Chapter 39: Motor Mechanisms and Muscle Contraction

This set covers muscle anatomy, the biochemical mechanisms of contraction, neurological control, fiber types, and the different types of skeletal systems.

Thin filaments

Protein structures consisting of two strands of actin coiled around one another.

Thick filaments

Protein structures composed of staggered arrays of myosin molecules.

Myofibrils

Elongated contractile threads found in striated muscle cells that contain bundles of thick and thin filaments.

Sarcomeres

The basic contractile units of myofibrils, composed of repeating sections.

Z lines

Structures located at the ends of sarcomeres where thin filaments are attached.

M lines

Structures located in the middle of the sarcomere where thick filaments are anchored.

Sliding-filament model

A model of muscle contraction where filaments slide past each other longitudinally, increasing overlap and shortening the muscle while the filaments themselves remain the same length.

Tropomyosin

A regulatory protein that, along with the troponin complex, binds to actin strands on thin filaments to prevent actin and myosin interaction when the muscle is at rest.

Troponin complex

A set of regulatory proteins on thin filaments that bind $Ca^{2+}$ to expose myosin-binding sites, allowing contraction to occur.

Acetylcholine (ACh)

The neurotransmitter released by a motor neuron that depolarizes a muscle fiber and triggers an action potential.

Transverse (T) tubules

Infoldings of the plasma membrane through which action potentials travel to reach the interior of the muscle fiber.

Sarcoplasmic reticulum (SR)

A specialized endoplasmic reticulum in muscle cells that releases $Ca^{2+}$ into the cytosol to initiate contraction.

ALS (Amyotrophic lateral sclerosis)

A disease where motor neurons in the spinal cord and brainstem degenerate, resulting in muscle fiber atrophy.

Myasthenia gravis

An autoimmune disease that reduces signal transmission by attacking acetylcholine receptors on muscle fibers.

Motor unit

A functional unit consisting of a single motor neuron and all the muscle fibers it controls.

Twitch

A muscle contraction resulting from a single action potential in a motor neuron.

Tetanus

A state of smooth, sustained contraction produced by a series of action potentials delivered at a high frequency.

Myoglobin

A protein found in oxidative muscle fibers that binds oxygen more tightly than hemoglobin does.

Oxidative fibers

Muscle fibers that rely primarily on aerobic respiration for ATP, containing many mitochondria and a rich blood supply.

Glycolytic fibers

Muscle fibers that use glycolysis as their primary ATP source and fatigue more easily than oxidative fibers.

Slow-twitch fibers

Oxidative muscle fibers that contract more slowly but are capable of sustaining longer contractions.

Fast-twitch fibers

Muscle fibers that contract rapidly for short durations; they can be either glycolytic or oxidative.

Cardiac muscle

Striated muscle found only in the heart with cells electrically connected by intercalated disks, capable of generating action potentials without neural input.

Smooth muscle

Muscle cells lacking striations found in the walls of hollow organs, characterized by slow contractions often initiated without neuron input.

Paramyosin

A protein in the muscles of invertebrates like clams that enables long-term contraction with low energy requirements.

Hydrostatic skeleton

A skeleton type consisting of fluid held under pressure in a closed body compartment, found in cnidarians, flatworms, and annelids.

Peristalsis

A type of movement on land produced by rhythmic waves of muscle contractions, commonly used by annelids.

Exoskeleton

A hard encasement deposited on the surface of an animal, such as the shells of molluscs or the cuticles of arthropods.

Chitin

A polysaccharide that makes up approximately $30-50\%$ of the arthropod cuticle.

Endoskeleton

A hard internal skeleton buried in soft tissue, found in organisms ranging from sponges to mammals.

Ligaments

Connective tissues that join bones at joints to allow for freedom of movement.