Chapter 12: Muscle Physiology

skeletal muscle

A muscle that is attached to the bones of the skeleton and provides the force that moves the bones.

striated

marked with thin, narrow grooves or channels

cardiac muscle

Involuntary muscle tissue found only in the heart.

smooth muscle

Involuntary muscle found inside many internal organs of the body

muscle fiber

long slender skeletal muscle cells

fascicles

Bundles of muscle fibers wrapped in perimysium

tendons

Connect muscle to bone

multinucleated

more than one nucleus per cell

sarcolemma

plasma membrane of a muscle fiber

transverse tubules

transmit muscle impulses into the cell interior.

sarcoplasmic reticulum

Organelle of the muscle fiber that stores calcium.

sarcoplasm

cytoplasm of a muscle cell

mitochondria

An organelle found in large numbers in most cells, in which the biochemical processes of respiration and energy production occur.

glycogen

An extensively branched glucose storage polysaccharide found in the liver and muscle of animals; the animal equivalent of starch.

neuromuscular junction

point of contact between a motor neuron and a skeletal muscle cell

myofibrils

Microscopic protein filaments that make up muscle cells.

actin (thin filaments)

A globular protein that links into chains, two of which twist helically about each other, forming microfilaments in muscle and other contractile elements in cells.

myosin (thick filaments)

a fibrous protein that forms (together with actin) the contractile filaments of muscle cells and is also involved in motion in other types of cells

sarcomere

a structural unit of a myofibril in striated muscle, consisting of a dark band and the nearer half of each adjacent pale band.

Z lines

define the boundaries of each sarcomere

I band

thin filaments (actin) only

A band

dark area; extends length of the thick filaments

H zone

thick filaments only

M line

supporting proteins that hold the thick filaments together in the H zone

G actin

a globular subunit of F actin with an active site for binding a myosin head

myosin binding site

where myosin head attaches during contraction

F actin

A fibrous protein made of a long chain of G actin molecules twisted into a helix; main protein of the thin myofilament

tropomyosin

A protein of muscle that forms a complex with troponin regulating the interaction of actin and myosin in muscular contraction

troponin

A protein of muscle that together with tropomyosin forms a regulatory protein complex controlling the interaction of actin and myosin and that when combined with calcium ions permits muscular contraction

Ca2+

Calcium ion

crossbridges

when myosin heads attach to actin

actin-binding site

specialized region of the myosin head capable of binding to actin

ATPase site

Located on heads of myosin molecules. Essential for producing contractile force

titin

a protein that positions the myosin filament to maintain equal spacing between actin filaments

sliding filament model

The theory explaining how muscle contracts, based on change within a sarcomere, the basic unit of muscle organization, stating that thin (actin) filaments slide across thick (myosin) filaments, shortening the sarcomere; the shortening of all sarcomeres in a myofibril shortens the entire myofibril

crossbridge cycle

the mechanism that drives muscle contraction

ATP (adenosine triphosphate)

Composed of a sugar ribose, nitrogenous base adenine, and a chain of three phosphate groups bonded to it.

power stroke

action of myosin pulling actin inward (toward the M line)

rigor

a state of rigidity in muscle tissues during which they are unable to respond to stimuli due to the coagulation of muscle protein

excitation-contraction coupling

sequence of events from motor neuron signaling to a skeletal muscle fiber to contraction of the fiber's sarcomeres

motor unit

A motor neuron and all of the muscle fibers it innervates

neuromuscular junction

point of contact between a motor neuron and a skeletal muscle cell

acetylcholine

A neurotransmitter that enables learning and memory and also triggers muscle contraction

nicotinic cholinergic receptors

agonistic receptors found in skeletal muscle, in CNS, and autonomic division of the PNS

motor end plate

the flattened end of a motor neuron that transmits neural impulses to a muscle

end-plate potential

The depolarization of the motor end plate on a muscle cell.

action potential

a neural impulse; a brief electrical charge that travels down an axon

acetylcholinesterase

the enzyme that breaks down acetylcholine in the synaptic cleft

muscle twitch

the response of a muscle to a single brief threshold stimulus

isometric twitch

a twitch during which a muscle generates force but does not shorten

isotonic twitch

a twitch during which a muscle shortens and lifts a constant load

treppe

Phenomenon in which each successive twitch contracts more forcefully than the previous one

summation

increased force of contraction by a skeletal muscle fiber when a twitch occurs before the previous twitch relaxes

tetanus

a sustained muscular contraction resulting from a rapid series of nerve impulses

length-tension curve

the curve that accounts for the active and passive elements of muscle tension and dictates that optimal tension is developed at one point known as the resting length, the point in its range where peak torque is developed

recruitment

an increase in the response to a stimulus owing to the activation of additional receptors, resulting from the continuous application of the stimulus with the same intensity

small motor units

fine degree of control

Three to six muscle fibers per neuron

Eye and hand muscles

large motor units

more strength than control

Powerful contractions supplied by large motor units with hundreds of fibers

Gastrocnemius of calf has 1,000 muscle fibers per neuron

size principle

motor units with larger and larger fibers are recruited as stimulus intensity increases

asynchronous activation of motor units

When the nervous system modulates the firing rates of the motor so that different motor units take turns maintaining muscle tension

creatine phosphate

An energy storage molecule used by muscle tissue. The phosphate from creatine phosphate can be removed and attached to an ADP to generate ATP quickly.

oxidative phosphorylation

When energy is released at each step of the chain is stored in a form the mitochondrion can use to make ATP.

fermentation (anaerobic respiration)

Process by which cells release energy in the absence of oxygen

fast twitch fibers

muscle fibers that contract rapidly and forcefully but fatigue quickly

fast glycolytic fibers

contract quickly, have fast myosin ATPase, and are easily fatigued

muscle fatigue

inability of muscle to maintain force of contraction after prolonged activity

muscle atrophy

loss of muscle bulk due to muscle disease, nervous system disease, or lack of use; commonly called muscle wasting

muscle hypertrophy

muscle enlargement from overuse

muscle spindle

a sensory receptor located in a muscle that senses its tension

Golgi tendon organ

the receptor organ at the junction of the tendon and muscle that is sensitive to stretch

muscle spindle stretch reflex

spinal reflex, is the only known monosynaptic reflex in the human body

For example, in the knee jerk reflex, tapping the patellar tendon stretches the quadriceps muscle, which excites muscle spindles in that muscle and triggers APs that travel to the spinal cord. Efferent neurons in the muscle stimulate the quadriceps to contract an the leg to "kick" forward, or extend.

tendon reflex

reflexive contraction of a muscle when its tendon is tapped

autonomic nervous system

the part of the peripheral nervous system that controls the glands and the muscles of the internal organs (such as the heart). Its sympathetic division arouses; its parasympathetic division calms.

paracrines

locally acting chemicals that affect cells other than those that secrete them

dense bodies

cytoplasmic structure to which thin filaments of a smooth muscle fiber are anchored

calmodulin

calcium binding protein

myosin light-chain kinase (MLCK)

A kinase in smooth muscle cells activated by calmodulin the presence of Ca2+. As its name implies, this kinase phosphorylates myosin, activating it so that muscle contraction can occur.

phosphatase

removes a phosphate group from a molecule

single unit smooth muscle

Smooth muscle with gap junctions linking the cells together so they function as a unit

gap junctions

Points that provide cytoplasmic channels from one cell to another with special membrane proteins. Also called communicating junctions.

pacemaker activity

some cells contain special ionic channels which open or close spontaneously and rhythmically without external stimulation which results in a threshold depolarization at the initial segment

pacemaker potentials

initiate the action potentials that spread out through the heart to trigger its rhythmic contractions

slow wave potentials

basic electrical rhythm of GI tract, spontaneous, rhythmic cycles of depolarization and repolarization

multi-unit smooth muscle

a type of smooth muscle found in the iris of the eye and in the walls of blood vessels

intercalated disks

These structures branch and connect cardiac cells. They contain specialized gap junctions and coordinate muscle contractions.

pacemaker cells

heart cells that regularly produce spontaneous electrical impulses

sinoatrial node

A small mass of tissue that is made up of Purkinje fibers, ganglion cells, and nerve fibers, that is embedded in the musculature of the right atrium, and that originates the impulses stimulating the heartbeat -- called also S-A node, sinus node.

atrioventricular (AV) node

neurological tissue in the center of the heart that receives and amplifies the conduction of impulses from the SA node to the bundle of His

refractory period

a period of inactivity after a neuron has fired