Muscular System

Muscles are responsible for all types of body movement (Machines of the body)Three basic muscle types are found in the

body: Skeletal muscle

Cardiac muscle Smooth muscle

Characteristics of MuscleSkeletal and smooth muscle cells are

elongated ,so called muscle fiber.All can shorten or contract due to the

movement of microfilamentsAll muscles share same terminology

◦Prefix myo refers to muscle◦Prefix mys refers to muscle◦Prefix sarco refers to muscle, for

example,sarcoplasm.

Types of Muscle

Types of Muscle, cont.

Skeletal Muscle Characteristics

Most are attached by tendons to bones Cells are multinucleate Striated – have visible banding Voluntary – subject to conscious control Cells are surrounded and bundled by connective tissue

Characteristics of Smooth Muscle

• Has no striations• Spindle-shaped cells• Single nucleus• Termed involuntary (no

conscious control) or visceral.• Found mainly in the walls of

hollow organs arranged in two layers :circular and longitudinal.

Cardiac Muscle Characteristics

• Has striations• has one or two

nuclei• Fibers are branched

and joined tighther at an intercalated disc

• Involuntary• Found only in the

heart• Arranged in spiral or

figure 8-shaped bundles.

Functions of Skeletal Muscles 1-Producing movement 2- Maintaining posture 3- Stabilizing joints 4-Generating heat: is a by-product of muscle activity.As

ATP is used to power muscle contraction, nearly three-quarters of its energy escapes as heat. This heat is vital in maintaining normal body temperature . Skeletal muscles accounts for at least 40% of body mass , so it is the muscle type most responsible for heat generation.

Connective Tissue WrappingsConnective Tissue Wrappings of Skeletal Muscleof Skeletal Muscle

Slide 6.4aCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Endomysium – around single muscle fiber

Perimysium – around a fascicle (bundle) of fibers

Epimysium – covers the entire skeletal muscle

Figure 6.1

Skeletal Muscle AttachmentsSkeletal Muscle Attachments

Slide 6.5Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Epimysia blends into a connective tissue attachments which are either: Tendons – cord-like structure which

besides anchoring muscles, it provides durability and conserving space.

Aponeuroses – sheet-like structure

Sites of muscle attachment Bones

Cartilages

Connective tissue coverings

Microscopic Anatomy of SkeletalMicroscopic Anatomy of Skeletal Muscle cell (fiber) Muscle cell (fiber)

Slide 6.9aCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

The nuclei are pushed aside by ribbonlike myofibrils

Cells(Fibers) are multinucleate, Nuclei are just beneath the sarcolemma (cell membrane)

Figure 6.3a

Myofibril

Bundles of myofilaments

Myofibrils are aligned to give distrinct bands

I band = light band

A band = dark band

Slide 6.10bCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Sarcomere Contractile unit of a muscle fiber

Figure 6.3b

Slide 6.11aCopyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings

Organization of the sarcomere Thick filaments = myosin filaments

Composed of the protein myosin

Thin filaments = actin filaments

Composed of the protein actin

Figure 6.3c

Striations in Skeletal Muscle

• A- Band- covers the length of the myosin filament

• I- Band- covers actin where there is no overlap with myosin

• H- Zone- the portion of myosin that does not overlap actin

• Z- Line- point where actin filaments anchor

Z line Z line

The sarcomere is the functional unit of a muscle cell

H Band

The area of the sarcomere has only myosin is called the H band.

Sarcomere Relaxed

The part of the sarcomere with only actin filaments is called the I band.

Sarcomere Partially Contracted

This sarcomere is partially contracted. Notice that the I bands are getting shorter.

Sarcomere Completely Contracted

The sarcomere is completely contracted. The I and H bands have almost disappeared.

Neuromuscular Components

Neuromuscular junction is the point where a motor neuron joins muscle fibers ,

Motor unit is a motor neuron and all the muscle fibers it innervates, may be few or hundreds.

All or none law :with adequate stimulation, a muscle cell will contract to its fullest extent or not at all.

Skeletal muscle activity

•Muscles have two special functions1-Irritability : the ability to receive and respond to

stimulus2-Contractility : the ability to shorten• Muscle cell must be stimulated by nerve impulse

to contract• There is a gap (synaptic cleft) between the nerve

axonal terminal &muscle cell

• This gap is crossed by a chemical transmitter called Acetylcholine (Ach)

• Sarcolemma becomes depolarised i.e., temporarily permeable to sodium

• This upset generates an electric current called action potential which is unstoppable leading to contraction of the whole muscle fiber such as lighting a match under a small dry twig.

MUSCLE TONE• Relaxed skeletal muscles are always slightly contracted

involantarily. – This state of continuous partial contraction is termed

“muscle tone”.• One group of muscle fibers are activated followed by

another group and so on.– Spinal reflexes continually activate an alternating subset

of motor neurons– No active movement produced– Muscles kept firm, healthy, and ready to respond to

stimulation– Helps stabilize joints and maintain posture

The Muscle Twitch   The response of a motor unit to a single action potential of its motor neuron is called a muscle twitch. The muscle fibers contract quickly and then relax. Twitch contractions of some muscles are rapid and brief, as with the muscles controlling eye movements.Graded Muscle Responses For proper skeletal movement, contractions must be relatively smooth and vary in strength. These variations are referred to as graded muscle responses. In general, muscle contraction can be graded in two ways: (1)by changing the frequency of stimulation and (2) by changing the strength of the stimulus.

1- Muscle Response to Changes in Stimulation Frequency   

The nervous system achieves greater muscular force by increasing the firing rate of motor neurons. the second twitch will be stronger than the first as the second twitch will appear to ride on the shoulders of the first .This phenomenon, called wave summation, occurs because the second contraction occurs before the muscle has completely relaxed. If the muscle is stimulated at an increasingly faster rate, the relaxation time between the twitches becomes shorter and shorter, progressing to a sustained but quivering contraction referred to as unfused or incomplete tetanus

Finally, as the stimulation frequency continues to increase, muscle relaxation disappears and the contractions fuse into a smooth, sustained contraction plateau called fused or complete tetanus. (This term is often confused with the bacterial disease called tetanus that causes severe involuntary contractions.)

2-Muscle Response to Stronger Stimuli   The force of contraction is controlled more precisely by delivering shocks of increasing voltage to the muscle, calling more and more muscle fibers into play. The stimulus at which the first observable contraction occurs is called the threshold stimulusBeyond this point, the muscle contracts more and more vigorously as the stimulus strength is increased until the point at which all the muscle’s motor units are recruited. Increasing the stimulus intensity beyond the maximal stimulus does not produce a stronger contraction.

MUSCLE CONTRACTIONS

1-Isotonic contraction Muscle length changes and moves the load.Once tension is sufficient to move a load, tension remains relatively constant.Bending the knee, rotating the arms and smiling are examples.

2- Isometric contraction

Tension builds but muscle length remains constant.Muscle attempts to move a load greater than the force the muscle is able to develop.

Trying to lift your car or push against immovable object or trying to lift 400kg are examples.

Effect of exercise on Muscle• Muscles are no exceptions to the saying –use

it or lose it-• Regular exercise increases muscle size,

strength and endurance1-Aerobic exercise (isotonic contraction)results

in stronger muscle with greater resistance to fatigue (endurance type of exercise).

• No increase in size but better heart &lungs functions.

• Examples are jogging or biking

2-Isometric(Resistance) exercise

• Require little time• No special equipment• Forcing the muscle to contract with as much as

possible leads to enlargement of muscle cell without increase in their number

• Examples are weigh lifting and body building.N.B., The best exercise program for most people is

one that include both types of exercise.

Energy for muscle contraction• ATP store in the muscle supplies energy ONLY

for 4-6 seconds• ATP then comes from:1- direct phosphorylation of ADP by reaction

with creatine phosphate(CP) -No need for oxygen -1 ATP for 1 CP -Provides energy for 15 seconds only

2-Anaerobic respiration: is the glycolysis and so lactic acid accumulates due to absence of oxygen.

• -2 ATP per glucose molecule• -lactic acid accumulates• - so muscle become sluggish or fatigued.• - Provides energy for 30-60 seconds only

3-aerobic mechanism (oxidative phosphorylation)

• Oxygen is needed to oxidize glucose, pyruvic acid, lactic acid, free fatty acids and aminoacids

• 38 ATP for 1 glucose molecule• Duration of energy is HOURS as long as oxygen

is available to the muscle.

Muscle fatigue.

Muscle fatigue occurs when an exercising muscle can no longer respond to stimulation with the same degree of contraction.

Causes for this fatigue is an accumulation of lactic acid, and the depletion of energy

reserves .Increased oxygen consumption is needed for a period of time to recover from exercise (paying off an oxygen debt) to get rid of accumulated lactic acid.

Naming of Skeletal Muscle

Action

Origin&

Insertion

ShapeNumber

OfOrigins

Size

Location

Direction ofMuscleFibers

SkeletalMuscle

Naming Skeletal Muscle1-Direction of Muscle Fibers

Relative to the Midline–RECTUS means parallel to

midline•Example:Rectus Abdominus

–TRANSVERSE means perpendicular to midline

•Example:Transversus Abdominus:

–OBLIQUE means diagonal to midline

• Example :External Oblique• Internal oblique

2-Location– Structure near which a

muscle is foundMuscle near frontal bone = Frontalis

–Muscle near the Tibia– = Tibialis

3-Relative Size of Muscle

– MAXIMUS means largest• Eg.Gluteus Maximus

– MINIMUS means smallest• Eg.Gluteus MinimusMEDIUS mens in between• Eg. Gluteus medius

– LONGUS means long• Eg.Fibularis Longus

BREVIS means short• Eg.Fibularis Brevis

4-Number of OriginsNumber of tendons or origins

- BICEPS means two Biceps Brachii

-TRICEPS means three Triceps Brach

-QUADRICEPS means four Quadriceps

Femoris

5 -Shape of the Muscle

-DELTOID means having a triangular shape

-TRAPEZIUS means having a trapezoid shape

-SERRATUS means having a saw-toothed shape (Serratus Ant.)

- RHOMBOIDEUS means having a diamond shape (Rhomboid Major)

6-Origin & Insertion

-STERNOCLEIDOMASTOID: attaches to the Sternum, Clavicle, and Mastoid Process

–-ILIO COSTALIS attaches to the ilium & ribs

7-ACTION of the muscle

NAME ACTION EXAMPLE

FLEXOR Decrease angle at a joint Flexor Carpi Radialis

EXTENSOR Increase angle at a joint Extensor Carpi Ulnaris

ABDUCTOR Move bone away from midline

Abductor Pollicis Longus

ADDUCTOR Move bone toward midline Adductor Longus

LEVATOR Produces upward movement

Levator Scapulae

DEPRESSOR Produces downward movement

Depressor Labii Inferioris

SUPINATOR Turn palm upward/anteriorly

Supinator

PRONATOR Turn palm downward/posteriorly

Pronator Teres

Types of Skeletal MusclePrime mover (Agonist) – muscle with the major

responsibility for a certain movement e.g,biceps is the prime mover of elbow flexion.

Antagonist – muscle that opposes or reverses a prime mover e.g,triceps is antagonist to bicepsi.e, prime mover of elbow extension

Synergist – muscle that aids a prime mover in a movement and helps prevent rotationFixator – stabilizes the origin of a prime mover

Types of body movementsEvery one of our 600 –odd skeletal muscle

is attached to bone or to other connective tissue structures ,at no fewer than two points;

1-origen,is attached to the immovable or less movable bone

2- insertionis attached to the movable bone,i.e.,it moves toward the origen.

Flexion – Movement that decreases angle between 2 bones.

–Extension – movement that increases angle between 2 bones

Abduction – movement away from the midline of the body

–Adduction – movement towards the midline of the body

Rotation – movement around a longitudinal axis

Circumduction-combination of flexion, extension, abduction, and adduction. seen in ball and socket joints as the shoulder

–Supination – occurs when palms rotate forward or upward

–Pronation – occurs when palms rotate downward or posteriorly

-Dorsiflexion standing on heal

-Plantar flexion – standing on toes

Inversion of the foot: turn the sole medially.

Eversion of the foot: turn the sole laterally

Opposition: moving the thumb to touch the tips of the other fingers

Divisions of the Muscular System

Axial muscles:position head and spinal columnmove rib cage

60% of skeletal musclesAppendicular muscles:

support pectoral and pelvic girdlessupport limbs

40% of skeletal muscles

Axial and Appendicular M

Figure 11–3a

Figure 11–3b

Muscles of the face

Figure 11–4a

Muscles of Facial xpression

Figure 11–4b

Extrinsic Eye Muscles Also called extra-ocular muscles

Figure 11–5a, b

Extrinsic Eye Muscles

Figure 11–5c

Inferior rectusSuperior rectusMedial rectusLateral rectusInferior obliqueSuperior oblique

Muscles of Mastication

Masseter:–the strongest jaw muscle

Temporalis:–helps lift the mandible

:Buccinator flattens the cheek,hold

the food between the teeth

Anterior Muscles of the Neck

MUSCLES OF RESPIRATION

Muscles of the anterior abdominal wall

Functions of Pelvic Floor Muscles

-Support organs of pelvic cavity-Flex sacrum and coccyx

-Control movement of materials through urethra and anus

The Appendicular Muscles

Figure 11–13a

The Appendicular Muscles

Figure 11–13b

Divisions of Appendicular MusclesDivisions of Appendicular Muscles

1(Muscles of the shoulders and upper limbs

2 (Muscles of the pelvis and lower limbs

The Intrinsic Muscles of the Foot

Figure 11–22b, c

Effects of Aging on the Muscular System-Skeletal muscle fibers become smaller in diameter

- Skeletal muscles become less elastic:develop increasing amounts of fibrous tissue (fibrosis)

- Decreased tolerance for exercise- Decreased ability to recover from muscular

injuries

Developmental Aspects of Muscles- The first movement of the fetus (quickening) usually occur by the eixteenth week of pregnancy.- At birth, a baby’s movements are uncoordinated and largely reflexive. - Muscular development reflects the level of neuromuscular coordination, which develops in a head-to-toe and proximal-to-distal direction. In other words, a baby can lift its head before it can walk, and gross movements precede fine ones.