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The human body is made up of more than 600 muscles that help us do everything from breathing to lifting heavy objects. There are three types of muscles: skeletal, smooth, and cardiac. Skeletal muscles are attached to the bones and allow us to perform a wide range of movements. Smooth muscles are found in the stomach, digestive system, bladder, and uterus, and they help with functions like pushing food through the digestive tract and regulating the flow of urine. Cardiac muscle is responsible for pumping blood throughout the body and is found only in the heart.
| Characteristics | Values |
|---|---|
| Number of muscles in the human body | 600+ |
| Types of muscles | Skeletal, Smooth, Cardiac |
| Skeletal muscle composition | Bones, tendons, ligaments |
| Skeletal muscle function | Support body weight, enable movement |
| Smooth muscle location | Digestive tract, uterus, blood vessels, liver, pancreas, intestines |
| Smooth muscle function | Contract in waves to enable movement |
| Smooth muscle control | Involuntary |
| Cardiac muscle location | Heart |
| Cardiac muscle function | Pump blood throughout the body |
| Cardiac muscle control | Involuntary |
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What You'll Learn
Skeletal muscles
The brain, nerves and skeletal muscles work together to cause movement, and this is known as the neuromuscular system. Skeletal muscles are serviced by 50-200 branches of specialised nerve cells called motor neurones. These nerve cells plug directly into the skeletal muscle, with the point of contact known as the neuromuscular junction.
Skeletal muscle mass varies from person to person, with males typically having more skeletal muscle mass than females. Additionally, taller individuals and those with overweight tend to have higher muscle mass. Muscle mass decreases with age.
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Cardiac muscles
There are about 600 muscles in the human body, and they can be categorised into three types: skeletal, smooth, and cardiac. Cardiac muscle, also called myocardium or heart muscle, is one of these three major categories of muscles found in the human body. It forms the bulk of the heart, constituting the main tissue of the heart wall.
The heart wall is a three-layered structure, with a thick layer of myocardium sandwiched between the inner endocardium and the outer epicardium (or visceral pericardium). The endocardium is not cardiac muscle, but rather simple squamous epithelial cells that form the inner lining of the heart chambers and valves. The pericardium, on the other hand, is a fibrous sac surrounding the heart, consisting of the epicardium, pericardial space, parietal pericardium, and fibrous pericardium.
Cardiac muscle cells, or cardiomyocytes, are the contractile myocytes of the cardiac muscle. They are striated, branched, and contain many mitochondria. Each cardiomyocyte is surrounded by an extracellular matrix produced by supporting fibroblast cells. These cells are joined together at their ends by intercalated discs to create a functional syncytium of cardiac cells. This allows for the propagation of coordinated action potentials from one cell to the next, enabling the heart to work as a pump.
The primary function of cardiac muscle is to pump blood into circulation by generating sufficient force. This involves the cardiac muscle and electrical impulses, with the contractile functions requiring ATP, which can be obtained through various substrates. The coordinated contraction of the cardiac muscle is vital for pumping blood throughout the cardiovascular system.
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Smooth muscles
Smooth muscle is one of the three main types of muscle in the human body, alongside skeletal and cardiac muscle. Smooth muscle is an involuntary muscle, meaning it works without conscious thought. It is controlled by the autonomic nervous system.
Smooth muscle is found throughout the body and serves a variety of functions. It is found in the stomach, intestines, and bladder, where it helps with digestion and nutrient collection, and in the urinary system, where it helps to rid the body of toxins and maintain electrolyte balance. Smooth muscle is also found in the walls of arteries and veins, where it plays a vital role in regulating blood pressure and tissue oxygenation.
Smooth muscle is involved in passage width control, with some smooth muscles managing the width of passages inside the body. For example, smooth muscle in sphincters, like the bladder or anus, stays flexed and only relaxes when needed. Smooth muscle is also found in the skin, where it causes hairs to stand on end, and in the eyes, where it controls how the pupils dilate or constrict and how the eyes focus.
Smooth muscle can be divided into two subgroups: single-unit and multi-unit smooth muscle. Most smooth muscle is of the single-unit type, which is found in the walls of most internal organs, blood vessels (excluding large elastic arteries), the urinary tract, and the digestive tract. Multi-unit smooth muscle is found in the trachea, in the iris of the eye, and lining the large elastic arteries.
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Voluntary and involuntary muscles
The human body has about 600 muscles that help with everything from breathing and swallowing to giving birth and lifting weights. These muscles can be grouped into three main types: skeletal, smooth, and cardiac.
Skeletal muscles are voluntary muscles that can be consciously controlled. They make up 30% to 40% of human body weight and are attached to bones by tendons. These muscles allow us to perform a wide range of movements and functions. For example, skeletal muscles in your neck, arm, and shoulder are used when reaching for a book on a shelf. Other examples of skeletal muscles include major muscle groups that control the movement of the biceps and triceps in your arms, quadriceps in the legs, or abdominals.
Smooth muscles and cardiac muscles are involuntary muscles. Smooth muscles are found in various internal structures, including the digestive tract, uterus, and blood vessels such as arteries. They are arranged in layered sheets that contract in waves along the length of the structure. Cardiac muscle, also known as myocardium, makes up the middle layers of the heart and is responsible for its rhythmic contractions. It is composed of a specialized type of muscle cell that is only found in the heart.
The brain, nerves, and skeletal muscles work together to cause movement, which is known as the neuromuscular system. Skeletal muscles receive signals from the brain that lead to muscle contraction and movement. Similarly, involuntary muscles are regulated by the autonomic nervous system, which controls the activity of organs and blood vessels needed for essential daily functions, such as digestion and breathing.
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Muscles in the neck
The muscles in the neck are responsible for movement of the head in all directions. They are divided into three or four main groups, depending on the source. The three groups are anterior, lateral, and posterior, based on their position in the neck. The four groups are the suboccipital muscles, the scalene muscles, the anterior group, and the posterior group.
The suboccipital muscles are innervated by the suboccipital nerve and act to rotate the head and extend the neck. They include the rectus capitis posterior major and rectus capitis posterior minor, which attach the inferior nuchal line of the occiput to the C2 and C1 vertebrae, respectively. The obliquus capitis superior extends from the occiput to C1, while the obliquus capitis inferior originates from C2 and attaches to C1.
The three scalene muscles form the floor of the posterior triangle. The anterior and middle scalenes originate from the transverse processes of certain cervical vertebrae and attach to the first rib. The posterior scalene also originates from the cervical spine but attaches to the second rib. Together, the scalenes flex the neck and can be recruited as accessory muscles of respiration.
The anterior neck muscles are a group of muscles covering the anterior aspect of the neck. The superficial muscles in this group include the platysma and sternocleidomastoid. The platysma is a sheet-like muscle that lies within the subcutaneous tissue of the anterior neck, originating from the skin and fascia overlying the region of the clavicle and passing superiorly along the neck. It inserts into several points, including the mandible, the skin of the lower face, the lower lip, and the corners of the mouth. The platysma functions mainly as a muscle of facial expression, contributing to expressions such as sadness by pulling the corners of the mouth downwards. The sternocleidomastoid is a large, two-headed muscle that originates from the medial third of the clavicle (clavicular head) and the manubrium of the sternum (sternal head). The heads come together and ascend diagonally to insert onto the mastoid process of the temporal bone. This muscle is involved in cervical side bending and flexing the neck and raising the sternum to assist in forced inhalation.
The suprahyoid muscles are found superior to the hyoid bone and include the digastric, mylohyoid, geniohyoid, and stylohyoid. The mylohyoid and geniohyoid originate from the internal surface of the mandible, with the former flattening the floor of the mouth and the latter depressing the mandible. The digastric muscle also depresses the mandible. The infrahyoid muscles, found inferior to the hyoid bone, consist of the sternohyoid, omohyoid, sternothyroid, and thyrohyoid.
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Frequently asked questions
Skeletal muscles are found throughout the body, attached to bones by tendons. They make up between 30% and 40% of total body mass. Examples include shoulder muscles, hamstring muscles, abdominal muscles, biceps, and triceps.
Cardiac muscle is found in the heart. It is responsible for pumping blood throughout the body.
Smooth muscle is found in the walls of blood vessels, the urinary bladder, intestines, stomach, eyes, and the female reproductive system. It helps regulate blood pressure and aids in digestion and reproduction.
The sternocleidomastoid, scalene, splenius capitis, and splenius cervicis muscles are all found in the neck.
