Human Anatomy Muscles

  • Three types of Muscle Tissues, Smooth, Cardiac & Skeletal

  • Cardiac muscle tissue is found only in heart

  • Smooth muscle tissue is found in walls of hollow organs

  • All muscles share four main characteristics:

    • Excitability: ability to receive and respond to stimuli

    • Contractility: ability to shorten forcibly when stimulated

    • Extensibility: ability to be stretched 

    • Elasticity: ability to recoil to resting length

  • Four important muscle functions:

    • Produce movement

    • Maintain posture and body position 

    • Stabilize joints

    • Generate heat as they contract

  • Muscles are made up of different tissues with three features: 

    • Nerve and blood supply

    • Attachments

    • Connective tissue sheaths

  • Muscles attach to bone in two places:

    • Insertion: attachment to movable bone

    • Origin: attachment to immovable bone

  • Muscles → Fascicles → Fibers → Fibrils → Actin & Myosin

  • Sarcolemma: Muscle fiber plasma membrane

  • Sarcoplasm: Muscle fiber cytoplasm

  • Myofibrils: Actin & Myosin myofilaments

  • Sarcoplasmic Reticulum: Endoplasmic reticulum

  • T - Tubules: Tube formed by protrusion of sarcolemma deep into cell interior

  • Myofibrils are densely packed

  • Striations: stripes formed from repeating series of dark and light bands 

    • A bands: dark regions

    • H zone: lighter region in middle of dark A bands

    • M line: line of protein that bisects H zone vertically 

    • I bands: lighter regions 

    • Z disc (line): coin-shaped sheet of proteins on midline of light I band

  • Myosin: Thick Filament {connected at M line}

  • Actin: Thin Filament {anchored to Z disc}

  • Sarcomere: Unit of function

  • Contraction: The activation of cross bridges to generate force 

  • Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening 

  • Contraction ends when cross bridges become inactive

  • Four steps must occur for skeletal muscle to contract: 

    •  Nerve stimulation

    •  Action potential, an electrical current

    •  Action potential must be propagated along sarcolemma

    •  Intracellular Ca²⁺ levels must rise briefly

  • Steps 1 and 2 occur at neuromuscular junction

  • Steps 3 and 4 link electrical signals to contraction, so referred to as excitation-contraction coupling

  • Ions carry an electrical charge

    • Cytoplasm and extracellular fluids near the plasma membrane contain electrically unequal distributions of ions

  • This difference in charge creates an electrical potential across the plasma  membrane, known as a membrane potential

  • Membrane potentials are measured in millivolts 

  • Most neurons are separated from one another by tiny spaces called synapses 

  • Neurotransmitters: Chemical messengers that transmit information from one neuron to another, muscle, or gland

  • The membranes of axons and the cells they communicate with are separated by a tiny space known as the synaptic cleft

  • The ends of axons contain synaptic vesicles that store neurotransmitters

  • At a synapse, the neuron that contains the synaptic vesicles is called the presynaptic neuron

  • The cell on the other side of the synaptic cleft is called the postsynaptic cell

  • Axon terminal and muscle fiber are separated by gel-filled space called synaptic cleft

  • Stored within axon terminals are membrane-bound synaptic vesicles

  • Synaptic vesicles contain neurotransmitter acetylcholine 

  • NMJ consists of axon terminals, synaptic cleft, and junctional folds

  • Nerve impulse arrives at axon terminal, causing ACh to be released into synaptic cleft

  • ACh is quickly broken down by enzyme acetylcholinesterase, which stops 

  • Contractions

  • Acetylcholine