muscular system

contractility

is the ability of muscle to shorten forcefully

excitability

is the capacity of muscle to respond to a stimulus

extensibility

means that a muscle can be stretched beyond its normal resting length and still be able to contract. If you stretch to reach a dropped pencil, your muscles are longer than they are normally, but you can still retrieve the pencil.

elasticity

is the ability of muscle to recoil to its original resting length after it has been stretched.

skeletal muscle

• — with its associated connective tissue, constitutes approximately 40% of body weight.

• is so named because many of the muscles are attached to the skeletal system. However, some — attaches to the skin or connective tissue sheets.

striated muscle

Skeletal muscle is also called —- because transverse bands, or striations, can be seen in the muscle under the microscope

muscle fiber

Each muscle cell is called a —

1. epimysium

2. perimysium

3. endomysium

Each skeletal muscle is surrounded by several connective tissue layers that support the muscle during contraction. A skeletal muscle has three layers of connective tissue:

epimysium

• forms a connective tissue sheath that surrounds each skeletal muscle.

• The protein fibers of the —- gradually merge with a layer of connective tissue between adjacent muscles and connecting the skin to superficial muscles.

perimysium

• — subdivides each whole muscle into numerous, visible bundles of muscle fibers

• — is a loose connective tissue serving as passageways for blood vessels and nerves that supply each fascicle.

fascicles

A skeletal muscle is subdivided into groups of muscle cells, termed —-

endomysium

• is a delicate layer of connective tissue that separates the individual muscle fibers within each fascicle.

• — serves as passageways for nerve fibers and blood vessels that supply each separate muscle fiber.

tendons

The protein fibers of the three layers of connective tissue blend into one another and merge at the ends of most muscles to form —, which attach muscle to bone

1. electrical component

2. mechanical component

There are two main aspects to muscle contraction:

1. sarcolemma

2. transverse tubules

3. sarcoplasmic reticulum

There are three muscle fiber components that respond to and transmit electrical signals:

sarcolemma

is the cell membrane of muscle fibers

transverse tubules

• are tubelike inward folds of the sarcolemma

• At regular intervals along the muscle fiber, the sarcolemma forms — by projecting and extending into the interior of the muscle fiber.

• — carry electrical impulses into the center of the muscle fiber so that the muscle fiber contracts as a whole

sarcoplasmic reticulum

is a highly specialized smooth endoplasmic reticulum in skeletal muscle fibers that stores high levels of Ca 2+

switch

Release of Ca 2+ from the sarcoplasmic reticulum is a “—-” for muscle contraction.

terminal cisternae

T tubules lie next to enlarged portions of the sarcoplasmic reticulum called —

triad

Two terminal cisternae and their associated T tubule form a critical structure for muscle contraction called a —

sarcoplasm

Muscle fibers contain other organelles, such as numerous mitochondria, as well as energy-storing glycogen granules. Together these organelles constitute the cytoplasm, called the — in muscle fibers

myofibrils

• are bundles of protein filaments

• Each muscle fiber has numerous — in its sarcoplasm.

• are long threadlike structures that extend the entire length of the muscle fiber

1. actin myofilaments (thin)

2. myosin myofilaments (thick)

2 types of myofilaments

sarcomeres

• the actin and myosin myofilaments are arranged into highly ordered units called —

• — are the structural and functional units of skeletal muscles.

• The myofilaments in the — provide the mechanical aspect of muscle contraction

Z disks

• Structures called — form a stationary anchor for actin myofilaments

• One sarcomere extends from one —- to the next —.

I bands

• Each of the two — includes a Z disk and extend to the ends of the myosin myofilaments.

• The —- contain only actin myofilaments (thin filaments) and thus appear lighter staining.

A band

• The darker-staining band in the center of each sarcomere is called an—

• Each— contains both actin and myosin myofilaments overlapping, except in the center

H zone

The center of each A band has a smaller band, called the —-, and contains only myosin myofilaments (thick filaments)

M line

• The middle of each H zone has a dark line, called the—-

• consists of delicate protein filaments that hold the myosin myofilaments in place

actin, tropomyosin, troponin

Each actin myofilament is composed of three separate proteins:

tropomyosin

• In a relaxed muscle, — is covering the attachment sites on the actin.

• A muscle cannot contract until the —- moves to uncover the active sites.

troponin

consists of three subunits:

• a subunit that anchors the troponin to the actin

• a subunit that prevents the tropomyosin from uncovering the actin attachment sites in a relaxed muscle

• a subunit that binds Ca 2+

myosin molecules

Myosin myofilaments are composed of many elongated —— shaped like golf clubs

myosin heads

— have three important properties:

• — bind to active sites on the actin molecules to form cross-bridges to contract the muscle

• — are attached to the rod portion by a hinge region that bends and straightens during contraction

• — break down adenosine triphosphate (ATP), releasing energy.

motor neurons

The — carry electrical signals called action potentials, which cause action potentials in the muscle fiber, followed by muscle contraction

neuromuscular junction

• The point of contact of motor neuron axon branches with the muscle fiber is called the —-

• — consists of a group of enlarged axon terminals that rests on a portion of the sarcolemma.

• — is a synapse where a neuron connects with a muscle fiber

synapse

refers to the cell-to-cell junction between a nerve cell and either another nerve cell or an effector cell, such as in a muscle or a gland

motor unit

is a group of muscle fibers that a single motor neuron stimulates

presynaptic terminal

• Each axon terminal is called the —-

• is the end of a neuron cell axon fiber

synaptic cleft

The space between the presynaptic terminal and the muscle fiber (postsynaptic membrane) is the

postsynaptic membrane

The muscle cell membrane in the area of the junction is called the motor end-plate, or the —-

synaptic vesicles

• Each presynaptic terminal contains numerous mitochondria and many small, spherical sacs, called —-

• a vesicle in the presynaptic terminal that stores and releases neurotransmitter chemicals

acetylcholine

• The synaptic vesicles contain the neurotransmitter —

• is the neurotransmitter that stimulates skeletal muscles.

neurotransmitter

• is a molecule that allows a neuron to communicate with its target.

• They are released from a presynaptic membrane and diffuse across the synaptic cleft to alter the activity of the muscle fiber.

• are chemicals that stimulate or inhibit postsynaptic cells

ligand-gated ion channels

• Neurotransmitters can stimulate or inhibit the production of an action potential in the motor end-plate (the sarcolemma) by binding to —-

• are specialized membrane transport proteins that are opened or closed by specific molecules, such as neurotransmitters

sliding filament model

• The parallel arrangement of myofilaments in a sarcomere allows them to interact, which causes muscle contraction. This interaction is described by the —-

• When a muscle contracts, the actin and myosin myofilaments in the sarcomere slide past one another and shorten the sarcomere.

• When sarcomeres shorten, myofibrils, muscle fibers, muscle fascicles, and muscles all shorten to produce muscle contraction

ions

The phospholipid bilayer interior is a hydrophobic environment, which inhibits the movement of charged particles, particularly —

ion channels

• Two types of membrane proteins, called —-

• — permit ions to pass through the membrane

leak and gated

• There are two major types of ion channels:

• These two types of channels contribute to the electrical properties of both a resting cell and a stimulated cell.

leak ion channels

In resting cells, the —- allow for the slow leak of ions down their concentration gradient

gated ion channel

• — are most important in stimulated cells.

• It is their presence that governs the production of action potentials.

• may open or close in response to various types of stimuli

resting membrane potential

• The electrical charge difference across the cell membrane of an unstimulated cell is called the

• is due to the inside of the membrane being negatively charged in comparison to the outside of the membrane which is positively charged

action potential

• occurs when the excitable cell is stimulated.

• is a reversal of the resting membrane potential such that the inside of the cell membrane becomes positively charged compared with the outside

depolarization

This increase in positive charge inside the cell membrane is called —

threshold

• If the depolarization causes the membrane potential to reach —-, an action potential is triggered.

• is the membrane potential at which gated Na + channels open

depolarization phase

the — of the action potential is a brief period during which further depolarization occurs and the inside of the cell becomes even more positively charged

repolarization

• As the inside of the cell becomes positive, this voltage change causes additional permeability changes in the cell membrane, which stop depolarization and start —-

• opening of gated K+ channels starts

repolarization phase

is the return of the membrane potential to its resting value. It occurs when ligand-gated Na + channels close and gated K + channels open.

acetylcholinesterase

• enzyme — rapidly breaks down acetylcholine in the synaptic cleft into acetic acid and choline.

• keeps acetylcholine from accumulating within the synaptic cleft, where it would act as a constant stimulus at the motor end-plate, producing continuous contraction in the muscle fiber

cross-bridge cycling

• The mechanical component of muscle contraction is called —

• This rapid sequence of events will cause the sarcomeres to shorten and the muscle will contract.

• The energy from one ATP molecule is required for each —-

rigor mortis

will occur when a person dies and no ATP is available to release cross-bridges.

muscle relaxation

occurs when acetylcholine is no longer released at the neuromuscular junction.

muscle twitch

The response of a muscle fiber to a single action potential along its motor neuron is called a —

1. latent or lag phase

2. contraction phase

3. relaxation

A twitch has three phases

latent or lag phase

is the gap between the time of stimulus application to the motor neuron and the beginning of contraction.

contraction phase

• commences once the Ca 2+ released from the sarcoplasmic reticulum initiates cross-bridge formation and cross-bridge cycling

• is the time during which the muscle contracts

relaxation phase

• is much longer than the contraction phase, because the concentration of Ca 2+ in the sarcoplasm decreases slowly due to active transport into the sarcoplasmic reticulum

• is the time during which the muscle relaxes

isometric contractions and isotonic contractions

two major types of muscle contractions:

isometric contraction

This type of contraction increases the tension in the muscle, but the length of the muscle stays the same

isotonic contraction

This type of contraction increases the tension in the muscle and decreases the length of the muscle.

summation and recruitment

The change in muscle contraction strength depends on two factors:

summation

the amount of force in an individual muscle fiber, is called —

recruitment

the amount of force in a whole muscle, called —

motor unit

• consists of a single motor neuron and all the muscle fibers it innervates.

• An action potential in the neuron of a — causes contraction of all the muscle fibers in that unit.

incomplete tetanus

occurs when the frequency of stimulation only allows for partial relaxation of the muscle fiber.

complete tetanus

is a sustained contraction that occurs when the frequency of stimulation is so rapid that no relaxation occurs

muscle tone

• is the constant tension produced by body muscles over long periods of time.

• is responsible for keeping the back and legs straight, the head in an upright position, and the abdomen from bulging

concentric contractions

• are isotonic contractions in which tension in the muscle is great enough to overcome the opposing resistance, and the muscle shortens.

• such as lifting a loaded backpack from the floor to a tabletop

eccentric contractions

• are isotonic contractions in which tension is maintained in a muscle but the opposing resistance is great enough to cause the muscle to increase in length.

• For example, occur when a person slowly lowers a heavy weight

slow-twitch fibers and fast-twitch fibers

two major types of skeletal muscle fibers

slow-twitch fibers

• contract slowly

• fatigue slowly

• have a considerable amount of myoglobin

• use aerobic respiration

• are dark in color

• used by long distance runners

fast-twitch fibers

• contract quickly

• fatigue quickly

• use anaerobic respiration

• energy from glycogen

• light color

• used by sprinters

hypertrophies

In response to exercise, a muscle increases in size, or

atrophies

a muscle that is not used decreases in size, or

Adenylate kinase

Conversion of two ADP to one ATP and one adenosine monophosphate (AMP) by the enzyme —

creatine

During periods of rest, muscle fibers accumulate extra ATP. This extra ATP is utilized in muscle fibers to transfer a phosphate from the ATP to a small protein synthesized by muscle fibers called —

creatine phosphate

• transfer of the phosphate creates the molecule —

• This molecule acts like a “bank” for “high-energy” phosphate.

creatine kinase

When ATP levels start to drop in a contracting muscle fiber, the enzyme —- will transfer a phosphate from creatine phosphate to ADP, immediately producing ATP

anaerobic respiration

• does not require O2 and involves the breakdown of glucose to produce ATP and lactate.

• produces only enough ATP to power muscle contractions for 30–40 seconds.

glycolysis

The first step of anaerobic respiration is the enzymatic pathway, called

pyruvate

In glycolysis, one glucose molecule is broken down into two molecules of —-, producing a net gain of two ATP molecules

lactate

In anaerobic respiration, the pyruvic acid is then converted to a molecule called —

aerobic respiration

• — requires O2 and breaks down glucose to produce ATP, CO2 , and H2O (

• The ATP from —- supplies 95% of the total ATP required by a cell and provides enough ATP for hours of muscle contraction as long as O2 is readily available

fatigue

• is a temporary state of reduced work capacity.

• Without —, muscle fibers would be worked to the point of structural damage to them and their supportive tissues

oxidative stress

which is characterized by the buildup of excess reactive oxygen species

interleukin 6

• oxidative stress trigger an immune system chemical called

• is a mediator of inflammation, which is the most likely cause of muscle soreness

physiological contracture

• An example of muscle fatigue occurs when a runner collapses on the track and must be helped off. The runner’s muscle can no longer function regardless of how determined the runner is. Under conditions of extreme muscular fatigue, muscle may become incapable of either contracting or relaxing.

• This condition, called —, occurs when there is too little ATP to bind to myosin myofilaments

psychological fatigue

• The most common type of fatigue, —-, involves the central nervous system rather than the muscles themselves.

• The muscles are still capable of contracting, but the individual “perceives” that continued muscle contraction is impossible.

1. oxygen deficit

2. excess postexercise oxygen consumption

When a person exercises, there are two distinct phases of O2 use