The Muscular System

The Muscular System
Muscles are responsible for all types of body movement

Three basic muscle types are found in the body
Skeletal muscle
Cardiac muscle
Smooth muscle

Characteristics of Muscles
Skeletal and smooth muscle cells are elongated (muscle cell = muscle fiber)
Contraction and shortening of muscles is due to the movement of microfilaments
All muscles share some terminology
Prefixes myo and mys refer to “muscle”
Prefix sarco refers to “flesh”

Skeletal Muscle Characteristics
Most are attached by tendons to bones
Cells are multinucleate
Striated—have visible banding
Voluntary—subject to conscious control

Connective Tissue Wrappings of Skeletal Muscle
Cells are surrounded and bundled by connective tissue
Endomysium—encloses a single muscle fiber
Perimysium—wraps around a fascicle (bundle) of muscle fibers
Epimysium—covers the entire skeletal muscle
Fascia—on the outside of the epimysium

Skeletal Muscle Attachments
Epimysium blends into a connective tissue attachment
Tendons—cord-like structures 
Mostly collagen fibers
Often cross a joint due to toughness and small size
Aponeuroses—sheet-like structures
Attach muscles indirectly to bones, cartilages, or connective tissue coverings

Sites of muscle attachment

Connective tissue coverings

Smooth Muscle Characteristics
Lacks striations
Spindle-shaped cells
Single nucleus
Involuntary—no conscious control
Found mainly in the walls of hollow organs

Cardiac Muscle Characteristics
Usually has a single nucleus
Branching cells
Joined to another muscle cell at an intercalated disc
Found only in the walls of the heart

Skeletal Muscle Functions
Produce movement
Maintain posture
Stabilize joints
Generate heat

Microscopic Anatomy of Skeletal Muscle
Sarcolemma—specialized plasma membrane
Myofibrils—long organelles inside muscle cell
Sarcoplasmic reticulum—specialized smooth endoplasmic reticulum

Myofibrils are aligned to give distinct bands
I band = light band
Contains only thin filaments
A band = dark band 
Contains the entire length of the thick filaments

Sarcomere—contractile unit of a muscle fiber
Organization of the sarcomere
Thick filaments = myosin filaments
Thin filaments = actin filaments

Thick filaments = myosin filaments
Composed of the protein myosin
Has ATPase enzymes
Myosin filaments have heads (extensions, or cross bridges)
Myosin and actin overlap somewhat

Thin filaments = actin filaments
Composed of the protein actin
Anchored to the Z disc
At rest, within the A band there is a zone that lacks actin filaments 
Called either the H zone or bare zone
Sarcoplasmic reticulum (SR) 
Stores and releases calcium
Surrounds the myofibril

Stimulation and Contraction of 

Single Skeletal Muscle Cells
Excitability (also called responsiveness or irritability)—ability to receive and respond to a stimulus
Contractility—ability to shorten when an adequate stimulus is received
Extensibility—ability of muscle cells to be stretched
Elasticity—ability to recoil and resume resting length after stretching

The Nerve Stimulus and Action Potential
Skeletal muscles must be stimulated by a motor neuron (nerve cell) to contract

Motor unit
—one motor neuron and all the skeletal muscle cells stimulated by that neuron

Neuromuscular junction

Association site of axon terminal of the motor neuron and muscle

Synaptic cleft 

Gap between nerve and muscle
Nerve and muscle do not make contact
Area between nerve and muscle is filled with interstitial fluid
Action potential reaches the axon terminal of the motor neuron
Calcium channels open and calcium ions enter the axon terminal

Transmission of Nerve Impulse to Muscle
Calcium ion entry causes some synaptic vesicles to release their contents (acetylcholine, a neurotransmitter) by exocytosis
Neurotransmitter—chemical released by nerve upon arrival of nerve impulse in the axon terminal
The neurotransmitter for skeletal muscle is acetylcholine (ACh)
ACh attaches to receptors on the sarcolemma of the muscle cell
In response to the binding of ACh to a receptor, the sarcolemma becomes permeable to sodium (Na+)
Sodium rushes into the cell generating an action potential and potassium leaves the cell
Once started, muscle contraction cannot be stopped

The Sliding Filament Theory of Muscle Contraction
Activation by nerve causes myosin heads (cross bridges) to attach to binding sites on the thin filament
Myosin heads then bind to the next site of the thin filament and pull them toward the center of the sarcomere
This continued action causes a sliding of the myosin along the actin
The result is that the muscle is shortened (contracted) 

Contraction of Skeletal Muscle
Muscle fiber contraction is “all or none”
Within a skeletal muscle, not all fibers may be stimulated during the same interval
Different combinations of muscle fiber contractions may give differing responses

Graded responses—different degrees of skeletal muscle shortening
Graded responses can be produced by changing:
The frequency of muscle stimulation
The number of muscle cells being stimulated at one time

Types of Graded Responses
Single, brief contraction
Not a normal muscle function
Summing of contractions
One contraction is immediately followed by another
The muscle does not completely return to a resting state due to more frequent stimulations
The effects are added
Unfused (incomplete) tetanus
Some relaxation occurs between contractions but nerve stimuli arrive at an even faster rate than during summing of contractions
Unless the muscle contraction is smooth and sustained, it is said to be in unfused tetanus
Fused (complete) tetanus
No evidence of relaxation before the following contractions
Frequency of stimulations does not allow for relaxation between contractions
The result is a smooth and sustained muscle contraction

Muscle Response to Strong Stimuli
Muscle force depends upon the number of fibers stimulated
More fibers contracting results in greater muscle tension
Muscles can continue to contract unless they run out of energy

Energy for Muscle Contraction
Initially, muscles use stored ATP for energy
ATP bonds are broken to release energy
Only 4–6 seconds worth of ATP is stored by muscles
After this initial time, other pathways must be utilized to produce ATP

Direct phosphorylation of ADP by creatine phosphate (CP)
Muscle cells store CP
CP is a high-energy molecule
After ATP is depleted, ADP is left
CP transfers a phosphate group to ADP, to regenerate ATP
CP supplies are exhausted in less than 15 seconds
About 1 ATP is created per CP molecule

Aerobic respiration
Glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP)
A series of metabolic pathways occur in the mitochondria
This is a slower reaction that requires continuous oxygen
Carbon dioxide and water are produced 

Anaerobic glycolysis and lactic acid formation
Reaction that breaks down glucose without oxygen
Glucose is broken down to pyruvic acid to produce about 2 ATP
Pyruvic acid is converted to lactic acid
This reaction is not as efficient, but is fast
Huge amounts of glucose are needed
Lactic acid produces muscle fatigue

Muscle Fatigue and Oxygen Deficit
When a muscle is fatigued, it is unable to contract even with a stimulus
Common cause for muscle fatigue is oxygen debt
Oxygen must be “repaid” to tissue to remove oxygen deficit
Oxygen is required to get rid of accumulated lactic acid
Increasing acidity (from lactic acid) and lack of ATP causes the muscle to contract less

Types of Muscle Contractions
Isotonic contractions
Myofilaments are able to slide past each other during contractions
The muscle shortens and movement occurs
Example: bending the knee; rotating the arm
Isometric contractions
Tension in the muscles increases
The muscle is unable to shorten or produce movement
Example: push against a wall with bent elbows
Muscle Tone
Some fibers are contracted even in a relaxed muscle
Different fibers contract at different times to provide muscle tone and to be constantly ready 

Effect of Exercise on Muscles
Exercise increases muscle size, strength, and endurance
Aerobic (endurance)
exercise (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue
Makes body metabolism more efficient
Improves digestion, coordination
Resistance (isometric) exercise (weight lifting) increases muscle size and strength

Five Golden Rules of Skeletal Muscle Activity

Muscles and Body Movements

Movement is attained due to a muscle moving an attached bone
Muscles are attached to at least two points
Attachment to a moveable bone
Attachment to an immovable bone

Types of Body Movements

Movement of a bone around its longitudinal axis
Common in ball-and-socket joints
Example is when you move atlas around 
the dens of axis (shake your head “no”)
Types of Body Movements

Movement of a limb away from the midline
Opposite of abduction
Movement of a limb toward the midline
Combination of flexion, extension, abduction, and adduction
Common in ball-and-socket joints

Special Movements
Lifting the foot so that the superior surface approaches the shin (toward the dorsum)
Plantar flexion
Depressing the foot (pointing the toes)
“Planting” the foot toward the sole
Turn sole of foot medially
Turn sole of foot laterally
Forearm rotates laterally so palm faces anteriorly
Radius and ulna are parallel
Forearm rotates medially so palm faces posteriorly 
Radius and ulna cross each other like an X
Move thumb to touch the tips of other fingers on the same hand

Types of Muscles
Prime mover—muscle with the major responsibility for a certain movement
Antagonist—muscle that opposes or reverses a prime mover
Synergist—muscle that aids a prime mover in a movement and helps prevent rotation
Fixator—stabilizes the origin of a prime mover

Naming Skeletal Muscles
By direction of muscle fibers
Example: Rectus (straight)
By relative size of the muscle
Example: Maximus (largest)
By location of the muscle
Example: Temporalis (temporal bone)
By number of origins
Example: Triceps (three heads)
By location of the muscle’s origin and insertion
Example: Sterno (on the sternum)
By shape of the muscle
Example: Deltoid (triangular)
By action of the muscle
Example: Flexor and extensor (flexes or extends a bone)

Head and Neck Muscles
Facial muscles
Frontalis—raises eyebrows
Orbicularis oculi—closes eyes, squints, blinks, winks
Orbicularis oris—closes mouth and protrudes the lips
Buccinator—flattens the cheek, chews
Zygomaticus—raises corners of the mouth

Chewing muscles
Masseter—closes the jaw and elevates mandible
Temporalis—synergist of the masseter, closes jaw
Head and Neck Muscles

Neck muscles
Platysma—pulls the corners of the mouth inferiorly
Sternocleidomastoid—flexes the neck, rotates the head

Muscles of Trunk, Shoulder, Arm
Anterior muscles
Pectoralis major—adducts and flexes the humerus
Intercostal muscles 
External intercostals—raise rib cage during inhalation
Internal intercostals—depress the rib cage to move air out of the lungs when you exhale forcibly

Muscles of the abdominal girdle
Rectus abdominis—flexes vertebral column and compresses abdominal contents (defecation, childbirth, forced breathing)
External oblique—flex vertebral column; rotate trunk and bend it laterally
Internal oblique—flex vertebral column; rotate trunk and bend it laterally
Transversus abdominis—compresses abdominal contents

Posterior muscles
Trapezius—elevates, depresses, adducts, and stabilizes the scapula
Latissimus dorsi—extends and adducts the humerus
Erector spinae—back extension
Quadratus lumborum—flexes the spine laterally
Deltoid—arm abduction

Muscles of the Upper Limb
Biceps brachii—supinates forearm, flexes elbow
Brachialis—elbow flexion
Brachioradialis—weak muscle; elbow flexion
Triceps brachii—elbow extension (antagonist to biceps brachii)

Muscles of the Lower Limb
Muscles causing movement at the hip joint include:
Gluteus maximus—hip extension
Gluteus medius—hip abduction, steadies pelvis when walking
Iliopsoas—hip flexion, keeps the upper body from falling backward when standing erect
Adductor muscles—adduct the thighs
Muscles causing movement at the knee joint
Hamstring group—thigh extension and knee flexion
Biceps femoris
Muscles causing movement at the knee joint
Sartorius—flexes the thigh
Quadriceps group—extends the knee
Rectus femoris
Vastus muscles (three)
Muscles causing movement at ankle and foot
Tibialis anterior—dorsiflexion, foot inversion
Extensor digitorum longus—toe extension and dorsiflexion of the foot
Fibularis muscles—plantar flexion, foot eversion
Soleus—plantar flexion