Chapter 6 The Muscular System
Muscles are responsible for all types of body movement – they contract (shorten) and are the machines
of the body
Muscle cells are elongated and for this reason are called muscle fibers
Muscle cell = muscle fiber
Three basic muscle types are found in the body
 Skeletal muscle
 Cardiac muscle
 Smooth muscle
All muscles share some terminology
 Prefix myo refers to muscle
 Prefix mys refers to muscle
 Prefix sarco refers to flesh






Skeletal Muscles
Most are attached by tendons to bones
Cells are multinucleated (many nuclei)
Striated – have visible banding
Voluntary – subject to conscious control
Cells are surrounded and bundled by connective tissue = great force, but tires easily
Cells can be huge, some up to 1 foot in length
Connective Tissue Wrappings of Skeletal Muscle (keep soft muscle cells from being ripped when
exerting force)
 Endomysium – around single muscle fiber
 Perimysium – around a (bundle) of fibers, this bundle of fibers is called a fascicle
 Epimysium – covers the entire skeletal muscle, the epimysium blends into either tendons or
aponeuroses
 Tendon – cord-like structure that connects muscle to bones or cartilage
 Aponeuroses – sheet-like structure the connects muscle to bones or cartilage
Smooth Muscle Characteristics
 Has no striations
 Spindle-shaped cells
 Single nucleus
 Involuntary – no conscious control
 Found mainly in the walls of hollow organs
 Slow, sustained and tireless
Cardiac Muscle Characteristics
 Has striations
 Usually has a single nucleus
 Joined to another muscle cell at an intercalated disc
 Involuntary
 Found only in the heart
 Steady pace!
Functions of Muscules
 Produce movement
 Maintain posture
 Stabilize joints
 Generate heat
Microscopic Anatomy of a Skeletal Muscle
Sarcolemma—plasma membrane of a muscle cell
Myofibrils-ribbon like organelles that fill the cells, made of alternating light (I) and dark (A) bands
These bands give a striated or striped appearance
Myofibrils are chains of sarcomeres –the contractile units of a muscle fiber
Sarcomeres have myofilaments, some are thick filaments that contain myosin and others are thin
filaments that contain actin
For a muscle to contract these thick and thin filaments must slide past each other
Contraction requires calcium and ATP (energy) molecules
Sliding Filament Theory




Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on the thin
filament
Myosin heads then bind to the next site of the thin filament
This continued action causes a sliding of the myosin along the actin
The result is that the muscle is shortened (contracted)
To contract skeletal muscle cells must be stimulated by nerve impulses. One motor neuron (nerve cell)
may stimulate a few muscle cells or hundreds of them. One neuron and all the skeletal muscle cells it
stimulates are a motor unit.
A neurotransmitter called acetylcholine of Ach stimulates skeletal muscle cells.
Muscle fiber contraction is “all or none” but different movements and forces happen because:
The frequency of stimulation can change
The number of muscle cells being stimulated at one time can change
Different combinations of muscle fiber contractions may give differing responses this is called a graded
response
1. Muscle force depends upon the number of fibers stimulated
2. More fibers contracting results in greater muscle tension
Because of rapid stimulus we have continuous contraction which makes movement smooth
Muscles can continue to contract unless they run out of energy
Initially, muscles use stored ATP for energy
 Bonds of ATP 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
Methods of Providing Energy for Muscle Contraction
1. Direct phosphorylation
 Muscle cells contain creatine phosphate (CP)
 CP is a high-energy molecule
 After ATP is depleted, ADP is left
 CP transfers energy to ADP, to regenerate ATP
2. Anaerobic glycolysis
 Reaction that breaks down glucose without oxygen
 Glucose is broken down to pyruvic acid to produce some 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
3. Aerobic Respiration
 Series of metabolic pathways that occur in the mitochondria
 Glucose is broken down to carbon dioxide and water, releasing energy
 This is a slower reaction that requires continuous oxygen
When a muscle is fatigued, it is unable to contract
 The common reason for muscle fatigue is oxygen debt
 Oxygen must be “repaid” to tissue to remove oxygen debt
 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


Isotonic contractions
 Myofilaments are able to slide past each other during contractions
 The muscle shortens
 Body part moves (lifting weights)
Isometric contractions
 Tension in the muscles increases
 The muscle is unable to shorten
 Body doesn’t move, (wall sits)

Muscles are attached to at least two points
 Origin – attachment to a immoveable bone
Insertion – attachment to an movable bone
Types of Ordinary Body Movements
 Flexion – decreases angle of joint and brings two bones closer together
 Extension- opposite of flexion
 Rotation- movement of a bone in longitudinal axis, shaking head “no”
 Abduction/Adduction (see slides)
 Circumduction (see slides)
 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
 Location of the muscles origin and insertion
 Example: sterno (on the sternum)
 Shape of the muscle
 Example: deltoid (triangular)
 Action of the muscle
Example: flexor and extensor (flexes or extends a bone)
Disorders relating to the Muscular System
• Muscular Dystrophy: inherited, muscle enlarge due to increased fat and connective tissue, but
fibers degenerate and atrophy
• Duchenne MD: lacking a protein to maintain the sarcolemma
• Myasthemia Gravis: progressive weakness due to a shortage of acetylcholine receptors
Direct Phosphorylation-uses creatine phosphate, fastest, quickly exhausted, i.e sprint
Aerobic respiration-oxygen needed, glucose completely broken down, slow, at rest or light exercise
Anaeorobic glycolysis-without oxygen, glucose becomes lactic acid, faster than aerobic, intense exercise
Fatigue-oxygen deficit, muscles are firing without oxygen, eventually they will stop
Twitch---jerky movement, abnormal
Tetanic-continuous, smooth movement, normal
Isotonic- movement
Isometric-no movement
4 ways we name muscles: # of origins, shape, size, action
Origin-where muscle is attached to immovable bone
Insertion-where muscle attaches to bone that moves
Muscle tone-sustained partial contraction
Prime mover-muscle with major responsibility for movement
Types of Body Movement
Dorsiflexion and plantar flexion-up and down of foot, dorsiflexion is up, plantar is down
Supination and pronation-movements of the radius around the ulna ,
supination palm front pronation palm back
Adduction/abduction-moving limb toward, moving limb away from midline
Extension/flexion-2 bones farther apart, 2 bones closer together