BIOS110 Exam 2 Review Guide

Muscle Structure and the Sliding Filament Mechanism

Sliding Filament Theory

The sliding filament mechanism explains how muscles contract at the molecular level, primarily through the interaction of actin and myosin filaments within the sarcomere.

• Thick Filaments (Myosin): Composed of myosin molecules, these filaments have a long shaft and rounded heads that bind to actin.

• Thin Filaments (Actin): Structurally linked to the Z-line, actin filaments slide past myosin filaments during contraction.

• Cross-Bridge Formation: Myosin heads attach to exposed binding sites on actin, forming cross-bridges, which is essential for muscle contraction.

• Sliding Filament Mechanism: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere, shortening the muscle.

• Muscle Contraction: Requires ATP for myosin head movement and cross-bridge cycling.

Action Potential and Muscle Contraction

Initiation of Action Potential

An action potential in a motor neuron is initiated when incoming signals from other neurons produce graded potentials. These are small, short-lived changes in the neuron's membrane potential.

• Graded Potentials: Short-lived changes that slightly alter the resting potential of the neuron (about -70 mV).

• Threshold: If graded potentials reach a specific membrane voltage, an action potential is triggered.

• Voltage-Sensitive Channels: When threshold is reached, sodium channels open, allowing sodium ions to rush into the neuron, causing depolarization.

Action Potential Sequence

• Depolarization: Sodium channels open, neuron becomes more positive.

• Repolarization: Potassium channels open, potassium ions exit, restoring negative charge.

• Resting Potential Reestablishment: Sodium-potassium pump restores resting potential.

This process is all-or-none; if the action potential occurs, it propagates along the neuron to transmit information.

Electrical Impulses in Muscle Units

Process of Generating Electrical Impulses

Electrical impulses within a muscle unit involve several steps:

1. Motor Neuron Activation: An action potential travels down the motor neuron to the neuromuscular junction.

2. Neurotransmitter Release: Acetylcholine is released into the synaptic gap.

3. Muscle Cell Membrane Activation: Acetylcholine binds to receptors, initiating an electrical impulse in the muscle cell.

4. T Tubules: Extensions of the muscle cell membrane that transmit the impulse deep into the muscle cell.

5. Sarcoplasmic Reticulum: Electrical impulse triggers release of calcium ions, facilitating contraction.

Substructures of a Sarcomere and Their Functions

Sarcomere Structure

The sarcomere is the smallest contractile unit of a muscle myofibril, extending from one Z-line to the next. It contains two main types of protein filaments:

• Thick Filaments (Myosin): Located near the middle of the sarcomere, responsible for muscle contraction by interacting with actin filaments.

• Thin Filaments (Actin): Structurally linked to the Z-line, slide past myosin filaments during contraction.

The interaction between myosin and actin filaments is the basis of the sliding filament mechanism.

Muscle Heat Generation and Thermoregulation

Role of Muscles in Body Temperature

Muscles generate most of the body heat through contraction. This process is vital for maintaining posture and regulating body temperature.

• Thermoregulation: Muscle activity helps maintain a stable internal temperature, especially in cold conditions.

• Normal Body Temperature: Remains around 98.6°F (37°C) despite external temperature changes.

Major Muscle Groups

Upper Body Muscles

• Pectoral Muscles: Located in the chest, responsible for movements of the shoulder and arm.

• Deltoids: Located in the shoulders, allow for arm rotation and lifting.

• Biceps and Triceps: Located in the upper arm, responsible for flexion and extension.

Core Muscles

• Abdominals: Support the trunk, allow movement, and hold organs in place.

• Obliques: Located on the sides of the abdomen, assist in twisting and torso movement.

Lower Body Muscles

• Quadriceps and Hamstrings: Located in the thighs, crucial for walking, running, and jumping.

• Calves: Located in the lower leg, help in movements like standing on tiptoe.

These muscle groups work together to produce coordinated movements and maintain posture.

Events Leading to Muscle Contraction

Sequence of Muscle Contraction

• Nerve Impulse Initiation: Nerve impulse travels to the muscle cell, triggering calcium release from the sarcoplasmic reticulum.

• Latent Period: Delay as calcium ions are released and bind to troponin, allowing myosin heads to attach to actin filaments.

• Contraction Phase: Myosin heads pull actin filaments toward the center of the sarcomere, shortening the muscle cell and causing contraction.

• Relaxation Phase: Calcium ions are transported back into the sarcoplasmic reticulum, and the muscle cell returns to its resting state.

Summation and Tetanus

• Summation: Repeated stimulation before relaxation increases contraction strength.

• Tetanus: Continuous stimulation without relaxation leads to maximum contraction.

Cell Structures Involved in Muscle Contraction

Sarcoplasmic Reticulum (SR)

• Definition: Specialized form of smooth endoplasmic reticulum in muscle cells, stores calcium ions ($Ca^{2+}$).

• Function: Releases calcium ions during muscle contraction, reabsorbs them during relaxation.

Muscle Cavities and Their Functions

Major Muscle Cavities

• Diaphragm: Dome-shaped skeletal muscle separating the thoracic and abdominal cavities, primary muscle for breathing.

• Intercostal Muscles: Located between the ribs, assist in expanding and contracting the chest cavity during breathing.

• Pelvic Floor Muscles: Support organs in the pelvic cavity, help maintain continence.

Types of Muscle Contractions

Isotonic vs. Isometric Contractions

• Isotonic Contractions: Muscle shortens while maintaining a constant force. Example: lifting a book.

• Isometric Contractions: Muscle length remains the same while force is generated. Example: holding a book steady without moving it.

Key Properties

• Constant Force: In isotonic contractions, force remains constant as muscle changes length.

• Movement: Isotonic contractions produce movement; isometric contractions do not.

Summary Table: Key Structures and Functions in Muscle Contraction

Structure	Function
Sarcomere	Smallest contractile unit; contains actin and myosin filaments
Myosin (Thick Filament)	Forms cross-bridges with actin; responsible for contraction
Actin (Thin Filament)	Slides past myosin during contraction
Sarcoplasmic Reticulum	Stores and releases calcium ions ($Ca^{2+}$)
T Tubules	Transmit electrical impulses into muscle cell
Motor Neuron	Initiates action potential for muscle contraction
Acetylcholine	Neurotransmitter that activates muscle cell membrane

Key Equations

• Resting Membrane Potential: $V_{rest} \approx -70\,\text{mV}$

• Action Potential Threshold: $V_{threshold} \approx -55\,\text{mV}$

Additional info:

• Muscle contraction and relaxation are tightly regulated by the nervous system and require precise coordination of electrical and chemical signals.

• ATP is essential for both contraction and relaxation phases.