2-5 Intro to Muscle

Muscle Types

• Skeletal Muscle

  • Voluntary movement

  • Striated appearance

  • Multinucleate cells

• Cardiac Muscle

  • Involuntary movement

  • Striated appearance

  • Branched and uninucleate with intercalated disks

• Smooth Muscle

  • Involuntary movement

  • Non-striated appearance

  • Found in hollow organs like the digestive tract

  • Single-unit smooth muscle: characterized by gap junctions and autonomic innervation

Neuron Classification

• Functional Classification of Neurons

  • Based on direction of impulse conduction

  • Sensory Neurons: carry impulses from sensory receptors to CNS (Central Nervous System)

  • Motor Neurons: convey impulses from CNS to target organs (muscles or glands)

  • Muscles are a key target for motor neurons

Motor Neuron Pathways

• Upper Motor Neuron:

  • Located in the brain and brain stem

• Lower Motor Neuron:

  • Located in spinal cord and peripheral nervous system

• Somatic Motor Neuron Pathway:

  • Involves neurotransmitters like Glutamate and Acetylcholine

  • Interaction with Nicotinic Acetylcholine Receptors

  • Reuptake managed by Acetylcholinesterase

Muscle Terminology

• General Terms

  • Sarco-: prefix relating to muscle

  • Muscle Cell: also called muscle fiber or myofiber

  • Muscle Equivalent Components:

    • Cell membrane → Sarcolemma

    • Cytoplasm → Sarcoplasm

    • Modified endoplasmic reticulum → Sarcoplasmic reticulum

    • Myofilaments and Nucleus present in fibers

Structure of Muscle Fibers

• Skeletal Muscle Fibers:

  • Large, multinucleate, striped appearance

• Cardiac Muscle Fibers:

  • Smaller, branched, striated, single nucleus

  • Connected by intercalated disks

• Smooth Muscle Fibers:

  • Small, lack striations, involuntary

Skeletal Muscle Organization

• Organ Level:

  • Composed of multiple myofibers bound in fasciculi

  • Tendon: connects muscle to bone

  • Includes nerve and blood vessels

Skeletal Muscle Fiber Structure

• Key Components:

  • Myofibrils: contain sarcomeres (contractile units)

  • Mitochondria: ATP generation

  • Sarcoplasmic Reticulum: stores Ca2+

  • T-tubules: extensions of sarcolemma that assist in action potential conduction

Excitation-Contraction Coupling

• Steps linking action potentials to muscle contraction:

  • Neuromuscular Junction (NMJ)

  • Involvement of T-tubules and sarcoplasmic reticulum

Role of Calcium in Muscle Contraction

• Sarcoplasmic Reticulum: stores Ca2+ at rest

• Upon action potential (AP) stimulation:

  • Ca2+ diffuses through calcium release channels

• After contraction:

  • Ca2+ is actively pumped back into sarcoplasmic reticulum via SERCA ATPase pump

Sarcomere Structure

• Contractile Units

  • I Bands: only thin filaments (Actin)

  • A Bands: thick (Myosin) and thin filament overlap

  • H Bands: center of A band with no thin filament overlap

  • Z Discs: center of each I band

Mechanism of Contraction

• Sliding Filament Mechanism:

  • Sarcomeres shorten as thin filaments are pulled inward

  • A Bands stay the same length, while I Bands shorten

  • Thin filaments slide over thick filaments

  • H Band: shortens or disappears as contraction occurs