Muscular System Anatomy and Physiology

Muscle Tissue

• The major functions of the muscular system include movement/mobility, maintaining posture, joint stability, and heat production.

Skeletal Muscle

• Skeletal muscles attach to bones and facilitate voluntary movements, such as walking and facial expressions.
• Microscopically, skeletal muscles exhibit a striated appearance with long, cylindrical fibers and multiple peripheral nuclei.
• Skeletal muscles contract to produce movement by pulling on bones through tendons.

Cardiac Muscle

• Cardiac muscle is located exclusively in the heart.
• It is responsible for pumping blood throughout the body.
• It has a striated appearance with branching fibers connected via intercalated discs for coordinated contractions.
• Cardiac muscle contraction is involuntary.

Smooth Muscle

• Smooth muscle is located in the walls of internal organs, such as the stomach, intestines, bladder, and blood vessels.
• It mediates involuntary movements like digestion, blood pressure regulation, and uterine contractions.
• Microscopically, smooth muscle appears smooth with spindle-shaped cells and a single nucleus per cell.

Microscopic Characteristics of Muscle

• Skeletal and cardiac muscle exhibit a striated appearance due to the organized arrangement of contractile proteins.
• Smooth muscle lacks striations and has a spindle-shaped appearance.

Organization of Skeletal Muscle

• Individual muscle fibers are surrounded by connective tissue called the endomysium, which form fascicles.
• Fascicles are bundled together by the perimysium.
• The entire muscle is encased by the epimysium.

Connective Tissue Layers of Skeletal

• Endomysium surrounds individual muscle fibers.
• Perimysium surrounds fascicles, the bundles of muscle fibers.
• Epimysium surrounds the entire muscle.

Skeletal Muscle Fiber Components

• Skeletal muscle fibers contain myofibrils, which consist of repeating units called sarcomeres.
• Sarcomeres are composed of thick filaments (myosin) and thin filaments (actin).
• These sarcomeres are enclosed within the sarcolemma and surrounded by the sarcoplasm.
• The sarcoplasm houses the sarcoplasmic reticulum (SR), which is involved in calcium release for muscle contraction.

Sarcomere Composition and Arrangement

• The sarcomere, as the functional unit of muscle fiber, is composed of repeating units of thick (myosin) and thin (actin) filaments arranged in parallel.
• Thin filaments are anchored to Z-discs.
• Thick filaments are centrally located, partially overlapping the thin filaments within the A-band.

Neuromuscular Junction Structure

• The neuromuscular junction is a specialized synapse where a motor neuron connects to a skeletal muscle fiber.
• Its structure includes the presynaptic terminal (motor neuron axon ending), the synaptic cleft (the gap between the nerve and muscle), and the postsynaptic membrane (muscle fiber membrane with acetylcholine receptors).
• This structure allows for the transmission of signals to initiate muscle contraction.

Skeletal Muscle Fiber Contraction

• A skeletal muscle fiber contracts when a motor neuron releases acetylcholine (ACh) at the neuromuscular junction.
• This triggers an action potential that travels along the muscle fiber membrane and into the T-tubules, causing calcium release from the sarcoplasmic reticulum (SR).
• Calcium binds to troponin, which exposes actin binding sites for myosin cross-bridges to attach and lead to the sliding filament movement (contraction) through cross-bridge cycling, causing muscle shortening.
• Relaxation occurs when calcium is actively pumped back into the SR, allowing tropomyosin to re-cover the actin binding sites, detaching the cross-bridges, and returning the muscle to its resting state.

Skeletal Muscle Fiber Relaxation

• Relaxation happens when the motor neuron stops releasing acetylcholine (ACh).
• This leads to the reuptake of calcium ions back into the sarcoplasmic reticulum, causing the actin-binding sites on the thin filaments to be covered again.
• Muscle fiber loses tension and relaxes.
• The removal of calcium ions from the sarcoplasm is essential for the muscle to return to its relaxed state.

ATP Sources for Muscle Fibers

• Skeletal muscle fibers primarily generate ATP for contraction and relaxation through three sources: creatine phosphate (immediate energy), anaerobic glycolysis (short-term energy), and aerobic respiration (long-term energy).

Muscle System Changes Due to External Factors

• Prolonged inactivity or bed rest can lead to muscle atrophy (decrease in muscle mass and strength), potentially causing reduced mobility, difficulty performing daily activities, and increased risk of falls.
• Regular exercise induces muscle hypertrophy (increase in muscle mass and strength), improving physical performance and overall health.

Origin (Muscle Term)

• The fixed attachment point of a muscle, usually on a stationary bone, where the muscle begins to contract; considered the "starting point" of muscle movement.

Insertion (Muscle Term)

• The moveable attachment point of a muscle, where the muscle pulls on a bone to create movement; typically located on the bone that moves during muscle contraction.

Prime Mover (Agonist)

• The primary muscle responsible for producing a specific movement at a joint.

Antagonist

• The muscle that opposes the action of the prime mover, relaxing to allow the prime mover to contract and create movement.

Synergist

• A muscle that assists the prime mover by stabilizing the joint and contributing to the same movement.

Fixator

• A muscle that contracts to stabilize the origin of the prime mover, preventing unwanted movement at the joint.

Muscle Fascicle Arrangement

• Muscle fibers are bundled together within a muscle; patterns include parallel (e.g., biceps brachii), convergent (e.g., pectoralis major), circular (e.g., orbicularis oculi), and pennate (e.g., extensor digitorum longus, rectus femoris).
• Pennate arrangements have fascicles attaching to a central tendon at an angle, allowing for greater force generation but less range of motion compared to parallel muscles.

Muscle Names

• Prefixes like "flexor," "extensor," "abductor," and "adductor" describe the primary movement a muscle performs.
• Muscles are named after the body region they are located in, like "rectus abdominis" (abdominal area) or "tibialis anterior" (front of the tibia bone).
• Some muscles are named based on their shape, such as "deltoid" (triangular) or "orbicularis oculi" (circular).
• "Maximus" is the largest muscle, "medius" is the middle, and "minimus" is the smallest.

Temporalis

• Location: Temple area
• Attachments: Temporal bone to mandible
• Action: Elevates mandible (chewing)

Masseter

• Location: Cheek
• Attachments: Zygomatic process to mandible
• Action: Elevates mandible (chewing)

Sternocleidomastoid

• Location: Neck
• Attachments: Sternum, clavicle to mastoid process
• Action: Flexes neck, rotates head contralaterally

Digastric

• Location: Under mandible
• Attachments: Hyoid bone to mandible
• Action: Elevates hyoid bone, depresses mandible

Deltoid

• Location: Shoulder
• Attachments: Scapula to humerus
• Action: Abducts shoulder

Biceps Brachii

• Location: Front of upper arm
• Attachments: Scapula to radius
• Action: Flexes elbow

Triceps Brachii

• Location: Back of upper arm
• Attachments: Scapula and humerus to ulna
• Action: Extends elbow

Pectoralis Major

• Location: Chest
• Attachments: Sternum, clavicle to humerus
• Action: Adducts and flexes shoulder

Muscle Actions

• Muscles within a specific compartment or region perform the same type of movement together, like flexion, extension, abduction, or adduction.
• Muscles in the anterior compartment of the thigh all work together to extend the knee joint.

Explore the primary functions of the muscular system, including muscle types, their unique structures, and control mechanisms. Discover skeletal muscle attachments, smooth muscle functions, and specific muscle actions like those of the deltoid and sternocleidomastoid.