Vet-Physio_Muscles

MUSCULAR SYSTEM

• Professor Josephine R. Flores

LEARNING OBJECTIVES

• At the end of the topics, students should be able to:

  • Classify muscles and describe the functions of different muscles.

  • Explain the excitation-contraction coupling mechanism.

  • Describe the mechanisms of contraction of skeletal and smooth muscles.

TYPES OF MUSCLES

• Skeletal: Comprises 40% of the body.

• Cardiac: Found in the heart.

• Smooth: Involuntary muscles found in organs.

SKELETAL MUSCLE

• Components: Includes muscle fibers, myofibrils, and sarcomeres.

  • Myofibrils: Composed of actin and myosin contractile proteins.

  • Sarcomere: The basic unit of muscle contraction defined by the pattern of H and Z bands seen under a microscope.

FUNCTIONS OF MUSCLES

• Body Movements:

  • Move joints (locomotion).

  • Changes in posture and facial expressions.

  • Relocation of materials within the body.

• Signals: Help sensors in joints, tendons, and muscles to maintain joint position.

• Stabilization: Skeletal muscles stabilize joints,** minimizing dislocations.

• Heat Production: Important for maintaining body temperature through processes like shivering.

CONTRACTION OF SKELETAL MUSCLE

• Contraction is the process by which muscle fibers shorten.

• Involves flexor muscles at joints to produce movement.

  • Decrease in flexor angle at one joint and increase at another (e.g., stifle and tarsal joints).

PROPERTIES OF MUSCLES

• Excitability (Irritability): Muscle cells maintain membrane potential and respond to stimuli.

• Contractility: Muscle cells contract as electrical impulses spread across the fiber.

• Extensibility: Muscles can lengthen in response to stretch, particularly smooth muscles.

• Elasticity: Muscles can recoil to their original length after being stretched due to elastic elements within.

MYOFIBRILS

• Composed of bundles of myofilaments (actin and myosin) packed tightly into muscle fibers.

• Myofibrils are attached to the inner surface of the sarcolemma.

• Cross-bridges: Myosin heads form bridges with adjacent actin filaments, enabling contraction.

TROPONIN COMPLEX

• Components:

  • TnT: Binds to tropomyosin.

  • Tnl: Binds to G actin.

  • TnC: Binds to calcium ions.

PROTEINS OF MYOFIBRILS

• Contractile Proteins: Actin and myosin generate force during contraction.

• Regulatory Proteins: Tropomyosin and troponin help to initiate and terminate contraction.

• Structural Proteins: Provide support and maintain alignment of filaments (includes titin and dystrophin).

SARCOPLASMIC RETICULUM

• Forms a tubular network surrounding each myofibril.

• Triad: The arrangement of terminal cisternae and T tubules facilitates excitation-contraction coupling.

SKELETAL MUSCLE CONTRACTION MECHANISM

• Involves voluntary stimulation of motor neurons.

• Release of neurotransmitter from motor neurons initiates excitation-contraction coupling.

• An action potential generated in the muscle fiber leads to calcium release from the sarcoplasmic reticulum, triggering contraction.

NEUROMUSCULAR JUNCTION

• The contact point where a motor neuron communicates with a muscle fiber.

• Involves the release of acetylcholine (ACh) which initiates muscle contraction.

EXCITATION-CONTRACTION COUPLING MECHANISM

• Involves action potential propagation along the sarcolemma and T tubules, leading to calcium release from the sarcoplasmic reticulum.

• Increase in cytosolic calcium ultimately promotes muscle contraction through the binding of calcium to troponin.

SLIDING FILAMENT THEORY

• Thin filaments slide between thick filaments as myosin attaches to actin.

• Z lines move closer together as the muscle contracts, shortening the sarcomere and myofibril.

LENGTH-TENSION RELATIONSHIPS

• The force a muscle can produce depends on its length prior to contraction.

• A sarcomere length of 2.0 - 2.4µm allows for optimal overlap between actin and myosin, maximizing tension.

• If too stretched or compressed, the ability to generate tension diminishes.

MUSCLE TWITCHES

• A single neuronal stimulation results in a muscle twitch, which encompasses a contraction and relaxation phase.

• Prolonged stimulation increases force and tension beyond initial twitch.

• Myogram measures individual muscle twitches, typically lasting from 20 to 200 milliseconds.

TREPPE

• A phenomenon where successive muscle twitches result in a steplike increase in tension due to increased calcium availability between twitches.

MOTOR UNIT OF SKELETAL MUSCLE

• Comprises a motor neuron and all muscle fibers it innervates, functioning together in muscle contractions.

SUMMATION

• Muscle contractions can be graded based on the frequency and strength of stimulation.

• Increased frequency leads to tetanization, with incomplete and complete forms observable.

MUSCLE TONE

• The degree of tension maintained by skeletal muscles, managed by the CNS through motor neuron activity.

• Helps stabilize joints and maintain posture.

CHARACTERISTICS OF MUSCLE CONTRACTIONS

• Concentric: Muscle shortens under tension to overcome resistance.

• Eccentric: Muscle lengthens while developing tension.

• Isometric: Muscle tension increases without changing length, sustained in postural muscles.

TYPES OF MUSCLE FIBERS

• Classified by speed (fast or slow) and metabolic pathways (oxidative or glycolytic).

• Different fiber types are identified by their myoglobin content: red (high) and white (low).

TYPES OF SMOOTH MUSCLES

• Single-unit (Visceral): Cells connected via gap junctions, functioning as a single unit (e.g., walls of digestive tract).

• Multiunit: Individual cells lack gap junctions and require separate innervation (e.g., iris of the eye).

SMOOTH MUSCLE CONTRACTION

• Calcium ions trigger contraction through calmodulin, unlike skeletal muscle which depends on troponin.

• Activation of myosin light-chain kinase occurs, leading to cross-bridge formation with actin.

LATCH MECHANISM

• Prolonged tonic contraction of smooth muscle utilizes less energy than skeletal muscle, allowing sustained contraction with minimal excitatory signals.