Muscle Types and Functions in Human Anatomy

Skeletal Muscle

Striated, voluntary muscle attached to bones, responsible for body movement.

Cardiac Muscle

Striated, involuntary muscle found only in the heart, responsible for pumping blood.

Smooth Muscle

Non-striated, involuntary muscle found in the walls of hollow organs, responsible for various internal movements.

Excitability

Ability to receive and respond to stimuli.

Contractility

Ability to shorten forcibly when stimulated.

Extensibility

Ability to be stretched or extended.

Elasticity

Ability to recoil to resting length after stretching.

Movement

Enables body locomotion and manipulation of objects.

Maintain posture/body position

Stabilizes the body against gravity.

Stabilize joints

Helps to hold bones together and prevent excessive movement.

Generate heat

Muscle contraction produces heat, contributing to body temperature homeostasis.

Sarcolemma

Plasma membrane of a muscle fiber.

Sarcoplasm

Cytoplasm of a muscle fiber.

Myofibrils

Densely packed, rod-like contractile elements that run the length of the muscle fiber.

Sarcoplasmic Reticulum (SR)

Smooth endoplasmic reticulum surrounding each myofibril; stores and releases calcium ions.

T Tubules (Transverse Tubules)

Inward extensions of the sarcolemma that penetrate into the muscle fiber.

Triad

Grouping of two terminal cisterns of the SR and a T tubule between them.

Sarcomeres

Smallest contractile unit of a muscle fiber, composed of myofilaments made of contractile proteins.

Striations

Repeating series of dark (A bands) and light (I bands) visible along the length of each myofibril.

H Zone

Lighter region in the midsection of the dark A band where filaments do not overlap.

M Line

Dark line in the center of the H zone formed by protein molecules that hold adjacent thick filaments together.

Z Disc (Z Line)

Coin-shaped sheet of proteins on the midline of the light I band that anchors the thin filaments and connects myofibrils to one another.

A Band

Dark region of a sarcomere that corresponds to the length of the thick filaments.

I Band

Light region of a sarcomere that contains only thin filaments.

Thick Filaments

Composed primarily of the protein myosin.

Myosin

Protein that makes up the thick filaments; has a rod-like tail and globular heads that form cross-bridges.

Thin Filaments

Composed primarily of the protein actin.

Actin

Protein that forms the core of the thin filaments; has binding sites for myosin heads.

Troponin

Regulatory protein associated with actin that binds to calcium ions.

Tropomyosin

Regulatory protein that spirals around the actin core and blocks myosin-binding sites.

Sliding Filament Model of Contraction

During contraction, thin filaments slide past thick filaments, causing the sarcomere to shorten.

Z Discs Toward M Line

As thin filaments slide, the Z discs are pulled closer to the M line.

Cross Bridges

Form when myosin heads attach to actin filaments.

Ratchet Motion

The myosin heads pivot and pull the thin filaments toward the center of the sarcomere, then detach and reattach further along the thin filament.

Neuromuscular Junction (NMJ)

Synapse between an axon terminal of a motor neuron and the sarcolemma of a muscle fiber.

Axon Terminal

End of a motor neuron axon that contains synaptic vesicles filled with neurotransmitter.

Acetylcholine (ACh)

Neurotransmitter released at the neuromuscular junction.

Acetylcholine Receptors

Proteins on the sarcolemma that bind to acetylcholine.

Synaptic Cleft

Space between the axon terminal and the sarcolemma.

Acetylcholinesterase

Enzyme located in the synaptic cleft that breaks down acetylcholine.

End Plate Potential

Local depolarization of the sarcolemma at the neuromuscular junction.

Threshold

Minimum level of depolarization required to generate an action potential.

Depolarization

Loss of the inside negativity of the plasma membrane.

Repolarization

Reestablishment of the resting membrane potential.

Refractory Period

Period of time following an action potential during which a muscle fiber cannot be stimulated again.

Latent Period

Brief delay between the stimulus and the beginning of contraction.

Role of Calcium in Contraction

Calcium ions bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

Cross Bridge Cycle

Sequence of events involving the attachment, pivoting, detachment, and resetting of myosin heads, leading to muscle contraction.

Rigor Mortis

Stiffening of muscles after death due to the lack of ATP, preventing myosin from detaching from actin.

Tension

Force exerted by a contracting muscle on an object.

Muscle Tone

Low level of contractile activity in relaxed muscle that keeps the muscle healthy and ready to act.

Direct Phosphorylation of ADP by Creatine Phosphate (CP)

Fastest way to regenerate ATP; CP donates a phosphate group to ADP.

Anaerobic Pathway (Glycolysis)

ATP generation that does not require oxygen; glucose is broken down to pyruvic acid, producing a small amount of ATP and lactic acid.

Aerobic Pathway

ATP generation that requires oxygen; glucose, pyruvic acid, fatty acids, and amino acids are broken down in the mitochondria, producing a large amount of ATP.

Slow Oxidative Fibers

Muscle fibers that contract slowly, rely on aerobic respiration, and are fatigue-resistant (high myoglobin, many mitochondria, rich capillary supply).

Fast Oxidative Fibers

Muscle fibers that contract quickly, rely on aerobic respiration, and have intermediate fatigue resistance (high myoglobin, many mitochondria, rich capillary supply).

Fast Glycolytic Fibers

Muscle fibers that contract quickly, rely on anaerobic glycolysis, and fatigue quickly (low myoglobin, few mitochondria, few capillaries).

Aerobic Exercise

Activities that increase endurance and promote changes in muscle fibers such as increased capillaries, mitochondria, and myoglobin synthesis.

Resistance Exercise

Activities that increase muscle strength and size by causing hypertrophy of muscle fibers.

Overload Principle

To improve muscle performance, muscles must be subjected to loads greater than those normally encountered.

Hypertrophy

Increase in muscle fiber size due to increased synthesis of contractile proteins.

Atrophy

Decrease in muscle fiber size due to disuse or denervation.

Pull, Never Push

Muscles can only contract and pull; they never push.

Prime Mover (Agonist)

Muscle that has the major responsibility for producing a specific movement.

Antagonist

Muscle that opposes or reverses a particular movement of the prime mover.

Synergist

Muscle that helps the prime mover by adding extra force to the same movement or by reducing undesirable or unnecessary movements.

Fixator

Synergist that immobilizes a bone or a muscle's origin so that the prime mover can act more efficiently.

Functional Groups

Muscles that work together to produce or control a movement.

Lever

Rigid bar (bone) that moves on a fixed point when a force is applied to it.

Fulcrum

Fixed point (joint) on which a lever moves.

Effort

Force (muscle contraction) applied to the lever.

Load

Resistance (bone + tissues + any added weight) moved by the effort.

First Class Lever

Fulcrum is located between the effort and the load (e.g., seesaw, nodding the head).

Second Class Lever

Load is located between the fulcrum and the effort (e.g., wheelbarrow, standing on tiptoes).

Third Class Lever

Effort is applied between the fulcrum and the load (e.g., tweezers, flexing the forearm).