Skeletal Muscle Contraction

Question 1

what are the two types of muscles?

Answer 1

striated and smooth. classified based on arrangement of actin and myosin

striated: striped appearance, seen in skeletal and cardiac muscle. actin and myosin arranged parallel
smooth: not striped. actin and myosin are not arranged parallel

Question 2

describe invertebrate muscles

Answer 2

have myosin based thick filaments and actin based thin filaments.
smooth, cross striated or obliquely striated muscles
sometimes intermediate between smooth and striated

Question 3

describe skeletal muscle structure

Answer 3

multinucleated, made of muscle fibres. these fibres are organized into bundles (fascicles) by connective tissue (epimysium, endomysium, perimysium)
myofibrils contain contractile proteins, actin and myosin

epimysium is film outside muscle, perimysium is just below that
endomysium is film inside muscle in bundles

Question 4

describe the filaments within muscle

Answer 4

thick filaments make up contractile elements

myocytes (muscle cells), contractile cells unique to animals

contractile elements:
- thick filaments (myosin polymers)
- thin filaments (actin polymers, with troponin and tropomyosin on outer surface)
filaments are arranged into sarcomeres, and sarcomeres arranged into myofibrils

Question 5

describe the sarcomere

Answer 5

sarcomere is the contractile unit of the muscle fiber

skeletal and cardiac muscle is started because of the arrangement of thick and think filaments in myofibrils.

sarcomeres are bordered by Z-disks which anchor the thin filaments. at contraction the Z discs are brought together
thick filaments are joined at M-line, anchored by titin (spring-protein)
A band is region of thick filaments
I band is region of thin filaments that dont overlap myosin

Question 6

describe neurogenic muscle

Answer 6

excited by neurotransmitters

receive signals from motor neurons from somatic system
the synapse with muscle fibres is called neuromuscular junction, and the region on the post synaptic membrane of the muscle is called the motor end plate.

sarcolemma is rich in Ach receptors

Question 7

what is the primary neurotransmitter at the vertebrate neuromuscular junction?

Answer 7

acetylcholine

but they can also release other neurotransmitters like glutamate and GABA

Question 8

explain Ach pathway between neurons

Answer 8

Acetylcholine CoA is made in mitochondria, is converted into Ach, packaged into vesicle and released onto synapse. There, it binds to receptor on postsynaptic cell. this increases cell permeability for Na ions, which rush in. AP spreads across sarcolemma surface as Ach is broken down and taken out of the cleft. diffusing out of synapse

Question 9

explain excitation-contraction coupling

Answer 9

depolarization of the muscle plasma membrane (sarcolemma)
intracellular Ca increases, which allows myosin to bind actin
contraction via sliding filaments
sliding filaments can be used to show how actin and myosin interact during muscle contraction

Question 10

what are T tubules

Answer 10

transverse (T) tubules are tubes in the sarcolemma that enhance penetration of APs into myocyte.

Question 11

what is sarcoplasmic reticulum

Answer 11

stores calcium, especially in terminal cisternae

Question 12

what ion is needed for muscle contraction and where does it come from?

Answer 12

Ca2+
most of it comes from sarcoplasmic reticulum. after contraction, Ca2+ is transported out of cytosol to relax

Ach released from axon terminal, binds to receptors in motor end plate. this elicits AP, which propagates down sarcolemma and T tubule, where it triggers Ca 2+ to release. Ca2+ is released and binds to troponin, exposing myosin binding sites. cross bridge is formed where muscle contracts, then tropomyosin blocks myosin binding sites for it to relax.

Question 13

explain the sliding filaments model and contraction

Answer 13

resting sarcomere, ATP is hydrolyzed when myosin head isn’t attached.
Troponin-Ca2+ complex pulls tropomyosin away, exposing myosin binding site
ADP+P bind to myosin as the myosin head attaches to actin
ADP+ P release to let head pivot and actin filament moves
ATP is bound and hydrolized, and myosin head returns to resting position

Question 14

describe the 3 phases of muscle contraction

Answer 14

latent period: AP moves through sarcolemma, Ca2+ is released from calcium stores
Contraction phase: Ca2+ binds to troponin, active sites revealed, and actin-myosin interaction, then tension builds
relaxation phase: Ca2+ is taken back into sarcoplasmic reticulum and is also removed extracellularly. active sites covered, tension falls back down to rest

Question 15

tension patterns

Answer 15

treppe (builds and relaxes, gets a little bigger as time goes on)
incomplete tetanus (builds, relaxes a bit and builds again, gets big)
complete tetanus (builds and builds, no relaxing, gets very big)

Question 16

describe the motor neurons within skeletal muscle

Answer 16

motor unit- all muscle fibers controlled by a single motor neuron
motor unit recruitment- activation of more motor units to produce more tension
asynchronous motor unit summation- motor units activated on rotating basis to maintain sustained connection (prevent exhaustion)
muscle tone- retain tension in skeletal muscle