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Bones, muscles, and joints make up the musculoskeletal system, which gives our bodies shape, structure, and support, and enables us to move. Bones are strong tissues made of collagen and calcium phosphate, and they are connected by joints, which allow for movement. Muscles are attached to bones by tendons and help control our movements. When a muscle contracts, it pulls a bone toward another across a joint. However, muscles can only contract and cannot push bones back to their original position, so they work in pairs of flexors and extensors.
| Characteristics | Values |
|---|---|
| How muscles move bones | Muscles contract and relax to move bones. |
| How muscles are attached to bones | Tendons attach skeletal muscles to bones. |
| Types of muscle attachments | Origin and insertion. |
| Types of muscles | Skeletal, smooth, and cardiac. |
| Types of joints | Synovial, cartilaginous. |
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What You'll Learn
Tendons and skeletal muscles
The musculoskeletal system comprises bones, skeletal muscles, joints, cartilage, and tendons, all working together to support the body and enable movement. Bones give the body shape, protect organs, and store minerals like calcium and phosphorus. They are strong tissues made of collagen and calcium phosphate. The spine, for instance, helps us walk, move, twist, and bend.
Skeletal muscles are the most common type of muscle in the body. They are voluntary muscles, meaning we control how and when they work. They are attached to bones by tendons, which are made of strong fibrous connective tissue. When we want to move, the brain signals a muscle to contract, and it pulls one bone towards another across a joint. Skeletal muscles work in pairs, with one muscle contracting and the other lengthening. For example, when the bicep contracts, the tricep lengthens, and vice versa.
Tendons serve as "mechanical bridges," transmitting muscle forces to the bones and joints. They are present throughout the entire length of a muscle, and their activity is influenced by the muscle, and vice versa. Tendons are classified as dense, regular connective tissues, integrating bones and skeletal muscles into functional units. Tendons and ligaments are essential for movement, positioning bones correctly, and ensuring skeletal muscles contract at the appropriate time.
Tendons adapt to their mechanical environment, and their ability to adapt changes with age. Mechanical tension from muscle contraction and relaxation increases collagen synthesis and tendon diameter. Aging alters the cellular structure of tendons, reducing their capacity for regeneration and their effectiveness in directing muscle forces toward bone tissue.
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How muscles contract
The musculoskeletal system, comprising bones, muscles, and joints, enables us to perform everyday physical activities. Bones provide the body with shape and structure, protect internal organs, and store minerals like calcium. Muscles, on the other hand, help control movements and certain body systems, such as digestion. They are attached to bones by tendons, which are made of strong fibrous connective tissue. When a muscle contracts, it shortens, pulling one bone towards another across a joint.
Skeletal muscles, the most common type of muscle in the body, are voluntary muscles that we can consciously control. They are responsible for various functions, including chewing and swallowing, expanding and contracting the chest cavity for breathing, and regulating blood sugar levels. When a skeletal muscle contracts, it works in pairs with another muscle. For example, when the bicep flexor muscle in the front of the upper arm contracts, the tricep extensor muscle at the back lengthens, allowing the elbow to bend.
Smooth muscles, on the other hand, are involuntary muscles that we cannot consciously control. They are found in the walls of organs like the stomach, intestines, and blood vessels, and are responsible for functions like breaking down food and moving it through the digestive system, as well as regulating blood pressure.
Cardiac muscle is a special type of muscle found in the heart. Like smooth muscle, it is also involuntary and contracts to make the heart beat.
The process of muscle contraction involves several key steps, including the Sliding Filament Theory, which explains how muscle length changes during contraction. Muscle cells contain a striped pattern formed by basic units called sarcomeres, which are composed of actin and myosin filaments. The interaction between these filaments leads to muscle contraction, with the energy for this process supplied by the hydrolysis of ATP. Calcium is also required for muscle contraction, as it regulates the binding of myosin to actin filaments.
There are different types of muscle contractions, including isometric, isotonic, concentric, and eccentric contractions. Isometric contraction generates tension without changing the muscle length, such as when gripping an object. In isotonic contraction, the tension remains constant despite changes in muscle length, while in concentric contraction, the muscle tension overcomes the load, causing the muscle to shorten. Eccentric contraction involves the muscle fibres lengthening as they contract, acting to decelerate the joint rather than pull it towards the contraction.
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Types of joints
Bones, muscles, and joints form the musculoskeletal system, which gives the body its structure and support, enables movement, and protects internal organs. Joints are the points in the body where two or more bones meet. They can be classified in various ways, including structural and functional classification, as well as by their range of motion and the type of tissue that holds the bones together.
Structural Classification
According to structural classification, there are three types of joints: fibrous or fixed joints, synovial or freely movable joints, and cartilaginous or slightly movable joints.
Fibrous or Fixed Joints
Fibrous joints are found only in three areas of the body: the skull, the distal tibiofibular and cuboideonavicular (cuboid and navicular bones) joints, and the mouth. In fibrous joints, bones are connected by fibrous connective tissue, and their motion is negligible. The skull, for instance, is made up of multiple fused bones.
Synovial or Freely Movable Joints
Synovial joints are the most common type of joint and are found in the arms and legs. They are characterised by the presence of a joint cavity filled with synovial fluid, which lubricates the cartilage covering the ends of the bones. Ligaments connect the bones and help stabilise the joint, allowing movement in certain directions. Gliding joints, hinge joints, pivot joints, and ball-and-socket joints are all types of synovial joints.
Cartilaginous or Slightly Moveable Joints
Cartilaginous joints are partly movable and are made up of either symphysis or synchondrosis joints. Symphysis joints, found primarily along the midline of the body, feature adjacent bone surfaces lined with hyaline cartilage and connected by fibrous tissue. Intervertebral joints and the pubic symphysis are examples of symphysis joints. Synchondrosis joints, on the other hand, are temporary cartilaginous joints present in young children until puberty.
Functional Classification
The functional classification of joints includes uniaxial and biaxial joints, which can be further categorised based on the movements they allow.
Uniaxial Joints
Elbows and knees are examples of uniaxial joints, specifically hinge joints, which permit flexion and extension movements.
Biaxial Joints
Saddle joints, a type of biaxial joint, allow movement in two planes: flexion/extension and abduction/adduction. The thumb is the only bone in the human body with a saddle joint.
Other types of joints include condyloid joints, which allow up-down and side-to-side movements, and are found in the index finger, wrist, and elbow.
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Muscle movement and the brain
The brain is the command centre of the body, controlling thought, memory, emotion, touch, motor skills, vision, respiration, and every process that regulates the body. The musculoskeletal system, which includes bones, muscles, joints, cartilage, tendons and ligaments, gives the body its structure and support, and enables us to move around.
The neuromuscular system connects muscles and nerves, which control body movements and functions. Nerves called motor neurons send messages from the brain to muscles, making them contract and move. When we want to move, the brain tells a muscle to contract, and it shortens, pulling one bone towards another across a joint. Bones, muscles and joints work together to enable movement. For example, when you bend your arm, your bicep contracts and your tricep lengthens. Muscles work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint, and when the movement is completed, the flexor relaxes and the extensor contracts to straighten the limb at the same joint.
Messages are sent from the brain to the muscles via electrical impulses. These messages are carried by neurons, which work together in complex rhythms to signal to muscles when and where to move. Each motor neuron ending sits very close to a muscle fibre, at a point called a neuromuscular junction. The motor neuron releases a chemical, which is picked up by the muscle fibre, signalling it to contract.
The brain's outer portion contains neurons, and the inner area communicates with the cerebral cortex. The cerebellum's function is to coordinate voluntary muscle movements and to maintain posture, balance and equilibrium.
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Smooth muscles
In the skin, smooth muscle cells such as arrector pili cause the hair to stand erect, resulting in goosebumps. Smooth muscles in the eyes, specifically the ciliary muscles, iris dilator muscle, and iris sphincter muscle, control how the eyes focus and dilate or constrict the pupils. Smooth muscles are also found in the uterus, where they contract during labour. Additionally, smooth muscles are involved in the respiratory and reproductive systems, as well as in the regulation of toxins and electrolyte balance in the urinary system.
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Frequently asked questions
Muscles move bones by contracting and then relaxing. When a muscle contracts, it pulls a bone towards another across a joint.
Joints are places where two bones meet. They make the skeleton flexible and allow our bodies to move in many ways. Synovial joints, for example, are fluid-filled joints that allow for the most movement.
Synovial joints are found in the arms and legs. The shoulder and knee are typical examples of synovial joints that show how muscles move bones.
Muscles work in pairs of flexors and extensors. The flexor contracts to bend a limb at a joint. Then, the flexor relaxes and the extensor contracts to extend or straighten the limb at the same joint.
Skeletal muscles are the most common type of muscles in the body. They are attached to the skeleton by tendons and cause bones to move when they contract.
