What are Two-Jointed Muscles?
Two-jointed muscles are the most common type of muscular contractile unit in human skeletal muscle tissue. These units consist of two or more distinct types of muscle fibers, each with its own motor fiber(s) and tendon(es). Each type of muscle fiber is called a myotendinous, meaning it contains both myofibrils (myosin filaments) and actin filaments. Myotendinous muscle fibers are the largest type of muscle fiber. Muscle fibers have a cross sectional area (the total surface area divided into equal parts by perpendicular planes) of approximately 10 square micrometers. Muscle cells contain between 100,000 and 200,000 myonuclei per milliliter of myofiber. There are several different types of myocytes in skeletal muscle tissue. Myocytes are specialized nuclei found only in myofibers. Myocyte number varies from cell to cell within a single muscle fiber. The amount of myonuclei per milliliter of muscle fibers increases with age and is higher in older individuals than younger ones.
The term “muscle” is often used to describe the entire muscular system: skeletal, smooth, and cardiac muscle. Muscle cells are highly specialized and are able to contract or shorten. This shortening happens when the myosin heads attach to the actin filaments and pulls them toward the center of the sarcomere. It is important to note that skeletal muscle fibers are striated.
This means that myofibrils are arranged in a specific pattern creating alternating light and dark bands. These bands are called sarcomeres. Recently, several studies have shown that human skeletal muscle tissue contains more than 200 different types of muscle fibers! Muscle cells are also surrounded by a basal lamina, an intercellular material composed of collagen and glycoproteins. Hormones bind to receptors on the cell membrane of a cell. The alternating banding that one sees under the microscope is referred to as striations or stripes. Striations are important because they play a major role in the sliding filament model of muscular contraction.
Most skeletal muscle is made up of two kinds of muscle fibers: Type I and Type II. Each has its own distinct features and physiological properties. The Type I (slow oxidative) fiber has a redder color and many blood vessels. The cell membrane, or plasma membrane, is a phospholipid bilayer that surrounds the entire muscle cell.
The contractile part of a muscle cell is the myofibril. It is surrounded by the sarcoplasm.
Skeletal muscles are able to contract and shorten. They are attached to bone by a tendon. There are several types of skeletal muscles:
Unitary Skeletal Muscle- These are the smallest muscles that are found within the body. Its oxidative characteristics support the higher mitochondria content, larger size, and abundant blood vessels of this type. The Type I (slow oxidative) fiber makes up about 55% of all skeletal muscle fibers. The type I (fast oxidative glycolytic) fiber is sometimes referred to as the fast twitch oxidative or red fiber.
It is able to sustain a moderate pace of activity for an extended period of time (upwards of 30 minutes). This fiber uses oxygen and glucose (blood sugar) as its primary energy source. It has an abundance of myoglobin, which is an oxygen-binding protein. Myoglobin is responsible for the red coloring in muscles, giving them a reddish color. The Type IIa (fast glycolytic) fiber is sometimes referred to as the fast twitch glycolytic or white fiber.
• Striated: The skeletal muscle cells are firm and have a striped appearance. This is due to the dense zigzag of the myofibrils (myo=muscle, fibril=small fiber).
Fascicle: Aggregates of muscle fibers which lie parallel to each other
Muscle fiber: A single unit of skeletal muscle. Each fiber is formed by bundles of myofibrils. This fiber uses glycogen and glucose as its primary energy source. It can only sustain a maximum pace of activity for about 10 minutes.
This fiber has a moderate amount of myoglobin. The Type IIx (intermediate) is sometimes referred to as the fast twitch oxidative glycolytic or intermediate fiber). This fiber can sustain a moderate pace of activity for an extended period of time (upwards of 30 minutes). This fiber uses oxygen and glucose as its primary energy source. It has a moderate amount of myoglobin.
Tendon: The thick band of white material that connects a muscle to the bone. It is surrounded by a dense net work of collagen fibers.
Skeletal muscles are attached to bones by tendons. There are three different types of skeletal muscles: striped, unstriated, and heart muscles. All skeletal muscles have the following features:
Joints (or articulations) are places where two or more bones meet. The main types of joints are the following:
Fibrous or joint capsules are connective tissue membranes in a joint that bind the opposing bones together. They are composed of dense irregular connective tissue, collagen fibers, and fibroblasts (cells that produce collagen fibers). These capsules contain synovial fluid, a viscous transparent fluid that reduces friction during movements.
Synovial joints are freely moveable. Most of the joints in the body are of this type. They allow for large movements and are usually fairly mobile. The bones are joined by fibrous capsules lined with a smooth membrane containing synovial fluid.
Synarthroses (immovable joints) are very firm and fixed joints. They do not have any movable parts. The only skeletal joint of this type is the pubic symphysis (or at least, the only one that is immovable in adults).
Amphiarthroses (semifreely movable joints) have a combination of fibrous and cartilage or cartilage and bone. These joints allow for some movement. The bones are connected by a thin layer of fibrocartilage or hyaline cartilage.
There are three basic types of joints: synarthroses, amphiarthroses, and diathroses.
Growth in length is called longitudinal growth and growth in width is called transverse growth. Longitudinal growth occurs at the epiphyseal plates at the ends of the tubular bones (e.g. the bones of your limbs).
Transverse growth occurs at the ends of the long bones(e.g. the head of your radius) and at the center of the flat bones (e.g. the sternum).
Bone is living tissue that is constantly changing in response to forces placed on it. Certain bones keep growing until you are about 25 years old.
Bones do not contain blood vessels or nerves but are supplied by nearby blood vessels and nerves. The bone cells known as osteocytes live inside the bone and respond to a break in the bone by sending out a chemical signal to start healing the break. Osteoblasts and osteoclasts have already begun the process of healing a broken bone before you even feel the pain. The area around a broken bone ends up being very thin due to the process of modeling.
Bone is very hard and brittle therefore, in order for it to be flexible and strong, it contains a large amount of collagen fibers in its matrix. Collagen fibers are arranged in parallel bundles that run in all directions. These fibers make bone very strong in all directions.
Long bones are cylindrical, have a medullary cavity inside (this is where red bone marrow is found), and contain blood vessels and nerves inside them. The main function of the flat bones (e.g. the skull) is for protection of certain organs(e.g.
In this lesson, you will learn about the structure of bone, its function, and a few common diseases.
The largest organ in the body to most people is actually the skin. To other people, it’s the liver, heart, or brain. But to medical students it is the skeleton.
Because you have to know it inside and out.
This lesson will cover:
Functions of the Skeleton
Common Diseases of the Skeleton
Structures closely related to the Skeleton (i.e. muscle, cartilage, ligaments, and fasciae)
The human skeleton is made up of two main parts: the axial skeleton and the appendicular skeleton. The axial skeleton consists of 80 bones and is the central structural support for the body. It also protects the brain and spinal cord. The appendicular skeleton consists of 126 bones and includes the shoulders, hips, limbs, and girdles for movement at these joints.
Bone growth occurs in two ways. Intramembranous ossification is the process by which mesenchymal tissue is converted into bone. This type of bone formation takes place in flat bones of the skull, the clavicles, and the ribs. Endochondral ossification involves the conversion of pre-existing cartilage into bone.
Cartilage is the flexible but durable substance that lines the epiphyses of long bones. Both of these processes begin in the mesenchyme tissue.
The flat bones of the skull, clavicles, and ribs all are formed by intramembranous ossification. These are the bones that are not contained within a joint or a limb. They form initially from mesenchymal cells which then turn into connective tissue and eventually into bone. After this process occurs, the bone is covered by an layer of connective tissue, the periosteum.
The periosteum contains nerves and blood vessels which help in the growth and development of the bone.
The periosteum is divided into two layers: the outer fibrous layer and the inner cambium layer. The fibrous layer contains longitudinal fibers aligned in the same direction as the bone. These fibers merge with collagen fibers in the developing bone.
Sources & references used in this article:
Spasticity & Gait Dysfunction in MS by A Shah – Multiple Sclerosis for the Physician Assistant – researchgate.net
Effects of long-term hypergravity on muscle, heart and lung structure of mice by M Frey, R von Känel-Christen… – Journal of Comparative …, 1997 – Springer
Walking exercise combined with neuromuscular electrical stimulation of antagonist resistance improved muscle strength and physical function for elderly people: A … by R Hashida, H Matsuse, Y Takano, M Omoto… – The Journal of …, 2016 – jstage.jst.go.jp
Exercise control system by C Eckler – US Patent 5,011,142, 1991 – Google Patents
A neural network model rapidly learning gains and gating of reflexes necessary to adapt to an arm’s dynamics by KT Kalveram – Biological Cybernetics, 1992 – Springer
Joint Pain and Weather: 4 Tips to Ease Discomfort by M Halpern – breakingmuscle.com
A neural-network model enabling sensorimotor learning: Application to the control of arm movements and some implications for speech-motor control and stuttering by KT Kalveram – Psychological research, 1993 – Springer