antomy lecture exam 3

Sliding Filament Mechanism

Process where thick (myosin) and thin (actin) filaments slide past one another, shortening the sarcomere and causing muscle contraction.

Myosin

Motor protein that forms thick filaments; its heads bind actin, hydrolyze ATP, and generate the power stroke.

Actin

Protein that forms thin filaments; contains binding sites for myosin heads.

Troponin

Regulatory protein that binds Ca²⁺, changes shape, and moves tropomyosin off actin’s binding sites.

Tropomyosin

Rod-shaped protein that covers actin’s myosin-binding sites when the muscle is at rest.

Sarcomere

Functional contractile unit of muscle fiber, bordered by Z discs and containing overlapping thick and thin filaments.

Z Disc

Protein boundary of a sarcomere where thin filaments anchor.

M Line

Central line in a sarcomere that stabilizes thick filaments.

Zone of Overlap

Region where actin and myosin filaments overlap; increases during contraction.

Myosin Head

Pivoting portion of myosin that binds actin, hydrolyzes ATP, and performs the power stroke.

Cross-Bridge

Connection formed when a myosin head binds to an actin binding site.

Power Stroke

Pivot of the myosin head that pulls actin toward the M line.

ATP (Adenosine Triphosphate)

Energy molecule that resets myosin heads and enables their release from actin.

ADP + Pi

Products of ATP hydrolysis that remain bound to myosin until the power stroke occurs.

Hydrolysis (of ATP)

Chemical splitting of ATP into ADP and inorganic phosphate, releasing energy for myosin head cocking.

Sarcoplasmic Reticulum (SR)

Specialized endoplasmic reticulum in muscle fibers that stores and releases Ca²⁺.

Terminal Cisternae

Dilated end sacs of the SR that release Ca²⁺ upon stimulation.

Transverse (T) Tubules

Invaginations of the sarcolemma that conduct action potentials into the cell interior.

Sarcolemma

Plasma membrane of a muscle fiber.

Cytosol

Intracellular fluid where Ca²⁺ binds troponin during contraction.

Calcium Ions (Ca²⁺)

Ions released from SR that bind troponin and initiate contraction.

Sodium Ions (Na⁺)

Ions that enter the muscle fiber via ligand-gated channels, generating the muscle action potential.

Neuromuscular Junction (NMJ)

Synapse between a motor neuron and a muscle fiber that initiates muscle action potentials.

Synaptic Bulb (End Bulb)

Swollen axon terminal containing synaptic vesicles filled with neurotransmitter.

Synaptic Vesicle

Membrane-bound sac in the synaptic bulb that stores neurotransmitter (acetylcholine).

Synaptic Cleft

Narrow space between synaptic bulb and muscle fiber where neurotransmitter diffuses.

Acetylcholine (ACh)

Neurotransmitter released at the NMJ that binds receptors on the motor end plate.

Acetylcholinesterase (AChE)

Enzyme in the synaptic cleft that rapidly breaks down acetylcholine, ending stimulation.

Motor End Plate

Specialized region of sarcolemma with ACh receptors and ligand-gated Na⁺ channels.

Action Potential

Rapid electrical signal traveling along neuron or sarcolemma, triggering Ca²⁺ release.

Voltage-Gated Calcium Channel

Channel in synaptic bulb that opens in response to an action potential, allowing Ca²⁺ entry.

Ligand-Gated Sodium Channel

Sarcolemma channel that opens when ACh binds, permitting Na⁺ influx.

Depolarization

Membrane potential change caused by Na⁺ influx, initiating the muscle action potential.

Calcium ATPase Pump

SR membrane pump that uses ATP to return Ca²⁺ to the SR, leading to relaxation.

Length-Tension Relationship

Correlation between sarcomere length at rest and the force it can generate during contraction.

Sarcomere

Smallest contractile unit of a myofibril; spans from one Z line to the next.

Z line (Z disc)

Boundary of a sarcomere; anchoring point for thin filaments and appears zig-zagged.

A band

Dark region of a sarcomere containing the entire length of thick filaments (myosin).

I band

Light region containing only thin filaments on either side of the Z line.

H zone

Central part of the A band where thick filaments have no thin-filament overlap.

M line

Midline of the sarcomere; proteins that hold thick filaments in place.

Zone of overlap

Area where thick and thin filaments interdigitate; increases during contraction.

Thin filament

Filament made primarily of actin plus regulatory proteins; appears red on model.

Thick filament

Filament composed of myosin molecules and their projecting heads; appears blue.

Actin

Globular protein forming two twisted chains; main component of thin filaments.

Myosin

Motor protein forming thick filaments; has projecting heads that bind actin.

Myosin head

Movable projection of myosin that attaches to actin to generate force.

Tropomyosin

Long regulatory strand that blocks myosin-binding sites on actin at rest.

Troponin

Three-part protein that controls tropomyosin position and regulates contraction.

Sarcoplasmic reticulum

Smooth ER of muscle fiber; stores and releases Ca²⁺ for contraction.

T tubule

Invagination of sarcolemma that brings action potentials into the fiber interior.

Myofibril

Cylindrical bundle of sarcomeres inside a muscle fiber.

Muscle fiber

Single skeletal muscle cell; wrapped in endomysium.

Fascicle

Bundle of muscle fibers surrounded by perimysium.

Epimysium

Connective tissue sheath encasing an entire skeletal muscle.

Perimysium

Connective tissue layer surrounding a fascicle.

Endomysium

Delicate connective tissue around an individual muscle fiber.

Neuromuscular junction

Point where a motor neuron communicates with a muscle fiber.

Axon

Long neuronal process transmitting impulses toward terminals.

Axon terminal

Distal portion of axon that forms synapses with a muscle fiber.

Synaptic end bulb

Bulbous enlargement at axon terminal containing synaptic vesicles.

Synapse (synaptic cleft)

Tiny space between neuron and motor end plate where neurotransmitter diffuses.

Motor end plate

Folded region of sarcolemma that houses ACh receptors at NMJ.

Sarcolemma

Plasma membrane of a muscle fiber.

Myelin sheath

Insulating wrapping around axons formed by Schwann cells.

Schwann cell

Glial cell that produces myelin around peripheral axons.

Terminal branches

Small branches of an axon leading to individual synaptic bulbs.

Synaptic vesicles

Membranous sacs storing neurotransmitter (ACh) in the synaptic bulb.

Flexion

Movement that decreases the angle between two bones.

Extension

Movement that increases the angle between two bones.

Hyperextension

Extension beyond the anatomical position.

Abduction

Movement away from the midline of the body.

Adduction

Movement toward the body’s midline.

Circumduction

Circular, cone-shaped motion combining flexion, extension, abduction, adduction.

Rotation

Bone turns around its own longitudinal axis.

Elevation

Lifting a body part superiorly (e.g., shrugging).

Depression

Moving a body part inferiorly.

Protraction

Anterior movement in a transverse plane (e.g., mandible forward).

Retraction

Posterior movement in a transverse plane.

Inversion

Turning sole of foot medially.

Eversion

Turning sole of foot laterally.

Dorsiflexion

Lifting foot so superior surface approaches shin.

Plantar flexion

Pointing foot downward, increasing angle at ankle.

Supination

Forearm rotation that turns palm anteriorly (radius & ulna parallel).

Pronation

Forearm rotation that turns palm posteriorly (radius crosses ulna).

Opposition

Thumb movement toward fingers enabling grasping.

Aponeurosis

Broad, flat tendon sheet connecting muscles to each other or bone.

Epicranial aponeurosis (Galea aponeurotica)

Tendon sheet linking frontalis and occipitalis across the skull roof.

Occipitofrontalis

Two-part scalp muscle consisting of frontalis and occipitalis bellies.

Frontalis

Anterior belly that raises eyebrows and wrinkles forehead.

Occipitalis

Posterior belly that pulls scalp posteriorly.

Temporalis

Fan-shaped muscle over temporal bone; elevates mandible for chewing.

Orbicularis oculi

Circular muscle around the eye; closes eyelids.

Nasalis

Muscle over bridge of nose; flares nostrils.

Orbicularis oris

Circular muscle around mouth; purses lips.

Masseter

Powerful jaw elevator on cheek; major muscle of mastication.

Platysma

Thin superficial neck muscle that tenses skin of neck and depresses mandible.

Sternocleidomastoid

Neck muscle from sternum & clavicle to mastoid; flexes & rotates head.

Deltoid

Triangular shoulder muscle; abducts arm.

Pectoralis major

Large chest muscle; flexes, adducts, and medially rotates arm.