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The human body has over 600 muscles, each with at least one origin and insertion point. The origin is the end of the muscle that attaches to the stable base (the proximal bone) and does not move when the muscle is contracted. The insertion point, on the other hand, attaches to the moving bone (the distal bone) and moves when the muscle contracts. For example, the biceps have two origin points that lead to two heads, which merge and finish with one insertion point. The triceps brachii, meanwhile, has four points of attachment: one insertion on the ulna and three origins on the humerus and scapula. The names of skeletal muscles are based on various factors, including their location, origin and insertion, number of origins, shape, size, direction, and function.
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What You'll Learn
Muscle attachments and actions
The human body has over 600 muscles, and learning about the muscular system involves memorising details about each muscle, including their attachments and joint motions. The origin is the end of the muscle that attaches to the stable base or proximal bone, which does not move when the muscle is contracted. The insertion is the end of the muscle that attaches to the moving or distal bone. The belly of the muscle is the fleshy part in between the tendons that contracts.
Muscles move by shortening their length, pulling on tendons, and moving bones closer to each other. The prime mover, or agonist, is the muscle that provides the primary force driving the action. An antagonist muscle resists or reverses this action. Prime movers and antagonists are often paired on opposite sides of a joint. For example, the triceps brachii and anconeus are muscles that extend the elbow, while the biceps brachii, brachialis, and brachioradialis flex the elbow.
Some muscles are named based on their connection to a stationary bone (origin) and a moving bone (insertion). For example, the sternocleidomastoid connects the sternum and clavicle to the mastoid process of the skull. Some muscles connect to more than one bone or to more than one place on a bone, and therefore have multiple origins and insertions. A muscle with two origins is a biceps, and a muscle with three origins is a triceps.
The actions of certain muscles are dependent on the position of the body. For example, the piriformis muscle laterally or externally rotates the hip when in anatomical position, but when the hip joint is flexed more than 90 degrees, the piriformis acts to internally rotate the hip.
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Types of muscle tissue
There are three types of muscle tissue in the human body: skeletal, smooth, and cardiac.
Skeletal muscle is a specialised tissue attached to bones that allows movement. Together, skeletal muscles and bones are called the musculoskeletal system, or locomotor system. Skeletal muscles are broadly classified into two fibre types: Type I (slow-twitch) and Type II (fast-twitch). Skeletal muscles are voluntary muscles, meaning they are under our conscious control. They are also known as striated muscles, as the tissue appears striped when viewed under a microscope.
Smooth muscle is located in the walls of organs and structures such as the oesophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin. It is non-striated and involuntary, meaning its motion happens without conscious awareness.
Cardiac muscle, also known as myocardium, is a special type of muscle tissue that makes up the middle layers of the heart. It is an involuntary, striated muscle that encloses the chambers of the heart. The cardiac muscle also contains specialised cardiac pacemaker cells that allow for cardiac tissue to depolarize without external stimuli.
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Muscle contractions
There are three types of muscles in mammals: skeletal, cardiac, and smooth. Skeletal muscles are attached to bones and provide the body with structure and strength. Cardiac muscles make up the heart's walls, allowing blood to be pumped through the vasculature. Smooth muscles are found in the blood vessels, gastrointestinal tract, bronchioles, uterus, and bladder.
Concentric contractions occur when muscle tension is sufficient to overcome the load, resulting in the muscle shortening as it contracts. This typically happens when lifting something heavy. An example is picking up a heavy box, where your arm muscles contract to hold the weight, and your leg muscles tighten as you stand up with the additional weight.
Eccentric contractions, on the other hand, occur when a muscle is actively lengthened during normal activities like walking. When you lower a heavy object, your muscle remains contracted but lengthens to manage and shift the weight.
The complex process of muscle contraction, known as excitation-contraction coupling, begins with an action potential causing depolarization in the myocyte membrane. This depolarization spreads via transverse (T) tubules, leading to conformational changes in dihydropyridine receptors and subsequent calcium release from the sarcoplasmic reticulum. Calcium binds to troponin C, causing a shift in tropomyosin and allowing myosin heads to attach to actin filaments, forming cross-bridges. ATP hydrolysis and subsequent conformational changes in the myosin heads lead to the breaking of these cross-bridges, resulting in muscle contraction.
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Muscle innervation
The number of nerve fibres that innervate a muscle is smaller than the number of muscle fibres. These nerve fibres branch out and can innervate several muscle fibres. The nerve and the group of muscle fibres that are innervated together are called a motor unit. The size of these motor units varies; for example, a motor unit in the eye may contain around seven muscle fibres, while a motor unit in the leg may contain over 1000.
The spinal cord plays a crucial role in muscle innervation. Upon exiting the spinal column, the anterior and posterior spinal nerve roots combine to form mixed spinal nerves, which contain both motor and sensory nerve fibres. These mixed spinal nerves then divide into posterior and anterior rami, which also contain both types of nerve fibres. Peripheral motor nerves can contain up to 40% sensory nerve fibres, while peripheral sensory nerves contain motor nerve fibres that innervate smooth muscles.
The autonomic nervous system (ANS) innervates smooth muscle organs and tissues, while the skeletal motor system innervates striated muscles. The abdominal muscles, in particular, are innervated by the anterior rami of the thoracic and lumbar spinal nerves. Different studies have provided varying descriptions of abdominal muscle innervation, which may be due to individual variability, anatomical features of the spine, or developmental factors.
Damage to muscles can result in denervation, which impairs muscle development and regeneration. This can lead to muscle atrophy and the loss of functional properties. However, recent studies have explored methods to promote the maintenance and regrowth of damaged axons in ischemic muscles, highlighting the importance of innervation for muscle health and function.
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Muscle names and locations
The human body has over 600 muscles, and memorising the muscular system involves learning details about each muscle, such as attachments and joint motions. Skeletal muscles are named based on various factors, including their location, origin and insertion, number of origins, shape, size, direction, and function. Many muscles derive their names from their anatomical region. For example, the rectus abdominis and transverse abdominis are found in the abdominal region. Some muscles are named after the part of the bone to which they are attached, such as the tibialis anterior, which is named after the anterior portion of the tibia.
Other muscles are named based on a hybrid of these factors, such as the brachioradialis, which is named after a region (brachial) and a bone (radius). Some muscles are also named based on their connection to a stationary bone (origin) and a moving bone (insertion). For example, the sternocleidomastoid connects the sternum and clavicle to the mastoid process of the skull. The occipitofrontalis connects the occipital bone to the frontal bone. Some muscles have multiple origin points, such as the Serratus Anterior, which attaches to the superior 8-9 ribs. Other muscles have one origin and one insertion, such as the Semitendinosus, one of the hamstring muscles. The bicep has two origin points leading to two "heads", which merge and finish with one insertion. The triceps brachii has four points of attachment: one insertion on the ulna and three origins (two on the humerus and one on the scapula).
The size of the muscle can also be used to distinguish between muscles in the same region. For example, in the gluteal region, the gluteus maximus is the largest muscle, the gluteus medius is medium-sized, and the gluteus minimus is the smallest. The direction in which the muscle fibres run can also be used to identify a muscle. In the abdominal region, the rectus abdominis runs straight up and down, the transverse abdominis runs transversely, and the obliques run at an angle.
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Frequently asked questions
Insertion refers to the end of a muscle that attaches to the moving bone when the muscle is contracted.
Origin points are the attachment points on the stable base, which does not move when the muscle is contracted. The insertion point is the attachment point on the moving bone.
Learning the origin and insertion points of muscles can be done by memorising the details of each muscle, including its attachments and joint motions. A useful memory tool is to say that the origin observes and the insertion inches closer.
An example of a muscle insertion point is the bicep brachii, where the insertion point is on the forearm, and the origin point is in the shoulder. Another example is the bicep femoris, where the insertion point is on the tibia and fibula, and the origin point is on the ischium and femur.
