Muscular System

“The function of muscle is to pull and not to push, except in the case of the genitals and the tongue.”- Leonardo da Vinci

Muscle Tissue•There are three types of

muscle tissue.•Smooth muscle lines the

walls of hollow organs.▫Involuntary.▫No striations, single

nucleus.

•Cardiac muscle forms the walls of the heart.▫Cells are branching and

striated.▫Single nucleus per cell.▫Intercalated discs divide

each cell.

•Skeletal muscle attached to bones.▫Voluntary.▫Long, striated, with

multiple nuclei.•The muscular system

refers to skeletal muscles only.

Muscular System Functions•Produce movement of

the skeleton.•Maintain body

position.•Support soft tissues.•Guard body openings.•Maintain body

temperature.

Muscle Fibers (cells)•Individual skeletal muscle cells are called

muscle fibers due to their length. •Skeletal muscle fibers are very long,

stretching the entire length of a muscle.•Each may contain

hundreds of nuclei.▫These nuclei are needed

to maintain control across the entire cell.

▫This arrangement makes mitosis impossible.

Muscle Anatomy – Connective Tissue

•The epimysium is a layer of collagen fibers that covers the entire muscle, separating it from surrounding organs.

•Each muscle is divided into bundles of muscle fibers, called a fascicle. ▫Each fascicle is divided by the

perimysium, which also contains blood vessels and nerves for the muscle.

•The endomysium surrounds and separates each individual muscle fiber (cell).

Tendon

Muscle FascicleFiber

Myofibril

Epimysium

Perimysium

Endomysium

Structures of the Muscle Fiber• The sarcolemma – The cell membrane of the

muscle fiber.• Sarcoplasm – The cytoplasm of a muscle fiber.• T-Tubules – Tubes that travel through the

sarcolemma to transmit signals to contract or relax.• Myofibrils – Bundles of protein filaments within

the muscle fiber.▫ Thin, light filaments, made of the protein actin▫ Thick, dark filaments, made of the protein myosin.▫ The alternation of these filaments creates the striated

pattern of skeletal muscle tissue.• Sarcoplasmic reticulum – A special type of

smooth ER found in muscle cells. Contains calcium needed for contraction.

T-Tubules Sarcoplasmic Reticulum Sarcolemma

MyosinActin

Muscle Contraction•Muscle contraction begins with a nerve

signal that is transmitted through the motor neuron.▫The neurotransmitter used as a signal is

acetylcholine.▫Contraction ends when the neurotransmitter is

broken down by an enzyme (acetylcholinesterase).

•This signal is sent to every fiber in the muscle simultaneously through the t-tubules.

•The sarcoplasmic reticulum releases calcium ions (Ca2+), initiating muscle contraction.

Muscle Contraction•The calcium influx stimulates the myosin

filaments to form connections to the actin filaments.

•The myosin filaments pull the actin filaments inward, causing the muscle to contract.

Muscle Fibers Before Contraction

Muscle Fibers After Contraction

Muscle Contraction•When a muscle contracts, it

pulls bones closer together, creating movement.▫Example: The biceps, the

scapula, radius, and ulna.•Contracted muscles become

more visible because all of the volume (cytoplasm) is forced outward.

Twitch•A twitch is a single sequence of a muscle

fiber that includes:▫Stimulus by a motor neuron.▫Contraction of the muscle.▫Relaxation of the muscle.

Twitch• A twitch begins

with a stimulus, or signal by a motor neuron.

• During the latent period, the signal is spreading across the muscle. No actual tension occurs. Resting

PhaseLatent Period

Twitch•In the

contraction phase, the myosin filaments pull on the actin, creating increasing amounts of tension. Resting

PhaseLatent Period

Contraction Phase

Twitch•Calcium levels

drop during the relaxation phase, and linkages between actin and myosin decline.

Resting Phase

Latent Period

Contraction Phase

Relaxation Phase

Summation•A single twitch is short

(milliseconds) and doesn’t really produce a useful movement.

•If a second stimulus arrives before the relaxation phase completes, a more powerful contraction occurs. This is summation.

Summation•Eventually this builds

into the maximum tension possible with the muscle. This is called incomplete tetanus.▫Relaxation phases still

occur, just not completely.

Summation•Complete tetanus

occurs if the stimulus frequency is so high that the relaxation phase is completely eliminated.

How long can muscles contract?•The duration of muscle contraction depends on

three factors:▫How long the stimulus from the neuron lasts.▫How many electrolytes needed for contraction (K+,

Na+, Ca2+) are available within the muscle.▫How much ATP energy is available within the

muscle.•ATP is the direct unit of energy used by muscle

fibers. If the supply of ATP is exhausted, the muscle becomes fatigued and will not contract.▫ATP is converted to ADP when it is used.▫An active skeletal muscle fiber may use 600

trillion molecules of ATP per second.

ATP while Resting•While at rest, muscles build up their

energy reserves.▫Energy in the form of fatty acids or glucose

is brought in by the blood.•Mitochondria convert the fatty acids and

glucose into ATP by cell respiration.

Cell Respiration•There are two main parts of cell

respiration:•Glycolysis, which occurs in the

cytoplasm.▫Breaks down glucose (6-carbon sugar) into

two molecules of pyruvate (3-carbon)▫Produces 2 molecules of ATP.▫Occurs quickly, does not require oxygen.

•The rest occurs in the mitochondria.▫Metabolizes the pyruvate into 3 molecules

of CO2.▫Produces 34 molecules of ATP.▫Requires oxygen.

Energy Storage in Muscles

•Creatine (C4H9N3O2) is an organic acid that can store energy in muscles as creatine-phosphate.

•Glycogen is a polysaccharide similar in structure to starch. ▫Made of multiple glucose

molecules bonded together. Some atheletes take supplements to increase creatine levels in muscle.

Blood VesselFatty Acids

Mitochondria

Glucose Glycogen

Creatine

This diagram shows a muscle fiber at rest.

ATP is generated in the mitochondria from fatty acids.Excess ATP is used to generate creatine-phosphate or glycogen.

ATP During Moderate Activity•During moderate activity (e.g. jogging),

the stored creatine phosphate and glycogen in the muscle is metabolized.▫CP can be directly used to generate ATP.▫Glycogen can be broken down into glucose,

which the mitochondria can use to generate ATP.

•Moderate activity can continue as long as:▫Oxygen levels are sufficient ( ↑ breathing

rate)▫Glycogen is not used up.

“Hitting the wall”

Blood VesselFatty Acids

Mitochondria

GlycogenGlucose

Pyruvate

ATP used as energy for contraction.This diagram shows a muscle fiber in moderate activity.

ATP is being generated in the mitochondria and cytoplasm from glucose and fatty acids.

ATP During Intense Activity•During intense activity (e.g. sprinting),

oxygen is unable to diffuse into the muscle fast enough to provide the needed ATP by cell respiration.

•The muscle begins relying on glycolysis to make up for this deficiency.▫This creates an excess of pyruvate, which is

converted to lactic acid.▫Lactic acid lowers the pH of the muscle and

interferes with normal function. Soreness.

Lactic Acid

GlycogenGlucose

Pyruvate

ATP used as energy for contraction.

Lactic Acid

This diagram shows a muscle fiber in intense activity.

Oxygen levels are insufficient; only glycolysis can occur. Excess pyruvate is converted to lactic acid.Creatine Phosphate is utilized as a source of ATP.

Fatigue•Fatigue occurs when the muscle can no

longer contract, despite stimulus from the brain.

•Caused by rapid and repeated contractions.▫ATP levels are too low.▫Lactic acid levels are too high.

•The muscles will not regain their normal ability to contract until pH, oxygen, and ATP levels have been restored.

Case Study: Rigor Mortis• When an organism dies, skeletal muscle is deprived

of nutrients and oxygen.• Calcium ions will leak out of the sarcoplasmic

reticulum, causing one last sustained muscle contraction.▫Because no ATP is left in the muscle, the myosin and

actin are unable to separate, causing rigor mortis.

Case Study: Rigor Mortis•As the cells of the body die, enzymes are

released from the lysosomes.•These enzymes break down the actin and

myosin filaments, and the muscles permanently go limp.

Endurance•Two factors influence

muscle endurance:▫The type of fibers within

the muscle.▫Physical conditioning.

Types of Muscle Fibers•Fast-twitch fibers are able to reach peak

tension within 0.01 seconds or less of neural stimulation.▫Large in diameter.▫Densely packed with myofibrils (actin and

myosin).▫Large glycogen reserves.▫Fewer mitochondria.

•Fast-twitch fibers produce the most tension, but get fatigued quickly.

Types of Muscle Fibers•Slow-twitch fibers can take three times

as long to reach peak tension.▫Half the diameter of fast-twitch fibers.▫Increased network of capillaries, allowing

for a greater and more reliable oxygen supply.

▫Contain a special protein called myoglobin that reserves additional oxygen within the muscle.

▫Higher numbers of mitochondria.

• Chicken meat is a good example of the differences between muscles with lots of slow-twitch or fast-twitch fibers.

• The breasts are “white meat” because they have mostly fast-twitch fibers and less myoglobin. ▫Wings are only used for short

intervals to escape predators.• The legs and thighs are “dark

meat” and have mostly slow-twitch fibers and more myoglobin.▫Used much more frequently during

the day, but not at the same intensity.

Physical Conditioning•The percentage of fast vs. slow-twitch

fibers in a muscle is genetically determined.

•The ability of fast-twitch muscles to resist fatigue can be increased through physical conditioning.

•Aerobic exercise focuses on improving endurance.▫Improved oxygen intake and delivery.▫Higher glycogen stores within muscle.

Physical Conditioning•Anaerobic

exercise focuses on improving strength.▫The number of

muscle fibers does not change, but their size can.

▫An increase in size is hypertrophy.

▫A decrease in size is atrophy.

Anabolic Steroids•Anabolic steroids are chemical compounds

that mimic the effects of testosterone.▫Increases protein synthesis, causing muscle

buildup.•The change in hormone levels has a lot of

other side effects:▫Increase in blood cholesterol.▫Acne▫High blood pressure▫Testicular atrophy▫Increase in male characteristics in women.

Testicular Atrophy?•One of the primary

functions of the testes is to produce the hormone testosterone.

•If anabolic steroids are taken externally, the testes will shrink in size, as the body attempts to compensate.▫This is called negative-

feedback inhibition.

Polio•Polio is a viral infection

that can enter the nervous system, specifically infecting and destroying motor neurons.▫This causes paralysis.

•Polio is now considered eradicated due to the use of a vaccine.

ALS (Lou Gehrig’s Disease)•Amyotrophic

Lateral Sclerosis (ALS) is a neurodegenerative disease that affects motor neurons in the brain and spinal cord.▫Muscle begins to

atrophy due to disuse.

Tetanus•Tetanus is a disease caused by a bacteria

called Clostridium tetanii.•The bacteria releases a toxin that stops

the normal motor neuron inhibition process.▫Muscles will uncontrollably contract and

maintain the contraction.▫The toxin spreads, causing symptoms such

as lockjaw.

Botulism•Botulism occurs when food contaminated

with a bacterial toxin are ingested.•The toxin prevents the release of

acetylcholine by motor neurons, causing paralysis.

Muscular Dystrophy

•Muscular dystrophy is a group of degenerative disorders that affect muscle tissue.▫Multiple causes, the

most common one is the lack of a protein needed to maintain muscle integrity.

Curare• This is a toxin produced

by poison dart frogs.• Blocks acetylcholine

receptors in muscles, causing paralysis.

• Neostigmine is a drug that prevents acetylcholine from breaking down. This makes it an effective treatment for curare.

• Discovered when it was observed that animals called peccaries were immune to the poison.

Muscle Anatomy

Muscle Attachment•All muscles are attached

to at least two points:▫The origin is an

attachment to a immoveable bone.

▫The insertion is an attachment to an movable bone.

Types of Muscle Movement• Flexion

▫Most often found in hinge joints.

▫Brings two bones closer together.

• Extension▫Opposite movement of

flexion.▫Pulls two bones farther

apart.• Hyperflexion or

hyperextension▫Flexion beyond the

normal position.

Types of Muscle Movement•Abduction

▫Pulls a limb away from the midline of the body.

•Adduction▫Pulls a limb toward

the midline of the body.

•Circumduction▫Rotation of a limb.

Types of Muscle Movement•Rotation is the

movement of a bone in a circular direction around a central axis.

•Medial rotation is toward the midline.

•Lateral rotation is away from the midline.

• Some muscles are named based on the direction of their fibers.

• Rectus means straight.▫Rectus abdominis.

• Oblique means diagonally arranged.▫External abdominal

oblique.

How Are Muscles Named?

•Muscles within a group may have different names based on their size.

•Maximus and longus indicates a larger muscle.▫Fibularis longus

•Minimus and brevis indicate a smaller muscle.▫Fibularis brevis

•Prefixes like bi- and tri- may be used to indicate multiple heads or attachment sites. ▫Triceps brachii

•Muscles may also be named based on their origin and insertion bones.▫Sternocleidomastoid: sternum, clavicle,

mastoid process of the temporal bone

•If a muscle resembles a shape, it can be named after that shape.▫The deltoid is named after the Greek letter

Delta, which is a triangle.

•Finally, muscle names may indicate a specific action they perform.▫Flexor carpi

ulnaris

Orbicularis Oris

Orbicularis Oculi

Sternocleidomastoid

TemporalisFrontalisZygomatic Bone

Buccinator Masseter

Trapezius

Zygomaticus

Head and Neck Muscles• The frontalis raises the eyebrows.• The masseter and temporalis both elevate the

mandible. ▫ Chewing muscles

• The buccinator flattens the cheeks during chewing, holding them against the teeth.

• The orbicularis oculi performs all eyelid movements, including opening, closing, blinking, etc.

• The orbicularis oris closes the mouth with the lips.• The zygomaticus raises the corners of the mouth

when smiling.• The sternocleidomastoid rotates the head and flexes

the neck.

Sternocleidomastoid

Internal Abdominal Oblique

SternumDeltoid

Serratus Anterior

Clavicle

External Abdominal Oblique

Rectus Abdominis

Pectoralis Major

Trapezius

Latissimus Dorsi

Muscles of the Trunk•The pectoralis major adducts the

humerus.•The rectus abdominis flexes the

vertebral column and compresses the contents of the abdomen.▫The “pushing” muscle of defecation, childbirth,

and forced breathing.▫The transversus abdominis also performs

this action.•The external and internal obliques

rotate the trunk.

Sternocleidomastoid

Trapezius

Deltoid

Teres MajorInfraspinatus

External Oblique

Latissimus Dorsi

Muscles of the Dorsal Trunk•The trapezius elevates and depresses the

scapula.•The latissimus dorsi adducts the

humerus.•The deltoid abducts the arm.

Deltoid

Brachioradialis

Biceps

Extensor Digitorum

Triceps

Extensor Carpi UlnarisExtensor Carpi Radialis

Anterior Muscles of the Arm•The biceps brachii and brachioradialis

flex the arm.•The triceps brachii extends the arm.•The extensor carpi radialis and ulnaris

extend the wrist.•The extensor digitorum extends the four

non-thumb digits.

Posterior Muscles of the Leg

Gluteus medius

Gluteus maximus

SemitendinosusBiceps femorisSemimembranosus

Gastrocnemius

Soleus

Anterior Muscles of the Leg

Gluteus mediusTensor Fascia Latae

Sartorius

Rectus FemorisVastus Medialis

FibularisTibialis Anterior

Vastus Lateralis

Soleus

Gracilis

Muscles of the Hip, Thigh, and Leg

•The gluteus maximus adducts and extends the leg.

•The gluteus medius abducts the leg.•The hamstring group flexes the knee.

▫Biceps femoris▫Semitendinosus▫Semimembranosus

•The gastrocnemius and soleus extend the foot.

Muscles of the Hip, Thigh, and Leg

•The sartorius flexes, abducts, and laterally rotates the thigh.▫Look at the bottom of your foot while

standing to demonstrate these actions.•The quadriceps group extends the knee.

▫Rectus femoris▫Vastus medialis▫Vastus lateralis▫Vastus intermedialis (a deep muscle)

Muscles of the Hip, Thigh, and Leg

•The tibialis anterior and fibularis muscles flex the foot.

Frontalis

MasseterSternocleidomastoid

External Oblique

Gluteus MediusTensor Fascia Latae

Rectus FemorisVastus LateralisVastus Medialis

Fibularis LongusSoleus

Tibialis Anterior

TemporalisOrbicularis Oculi

ZygomaticusOrbicularis Oris

Pectoralis Major

Biceps Brachii

Deltoid

Rectus AbdominisInternal Oblique

Adductor (Groin)Gracilis

Sartorius

TrapeziusDeltoid

Triceps BrachiiLatissimus Dorsi

External ObliqueGluteus Medius

Soleus

Gastrocnemius

SemitendinosusBiceps Femoris

AdductorGluteus Maximus