Anatomy: muscle tissues

functions of muscle tissues

muscle tissues contract in response to a stimulus from the nervous system in order to:

- move material through the body

- move parts of the body and produce movement

- generate heat

excitability

motor signal to contract reaches muscle and initiates a contraction

- muscle cell can respond to a stimulus (electrical impulse)

contractility

muscle contracts and shortens

- in response to stimulus, muscle cell contracts (shortens or attempts to shorten)

extensibility

motor signal to and contraction of muscle stops and muscle is pulled back to resting length (by gravity and/or an antagonist)

- a contraction of the triceps brachii is responsible for pulling (or extending) the biceps brachii

- when contraction ends, muscle cell can be pulled back to resting length

elasticity

muscle is pulled beyond resting length by antagonistic muscles and is able to regain resting length after this stretch

1. muscle has been stretched beyond resting length

2. muscle is able to regain original shape (titin plays a key role)

- muscle cell can be stretched beyond its resting length and then shorten back to resting length

gastrocnemius

- medial head

- lateral head

3 muscle tissue types

1. skeletal muscle

2. cardiac muscle

3. smooth muscle

skeletal muscle

- moves the skeleton

- ~40% of body weight

- under voluntary control

- striated

- typically attaches bone -> bone

-> may also attach bone to skin or CT

- 1 muscle fiber can be the length of entire muscle

- can range in length from <1 inch to over a foot

- long, cylindrical, multinucleated cells

cardiac muscle

- only found in heart wall

- under involuntary control

- striated

smooth muscle

- found within the walls of most internal organs

- under involuntary control

fiber

a muscle cell

sarcolemma

plasma membrane of a muscle cell

myofilaments

cytoskeletal units

- actin and myosin

- thin (actin) filament

- thick (myosin) filament

stapedius

shortest skeletal muscle

- in middle ear

sartorius

longest muscle

endomysium

loose (areolar) CT surrounding a single muscle fiber

perimysium

dense CT surrounding a muscle fascicle

fascicle

collection of muscle fibers

- wrapped by perimysium

epimysium

dense irregular CT surrounding a muscle

what run through muscle?

- arteries

- veins

- nerves

arteries

provide oxygen and nutrients

veins

remove cellular waste

nerves

innervate muscle cells

muscle fiber

single muscle cell

sarcoplasm

cytoplasm of a muscle cell

myofibrils

rodlike bundles of actin and myosin that run parallel within the muscle cell

T tubule

extension of sarcolemma that extends into the cell and wraps around myofibrils

- carry electrical stimulus into cell

sarcoplasmic reticulum

modified endoplasmic reticulum that stores and pumps calcium ions

structures of a myofibril

- sarcomere

- myofilaments

- Z disc (line)

- A band

- I band

sarcomere

- functional units within a myofibril, repeat the entire length of each myofibril

- organization within each sarcomere gives muscle a striated appearance

Z disc (line)

protein disc joining adjacent sarcomeres

- associates with T tubule

A band

primarily myosin (thick filaments), but some overlapping actin

I band

primarily actin (thin filaments)

sarcomere shortening

when a contraction occurs, each sarcomere shortens a little, the result adding to the total shortening of the myofibril

- overall shortening of length is about 1/3

titin

large, spring-like protein that attaches Z disc to myosin

- elastic (titin) filaments

which characteristic of skeletal muscle does titin allow for?

elasticity

sliding filament theory

during a contraction, actin and myosin filaments slide across one another

motor units within a muscle

- within a muscle -> muscle fibers work together to perform an action

- changes in force, and fine muscle control are possible because of distinct motor units within a muscle

motor unit

1 motor neuron and all the muscle fibers it innervates

neuron

single nerve cell

force and motor units

- all fibers innervated by a single motor neuron (motor unit) contract at the same time

- activating more motor units within a muscle increases the force exerted by that muscle

large muscles

~2000 fibers/motor unit

smaller muscles

~10 fibers/motor unit

- fine motor control

neuromuscular junction

where neuron (axon terminal) stimulates the muscle cell

what would make a muscle be able to shorten most during a contraction?

longest fibers

what would create the most force during a contraction?

greater number of fibers (strongest)

parallel muscles

- muscle fascicles run parallel to axis of muscle

- tendon on either side

- look long and ropelike

- fewer fibers than other types

- longer fibers so able to shorten more

pennate muscles

- tendon runs the whole length of muscle

- fascicles attach to tendon at an angle

- resemble a feather

- 3 subtypes

pennate muscles fascicles attach to tendon at an angle

- shorter fibers than parallel muscles

- allows for more fibers so stronger than parallel

subtypes of pennate muscles

1. unipennate

2. bipennate

3. multipennate

unipennate muscles

fascicles attach to one side of tendon

bipennate muscles

fascicles attach to both sides of tendon

multipennate muscles

branching tendon with fascicles attaching at many points

- strongest

convergent muscles

- origin of muscle is long and broad

- muscle fascicles collected into tendon at insertion

- fan shaped

convergent muscles relationship to other muscle types

- more fibers than parallel

- longer fibers than pennate

relative strength of muscles: ranking

pennate > convergent > parallel

relative shortening ability of muscles

parallel > convergent > pennate

circular muscles

- fascicles arranged as a ring

- sphincter muscles

sphincter muscles

when contracted the muscle constricts an orifice (opening) keeping it closed

the pectoralis major is what kind of muscle

convergent

the orbicularis oris is what kind of muscle

circular

the deltoid is what kind of muscle

multipennate

the biceps brachii is what kind of muscle

parallel

the rectus femoris is what kind of muscle

bipennate

the sartorius is what kind of muscle

parallel

the extensor digitorium longus is what kind of muscle

unipennate

origin

attachment site that is not moved during a muscle action

insertion

attachment site that is moved when muscle shortens

action

the resulting movement of a muscle contraction

tendon

the dense reticular CT that connects muscle to bone

tendons are a dense regular connective tissue. with which structure of bones do you think tendons are continuous?

periosteum (dense irregular CT)

tendons are a dense regular connective tissue. with which structure of muscles do you think tendons are continuous?

epimysium (dense irregular CT)

direct attachment

short, dense regular CT fibers connect muscle to bone

indirect attachment

long, dense, regular CT fibers connect muscle to bone

- ex: tendon and aponeurosis

aponeurosis

flat sheet (different from tendon, which is more rope like)

upraised bone markings are visible where?

bone where muscles attach

synergists

muscles that work together to perform an action

antagonists

muscles that perform opposite functions

prime mover (agonist)

muscle that is primarily responsible for a movement

fixator

a synergist that assists by holding a bone firmly in place to allow the prime mover to work more effectively

elbow flexion: synergists

biceps brachii, brachialis, and brachioradialis muscles work together to flex elbow

elbow flexion: antagonists

triceps brachii muscle reverses the movement and extends elbow

elbow flexion: prime mover (antagonist)

brachialis muscle is the primary muscle of elbow flexion

elbow flexion: fixator

muscles of scalpula stabilize the shoudler to make elbow flexion more efficient

true statements about muscle fibers

- may be as long as the muscle body

- contain sarcomeres as their contractile unit

- are bundled into fascicles

- contain myofibrils

cardiac muscle tissue

thick muscle of heart wall

- striated

- branched

- most cells are uninucleated, but MAY have 2 large nuclei

- cells have some regenerative ability (perhaps 1% a year)

- involuntary

- surrounded by endomysium

- connected at intercalated discs

what type of cellular junction allows cardiac muscle cells to contract in a coordinated fashion?

gap junction

structures of an intercalated disc

- gap junction

- desmosomes

gap junction of an intercalated disc

allow for coordinated contractions by allowing action potentials to quickly spread from cell to cell

desmosomes of an intercalated disc

provide strength, site where intermediate dilaments are attached (fasciae adherens)

inherent rhythmicity (automaticity) of cardiac muscle

- cardiac muscle cells are able to initiate their own contraction without stimulation from the nervous system

- note: our heart does rely on the NS to control things such as heart rate

what type of muscle would you expect to find within the walls of blood vessels?

smooth

smooth muscle tissue

- small, spindle shaped cells

- uninucleated

- NO striations -> they do contain myofilaments, but they are not arranged into sarcomeres

- each cell covered by endomysium

- involuntary

- regenerate

smooth muscle cells

- intermediate filaments anchor actin and myosin

- a contraction involves myosin and actin fibers moving against one another

smooth muscle is typically arranged into two distinct layers:

- circular layer

- longitudinal layer

- layers differ in orientation of cells: typically perpendicular to one another

circular layer of smooth muscle

closest to lumen of organ

longitudinal layer of smooth muscle

wraps around circular layer