The Muscular System

Types of Muscle Tissue

The human body contains three distinct types of muscle tissue, each with specialized structure and function.

• Skeletal Muscle: Voluntary muscle attached to bones; responsible for moving the body.

• Cardiac Muscle: Involuntary muscle found only in the heart; enables heart contractions.

• Smooth Muscle: Involuntary muscle located in organs and blood vessels; controls movements such as digestion and blood flow.

Characteristics of Muscle Tissue

Muscle tissue exhibits four essential properties that enable its function:

• Excitability: Ability to respond to stimulation, typically from nerves.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to stretch beyond resting length.

• Elasticity: Ability to return to original shape after stretching or contracting.

Functions of Muscles

Muscles perform several vital functions in the body:

• Produce movement: Facilitate locomotion and manipulation of the environment.

• Maintain posture: Stabilize the body and keep it upright.

• Stabilize joints: Support and reinforce joints.

• Produce heat: Generate heat as a byproduct of contraction, helping regulate body temperature.

• Support soft tissues: Protect internal organs.

Connective Tissue Coverings of Muscle

Muscles are organized and protected by layers of connective tissue:

• Tendon: Connects muscle to bone, transmitting force for movement.

Muscle Fiber Structure

Each muscle fiber (cell) contains specialized structures for contraction:

• Muscle fiber: The basic muscle cell.

• Sarcolemma: Cell membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle cell.

• Myofibrils: Rod-like structures within muscle fibers, composed of repeating units called sarcomeres.

• Sarcomere: The basic functional unit of contraction.

• Myofilaments: Protein filaments (actin and myosin) responsible for contraction.

Actin and Myosin

Muscle contraction depends on the interaction between two main types of myofilaments:

• Actin: Thin filament.

• Myosin: Thick filament.

• Sarcomere: Functional contractile unit, defined by Z discs.

Sliding Filament Theory

The sliding filament theory explains how muscles contract at the molecular level:

• Calcium is released from the sarcoplasmic reticulum.

• Calcium binds to troponin, causing tropomyosin to move away from actin's binding sites.

• Myosin heads attach to actin, forming cross-bridges.

• Myosin pulls actin inward, shortening the sarcomere.

• Muscle contracts as actin slides past myosin; the filaments themselves do not shorten.

Equation for Muscle Contraction Energy:

ATP is required for muscle contraction and relaxation:

Neuromuscular Junction (NMJ)

The neuromuscular junction is where a motor neuron communicates with a muscle fiber:

• Motor neuron: Nerve cell that stimulates muscle contraction.

• Acetylcholine (ACh): Neurotransmitter released by the neuron to initiate contraction.

• Acetylcholinesterase: Enzyme that breaks down ACh, ending the signal.

Role of ATP and Calcium in Muscle Contraction

• ATP: Provides energy for muscle activity; allows myosin to detach from actin after contraction.

• Calcium: Enables actin and myosin interaction by exposing binding sites on actin.

Types of Muscle Contractions

Muscle System Organization and Movement

Muscle Terminology

• Example: When bending the elbow, the biceps is the agonist, and the triceps is the antagonist.

Muscle Movements

Muscle Naming Conventions

Muscles are named based on several criteria:

• Location: Where the muscle is found (e.g., brachialis in the arm).

• Shape: The muscle’s shape (e.g., deltoid is triangular).

• Size: Maximus means largest; minimus means smallest.

• Direction: Rectus means straight.

• Number of origins: Biceps means two heads; triceps means three.

• Action: Flexor bends; extensor straightens.

Major Muscles of the Body

Major Anterior Muscles (Front)

Major Posterior Muscles (Back)

Important Muscle Groups

• Quadriceps: Located at the front of the thigh; extends the knee.

• Hamstrings: Located at the back of the thigh; flexes the knee.

• Abdominals: Form the abdominal wall; help move and stabilize the trunk.

• Rotator cuff: Four muscles that stabilize the shoulder joint.

Rotator Cuff Muscles (SITS)

• Supraspinatus

• Infraspinatus

• Teres minor

• Subscapularis

The rotator cuff muscles stabilize the shoulder joint and allow a wide range of arm movements.

Summary: Key Concepts for Exam Preparation

• Three types of muscle tissue

• Four characteristics of muscle tissue

• Epimysium, perimysium, endomysium

• Actin versus myosin

• Parts of a sarcomere

• Sliding filament theory

• Neuromuscular junction

• ATP and calcium

• Isometric versus isotonic contractions

• Origin versus insertion

• Agonist versus antagonist

• Flexion, extension, abduction, adduction

• Major anterior and posterior muscles

• Quadriceps versus hamstrings

• Four rotator cuff muscles

Study advice: Learn the location and function of each muscle, not just their names, to master the muscular system for exams.