The Muscular System

Overview of the Muscular System

The muscular system is responsible for producing body movements through the contraction of skeletal muscles. These muscles typically work in coordinated groups, allowing for complex and controlled actions.

• Body Movements: Result from the contraction of skeletal muscles.

• Group Actions: Muscles often function together in groups to perform specific movements.

Muscle Attachments and Movements

Terms Relative to Skeletal Muscle Movements

Understanding muscle attachments is essential for describing how muscles produce movement at joints.

• Origin: The attachment of the muscle to the bone that remains stationary during movement.

• Insertion: The attachment of the muscle to the bone that moves during contraction.

• Belly: The fleshy, central part of the muscle located between the origin and insertion.

• Example: The biceps brachii originates at the scapula (origin) and inserts on the radius (insertion), with the belly in between.

Muscle-Bone Attachment Structures

Connective Tissue Attachments

Muscles attach to bones via specialized connective tissue structures that transmit the force of contraction.

• Tendon: A cord of dense regular connective tissue that attaches muscle to bone.

• Aponeurosis: A broad, flat sheet of dense regular connective tissue that connects muscles to other structures.

• Additional info: Tendons are strong and can withstand tension, while aponeuroses are found in areas requiring broad attachment, such as the abdominal wall.

Nerve and Blood Supply to Muscles

Innervation and Vascularization

Each muscle fiber and muscle receives a dedicated supply of nerves and blood vessels to support its function.

• Nerve Supply: Each muscle fiber is innervated by a branch of a motor nerve, allowing for precise control.

• Blood Supply: Each muscle is supplied by its own arteries and veins, ensuring delivery of oxygen and nutrients and removal of waste.

• Capillary Networks: Blood vessels branch extensively to provide each muscle fiber with a direct blood supply.

Naming Skeletal Muscles

Criteria for Muscle Naming

Skeletal muscles are named according to several anatomical and functional characteristics.

• Location: Indicates where the muscle is found (e.g., tibialis anterior is located in front of the tibia).

• Shape: Describes the muscle's form (e.g., deltoid is triangular, trapezius is trapezoidal).

• Relative Size: Compares the muscle's size to others (e.g., gluteus maximus is larger than gluteus minimus).

• Direction/Orientation of Fibers: Refers to the alignment of muscle fibers (e.g., rectus abdominis runs straight, transversus abdominis runs transversely).

• Number of Origins: Indicates how many tendons of origin a muscle has (e.g., biceps has two, triceps has three, quadriceps has four).

• Location of Attachments: Names may reflect the bones of origin and insertion (e.g., sternocleidomastoid originates from the sternum and clavicle, inserts on the mastoid process).

• Action: Describes the primary movement produced (e.g., flexor carpi radialis flexes the wrist, extensor digitorum extends the fingers).

Examples of Muscle Naming

Muscles Named by Location, Shape, and Action

Specific examples illustrate the application of naming conventions.

• Location: Frontalis (frontal bone), tibialis anterior (front of tibia).

• Shape: Deltoid (triangle), trapezius (trapezoid), rhomboideus (rhomboid), serratus (saw-toothed).

• Action: Flexor carpi radialis (flexes wrist), extensor digitorum (extends fingers), adductor magnus (adducts thigh).

• Number of Origins: Biceps (2), triceps (3), quadriceps (4).

• Attachments: Sternocleidomastoid (sternum, clavicle, mastoid process).

Muscle Contraction Physiology

Calcium Movements Inside Muscle Fibers

Muscle contraction is regulated by the movement of calcium ions within muscle fibers.

• Action Potential: Triggers the release of Ca2+ ions from the sarcoplasmic reticulum (SR).

• Troponin-Tropomyosin Complex: Ca2+ binds to troponin, causing a shift in tropomyosin and exposing binding sites for myosin on actin filaments.

• Cross-Bridge Formation: Myosin binds to actin, resulting in muscle contraction (sliding filament mechanism).

Calcium Removal: Ca2+ is pumped back into the SR when the stimulus ends, leading to muscle relaxation.

• Equation:

• Additional info: Calcium acts as an "on-off switch" for muscle contraction.

Recording Muscle Contraction: The Myogram

Phases of a Muscle Twitch

A myogram records the electrical and mechanical events during muscle contraction.

• Latent Period: Delay between stimulus and onset of contraction.

• Contraction Phase: Muscle shortens as tension increases.

• Relaxation Phase: Muscle returns to its resting length as tension decreases.

• Refractory Period: Time after excitation when the muscle will not respond to a new stimulus.

• Example: Extraocular muscles have a short latent period, gastrocnemius and soleus have longer periods.

Graded Muscle Responses

Motor Unit Summation and Recruitment

The force of muscle contraction can be varied by changing the number and frequency of motor units activated.

• Graded Response: Achieved by varying the number of muscle fibers (motor units) contracting and/or the frequency of stimulation.

• Motor Unit Summation (Recruitment): Increasing the number of active motor units leads to stronger muscle contractions.

• Size Principle: As greater force is needed, larger motor units with more and larger fibers are recruited.

• Equation:

Muscle Tone

Definition and Importance

Muscle tone refers to the continuous and passive partial contraction of muscles, which helps maintain posture and readiness for action.

• Muscle Tone: Small, involuntary contractions caused by spinal reflexes that rotate different motor units over time.

• Constant Tension: Muscles remain firm but do not shorten, maintaining posture (especially in neck, back, and leg muscles).

• Example: Postural muscles maintain muscle tone to support the body's position against gravity.