Physio Exam 1 Study Guide Part 2

Muscle Structure and Function

Levels of Muscle Structure

Muscle tissue is organized into hierarchical levels, each with distinct structural and functional properties.

• Muscle (Organ): Composed of bundles of fascicles, surrounded by the epimysium.

• Fascicle: A bundle of muscle fibers (cells), surrounded by the perimysium.

• Muscle Fiber (Cell): A single multinucleated cell, surrounded by the endomysium.

• Myofibril: Cylindrical structures within muscle fibers, composed of repeating sarcomeres.

• Sarcomere: The functional contractile unit of muscle, defined by Z-lines.

Motor Units and Recruitment

A motor unit consists of a single motor neuron and all the muscle fibers it innervates. Recruitment refers to the activation of additional motor units to increase muscle force.

• Significance: Allows graded control of muscle contraction strength.

• Small motor units: Fine control (e.g., eye muscles).

• Large motor units: Gross movements (e.g., thigh muscles).

Myofibril Banding Pattern

Myofibrils display a characteristic banding pattern due to the arrangement of thick (myosin) and thin (actin) filaments.

• A band: Dark region; length of thick filaments.

• I band: Light region; contains only thin filaments.

• H zone: Central part of A band; only thick filaments.

• Z line: Defines the boundary of a sarcomere.

• Band changes during contraction: I band and H zone shorten, A band remains constant.

Cross-Bridge Cycle and Sliding Filament Theory

The cross-bridge cycle describes the interaction between actin and myosin during muscle contraction, forming the basis of the sliding filament theory.

• Steps:

  1. Myosin head binds to actin (cross-bridge formation).

  2. Power stroke: Myosin head pivots, pulling actin filament.

  3. ATP binds to myosin, causing detachment from actin.

  4. ATP hydrolysis re-cocks the myosin head.

• Sliding filament theory: Muscle contraction results from actin filaments sliding over myosin filaments, shortening the sarcomere.

Excitation-Contraction Coupling

This process links the electrical excitation of the muscle fiber to contraction.

• Action potential travels along sarcolemma and down T-tubules.

• Triggers release of Ca2+ from the sarcoplasmic reticulum.

• Ca2+ binds to troponin, exposing binding sites on actin.

• Initiates cross-bridge cycling.

Length-Tension Relationship

The force a muscle can generate depends on its length at the time of stimulation.

• Optimal length: Maximum overlap of actin and myosin allows greatest force.

• Too stretched or compressed: Fewer cross-bridges form, reducing force.

Creatine and Creatine Phosphate in Muscle Physiology

Creatine phosphate serves as a rapid source of high-energy phosphate to regenerate ATP during the initial seconds of intense muscle activity.

• Reaction: $\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

• Provides energy for short bursts of activity (e.g., sprinting).

Types of Skeletal Muscle Fibers

Skeletal muscle fibers are classified based on contraction speed and metabolic properties.

• Type I (Slow-twitch): Fatigue-resistant, oxidative metabolism, endurance activities.

• Type IIa (Fast-twitch, oxidative-glycolytic): Intermediate properties.

• Type IIb/x (Fast-twitch, glycolytic): Fatigue quickly, anaerobic metabolism, short bursts of power.

Muscle Fatigue

Muscle fatigue is the decline in ability of a muscle to generate force.

• Causes: Depletion of ATP, accumulation of lactic acid, ionic imbalances.

• Results in reduced performance and increased risk of injury.

Monosynaptic Muscle Stretch Reflexes

These reflexes help maintain muscle tone and posture. The muscle spindle is the sensory receptor involved.

• Gamma motor neurons: Adjust sensitivity of muscle spindles.

• Example: Knee-jerk (patellar) reflex.

Golgi Tendon Organs

Golgi tendon organs are proprioceptors that detect changes in muscle tension.

• Activation inhibits muscle contraction to prevent damage from excessive force.

Reciprocal Innervation of Skeletal Muscles

During movement, activation of one muscle group is accompanied by inhibition of the opposing group.

• Ensures smooth and coordinated motion.

• Example: Flexor contracts while extensor relaxes during elbow flexion.

Alpha and Gamma Motor Neurons

Alpha motor neurons innervate extrafusal muscle fibers (force generation), while gamma motor neurons innervate intrafusal fibers (muscle spindle sensitivity).

• Both are active during voluntary contraction to maintain muscle tone and proprioception.

Neural Pathways of Reflexes

Reflexes are rapid, involuntary responses to stimuli, involving specific neural circuits.

• Monosynaptic reflex: One synapse between sensory and motor neuron (e.g., stretch reflex).

• Polysynaptic reflex: Multiple synapses, involving interneurons (e.g., withdrawal reflex).

Cardiac and Smooth Muscle vs. Skeletal Muscle

Muscle tissue types differ in structure, function, and control mechanisms.

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Present	Present	Absent
Control	Voluntary	Involuntary	Involuntary
Location	Attached to bones	Heart	Walls of hollow organs
Contraction Speed	Fast	Intermediate	Slow

Smooth Muscle Structure and Regulation

Smooth muscle lacks striations and is found in the walls of hollow organs. Its contractions are regulated by autonomic nervous system, hormones, and local factors.

• Contraction involves Ca2+-calmodulin interaction, activating myosin light-chain kinase.

• Allows for slow, sustained contractions (e.g., peristalsis in intestines).