Athena Vale
Understanding how to control muscle fibres is essential for fitness trainers and athletes, but also for anyone looking to improve their health through exercise. Muscle fibres are responsible for controlling physical forces moving through the body, and they can be found in skeletal, cardiac, and smooth muscles. Skeletal muscles are the most common type of muscle in the body, and they are also under voluntary control, meaning that we can control how and when they work. The speed and strength of muscle contractions depend on the type of muscle fibre and how it is activated.
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What You'll Learn
- Skeletal muscle fibres are controlled voluntarily by the individual
- Cardiac muscle fibres are controlled involuntarily by the autonomic nervous system
- Smooth muscle fibres are controlled involuntarily by the autonomic nervous system
- Muscle fibres can be lengthened or shortened to control movement
- Muscle fibres are activated by a motor neuron
Skeletal muscle fibres are controlled voluntarily by the individual
Skeletal muscles are the most common muscles in the human body, comprising between 30% and 40% of total body mass. They are attached to bones via tendons and allow us to perform a wide range of movements and functions. Unlike smooth and cardiac muscles, skeletal muscles are under voluntary control, meaning an individual can control how and when they work.
Skeletal muscles are composed of cells called muscle fibres, which are multinucleated and range from 10 to 100 micrometres in diameter and several centimetres in length. Each muscle fibre is made up of several hundred to several thousand myofibrils, which are the basic units of the muscle fibre. The myofibrils are surrounded by the muscle cell membrane (sarcolemma), which forms deep invaginations called transverse tubules (T-tubules) within the myofibril. The nuclei are located in the cell's periphery, adjacent to the sarcolemma.
Each muscle fibre is a layer of connective tissue called the endomysium, which contains capillaries and nerve tissue to supply the individual muscle fibres. Multiple muscle fibres join to form fascicles, which are encased by another connective tissue covering called the perimysium. The perimysium may surround anywhere from 10 to 100 fascicles. Muscle fascicles are further grouped to form a muscle, which is encased by a fibrous tissue envelope called the epimysium.
Motor units are a single motor neuron and the muscle fibres it innervates. When a motor unit is signalled to contract, it activates all of its attached muscle fibres. The number of muscle fibres in a motor unit depends on the function of the muscle. Muscles that require fine motor control will involve fewer fibres, whereas larger muscle groups will involve significantly more muscle fibres.
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Cardiac muscle fibres are controlled involuntarily by the autonomic nervous system
The human body's autonomic nervous system (ANS) is a component of the peripheral nervous system that controls cardiac muscle contraction, visceral activities, and glandular functions. The ANS regulates heart rate, blood pressure, respiration rate, body temperature, sweating, gastrointestinal motility, and other visceral activities that maintain homeostasis. It functions continuously without conscious effort, controlled by centres in the spinal cord, brain stem, and hypothalamus.
Cardiac muscle fibres are a part of the human body's skeletal muscle system, which serves various purposes, including producing movement, maintaining body posture and position, and stabilising joints. Unlike skeletal muscle contraction, which is under voluntary control, cardiac muscle fibres are controlled involuntarily by the autonomic nervous system.
The ANS has two interacting systems: the sympathetic and parasympathetic systems, which have opposing effects on the heart. The sympathetic system prepares the body for energy expenditure and emergency or stressful situations, often referred to as the "fight or flight" response. This system increases cardiac output and causes vasodilation, with vessels in different locations reacting differently to sympathetic stimulation.
The parasympathetic system, on the other hand, provides the ability to decrease heart rate through the vagus nerves. Damage to these nerves can lead to tachycardia, while injury to the sympathetic fibres can impair the ability to increase heart rate, resulting in bradycardia.
The network of nerves supplying the heart is called the cardiac plexus, which includes both sympathetic and parasympathetic fibres. The cardiac plexus receives input from the right and left vagus nerves, as well as the sympathetic trunk, influencing heart rate, cardiac output, and contraction forces.
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Smooth muscle fibres are controlled involuntarily by the autonomic nervous system
Smooth muscle fibres differ from skeletal muscle fibres in that they are controlled involuntarily by the autonomic nervous system. Smooth muscle fibres are not organised into sarcomeres and do not contain the troponin complex required for skeletal muscle contraction. Instead, they are controlled by the autonomic nervous system, which uses hormones, neurotransmitters, and other receptors to regulate their contraction and relaxation. This allows the body to tightly regulate many of its subsystems for life without conscious thought. For example, a person does not need to consciously think about their blood pressure for it to adapt to increasing oxygen demands from exercise.
The autonomic nervous system, which is made up of the sympathetic and parasympathetic nervous systems, controls smooth muscle tissue through nonadrenergic noncholinergic (NANC) nerve fibres. NANC neurotransmitters such as ATP, nitric oxide (NO), and peptides like VIP and substance P are involved in NANC neurotransmission in the digestive, respiratory, and urinary tracts. For instance, in the digestive tract, inhibitory NANC innervation with ATP is responsible for fast relaxation, while VIP and possibly NO are responsible for the slow response. In the urinary bladder, pelvic nerve stimulation causes a large, transient atropine-resistant contraction, which is blocked by alpha, beta-methylene ATP, suggesting that it is due to ATP.
The parasympathetic nervous system, a component of the autonomic nervous system, regulates specific portions of the body through its nerves. For example, the vagus nerve innervates the gastrointestinal tract from the oesophagus to the proximal portion of the large intestines and also sends out branches to the heart, larynx, trachea, bronchi, liver, and pancreas. Smooth muscle plays a crucial role in the disease process throughout the body, and its regulation by the autonomic nervous system is essential for maintaining vital signs during surgery. For instance, bronchodilators that relax airway smooth muscle are a life-saving treatment for asthmatics, and medications like metoclopramide stimulate gastric emptying by increasing smooth muscle signalling.
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Muscle fibres can be lengthened or shortened to control movement
The cross-bridge cycling begins when ATP binds to an ATP-binding domain on the myosin head. The myosin heads then move toward the negative end of actin (toward the centre of the sarcomere), displacing the actin filament and shortening the sarcomere. This cycle repeats as long as Ca is bound to troponin C and ATP is available. The force of contraction is a summation of the number of motor units recruited and the frequency of action potentials that reach those motor units.
To lengthen muscle fibres, eccentric exercises can be performed, which stimulate the addition of parallel-arranged sarcomeres within the muscle fibres. This results in increases in both fCSA and PCSA, with higher increases in PCSA when performed at longer muscle lengths. Immobilization in a lengthened position also results in a considerable addition of serial sarcomeres, ensuring that the optimum muscle length is attained in the immobilized joint position.
It is important to note that muscle shortening and muscle contraction are not synonymous. Tension within a muscle can be produced without changes in the length of the muscle, such as when holding a dumbbell in the same position.
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Muscle fibres are activated by a motor neuron
Motor units are defined as a single motor neuron and all the muscle fibres it innervates, with multiple motor units stimulating a single muscle. The number of muscle fibres comprising a motor unit depends on the function of the muscle. Muscles that require fine motor control will involve fewer fibres, whereas larger muscle groups will involve significantly more muscle fibres. For example, the extraocular muscle motor units are extremely small and have a very high proportion of muscle fibres capable of contracting with maximal velocity.
The central nervous system has two distinct ways of controlling the force produced by a muscle through motor unit recruitment: spatial recruitment and temporal recruitment. Spatial recruitment is the activation of more motor units to produce a greater force. Larger motor units contract along with small motor units until all muscle fibres in a single muscle are activated, thus producing the maximum muscle force. Temporal motor unit recruitment, or rate coding, deals with the frequency of activation of muscle fibre contractions.
The rate code is a way for motor neurons to signal the amount of force to be exerted by a muscle. An increase in the rate of action potentials fired by the motor neuron causes an increase in the amount of force that the motor unit generates. When the motor neuron fires a single action potential, the muscle twitches slightly, and then relaxes back to its resting state. If the rate of firing of the motor neuron increases, such that a second action potential occurs before the muscle has relaxed back to baseline, then the second action potential produces a greater amount of force.
Motor units and the motor neurons themselves vary in size. Small motor neurons innervate relatively few muscle fibres and form motor units that generate small forces, whereas large motor neurons innervate larger, more powerful motor units. Motor units also differ in the types of muscle fibres that they innervate. In most skeletal muscles, the small motor units innervate small “red” muscle fibres that contract slowly and generate relatively small forces; but, because of their rich myoglobin content, plentiful mitochondria, and rich capillary beds, such small red fibres are resistant to fatigue. These small units are called slow (S) motor units and are especially important for activities that require sustained muscular contraction, such as the maintenance of an upright posture. Larger motor neurons innervate larger, pale muscle fibres that generate more force; however, these fibres have sparse mitochondria and are therefore easily fatigued.
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Frequently asked questions
Muscle fibers are the flexible fibers that make up muscles and allow for movement in the body. They are connected to the bones and allow for a wide range of movements and functions.
There are three main types of muscle fibers: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Each type has a different function and energy-generation method.
Muscle fibers contract (tighten) and relax to allow for movement. The speed of contraction depends on how quickly myosin's ATPase hydrolyzes ATP. This process is influenced by the type of muscle fiber and the number of motor units involved.
Skeletal muscle fibers are under voluntary control, meaning you can control how and when they work. However, cardiac and smooth muscle fibers are involuntary and controlled by the autonomic nervous system.
Muscle fibers adapt to the specific type of exercise stimulus imposed during training. For example, endurance training can increase the endurance level of fast-twitch fibers, while sprint training can improve the power generated by slow-twitch fibers.
