Chapter 10: Muscle System

Learning Objectives

• Describe the relationship between body movement and muscle action.

• Discuss the terms origin and insertion as they relate to muscle contraction.

• Identify major muscles, their points of attachment, and their function.

Relationship Between Body Movement and Muscle Action

Effects of Exercise on Muscles

Regular exercise has significant effects on muscle structure and function, influencing endurance, strength, and resistance to fatigue.

• Aerobic exercise (e.g., jogging, swimming, biking):

  • Increases number of capillaries and mitochondria in muscle fibers

  • Enhances synthesis of myoglobin

  • Improves endurance, strength, and resistance to fatigue

• Anaerobic exercise (e.g., weight lifting, isometric exercises):

  • Leads to muscle hypertrophy (increase in fiber size)

  • Increases mitochondria, myofilaments, glycogen stores, and connective tissue

  • Results in greater muscle strength and size

• Muscle atrophy (degeneration and loss of mass):

  • Occurs due to immobilization or loss of neural stimulation

  • Can begin almost immediately; strength can decline 5% per day

Factors Affecting Muscle Contraction

The force and duration of muscle contraction depend on several physiological factors:

• Number of muscle fibers stimulated (recruitment): More motor units activated results in greater force.

• Relative size of fibers: Bulkier muscles can develop more tension. Muscle cells can increase in size (hypertrophy) with regular exercise.

• Frequency of stimulation: Higher frequency of nerve impulses leads to greater force.

• Degree of muscle stretch: Muscle fibers with sarcomeres at 80–120% of normal resting length generate more force.

Muscle Fiber Types and Contraction Characteristics

Muscle fibers are classified according to contraction speed and metabolic pathway:

Characteristics of Smooth Muscle

Location and Structure

Smooth muscle is found in the walls of most hollow organs, except the heart (which contains cardiac muscle). Examples include organs of the respiratory, digestive, urinary, reproductive, and circulatory systems.

• Organs contain two layers of sheets with fibers oriented at right angles:

  • Longitudinal layer: fibers run parallel to the long axis of the organ; contraction shortens the organ.

  • Circular layer: fibers run around the circumference; contraction constricts the lumen.

• Alternating contractions and relaxations of these layers mix and propel substances through the lumen.

Microscopic Features

• Varicosities: Bulbous swellings of autonomic nerve fibers, instead of neuromuscular junctions, store and release neurotransmitters into a wide synaptic cleft (diffuse junction).

• Innervated by the autonomic nervous system.

• No striations and no sarcomeres, but contain overlapping thick and thin filaments.

Functional Properties

• Cells are electrically coupled by gap junctions (cell junctions allowing passage of ions and electrical signals).

• Action potentials can be transmitted from fiber to fiber.

• Some cells are self-excitatory (depolarize without external stimuli).

• Can contract and relax but maintain contraction for prolonged periods with little energy cost.

• Maintain moderate degree of contraction constantly without fatiguing.

Regulation of Contraction

• Controlled by nerves, hormones, or local chemical changes.

• Neurotransmitter binding can cause either graded (local) potentials or action potentials.

• The same neurotransmitter may have stimulatory effects in one organ and inhibitory effects in another.

Muscle Contraction and Movement

Muscle Contractions and Force Generation

Muscle contraction is essential for body movement and is determined by the number of cross bridges formed between actin and myosin filaments.

• Muscles work in groups to produce movement, with prime movers (agonists), antagonists, synergists, and fixators playing distinct roles.

• Prime movers provide the main force for movement; antagonists oppose or reverse movement; synergists assist prime movers; fixators stabilize the origin of the prime mover.

Origin and Insertion

Muscles attach to bones at two points:

• Origin: The attachment site that does not move during contraction (usually proximal).

• Insertion: The attachment site that moves during contraction (usually distal).

During contraction, the insertion moves toward the origin, producing movement at the joint.

Major Muscles, Points of Attachment, and Function

Head and Neck Muscles

• Facial muscles: Responsible for facial expression; insert into skin rather than bone.

• Mastication muscles: Temporalis and masseter (jaw closure); pterygoids (jaw movement); buccinator (chewing).

• Tongue muscles: Genioglossus, hyoglossus, styloglossus anchor and move the tongue.

• Swallowing muscles: Push food toward the pharynx; complete swallowing.

• Neck muscles: Move the head; e.g., sternocleidomastoid.

Thorax and Abdominal Muscles

• Respiratory muscles: Diaphragm and external intercostals enlarge the rib cage for inspiration; internal intercostals and abdominal muscles assist in expiration.

• Abdominal wall muscles: External oblique, internal oblique, transversus abdominis, rectus abdominis; support abdominal wall, trunk movement, and increase intra-abdominal pressure for functions like defecation and childbirth.

• Pelvic diaphragm: Supports pelvic organs and helps release feces; increases intra-abdominal pressure.

Shoulder and Arm Muscles

• Shoulder muscles: Fix shoulder girdle and increase range of arm movement; include muscles of anterior and posterior thorax.

• Arm muscles: Deltoid (abduction), latissimus dorsi (extension, adduction), rotator cuff muscles (synergists and fixators).

• Forearm muscles: Biceps brachii (flexes and supinates forearm), brachialis (flexes forearm), triceps brachii (extends forearm).

Hip and Leg Muscles

• Thigh muscles: Anterior, medial, and posterior compartments; flex femur at hip, extend leg at knee (walking); extend thigh, flex leg (backswing).

• Adductors: Adductor magnus, longus, brevis; press thighs together.

• Rotators: Lateral and medial rotators of the thigh.

• Hamstrings: Posterior thigh muscles; prime movers of thigh extension and knee flexion; common site of sports injury.

Lower Leg Muscles

• Movements: Dorsiflexion, plantar flexion, inversion, eversion of the foot; extension of toes.

• Anterior compartment: Tibialis anterior, extensor digitorum longus, extensor hallucis longus (dorsiflexors).

• Posterior compartment: Gastrocnemius, soleus (plantar flexors; gastrocnemius attaches to Achilles tendon).

Summary Table: Factors Affecting Muscle Contraction

Key Terms and Definitions

• Hypertrophy: Increase in muscle fiber size due to exercise.

• Atrophy: Degeneration and loss of muscle mass.

• Origin: Fixed attachment point of a muscle.

• Insertion: Movable attachment point of a muscle.

• Prime mover (agonist): Muscle that provides the main force for movement.

• Antagonist: Muscle that opposes or reverses a particular movement.

• Synergist: Muscle that assists the prime mover.

• Fixator: Muscle that stabilizes the origin of the prime mover.

• Varicosity: Bulbous swelling of autonomic nerve fibers in smooth muscle.

• Gap junction: Cell junction allowing passage of ions and electrical signals between cells.

Relevant Equations

• Force of muscle contraction is proportional to the number of cross bridges formed:

• Optimal sarcomere length for maximal force: of resting length

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and the summary table.