HESI A2 - Muscular System - Key Terms and Concepts

Contractile tissue

A type of tissue that has the ability to shorten and generate force, enabling movement.

Excitability

The ability of muscle tissue to respond to nerve signals or stimuli by generating an electrical impulse.

Extensibility

The capacity of muscle fibers to stretch beyond their normal resting length without being harmed.

Muscle fiber

A long, cylindrical cell that makes up skeletal muscle tissue and contains multiple nuclei.

Myofibril

A thread-like structure within muscle fibers that contains the contractile proteins needed for muscle contraction.

Sarcolemma

The cell membrane of a muscle fiber that helps regulate ion exchange and transmits action potentials.

Sarcoplasm

The cytoplasm of a muscle fiber that contains organelles, glycogen, and myoglobin.

Myoglobin

A protein in muscle cells that binds oxygen and helps store it for energy production.

T-tubule

A deep fold of the muscle cell membrane (sarcolemma) that carries electrical signals into the interior of the muscle fiber.

Sarcoplasmic reticulum

A specialized organelle in muscle fibers that stores and releases calcium to regulate muscle contraction.

Actin

A thin filament protein in the sarcomere that interacts with myosin to produce muscle contraction.

Myosin

A thick filament protein in the sarcomere that pulls actin filaments to generate muscle contraction.

Tropomyosin

A regulatory protein that covers actin’s binding sites, preventing contraction until activated.

Troponin

A regulatory protein that binds calcium and shifts tropomyosin, allowing myosin to attach to actin.

Sliding filament theory

The explanation of how muscles contract by actin and myosin filaments sliding past each other.

Neuromuscular junction

The site where a motor neuron communicates with a muscle fiber to initiate contraction.

Acetylcholine

A neurotransmitter that transmits signals from nerves to muscles to trigger contraction.

Excitation-contraction coupling

The process by which an electrical impulse leads to muscle contraction.

Cross-bridge formation

The process in which myosin heads bind to actin filaments, enabling muscle contraction.

ATP hydrolysis

The breakdown of ATP to release energy for muscle contraction and relaxation.

Isotonic contraction

A type of muscle contraction where the muscle shortens or lengthens while generating consistent tension.

Isometric contraction

A muscle contraction where the muscle generates force without changing length.

Concentric contraction

A type of isotonic contraction where the muscle shortens as it generates force.

Eccentric contraction

A type of isotonic contraction in which the muscle lengthens while resisting force.

Agonist

The primary muscle that generates a particular movement.

Antagonist

A muscle that opposes the action of the agonist to control movement.

Synergist

A muscle that helps the agonist carry out an action.

Fixator

A muscle that stabilizes a joint to prevent unwanted movement.

Flexion

A movement that decreases the angle between two body parts.

Extension

A movement that increases the angle between two body parts.

Fast-twitch fibers

Muscle fibers that contract quickly and produce high force but fatigue rapidly.

Slow-twitch fibers

Muscle fibers that contract slowly and generate less force but have greater endurance.

Intermediate fibers

Muscle fibers with characteristics of both fast-twitch and slow-twitch fibers.

Superficial muscles

Muscles located close to the skin that contribute to movement and shape.

Deep muscles

Muscles located beneath superficial muscles that provide stability and support.

Facial muscles

A group of muscles responsible for expressions, chewing, and movement of the face.

Rotator cuff

A group of muscles and tendons that stabilize the shoulder joint and enable rotation.

Hamstrings

A group of muscles on the back of the thigh responsible for knee flexion and hip extension.

Quadriceps

A group of muscles on the front of the thigh responsible for knee extension.

Calf muscles

Muscles located in the lower leg that assist in walking, running, and jumping.

Muscle hypertrophy

The increase in muscle size due to strength training and resistance exercises.

Muscle atrophy

The decrease in muscle size and strength due to disuse, injury, or disease.

Tendonitis

Inflammation of a tendon caused by repetitive stress or overuse.

Muscle strain

A stretching or tearing of muscle fibers due to excessive force or overstretching.

Cramps

Sudden, involuntary muscle contractions that cause pain and stiffness.

Muscular dystrophy

An inherited condition marked by the gradual weakening and breakdown of skeletal muscles.

Rhabdomyolysis

A condition in which muscle fibers break down and release harmful substances into the bloodstream.

Fibromyalgia

A chronic disorder characterized by widespread muscle pain and fatigue.

Myasthenia gravis

An autoimmune disorder that causes muscle weakness due to impaired nerve signaling.

Muscle fatigue

The decline in muscle ability to generate force due to prolonged activity and energy depletion.

Skeletal muscle

A voluntary muscle with striations and peripherally situated nuclei that functions for voluntary movement and organ protection.

Cardiac muscle

An involuntary muscle with striations and a single central nucleus that functions by contractions to pump blood to and from the heart.

Smooth muscle

A non-striated, involuntary muscle characterized by a single central nucleus in the walls of organs that regulate respiration, blood flow, and the movement of internal contents.

Calcium

Triggers muscle contraction by allowing interaction between actin and myosin filaments.

Adenosine triphosphate (ATP)

Provides the energy needed for muscle contraction and relaxation phases.

Extensors

Muscles that increase the angle between two body parts by straightening a joint, moving the parts farther apart.

Flexors

Muscles that decrease the angle between two body parts by bending a joint, bringing the parts closer together.

Hyperextension

Excessive extension of a joint beyond its normal range of motion, often leading to injury.

Hyperflexion

Excessive flexion of a joint beyond its normal range of motion that can cause an injury.

Sarcomeres

The fundamental units of contraction in muscle fibers, made up of protein filaments known as actin and myosin.

Myofilament

Protein filaments (thin actin and thick myosin) within muscle fibers that interact to enable muscle contraction through the sliding filament mechanism.

Masseter

The most powerful muscle, responsible for elevating the jaw during chewing.

Stapedius

Located in the ear and is the smallest muscle found in the human body. It is responsible for dampening sound vibrations.

Strain

Injury to a muscle or tendon due to overstretching or overuse.

Sprain

Injury to a ligament caused by overstretching or tearing.

Muscle functions

1. Movement

2. Posture Maintenance

3. Joint Stabilization

4. Heat Generation

Skeletal muscle structural organization (from largest to smallest)

Muscle → Fascicle → Muscle fiber (cell) → Myofibril → Sarcomere → Myofilament (actin & myosin)

Contractility

Ability of muscles to shorten and exert force.

Elasticity

The property of muscles that allows them to return to the original shape after being stretched.

Abduction

It refers to the lateral movement of the limb, which moves away from the midline.

Adduction

It refers to the limb's medial movement, which moves towards the midline.

Rotation

A circular motion where a body part turns around its axis (e.g., turning the head or rotating the arm).

Circumduction

It refers to the circular movement of a body part using the combination of flexion, extension, abduction, and adduction.

Pronation

A movement where the palm faces downward, and the radius rotates medially.

Supination

A movement where the palm faces upward, and the radius rotates laterally.

Deltoid

A muscle forming the shoulder's rounded contour, which is responsible for flexion, extension, and abduction.

Pectoralis major

The largest chest muscle, responsible for arm adduction and rotation.

Pectoralis minor

A small muscle located beneath the pectoralis major, responsible for scapular depression, protraction, and downward rotation.

Biceps brachii

It is one of the arm's chief muscles, and it is found in the upper arm's ventral portion. It is responsible for the flexion of the elbow.

Triceps brachii

A large and thick three-headed muscle found on the upper arm's dorsal portion. Its primary function is the extension of the elbow.

Rectus abdominis

Known as "six-pack abs," it flexes the trunk by moving the ribs and pelvis.

Trapezius

A large upper back muscle that stabilizes and moves the scapula and helps with neck movement.

Gluteus

It comprises three (3) muscles that can be found on the buttock. It plays a crucial role in the rotation and extension of the hip joint, as well as stabilizing the pelvis and the upper body.