Dominic Stone
There are over 600 muscles in the human body, making up 30-40% of our total body mass. They are made of thousands of small fibres woven together, and they help us do everything from holding our bodies still to running marathons. There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles that we can control, while smooth and cardiac muscles are involuntary and work without our input. Skeletal muscles are the most common type and help us move, sit up straight, and keep our balance. Smooth muscles line the inside of some organs and help with essential jobs like moving waste through our intestines and helping our lungs expand when we breathe. Cardiac muscles make up the heart and pump blood throughout our bodies.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | More than 600 |
| Muscle composition | Thousands of small elastic fibres |
| Types of muscle tissue | Skeletal, cardiac, and smooth |
| Skeletal muscle composition | Elongated, multinucleate muscle fibres |
| Skeletal muscle movement | Voluntary |
| Cardiac muscle movement | Involuntary |
| Smooth muscle movement | Involuntary |
| Types of skeletal muscle fibres | Type I (slow-twitch) and Type II (fast-twitch) |
| Types of Type II fibres | Type IIA, Type IIx, and Type IIB |
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What You'll Learn
Skeletal muscles
The human body is an amazing machine, made up of many different types of muscles that work together to enable movement and support our bodies. One of the most prevalent and important types of muscle in our bodies is skeletal muscle. These muscles are attached to bones by tendons and are responsible for generating the force and movement that enables us to carry out a wide range of actions, from walking and running to lifting objects and even smiling.
Making up around 40% of our total body weight, skeletal muscles are essential for our posture, movement, and stability. They work in pairs to enable fluid movement; for example, when one muscle contracts to bend a joint, its opposing muscle relaxes to allow the movement, then contracts to return the joint to its starting position. This push-and-pull system is essential for maintaining balance and ensuring our movements are smooth and controlled.
The strength and endurance of skeletal muscles can be improved through exercise and training. Strength training, for instance, involves challenging the muscles with resistance to stimulate growth and increase strength. Similarly, endurance exercises like long-distance running can improve a muscle's ability to withstand fatigue and continue working efficiently. A balanced diet, adequate protein intake, and proper hydration also play a key role in maintaining and improving skeletal muscle health.
In summary, skeletal muscles are a vital component of our musculoskeletal system, enabling us to move with precision and power. Their voluntary nature gives us a great deal of control over our actions, and their adaptability means we can train them to become stronger and more resilient. Taking care of our skeletal muscles through exercise, nutrition, and hydration is key to maintaining overall health and ensuring these remarkable muscles can continue to serve us well throughout our lives.
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Cardiac muscles
If you're a cardiac muscle, you're the untiring drummer setting the rhythm of the heart. You're unique, powerful, and involuntary—and that's a good thing, because who'd remember to keep drumming 24/7?
Also known as the myocardium, cardiac muscle is one of three major categories of muscles in the human body, along with smooth muscle and skeletal muscle. The heart is made up of three layers—the pericardium, myocardium, and endocardium—and the myocardium forms the thick middle layer. The inner endocardium lines the cardiac chambers, covers the cardiac valves, and joins with the endothelium that lines the blood vessels connected to the heart. The outer aspect of the myocardium is the epicardium, which forms part of the pericardial sac that surrounds, protects, and lubricates the heart.
Cardiac muscle is made up of sarcomeres that allow for contractility. Each cardiomyocyte (cardiac muscle cell) contains a single, centrally located nucleus surrounded by a cell membrane known as the sarcolemma. The sarcolemma contains voltage-gated calcium channels, which are specialized ion channels that skeletal muscle does not possess. The sarcolemma of cardiac muscle cells contains highly branched invaginations known as T-tubules, which function in excitation-contraction coupling, action potential initiation and regulation, maintaining the resting membrane potential, and signal transduction.
Cardiac muscle cells contain branched fibers connected via intercalated discs that contain gap junctions and desmosomes. These interconnections allow the cardiomyocytes to contract together synchronously to enable the heart to work as a pump. The coordinated contraction of cardiac muscle involves electrical impulses and requires ATP, which can be obtained through various substrates, including fatty acids, carbohydrates, proteins, and ketones.
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Smooth muscles
Additionally, smooth muscles are responsible for important functions in the respiratory system, such as helping the lungs expand during breathing and controlling the dilation and constriction of the pupils in the eyes. In the female reproductive system, smooth muscles are involved in contractions during labour. Smooth muscles also have specific roles in the skin, such as causing hairs to stand on end when experiencing goosebumps.
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Voluntary movements
The premotor cortex plays a crucial role in selecting movements based on external information from the parietal cortex. It works together with the presupplementary and supplementary motor areas to generate readiness potentials. These signals are then sent through the motor cortex to the basal ganglia and cerebellum for checking and modulating motor control. The basal ganglia and cerebellum loops are essential in regulating and modulating movements, and their dysfunction can lead to movement disorders.
Skeletal muscles are responsible for voluntary movements. They contract and relax to facilitate intentional actions. Some skeletal muscles, known as fast-twitch muscles, contract quickly and use short bursts of energy. In contrast, others, like back muscles, move slowly and help maintain posture.
Understanding the distinction between voluntary and involuntary movements is crucial for comprehending the complexity of human behaviour and movement disorders. Voluntary movements are a result of the intricate coordination between various brain regions and the skeletal muscles, allowing for intentional and willed actions.
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Involuntary movements
One example of an involuntary movement disorder is myoclonus, characterised by sudden, jerky, irregular contractions of a single muscle or group of muscles. Myoclonus can be focal, affecting one muscle, or multifocal, involving multiple muscle groups. It is often associated with low oxygen levels in the brain (hypoxia) or metabolic processes like kidney or liver failure. Spinal myoclonus may be caused by multiple sclerosis, syringomyelia, ischemic myelopathy, spinal trauma, or infection. Medication side effects are also a common cause of myoclonus.
Another type of involuntary movement is tardive dyskinesia (TD), which arises from long-term use of antipsychotic drugs. TD is characterised by repetitive movements such as grimacing or eye blinking and can be mild or more chronic. It is considered a neurological condition originating in the brain.
Athetosis is another example of an involuntary movement disorder, characterised by slow, writhing movements that typically affect the arms and hands. It often leads to uncomfortable, twisted body positions and is associated with chronic conditions such as cerebral palsy, Huntington's disease, and Wilson's disease. Athetosis is caused by irregularities in the basal ganglia region of the brain.
Tics are also considered involuntary movements and can be either motor or vocal-based. They are classified as simple or complex, depending on the number of muscle groups involved. Simple tics may include excessive shoulder shrugging or finger flexing, while complex tics involve multiple muscle groups, such as repetitively hopping and flapping one's arms. In young people, tics are commonly associated with Tourette syndrome, while in adults, they may be a symptom of Parkinson's disease.
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Frequently asked questions
Muscles are pieces of soft tissue made up of thousands of small fibres woven together. They help your body perform a wide range of movements and functions.
There are more than 600 muscles in the human body. They are located almost everywhere throughout your body.
There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are voluntary muscles, while smooth and cardiac muscles are involuntary.
Skeletal muscles help with movement, sitting up straight, and keeping your balance. Smooth muscles are found in the bladder, uterus, eyes, and the walls of organs and structures such as the oesophagus, stomach, intestines, and bladder. They help with focusing your vision, pushing food through your body, and controlling urine. Cardiac muscles make up the heart and contract and relax to pump blood through your body.
