Alec Titan
The human body has over 600 muscles that help us do everything from moving our bodies to breathing and keeping us alive. Skeletal muscles are voluntary muscles stimulated and controlled by the brain and the somatic nervous system. When we think about moving, our brain decides which muscles are necessary to make that movement happen. Electrical impulses are sent via the spinal cord and nerves to the appropriate muscles. The nervous system then triggers chemical reactions, leading to the muscle fibers reorganizing themselves in a way that shortens the muscle, resulting in a contraction.
| Characteristics | Values |
|---|---|
| Type of muscle | Skeletal muscle |
| Control | Voluntary, controlled by the brain and the somatic nervous system |
| Function | Allow the body to move, in conjunction with the bones of the skeleton |
| Movement | Contraction and relaxation |
| Contraction | Caused by the release of chemical neurotransmitters including acetylcholine and electrolytes like sodium and calcium |
| Relaxation | When the nervous system signal is no longer present, the chemical process reverses, and the muscle fibers rearrange and the muscle relaxes |
| Contraction types | Concentric (muscle shortens) and eccentric (shortens and lengthens at the same time) |
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What You'll Learn
Muscles respond to electrical impulses from the brain
The human body is capable of incredible feats of movement and coordination, and at the heart of this are our muscles. Skeletal muscles, which allow the body to move, are stimulated and controlled by the brain and the somatic nervous system. When we think about moving, our brain decides which muscles are necessary and sends electrical impulses via the spinal cord and nerves to the appropriate muscles.
This process, known as muscle contraction, involves the shortening and pulling of muscles to create movement. For example, when performing a bicep curl, the bicep muscle contracts, the elbow flexes, and the weight is lifted. This movement is made possible by the contraction of the bicep muscle, which pulls on the bone it is attached to, causing the joint to move.
The electrical impulses from the brain cause the release of chemical neurotransmitters, including acetylcholine and electrolytes like sodium and calcium, which stimulate the muscle to contract. Acetylcholine, for instance, binds to receptors on the outside of the muscle fiber, starting a chemical reaction within the muscle. This reaction leads to the reorganisation of muscle fibers in a way that shortens the muscle, resulting in contraction.
The process of muscle contraction is complex and involves the interaction of various proteins and filaments within the muscle cells. The thin filaments, composed of actin, tropomyosin, and troponin, interact with the thick filaments made of the protein myosin. During muscle contraction, these filaments slide past each other, causing the muscle to shorten and generate force.
In summary, muscles respond to electrical impulses from the brain, which initiate a series of chemical and physical reactions that result in muscle contraction and, ultimately, movement. This intricate process allows us to perform a wide range of physical activities and showcases the remarkable capabilities of our bodies.
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Muscles contract and relax to move the body
The process of muscle contraction involves three steps. Firstly, a message is sent from the nervous system to the muscular system, triggering chemical reactions. Secondly, these chemical reactions lead to the muscle fibres reorganizing themselves in a way that shortens the muscle, resulting in a contraction. Finally, when the nervous system signal stops, the chemical process reverses, the muscle fibres rearrange, and the muscle relaxes.
The nervous system generates an electrical impulse called an action potential, which travels through a motor neuron to the muscle. When the nervous system signal reaches the neuromuscular junction, a neurotransmitter called acetylcholine is released, which binds to receptors on the outside of the muscle fibre. This starts a chemical reaction within the muscle, which leads to the muscle fibres reorganizing and shortening.
The thin and thick filaments of myofibrils are arranged in units called sarcomeres. The thick filaments are made from the protein myosin, and the thin filaments are composed of actin, tropomyosin, and troponin. When the muscle is inactive, tropomyosin blocks the interaction between myosin and actin. However, when an action potential causes depolarization in the myocyte membrane, it leads to the release of calcium ions, which initiate muscle contraction.
The contraction of skeletal muscles is a voluntary process, controlled by the brain and the somatic nervous system. The brain decides which muscles are necessary for a particular movement and sends electrical impulses via the spinal cord and nerves to the appropriate muscles. This causes the release of neurotransmitters, stimulating the muscle to contract.
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Muscles respond to resistance training
Resistance training is a form of exercise that is essential for overall health and fitness, as well as athletic performance. It is based on the principle that muscles will work to overcome a resistance force when they are required to do so. When you do resistance training repeatedly and consistently, your muscles become stronger.
Resistance training increases muscle strength by making muscles work against a weight or force. Different forms of resistance training include using free weights, weight machines, resistance bands, and your own body weight. When you lift weights, you recruit new patterns of communication between the brain, nerves, neuromuscular junction, and muscle fibres. Every time you lift weights and engage those muscles, you lay down new neuromuscular patterns and get stronger.
Motor neurons in the muscle and nervous system die as people get older and do not regenerate, resulting in a loss of strength. However, exercise can reverse this process. Geriatric patients can increase motor neuron firing by as much as 20% and increase stretch by more than one-third in just six weeks of weight training.
Muscle hypertrophy, or an increase in cell size, is a mechanism that increases muscle strength. When you lift weights, microscopic damage (microtears) occurs to the myofibrils within the muscle fibre. These microtears stimulate the body's repair response, causing more myofibrils to grow. The increased number of myofibrils causes muscle fibres to enlarge, increasing their volume and size. The chronic adaptation of increased cross-sectional size of the muscle fibres results in an increase in muscle strength and power.
Another chronic adaptation to the muscles is hyperplasia, which occurs when the number of muscle fibres increases. This results in a relative increase in protein synthesis, which is essential for the repair of muscle fibres in acute response to resistance training. Resistance training acutely increases the concentration and release of anabolic and catabolic proteins and steroid hormones.
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Muscles respond to chemical messages
Skeletal muscles are voluntary muscles that are stimulated and controlled by the brain and the somatic nervous system. The brain is the central processing unit, and nerve fibres from the brain run down the spinal cord, branching out in networks to every skeletal muscle that moves. When you think about moving, your brain decides which muscles are necessary to make that movement happen. Electrical impulses are sent via the spinal cord and nerves to the appropriate muscles.
The site where the motor neuron reaches a muscle cell is called the neuromuscular junction. When the nervous system signal reaches the neuromuscular junction, a chemical message is released by the motor neuron. This chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fibre. This starts a chemical reaction within the muscle. A multistep molecular process within the muscle fibre begins when acetylcholine binds to receptors on the muscle fibre membrane. The proteins inside muscle fibres are organised into long chains that can interact with each other, reorganising to shorten and relax.
When acetylcholine reaches receptors on the membranes of muscle fibres, membrane channels open and the process that contracts a relaxed muscle fibre begins. Open channels allow an influx of sodium ions into the cytoplasm of the muscle fibre. This sodium influx also sends a message within the muscle fibre to trigger the release of stored calcium ions. The calcium ions diffuse into the muscle fibre, and the relationship between the chains of proteins within the muscle cells changes, leading to the contraction.
When the stimulation of the motor neuron providing the impulse to the muscle fibres stops, the chemical reaction that causes the rearrangement of the muscle fibres' proteins is stopped. This reverses the chemical processes in the muscle fibres and the muscle relaxes.
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Muscles respond to calcium ions
Calcium ions play a crucial role in muscle function, plasticity, and disease. Muscle contraction is regulated by calcium. An action potential generated by a motor neuron on the muscle cell surface activates voltage-gated calcium channels, allowing calcium to enter the muscle cell. This calcium then activates another ion channel, the ryanodine receptor (RyR1 in muscle cells), which releases more calcium stored in the sarcoplasmic reticulum into the cell's cytoplasm.
Calcium diffusing in the cytoplasm between myosin and actin filaments causes them to slide into each other, triggering the contraction of the entire muscle fiber. As the action potential decays, calcium ions are pumped back into the sarcoplasmic reticulum by the SERCAs pump (Sarcoplasmic/endoplasmic reticulum calcium ATPase). Calcium ions (Ca2+) are the main regulatory and signaling molecule for all muscle fibers. The contractile properties of muscle fibers depend on the variable expression of proteins involved in Ca2+ signaling and handling.
Calcium ions are also involved in muscle relaxation. As calcium levels in the myocyte fall, tropomyosin covers the actin filaments' myosin-binding sites, preventing actin and myosin from interacting and thus inhibiting muscle contraction. Calcium ions further influence muscle contraction through their binding to various proteins in muscle tissue, including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca2+-binding proteins may play a role in Ca2+-triggered muscle contraction or modulate other muscle activities such as protein metabolism, differentiation, and growth.
Additionally, calcium ions are implicated in muscle diseases. For example, Brody's disease is caused by mutations in SERCA1, resulting in impaired calcium release and slowed muscle relaxation. Certain anesthetics can also cause excessive Ca2+ release during general anesthesia, leading to sustained muscle contraction, rigidity, and increased energy production and consumption.
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Frequently asked questions
Muscles are pieces of soft tissue throughout your body that help you do everything from holding your body still to moving your body and even running a marathon. There are more than 600 muscles in the human body.
Muscles contract and relax to mechanically move the body. Messages from the nervous system cause these muscle contractions. Skeletal muscles are voluntary muscles stimulated and controlled by the brain and the somatic nervous system.
Muscles respond to messages from the nervous system. When the nervous system generates a signal, it travels through a type of nerve cell called a motor neuron. When the nervous system signal reaches the neuromuscular junction, a chemical message is released by the motor neuron. This chemical message, a neurotransmitter called acetylcholine, binds to receptors on the outside of the muscle fiber. That starts a chemical reaction within the muscle.
The agonist/antagonistic system is the "reciprocal" synergy between muscle groups. For instance, your biceps shorten and bend your elbow, while the triceps on the other side of the arm shorten and return the elbow to its original position.
Eccentric contractions are when the muscle shortens and lengthens at the same time. For example, when you lift the final bicep curl of your set and then lower it slowly, your bicep is contracting to lower it slowly and prevent the dumbbell from falling, but it's lengthening at the same time to allow your arm to straighten and return to the starting position.
