Dominic Stone
Bones and muscles are both essential components of the human body, each serving critical functions. Bones provide the structural framework for the body, supporting and protecting our organs, while muscles facilitate movement and enable essential functions like chewing, digestion, and communication. The musculoskeletal system, comprising bones, muscles, and cartilage, develops through complex processes, with bones and muscles each deriving from distinct types of tissue. Understanding the development and interplay between bones and muscles is crucial in comprehending the body's remarkable capacity for movement and function.
| Characteristics | Values |
|---|---|
| Bones | Provide support for our bodies and help form our shape |
| Protect our organs | |
| Bones are made up of a framework of protein called collagen, with a mineral called calcium phosphate that makes the framework hard and strong | |
| Bones store calcium and release some into the bloodstream when it's needed by other parts of the body | |
| Muscles | Help the body with everyday movements |
| Help the body with chewing food and moving it through the digestive system | |
| Help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood | |
| Help us communicate | |
| Help us stay physically fit and healthy |
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What You'll Learn
Bones are formed through a process of ossification
Bones are formed through a process called ossification, which is a synonym for osteogenesis. This process begins between the sixth and seventh weeks of embryonic development and continues until about the age of twenty-five, though this may vary between individuals.
There are two types of bone ossification: intramembranous and endochondral. Both processes begin with a mesenchymal tissue precursor, but the transformation into bone differs. Intramembranous ossification directly converts the mesenchymal tissue to bone and forms the flat bones of the skull, clavicle, and most of the cranial bones. This process begins when mesenchymal cells in the embryonic skeleton gather together and begin to differentiate into specialized cells called osteoblasts. These osteoblasts group into clusters and form an ossification center. They then begin secreting osteoid, an unmineralized collagen-proteoglycan matrix that can bind calcium. The binding of calcium to osteoid results in the hardening of the matrix and the transformation of osteoblasts to osteocytes. These vessels will eventually form the red bone marrow.
Endochondral ossification, which forms the remainder of the axial skeleton and the long bones, takes much longer than intramembranous ossification. This process begins with mesenchymal tissue transforming into a cartilage intermediate, which is later replaced by bone. In a long bone, for example, some of the mesenchymal cells differentiate into chondroblasts (cartilage cells) that form the hyaline cartilaginous skeletal precursor of the bones. This cartilage is a flexible, semi-solid matrix produced by chondroblasts and consists of hyaluronic acid, chondroitin sulfate, collagen fibers, and water. As the matrix surrounds and isolates chondroblasts, they are called chondrocytes. Blood vessels then infiltrate the cartilage, changing it into a periosteum. The osteoblasts form a collar of compact bone around the diaphysis, and the cartilage in the center begins to disintegrate. Osteoblasts penetrate the disintegrating cartilage and replace it with spongy bone, forming a primary ossification center. Ossification continues from this center toward the ends of the bones. After spongy bone is formed in the diaphysis, osteoclasts break down the newly formed bone to open up the medullary cavity.
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The skull is formed differently to the rest of the skeleton
Bones always come before muscles. The first stage of any type of bone formation involves a mesenchymal condensation, where cells become densely packed together. From this point, osteogenesis occurs through intramembranous or endochondral ossification. Myotome cells then differentiate into myoblasts, forming skeletal muscle.
The skull is formed differently from the rest of the skeleton. The skull is composed of 22 bones, divided into two regions: the neurocranium and the viscerocranium. The neurocranium, or braincase, forms a protective cranial cavity that surrounds and houses the brain and brainstem. The viscerocranium, or facial skeleton, forms the skeleton of the face and includes the mandible. The skull roof bones, including the bones of the facial skeleton and the sides and roof of the neurocranium, are dermal bones formed by intramembranous ossification. The endocranium, the bones supporting the brain, are largely formed by endochondral ossification. The frontal and parietal bones are purely membranous.
The skull is a complex structure, and its bones are formed through both intramembranous and endochondral ossification. The bones that make up the skull include cranial bones, facial bones, and ossicles, which are made up of a number of fused flat and irregular bones. The cranial bones are joined at firm fibrous junctions called sutures, which are synarthrodial (immovable) joints formed by bony ossification. The skull base allows the passage of various neurovascular structures and is composed of the sphenoid and ethmoid bones, as well as parts of the frontal, temporal, and occipital bones.
The neurocranium itself has two parts: the membranous part that surrounds the brain as a vault, and the cartilaginous part (chondrocranium) that forms the base of the skull. The facial skeleton is formed by the bones supporting the face, including the mandible, the largest bone in the facial skeleton. The zygomatic arch is a bridge of bone that spans from the area of the cheek to just above the ear canal and is formed by the junction of the zygomatic bone (the cheekbone) and the temporal bone.
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Muscles are attached to bones by tendons
Bones form the structure of the human body, and muscles are attached to these bones by tendons. Tendons are tough, flexible bands of fibrous tissue that act as intermediaries between muscles and bones. They enable movement by pulling on the bones.
The musculoskeletal system is a complex network of bones, muscles, tendons, and other connective tissues that work together to provide structure, support, and mobility to the human body. The development of the musculoskeletal system begins in the early stages of embryonic development and continues throughout childhood and adolescence.
The first stage of bone formation involves a process called mesenchymal condensation, where cells become densely packed together. This is followed by either intramembranous ossification or endochondral ossification. During skeletal muscle development, myotome cells differentiate into myoblasts, which then form skeletal muscles.
Tendons play a crucial role in the musculoskeletal system by attaching muscles to bones. They are composed of strong connective tissue, primarily made of collagen fibers, creating tough bands that can withstand tension. Tendons are flexible, allowing for a range of motion at the joints. They also have some elasticity, enabling them to stretch and lengthen over time, thus increasing overall flexibility.
Injuries to tendons, such as tears or tendonitis, can occur, especially in areas like the Achilles tendon. Therefore, it is important to maintain muscle and tendon health through proper stretching and exercise.
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Bones provide support and protection for the body
Bones provide the body with a frame that is both light and strong. They are made up of a framework of collagen, a protein that is hardened and strengthened by calcium phosphate. This composition makes bones strong enough to support our entire weight, while also being very light. The skeletal system, formed by bones, gives our bodies shape and helps us move. It also protects our vital organs from injury. For example, the skull protects the brain and forms the shape of the face, the spinal cord is protected by the backbone, and the heart and lungs are sheltered by the ribs. The pelvis also helps protect the bladder, part of the intestines, and in women, the reproductive organs.
The skeleton is remarkably adapted to provide adequate strength and mobility so that bones do not break when subjected to substantial impact or the loads placed on them during vigorous physical activity. The shape or structure of a bone is at least as important as its mass in providing this strength. Bones are hollow, with an outer dense shell called cortical bone, which makes up about three-quarters of the total skeletal mass. The cortical shell is essential for providing strength and protection without excessive weight.
The inner network of trabecular bone, which makes up the remaining one-quarter of skeletal mass, has two important functions. Firstly, it provides a large bone surface for mineral exchange, and secondly, it helps maintain skeletal strength and integrity, especially in areas under continuous stress from motion and weight-bearing, such as the spine and the ends of long bones.
In addition to their structural and protective roles, bones also play a crucial role in blood cell production, the immune system, the storage of minerals like calcium and phosphorus, the release of essential hormones, and many other functions. Bone marrow, found inside many bones, is responsible for producing most of the body's red blood cells, white blood cells, and platelets. Bones are also important for storing minerals, particularly calcium and phosphorus, which are vital for the functioning of other body systems.
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Smooth muscle is controlled by the nervous system
The development of bones and muscles is a complex process. The first stage of bone formation involves a mesenchymal condensation, where cells become densely packed together. This is followed by either intramembranous ossification or endochondral ossification. The former involves mesenchymal cells ensheathed in membranous tissue directly undergoing ossification, while the latter involves these cells first differentiating into cartilage models before ossification. The skull, for example, is formed through both processes, while the vertebral column, ribs, and sternum form only through endochondral ossification.
Skeletal muscle, on the other hand, is derived from the mesoderm. The paraxial mesoderm forms a segmented series of tissue blocks on each side of the neural tube, called somites. Cells in the ventromedial part of the somite form the sclerotome, while those in the dorsal part form the dermatome. Cells from the two edges of the somite, the ventrolateral lip and the dorsomedial lip, migrate ventral to the dermatome and proliferate to form muscle cell precursors. Collectively, these structures form the dermomyotome, which will differentiate into dermatome cells and myotome cells. The dermatome cells form the dermis of the back and neck, while the myotome cells form the skeletal muscles.
Now, focusing on smooth muscle, it is found throughout the body and serves a variety of functions. Smooth muscle is present in the stomach, intestines, urinary system, arteries, and veins. It helps with digestion, nutrient collection, toxin removal, electrolyte balance, and blood pressure regulation. Unlike skeletal muscle, smooth muscle can be contracted and controlled involuntarily by the nervous system. This allows the body to adapt to increasing oxygen demands during exercise without conscious thought.
The nervous system uses hormones, neurotransmitters, and other receptors to control smooth muscle spontaneously. The sympathetic, parasympathetic, and enteric nervous systems work together to contract smooth muscle. Sympathetic stimulation of smooth muscle is received from spinal levels T1 to L2, which then routes autonomic nervous supply to organs and tissues. The parasympathetic nervous system, composed of the cranial nerves, vagus nerve, and pelvic splanchnic nerves, regulates specific body portions. For example, the vagus nerve innervates the gastrointestinal tract and sends branches to the heart, larynx, trachea, bronchi, liver, and pancreas.
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Frequently asked questions
Yes, bones come before muscles. Bone is a connective tissue that develops from the differentiation of mesoderm cells. The process of bone formation involves ossification of cartilage formed from mesenchyme. Two main forms of ossification occur in different bones: intramembranous and endochondral. The skull, for example, forms through intramembranous ossification. On the other hand, skeletal muscle is derived from the differentiation of myotome cells into myoblasts, which then fuse to form mutinucleated myotubes.
Bones provide support and help form the shape of our bodies. Although they are lightweight, bones are strong enough to support our entire weight. Additionally, bones protect vital organs. For instance, the skull safeguards the brain and shapes the face, while the spinal cord is protected by the backbone.
Muscles facilitate movement by pulling on the joints. They also assist in various essential functions, such as chewing food and moving it through the digestive system. Even when we are still, muscles throughout our body are constantly active. For example, muscles help the heart beat, enable breathing by allowing the chest to rise and fall, and regulate blood pressure and blood flow by acting on blood vessels.
