FreeMCATPrep Logo

Support us and cryptocurrency!
Try a browser that's faster, safer, ad-free, and earns you cryptocurrency for using it! W3Schools

More Than 475 Free MCAT Questions with Detailed Answers!

Click HERE for your Random Question from our MCAT Question A Day Archive

Biology - Musculoskeletal System

Bone is made of two substances mixed together. The substances are collagen and something called hydroxyapatite. Hydroxyapatite is a combination of calcium salt crystals. These are deposited on the collagen, which is a protein. There are two types of bone, spongy bone and compact bone. Bones are held together by joints. Joints come in three main varieties. Those made out of collagen fibers called fibrous and those made out of cartilage called cartilaginous and the variety called synovial. 

Synovial joints are found all over the body; in the shoulder, in the knee, and in the jaw. These kinds of joints are covered by a capsule and inside there is a synovial membrane that makes synovial fluid. This fluid keeps the bones from rubbing against each other. The ends of the bones are also covered by something called articular cartilage, which protects against rubbing. When you look closely at compact bone under the microscope, you see a bunch of circles and each circle is called a Haversian system. At the center of the Haversian system is a canal, it is called a Haversian canal. 

Blood vessels and nerves run through the Haversian canal. On the periphery of the Haversian canal are little wells called lacunae in which bone cells live. Bone cells are called osteocytes. Bone also contains osteoblast and osteoclast, which are involved in bone remodeling. Osteoblasts are bone building cells and osteoclast breakdown bone. 

Bones can move because of the muscles that are attached to them. Muscles are attached to bones by tendons. Ligaments, however, are for bone to bone attachments. Note that the muscles that allow us to move our skeletons are the ones over which we conscious voluntary control. They are called skeletal muscle or voluntary muscle. When we look at skeletal muscle under the microscope, we see striations, stripes. Know that skeletal muscle is striated. 

How skeletal muscles make us move? Let us say that muscle A is attached to two bones across the front side of the elbow joint. When this muscle gets shorter, the bones move closer to each other and the arm bends at the elbow joint. Let us look at another scenario that involves muscle B. Muscle B is attached to the same set of bones that muscle A is attached to, but muscle B crosses the back side of the elbow joint. If muscle B contracts, its contraction causes the arm to open because muscle B is attached across the back side of the elbow joint. Muscle B and muscle A have opposite actions because they are attached across opposite sides of the same joint. Muscle B and muscle A have antagonistic actions. They do opposite things. One muscle opens the arm at the elbow joint and another closes it. 

Muscle is composed of cells. These muscle cells are called muscle fibers. A muscle fiber is composed of sarcomeres. Sarcomeres are responsible for contraction. Sarcomere has filaments in it and the filaments are made of protein. The thin filaments are made of a protein called actin and the thick filaments are made of a protein called myosin. The thin filaments are attached to rods called Z-lines. Actin and myosin give a muscle the ability to contract. The entire length of the thick filament is called the A-band. The space between thick filaments is the I-band and the space between thin filaments is the H-band. 

The actin filaments and the myosin filaments can be connected by cross bridges, which move like hinge levers. When these hinge movement occurs, the actin and myosin slide past each other and the sarcomere gets shorter. 

Remember that during contraction only the H and I bands shorten and the A-band remains the same. When a sarcomere gets shorter, so, does the muscle fiber and consequently the whole muscle. In a muscle fiber, an endoplasmic reticulum is called the sarcoplasmic reticulum. When you want to move a part of your body, you send a nerve signal down a motor neuron to the appropriate skeletal muscle. When the muscle receives the nerve signal, a number of things happened. First, the nerve releases acetylcholine. The muscle undergoes an action potential that causes the sarcoplasmic reticulum to release calcium ions. 

The calcium ions caused the actin and myosin to slide past each other, which will cause the sarcomere to shorten. The excess acetylcholine is broken down by the enzyme cholinesterase. Muscle fibers have a lot of mitochondria to produce enough energy for muscle contraction. When muscle is contracting actively, it needs a big supply of oxygen and it produces a lot of carbon dioxide. Therefore, when a muscle is actively contracting, the blood vessels that supply it will dilate to allow the muscle to get more blood and of course more oxygen. In cases where the muscle contracts so actively that it cannot get enough oxygen to met it needs, it cannot take pyruvic acid and put it into the Krebs cycle. Therefore, the muscle reduces pyruvic acid without oxygen by anaerobic oxidation. In this reaction, the pyruvic is converted to lactate. Lactate will then build up in the muscle and will eventually cause some pain. This is called oxygen death. When a muscle is unable to get all of the oxygen it needs and so it starts to produce lactic acid. 

There are three types muscle; skeletal muscle, smooth muscle, and cardiac muscle. Skeletal muscle is the muscle through which we undertake voluntary movements of our skeleton. Cardiac muscle is the muscle that the heart is made of. It is striated and has special junctions between muscle cells called intercalated disks. It is under involuntary control. Smooth muscle is all other muscle. They function in every other part of the body. The smooth muscles are found inside the blood vessels and glands, our internal organs and ducts. Smooth muscle is responsible for involuntary movements within our body, it is under involuntary control. 

Smooth muscle and cardiac muscle get their nerve supply from the autonomic nervous system. Skeletal muscle gets its nerves supply from the somatic system. Remember, under the microscope, smooth muscle does not have any stripes, it is not striated, but cardiac muscle and skeletal muscle are striated.