Muscular System

are responsible for all types of body movements

Muscles

Three basic muscle types are found in the body

Skeletal, Cardiac, and Smooth Muscles

Contraction and shortening of muscles are due to the movement of _____________

myofilaments

Prefixes that refer to “muscle”

myo- and mys-

Prefixes that refer to “flesh”

sarco-

Most skeletal muscle fibers are attached by

tendons to bones

are large, cigar-shaped, and multinucleate.

Skeletal muscle cells

Skeletal muscle is also known as _______ because of

its obvious stripes

striated muscle

Skeletal muscle is also known as a ________ because it is the only muscle tissue subject to conscious control.

voluntary muscle

Skeletal muscle cells surrounded by connective tissues: encloses a single muscle fiber

Endomysium

Skeletal muscle cells surrounded by connective tissues: wraps around a fascicle (bundle) of muscle fibers

Perimysium

Skeletal muscle cells surrounded by connective tissues: covers the entire skeletal muscle

Epimysium

Skeletal muscle cells surrounded by connective tissues: on the outside of the epimysium

Fascia

• cordlike structures

• Mostly collagen fibers

• Often cross a joint because of their toughness and small size

Tendons

• sheetlike structures

• Attach muscles indirectly to bones, cartilages, or connective tissue coverings.

Aponeuroses

• No striations

• Involuntary—no conscious control

• Found mainly in the walls of hollow visceral organs (such as the stomach, urinary bladder, and respiratory passages)

• Spindle-shaped fibers that are uninucleate

• Contractions are slow and sustained.

Smooth Muscles

• Striations

• Involuntary

• Found only in the walls of the heart

• Uninucleate

• Branching cells joined by gap junctions, called intercalated discs

• Contracts at a steady rate set by the pacemaker.

Cardiac muscles

skeletal muscle has three other important roles:

Maintain posture and body position, Stabilize joints, Generate heat

Microscopic Anatomy of Skeletal Muscle: specialized plasma

membrane

Sarcolemma

long organelles inside a muscle cell

Myofibrils

Contains only thin filaments

Z disc is a midline interruption

I band/ light band

Contains the entire length of the thick filaments

H zone is a lighter central area

M line is in center of H zone

A band/ dark band

contractile unit of a muscle fiber

Structural and functional unit of skeletal muscle

Sarcomere

produce the banding (striped) pattern

Myofilaments

Thick myofilaments

myosin filaments

Thin myofilaments

actin filaments

• Composed of the protein myosin

• Contain ATPase enzymes to split ATP to release energy for muscle contractions

• Possess projections known as myosin heads

• Myosin heads are known as cross-bridges when they link thick and thin filaments during contraction.

myosin filaments

• Composed of the contractile protein actin

• Actin is anchored to the Z disc

• At rest, within the A band, there is a zone that lacks actin filaments called the H zone

• During contraction, H zones disappear as actin and myosin filaments overlap

actin filaments

• Specialized smooth endoplasmic reticulum

• Surrounds the myofibril

• Stores and releases calcium.

Sarcoplasmic reticulum (SR)

Stimulation and Contraction of Single Skeletal Muscle Cells: ability to receive and respond to a stimulus

Irritability/ responsiveness

Stimulation and Contraction of Single Skeletal Muscle Cells: ability to forcibly shorten when an adequate stimulus is received

Contractility

Stimulation and Contraction of Single Skeletal Muscle Cells: the ability of muscle cells to be stretched

Extensibility

Stimulation and Contraction of Single Skeletal Muscle Cells: ability to recoil and resume resting length after stretching.

Elasticity

Skeletal muscles must be stimulated by a ____________ to contract

motor neuron (nerve cell)

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

Motor unit

Association site of axon terminal of the motor neuron and sarcolemma of a muscle

Neuromuscular junction

• Chemical released by nerve upon arrival of nerve impulse in the axon terminal

• Acetylcholine (ACh) is the neurotransmitter that stimulates skeletal muscle.

Neurotransmitter

is the neurotransmitter that stimulates skeletal muscle.

Acetylcholine (ACh)

• Gap between nerve and muscle filled with interstitial fluid

• Although very close, the nerve and muscle do not make contact

Synaptic cleft

When a nerve impulse reaches the axon terminal of the motor neuron: Step 1

Calcium channels open, and calcium ions enter the axon terminal

When a nerve impulse reaches the axon terminal of the motor neuron: Step 2

Calcium ion entry causes some synaptic vesicles to release acetylcholine (ACh)

When a nerve impulse reaches the axon terminal of the motor neuron: Step 3

ACh diffuses across the synaptic cleft and attaches to receptors on the sarcolemma of the muscle cell

When a nerve impulse reaches the axon terminal of the motor neuron: Step 4

If enough ACh is released, the sarcolemma becomes temporarily more permeable to sodium ions (Na+)

• Potassium ions (K+) diffuse out of the cell

• More sodium ions enter than potassium ions leave

• Establishes an imbalance in which interior has more positive ions (depolarization), thereby opening more Na+ channels

When a nerve impulse reaches the axon terminal of the motor neuron: Step 5

Depolarization opens more sodium channel that allow sodium ions to enter the cell

• An action potential is created

• Once begun, the action potential is unstoppable

• Conducts the electrical impulse from one end of the cell to the other

When a nerve impulse reaches the axon terminal of the motor neuron: Step 6

Acetylcholinesterase (AChE) breaks down acetylcholine into acetic acid and choline

• AChE ends muscle contraction

• A single nerve impulse produces only one contraction

Cell returns to its resting state when:

1. Potassium ions (K+) diffuse out of the cell

2. Sodium-potassium pump moves sodium and potassium ions back to their original positions

A&P Flix™: Events at the Neuromuscular Junction

Mechanism of Muscle Contraction: The Sliding Filament Theory

What causes filaments to slide?

• Calcium ions (Ca2+) bind regulatory proteins on thin filaments and expose myosin-binding sites, allowing the myosin heads on the thick filaments to attach

• Each cross-bridge pivots, causing the thin filaments to slide toward the center of the sarcomere

• Contraction occurs, and the cell shortens

• During a contraction, a cross-bridge attaches and detaches several times

• ATP provides the energy for the sliding process, which continues as long as calcium ions are present.

A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Graded responses

• Muscle fiber contraction is “all-or-none,” meaning it will contract to its fullest when stimulated adequately

• Within a whole 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.

A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Graded responses can be produced in two ways

• By changing the frequency of muscle stimulation

• By changing the number of muscle cells being stimulated at one time

A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Muscle response to increasingly rapid stimulation

• Single, brief, jerky contraction

• Not a normal muscle function

A. Muscle twitch

A&A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Muscle response to increasingly rapid stimulation

• In most types of muscle activity, nerve impulses are delivered at a rapid rate

• As a result, contractions are “summed” (added) together, and one contraction is immediately followed by another

B. Summoned Contractions

A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Muscle response to increasingly rapid stimulation

• When stimulations become more frequent, muscle contractions get stronger and smoother

C. Unfused/Incomplete Tetanus

A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Muscle response to increasingly rapid stimulation

• is achieved when the muscle is stimulated so rapidly that no evidence of relaxation is seen

• Contractions are smooth and sustained

Fused (complete) tetanus

A&P Flix™: The Cross Bridge Cycle

Contraction of a Skeletal Muscle as a Whole

Muscle response to stronger stimuli

• Muscle force depends upon the number of fibers stimulated

• Contraction of more fibers results in greater muscle tension

• When all motor units are active and stimulated, the muscle contraction is as strong as it can get.

Providing Energy for Muscle Contraction:

• The only energy source that can be used to directly power muscle contraction

• Stored in muscle fibers in small amounts that are quickly used up

• After this initial time, other pathways must be utilized to produce ATP.

ATP

Providing Energy for Muscle Contraction:

Three pathways to regenerate ATP:

• fastest

• Muscle cells store CP, a high-energy molecule

• After ATP is depleted, ADP remains

• CP transfers a phosphate group to ADP to regenerate ATP

• CP supplies are exhausted in less than 15 seconds

• 1 ATP is produced per CP molecule

Direct phosphorylation of ADP by creatine phosphate (CP)

Providing Energy for Muscle Contraction:

Three pathways to regenerate ATP:

• Supplies ATP at rest and during light/moderate exercise

• A series of metabolic pathways, called oxidative phosphorylation, use oxygen and occur in the mitochondria

• Glucose is broken down to carbon dioxide and water, releasing energy (about 32 ATP)

• This is a slower reaction that requires continuous delivery of oxygen and nutrients

Aerobic pathway

Providing Energy for Muscle Contraction:

Three pathways to regenerate ATP:

• A 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, which causes muscle soreness

• This reaction is not as efficient, but it is fast

• Huge amounts of glucose are needed

Anaerobic

If muscle activity is strenuous and prolonged, _____ occurs

muscle fatigue

Suspected factors that contribute to muscle fatigue include:

• Ion imbalances (Ca2+, K+)

• Oxygen deficit and lactic acid accumulation

• Decrease in energy (ATP) supply

After exercise, the oxygen deficit is repaid by _______

rapid, deep breathing

Types of Muscle Contractions

• Myofilaments are able to slide past each other during contractions

• The muscle shortens, and movement occurs

• Example: bending the knee; lifting weights, smiling

Isotonic contractions

Types of Muscle Contractions

• Muscle filaments are trying to slide, but the muscle is pitted against an immovable object

• Tension increases, but muscles do not shorten you.

Isometric contractions

• State of continuous partial contractions

• Result of different motor units being stimulated in a systematic way

• Muscle remains firm, healthy, and constantly ready for action

Muscle tone

increases muscle size, strength, and endurance

exercise

Effect of Exercise on Muscles

• (biking, jogging) results in stronger, more flexible muscles with greater resistance to fatigue

• Makes body metabolism more efficient

• Improves digestion, coordination

Aerobic (endurance) exercise

Effect of Exercise on Muscles

• (weight lifting)increases muscle size and strength

• Individual muscle fibers enlarge

Resistance (isometric) exercise

Types of Body Movements

Muscles are attached to no fewer than two points:

attachment to an immovable or less movable bone

Origin

Types of Body Movements

Muscles are attached to no fewer than two points:

attachment to a movable bone

Insertion

occurs when muscles contract across joints

Body movement

When the muscle _______, the insertion moves______ the origin

contracts, toward

Types of Body Movements

• Decreases the angle of the joint

• Brings two bones closer together

• Typical of bending hinge joints (e.g., knee and elbow) or ball-and-socket joints (e.g., the hip)

Flexion

Types of Body Movements

• Opposite of flexion

• Increases angle between two bones

• Typical of straightening the elbow or knee

• Extension beyond 180º is ________

Extension, hyperextension

Types of Body Movements

• Movement of a bone around its longitudinal axis

• Common in ball-and-socket joints

• Example: moving the atlas around the dens of axis

• (i.e., shaking your head “no”)

Rotation

Types of Body Movements

• Opposite of abduction

• Movement of a limb toward the midline.

Adduction

Types of Body Movements

Movement of a limb away from the midline

Abduction

Types of Body Movements

• Combination of flexion, extension, abduction, and adduction

• Common in ball-and-socket joints

• The proximal end of the bone is stationary, and the distal end moves in a circle.

Circumduction

Special Movements

Lifting the foot so that the superior surface approaches the shin (toward the dorsum)

Dorsiflexion

Special Movements

Pointing the toes away from the head

Plantar flexion

Special Movements

Turning sole of foot medially

Inversion

Special Movements

Turning sole of foot laterally

Eversion

Special Movements

• Forearm rotates laterally so palm faces anteriorly

• Radius and ulna are parallel

Supination

Special Movements

• Forearm rotates medially so palm faces posteriorly

• Radius and ulna cross each other like an X

Pronation

Special Movements

Moving the thumb to touch the tips of other fingers on the same hand

Opposition

Interactions of Skeletal Muscles in the Body

Muscles can only ___ as they contract

pull

Interactions of Skeletal Muscles in the Body

In general, groups of muscles that produce opposite actions lie on ______ sides of a joint.

opposite

Interactions of Skeletal Muscles in the Body

muscle with the major responsibility for a certain movement

Prime mover

Interactions of Skeletal Muscles in the Body

muscle that opposes or reverses a prime mover

Antagonist

Interactions of Skeletal Muscles in the Body

muscle that aids a prime mover in a movement or reduces undesirable movements

Synergist

Interactions of Skeletal Muscles in the Body

Specialized synergists that hold a bone still or stabilize the origin of a prime mover

Fixator