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Questions and Answers

What causes the muscle fiber to return to its resting length after contraction?

Stretching of series-elastic components (correct)

Loss of ATP during muscle activity

Accumulation of calcium in the sarcoplasm

Contraction of synergistic muscles

Which of the following is a consequence of cholinesterase inhibitors found in pesticides?

Increased muscle relaxation

Flaccid paralysis due to muscle overactivity

Cramps in respiratory muscles

Spastic paralysis and possible suffocation (correct)

What is the primary cause of rigor mortis after death?

Decreased extracellular calcium levels

Deterioration of the muscle cell membrane

Depleted ATP levels (correct)

Release of myoglobin into the bloodstream

What type of muscle contraction occurs when tension is developed while shortening?

Isotonic concentric contraction Signup and view all the answers

Which neuromuscular toxin blocks the release of acetylcholine, leading to flaccid paralysis?

Botulinum toxin Signup and view all the answers

Which process produces ATP using Pi from creatine phosphate in muscles?

Phosphagen system Signup and view all the answers

What happens during an isometric muscle contraction?

Muscle length remains constant while developing tension Signup and view all the answers

What is the immediate effect of an increase in cytosolic calcium after death?

Activation of myosin-actin cross bridging Signup and view all the answers

What is the primary function of Schwann cells at the neuromuscular junction?

They envelope and isolate the neuromuscular junction. Signup and view all the answers

What is the primary function of the endomysium in skeletal muscle?

To allow room for capillaries and nerve fibers Signup and view all the answers

Which step follows the binding of acetylcholine (ACh) to its receptors on the muscle cell surface?

End-plate potential (EPP) is generated. Signup and view all the answers

What is the primary source of ATP during the first 6 seconds of intense exercise?

Phosphagen system Signup and view all the answers

Which of the following best describes the epimysium?

It encompasses groups of muscle fibers known as fascicles. Signup and view all the answers

What is the resting membrane potential of muscle and nerve cells typically?

-90 mV Signup and view all the answers

What byproduct is generated during anaerobic respiration in muscles?

Lactic acid Signup and view all the answers

What are the two ways a muscle can attach to a bone?

Direct and indirect attachments Signup and view all the answers

What initiates the release of acetylcholine at the neuromuscular junction?

Voltage-gated calcium channels opening. Signup and view all the answers

During depolarization of the muscle cell membrane, which ion primarily enters the cell?

Sodium (Na+) Signup and view all the answers

What is the role of the perimysium in muscle structure?

It separates and bundles groups of muscle fibers into fascicles. Signup and view all the answers

Which enzyme transfers Pi groups from one ADP to another in muscle phosphorylation?

Myokinase Signup and view all the answers

How do the striations in skeletal muscle appear?

From the arrangement of internal contractile proteins Signup and view all the answers

After how many seconds of exercise does the respiratory and cardiovascular systems begin to support aerobic respiration?

40 seconds Signup and view all the answers

What is the result of an action potential traveling along the plasma membrane of the muscle cell?

It self-propagates and continues to the T tubules. Signup and view all the answers

What is the maximum number of ATP molecules produced per glucose molecule during aerobic respiration?

36 Signup and view all the answers

What general process occurs following the end-plate potential in the muscle fiber?

Excitation-contraction coupling begins. Signup and view all the answers

What is the characteristic structure of a skeletal muscle fiber?

It can range from 10 to 100 micrometers in diameter. Signup and view all the answers

What typically happens when stress is applied to a tendon during muscular contraction?

The tendon will tear before detaching from the muscle or bone. Signup and view all the answers

What ultimately limits ATP production during prolonged-duration exercise?

Depletion of glycogen and blood glucose Signup and view all the answers

What characterizes the repolarization phase of the action potential in muscle cells?

Potassium ions exit the cell. Signup and view all the answers

What is the role of acetylcholinesterase at the neuromuscular junction?

It breaks down acetylcholine. Signup and view all the answers

During anaerobic respiration, which source of glucose is primarily used for ATP production?

Both blood glucose and stored glycogen Signup and view all the answers

What type of muscle is considered voluntary and striated, commonly attached to bones?

Skeletal muscle Signup and view all the answers

What process is initiated by the depletion of stored ATP and phosphates during intense exercise?

Glycolysis Signup and view all the answers

Which component of the neuromuscular junction contains acetylcholine?

Synaptic knob Signup and view all the answers

How many muscle fibers does one motor neuron innervate in the gastrocnemius muscle?

1000 fibers Signup and view all the answers

What occurs at the axon terminal of the motor neuron during muscle activation?

Acetylcholine is released into the synaptic cleft. Signup and view all the answers

What is the purpose of the motor end plate in a neuromuscular junction?

To receive acetylcholine and stimulate muscle contraction. Signup and view all the answers

What happens at the synaptic cleft during muscle stimulation?

A gap exists where neurotransmitters diffuse. Signup and view all the answers

What is the significance of myelinated axons in muscle movement?

They allow quicker transmission of action potentials. Signup and view all the answers

What initiates the process of muscle contraction at the neuromuscular junction?

Release of acetylcholine from synaptic vesicles. Signup and view all the answers

What is a primary characteristic of slow-twitch fibers?

High resistance to fatigue Signup and view all the answers

Which factor contributes to muscle fatigue during prolonged exercise?

Accumulation of extracellular K+ Signup and view all the answers

Resistance training primarily stimulates which of the following?

Synthesis of more myofilaments Signup and view all the answers

What occurs during the repayment of oxygen deficit after exercise?

Oxygen is replenished from myoglobin Signup and view all the answers

Why do fast-twitch fibers typically fatigue more quickly than slow-twitch fibers?

They are enriched with glycogen and lactic acid systems. Signup and view all the answers

How does endurance training primarily benefit muscle tissue?

It enhances the density of capillaries. Signup and view all the answers

What is one effect of ATP shortage during muscle fatigue?

Diminished motor nerve excitability Signup and view all the answers

Which statement is true regarding the role of motor units in muscle contraction?

All fibers within a single motor unit are similar in type. Signup and view all the answers

Study Notes

Muscles & Muscle Tissue

Muscles are responsible for movement, stability, communication, control of openings, and body heat production.
Myology is the study of muscles.
There are 600 skeletal muscles in the human body and most are attached to bones.
Muscles shorten by converting chemical energy of ATP into mechanical energy.

Types of Muscle Tissue

There are three types of muscle tissue: skeletal, cardiac, and smooth.
Skeletal muscles are voluntary, striated, and multinucleated.
Cardiac muscles are found in the heart, striated, branched, and involuntary.
Smooth muscles are found in hollow organs, involuntary, and non-striated.

Functions of Muscles

Muscles allow movement of body parts and organ contents.
Muscles aid in maintaining posture and preventing unwanted movements (fixing a joint).
Muscles facilitate communication through speech, expression, and writing.
Muscles control openings and passageways.
Muscles generate heat, with skeletal muscles contributing up to 85% of body heat.

Characteristics of Muscle Tissue

Responsiveness (excitability or irritability): The ability to receive and respond to stimuli.
Conductivity: The ability for a local electrical change to trigger a wave of excitation travelling along the muscle fiber.
Contractility: The ability to shorten when stimulated.
Extensibility: The ability to be stretched.
Elasticity: The ability to return to its original resting length after being stretched.

Anatomy of Skeletal Muscle

Skeletal muscle cells (also called muscle fibers) are 10 to 100 µm in diameter and up to 30 cm long.
Muscle fibers are bundled together in groups called fascicles.
Skeletal muscle is composed of two types of tissue: muscular tissue and connective tissue.

Connective Tissues of a Muscle

Epimysium: The outermost layer that covers the entire muscle belly and blends into connective tissue separating muscles.
Perimysium: A slightly thicker layer of connective tissue surrounding a bundle of muscle cells, called a fascicle.
Endomysium: A thin layer of tissue surrounding each muscle cell/fiber, providing room for capillaries and nerve fibers.

Muscle Attachments

Muscles can attach to bone in two ways:
- Direct (fleshy): Epimysium is directly continuous with the periosteum.
- Indirect: Epimysium continues as a tendon that merges into the periosteum.
- Stress tears the tendon before pulling it from either bone or the muscle.

Anatomy of Skeletal Muscle (continued)

Skeletal muscles are voluntary and striated.
Muscle fibers are 10 to 100 µm in diameter and up to 30 cm long.
Alternating light and dark bands (striations) reflect the overlapping arrangements of internal contractile proteins.

Muscle Fibers (Form follows Function)

Muscle fibers have multiple nuclei due to fusion of multiple myoblasts during development.
Sarcolemma: Muscle cell membrane with tunnel-like folds called transverse (T) tubules. These carry electrical current into the cell.
Sarcoplasm: Cytoplasm of the muscle cell containing myofibrils (bundles of microfilaments called myofilaments), glycogen for stored energy and myoglobin for binding oxygen.
Sarcoplasmic reticulum: Specialized endoplasmic reticulum of muscle cells. It's a series of interconnected storage sacs called terminal cisternae that store calcium.

The Muscle Fiber

Muscle fibers consist of myofibrils.
Myofibrils contain myofilaments (actin and myosin).
The overlapping arrangement creates striations.
The sarcomere is the functional unit of the myofibril.
Z disc, H zone, I band, and A band are visible structures in a sarcomere.

Muscle Proteins

Muscle tissue contains contractile and regulatory proteins.
Contractile proteins (myosin and actin) are responsible for muscle contraction.
Regulatory proteins (troponin and tropomyosin) regulate muscle contraction.
- Troponin is a switch that starts and stops muscle cell shortening. Calcium binds to it causing contraction.
- Tropomyosin serves to cover or uncover the active sites on actin, regulating muscle contraction.
Thick filaments are made of 200-500 myosin molecules.
Thin filaments are made of two intertwined strands of fibrous (F) actin. Globular (G) actin subunits with active sites and tropomyosin proteins.

Muscle Filaments

Thick filaments have tails with heads (cross bridges) directed outward around the tails in bundle.
Thin filaments are two intertwined strands of fibrous actin.
Elastic filaments (titin) connect thick filaments to the Z disc structure to keep thick and thin filaments aligned.
Elastic filaments resist overstretching the muscle.

Overlap of Thick & Thin Filaments

Thick and thin filaments overlap, which is key in muscle contraction.

Striations and Sarcomeres

Striations in muscle are due to the organization of filaments in sarcomeres.
A bands are thick filament regions, including the H band.
I bands are thin filament regions bisected by Z discs.
Sarcomeres are from one Z disc to the next.

Relaxed versus Contracted Sarcomere

Muscle contraction occurs by shortening of individual sarcomeres.
Z discs become closer together, pulling on the sarcolemma.
Thick and thin filaments change their overlap as sarcomeres shorten, but the filaments themselves do not change length.

Sliding Filament Theory

The filaments within the sarcomeres slide past one another during muscle contraction.
The filaments don’t shorten, rather they slide.
The overlap changes during contraction.

Nerve-Muscle Relationships

Muscles require stimulation by nerves to contract.
Cell bodies of motor neurons are in the brainstem or spinal cord.
Axons of somatic motor neurons are called somatic motor fibers.
Each motor neuron branches and supplies one or more muscle fibers, forming a motor unit.
Motor units are dispersed throughout muscles.

Motor Units

Motor unit = A single motor neuron and all the muscle fibers it innervates.
Motor units are dispersed throughout muscles to control fine movements and strength.
Fine control = few muscle fibers per nerve fiber
Strength control = many muscle fibers per nerve fiber
- Examples of these include eye muscles (fine control) and gastrocnemius muscles (strength control).

Neuromuscular Junction

The neuromuscular junction is where the motor neuron axon terminal meets a muscle fiber.
The axon terminal contains synaptic vesicles filled with acetylcholine (ACh).
ACh is released into the synaptic cleft, stimulating the muscle fiber.
Acetylcholinesterase breaks down ACh, ending the stimulation and allowing relaxation.

Neuromuscular Junction (continued)

Synaptic knob: Swollen end of the nerve fiber containing ACh.
Motor end plate: Specialized region of the muscle cell surface with ACh receptors.
Synaptic cleft: Tiny gap between nerve and muscle cells.
Schwann cells: Envelop and isolate the neuromuscular junction.

Muscle Contraction & Relaxation

Excitation: Action potentials in the nerve lead to action potentials in the muscle.
Excitation-contraction coupling: Action potentials on the sarcolemma activate myofilaments (actin and myosin).
Contraction: Shortening of muscle fibers or formation of tension.
Relaxation: Return of the fiber to its resting length.

Excitation of a Muscle Fiber

Nerve signal arrives at the synaptic knob.
Acetylcholine (ACh) is released into the synaptic cleft.
ACh binds to receptors on the sarcolemma.
This opens ion channels, creating an end-plate potential (EPP).
Nearby voltage-gated channels open, creating action potentials in the muscle fiber itself.

Excitation (steps 1 & 2)

Nerve signal stimulates voltage-gated calcium channels.
Calcium enters the synaptic knob, triggering ACh release.

Excitation (steps 3 & 4 )

ACh binds to receptors on muscle cells, which opens acetylcholine receptor that lets sodium (Na+) enter and K+ exit the cell.
This causes a action potential at the end plate called EPP.

Excitation (step 5)

The end-plate potential opens nearby voltage-gated channels in the plasma membrane, producing an action potential in the muscle fiber itself.

Electrically Excitable Cells

Plasma membranes are polarized; resting membrane potential has high Na+ outside and high K+ inside the cell.
Membrane potential changes in response to stimulus (Na+ rushes in, K+ rushes out).
This rapid change is the action potential, which travels along the sarcolemma.

Membrane Potential

Action potentials are rapid up-and-down changes in membrane potential due to Na+ rushing into the cell and K+ rushing out of the cell along the sarcolemma.

Excitation-Contraction Coupling (steps 6&7)

Action potentials propagate along T tubules.
Calcium is released from the terminal cisternae of the sarcoplasmic reticulum.

Excitation-Contraction Coupling (steps 8&9)

Calcium binds to troponin.
The troponin-tropomyosin complex changes shape, exposing active sites on actin.

Contraction (steps 10 & 11)

Myosin ATPase in the myosin head hydrolyzes an ATP molecule.
The myosin head "cocks" and binds to an active site on actin, forming a cross-bridge.

Contraction (steps 12 & 13 )

Myosin head releases ADP + phosphate as it flexes pulling the thin filament.
More ATP binds – this is necessary to break the cross-bridge.
Repeating the cycle pulls the thin filament. Half of the heads remain attached, preventing slippage.
Thin and thick filaments slide past each other, shortening the muscle.

Relaxation (steps 14 & 15 )

Nerve stimulation ceases, ACh is removed, and acetylcholinesterase breaks down ACh.
Causes stimulation to cease in the muscle cell.

Relaxation (step 16)

Active transport pumps calcium back into the sarcoplasmic reticulum.

Relaxation (steps 17 & 18)

Calcium ions are lost from troponin.
Tropomyosin returns to block active sites on actin.
The muscle fiber returns to its resting length.

Muscle Fatigue

Fatigue is progressive weakness and loss of contractility from prolonged muscle use.
Causes include:
- Glycogen depletion: Decreasing ATP synthesis.
- ATP shortage: Sodium-potassium pumps fail to maintain membrane potential, affecting excitability.
- Lactic acid accumulation: Lowering pH and inhibiting enzyme function.
- Extracellular K+ accumulation: Lowering membrane potential.
- ACh depletion in motor nerve fibers.

Slow- and Fast-Twitch Fibers

Not all muscle fibers are metabolically identical.
- Slow-twitch fibers (type I or red): Adapted for aerobic respiration, resistant to fatigue. (e.g. Soleus, postural muscles).
- Fast-twitch fibers (type II or white): Adapted for anaerobic respiration, more prone to fatigue. (e.g. Extraoccular eye muscles, gastronemius, biceps brachii).

Strength and Conditioning

Strength training increases muscle size, fascicle arrangement, motor unit recruitment and stimulation frequency, and muscle length at start of contraction. This helps to increase strength of contraction.
Resistance training stimulates cells to enlarge due to increased myofilament synthesis.
Endurance training increases mitochondria, glycogen, and capillary density.

Energy Needs of Muscle

Muscles need ATP for contraction, which is generated through various systems:
- Phosphagen system (creatine kinase and myokinase) to generate ATP quickly.
- Aerobic respiration for continued ATP production.
- Anaerobic respiration (glycolysis) for quick ATP production in the short term, producing lactic acid as byproduct.
- The amount of ATP used depends on the duration and intensity of an exercise.

Muscle Short-Term Energy Needs (Anaerobic Respiration)

Glycolysis breaks down glucose for ATP without oxygen, creating lactic acid.
ATP is produced quickly.

Muscle Long-Term Energy Needs (Aerobic Respiration)

Respiratory and cardiovascular systems must "catch up" to provide oxygen for aerobic respiration to generate ATP, once the initial 40 seconds are over.
Oxygen consumption increases and then levels off to a steady state.
ATP production keeps pace with demand.

Rigor Mortis

Stiffening of the body.
Results from increased cytosolic calcium upon death. This causes deterioration in the sarcoplasmic reticulum releasing calcium and activating myosin-actin cross-bridges.
Muscle relaxation requires ATP, which is no longer produced after death.

Isometric & Isotonic Contractions

Isometric: Develops tension but does not change length.
Isotonic: Tension develops while shortening or lengthening.
- Concentric: Muscle shortens while generating tension.
- Eccentric: Muscle lengthens while generating tension.

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