Dominic Stone
The human body is made up of over 600 muscles, which work with the skeleton to allow us to do things like standing, sitting, walking, and running. In most cases, skeletal muscles are attached to the bones with the help of tendons, which are cords made of tough tissue. However, facial muscles are unique in that they originate from bones and insert into the dermis, manipulating the skin and soft tissues. This allows for the small movements that enable us to make a wide range of facial expressions.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600+ |
| Types of muscles | Skeletal, Smooth, Cardiac |
| Types of filaments in muscles | Actin, Myosin |
| Types of muscle cells | Voluntary, Involuntary |
| Muscles attached to skin | Facial muscles |
| Muscles attached to bones | Skeletal muscles |
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What You'll Learn
Facial muscles attach to the skull bone
The human face has around 20 flat skeletal muscles that attach to different places on the skull. These craniofacial muscles are essential for chewing and making facial expressions. They originate from bone or fascia and insert into the skin. Facial muscles are striated muscles that link the facial skin to the skull bone to perform important daily life functions, such as mastication and emotion expression. The muscles of facial expression outnumber the mastication muscles.
Facial muscles produce various movements and are often categorized into facial expression (mimetic) and mastication muscles. The facial nerve (cranial nerve or CN VII) innervates the facial expression muscles, while the trigeminal nerve's mandibular division (CN V3) innervates the muscles of mastication. The primary difference between facial expression and mastication muscles, besides their function, is their embryological origin, which determines their innervation. CN VII innervates facial expression muscles, while mastication muscles receive innervation from CN V3.
The occipitofrontalis muscle, for example, extends from the eyebrows to the top of the skull and can raise the eyebrows and wrinkle the forehead. The orbicularis oculi closes the eyelids, while the orbicularis oris is a circle of muscle around the mouth that closes or purses the lips. The buccinator is a thin muscle in the cheek that holds each cheek toward the teeth. The masseter runs from each cheek to each side of the jaw and helps the jaw close. The lateral pterygoid is a fan-shaped muscle that helps the jaw open.
Damage to the facial nerve and problems with the facial muscles can be caused by autoimmune diseases, Bell's palsy, head and neck cancer, infection, and injury to the head or face.
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Tendons connect muscle to bone
The musculoskeletal and skin systems are made up of specialised tissues. Bones, muscles, and joints work together to move our bodies and give them stability. Tendons and ligaments play a crucial role in this system. Tendons connect muscles to bones, allowing us to move, and ligaments help to stabilise the joints by holding things in place.
Tendons are made of strong, fibrous connective tissue with a high collagen fibre content. They act as mechanical bridges that transmit muscle forces to the bones and joints. This tough structure also helps muscles complete joint movements along a plane. Tendons are present throughout an entire muscle's length, not just at the tips.
At the musculotendinous junction, the muscle pierces the tendon, and at the osteotendinous junction, the tendon inserts into the bone. One end of the tendon is attached to a muscle, and the other end is firmly attached to the periosteum (the membrane covering the bone) or directly to the bone itself. Tendons act as space-saving "connectors" that transfer the movement of the muscle to the bone.
Tendons respond to changes in mechanical loading with growth and remodelling processes, much like bones. Tendons' ability to adapt decreases with age, as cellular structure changes and regeneration capacity diminishes. Collagen fibres become less organised, and calcification can occur, leading to reduced viscoelasticity and strength.
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Muscles don't turn into fat
It is a common misconception, especially among adolescents and adults, that muscles turn into fat when one stops exercising. However, this is not true. Muscle cells and fat cells are different; they have different chemical compositions, and one cannot be converted into the other.
When a person stops exercising, their muscle cells may decrease in volume and become smaller. Simultaneously, the volume of fat cells in the body may increase. This concurrent change may give the impression that muscle is transforming into fat. But this is a misunderstanding of how the body works.
Fat and muscle are two distinct tissues with different cellular makeups. Muscle comes in three forms: skeletal, cardiac (heart), and smooth (mostly found in the intestine). Skeletal muscle, which is attached to bones by tendons, is the type of muscle that we typically think of in relation to body composition. It allows for the voluntary movement of the body.
To lose weight, one must achieve a calorie deficit by consuming fewer calories than the body requires daily, increasing physical activity to burn calories, or a combination of both. However, a too-large calorie deficit can lead to rapid muscle mass loss, as the body will break down muscle to be used as an emergency fuel source. Therefore, it is recommended to maintain a moderate calorie deficit and engage in strength training several times a week to preserve muscle mass during weight loss.
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Smooth muscle is involuntary
Smooth muscle is one of the three major types of vertebrate muscle tissue, the other two being skeletal and cardiac muscle. Smooth muscle is found throughout the body and serves a variety of functions. It is present in the stomach, intestines, bladder, uterus, and the walls of blood and lymph vessels. Smooth muscle also exists in the tracts of the respiratory, urinary, and reproductive systems, as well as in the eyes.
Smooth muscle is unique in that it is involuntary. This means that it can be contracted and controlled without conscious thought. For example, a person does not need to actively think about their blood pressure for their body to adjust it according to the oxygen demands of exercise. The nervous system uses smooth muscle to regulate many of the body's subsystems, ensuring the body's basic functions are maintained.
Smooth muscle can be divided into two main types: single-unit and multi-unit. Most smooth muscle is of the single-unit type, found in the walls of internal organs, blood vessels (except large elastic arteries), the urinary tract, and the digestive tract. In single-unit smooth muscle, a single cell in a bundle is innervated by an autonomic nerve fiber, allowing an action potential to be propagated through neighbouring muscle cells due to the presence of gap junctions. This results in a coordinated contraction or relaxation of the entire muscle.
Multi-unit smooth muscle, on the other hand, is found in the trachea, the iris of the eye, and lining the large elastic arteries. However, it is important to note that these terms are oversimplifications as smooth muscles are typically influenced by a combination of different neural elements. Smooth muscle also plays a significant role in the disease process throughout the body, with treatments for conditions such as asthma, gastric issues, and ischemic heart disease targeting the modification of signalling pathways that affect smooth muscle.
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Cardiac muscle is also involuntary
Cardiac muscle, also known as myocardium or heart muscle, is one of three types of muscle tissues in the body, the others being skeletal and smooth muscle. It is an involuntary, striated muscle that forms the heart's walls and keeps the heart pumping blood around the body. The cardiac muscle is composed of individual cardiac muscle cells or cardiomyocytes, joined by intercalated discs and encased in collagen fibres and other substances that form the extracellular matrix.
Cardiac muscle cells are the contracting cells that allow the heart to pump blood. Each cell needs to contract in coordination with its neighbouring cells, working together to efficiently pump blood from the heart. If this coordination breaks down, the heart may not pump at all, as can occur during abnormal heart rhythms such as ventricular fibrillation. The Purkinje fibres rapidly conduct electrical signals, and coronary arteries bring nutrients to the muscle cells, while veins and a capillary network remove waste products.
The sheets of muscle that wrap around the left ventricle closest to the endocardium are oriented perpendicularly to those closest to the epicardium. When these sheets contract in a coordinated manner, they allow the ventricle to squeeze in several directions simultaneously – longitudinally (becoming shorter from apex to base), radially (becoming narrower from side to side), and with a twisting motion (similar to wringing out a damp cloth) to squeeze the maximum possible amount of blood out of the heart with each heartbeat.
Contracting heart muscle uses a lot of energy and requires a constant flow of blood to provide oxygen and nutrients. Blood is brought to the myocardium by the coronary arteries, which originate from the aortic root and lie on the outer or epicardial surface of the heart. Cardiac muscle tissue gets its strength and flexibility from its interconnected cardiac muscle cells, or fibres. Most cardiac muscle cells contain one nucleus, but some have two. The nucleus houses all of the cell’s genetic material.
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Frequently asked questions
Yes, muscles attach to the skin. Facial muscles, for example, are striated muscles that link the facial skin to the skull bone. Many facial muscles attach under the skin, allowing for various facial expressions.
The human face has around 30 muscles on each side. Some examples of facial muscles include the orbicularis oculi, nasalis, depressor septi, and dilator naris.
There are three main types of muscles in the human body: skeletal, smooth, and cardiac.
Skeletal muscles are voluntary muscles that you can control. They are attached to bones via tendons, which are made of tough tissue.
Smooth and cardiac muscles are involuntary muscles, meaning they are not under conscious control. Smooth muscles are found within the walls of internal organs, while cardiac muscles are specialized muscles found only within the heart.
