Skeletal System and Muscles in Human Body

The main organs and tissues of the musculoskeletal system in humans are bones, cartilages and muscles. These systems provide support and protection to organs; maintain structure of the body; help in movement of organs and limbs; and store nutrients {muscles store glycogen; while bones store calcium and phosphorus}. Further bones also have function of Hematopoiesis {making of blood cells} in bone marrow (mainly within flat bones).

Bone

Bone is a highly mineralized tissue in which connective tissue part is 1/3rd while mineral part is 2/3rd portion. Apart from providing mechanical strength, the bones work as homeostatic reservoir for ions such as calcium, magnesium and phosphorous. Thus, bones have a very important function in acid base balance in the body. There are 270 bones in a new born baby and 206 in an adult human.

Bone as connective Tissue

Formation of bones is called Osteogenesis or Ossification.  Bones are made of three types of specialized cells called osteoblasts, osteocytes and osteoclasts. Osteoblasts make the bone while Osteoclasts break / remodel the osteous tissue. They create canals in bones. Osteocytes provide support.

The intercellular part between Osteocytes is made of casein protein and inorganic phosphates. We note here that bony tissue is highly vascular and has greater regenerative power than any other tissue of the body except Blood. Bones have narrow tubes called Haversian Canals and Volkmann’s Canals apart from the network of blood vessels.

Number of Bones

An adult human has 206 bones while a new born baby has 300 bones. 94 bones fuse as a baby grows. The total number of bones in human skull is 29. The face of a man is made up of 14 bones. Largest and longest bone is femur (thigh bone).The shortest bone in the human body is stapes or stirrup bone in the middle ear.

Cartilage

Cartilage is softer, elastic tissue that makes joints between bones, rib cage, ear, nose, bronchial tubes, intervertebral discs etc.

Muscles

The human body has more than 650 muscles, which make up half of a person’s body weight. Humans have three different kinds of muscle: Skeletal, Involuntary and Cardiac muscles.

Skeletal muscle

These are voluntary muscles that hold the skeleton together, give the body shape, and help it with everyday movements. They are striated because they are made up of fibers that have horizontal stripes when viewed under a microscope. They can contract quickly and powerfully, but they tire easily and have to rest between workouts.

Involuntary or smooth muscle:

It is also made of fibers, but looks smooth, not striated and they’re controlled by the nervous system automatically. Walls of the stomach and intestines, walls of blood vessels are the examples of involuntary muscles. Smooth muscles take longer to contract than skeletal muscles but they can stay contracted for a long time because they don’t tire easily.

Cardiac muscle

It is an involuntary type of muscle found in the heart. Its rhythmic, powerful contractions force blood out of the heart as it beats. Cardiac muscle contraction is totally involuntary, meaning it occurs without nervous stimulation and doesn’t require conscious control.

Sarcomeres

Sarcomeres are the functional units of muscle fibers. Within them, the blocks of actin and myosin molecules are placed in an organized manner. Sarcomeres are the contractile units of muscle tissue, formed of alternating actin blocks (thin filaments) and myosin blocks (thick filaments).

Myoglobin

Myoglobin is a pigment similar to hemoglobin which is present in muscle fibers. Myoglobin has a large affinity to oxygen. It keeps oxygen bound and releases the gas under strenuous muscle work. Therefore, myoglobin acts as an oxygen reserve for muscle cells. If oxygen from hemoglobin or myoglobin is not enough to supply energy to the muscle cells, the cell begins to use lactic fermentation in an attempt to compensate for that deficiency. Lactic fermentation releases lactic acid and this substance causes muscle fatigue and predisposes the muscles to cramps.

Joints

Joints are structures where two bones are attached so that bones can move relative to each other. Bones are held together at joints by ligaments,  which are strong, fibrous, connective tissues. Joints are classified into three groups:

• Immovable (fibrous) joints, e.g. skull bones;
• Slightly movable (cartilagenous) joints, e.g. intervertebral discs;
• Freely movable (synovial) joints, e.g. limb joints.

Synovial joints permit the greatest degree of flexibility and have the ends of bones covered with a connective tissue (synovial membrane) filled with joint (synovial) fluid.

Vertebral Column

The spinal cord runs along the dorsal side of the body and links the brain to the rest of the body.  Vertebrates have their spinal cords encased in a series of (usually) bony vertebrae that comprise the vertebral column. Our back is composed of 33 bones called vertebrae, 31 pairs of nerves, 40 muscles and numerous connecting tendons and ligaments running from the base of your skull to your tailbone. Between our vertebrae are fibrous, elastic cartilage called discs known as shock absorbers.