Tutorial: Muscle Physiology
The Skeleton | Divisions | Compact Bone | Spongy Bone | The Joints | Muscles | Muscle and Tissue | Muscle as an Organ | Myofibril | Sarcoplasmic Reticulum | Sarcomere | Actin and Myosin | Cross-bridge | Sarcomere Animation | Neuromuscular Junction | Neuromuscular Junction Animation | Neurotransmitter | All-or-None | Recruitment | Twitch | Summation | Tetanus | Tone | Muscle movements | Quiz

Copyright © Steve Kuensting, 2004, All Rights Reserved.
This web tutorial may not be distributed by any means
without the expressed permission of the author!

Muscle and Tissue

There are 3 types of muscle tissue in the
human body - skeletal, smooth, and cardiac.
Their differences are listed below. The
contraction of skeletal muscle is the best
documented of the 3 so we will study its
contraction here in this tutorial.

Muscle tissue types

The Skeleton

Skeletal muscle usually moves bones. The skeleton consists of 206 separate bones.

The SkeletonBall and Socket Joint
Human Skeleton, Anterior view
By LadyofHats Mariana Ruiz Villarreal [Public domain], via Wikimedia Commons
Skeleton posterior view
Human Skeleton, Posterior view
By LadyofHats Mariana Ruiz Villarreal [Public domain], via Wikimedia Commons


Bones are moved by muscles because muscles connect them together at places called joints, which allow motion between the bones.
Hip joint
Hip joint

I [GFDL, CC-BY-SA-3.0 or GFDL], from Wikimedia Commons
Hip x-ray
X-ray, Hip

By Scuba-limp [Creative Commons Attribution-Share Alike 3.0 Unported or GNU] via Wikimedia Commons.

Divisions of the Skeleton

The 206 bones of the adult human skeleton are divided into the axial and appendicular subdivisions. The axial skeleton consists of the skull (cranium and facial bones), vertebra, ribs, and sternum. The appendicular skeleton consists of the bones of the pectoral and pelvic girdle, as well as all of the arm and leg bones. There are 7 bones that fit into neither category. The table below summarizes the locations of the different bones.

Bones of the Human Skeleton
Axial Skeleton
73 bones
Cranium8 bones: frontal (1), parietal (2), occipital (1), temporal (2), ethmoid (1), sphenoid (1)
Facial Bones14 bones: maxillary (2), mandible (1), inferior nasal conchae (2), lacrimal (2), nasal (2), palatine (2), vomer (1), zygomatic (2)
Vertebra26 bones: cervical (7), thoracic (12), lumbar (5), sacrum (1), caudal (fusion of 3)
Ribs and Sternum25 bones: ribs (24 - one pair per thoracic vertebra), sternum (1)
Appendicular Skeleton
126 bones
Girdles4 bones: Pectoral: clavicle (2) and scapula (2)
2 bones: Pelvic: Coxal bone (2), each consisting of the ilium, pubis, and ischium
Arm30 bones (per arm): humerus (upper arm), radius and ulna (forearm), carpals (8, wrist), metacarpals (5, palm), phalanges (14, fingers)
Leg30 bones (per leg): femur (thigh), tibia and fibula (lower leg), patella (kneecap), tarsals (7, ankle), metatarsals (5, arch), and phalanges (14, toes)
7 bones
Ear3 bones (per ear): malleus, incus, stapes - forming the bones of the middle ear
Hyoid bone1 bone: floating bone in the human neck


Bone cells are called osteocytes. They produce a hard material around themselves called the matrix that consists of calcium salts and collagen protein. They are arranged in cylindrical patterns throughout bone around thin tubes called Haversian canals. Tiny cytoplasmic extensions called canaliculi connect the osteocytes to one another and the Haversian canals. The Haversian canals contain nerves and blood vessels that nourish the osteocytes.


By BDB [CC-BY-SA-2.5], via Wikimedia Commons

Compact Bone

The dense outer layer of all human bones is called compact bone. It is not solid, though it appears so. It has Haversian Canals and canaliculi that radiate through the dense matrix to feed the osteocytes. The collagen and calcium salts of the compact bone give it most of its strength.

Haversian canal
Bone Anatomy

By Sunshineconnelly at en.wikibooks [CC-BY-3.0], from Wikimedia Commons

Compact and spongy bone
Bone Structure

By SEER [Public domain], via Wikimedia Commons

Spongy Bone

Bones are not entirely compact bone. At the center of long bones there is usually a large-diameter flesh-filled cavity filled with blood vessels or fat, called marrow. Red marrow produces blood cells, yellow marrow stores fat. At the ends of bones there is usually a network of marrow and compact bone strands called spongy bone. Spongy bone reinforces the ends of bones and the interiors of bones such as facial bones to keep them from collapse when they are stressed by motion or collision.

Spongy bone
Spongy Bone

By Sunshineconnelly at en.wikibooks [CC-BY-3.0], from Wikimedia Commons

The Joints

Bones meet bones at places called joints. The ends of bones are usually covered by cartilage for protection, and many bones have cartilage cushions that further lessen the wear of the bone ends. The meniscus in the knee or disks in the vertebral column are examples of such cushions. The bones at joints are attached together by thin elastic straps called ligaments which keep the bones from coming apart. Muscles generally reach across joints to pull and move the bones. The muscles are attached to the bones via tendons. Tendons are much less elastic than ligaments, so that as a muscle pulls, a bone actually moves.

Joint CategoriesLocations
immovable jointscranial to cranial/facial bones; facial to facial bones; pubis to ischium and ilium
slightly movable jointscoxal bone to sacrum; vertebra to vertebra; carpal to carpal; tarsal to tarsal; tibia to fibula
freely movable jointshinge: humerus to ulna; femur to tibia
ball-and-socket: humerus to scapula, femur to coxal bone
pivot: radius to ulna, atlas to axis (1st to 2nd cervical vertebra)
saddle: carpal to thumb metacarpal

Knee structure
Knee Structure

By Mysid [Public domain], via Wikimedia Commons


Muscles are usually anchored to one bone near a joint - called their origin. The muscle then reaches across the joint, often with a tendon, to attach to another bone that it moves - called their insertion. The nerve that controls the muscles action (motor nerve) is called the muscle's innervation. All skeletal muscles have origins, insertions, and innervations. For example, the biceps brachii muscle originates on the scapula (supraglenoid tubercule and coracoid process), inserts on the radius (radial tuberosity) and is innervated by the musculocutaneous nerve.

Biceps brachii muscle

By Pearson Scott Foresman [Public domain], via Wikimedia Commons

Muscle as an Organ

A muscle such as the biceps brachii is a whole organ, not a tissue. It is composed of skeletal muscle tissue, connective tissue, nervous tissue, and epithelial tissue. Most of a muscle is skeletal muscle cells that are long and thin that lie parallel to each other in bundles called fasciculi. A skeletal muscle cell is also called a muscle fiber. Each fiber has more than one nucleus because it formed embryonically by the fusion of cells. The muscle is attached to bone via a tendon and is completely wrapped in connective tissue called fascia.

Muscle anatomy

Myofibril/Sarcoplasmic Reticulum

A whole muscle consists of bundles of fibers (cells) called fasciculi. Each fasciculus consists of a bundle of fibers. Each fiber (muscle cell) consists of a bundle of myofibrils, along with other organelles such as nuclei and mitochondria. The myofibril contains sarcomeres and is completely wrapped in special tubules called the sarcoplasmic reticulum (like endoplasmic reticulum). Thus, a muscle is a bundle of fibers, which are a bundle of myofibrils, which contains chains of sarcomeres.


Myofibril Structure

Myofibrils are the organelles that allow a muscle/muscle fiber to contract. Each myofibril (a muscle fiber has numerous myofibrils) are arranged parallel and run lengthwise in the fiber. Within the myofibrils are proteins that make the myofibril shorten. The arrangement of the proteins give the skeletal muscle fiber a striped or striated appearance.



Note the names of the different sections of the myofibril below. The most important section of the myofibril is the sarcomere. Note that the myofibril is actually a "string" of sarcomeres that are connected end to end. The Z-line is the area where sarcomeres are joined together. Each sarcomere has dark bands within it called "A bands" and lighter bands called "I" and "H" bands. Note the locations of these bands below.


Actin and Myosin

The bands within the sarcomere are due to the presence of 2 proteins: actin and myosin. The thicker of the two is myosin and it makes up threads in the sarcomere called thick filaments. The thinner is actin and it makes up the thin filaments. Myosin is primarily in the A band and actin is primarily in the I band, but they both overlap each other in the bands.


Sarcomere Shortening

Sarcomeres are the structures within myofibrils that allow the myofibril and fiber to shorten. Sarcomeres shorten because the filaments inside of them slide past each other - not because the filaments shorten. Thus, when a fiber contracts, chemicals are not shortening. Instead, chemicals are sliding past each other and thus shortening the distance that they span.

Sarcomere shortening


What allows a sarcomere to shorten is the interaction of actin and myosin. Myosin has special bristles located along its sides called cross-bridges. Cross-bridges can grab onto actin and pull on it, causing actin to slide along myosin. This sliding of actin past myosin allows the sarcomere to shorten. Note that in each sarcomere the Z-lines come together because the cross-bridges pull the actin on each side towards the middle. This explanation, of actin sliding past myosin, is called the Sliding Filament Theory of Muscle Contraction.

Actin and Myosin

Summary of Muscle Anatomy and Sarcomere Construction

Below is a summary of the anatomy of a muscle from the whole organ to the sarcomere.
Sarcomere construction Skeletal Muscle Structure
By Raul654 [Creative Commons Attribution-Share Alike 3.0 Unported or GNU] via Wikimedia Commons.

By David Richfield (Slashme user) (http://en.wikipedia.org/wiki/Sarcomere) [GFDL], via Wikimedia Commons

Sarcomere Animation

The chemical stimulus that causes cross-bridges to pull on actin is the presence of calcium ions. When the muscle fiber is stimulated to shorten, it releases calcium ions into the myofibrils to allow cross-bridges to pull on actin. The calcium is stored in the sarcoplasmic reticulum just for this purpose. The hyperlinks below animate the release of calcium, the sliding of actin, and the shortening of a sarcomere.


Below is the first frame of the animation of the activity of a single sarcomere. Study it before you click on the animation links below.

Sarcomere animation
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =

Neuromuscular Junction

Muscles are always under nervous system control, otherwise they would be contracting all of the time. A neuron that controls a muscle fiber is called a motor neuron. The motor neuron forms a special junction called the neuromuscular junction where it interacts with the muscle fiber.

Neuromuscular junction

This diagram below is a simplification of all of the important parts we have discussed so far. Draw this picture in your notes and label it for reference!

Neuromuscular junction


Action potentials (nerve impulses) travel to the end of the motor neuron. The axon terminal of the motor neuron then releases neurotransmitter onto the muscle fiber at the neuromuscular junction. The neurotransmitter causes a muscle impulse like the nerve impulse - a muscle action potential.


Calcium Release

The muscle impulse causes the sarcoplasmic reticulum to release calcium ions into the myofibrils of the muscle fiber. The calcium interacts with actin and allows the cross-bridges of myosin to pull on the actin. As the cross-bridges pull, the actin slides past the myosin and the sarcomeres shorten, which causes the myofibrils to shorten, which causes the fibers to shorten.

Calcium storage and action

Neurotransmitter Animations

When the motor neuron stops releasing neurotransmitter, the muscle action potentials stop, which allows the sarcoplasmic reticulum to pull the calcium out of the myofibrils. Without calcium interacting with the actin, cross-bridges of myosin cannot pull on actin and contraction stops. The muscle fiber is then relaxed.

Cross bridge function

Below is the first frame of the animation. Study it, before you click on the animation links below.
Neuromuscular junction animation start
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =


Muscle fibers are controlled in an all-or-none fashion. This means that they contract as much as they can or not at all - there is no ability to contract variably. Whole muscles can contract partially through recruitment - by the brain varying the number of fibers within a muscle that are stimulated. For mild/weak contractions only a small percentage of fibers are recruited while a strong contraction may require almost 100% of all fibers to actively pull.

Graded contractions


Whole muscles can be removed from animals and studied. If the muscle is stimulated electrically, a computer can graphically display the contractions as a myogram. A twitch is a contraction of the whole muscle due to a single stimulus of electricity. Note that there is a latent period after stimulation where the muscle has not yet contracted - this is when calcium is being released and the actin and myosin begin interaction.



Following the latent period there is a period of contraction followed by a period of relaxation. The entire action is called a twitch.

Twitch action


If electrical stimuli given so that full relaxations are not allowed, a phenomenon called summation occurs. The whole muscle begins to create more tension with each new stimulus (without full relaxation). This is because the muscle is beginning each new contraction from a semi-shortened state. If a constant stimulus is supplied, tetanus results - the whole muscle (all fibers) contract fully.



Whole muscles that exist in living organisms behave according to the same principles that are observed in a myograph. Even when whole muscles are relaxed, the spinal cord is stimulating a low number of muscle fibers at all times - creating a constant muscle tone. Tone is required to maintain posture - in holding the head and torso upright. Tone increases after vigorous exercise - the muscles are even "tighter" than they normally are.


Prime Movers

As a last consideration of this program, we will briefly look at some of the prime movers (major muscles involved in bone movement) in skeletal muscle contraction. Below is a diagram that labels many of the prime movers you should know. Draw a simple diagram and label the muscles in your notes. Muscles are often found to work against each other. For example, biceps brachii and triceps brachii oppose one another. Muscles that oppose one another are called antagonists. There are other muscles that aid each other - they pull on the same bone and move it the same way. Such muscles are called synergists. Biceps brachii and brachialis are synergists.
Human Skeletal muscles
By KVDP (Own work) [Public domain], via Wikimedia Commons

The Major Prime Movers

Muscle Movements

Muscle movements are described with specific words. Extension is the action of increasing a joint angle while flexion is decreasing a joint angle. Adduction means to bring closer to the body while abduction means to move further from the body.

Prime movers

The Prime Movers

The different prime movers have different functions. In the table below you will find the functions of some different muscles. List the muscles and the functions in your notes! The underlined portion is the part you need to know. Click the names of the muscles to view them anatomically.

•frontalis: raises eyebrow.
•trapezius: rotates the scapula, raises scapula, pulls the scapula medially, or pulls the scapula downward.
•pectoralis major: flexes, adducts, and rotates the humerus at the shoulder, or adducts the arm; pulls the arm across the chest.
•teres major: adducts, extends, rotates arm medially.
•latissimus dorsi: extends and adducts the arm and rotates the humerus inwardly at the shoulder, or pulls the humerus downward and back.
•deltoid: abducts the upper arm, extends the humerus, or flexes the humerus at the shoulder.
•biceps brachii: flexes the arm at the elbow and rotates the hand laterally.
•triceps brachii: extends the arm at the elbow.
•rectus abdominis: tenses the abdominal wall, compresses the abdominal contents, and flexes the vertebral column at the waist.
•erector spinae: extension of the vertebral column.
•gluteus maximus: extends the leg at the hip.
•biceps femoris: flexes and rotates the lower leg at the knee and and extends the thigh.
•psoas major: Flexion of the femur at the hip.
•sartorius: flexes the leg and thigh, abducts the thigh, and rotates the thigh laterally at the hip.
•rectus femoris: extend the lower leg at the knee.
•tensor facia latae: abducts and rotates the femur inward.
•gracilis: adducts thigh, flexes knee joint.
•gastrocnemius: plantar flexion of the foot at the ankle and flexion of the leg at knee.
•tibialis anterior: dorsiflexion of the foot at the ankle and flexion of the leg at the knee.
•sternocleidomastoid: flexes vertebral column and rotates the skull.
•masseter: raises mandible.


  1. What type of muscle tissue has branched fibers?

  2. What type of muscle tissue is voluntary?

  3. What type of muscle is found within the heart?

  4. What type of protein is found in the A band?

  5. What type of protein is found in the I band?

  6. What thin organelles within a fiber are responsible for shortening, and contain sarcomeres?

  7. What sac-like organelles store calcium?

  8. What is the general name of all chemicals used by neurons to stimulate muscles?

  9. What is the name of the junction of a motor neuron and a muscle fiber?

  10. What protein makes up the thin filaments?

  11. What protein makes up the thick filaments?

  12. What types of small paddles are located on myosin?

  13. What mineral stimulates cross-bridges to interact with actin?

  14. What muscle flexes the arm at the elbow?

Diagram 1: Name the structures:
Quiz 1

Diagram 2: Name the structures:
Quiz 2

Diagram 3: Describe the motion depicted in each diagram (using motion, bone, joint) and, whenever possible, name the prime mover:
Quiz 3

Diagram 4: Name the prime movers and describe the motion:
Quiz 4

Diagram 5: Name the prime movers and describe the motion:
Quiz 5