Skeletal, Muscular, and Nervous Systems

The Skeletal System

Marathon runners travel more than 26 miles from start to finish. Running that far challenges the body both physically and mentally. Those who finish can take pride in achieving such a difficult goal. Running a marathon requires muscle strength and endurance. It also requires a sturdy skeletal system to support those muscles.

The skeletal system is a framework of bones that provides structure to your body and protects your internal organs. The skeleton also serves as a reservoir for the storage of minerals, produces blood cells, and allows movement of the body. A typical human skeleton is made up of 206 bones.

The skull contains bones that surround and protect the brain, while bones of the spine support the skull and allow movement in the back. The ribs protect important internal organs. Bones in the hand allow for many tasks, and bones in the feet support the body when walking or running.

Each bone is composed of living, growing tissue. If a bone breaks, one type of bone cell breaks down the damaged tissue while another type of bone cell begins to rebuild the bone. Calcium and vitamin D are nutrients that help build strong bones. You may think of bones as hard, solid objects. Although the outside of a bone is hard, the inside is filled with soft tissue called marrow. Bone marrow produces new blood cells.

Two types of bone marrow, red and yellow, run through the center of many long bones, such as those in the legs and arms. Red bone marrow produces red blood cells, which transport oxygen throughout the body. It also produces certain types of white blood cells, which help to fight disease. Yellow bone marrow is made up mostly of fat cells that are a source of stored energy.


Bones come in different shapes and sizes, and are based on their functions. For example, short, slender bones in your fingers allow your fingers to move and grasp things. Bones are classified as:

  • long bones, found in the legs and arms
  • short bones, found in fingers and toes
  • flat bones, found in the skull and pelvis
  • irregular bones, found in the backbone and ears


Joints are places where two or more bones meet. Tough strands of connective tissue called ligaments connect bones at most joints. Bones at joints are covered in flexible cartilage, which is a tough, elastic connective tissue. Cartilage covers bones to prevent them from rubbing against each other.


The amount of bone movement at a joint varies. For example, joints in the skull are not movable, while joints in the shoulder allow a wide range of motion. Joints are classified by the type of movement they allow and the shapes of their parts.

  • Ball-and-socket joints, found in the shoulders and hips, allow for movement in almost all directions.
  • Hinge joints, found in elbows and knees, allow for movement in one direction.
  • Gliding joints, found in wrists and ankles, allow limited movement in many directions.
  • Pivot joints, found between vertebrae in the spine, mainly allow rotating movement from side to side.
  • Fixed joints, found in the skull, hold the bones of the skull together and do not allow for any movement.

The Muscular System

The skeletal and muscular systems are interrelated. The muscular system uses muscles to move the bones of the skeletal system. Muscles are tissues that contract. Contraction occurs when muscle fibers shorten and pull together. Many muscles attach to bones, allowing you to walk, run, throw, dance, or do any other type of activity. Other muscles allow your heart to beat or move food through your body.

Voluntary Muscles

Skeletal muscles, or the muscles that are used to control bone movement, are all voluntary muscles you can consciously control. These muscles are connected to bones by tendons, which are bands of strong, fibrous connective tissue. Because a muscle can only contract, every joint is controlled by opposing muscles. This allows back-and-forth movement to occur. As shown in the image, you contract your biceps muscle to bend your arm. When your biceps muscle contracts, your triceps muscle relaxes and becomes stretched. You contract your triceps muscle to straighten your arm. When your triceps muscle contracts, your biceps muscle relaxes and becomes stretched.


Involuntary Muscles

Muscles in the lungs, intestines, bladder, blood vessels, and heart are all involuntary muscles. You have limited control over involuntary muscles. In other words, these muscles function without you consciously controlling them. Some involuntary muscles move materials into, through, or out of the body, such as the smooth muscles in the blood vessels, lungs, or intestines. The involuntary muscle in the heart is called cardiac muscle. Cardiac muscle causes the heart to beat, which pumps blood throughout the body.

The Nervous System

When you want to move, what causes your muscles to contract? Muscles move in response to electrical and chemical signals that travel through the nervous system. The nervous system includes the brain, spinal cord, and a network of specialized nerve cells called neurons. When you decide to move your arm or leg, your brain sends a nerve impulse down the spinal cord. The impulse reaches the muscles involved, causing muscle contraction and movement.


A neuron consists of three main parts: dendrites that receive messages from other neurons, a cell body that contains the nucleus, and an axon that allows an electrical signal to travel to other neurons and muscles. Axons are surrounded by a myelin sheath that insulates the axon and improves the speed at which impulses are conducted. At the gap between neurons, called a synapse, the electrical message is changed to a chemical message that is picked up by the next nerve cell.


The Spinal Cord

The spinal cord consists of nerve fibers that send impulses to and from the brain to the rest of the body. The spinal cord sends many messages from the brain to allow the body to function. At times, however, the spinal cord functions as a body defense system by immediately acting on an emergency nerve-cell message without involving the brain. For example, when you touch a hot stove, a sensory neuron sends a message to your spinal cord that the stove is hot. The spinal cord immediately sends a message to the motor neurons in your arm and hand that causes you to pull your hand away from the stove. This reflex response is used to prevent injury when it would take too long for the message to travel to the brain, down the spinal cord, and then out to the affected area.

The Brain

The brain is a complex organ that contains 90 percent of the neurons in the body. The brain controls all actions except reflex responses. Different portions of the brain control different body functions.

There are three main structures at the base of the brain. The cerebellum controls coordination, posture, and balance. The medulla oblongata governs involuntary body functions like breathing and digestion. The pons helps control the rate of breathing and relays signals between the cerebellum and the cerebrum.

The cerebrum is the large structure at the top of the brain. It controls motor coordination and interpretation of sensory information from inside and outside the body. The cerebrum is divided into two halves called hemispheres.

Each hemisphere contains four lobes that perform specific functions:

  • The frontal lobe regulates voluntary movements and is involved with decision making and problem solving.
  • The temporal lobe regulates memory, emotions, hearing, and language.
  • The parietal lobe processes sensory signals from the body.
  • The occipital lobe is involved with sight and visual memory.