weight. Cycling across a bridge shortens the sarcomere, muscle fibre and muscle.
Why do muscles contract?
Muscles perform various functions in the body. The primary reasons for muscle contraction are,
- To maintain posture – Muscles assists in maintaining a position such as sitting or standing.
- To provide stability to the joints as the muscles lengthen and shorten as the body requires them.
- To produce heat to regulate body temperature. Muscle work accounts for approximately 40% of the body’s temperature. Shivering causes the skeletal muscles to contract in order to warm up.
Causes of muscle contraction
Muscle spasms, commonly known as involuntary muscle contraction, can be caused due to the following reasons.
The quick muscle spasms serve as reminders that the muscles are tired and require rest to function optimally.
A muscle contraction can be due to dehydration, muscle strain, or simply holding a position for an extended period.
A muscle’s sudden and painful contraction during muscle spasm usually occurs after overusing or injuring the muscle through exercise or stress.
How does your muscle contract?
A signal from the brain and the nervous system initiates muscle contraction. The signal then transfers to the muscles, directing them to contract. The process is really swift.
Process of muscle contraction
1. Depolarisation and Calcium ion release
- An action potential generated by a motor neuron causes acetylcholine to be released into the motor end plate.
- Acetylcholine brings about depolarisation in the sarcolemma, which spreads through the muscle fibre via the T tubules.
- When the cell is depolarised, the sarcoplasmic reticulum releases the stored calcium ions (Ca2+).
- Calcium ions are crucial in the initiation of muscular contractions.
2. Actin and myosin cross-bridge formation
- A blocking complex consisting of troponin and tropomyosin protects the binding sites for myosin heads on actin.
- Calcium ions bind to troponin and reorganise the complex, exposing myosin head binding sites.
- Myosin heads, along with the actin filaments, form a cross-bridge.
3. Sliding mechanism of actin and myosin
- ATP binds to the myosin head, breaking the actin-myosin cross-bridge.
- ATP hydrolysis makes myosin heads swivel and change position, directing them to the next actin-binding site.
- Myosin heads attach to the new actin sites and revert to their original shape.
- In a sliding mechanism, this reorientation drags actin along with myosin.
- The myosin heads propel the actin filaments like a row boat.
4. Sarcomere shortening
- The actin filaments are dragged along the length of the myosin by the repeated reorientation of the myosin heads.
- The Z lines are pulled closer together as the actin filaments are anchored to Z lines, thereby shortening the sarcomere.
- The muscle fibres as a whole contract as the individual sarcomeres shorten in length.
Muscle contraction is characterised by an increase in tension or a decrease in muscle length.
Isometric muscle contractions occur when muscle tension changes but muscle length remain constant. Isotonic means that muscle length changes, but muscle tension remains constant.
A motor neuron electrochemically stimulates a muscle fibre to initiate muscle contraction. This happens at a neuromuscular junction, which is a chemical synapse.
Acetylcholine is a neurotransmitter that diffuses across synaptic clefts and binds to receptors on muscle fibres. This causes muscle contraction to occur.
Myosin heads repeatedly bind with and pull-on actin filaments using ATP energy. This causes actin filaments to move toward the centre of a sarcomere, shortening it and causing the muscle to contract.
What works to affect muscle contractions?
Size and length of muscle and fibre affect the muscle contraction along with the angle and physical properties of the fibre-tendon attachment, the type of fibre, number of parallel cross-bridges and force.
What can cause a muscle contraction to stop?
When signalling from the motor neuron ends, muscle contraction usually stops.
What is muscle contraction, and why is it important?
Muscle contraction refers to the shortening or lengthening of muscles during an activity.
Muscle contraction serves several essential functions in the body, including movement, joint stability, posture and heat production.
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