3 Types of muscles
-categorized by the location, histology and nervous or other modes of control

1. Skeletal muscle tissue

-named for the location
-attached to bones, skin and deep fascia

-striated (dark and light bands)
-voluntary

2. Cardiac muscle tissue

-walls of the heart
-striated

-involuntary “natural pacemaker”

3. Smooth muscle tissue

-involved with processes related to maintaining the internal environment

-found in the walls of hollow organs, attached to hair follicles

-non striated
-involuntary

4 muscle tissue characteristics

1. Excitability
-similar to nervous tissue
-stimulus initiates action potential (impulse)

2. Contractility
-shorten and thicken
-actively do work when stimulus is received

3. Extensibility
-ability to be stretched (paired muscle groups)

4. Elasticity
-ability to return to its original shape after contracting or extending


Functions

motion

maintenance of posture

heat production (85% of heat generated from muscles)

Connective tissue components

Superficial fascia (subcutaneous layer)
-immediately deep to the skin

-stores fat, insulates, protects, provides pathway for nerves and blood vessels

Deep fascia
-lines body walls, extremities and holes muscles together

-splits muscles into functional groups (pectoralis major and pectoralis minor)
function:
-allows free movement of muscles
-fills space

-carries nerve and vascular supply

-sometimes provides origin for muscles

Epimysium
-wraps the entire muscle bundle

Perimysium
-covers muscle fiber bundles which are called fasciculi

Endomysium
-covers individual fibers within the fasciculi

the 3 “mysiums” may extend beyond the muscle, those fibers that extend beyond the muscle become the tendon

Skeletal muscle organization
(from biggest to smallest)

Epimysium
-connective tissue that covers the entire muscle

muscles are bundles of fasciculi

Perimysium
-C.T. that covers the individual fascicles

Each fascicle contains many bundles of fibers

Each fiber contains many myofibrils

Each myofibril contains many myofilaments

myofilament
-smallest part of a “muscle”
-DO NOT extend the entire length of the muscle
-fit into compartments called sarcomeres

sarcomeres
-contraction of muscle occurs in these compartments
-lie end to end within the myofibril

The myofibril shortens because of the action of the myofilaments in the sarcomere

2 types of myofilaments

myosin--
thick contractile protein

actin--
thin contractile protein

The ability for actin and myosin to change shape allows for the 2 myofilaments to be pulled (slide) over each other


Myofilaments form patterns in the sarcomeres

1. “I” band consist of the Z disc (stationary) and actin filaments

2. “A” band consist of myosin overlapping with the ends of the actin filaments

H zone-- is within “A” band and contains only myosin

3. Another “I” band

**Structures between the Z disc make up the sarcomere

**Heads or hooks on the myosin bind with actin to pull the myosin filament over the actin

**Tropomyosin blocks the binding sites on the actin when the muscle is relaxed

1. ACH released by the nerve binds on the sarcolemma (covering) of the muscle

2. ACH diffuses across neuromuscular junction. This initiates a muscle action potential that spreads over the sarcolemma.

3. Muscle action potential releases calcium from storage in the sarcolemma.

4. Increased permeability of Ca⁺

5. Ca⁺ bind to the tropomyosin thus exposing the myosin binding sites on actin and allowing myosin “heads” to bind to the tropomysium

6. Tropomyosin slides over and allows the myosin heads to bind to the actin
myofilaments

7. ATP energy provides the fuel for the movement of the myosin head

8. The myosin head bends and pulls the myofilaments over each other, this is called the power stroke

9. Sliding movement draws the Z disc toward each other, sarcomere shortens, fibers contract and the muscle contracts.


RELAXATION

1. ACH “wears off”

2. No more muscle action potential. Ca⁺ transported back to the sarcoplasmic reticulum.

3. Low Ca⁺ concentration allows tropomysium to slide over the binding sites.

4. Sarcomeres return to their normal length.

spasm
-sudden involuntary contraction of a large group of muscles

tremor
-involuntary contraction of opposing muscle groups

fasiculation
-involuntary, brief twitch of a muscle visible under the skin
-occurs irregularly and doesn’t move the effected muscle

fibrillation
-similar to fasiculation except it is not visible under the skin

tic
-twitch made involuntarily by muscles under voluntary control
-eyelids or facial muscles are good examples

-generally tics are of psychological origin

-may involve lack of nutrients, disease, injury, atrophy, neurological problem, or accumulation of toxic products

fibrosis
-formation of fibrous connective tissue where it normally does not exist

-mature skeletal and cardiac muscle can not undergo mitosis. Damaged fibers are replaced with fibrous connective tissue
-most often the consequences of muscle injury or degeneration

fibromyalgia (algia= painful condition)
-refers to a group of common non articular rheumatic disorders characterized by pain, tenderness, and stiffness of muscles, tendons and surrounding soft tissue

-effects the fibrous connective tissue components of tendons and ligaments

-may be caused or aggravated by physical or mental stress, trauma, exposure to dampness or cold, poor sleep

-relieved by heat, massage and rest.

-muscle destroying diseases

-degeneration of individual muscle fibers which leads to a progressive atrophy of the skeletal muscle

-skeletal muscles affected bilaterally

-classified by mode of inheritance, age of onset, and clinical characteristics

1. Duchenne Muscular Dystrophy (DMD)
-most common form
-genetic
-gene identified and DNA sequence worked out (could lead to replacement therapy to prevent muscle loss)

2. Myasthenia Gravis (MG)
-weakness of skeletal muscle contraction

-auto immune disorder caused by antibodies directed against ACH receptors on the sarcolemma, prevents ACH from being receipted

treatments
-drugs to increase levels of ACH
-steroids to reduce antibody levels

-plasmapheresis--separating the unwanted antibodies from the blood plasma

-thymectomy--removal of thymus to halt antibody production

-fibers are quadrangular

-single nucleus

-more mitochondria and larger mitochondria

-contain actin and myosin

-fibers branched an interconnected

-2 separate networks (atria and ventricles) intercalated disc separate each fiber in a network

-impulse stimulates the entire network, contraction of the entire network

-atria contracts--blood to the ventricles

-ventricles contract--blood to the arteries and through the body

-auto rhythmicity--nerve impulses only increase or decrease the rhythmic contractions

-remains contracted 10 to 15 times longer

-extra long refractory period--allow heart to rest and prevents tetanus

-fibers smaller than skeletal

-appears “unorganized”

-contractions in a “corkscrew” manner

2 types of smooth muscle

1. Visceral (single unit)

-found in wrap around sheets that form walls of arteries and hollow viscera

-fibers form a continuous network--one impulse can cause action potential for the entire network

2. Multi Unit Smooth Muscle

-each fiber with own nerve ending--(like skeletal muscle)

-arrector pili, internal eye muscles