The Muscular System Flashcards
skeletal muscle
aka voluntary muscle, its role is to contract in response to conscious intent, striated
cardiac muscle
found only in the wall of the heart, striated, autonomic
smooth muscle
found in the walls of all hollow organs such as the GI tract, the urinary system, the uterus, etc., autonomic
tendons
attack muscles to bones, strong connective tissue formed primarily of collagen
flexing
reducing the angle of the joint by muscle contraction
extending
increasing the angle of a joint by muscle contraction
abducting
moving away from the body’s midline by muscle contraction
adducting
moving toward the body’s midline by muscle contraction
origin
the point on the bone closer to the centre of the body where the skeletal muscle attaches
insertion
the point where the skeletal muscle attaches on the bone that is more distant from the centre of the body
what happens when a muscle contracts?
its insertion point on bone farther from body is brought closer to the origin point on bone closer to body
antagonist
muscles that are responsible for movement in opposite directions
synergistic
muscles that move a joint in the same direction
fascicles
bundles of contractile tissue held together by connective tissue, allows flexibility within the muscle
muscle fibers/myofibers
a single skeletal muscle cell
characteristics of skeletal muscle cells:
1) multinucleate syncytia formed by the fusion of individual cells during development
2) innervated by a single nerve ending
sarcolemma
cell membrane of myofiber, made of plasma membrane and an additional layer of polysaccharide and collagen
myofibril
smaller units within a myofibre, acts like a specialized organelle, responsible for the striated appearance of skeletal muscle and generates the contractile force of skeletal muscle
what are the proteins in the myofibril that generate contraction?
polymerized actin and myosin
thin filaments
actin polymers
thick filaments
myosin polymers
striated appearance of cardiac and skeletal muscles is caused by:
overlapping arrangement of bands of thick and thin filaments in sarcomeres
each sarcomere is bound by two:
Z-lines where thin filaments attach and overlap with thick filaments in the middle of each sarcomere (thick filaments are not attached to Z lines)
I bands
the regions of the sarcomere composed only of thin filaments
A band
the full length of the thick filament, including regions of overlap with thin filaments, where contraction is generated
H zone
area composed of only thick filaments, only seen in resting sarcomeres
describe action of sarcomere during contraction:
thin and thick filaments slide across each other, drawing the Z lines of each sarcomere closer together and shortening the length of the muscle cell
cross bridge
attachment of myosin head to actin
myosin is an :
enzyme, ATPase, uses ATP hydrolysis to power filament sliding
what are the four steps of filament sliding/the contractile cycle?
1) cross bridge formation: binding of the myosin head (with bound ADP and P) to a myosin binding site on actin
2) power stroke: myosin head moves to a low energy conformation and pulls the actin chain toward the centre of the sarcomere, ADP is released
3) binding of a new ATP molecule causes release of actin by myosin head
4) ATP hydrolysis causes myosin head to move to high-energy cocked conformation
what are the basic requirements for contractile cycle?
myosin, actin, ATP, Mg2+ (in the myofiber, cytoplasmic calcium is required due to troponin-tropomyosin complex)
troponin-tropomyosin complex prevents:
contraction when calcium is not present
tropomyosin
a long fibrous protein that winds around the actin polymer, blocking all the myosin binding sites
troponin
a globular protein bound to the tropomyosin that can bind calcium, changes conformation when troponin binds calcium which exposes myosin binding sites on actin)
neuromuscular junction (NMJ)
the synapse between an axon terminus (synaptic knob) and a myofiber, a long trough or invagination of the cell membrane that allows for depolarization of a large region of the postsynaptic membrane at once
motor end plate
the postsynaptic membrane/myofiber cell membrane
what is the neurotransmitter at a neuromuscular junction?
acetylcholine
what kind of impulse transmission occurs at a neuromuscular junction?
chemical synaptic transmission
end-plate potential (EPP)
depolarization of postsynaptic membrane due to postsynaptic sodium influx when ACh binds to ACh receptors (ligand gated sodium channels)
miniature EPP (MEPP)
the smallest measurable EPP, caused by exocytosis of a single ACh vesicle
how is the action potential propagated in myofibers?
by a continuing wave of voltage-gated sodium channels opening, must depolarize the entire myofiber if contraction is to occur
transverse tubules (T-tubules)
deep infoldings of the cell membrane which allow the action potential to travel into the thick myofiber cell
sarcoplasmic reticulum (SR)
specialized membrane in the myofiber that is a huge, specialized smooth endoplasmic reticulum which enfolds each myofibril in the cell. specialized to sequester (active transporters remove calcium from the sarcoplasm) and release calcium (voltage-gated calcium channels open when action potential travels down T-tubular network)
saroplasm
myofiber cytoplasm
muscle twitch
the smallest measurable muscle contraction
how can the nervous system increase the force of contraction?
1) motor unit recruitment (activating more motor neurons, and thus more myofibers)
2) frequency summation (amount of time between successive stimulations is greater than the duration of the refractory period but before cytoplasmic calcium has been returned to its low resting level)
motor unit
a group of myofibers innervated by the branches of a single motor neuron’s axon
tetanus
the strongest possible contraction
length-tension relationship
a muscle contracts most forcefully at an optimum length, allowing for a maximum degree of overlap between thick and thin filaments
creatine phosphate
intermediate-term energy storage molecule, hydrolyzed during contraction to drive regeneration of ATP
myoglobin
globular protein in muscle that provides an oxygen reserve by taking oxygen from hemoglobin and then releasing it as needed
cramps are a result of:
exhaustion of energy supplies (low ATP)
rigor mortis
rigidity of skeletal muscles after death, result of complete ATP exhaustion where myosin heads cannot release actin so muscle can neither contract nor relax
lactic acid produced in muscle is transported to:
liver where it is converted to pyruvate
skeletal muscle fibers can be:
slow twitch fibers or fast twitch fibers (different contractile speeds)
type I/ slow twitch/red slow twitch/red oxidative fibers
high myoglobin content, better blood supply due to extensive surrounding capillary network, good oxygen delivery, able to maintain contraction for extended periods of time without fatigue
type II/fast twitch fibers
two types, (A and B) which have different abilities to resist fatigue
type IIA (fast twitch oxidative fibers)
somewhat resistant to fatigue, can maintain longer duration of activity, more mitochondria
type IIB (white fast twitch fibers)
lack mitochondria, contract very quickly with great force, not fatigue resistant
what are four similarities between cardiac and skeletal muscle?
1) sarcomere organization/striated appearance
2) T-tubules
3) troponin-tropomyosin complex
4) length-tension relationship
what are five differences between cardiac and skeletal muscles?
1) cardiac muscle is a functional syncytium (only have one nucleus)
2) cardiac muscle cells connected to other cells by intercalated disks (gap junctions)
3) cardiac muscle cell uses extracellular and intracellular calcium, skeletal muscle only uses internal calcium (sarcoplasmic reticulum stores)
4) cardiac muscle contraction does not depend on stimulation by motor neurons (SA node is the pacemaker)
5) action potential depends on both fast voltage gated sodium channels and voltage-gated calcium channels (slow) which creates a plateau phase
the vagus nerve (parasympathetic nerve, releases ACh) synapses with:
the sinoatrial node to inhibit spontaneous depolarization, slows heart rate aka vagus tone
what is the purpose of a plateau phase in cardiac muscle cells?
1) longer duration of contraction facilitates ventricular emptying
2) longer refractory period helps prevent disorganized transmission of impulses and prevents summation or tetanus
how is smooth muscle different from skeletal muscle?
1) smooth muscle cells are smaller
2) no T-tubules
3) only one nucleus, functional syncytia
4) no sarcomeres, thick and thin filaments are dispersed in the cytoplasm
5) no troponin-tropomyosin complex, contraction is regulated by calmodulin and myosin light-chain kinase
6) relies heavily on extracellular stores of calcium for contraction
7) have almost no sodium fast channels, action potential is determined by slow channels only
8) some smooth muscle must sustain prolonged contractions, less sharp spike and plateau phase similar to cardiac cells
9) constantly fluctuating resting potential (slow waves, not spike potentials and do not elicit muscle contraction)
10) innervated by autonomic motor neurons and action potential is spread from cell to cell (each action potential is limited to one large myofiber in skeletal cells)
calmodulin and myosin light-chain kinase (MLCK) regulation of smooth muscle
calmodulin binds calcium and activates myosin light-chain kinase which phosphorylates a portion of the myosin molecule, thus activating its enzymatic/mechanical activity
amplitude of slow waves in smooth muscle is increased by:
ACh, decreased by NE (sympathetic stimulation)
