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BIOL 215/216 - Anatomy & Physiology I & II: Muscular System

Human Anatomy & Physiology: Comprehensive study of the structure and function of the human organism, including but not limited to the integumentary, skeletal, muscle, nervous and endocrine systems.

Get Body Smart

Get Body Smart: GetBodySmart represents the creator Scott Sheffield's attempt to create a fully animated and interactive eBook about human anatomy and physiology. The contents and design of this long-term project are based on his 21+ years of teaching this material at the university level.

This site not only includes images, but also self-quizzing options for muscles as well.  

Web Anatomy Muscular System

Muscular System:  A series of quizzes that not only tests you on your knowledge of the various muscles in the body, but also checks to see if you are spelling them correctly.  Note:  this site tests you on posterior muscles, anterior muscles, arms, legs, as well as muscle physiology and micro anatomy.  

Master Muscle List

Master Muscle List:  The Master Muscle list represents a quick and easy reference for accessing information on the origin, insertion, nerve supply, and action of a given muscle.  Selecting an individual muscle, listed alphabetically or by anatomical region, will display an illustration of that muscle along with related functional information.  

Human Physiology

Human Physiology:  An open textbook via the platform Wikibooks.  This book was developed with the assistance of pre-nursing students at Utah State University. Covers topics including Homeostasis, Cell Physiology, Integumentary System, Nervous System, Senses, Muscular System, Blood Physiology, Cardiovascular System, Immune System, Urinary System, Respiratory System, Gastrointestinal System, Nutrition, Endocrine System, Male Reproductive System, Female Reproductive System, Pregnancy and Birth, Genetics and Inheritance, Development: Birth through Death.  A searchable PDF and printable version of this book is available via the Wikibooks site.  

Inner Body

Inner Body:  With both male and female examples as well as 2D and 3D interactive views, the InnerBody website allows for an in-depth examination and navigation of the human muscular system.


BioDigital Systems:  Availble for free (individual) or for a fee (groups/businesses), this interactive system will first require you to sign in via your Facebook or Google account to gain access.  Once inside the system, you can zoom and rotate your virtual skeleton.  Eleven systems in total are able to view and examine.  Note:  this site allows you to repeatedly quiz yourself on all eleven systems that are covered within the site.  


Anatomy Drill & Practice

Anatomy Drill & Practice:  This site covers the human body, the chemical, cellular, and tissue levels of organization, the integumentary system, skeletal system, muscular system, nervous system, cardiovascular system, respiratory system, digestive system, excretory system, and reproductive system.  

This site not only includes images of and information on the above listed systems, but it also includes interactive drills and practice questions for students.  NOTE:  Flash required for the quizzes/practice questions.  

AK Lectures on the Muscular System

The AK Lectures are a series of lectures from a (external) educational platform designed to "promote collaboration between our users and help spread knowledge to every part of the world."

These lectures vary in length, and will open in a new window when you click on the provided link.

Sarcomere:  The basic unit of skeletal and cardiac muscle is the sarcomere. The sarcomere consists of thin filaments and thick filaments. The thin filaments themselves consist of globular proteins called actin that join together to form long polymer chains. Two of these polymer chains intertwine in a helical fashion to form the thin filament. The thin filament also contains two other proteins called troponin and tropomyosin, which are involved in muscle contraction. The thick filament consists of a protein called myosin. Many of these myosin twist around one another to form the thick filament. At both ends of the thick filament are extensions called myosin heads, which are involved in binding to the thin filaments to cause the muscle contraction. The H-zone is the region that only contains the thick filaments, the I-band only contains the thin filaments, the A-band contains the thick-filaments in their entirety, which includes a small portion of the thin filaments as well. The Z-lines are the boundary lines for the sarcomere. When the muscle contracts, the distance between the two Z-lines decreases, the I-band and H-zone also decrease in length but the A-band remains unchanged because the thick filament does not change in size.

Structure of Skeletal Muscle:  Skeletal muscle appears striated (striped) and this is due to the fact that is consists of individual units called sarcomeres. Each sacromere is connected end-to-end to create a long fiber called the myofibril. Many of these myofibrils are placed into the cytoplasm of the cell called the sarcoplasm. Each muscle cell, also called a muscle fiber or myocte, contains a specialized endoplasmic reticulum called the sarcoplasmic reticulum, which contains a high concentration of calcium ions that are needed for muscle contraction to occur. The muscle cell also contains a specialized cell membrane called the sarcolemma, which contains deep invaginations called T-tubules (transverse tubules) that allow for a quick and uniform propagation of the action potential across the cell. Each muscle fiber is packed into a bundle called the fascicle, and many of these fascicles are bundled even further to create the actual muscle as it appears on the macroscopic level. Skeletal muscles are multinucleated (many nuclei per cell) and are controlled by the somatic nervous system. Skeletal muscles are usually found close to blood and lymph vessels and this implies that skeletal muscle contraction aids the movement and flow of blood and lymph fluid through the vessels. Shivering is a process that allows us to maintain our core body temperature under cold conditions and shivering is a result of the contraction of skeletal muscle that is initiated by the hypothalamus. Shivering produces a good deal of heat and this heat can be used to warm the body.

Contraction of Skeletal Muscle:  Before skeletal muscle can undergo contraction, an electrical signal must be created in the form of an action potential by the brain and then sent to the motor neuron of the somatic nervous system that innervates that particular muscle. It travels along the axon of the motor neuron and eventually reaches the motor end plate (neuromuscular junction), which is the synapse between the axon terminal of the motor neuron and the muscle cell's membrane. The neurotrasmitter acetylcholine depolarizes the membrane and the action potential travels through the transverse tubules (t-tubules) and into the sarcoplasmic reticulum. TThe sarcoplasmic reticulum then releases calcium ions into the cytosol of the cell, which go on to bind to the troponin found on the thin filament. Once calcium binds to troponin, it shifts the tropomyosin and exposes the myosin binding sites. At the same time, the myosin head of the thick filament hydrolyzes ATP into ADP and Pi, which remain attached to the myosin head. The myosin head then orients itself at a ninety degree angle with respect to the thick filament and binds onto the exposed myosin binding site on the thin filament. As soon as the ADP and P are released by the myosin head, the myosin head once more shifts itself and this creates the power stroke that moves the thin filament. This causes the muscle to contract. In order for the myosin to detach itself from the thin filament, ATP must bind to the myosin head. Once ATP binds, calcium detaches from the troponin and the calcium ions are then pumped right back into the sarcoplasmic reticulum. This process can now be repeated.

The Motor Unit:  Our body organizes neurons and muscles into something called motor units. A motor unit is simply the motor neuron and all the muscle cells that the motor neuron innervates. A single motor neuron can innervate thousands of individual muscle cells. The amount of force that is produced by any given muscle depends on three factors (1) the size of the motor neuron (2) the number of motor neurons and (3) the thickness of the particular muscle cell.  

Cardiac Muscle:  Cardiac muscle is the type of muscle that makes up the heart. Just like skeletal muscle, cardiac muscle is striated because it is composed of individual units called sarcomeres. These sarcomeres are connected end-to-end to form myofibrils and many of these myofibrils are found in the sarcoplasm of the cell. Cardiac muscle also contains a sarcolemma that has deep invaginations called T-tubules. So we see that many of the same features that are found in skeletal muscle are also found in cardiac muscle. However, there are some important differences. For instance, adjacent cardiac muscle cells are connected to one another via special regions called intercalated discs. These contain gap junctions and desmosomes. The gap junctions allow for a quick and uniform propagation of action potential between cells because they allow the passage of ions. Desmosomes hold the cells together. Cardiac muscles only contain one nucleus per cell and contain relatively large mitochondria. The autonomic nervous system is in control of cardiac muscle, which means its control is involuntary. However, certain cardiac muscle cells are capable of displaying myogenic activity, which means that they can actually contract without the input of any sort of signal from the nervous system. Cardiac muscle cells also contain a longer than normal depolarization period during the muscle contraction process. This is due to calcium voltage-gated channels found on the cell membrane that open up following the closure of sodium voltage-gated channels. This leads to a longer contraction and allows the heart of contract in a long, steady and forceful fashion.

Smooth MuscleUnlike cardiac or skeletal muscle, smooth muscle is not striated because it does not consist of sarcomeres. Instead, a network of different filaments are found throughout the entire cell body and this helps the cell contract. The three types of filaments are thin filaments, thick filaments and intermediate filaments. Thin filaments are usually connected to regions called dense bodies while the thick filament is found in between thin filaments. The movement of the thick and thin filaments causes the dense bodies to move together, which in turn pulls on the intermediate filaments, which brings all the dense bodies inside the cell closer together. This ultimately causes the cell to shrink and this is known as the muscle contraction. Smooth muscles can be arranged into two ways. We have single-unit smooth muscle, also known as visceral smooth muscle and we also have multi-unit smooth muscle. Single-unit smooth muscles consist of a collection of different smooth muscle cells that are connected via gap junctions. Only a few of these muscles are actually innervated by a neuron. When a signal arrives at the innervated smooth muscle, the gap junctions cause the other cells to contract in a uniform fashion. Therefore, single-unit smooth muscles contract together, as a single unit. These smooth muscles are also capable of myogenic activity. Multi-unit smooth muscle consist of a bundle of smooth muscle cells that are all innervated by a neuron. This means that the contraction of a muscle in a multi-unit system is independent of the contract of some adjacent cell. Smooth muscles are control by the autonomic nervous system and are found in places like blood vessels, stomach, small intestine, uterus, bladder and many other places. Smooth muscles are uninucleated, which means that they have one nucleus per cell.

Agonist-Antagonist Muscle Pairs:  The muscular and skeletal system work together to coordinate the voluntary movement of our body, which is ultimately controlled by the nervous system. Skeletal muscle attaches to our bones not directly but rather via fibrous structures called tendons, which consist predominately of collagen fibers. Tendons should not be confused with ligaments, which connect bone to other bone. The biceps-triceps system consists of these two muscles as well as a collection of bones (humerus, radius, ulna and others) that are responsible for the voluntary movement of our arms. In most of these systems, there is a large bone that does not actually move and this is called the immovable bone while the bones that do move are called the movable bones. In the case of the biceps-triceps system, the humerus does not move while the radius and ulna do move. The point where the muscle-tendon attach to the immovable bone is called the origin and this is the proximal end of the muscle. On the other hand, the point where the muscle-tendon attached to the movable bone is called the insertion and this is the distal end of the muscle. The biceps-triceps system works antagonistically. This means that when one of the muscle contracts, the other muscle elongates (stretches out) and vice versa. The muscle that contracts is called the agonist while the muscle that lengthens is called the antagonist. When we flex our biceps and move the radius and ulna bones towards our body, the biceps acts as the agonist while the triceps acts as the antagonist. On the other hand, if we reverse this motion and move the bones away from the body, the biceps will be the antagonist while the triceps will be the agonist. A muscle that flexes and contracts to decrease the angle in the joint is called the flexer while a muscle that increases the angle when it contracts is called an extensor.