Muscular System Overview

Muscular System

• The muscular system consists of skeletal, smooth, and cardiac muscle tissues. These are all responsible for body movements.

Skeletal Muscle Tissue

• Attached to bones and skin
• Responsible for voluntary movements, such as walking and talking
• Contain striations, which give them a striped appearance

Skeletal Muscle Organization

• Muscle fascicles: Bundles of muscle fibers (cells)
• Muscle fibers: Long, cylindrical cells with multiple nuclei
• Myofibrils: Contractile units within muscle fibers, made up of myofilaments
• Myofilaments: Thin (actin) and thick (myosin) protein filaments responsible for muscle contraction

Muscle Attachments

• Origin: Attachment site of a muscle that remains stationary during contraction.
• Insertion: The structure that moves during muscle contraction, usually a bone, that moves.

Smooth Muscle Tissue

• Found in the walls of hollow organs, such as the stomach and blood vessels
• Responsible for involuntary movements, such as digestion and blood pressure regulation
• Lack striations, contributing to their smooth appearance

Cardiac Muscle Tissue

• Found only in the heart
• Responsible for pumping blood throughout the body
• Contains striations, similar to skeletal muscle.
• Involuntary, and under the control of the nervous system.

Muscle Functions

• Movement: Skeleton muscles facilitate movement of the body, with skeletal muscles responsible for most of the movement.
• Maintaining posture: Skeletal muscles continuously contract to help maintain posture and stabilize joints.
• Stabilizing joints: Skeletal muscles support and stabilize joints, preventing dislocation and injury. Muscle tension helps maintain joint integrity.
• Generating heat: Muscle contractions generate heat which helps regulate body temperature

Muscle Fiber Contraction

• Sliding filament theory: Muscle contraction occurs when thin (actin) filaments slide past thick (myosin) filaments.
• Role of calcium (Ca2+): Calcium ions bind to troponin, which initiates the uncovering of binding sites on actin, allowing myosin to bind and pull.

Muscle Contraction & Relaxation

• Neurotransmitter (acetylcholine) at neuromuscular junction: The nerve impulse stimulates the release of acetylcholine, a neurotransmitter, at the neuromuscular junction.
• Action potential in muscle fiber: Acetylcholine binds to receptors, triggering an action potential in the muscle fibers.
• Calcium (Ca2+) release: The action potential travels through the muscle fiber, causing the release of calcium ions (Ca2+) from the sarcoplasmic reticulum.
• Sliding filament mechanism: Calcium ions bind to troponin, exposing myosin-binding sites on actin. Myosin heads bind to actin, initiating the sliding of filaments.
• Muscle relaxation: When the nerve impulse stops, acetylcholine is broken down, Ca2+ is pumped back, and the myosin-binding sites are covered again, ending muscle contraction.

Muscle Energy Sources

• ATP: Directly fuels muscle contraction.
• Creatine phosphate: Short-term reserve of energy that can be used to quickly regenerate ATP.
• Glycogen: Stored glucose that can be used to produce ATP through anaerobic glycolysis.
• Aerobic respiration: Primary energy source for sustained muscle activity, requiring oxygen to produce ATP.

Muscle Fatigue and Oxygen Debt

• Muscle fatigue: The inability of a muscle to contract forcefully after prolonged or strenuous activity.
• Causes: Accumulation of lactic acid, depletion of ATP, and disruption of electrolyte balance.
• Oxygen debt: The amount of oxygen that is needed to restore muscle cells to their pre-exercise state after strenuous activity

Types of Muscle Contractions

• Isometric contractions: Muscle tension increases, but muscle length stays the same. Example: Holding a heavy object.
• Isotonic contractions: Muscle tension remains constant, but muscle length changes. Example: Lifting a weight.
• Concentric contractions: Muscle shortens as it contracts. Example: Curling a dumbbell.
• Eccentric contractions: Muscle lengthens as it contracts. Example: Lowering a dumbbell.

Types of Muscle Fibers

• Slow-twitch (type I) fibers: Red muscle fibers that are fatigue-resistant, use aerobic metabolism and are designed for endurance activities.
• Fast-twitch (type IIa) fibers: Pink muscle fibers that are fast-twitch and fatigue resistant, use both aerobic and anaerobic metabolism.
• Fast-twitch (type IIb) fibers: White muscle fibers that are fast, powerful, and easily fatiguable, use anaerobic metabolism.

Factors Affecting Muscle Tension

• Number of muscle fibers: More muscle fibers involved, the stronger the contraction.
• Size of muscle fibers: Larger muscle fibers generate more force.
• Frequency of stimulation: Increased frequency of stimulation results in stronger contractions.
• Length-tension relationship: Muscle fibers contract most forcefully at their optimal length.

Muscle Movements

• Flexion: Decreases the angle between two bones.
• Extension: Increases the angle between two bones.
• Abduction: Movement away from the midline of the body.
• Adduction: Movement toward the midline of the body.
• Rotation: Turning a bone around its longitudinal axis.
• Circumduction: Movement in a circular motion.
• Pronation: Turning the palm of the hand downward.
• Supination: Turning the palm of the hand upward.
• Inversion: Turning the sole of the foot inward.
• Eversion: Turning the sole of the foot outward.
• Dorsiflexion: Bending the foot upward.
• Plantar flexion: Bending the foot downward.

Muscle Naming Criteria

• Location: Based on the region of the body where the muscle is located. Example: Pectoralis muscle, located in the chest. 
• Size (large, small, long, short) Examples: Gluteus maximus, a large muscle in the buttocks, Sartorius, a long muscle in the thigh.
• Shape: Based on the muscle's shape. Examples: Deltoid, triangular shaped muscle in the shoulder, Trapezius, trapezoid shaped muscle in the back.
• Number of Origins: Based on the number of attachment points that the muscle has. Example: Biceps brachii, has two heads, or origins
• Direction of Fibers: Based on the direction of muscle fibers, such as rectus for straight. Example: Rectus abdominis, a straight muscle in the abdomen.
• Action: Based on the primary movement that the muscle performs. Example: Flexor digitorum superficialis, a muscle that flexes the fingers.

Skeletal Muscle Groups

• Head and neck: Muscles responsible for facial expressions, chewing, and head movement.
• **Trunk: ** Muscles responsible for movements of the spine, ribs, and abdomen.
• Upper limb: Muscles responsible for movements of the shoulder, arm, forearm, and hand.
• Lower limb: Muscles responsible for movements of the hip, leg, and foot.

Muscles of the Head and Neck

• Facial muscles: Control facial expressions. Examples: Frontalis, Orbicularis oculi, Zygomaticus major.
• Chewing muscles: Control the movement of the jaw.
• Neck muscles: Control head movements, posture, and swallowing.

Muscles of the Trunk

• Back muscles: Responsible for posture, extension, and lateral flexion of the spine. Examples: Trapezius, Latissimus dorsi, Erector spinae.
• Chest muscles: Responsible for breathing, pushing, and lifting. Examples: Pectoralis major, Pectoralis minor, Intercostal muscles.
• Abdominal muscles: Responsible for trunk flexion, rotation, and maintaining the abdominal cavity. Example: Rectus abdominis, External oblique, Internal oblique.

Muscles of the Upper Limb

• Shoulder: Muscles responsible for movement of the shoulder joint. Examples: Deltoid, Supraspinatus, Infraspinatus, Teres minor, Subscapularis.
• **Arm: ** Muscles responsible for movements of the elbow and forearm. Examples: Biceps brachii, Triceps brachii, Brachialis.
• Forearm: Muscles responsible for movements of the wrist and hand. Example: Flexor carpi radialis, Extensor carpi radialis, Flexor digitorum superficialis.
• Hand: Muscles responsible for fine motor movements of the fingers.

Muscles of the Lower Limb

• Hip: Muscles responsible for movement of the hip joint. Examples: Gluteus maximus, Gluteus medius, Gluteus minimus, Iliopsoas, Adductor muscles.
• Thigh: Muscles responsible for movements of the knee joint. Examples: Quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), Hamstring muscles (biceps femoris, semitendinosus, semimembranosus).
• Leg: Muscles responsible for movements of the ankle and foot. Examples: Gastrocnemius, Soleus, Tibialis anterior.

Developmental Aspects of the Muscular System

• Muscle development: Muscle control progresses in a superior to inferior and proximal to distal direction.
• Peak muscle development: Muscle growth peaks during mid-adolescence.
• Muscle atrophy: Muscles decrease in size and strength with lack of exercise
• Muscle hypertrophy: Muscles increase in size and strength with vigorous exercise.
• Aging: Muscles lose mass and strength with age. Exercise helps retain muscle mass and strength.

Muscle Disorders

• Muscle strain
• Muscle cramps
• Myalgia
• Tendinitis
• Fibromyalgia
• Muscular dystrophy
• **Amyotrophic lateral sclerosis (ALS) **
• Botulism
• Tetanus
• Myasthenia gravis