Ch 9a: Muscles and Muscle Tissue

what are the 3 types of muscle tissue?

Skeletal

Cardiac

Smooth

what is skeletal muscle?

longest fibers, striated, voluntary, multinucleated

what is cardiac muscle?

only found in heart, striated, involuntary, intercalated discs help heart contract

what is smooth muscle?

found in walls of hollow organs (visceral), nonstriated, involuntary

what are the 4 main characteristics shared by all muscles?

Excitability, Contractility, Extensibility, Elasticity

what is excitability?

responsiveness

ability to receive and respond to stimuli

what is contractility?

ability to shorten forcibly when stimulated

what is extensibility?

ability to be stretched

what is elasticity?

ability to recoil to resting length

what are the 4 important functions of muscles?

Produce movement: responsible for all locomotion and manipulation

Maintain posture and body position

Stabilize joints

Generate heat as they contract

what is the order of muscle from largest to smallest?

1. fascicle

2. muscle fiber

3. myofibril

4. myofilaments

what is the epimysium?

connective tissue surrounding entire muscle

what is the perimysium?

connective tissue surrounding fascicles (groups of muscle fibers)

what is the endomysium?

connective tissue surrounds each muscle fiber

what is the attachment to moveable bone called?

insertion

what is the attachment to immoveable/less moveable bone called?

origin

what is direct attachment?

epimysium fused to the bone

what is indirect attachment?

muscle attached by tendon or sheetlike aponeurosis

Indirect is the most common type of attachment

what is the sarcolemma?

muscle fiber plasma membrane

what is the sarcoplasm?

cytoplasm, contains a lot of glycosomes that store glycogen which provide glucose during muscle cell activity for ATP, contains myoglobin for oxygen storage

what are myofibrils?

80% of cell volume, hold actin and myosin, they dont ever shorten just slide

what is a sarcomere?

The functional unit of skeletal muscle, classified from z-disc to z-disc, this is the actual part of the muscle that actually shortens

Contains A band with half of an I band at each end

what is the purpose of t tubules?

increase muscle fiber surface area, allos changes in membrane potential to rapidly penetrate the fibers

what are the 2 types of myofilaments?

myosin and actin

what is the central thick myofilament?

myosin → “dark” A-bands

attached by the H zone, connected at the M line

what are the thin myofilaments?

actin → “light” I-bands

attached by z discs

what does each myosin filament contain?

head and tail (myosin head that links myosin and actin together)

what are the regulatory proteins bound to actin?

Tropomyosin and troponin

what blocks myosin from binding to actin?

Tropomyosin

what facilitates binding along with the help of calcium ions?

troponin

what is the sarcoplasmic reticulum?

network of smooth endoplasmic reticulum tubules surrounding each myofibril

Main function is storage and regulation of calcium ions

what is acetylcholine (ACh)?

the neurotransmitter that allows an action potential to cross from neuron to muscle cell

what is not visible when muscles contracts?

the M-line

what is the sliding filament model of contraction?

states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more

Thick (myosin) and thin (actin) filaments NEVER change length, just overlap more

how are thin filaments pulled closer towards the center of a sarcomere?

Cross bridge attachments form and break several times

• Z discs are pulled toward M line

• I bands shorten

• Z discs become closer

• H zones disappear

• A bands move closer to each other

***actin and myosin never shorten the only slide past one another***

what happens when there is a low concentration of calcium (Ca2+)

Tropomyosin blocks active sites on actin

Myosin heads cannot attach to actin

Muscle fiber remains relaxed

what happens that causes the sarcoplasmic reticulum (SR) to release Ca2+ to cytosol?

Voltage-sensitive proteins in T tubules change shape

what happens so that tropomyosin moves away from myosin-binding sites

Ca2+ binds to troponin and changes shape

Step 1 of Cross Bridge Cycling

Cross bridge formation: high-energy myosin head attaches to actin thin filament active site

Step 2 of Cross Bridge Cycling

Working (power) stroke: myosin head pivots and pulls thin filament toward M line

Step 3 of Cross Bridge Cycling

Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach

Step 4 of Cross Bridge Cycling

Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state

• This energy will be used for power stroke in next cross bridge cycle

what does a skeletal muscle fiber NEED to contract?

Calcium

ATP

Stimulation from a motor neuron

what is a chemically gated ion channel?

opened by chemical messengers such as neurotransmitters

• Example: ACh receptors on muscle cells

what is a voltage-gated ion channel?

open or close in response to voltage changes in membrane potential

how are skeletal muscles stimulated?

somatic motor neurons

what is a neuromuscular junction?

an axon splitting a bunch (getting smaller and smaller) and then coming into “contact” with a muscle fiber.

what is the synaptic cleft?

gel-filled space between axon terminal and muscle fiber

what are the purpose of junctional folds in the sarcolemma?

increase the surface area to contain millions of ACh receptors

what are the events at the neuromuscular junction?

1. AP arrives at axon terminal

2. Voltage-gated calcium channels open, calcium enters motor neuron

3. Calcium entry causes release of ACh neurotransmitter into synaptic cleft

4. ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma

5. ACh binding to receptors, opens gates, allowing Na+ to enter resulting in end plate potential

6. Acetylcholinesterase degrades ACh

what is end plate potential

ACh released from motor neuron binds to ACh receptors on sarcolemma

Causes chemically gated ion channels on sarcolemma to open

what is depolarization?

generation and propagation of an action potential (AP)

If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open

what is repolarization?

Restoration of resting conditions

Na+ voltage-gated channels close, and voltage-gated K+ channels open

K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage

what is the refractory period?

muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete

• Ionic conditions of resting state are restored by Na+-K+ pump

• Na+ that came into cell is pumped back out, and K+ that flowed outside is pumped back into cell

what is excitation-contraction (E-C) coupling?

events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction)

AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR

• Ca2+ release leads to contraction

what is rigor mortis?

3–4 hours after death, muscles begin to stiffen

• Peak rigidity occurs about 12 hours postmortem

Intracellular calcium levels increase because ATP is no longer being synthesized, so calcium cannot be pumped back into SR

• Results in cross bridge formation