Chapter 10: The Muscular System – Structure, Function, and Major Muscles

Muscle System Overview

Introduction

The muscular system is essential for body movement, posture, and vital functions such as breathing and circulation. This chapter explores the relationship between muscle action and body movement, the characteristics of smooth muscle tissue, and the identification of major muscles, their points of attachment, and their functions.

Relationship Between Body Movement and Muscle Action

Types of Exercise and Muscle Adaptation

• Aerobic (endurance) exercise: Activities like jogging, swimming, and biking increase muscle capillaries, mitochondria, and myoglobin synthesis, resulting in greater endurance and resistance to fatigue.

• Resistance exercise: Weight lifting and isometric exercises lead to muscle hypertrophy (increase in fiber size), increased strength, and connective tissue reinforcement.

• Muscle health: Muscles must remain active to prevent atrophy (degeneration and loss of mass). Disuse atrophy can begin after immobilization or loss of neural stimulation, with muscle strength declining up to 5% per day.

Force of Muscle Contractions

• Number of muscle fibers stimulated: More motor units recruited, greater force.

• Relative size of fibers: Bulkier muscles develop more tension.

• Frequency of stimulation: Higher frequency, greater force.

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

Factors Affecting Muscle Contraction

Factors	Velocity & Duration	Force
Muscle fiber type	Classified by speed of contraction: slow vs fast fibers	Number of muscle fibers stimulated: more units, greater force
Load	Muscles contract faster with less load; greater load slows contraction	Relative size of fibers: bulkier muscle, more tension
Recruitment	More motor units contracting, faster and more prolonged contraction	Frequency of stimulation: higher frequency, greater force
Degree of stretch	Optimal sarcomere length (80–120%) for maximal force	Degree of muscle stretch: optimal length, more force

Functional Groups of Muscles

• Prime mover (agonist): Major responsibility for producing specific movement.

• Antagonist: Opposes or reverses particular movement.

• Synergist: Helps prime movers by adding force or reducing unnecessary movement.

• Fixator: Stabilizes origin of prime mover.

Example: Biceps brachii (prime mover of elbow flexion), triceps brachii (antagonist).

Characteristics of Smooth Muscle Tissue

Location and Structure

• Found in walls of most hollow organs (respiratory, digestive, urinary, reproductive, circulatory except smallest blood vessels).

• Not found in heart (heart contains cardiac muscle).

• Most organs have two layers: longitudinal (fibers run parallel to organ axis) and circular (fibers run around circumference).

• Alternating contractions mix and squeeze substances through organ lumen.

Cellular Features

• Contain varicosities (bulbous swellings) that release neurotransmitters into a wide synaptic cleft (diffuse junction).

• Innervated by autonomic nervous system.

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

• Cells electrically coupled by gap junctions (allow ions and electrical signals to pass between cells).

• Slow, synchronized contractions; some cells are self-excitatory.

• Most smooth muscle maintains moderate contraction without fatiguing.

Regulation of Contraction

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

• Neural regulation: Neurotransmitter binding causes graded or action potentials. One neurotransmitter can have stimulatory effect in one organ, inhibitory in another.

Origin and Insertion in Muscle Contraction

Definitions and Differences

• Origin: Attachment point of skeletal muscle that does not move during contraction (usually more proximal).

• Insertion: Attachment point that moves during contraction (usually more distal).

• Muscles attach to bones via tendons.

Example: Biceps brachii origin at scapula, insertion at radius.

Naming Muscles

• Location: Bone or region associated (e.g., temporalis over temporal bone).

• Shape: Deltoid (triangle), trapezius (trapezoid).

• Size: Maximus (largest), minimus (smallest), longus (long).

• Direction of fibers: Rectus (straight), transversus (right angles), oblique (diagonal).

• Number of origins: Biceps (two), triceps (three).

• Location of attachments: Sternocleidomastoid (sternum, clavicle, mastoid).

• Action: Flexor, extensor, adductor.

Fascicle Arrangement

• Circular: Concentric rings (e.g., orbicularis oris).

• Convergent: Broad origin, fascicles converge to single tendon (e.g., pectoralis major).

• Parallel: Fascicles parallel to long axis (e.g., sartorius).

• Pennate: Fascicles attach obliquely to central tendon (e.g., rectus femoris).

Leverage and Muscle Movement

• Muscles act as levers with three components:

  • Lever: Rigid bar (bone) that moves on a fixed point (fulcrum).

  • Effort: Force applied by muscle contraction.

  • Load: Resistance moved by the effort.

• Three classes of levers:

  • First-class: Fulcrum between effort and load (e.g., neck extension).

  • Second-class: Load between fulcrum and effort (e.g., standing on tiptoe).

  • Third-class: Effort between fulcrum and load (e.g., biceps curl).

Equation: $\text{Mechanical Advantage} = \frac{\text{Load}}{\text{Effort}}$

Major Muscles: Points of Attachment and Function

Muscles of the Head and Face

• Facial expression muscles insert into skin, not bone; important for nonverbal communication.

• Groups: Muscles of the scalp, muscles of the face.

Muscles of Mastication and Tongue Movement

• Jaw closure: Temporalis, masseter.

• Grinding movements: Pterygoids.

• Chewing role: Buccinator.

• Tongue movement: Genioglossus, hyoglossus, styloglossus.

Muscles of the Neck and Throat

• Sternocleidomastoid: Divides neck into anterior and posterior triangles.

• Epiglottis: Covers larynx during swallowing.

• Groups: Anterior neck muscles (move head), intrinsic muscles of the back (extend trunk, maintain posture).

• Erector spinae muscles: Iliocostalis, longissimus, spinalis.

• Quadratus lumborum: Lateral flexion of spine.

Muscles of Respiration

• Inspiratory muscles: Diaphragm, external intercostals.

• Expiration: Relaxation of inspiratory muscles, contraction of internal intercostals, abdominal muscles for forced expiration.

Muscles of the Abdominal Wall

• Four paired muscles: Rectus abdominis, external obliques, internal obliques, transversus abdominis.

• Actions: Lateral flexion, rotation of trunk, promotion of urination, defecation, childbirth, vomiting, coughing, screaming.

Muscles of the Pelvic Diaphragm

• Functions: Seals inferior outlet of pelvis, supports pelvic organs, lifts pelvic floor, resists increased intra-abdominal pressure.

Muscles of the Shoulder and Arm

• Extrinsic shoulder muscles act to fix shoulder girdle and increase range of arm movements.

• Groups: Muscles of anterior thorax, muscles of posterior thorax.

• Prime movers of arm: Pectoralis major, latissimus dorsi, deltoid.

• Rotator cuff muscles: Supraspinatus, infraspinatus, teres minor, subscapularis, coracobrachialis, teres major.

Muscles of the Arm and Forearm

• Anterior compartment: Biceps brachii, brachialis, brachioradialis (flex and supinate forearm).

• Posterior compartment: Triceps brachii (forearm extensor).

Muscles of the Thigh

• Grouped as anterior, medial, or posterior.

• Anterior: Flex femur at hip, extend leg at knee (iliopsoas, tensor fasciae latae, rectus femoris).

• Medial: Adduct thigh (adductors, sartorius).

• Posterior: Extend thigh, flex leg (hamstrings: biceps femoris, semitendinosus, semimembranosus).

Adductors and Rotators of the Thigh

• Adductors: Adductor magnus, adductor longus, adductor brevis, pectineus, gracilis.

• Abductors and lateral rotators: Gluteus maximus, gluteus medius, gluteus minimus, piriformis, obturator externus/internus, gemellus.

Quadriceps and Hamstrings

• Quadriceps: Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius (powerful knee extensors).

• Hamstrings: Biceps femoris, semitendinosus, semimembranosus (thigh extension, knee flexion).

Muscles of the Leg and Foot

• Movements: Ankle dorsiflexion/plantar flexion, inversion/eversion of foot, toe flexion/extension.

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

• Posterior compartment: Soleus, tibialis posterior, gastrocnemius.

Additional info: The notes include diagrams and tables for muscle groups, points of attachment, and functional roles, supporting visual learning and clinical application.