Muscular System

what are the 3 types of muscle?

smooth; cardiac; skeletal

where is smooth muscle found?

walls of organs, airways, and blood vessels

smooth muscle contracts and relaxes _____ (voluntarily/involuntarily)

involuntarily

why is smooth muscle not striated?

it lacks sarcomeres

describe the structure of smooth muscle cells

they are short, tapered, and have only one nucleus per cell

cardiac muscle is found in the heart, and it contracts _____ (voluntarily/involuntarily)

involuntarily

describe the structure of cardiac muscle cells

they are striated (have sarcomeres) and have one nucleus per cell

intercalated discs are found in _____ muscle

cardiac

intercalated discs hold adjacent cardiac muscle cells (also known as _____) together, which allows for the heart _____

cardiomyocytes; syncytium

what are the components of an intercalated disc?

desmosomes and gap junctions

_____ cells are long, multinucleate, and striated

skeletal muscle

skeletal muscles are under _____ (voluntary/involuntary) control

voluntary

what are the "levels of bundles" in skeletal muscles?

muscle belly --> fascicles --> muscle fibers --> myofibrils

what is the protective sheath encasing all the fascicles contained in a muscle belly?

epimysium

what is the protective sheath found between the fascicles of a muscle belly?

perimysium

what is the protective sheath found between muscle fibers of an individual skeletal muscle fascicle?

endomysium

_____ are skeletal muscle cells

muscle fibers

what is the plasma membrane of a muscle fiber called?

the sarcolemma

what is the cytoplasm of a muscle fiber called?

the sarcoplasm

muscle fibers contain an array of organelles arranged in parallel, what are they called?

myofibrils

sarcomeres contain protein _____

myofilaments

myofibrils are composed of many repeating functional units, called _____

sarcomeres

what are the main myofilament proteins of a myofibril?

actin and myosin

what is the functional unit of a muscle fiber and how does it facilitate muscle contraction?

sarcomeres; they shorten

the _____ model explains how myofilaments slide past one another to shorten the sarcomeres of a muscle fiber

sliding filament

do myofilaments shorten during contraction?

no, the sarcomere shortens because myofilaments slide past each other

what is the space between the presynaptic motor neuron and postsynaptic muscle fiber called?

neuromuscular junction

what neurotransmitter is released into neuromuscular junctions?

acetylcholine

acetylcholine facilitates the opening of _____ channels on the muscle fiber

ligand-gated Na+

what is the result of acetylcholine on the muscle fiber?

it allows some sodium to enter the muscle fiber and create a graded potential. This depolarization opens nearby voltage-gated Na+ channels to transfer the action potential to muscle

_____ allow action potentials to spread through muscle fibers quickly, coordinating the contraction

t-tubules

where are t-tubules found?

the sarcolemma

what is the specialized smooth endoplasmic reticulum (ER) found in muscle fibers?

the sarcoplasmic reticulum (SR)

the sarcoplasmic reticulum stores _____ ions

Ca2+

what causes voltage-gated Ca2+ channels to open on the sarcoplasmic reticulum (SR)?

a wave of depolarization traveling down t-tubules

where does Ca2+ flow after it leaves the SR?

the sarcoplasm

what is the resting state of a muscle?

relaxed

_____ covers actin's binding site for myosin when muscle fibers are not stimulated

tropomyosin

_____ hold tropomyosin in place over actin's binding site for myosin

troponin complexes

how does troponin move tropomyosin?

Ca2+ released from the SR binds to troponin-C, which triggers a shape change that moves troponin and therefore tropomyosin

what type of enzyme are myosin heads and what is their function?

ATPases; hydrolyze an ATP molecule into ADP and Pi

a _____ forms when myosin (thick myofilaments) bind to actin (thin myofilaments) after muscle fiber stimulation

cross-bridge

cross-bridges form when myosin ATPases _____ and enter their high-energy state

hydrolyze ATP

the power-stroke occurs when myosin _____ to enter a low-energy conformation

releases ADP + Pi

(the power-stroke occurs after the initial formation of a cross-bridge)

what releases the cross-bridge after the power-stroke has occurred?

ATP binding to myosin

why does rigor mortis occur in dead animals?

no ATP is being produced, so cross-bridges cannot be released

when the brain stops sending nerve signals for contraction, contraction stops - what happens to Ca2+?

it re-enters the SR by active transport

what is the result of the active transport of Ca2+ back into the sarcoplasmic reticulum?

troponin returns to its original shape and pulls tropomyosin back over the actin binding site

the _____ represents the periphery (end) of each sarcomere

Z line

the _____ represents the midpoint of each sarcomere

M line

_____ branch from the Z line toward the M line

thin actin myofilaments

_____ branch from the M line toward Z line

thick myosin myofilaments

_____ are areas where only actin is present (includes the Z disk)

I bands

Mnemonic: I is a thin letter, so the I band only has thin filaments (actin)

_____ are areas where myosin and actin overlap

A bands

all bands and zones of a sarcomere shorten/disappear during a muscle contraction, with the exception of _____

the A band

the _____ is an area where only myosin is present

H zone

Mnemonic: H is a thick letter, so the H zone only has thick filaments (myosin)

a _____ is all the muscle fibers innervated by one motor neuron

motor unit

describe the general concept of a small motor unit

1 motor unit with few muscle fibers

_____ movements are created by small muscles with many small motor units

precision

describe the general concept of a large motor unit

1 motor unit with many muscle fibers

_____ movements are created by large muscles with few large motor units

powerful

a _____ is a brief muscle fiber contraction after a single action potential stimulates an entire motor unit

twitch

describe the all-or-nothing principle of muscle contraction

either the depolarization is above threshold (all fibers in the unit twitch) or it is below threshold (no fibers in the unit twitch)

what are the two main types of voluntary skeletal muscle contractions?

isotonic & isometric

_____ are those where the muscle contracts against the same weight to product a constant tension

isotonic contractions

what are the two types of isotonic contractions?

concentric and eccentric

_____ occur when a muscle shortens as it pulls the bone it inserts onto closer to the bone it originates from

concentric contractions

_____ occur when a muscle lengthens

eccentric contractions

_____ occur when the muscle length and angle across the joint do not change

isometric contractions

what are the three main functional groups of skeletal muscles?

agonists (primary movers); antagonists; synergists

_____ are muscles involved in preforming a desired motion

agonists

_____ produce the opposite motion to agonists

antagonists

_____ are muscles that help primary movers and provide support to joints

synergists