Lecture 18- Skeletal muscle Flashcards
what is skeletal muscle controlled by?
neurons in the CNS
what two neurons in the CNS is skeletal muscle controlled by?
upper motor neurons
lower motor neurons
what do upper motor neurons do?
in the motor cortex, synapse on motor neurons in the spinal cord
what do lower motor neurons do?
in spinal cord, send axons out the ventral roots and make synapses on muscle cells
what is the synapse between the lower motor neurons and muscle cell called?
neuromuscular junction (NMJ)
what is the neuromuscular junction synapse like in comparison to a central synapse?
NMJ is a huge synapse (20000 times larger)
what is the difference between central synapse postsynaptic membrane and NMJ postsynaptic membrane?
NMJ membrane is highly highly folded and has many nicotinic acetylcholine receptors and Na+ channels. its very folded to increase SA. they also always cause an AP in the postsynaptic muscle cell (because such a large synapse)
what is the group of muscle cells controlled by a lower motor neuron called?
motor unit
what is a motor unit?
one motor neuron and all of the muscle cells it controls
what are the two different ways a motor unit will stimulate other muscle cells?
can stimulate only one muscle cell
can stimulate many (with big muscles)
how many synapses does the muscle cell receive?
ONE
what is the structure like in skeletal muscles?
consists of a number of muscle fibers lying parallel to one another and held together by connective tissue
what is a single skeletal muscle cell known as?
muscle fiber
what are 3 descriptors used to describe skeletal muscles structure?
multinucleated
large, elongated and cylindrically shaped
fibers usually extend the entire length of muscle
what is a muscle fascicle and what is it surrounded by?
bundle of fibers surrounded by connective tissue (gives the muscle physical strength
what is the sarcolemma?
cell membrane of a muscle cell
what are T- tubules?
network of tubes that go through muscle cell (continuous with the outside)
what is the sarcoplasmic reticulum?
storage bag for calcium
what is the myofibril?
basic unit of muscle contraction made of sarcomere
what is the sarcomere made of?
Z disks (zig zag)
what is the M line?
middle line
what are two filaments the myofibril is made of?
thick filaments
thin filaments
what are thick filaments made of?
myosin molecule (myosin heads, myosin tail)
what are thin filaments made of?
actin chain (troponin, nebulin, tropomyosin, G-actin molecule)
describe the structure of thick filaments
shaped like golf club
tail ends are intertwined around each other and oriented toward center
globular heads project out at one end and form cross bridges with actin chain
myosin head has two important sites critical to contractile process, what are they?
actin binding site
myosin ATPase (hydrolyses atp to use it for energy)
what is G- actin?
globular actin that is spherical but assembles into long chains
what does the actin have as apart of its structure to attach to myosin creating cross bridging formation?
binding site to attach to myosin head
what kinds of proteins are tropomyosin and troponin?
regulatory proteins
what is tropomyosin?
thread like molecules that interact with actin along its spiral groove
what is the function of tropomyosin binding sites?
cover myosin binding sites (thus protecting actin)
troponin is made of 3 polypeptide units, what are they? what do they bind to?
one binds to tropomyosin
one binds to actin
one can bind with Ca++
what does troponin do when not bound to calcium?
stabilizes tropomyosin in blocking position over actins cross bridge binding sites
what happens when troponin binds to calcium?
- tropomyosin moves away from blocking position
- actin and myosin bind and interact
- creates cross bridges
what connects the thick and thin filaments together?
titin (joins M- lines to Z lines at opposite ends of sarcomere)
what are the two important roles of titin?
stabilize position of thick filaments in relation to thin filaments
improves muscles elasticity
where is the I band located?
between A bonds
function of nebulin
aligns actin filaments
function of myomesin
M- line (middle line)
what happens when actin and myosin slide past each other?
muscle shortens (contracts), the A band doesnt move but the Z disks slide into A band interconnecting them stronger
what is the process of initiating contraction? 5
- Ca++ levels increase in cytosol
- Ca++ binds to troponin
- tropomyosin shits exposing binding site on actin
- myosin binds strongly to actin and completes power stroke
- actin filament moves
what is a power stroke?
cross bridge bends, pulling thin myofilament inward
do actin and myosin contract?
NO
myosin is properly called a motor protein, what is that?
a protein that hydrolyzes ATP to convert chemical energy to carry out mechanical work
what does the binding of calcium do?
shortens sarcoma
