A&P Exam 3 Muscles

3 types of muscles

skeletal cardiac and smooth

skeletal and smooth muscle

elongated and called muscle fibers

myo mys and sarc

prefixes for muscle

skeletal muscle

• Organs attached to and cover bony skeleton

• Elongated cells called muscle fibers

• Striated (striped)

• Voluntary (i.e., conscious control)

• Responsible for overall motility

• Contract rapidly; tire easily; powerful

• Require nervous system stimulation

cardiac muscle

• Only in heart; bulk of heart walls

• Striated

• Can contract without nervous system

stimulation

• Involuntary

Smooth Muscle

• In walls of hollow organs, e.g., stomach,

urinary bladder, and airways

• Not striated

• Can contract without nervous system

stimulation

• Involuntary

Special Characteristics of Muscle

Tissue

• Excitability (responsiveness): ability to receive

and respond to stimuli

• Contractility: ability to shorten forcibly when

stimulated

• Extensibility: ability to be stretched

• Elasticity: ability to recoil to resting length

Muscle Functions

– Movement of bones or fluids (e.g., blood)

– Maintaining posture and body position

– Stabilizing joints

– Heat generation (especially skeletal muscle

Additional muscle functions

– Protects organs, forms valves, controls pupil size,

causes "goosebumps"

Gross Anatomy of skeletal muscle

Predominantly muscle fibers however blood

vessels, nerve fibers, and connective tissue

are present

Each skeletal muscle served by one artery, one nerve,

and one or more veins

– Enter/exit near central part and branch through

connective tissue sheaths

– Every skeletal muscle fiber supplied by nerve

ending that controls its activity

– Huge nutrient and oxygen need; generates large

amount of waste

Epimysium

dense irregular connective tissue

surrounding entire muscle; may blend with fascia

Perimysium

fibrous connective tissue surrounding

fascicles (groups of muscle fibers)

Endomysium

fine areolar connective tissue

surrounding each muscle fiber

Direct skeletal muscle attachment

epimysium fused to periosteum of bone

or perichondrium of cartilage

Indirect skeletal muscle attachment

connective tissue wrappings extend

beyond muscle as ropelike tendon or sheetlike

aponeurosis

Microscopic Anatomy of skeletal muscle

• Long, cylindrical cell

– 10 to 100 μm in diameter; up to 30 cm long

• Multiple peripheral nuclei

• Sarcolemma = plasma membrane

• Sarcoplasm = cytoplasm

– Contains large amounts of Glycosomes for

glycogen storage, myoglobin for O2 storage

• Modified structures: myofibrils, sarcoplasmic

reticulum, and T tubules

Myofibrils

• Densely packed, rodlike elements

• Each cell contains hundreds of these

• ~80% of cell volume

• Contain sarcomeres - contractile units

– Sarcomeres contain myofilaments

• Exhibit striations - perfectly aligned repeating

series of dark A bands and light I bands

H zone:

lighter region in midsection of dark A band

where filaments do not overlap

M line

line of protein myomesin bisects H zone

Z disc (line)

coin-shaped sheet of proteins on

midline of light I band that anchors thin filament and connects myofibrils to one another

Thick filaments

run entire length of an A band

Thin filaments

run length of I band and partway into

A band

Sarcomere

region between two successive Z discs. Composed of thick and thin myofilamentsmade of contractile proteins

Excitability (responsiveness

ability to receive

and respond to stimuli

Contractility

ability to shorten forcibly when

stimulated

Extensibility

ability to be stretched

Elasticity

ability to recoil to resting length

T tubules

send signals conduct impulses deep into the muscle fiber

Elastic filament

– Composed of protein titin

– Holds thick filaments in place; helps recoil after

stretch; resists excessive stretching

Dystrophin

Links thin filaments to proteins of sarcolemma

Sarcoplasmic Reticulum (SR)

• Network of smooth endoplasmic reticulum

surrounding each myofibril

– Most run longitudinally

• Pairs of terminal cisterns form perpendicular

cross channels

• Functions in regulation of intracellular Ca2+

levels

– Stores and releases Ca2+

cross

bridges

Occurs when myosin heads bind to actin

Activation (at neuromuscular junction)

• Must be nervous system stimulation

• Must generate action potential in sarcolemma

Excitation-contraction coupling

• Action potential propagated along sarcolemma

• Intracellular Ca2+ levels must rise briefly

The phases leading to muscle fiber contraction

contraction

ap in a neuron → ap axon terminal → release of ACH→ ACH binds to sarcolemma ap → AP travels along sarcolemma down the t tubles → Ca+ dumped into cell by SR → Ca+ binds to troponin→ Tropomyosin moved out of the way → Myosin heads bind to actin and pull if ATP is present (cross bridging)

generation of a motor neuron action potential, neurotransmitter release, generation of a muscle cell action potential, release of calcium ions from SR, ATP-driven power stroke, sliding of myofilaments