Structure of a Skeletal Muscle
Connective Tissue Coverings, Skeletal Muscle Fibers, Neuromuscular Junction and Motor Units
Smooth Muscle Fibers and Smooth Muscle Contraction
Origin and Insertion and Interaction of Skeletal Muscle
Muscles of Facial Expression Muscles of Mastication
Muscles that Move the Head Muscles that Move the Pectoral Girdle
Muscles that Move the Arm Muscles that Move the Forearm
Muscles that Move the Wrist, Hand, and Fingers
Muscles that Move the Thigh Muscles that Move the Leg
Muscles that Move the Ankle, Foot, and Toes
Skeletal muscles are the major organs that make up the muscular system. A skeletal muscle consists of connective tissues, skeletal muscle tissue, blood vessels and nerves.
The following are connective tissue coverings that are associated with skeletal muscles. When you look at the "marbling" in a steak, you are actually viewing connective tissues in the steak. The red portion of the steak is the muscle tissue.
Fascia - This covers entire skeletal muscles and separates them from each other.
Tendon - This is a cord-like structure made of fibrous connective tissue that connects muscles to bones.
Aponeurosis - This is a sheet-like structure made of fibrous connective tissue. It typically attaches muscles to other muscles.
Epimysium - This is a thin covering that is just deep to the fascia of a muscle. It surrounds the entire muscle.
Perimysium - This is a connective tissue that divides a muscle into sections called fascicles.
Endomysium - This is a connective tissue covering that surrounds individual muscle cells which are also called muscle fibers. Therefore, every muscle cell in a muscle is surrounded by connective tissue!
Click here to view tendons and aponeuroses.
Click here to view the various connective tissues associated with skeletal muscles.
Click here to view a micrograph of perimysium within skeletal muscle.
Skeletal muscle fibers are called muscle cells because of their long lengths. Muscle fibers are cells that specialize in contracting (shortening) and relaxing (lengthening). The cell membrane of a muscle fiber is called a sarcolemma. The cytoplasm of this cell type is called sarcoplasma and the endoplasmic reticulum is called sarcoplasmic reticulum. A skeletal muscle fiber has more than one nucleus, therefore we say it is multinucleated.
The sacrcopplasm is filled with myofibrils. Myofibrils are thread-like stuctures made of two kinds of protein filaments. The protein filaments that make up myofibrils are called actin filaments and myosin filaments. The actin filaments are thin while the myosin filaments are thick. These two filaments form a very distinct pattern over and over again in a myofibril. The pattern is called a sarcomere which is shown below. So a sarcomere can be simply defined as a segment of a myofibril.
A sarcomere is divided up into areas or bands. The A band of a sarcomere is where the actin and myosin filament overlap. The I band is the area that contains actin filaments only. When you see striations (stripes) in the sarcoplasm of a skeletal muscle cell, you are actually seeing A bands and I bands. A bands are darker while I bands are lighter.
As mentioned previously, sarcoplasmic reticulum is the endoplasmic reticulum of a muscle fiber. It is specialized to store calcium. Sarcoplasmic reticulum also associates with transverse tubules which are tubules that extend inward from the cell membrane (sarcolemma).
Click here to view a muscle fiber
Click here to view the structure of a sarcomere.
A neuromuscular junction is where a neuron communicates with a muscle fiber. This is where a neuron stimulates a muscle fiber to contract.
If a neuron stimulates a muscle fiber, it is called a motor neuron. Motor neurons are located in the brain and spinal cord but they have processes or fibers that reach out to muscle fibers. The motor neuron fiber contacts a muscle cell on the muscle cell's motor end plate. The motor end plate is simply an area on the sarcolemma that is specialized to receive messages from motor neuron fibers. The motor neuron fiber will release chemicals onto the motor end plate to make the muscle fiber contract. The chemicals released are called neurotransmitters.
Click here to view a neuromuscular junction.
One motor neuron and all the muscle fibers it communicates with is called a motor unit. A motor neuron can communicate with many muscle fibers because the fiber of a motor neuron branches many times. So, one motor neuron can stimulate hundreds or thousands of muscle fibers to contract at the same time!
Click here to view a motor unit.
Muscle contraction starts at the sarcomere level. When all the sarcomeres in a myofibril contract (shorten), the entire myofibril will shorten. When myofibrils shorten, the muscle fiber shortens. When muscle fibers shorten, the muscle shortens.
In order for a sarcomere to shorten, myosin and actin filaments must slide against each other. Portions of the myosin filaments called mysosin heads can "grab" actin filaments and pull on them. When the myosin heads pull on actin, the actin filaments slide closer together and the length of the sarcomere shortens. However, myosin heads will not "grab" the actin filaments unless calcium is available.
When ATP is available, the myosin heads will "let go" of the actin filaments. As long as calcium is still available, the myosin heads will immediately "grab" the actin filaments again and pull them closer together. Therefore the myosin heads will "grab", release, and pull on the actin filaments over and over again as long as calcium and ATP are available. The result is that the sarcomeres will shorten more with each pull of the myosin heads. Once calcium is no longer available, the myosin heads will "let go" of the actin filaments and the sarcomeres will lengthen back out to their original lengths allowing the myofilaments and muscle fibers to lengthen (relax).
Click here to view actin and myosin filaments
Click here to view how actin and myosin interact in muscle contraction
When a motor neuron generates a nerve impulse, the impulse travels to the end of its fibers. The impulse causes the fibers to release neurotransmitters. Acetylcholine is the neurotransmitter that is released onto skeletal muscle fibers. Acetycholine will generate a muscle impulse that travels down the transverse tubules of the sarcolemma. The transverse tubules now stimulate the sacroplasmic reticulum to release calcium. The calcium then causes the myosin to bind to actin and contraction begins.
When the motor neuron no longer sends the nerve impulse, the muscle fiber releases an enzyme called acetycholinesterase. Acetylcholinesterase breaks down acetylcholine so the muscle impulse is stopped. When the muscle impulse stops, calcium returns to the sarcoplasmic reticulum. Since calcium is no longer available, myosin "lets go" of actin and the sarcomeres lenghten. When sarcomeres lengthen, the myofibrils and hence the muscle fibers lengthen and relax.
Because a lot of ATP is needed for sustained or repeated muscle contractions, a muscle cell must have multiple ways to store or make ATP.
Muscle fibers contain a protein called creatine phosphate. This protein stores extra phosphate group. Recall that when an ATP (adenosine triphosphate) molecule is used for energy, it loses a phosphate group and becomes ADP (adenosine diphosphate). Creatine phosphate can donate a phosphate group to ADP so it becomes ATP again. This is a rapid way for muscle fibers to regenerate their ATP supplies.
Just like other cells in body, muscle fibers can also oxidize glucose to make ATP. Recall that in this process glucose is converted to pyruvic acid, pyruvic acid is converted to acetyl coenzyme A and then coenzyme A enters the citric acid cycle where a lot of ATP is produced. Also recall that the citric acid cycle will not take place unless oxygen is available. Therefore, muscle fibers need a large supply of oxygen.
Oxygen is carried in the blood to muscle cells by a pigment called hemoglobin. Muscle cells make another pigment called myoglobin that stores extra oxygen for the cell.
When skeletal muscles are used strenuously for a minute or two, a condition of oxygen debt develops. This condition develops when oxygen supplies in the muscle are low and the citric acid cycle can no longer be used to produce ATP. When oxygen is low, muscle fibers convert pyruvic acid to lactic acid. Lactic acid causes muscle fatigue.
Lactic acid is carried by the blood stream to the liver where it can be converted back into glucose. However, this process requires energy. The oxygen debt is the amount of oxygen the liver cells need to make enough ATP to convert lactic acid into glucose. This is why you are still burning glucose after you exercise!
Click here for summary figures of how muscles cells generate energy with oxygen and without oxygen.
Muscle fatigue is a condition in which the a muscle has lost its ability to contract. It usually develops because of an accumulation of lactic acid. It can occur also if the blood supply to a muscle is interruped or if a motor neuron loses its ability to release acetlycholine onto muscle fibers. Cramps can accompany muscle fatigue. Cramps are sustained involuntary contractions of muscles.
As the reactions of the citric acid cycle occur in a muscle fiber, ATP is released. However, heat is also released. Therefore, active muscles are sources of heat for the body! The heat is released into the blood stream where it is transported to other body cells.
Muscle contraction responses can be measure in a laboratory by removing an individual muscle fiber. The muscle fiber is connected to a device that measures its length. The muscle fiber is stimulated to contract by an electrical current that mimics a nerve impulse.
The minimum amount of stimulus needed to cause a muscle fiber to contract is called the threshold stimulus. In a laboratory setting, an individual muscle fiber will not contract until a strong enough electrical current is applied to it. In the body, an individual muscle fiber will not contract until enough acetylcholine is released onto it.
Once a muscle fiber is exposed to its threshold stimulus or a stronger stimulus, it will contract completely. If a muscle fiber is exposed to a stimulus weaker than its threshold stimulus, it will not contract at all. Therefore we say a muscle fiber contracts completely or not at all; this is called an all-or-none response.
When muscle contraction is recorded, the resulting pattern is called a myogram. When a muscle is stimulated to contract (by its threshold stimulus or a stronger stimulus) it does not contract right away. The period from when the stimulus is applied to the time it takes for contraction to begin is called the latent period. The period when the muscle is shortening is called the period of contraction. The period when the muscle is lengthening to its original length is called the period of relaxation.
If only one threshold stimulus is applied to a group of muscle fibers, the muscle fibers will contract and relax. This action that lasts less than a second is called a twitch.
Click here for examples of myograms
A single muscle twitch does not generate much strength. If a group of muscle fibers are stimulated to contract and then stimulated again to contract before they relax, the second contraction will be stronger than the first because the contraction of the two twitches are added together in a process called summation. When muscle fibers are continually stimulated to contract, the resulting contraction is called tetanic. Tetanic contractions do not allow muscle fibers to contract at all!
A single muscle is organized into many motor units. Recall that a motor unit contains one motor neuron and all the muscle fibers the neuron stimulates. If an entire muscle wants to contract, many motor units will have to be activated. An increase in the number of activated motor units is called recruitment. The stronger the contraction of the muscle, the more motor units that have been recruited.
Even when a muscle appears to be totally relaxed, it has some muscle fibers that are contracting. Therefore, all muscles are in a state of partial contraction. These partial contractions produce muscle tone. When a muscle appears to be relaxed, most of the motor units are not active. However, a few motor units are always active which produces sustained or continual contractions in the muscle. Muscle tone is maintained by sustained contractions!
Recall that smooth muscle fibers lack striations and are not under voluntary control. There are two types of smooth muscles.
1) Multiunit smooth muscle - This type of smooth muscle is found in the iris of the eye and in the walls of blood vessels. This muscle type contracts in response to motor neurons or hormones.
2) Visceral smooth muscle - This muscle type contains sheets of muscle cells that closely contact each other. It is found in the walls of hollow organs such as the stomach, intestines, bladder, uterus,etc. Muscle fibers in visceral smooth muscles stimulate each other to contract. Therefore the muscle fibers tend to contract and relax together. This type of muscle produces an action called peristalsis. Peristalsis is a rhythmic contraction that pushes substances through tubes of the body. The stomach and intestines exhibit peristalsis.
Two neurotransmitters are involved in smooth muscle contraction, 1) acetylcholine and 2) norepinephrine. Depending on the smooth muscle type, these neurotransmitter can cause smooth muscle contraction or inhibit smooth muscle contraction. Hormones also affect the contraction of smooth muscle. Smooth muscle fibers are slower to contract and relax than skeletal muscle fibers.
Recall that cardiac muscle is only found in the wall of the heart, is striated and not under voluntary control. Groups of cardiac muscle are connected to each other through intercalated discs. These discs allow the fibers in that group to contact and relax together. This design allows the heart to work as a pump. Cardiac muscle is also self -exciting (it does not need nerve stimulation to contract).
Click here for a summary table of the characteristics of the three muscle types.
The action a skeletal muscle depends largely on what the skeletal muscles are attached to.
Origins and insertions are sites of attachments for skeletal muscles. The attachment sites that move when the muscle contracts is called the insertions. The attachment sites that do not move is called the origins. For example, the biceps brachii (muscle on the front of the upper arm) attaches to two places on the scapula and one site on the radius. When the biceps brachii contracts the radius moves ( the arm bends at the elbow). Therefore, the site where the biceps brachii attaches to the radius is its insertion. The sites where the biceps brachii attaches to the scapula are its origins.
Click here to view the origins and insertions of the biceps brachii.
Most of the time when a body movement is produced a group of muscles contract, not just one individual muscle. One muscle however is responsible for most of the movement; this muscle is called the prime mover. Other muscles help the prime mover by stablilzing joints; these muscles are called synergists. An antagonist is a muscle that produces the opposite movement of the prime mover. When the prime mover contracts, the antagonist must relax in order to produce a smooth body movement.
For example, when a person bends their arm at the elbow, the prime mover is the biceps brachii. The synergist muscles are the brachialis and brachioradialis. The antagonist is the triceps brachii because its action is to extend the arm at the elbow.
Frontalis - attaches to the frontal bone and skin above the eyes; raises the eyebrows
Orbicularis oris - attaches to muscles near the mouth and skin around mouth; puckers the lips (kissing muscle)
Orbicularis oculi - attaches to maxillary and frontal bones; also attaches to skin around the eyes; closes the eyes (winking muscle)
Zygomaticus - attaches to the zygomatic bone and orbicularis oris; pulls corners of the mouth up (smiling muscle)
Platysma - attaches to skin of upper chest and lower part of mandible; pulls corners of the mouth down (frowning muscle)
Masseter - attaches to lower border of zygomatic arch and lateral surface of mandible; closes the jaw
Temporalis - attaches to the temporal bone and lateral surface of mandible; closes the jaw
Sternocleidomastoid - attaches to the sternum, clavicle and mastoid process of temporal bone; pulls head to one side and pulls head to chest
Splenius capitis - attaches to cervical and thoracic vertebrae; also attaches to mastoid process of temporal bone; rotates the head and bends the head to the side
Trapezius - attaches to the occipital bone, cervical vertebrae and thoracic vertebrae; also attaches to the clavicle and scapula; raises the arm and pulls shoulder downward
Pectoralis minor - attaches to the sternal ends of the upper ribs and the lateral part of the scapula; pulls scapula downward or it can raise the ribs
Pectoralis major - attaches to clavicle, sternum and costal cartilages of upper ribs; also attaches to humerus; pulls arm across the chest; rotates arm; can adduct arm
Latissimus dorsi - attaches to spines of sacrum, lumbar vertebrae, thoracic vertebrae, iliac crest of coxal bone, and lower ribs; also attaches to humerus; acts to extend, adduct, and rotate arm inwardly.
Deltoid - attaches to top of scapula and clavicle; also attaches to humerus; acts to abduct and extend the arm at the shoulder
Subscapularis - attaches to anterior surface of scapula and proximal end of humerus; rotates the arm medially
Infraspinatus - attaches to posterior surface of scapula and proximal end of humerus; rotates the arm laterally
Biceps brachii - attaches to top of scapula and the radius; flexes arm at the elbow and rotates hand laterally
Brachialis - attaches to anterior surface of humerus and the ulna; flexes arm at the elbow
Brachioradialis - attaches to distal end of the humerus and the distal end of the radius on its lateral side; flexes forearm at the elbow
Triceps brachii - attaches to the top of the scapula and the humerus; also attaches to the posterior surface of the ulna; extends the arm at the elbow
Supinator - attaches to the lateral surface of the humerus at its distal end; also attaches to the ulna and the lateral surface of the radius; rotates the forearm laterally which is suppination
Pronator teres - attaches to the medial surface of the humerus at its distal end; also attaches to the ulna and lateral surface of the radius; rotates the forearm medially which is pronation
Flexor carpi radialis - attaches to the medial side of the humerus on its distal end and the bases of the second and third metacarpals; acts to flex the wrist and to abduct the wrist
Flexor carpi ulnaris - attaches to the medial surface of the humerus on its distal end and the proximal end of ulna; also attaches to the carpals and metacarpals; acts to flex the wrist and adduct the wrist
Palmaris longus - attaches to medial surface of humerus on its distal end and the fascia of the palm; acts to flex the wrist
Flexor digitorum profundus - attaches to the anterior surface of the ulna and the bases of distal phalanges in fingers 2,3,4, and 5; acts to flex the distal joints of fingers but not the thumb
Extensor carpi radialis longus and brevis (two muscles- brevis means short and longus means long) - attaches to the distal end of the humerus on its lateral surface and the posterior surfaces of the bases of the second and third metacarpals; acts to extend the wrist and to abduct the hand
Extensor carpi ulnaris - attaches to the distal end of the humerus on its lateral surface and the posterior surface of the base of the fifth metacarpal; acts to extend the wrist
Extensor digitorum - attaches to the distal end of humerus on its lateral surfce and the posterior surfaces of phalanges in fingers 2,3,4, and 5; acts to extend the fingers
External oblique - attaches to the outer surfaces of the lower ribs and the tops of the iliac bones; also attaches to the linea alba ( a band of tough connective tissue that connects the sternum to the pubic symphysis); compresses abdominal wall
Internal oblique - this muscle is deep to the external oblique; it attaches to the tops of the iliac bones, the lower ribs and the linea alba; compresses abdominal wall
Transverse abdominis - this muscle is deep to the internal oblique; it attaches to the lower ribs, the lumbar vertebrae, and the tops of the iliac bones; also attaches to the linea alba and the top of the pubis bones; compresses abdominal wall
Rectus abdominis - attaches to pubis bones, xiphoid process of the sternum, and costal cartilages; acts to flex vertebral column and to compress the abdominal wall
Psoas major - attaches to lumbar vertebrae and proximal end of femur; flexes the thigh
Iliacus - attaches to the ilium and the proximal end of femur; flexes the thigh
Gluteus maximus - attaches to the sacrum, coccyx, and posterior surface of the ilium; also attaches to the posterior surface of femur; extends the thigh
Gluteus medius and minimus ( two muscles -medius means medial and minimus means small) - attach to lateral surface of ilium and the proximal ends of femur; abducts the thighs and rotates thighs medially
Adductor longus and magnus (two muscles - longus means long and magnus means large) - attach to the pubis bone and the ischium; adducts the thigh; also rotates thigh laterally
Sartorius - attaches to top of ilium and medial surface of tibia; flexes the leg at the knee and thigh, abducts the thigh, rotates the thigh laterally but it rotates the lower leg medially; it carries out the act of sitting "cross - legged"
Hamstring group - this is a group of three muscles (biceps femoris, semitendinosus, and semimembranosus); attaches to ischium and posterior surface of femur; also attaches to upper surfaces of tibia and fibula; acts to flex the leg at the knee and extend the leg at the thigh
Quadriceps group - this is a group of four muscles (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius); attaches to the ilium and femur; also attaches to the patella and anterior surface of tibia; acts to extend the leg at the knee
Tibialis anterior - attaches to the lateral surface of the tibia; also attaches to the medial tarsals and the first metatarsal; acts to invert the foot and point the foot up (dorsiflexion)
Extensor digitorum longus - attaches to the lateral surface of the tibia at its distal end and anterior surface of the fibula; also attaches to to the dorsal surfaces of the phalanges of the four lateral toes (all toes but the big toe); acts to extend the toes and dorsiflex the foot.
Gastrocnemius - attaches to femur and the calcaneus (heel bone); acts to plantarflex the foot and flex the leg at the knee
Solues- this muscle is deep to the gastrocnemius; attaches to the fibula and posterior surface of the tibia; also attaches to the calcaneus; acts to plantarflex the foot
Flexor digitorum longus - attaches to the posterior surface of the tibia and the phalanges of the four lateral toes; acts to plantarflex the foot and flex the toes
Click below to see the following muscle groups
Muscles of facial expression Buttock muscles
Muscles of facial expression and arm muscles
Superficial muscles Arm and hand muscles
Superficial muscles 2 Leg muscles
Abdominal muscles Leg and foot muscles