Transverse tubules:openings scattered across the surface of the sarcolemma. -filled with extracellular fluid. -plays a major role in coordination of muscle contraction.
How does an electrical impulse of a muscle contraction reach the inside of a muscle fiber?
The electrical impulse travels along the T-tubules (which extend deep into the sarcoplasm).
Cylindrical structures encircled by T-tubules branches within the muscle fiber.
What are myofibrils made of?
thick and thin myofilaments.
What are myofilaments?
Protein filaments consisting primarily of actin or myosin.
What protein makes up thin filaments in myofibrils?
What protein makes up thick filaments in myofibrils?
What are the myofibrils attached to?
Myofibrils are attached to the sarcolemma at each end of the cell.
What provides the myofibrils the ATP needed for muscular contraction?
The breakdown of glucose and the activity of mitochondria which are scattered among the myofibrils.
What is the sarcoplasmic reticulum (SR)?
A specialized form of smooth endoplasmic reticulum which is bonded to the T-tubule in the muscle cell.
What is the Terminal Cisternae?
Expanded chambers of the SR lying on either side of a T-tubule.
What is a triad?
A transverse tubule sandwiched between a pair of terminal cisternae.
What does a terminal cisternae have to do to begin a muscle contraction?
Release the calcium ions it is storing.
What are sarcomeres?
-The smallest functional unit of the muscle fiber. -consists of repeating units of thin and thick myofilaments.
What are the interconnecting proteins that make up the boundaries of each sarcomere and cross the center of each I band?
What protein structure connects the central portions of each thick filament to its neighbor?
What is the dark band of thick filaments called?
What is the light region between two successive A bands?
The I band (it includes the z bands)
What are the thin filaments made of?
A twisted strand of actin molecules with an active site capable of interacting with myosin.
In a resting muscle, what are the active sites in the thin filaments covered by?
strands of the protein tropomyosin.
What holds the tropomyosin in position on the active sites of the thin filaments?
molecules of troponin
What are thick filaments composed of?
Myosin molecules (each with a head and tail)
How are the Myosin molecules oriented?
They are oriented away from the center of the sacomere, with the heads projecting outward.
What has to change position for a myosin head to attach to the active site on a strand of actin?
The troponin has to change position
What is the key that unlocks the active sites and starts a muscle contraction?
Calcium (ions bind to the troponin causing it to change shape and swing the tropomyosin away from the active sites)
What is the source of the calcium that triggers a muscle contraction?
terminal cisternae of the sarcoplasmic reticulum
What happens to the I bands when a sarcomere contracts?
They get smaller
What happens to the Z lines when a sarcomere contracts?
they move closer together.
What happens to the H bands when a sarcomere contracts?
What happens to the zones of overlap when a sarcomere contracts?
they get larger
What happens to the A bands when a sarcomere contracts?
they do not change
What is the sliding filament theory?
when sarcomeres contract the thin filaments slide toward the center along side the thick filaments.
what are cross-bridges?
The myosin heads of the thick filaments when they are connected to thin filaments.
Where does communication between the nervous system and skeletal muscle fiber occur?
NMJ (neuromuscular junction)- A specialized intercellular connection.
What controls each skeletal muscle fiber?
a motor neuron
What neurotransmitter is present at the synaptic terminal in a motor neuron?
What is a neurotransmitter?
a chemical released by a neuron to communicate with other cells.
What neurotransmitter produces a change in the sarcolemma that triggers the muscle contraction?
What is the synaptic cleft?
a narrow space that separates the synaptic terminal from the sarcolemma.
What is the motor end plate?
the portion of the sarcolemma containing receptors for ACh.
What enzyme breaks down ACh?
Acetylcholinesterase (AChE or cholinesterase) contained in the synaptic cleft and the motor end plate.
What is action potential?
an electrical impulse produced in the sarcolemma, from a neuron.
What triggers the massive release of Calcium ions by the terminal cisternae before a muscle contraction?
the passage of an action potential along the t tubules
When do the cross-bridges detach from the actin during a contraction cycle?
When the myosin head binds to another molecule of ATP
What has to happen before the myosin can repeat the contraction cycle?
It must split the ATP and capture the released energy.
When is a contraction cycle broken?
When calcium ion concentrations return to normal resting levels by active transport into the sarcoplasmic reticulum.
What is tension?
When muscle cells contract, pulling on collagen fibers, it produces an active force called tension.
What direction does tension generaly pull?
Tension applied to an object tends to pull the object toward the source of the tension.
What determines the amount of tension produced by an individual muscle fiber?
The number of cross-bridges it contains
What is a twitch?
A single stimulus-contraction-relaxation sequence in a muscle fiber.
What is summation?
when a second stimulus arrives before the relaxation phase has ended (in a twitch)
What is incomplete tetanus?
When the muscle is stimulated repeatedly and never completely relaxes.
What is complete tetanus?
when the rate of stimulation in a muscle is increased until the relaxation phase is eliminated all together.
What is a motor unit?
All the muscle fibers controlled by a single motor neuron.
What is recruitment?
The activation of more and more motor units resulting in smooth steady increase in muscle tension.
What is an isotonic muscle contraction?
Tension rises and the skeletal muscle's length changes. Tension in the muscle remains at a constant level until relaxation occurs.
What is isometric muscle contraction?
Tension never exceeds the resistance and the muscle as a whole doesn't change length.
What 3 things will help return a muscle to its original length after a contraction?
It is a combination of elastic forces, movements of opposing muscles and gravity.
What is the energy source for muscular activity?
What is the primary function of ATP?
the transfer of energy from one location to another.
When a muscle is at rest, what does ATP transfer energy into for storage?
creatine phosphate molecules and glycogen.
During muscle contraction what happens to the energy being stored in creatine phosphate?
The energy stored in creatine phosphate is transferred back to ATP.
How does the body feel fatigue from repeated muscle contractions?
From the lactic acid diffusing from the cell into the blood, which lowers the pH of the blood.
What is the Krebs cycle?
Organic substrates that are broken down by a series of chemical reactions during aerobic metabolism.
A common carbohydrate substrate that is broken down in Krebs cycle, producing 17 ATP molecules.
What process provides pyruvic acid to the mitochondria during muscle contraction?
What molecule is the limiting factor for ATP production by the mitochondria?
Oxygen (bottom of pg 209)
What is the breakdown of glucose to pyruvic acid in the cytoplasm of the cell?
What can continue to provide ATP when the availability of oxygen limits the rate of mitochondrial ATP production?
What happens when the muscle fiber begins to run short of ATP and CP during muscle contraction?
Enzymes break the glycogen molecules apart to release glucose that can be used to generate more ATP.
About how much ATP does mitochondrial activity provide during periods of peak muscle activity?
During periods of peak muscle activity what is the primary source of ATP?
The anaerobic process of glycolysis.
What is lactic acid?
An organic acid that dissociates into a hydrogen ion and lactate ion in body fluids, lowering the pH levels.
How many ATP molecules are generated from the 2 pyruvic acids under anaerobic (glycolysis) conditions versus aerobic (mitochondrial) conditions?
2 ATP molecules under anaerobic conditions vs 34 molecules under aerobic conditions.
Is the Krebs cycle Aerobic or anaerobic?
What type of muscle fatigue is the result of reserves of glycogen, lipids, and amino acids being depleted? ATP is used at or below the maximum rate of mitochondrial ATP generation. (the muscle fiber is functioning aerobically)
fatigue after long endurance activities.
What type of fatigue happens when the ATP is provided by glycolysis and lactic acid levels lower pH levels?
when a muscle produces a sudden, intense burst of activity.
During the recovery period of a muscle what is the muscle's metabolic activity focused on?
The removal of lactic acid and the replacement of intracellular energy reserves.
What is oxygen debt?
The additional oxygen required during the recovery period to restore the normal preexertion levels.
What are the 2 types of skeletal muscle fibers?
fast and slow
Describe Fast muscle fibers
white muscle, contract rapidly, fatigue rapidly
describe slow fibers
red muscles, contract slowly, contain myoglobin (serving as a source of O2), has more extensive blood vessels, slower to fatigue
what type of muscle fibers are make up most of the skeletal muscle fibers in the body?
What is Anaerobic conditioning?
the length of time muscle contractions can be supported by glycolysis and existing energy reserves of ATP and CP.
What is Aerobic endurance?
The length of time a muscle can continue to contract while being supported by mitochondrail activities.
For each pyruvic acid molecule broken down in the TCA cycle (or Krebs cycle, or citric acid cycle), the cell gains how many ATP molecules?
__________ is the length of time muscle contractions can be supported by glycolysis and existing energy reserves of ATP and creatine phosphate (CP).
What concept is defined as the maximum amount of tension produced by a particular muscle or muscle group?
There are no slow fibers in muscles of the______&_______.
hand and eye
Aerobic endurance is determined by the availability of substrates for aerobic metabolism from the breakdown of which of the following substances?
amino acids, lipids, carbohydrates.
Cardiac muscle tissue contracts without neural stimulation, a property called____________.
describe intercalated discs.
They are specialized sites of contact between cardiac muscle cells
What is the source of most of the calcium ions that enter smooth muscle cells to trigger contractions?
the extracellular fluid
In the digestive and urinary systems, rings of smooth muscles called ________ regulate movement along internal passageways
Describe the nucleus of cardiac muscle cell
It is relatively small and centrally located
About how much longer does a cardiac muscle contraction last compared to a skeletal muscle?
About 10 times longer.
What do cardiac muscles rely on for energy to continue contracting?
oxygen (aerobic metabolism)
What does the cardiac sarcoplasm contain to store oxygen?
large numbers of mitochondria and abundant reserves of myoglobin.
Cardiac muscle contractions result from ______ interaction.
What is contraction initiated by in cardiac muscles?
pacemaker (SA node)
What can cardiac muscles not do?
They cannot undergo tetany (sustained contraction)
What is Tetany?
What system has control of the muscle neurons?
Somatic Nervous System
What are the intercalated discs of the cardiac muscles?
The cellular connections that contain gap junctions which allow for the rapid movement of ions and small molecules and the rapid passage of action potentials from cell to cell, resulting in simultaneous contraction.
What is the most obvious structural difference between cardiac and skeletal muscle fibers?
Cardiac muscle fibers are branched and each cardiac cell contacts several others at intercalated discs.
What is muscle tension dependent on?
the frequency of stimulation and the number of muscle fibers being stimulated.
What is a twitch?
the contraction of a single muscle fiber
How long does a twitch last?
Does a twitch include a complete sequence?
Yes- stimulus, contraction, relaxation
What happens in the latent phase of the twitch sequence?
The impulse penetrates the muscle fiber and Ca ions are released (about 2msec)
What happens during the contraction phase in the twitch sequence?
tension rises to a peak. The cross bridges are interacting with the active sites on the actin filaments. (max tension is reached ~15msec after stimulation)
What happens during the relaxation phase in the twitch sequence?
muscle tension falls to resting levels as calcium levels drop, active sites are being covered and cross bridges are declining. (lasts ~25msec)
How does the internal organization of a smooth muscle cell differ from skeletal and cardiac muscle cells?
smooth muscle cells have NO myofibrils, sarcomeres, or striations, myofiliments are scattered throughout sarcoplasm.
Smooth muscle cells contain a ___________ nucleus.
single, centrally located
What shape is a smooth muscle cell?
What is muscle origin?
The point the muscle begins, The stationary end, usually the proximal end
What is the muscle insertion point?
The point that moves (usually the distal end), contraction of muscle moves insertion toward origin
What is the belly of a muscle?
The widest portion of a muscle
What is a Prime mover/Agonist muscle?
It is the muscle responsible for particular movement
What is an Antagonist muscle?
it's the muscle that provides resistance to the Prime mover and allows for controlled movement
What is a synergist muscle?
It's a muscle that assists the Prime Mover by providing additional pull or stabilizes the point of origin.
What is muscle tone?
The resting tension in a skeletal muscle that tense and firm a muscle, but does not cause movement.