Investigation One introduced students to the nervous system, which serves as the body’s control center. Students discovered how the sensory and motor pathways work together to react to a stimulus by testing reaction time. Students explored memory and thought through experiments to remember a sequence of patterns and identify dropped objects by their sound. Finally, students explored the autonomic nervous system by counting naturally-occurring eye blinks. In Investigation Two, students continue their exploration of the human body by studying the systems which give the body its structure and control its movement.

The skeletal and muscle systems are often referred to as one system: the skeletomuscular system. This designation arises from the fact that the two systems work together to give the body support and structure, as well as to move the body through space. The skeletal system is made up of 206 bones. Bones are made of a protein framework called a matrix that is filled with calcium and phosphorus. The protein matrix gives bones strength, while the minerals give them hardness and durability. The body regulates the mineral content of bone by removing and replacing minerals as needed. Bone also serves as a storage site for minerals, which are used by all cells in the body for other functions.

Bone also has another role. A spongy tissue called bone marrow is located inside the long bones of the body. The long bones are the humerus, ulna, and radius in the arms and the femur, tibia, and fibula in the legs. Marrow is also found in the ribs and vertebrae. The marrow is responsible for manufacturing blood cells, which must be replaced every four months.

The area where two bones meet is called a joint. The ends of bones are covered in a soft tissue called cartilage, which acts as a cushion to keep bones from eroding each other when they rotate against each other. Sometimes this cartilage wears away, causing the hard ends of the bones to rub against each other. This condition is called arthritis.

The skeleton is held together by the muscles and ligaments. A ligament is a tough, pliable tissue that connects bone to bone. A muscle attaches to the bones with a similar tissue called a tendon. Muscle contractions cause tendons to pull on bones, resulting in movement. The ligaments help keep joints aligned during muscle contraction. Damage to ligaments is a common result of athletic injuries.

Skeletal Muscle

There are three types of muscle tissue in the body: skeletal (striated), smooth and cardiac. Skeletal muscle is made of bundles of cells arranged in fibers. Skeletal muscle is also called striated muscle because the arrangement of cells in the muscle gives it a striped appearance under the microscope. Nervous stimulation of these bundles causes the cells to contract together to move various parts of the body. When we eat meat – a beef steak, ham, or chicken, for example – what we call “meat”, is generally striated skeletal muscle.

Smooth Muscle

Most skeletal muscle contractions are the result of voluntary nervous system activity. In other words, skeletal muscle contractions are the result of conscious thought. In contrast, contraction of smooth muscle is generally the result of involuntary nervous system activity. Smooth muscle is not arranged in the same type of bundles of muscle fibers as skeletal muscle. Instead, smooth muscle cells are frequently scattered throughout other tissues, or occur in a layer between other tissues. These cells contract and relax as needed to control life processes such as digestive system activity or movement of blood through the arteries.

When we hear groaning sounds from our abdomen, sounds we refer to as “hunger pains”, it is likely the result of local squeezing and contractions in our empty stomachs. LabLearner students might take a moment and listen to their own abdomens with a stethoscope. The gurgling sounds that you hear comes from involuntary smooth muscle contractions generated from the small intestine as it squeezes its contents along. Thus, most smooth muscle contractions are the result of unconscious brain activity. Therefore, conscious thought is not required for smooth muscle contraction to occur.

Cardiac Muscle

The third type of muscle is called cardiac muscle. Cardiac muscle tissue is found only in the heart. Contraction and relaxation of cardiac muscle is rhythmic and causes blood to move into and out of the heart with each contraction/relaxation cycle. Cardiac muscle is under the control of the autonomic nervous system. The autonomic nervous system works through a special bundle of nerve fibers in the heart that sends waves of electrical impulses to cause rhythmic contractions. The beating heart cells shown here are actually growing in a petri dish and viewed under a microscope. Notice that the rhythmic contraction of these cardiomyocyte cells occurs without any involvement of a nervous system at all! Our heart cells began their rhythmic contractions in our mother’s womb within the first couple of months of fetal development and continue to the very last moment of our lives.

Disruption of normal waves of contraction causes the cardiac muscle cells to beat out of sync. This causes a fluttering type of contraction called fibrillation. Doctors and emergency medical personnel use a special machine called a defibrillator to shock the heart back into the correct rhythm when fibrillation occurs. Abnormal heart rhythms often occur due to damage of the heart tissue resulting from a heart attack. Heart attacks occur when circulation is lost to an area of heart muscle, causing the tissue to die.

When we hear or feel a heartbeat, what we are hearing and feeling is the coordinated contraction of millions of heart cells that compose the cardiac muscle of the heart. Involuntary responses to the environment around us can automatically change the heart’s rate of contraction. We are able to feel and hear these changes if we pay attention or use a stethoscope. Play the video below to watch an animation of the contractions and sounds of a beating heart. Notice how it speeds up and slows down. This happens to our own hearts all day long in response to exercise and emotions. – every second of our lives.

Investigation Two focuses on skeletal muscle and the skeleton itself. Students will explore the skeletal and muscle systems by testing a model of a joint to learn how muscles work to change bone position. Students will also examine a model of a human torso to learn how muscles and bones are arranged in the body.