Muscular System: Skeletal, Smooth, and Cardiac Muscle Physiology

Skeletal Muscle

Skeletal muscles attach to bones, enable body movements.

Muscle Fibers

Threadlike, multinucleated, variable length/diameter.

Striated Muscle

Striated due to cross-stripes.

Voluntary Muscle

Controlled by nerves.

Endomysium

Covers skeletal muscle fibers and insulates them electrically.

Perimysium

Bundles muscle fibers into fascicles.

Epimysium

Covers fascicles and blood vessels, connects with tendons or aponeuroses.

Tendons/Aponeuroses

Connect muscle to bone, cartilage, or connective tissue.

Skeletal Muscle Functions

Movement, posture stabilization, and heat production during contraction.

Smooth Muscle

Small, spindle-shaped, non-striated, single nucleus, involuntary control.

Cardiac Muscle

Walls of the heart, branched, striated, single nucleus, involuntary control.

Intercalated Discs

Connect cardiac muscle cells.

Pacemaker Cells

Cells in the heart that initiate contraction.

Myofibrils

Long cylindrical structures in muscle fibers, composed of myofilaments.

Sarcolemma

Plasma membrane of skeletal muscle fiber.

Sarcomere

Basic unit of muscle contraction.

Actin

Thin filament protein in muscle contraction.

Myosin

Thick filament protein in muscle contraction.

ATPase Site

Site on myosin heads that breaks down ATP.

Sarcoplasmic Reticulum

Network around each myofibril that stores calcium ions.

Calcium Release

Triggers muscle contraction.

Extensibility

Ability to be stretched.

Elasticity

Ability to return to normal length.

Irritability

Ability to respond to stimulus.

Contractility

Ability to contract or shorten.

Neuromuscular system

Muscular and nervous systems.

Motor neuron

Nerve stimulating skeletal muscle.

Motor unit

Motor neuron and all its muscle fibers.

Neuromuscular junction

Synapse between motor neuron and muscle fiber.

Axon terminal

Releases acetylcholine.

Acetylcholine

Binds to muscle fiber receptors, exciting the fiber.

Synaptic cleft

Gap between neuron and muscle fiber membranes.

Calcium channels

Open, releasing calcium ions.

Depolarization

Positive charge inside fiber due to sodium ion influx.

Action potential

Created by sodium ion influx, causes muscle contraction.

Sodium-potassium pumps

Transport sodium out, potassium into muscle fibers during relaxation.

Calcium ions

Bind to thin filaments, exposing myosin-binding sites.

Cross bridges

Form between myosin heads and actin filaments.

All-or-none principle

Applies to muscle fibers, not whole muscle.

Graded responses

Whole muscle shows variable shortening and tension.

Muscle twitches

Brief force development and relaxation.

Summation

Results in greater tension development from rapid stimuli.

Fused or complete tetanus

Results from very high stimulation frequency, smooth, sustained contractions.

Unfused or incomplete tetanus

Results from higher stimulation frequencies, stronger, smoother contractions.

Muscle activity

Rapid frequency of stimulation without complete relaxation.

Tetanus

Allows for smooth and prolonged contractions.

Main force production factor

Number of stimulated fibers.

Few fibers stimulated

Results in small force.

All motor units active

Results in maximum force.

Muscle force variation

Varies with the required work.

ATP breakdown

Muscle contraction depends on ATP breakdown for energy.

ATP for myosin heads

ATP is needed for myosin heads to release and re-cock.

Muscle ATP storage

Muscles store enough ATP for a few seconds of contraction.

Direct energy source

ATP is the only direct energy source for muscle contraction.

Regeneration of ATP

Muscle cells must regenerate ATP continuously during contraction.

Metabolic pathways for ATP

Muscles use three metabolic pathways to produce ATP.

Creatine phosphate

High-energy molecule in muscle fibers that donates phosphate to ADP to regenerate ATP rapidly.

Aerobic respiration

Generates most ATP at rest and during light to moderate exercise.

Glucose and fat breakdown

Broken down with oxygen to produce ATP, carbon dioxide, and water.

Aerobic respiration characteristics

Is slow and requires a constant supply of oxygen and nutrients.

Mitochondria function

Use fatty acids, blood glucose, glycogen, and amino acids to produce ATP.

Process of aerobic respiration

Supports longer muscle contractions during endurance exercise.

Glucose breakdown

Produces pyruvic acid and a little ATP.

Pyruvic acid in respiration

Used in aerobic respiration if oxygen is present.

Lactic acid formation

Occurs when pyruvic acid converts to lactic acid during intense activity when oxygen is limited.

Anaerobic glycolysis

The process by which pyruvic acid converts to lactic acid.

Skeletal muscles

Attached to bones and primarily facilitate body movements.

Muscle fiber

An individual skeletal muscle cell, elongated shape, containing multiple nuclei.

Smooth muscle cells

Small, spindle-shaped, non-striated, have a single nucleus, and operate under involuntary control.

Contraction mechanism

Involves interaction between actin and myosin proteins within the sarcomeres of myofibrils.

Role of ATP and calcium ions

Facilitates contraction and is regulated by calcium ions from the sarcoplasmic reticulum.

Nervous stimulation response

Involves acetylcholine at the neuromuscular junction influencing muscle contraction and relaxation.

Muscle force production regulation

Regulated by the frequency and number of active fibers stimulated by the nervous system.

Energy sources for muscle contraction

Sourced from ATP, generated through creatine phosphate, aerobic respiration, and anaerobic glycolysis.