Muscular System Notes

Muscular System

Overview

• The muscular system is an organ system composed of skeletal, smooth, and cardiac muscles.
• There are over 600 muscles in the human body, constituting approximately 40% of an adult's body weight.
• Muscle Characteristics:
  • Excitable: Capable of responding to stimuli.
  • Contractile: Able to shorten and generate force.
  • Extensible: Can be stretched without damage.
  • Elastic: Able to recoil to original length after stretching.
• Functions of the Muscular System:
  • Produce locomotion (movement).
  • Stabilize joints.
  • Maintain posture.
  • Produce heat.

Types of Muscle

• Cardiac Muscle:
  • Found exclusively in the heart.
  • Characterized by short, branching fibers connected by intercalated disks.
  • Appears striated (striped).
  • Involuntary muscle, contracts automatically.
• Smooth Muscle:
  • Located in the walls of digestive tract, blood vessels, bladder, airways, and uterus.
  • Nonstriated in appearance.
  • Involuntary muscle, contracts automatically.
• Skeletal Muscle:
  • Attached to bones and responsible for body movement.
  • Markedly striated under a microscope.
  • Voluntary muscle, can be contracted at will.

Skeletal Muscle Structure

• Organization:
  • Muscle consists of muscle fibers.
  • Muscle fibers are organized into fascicles.
  • The entire muscle is surrounded by epimysium.
  • Fascicles are surrounded by perimysium.
  • Individual muscle fibers are surrounded by endomysium.
  • Fascia: Connective tissue that surrounds muscles and separates them from other tissues.
  • Tendons attach muscle to bone.
• Arteries, veins, and nerves run through the skeletal muscle

Structure of Muscle Fibers

• Muscle fibers contain myofibrils.
• Myofibrils are composed of myofilaments (thick and thin filaments).
• Sarcolemma: The cell membrane of a muscle fiber.
• Sarcoplasmic Reticulum: Stores calcium ions ($Ca^{2+}$) needed for muscle contraction.
• Transverse (T) Tubules: Invaginations of the sarcolemma that transmit electrical impulses deep into the muscle fiber.
• Mitochondria: Produce ATP (energy) for muscle contraction.
• Nucleus: Contains the genetic material of the muscle fiber.

Sliding-Filament Model of Contraction

• Myofibrils consist of myofilaments arranged in a lattice-like pattern to form sarcomeres.
• Sarcomere: The functional unit of muscle contraction, located between two Z discs.
• Thin Filaments: Primarily composed of actin.
• Thick Filaments: Primarily composed of myosin.
• During contraction, the thin filaments slide past the thick filaments, shortening the sarcomere.

Muscle Contraction

• Motor Neuron: A nerve cell that stimulates muscle contraction.
• Neuromuscular Junction: The connection between a motor neuron and a muscle fiber.
• Acetylcholine (ACh): A neurotransmitter released by the motor neuron that binds to receptors on the sarcolemma.
• Synaptic Cleft: The space between the motor neuron and the sarcolemma.
• When a motor neuron stimulates a muscle fiber:
  1. Vesicles of acetylcholine are released into the synaptic cleft.
  2. ACh binds to receptors on the sarcolemma.
  3. Electrical impulse travels along the T-tubules.
  4. Sarcoplasmic reticulum releases calcium ions ($Ca^{2+}$).
  5. Calcium binds to myofilaments, triggering muscle contraction.

How Muscle Fibers Contract and Relax

• A skeletal muscle must be stimulated by a motor neuron to contract.
• The connection between a motor neuron and a muscle fiber is called a neuromuscular junction.

Cross Bridge Cycle

• Calcium ($Ca^{2+}$) and ATP are essential for the cross-bridge cycle.

Disorders of Neuromuscular Junction

• Botulism: Food poisoning caused by Clostridium botulinum toxins that prevent the release of acetylcholine (ACh), leading to muscle paralysis.
• Myasthenia Gravis: An autoimmune disease where the body produces antibodies against ACh receptors, resulting in muscle weakness.
• Tetanus (Lockjaw): A serious infection caused by Clostridium bacteria that causes painful muscle tightening.
• Curare: A plant extract that binds to ACh receptor sites, blocking nerve transmission and causing paralysis.

Muscle Function

• Origin: The point where the muscle attaches to the more stationary bone.
• Insertion: The point where the muscle attaches to the more mobile bone.
• Belly: The main body of the muscle.
• Agonist (Prime Mover): The main muscle responsible for a particular movement.
• Synergist: A muscle that assists the agonist in performing a movement.
• Antagonist: A muscle that opposes the action of the agonist.

Body at Work

• Strength Training: Causes muscles to enlarge (hypertrophy) due to the repair of injured muscle fibers.
• Lack of Use: Causes muscles to shrink (atrophy).
• Endurance Exercise: Stimulates the growth of blood vessels in the muscle

Major Muscles of the Body

• Muscles are named according to:
  • Size
  • Shape
  • Location
  • Number of origins
  • Direction of muscle fibers
  • Actions

Muscles of the Head and Neck

• Frontalis
• Orbicularis oculi
• Zygomaticus
• Orbicularis oris
• Buccinator
• Temporalis
• Masseter
• Sternocleidomastoid
• Trapezius

Muscles of the Trunk

• External intercostals
• Internal intercostals
• Diaphragm

Muscles That Form the Abdominal Wall

• External oblique
• Internal oblique
• Rectus abdominis
• Transversus abdominis
• Linea alba

Muscles of the Shoulder and Upper Arm

• Deltoid
• Pectoralis major
• Serratus anterior

Muscles of the Neck, Shoulder, and Back

• Trapezius
• Latissimus dorsi
• Supraspinatus
• Infraspinatus
• Teres minor
• Rotator cuff

Muscles That Move the Forearm

• Brachialis
• Biceps brachii
• Brachioradialis
• Triceps brachii
• Pronator muscle

Muscles of the Thigh (Anterior View)

• Iliopsoas
• Iliacus
• Psoas major
• Sartorius
• Adductor magnus
• Adductor brevis
• Adductor longus
• Gracilis
• Rectus femoris
• Vastus lateralis
• Vastus medialis

Muscles of the Hip and Thigh (Posterior View)

• Gluteus medius
• Gluteus maximus
• Adductor magnus
• Gracilis
• Biceps femoris
• Semitendinosus
• Semimembranosus

Muscles That Act on the Foot

• Gastrocnemius
• Soleus
• Calcaneal (Achilles) tendon
• Tibialis anterior
• Extensor digitorum longus

Major Muscle Groups

• Deltoids/Shoulders
• Pectorals/Chest
• Biceps
• Abdominals
• Obliques
• Traps
• Triceps
• Lats
• Erector Spinae
• Glutes
• Hamstrings

Review Questions

• A single muscle cell is called a muscle fiber.
• External intercostals elevate the ribs during inspiration.
• During muscle contraction, the sarcoplasmic reticulum releases calcium.
• At rest, muscles obtain most of their energy by metabolizing fatty acids.
• The end of the muscle attached to the more mobile bone is called the insertion.
• Skeletal muscle is known as voluntary muscle.
• Epimysium is the connective tissue that surrounds the muscle as a whole.
• The chief function of T tubules is to allow electrical impulses to travel deep into the cell.
• Besides ATP, muscle contraction requires calcium.
• When extending the forearm, the triceps brachii is the prime mover, and the brachialis is the antagonist.