The Muscular System

I. Types of muscles

A. Skeletal1. Voluntary…under

conscious control2. Striated3. Result in the movement

of bones at articulations

4. Contracts at the fastest rate

5. Has the greatest strength of contraction

6. Can remain contracted for the shortest length of time

B. Visceral1. Involuntary…not able

to be consciously controlled

2. Nonstriated (smooth)3. Located within the

walls of some organs

4. Contracts at the slowest rate

5. Has the smallest strength of contraction

6. Can remain contracted for the longest amount of time

C. Cardiac1. Involuntary2. Has striations similar to

skeletal muscles3. Has an intermediate speed

and strength of contraction4. Can remain contracted for

an intermediate amount of time

II. Functional Characteristics

1. Excitability: (responsiveness/irritability) the ability to receive and respond to a stimulus

2. Contractility: the ability to shorten forcibly when adequately stimulated

3. Extensibility: the ability to be stretched or extended

4. Elasticity: the ability of a muscle fiber to recoil and resume its resting length after being stretched

III. Muscle Functions

1. Producing movement: all movements of the body and its parts are due to muscular contractions

2. Maintaining posture

3. Stabilizing and strengthening joints

4. Generating heat: heat is produced when muscles contract…maintaining a normal body temperature…a vital homeostatic process

5. Muscles also protect fragile internal organs and regulate the movement of substances through the various body openings by forming valves.

IV. Skeletal Muscles

•Each skeletal muscle is an individual organ consisting of muscle fibers, blood vessels, nerve fibers and connective tissue

•Skeletal muscles are the body’s primary type of muscles

•They provide motion through the process of contracting

•They attach to bones either by a direct connection with the bone and the outer layer of the muscle

or by a connection through a tendon

A. Connective Tissue

•The most abundant connective tissue associated with muscles isfascia…

•Fascia is: “a sheet or broad band of dense connective tissue that may occupy the narrow space between skin and muscle or may surround muscles or other organs.”

•There are two types of fascia: superficial and deep.

•Deep fascia is the type associated most commonly with muscles.

•Deep fascia surrounds a muscle, supporting it and holding it together as a single unit.

•It also provides a passageway for nerves and blood vessels.

1. Sheaths

a. Epimysium: the outer covering of dense, irregular connective tissue that surrounds the entire muscle. It is continuous with adjacent muscles or tendons

b. Perimysium: fibrous connective tissue that surrounds each bundle of muscle fibers (fascicle)

c. Endomysium: fine sheath of areolar and reticular connective tissue surrounding each individual muscle fiber

2. Tendons

•Thick bands of dense connective tissue

that forms most of the connections between bones and muscles

Tendons form when the three layers of connective tissue converge and extend away from the fleshy part of the muscle to attach to the periosteum of the bone

•a. A broad sheet-like tendon extending away from the fleshy part of some muscles is called an aponeurosis

•This can attach muscles to bones or muscles to muscles.

B. Microscopic Anatomy

•The sarcolemma is the cell membrane of a muscle fiber.

•Each muscle fiber is a long, cylindrical cell with multiple nuclei

•Skeletal muscle fibers are extremely large ranging in diameters of up to 10 times that of an average cell and up to 30 cm in length.

•The cytoplasm of a muscle cell is called sarcoplasm

•Myoglobin is a red pigment in the sarcoplasm that transports oxygen

•Myofibrils and sarcoplasmic reticulum are highly specialized organelles in the sarcoplasm.

•T tubules are modifications to the sarcolemma.

1. Myofibrils

•Rod-like organelles that run parallel to the length of the muscle fiber

•They contain the contractile elements of skeletal muscle cells

•Skeletal muscles are characterized by striations…

•A repeating series of dark A bands and light I bands

•The light stripe in the center of an A band is the H zone which is bisected by a dark M line

•The dark area in the I band is the Z disk.

•A sarcomere is the region of a myofibril between 2 successive Z disks…and is the smallest contractile unit of a muscle fiber

•There are two smaller structures in myofibrils called myofilaments. There are two types: thick and thin filaments

•Thick filaments are made mostly of myosin which has specific features that form the cross bridges that generate the tension developed by a contracting muscle cell.

•Thin filaments are made mostly of actin to which the myosin cross bridges attach during contraction

•Elastic fibers are a third (relatively newly discovered filament)

•It is made mostly of the protein titin

•Elastic fibers perform 2 functions:–Holding thick filaments in place

–Assisting the muscle cell in springing back to shape after stretching or shortening

C. Regulation of contraction

•Two sets of tubules regulate muscle contraction:

1. Sarcoplasmic reticulum: SER of muscle cells – it regulates intracellular levels of calcium

•Storing and releasing Ca on demand when the muscle fiber is stimulated to contract.

2. T Tubules: conduct nervous impulses to the deepest regions of the muscle cells and to every sarcomere signaling the release of Ca…ensuring coordinated muscular contraction

D. Muscles and Motion

1. Sliding Filament ModelContraction occurs when the cross bridges create enough tension to exceed the forces opposing contraction

•This occurs in a ratcheting type of motion…with the attachments formed and broken multiple times during a single contraction

•Contraction ends when the cross bridges become inactive and the tension decreases inducing relaxation

•In order to cause movement, the muscle must have two points of attachment:

Origin:The proximal attachment onto the bone that does NOT move

InsertionThe distal attachment onto the bone that moves

F. Muscle Response

1. Action PotentialThe result of a series of electrical changes that occurs along the entire length of the sarcolemma

•The resting state of the sarcolemma is polarized…the outside face is positivethe inside face is negative

•The action potential initiates and propagates depolarization which leads to the sliding of myofilaments and the contraction of the muscle

•Depolarization waves are quickly followed by repolarization waves…during which the initial, resting, electrical conditions of the polarized state is restored

•The muscle cell cannot be stimulated again until repolarization is complete.

•During this time, the muscle is said to be in a refractory period

2. The role of ionic calcium•Because calcium plays numerous, vital, physiological roles, the cytoplasmic concentrations of calcium is finely regulated by intracellular proteins

•When intracellular calcium levels are low, the muscle cell is relaxed. As calcium ion levels rise, the ions bind to sites causing cross bridge formation, attachments and detachments which result in muscular contractions

•As calcium ions are reclaimed by the sarcoplasmic reticulum, the contraction ends and the muscle fiber relaxes

3. All or none response

The threshold stimulus is the weakest stimulus capable of initiating muscular contractions

Either the threshold stimulus is reached and the muscle contracts

or it is not and the muscle does not contract.

There is no partial contraction!

The strength of the contraction results from the number of and specific muscles stimulated.

G. Additional Terms

1. Rigor mortis: most muscles begin to stiffen 3 – 4 hours after death…and relax over the following 48 – 60 hours

•This occurs because dying cells are unable to keep calcium ions out of the muscle cell…resulting in the formation of cross bridges and muscular contraction

•Relaxation cannot occur until the muscle proteins break down and the cross bridges detach

2. Muscle tension is the forces exerted by a contracting muscle

3. Load is the opposing force exerted on the muscle by the weight of the object being moved

4. Muscle twitch: the response of a motor unit to a single action potential…the muscle fibers contract quickly and then relax

5. Aerobic respiration: provides most of the ATP used for muscle activitythis takes place in the mitochondria…glucose combines with oxygen and breaks down to form water, carbon dioxide and ATP

•The carbon dioxide released diffuses out of the muscle tissue into the blood and is removed through exhalation

•The process of aerobic respiration provides high levels of ATP but requires a continuous source of oxygen and nutrients

6. Oxygen debt:occurs when the respiratory/cardiovascular system cannot bring in enough oxygen to meet the demand of the activity

This leads to an accumulation of lactic acid in the muscle fibers

Prolonged build up of lactic acid causes muscle fatigue

which may make the muscle unresponsive to stimuli

7. Muscle cramps occur when a muscle

contracts spasmodically without relaxing

8. Isotonic contractions produce movement as

the muscle pulls the bone towards a stationary structure

9. Isometric contractions produces muscle tension,

but does not shorten the muscle – therefore, does not produce movement.

This can occur when you push against an immovable object such as a wall.

V. Skeletal Muscle Nomenclature

1. Location of the muscle:some muscles are named for the bone or region with which the muscle is associated

2. Shape of the muscle:some muscles are named for their distinctive shapes (deltoid – triangular shape; trapezius – both right and left form a trapezoid)

3. Relative size:maximus – largestminimus – smallestlongus – longbrevis - short

4. Direction of muscle fibers:rectus – straight…fibers run parallel to the axistransversus – fibers run at right angles to the axisoblique – fibers run obliquely to the axis

5. Number of origins:biceps – two origins (heads)triceps – three originsquadriceps – four origins

6. Location of the attachments:some muscles are named for both their origins and insertions…the origin is named firststernocleidomastoid: two origins: sternum and clavicle and an insertion on the mastoid process

7. Action:flexorextensoradductorsuppinatoretc…

VI. Some “fun” muscles

A. Muscles of the face:1. Frontalis: raises the eyebrows2. Masseter:

elevates the mandible

3. Orbicularis oculi: controls the eyelids4. Orbicularis oris: controls the lips –

facilitates speech

5. Zygomaticus: raises the corners of

the lips…smiling6. Platysma: depress mandible…

lower lip back and down

7. Buccinator:whistling, sucking, holding food between teeth

8. Mentalis:protrudes lower lip…pout

B. Muscles of the back1. Rhomboids: adducts and rotates

the scapula2. Deltoids: abducts the arm, aids

in extending and flexing the humerus

3. Trapeziuselevates and rotates the scapula; adducts the scapula; depresses the shoulder; extends the head

4. Latissimus dorsiextends the arm; adducts and medially rotates the arm; pulls the shoulder downward and back

C. Muscles of the torso

1. Rectus abdominuscompresses the abdomen by flexing the vertebral column

2. Pectoralis majorflexes, adducts and medially rotates the arm

3. Internal and External obliquestogether they flex the vertebral column and compress the abdominal wall. Individually they aid back muscles in trunk rotation and lateral flexion

                                               

             

Linea Alba

•The tendinous seam that runs from the sternum to the pubic symphysis.

4. Diaphragm

•Inspiration…flattens on contraction increasing the vertical dimensions of the thorax

•Center is a tendon…allowing for the passage of the vena cava

D. Muscles of the arm

1. Biceps brachii:two heads of originflexes the forearm at the elbow; supinates the hand

2. Triceps brachii:three heads of originextends the forearm

E. Muscles of the leg

1. Adductor longus:adducts, flexes and laterally rotates the thigh

2. Quadraceps femoris:a group of muscles that extend the leg

3. Gastrocnemius:the major calf muscle; facilitates plantar flexion, and flexion of the leg at the kneethe distal tendon is the Achilles tendon

4. Sartorius:flexes, abducts and laterally rotates the thighweakly flexes the knee

5. Soleus:Plantar flexes foot…important locomotor and postural muscle during walking, running and dancing

F. Muscles of the lower torso

1. Gluteus maximus

Extends the thigh2. Gluteus medius

abducts and medially rotates the thigh