Muscles and Muscle Tissue

key words for cardiac muscle tissue

• involuntary

• striated

• walls of the heart

key words for smooth muscle tissue

• involuntary

• nonstriated

• walls of hollow visceral organs (stomach)

key words for skeletal muscle tissue

• voluntary

• striated

• attached to bones or skin

four important functions of skeletal muscle tissue in the body

1. produce movement

2. maintain posture and body position

3. stabilize joints

4. generate heat

which connective tissue sheath (perimysium, endomysium, epimysium) surrounds a muscle fibre (cell)?

endomysium

which connective tissue sheath (perimysium, endomysium, epimysium) surrounds the muscle (entire organ)?

epimysium

which connective tissue sheath (perimysium, endomysium, epimysium) surrounds a fascicle (bundle of muscle fibres)?

perimysium

what does a tendon connect?

connects a muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles

what are myofilaments?

filaments that constitute myofibrils; of two types: actin (thin filament) and myosin (thick filament)

what are myofibrils?

rodlike bundles of contractile filaments (myofilaments) found in muscle fibres (cells)

what are sarcomeres?

the smallest contractile units of muscles; extends from one Z disc to the next

what are A bands?

regions of the sarcomeres that contain thick filaments, including where the thick and thin filaments overlap; the dark bands of the striations seen along the length of the skeletal muscle fibre

what are I bands?

regions of the sarcomeres that contain only thin filaments; the light bands of the striations seen along the length of a skeletal muscle fibre

what is the H zone?

lighter-appearing region in the central portion of the A band (dark band) of a relaxed sarcomere; contains thick filaments but no thin filaments

function of myosin

makes up most of the thick filament

function of actin

makes up most of the thin filament

function of troponin

binds calcium ions

function of tropomyosin

blocks binding sites on the thin filament

what happens when the muscle fibre contracts?

• sarcomeres shorten

• I bands shorten

• distance from one Z disc to the next Z disc decreases

• H zone disappears during maximal contraction / thin filaments slide past the thick filaments

• thick / thin filaments overlap each other more

what opens a chemically gated channel?

• a chemical messenger (neurotransmitter)

• voltage gated ion channels open and close in response to changes in membrane potential

events of chemically gated channels opening

1. Action potential arrives at the axon terminal of the motor neuron

2. Calcium ions enter the axon terminal through voltage-gated channels

3. Influx of calcium ions causes the release of ACh from the axon terminal

4. ACh diffuses across the synaptic cleft from the axon terminal to the sarcolemma

5. Chemically gated channels open, ions pass into and out of the muscle fibre

6. ACh is broken down by the enzyme acetylcholinesterase

what is the end plate potential (EEP)?

Local depolarization of the junctional folds of the sarcolemma that can trigger an action potential

what is the role of EEP?

EEP spreads to the adjacent sarcolemma and triggers an AP there (when voltage-gated ion channels open). This AP leads to excitation-contraction coupling and so contraction occurs

how can an action potential travelling along the sarcolemma be able to penetrate deep into the muscle fibre?

AP reaches the cell interior by propagating down the T tubules. The T tubules ensure that every myofibril in the muscle fibre contract at the same time

What are triads?

skeletal muscle fibre structures that each consist of a T tubule and two adjacent terminal cisterns of the sarcoplasmic reticulum (SR)

what protein on the thin filament do calcium ions specifically bind to and how does this binding lead to the formation of cross bridges?

Calcium ions bind to troponin on the thin filament; this binding exposes the myosin-binding sites on actin by removing the blocking action of tropomyosin; cross bridges form when myosin heads bind to actin

what happens to calcium ions in the sarcoplasm after the muscle action potential ceases?

calcium ions are pumped back into the SR by active transport

steps of the cross bridge cycle

1. Cross bridge formation: myosin head binds to actin, forming a cross bridge

2. The power (working) stroke: myosin head pivots, pulling the thin filament toward the M line

3. Cross bridge detachment: ATP binds to myosin causing the myosin head to detach from actin

4. Cocking of the myosin head: myosin hydrolyzes ATP; the myosin head returns to its pre stroke (cocked) position

what prevents the thin filament from sliding backward when the myosin heads bind detaches from actin?

Many myosin heads along the length of a thick filament and at any given time, about half of them are in contact with the actin on the thin filament