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

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Muscle System Overview

Introduction

The muscle system is essential for body movement, posture, and vital functions. Muscles work by contracting and relaxing, enabling movement of body parts and maintenance of physiological processes. 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 immediately, 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 resting length generate 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: Synergist that immobilizes bone or muscle origin, stabilizing the prime mover.

Table: Factors Affecting Muscle Contraction

Factors	Velocity & Duration	Force
Muscle fiber type	Classified as slow or fast fibers	Number of muscle fibers stimulated (recruitment)
Load	Muscles contract faster when no load is added; 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 muscle stretch	—	Muscle fibers with sarcomeres at 80–120% resting length generate more force

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 of smooth muscle fibers: longitudinal (parallel to organ axis) and circular (around circumference).
Alternating contractions of these layers mix and propel substances through organ lumens.

Cellular Features

Contain varicosities (bulbous swellings) of nerve fibers instead of neuromuscular junctions.
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 constantly without fatigue.

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 at these points.

Naming Muscles

Location: Bone or body region (e.g., temporalis, brachialis).
Shape: Deltoid (triangle), trapezius (trapezoid).
Size: Maximus (largest), minimus (smallest), longus (long).
Direction of fibers: Rectus (straight), transversus (right angles), oblique (angles).
Number of origins: Biceps (two), triceps (three), quadriceps (four).
Location of attachments: Named according to origin and insertion (e.g., sternocleidomastoid).
Action: Flexor, extensor, adductor.

Fascicle Arrangement

Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).
Convergent: Broad origin, fascicles converge toward 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 Movement

Muscles use levers to move bones.
Lever: Rigid bar (bone) that moves on a fixed point (fulcrum).
Effort: Force applied by muscle contraction.
Load: Resistance moved by the effort.
Levers can operate at mechanical advantage or disadvantage depending on the position of effort, fulcrum, and load.

Equation:

Major Muscles, Points of Attachment, and Function

Muscles of Facial Expression

Insert into skin, not bone; important for nonverbal communication.
Groups: Muscles of the scalp and muscles of the face.

Muscles of Mastication

Temporalis and Masseter: Prime movers of jaw closure.
Pterygoids: Grinding movements.
Buccinator: Chewing role.

Muscles Promoting Tongue Movements

Three extrinsic muscles anchor and move tongue: Genioglossus, Hyoglossus, Styloglossus.

Muscles of the Neck and Back

Sternocleidomastoid: Divides neck into two triangles; moves head.
Epiglottis: Covers larynx during swallowing.
Anterior neck muscles: Move head.
Intrinsic muscles of the back: Extend trunk and maintain posture (Erector Spinae group: Iliocostalis, Longissimus, Spinalis; Quadratus Lumborum).

Muscles of Respiration

Diaphragm and External intercostals: Inspiration (enlarge rib cage).
Expiration: Relaxation of inspiratory muscles and contraction of internal intercostals.

Muscles of the Abdominal Wall

Four paired muscles: Rectus abdominis, External obliques, Internal obliques, Transversus abdominis.
Actions: Lateral flexion, rotation of trunk, and assistance in various bodily functions.

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: Move and stabilize shoulder girdle.
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 (powerful 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 (Adductor magnus, Adductor longus, Adductor brevis, Pectineus, Gracilis).
Posterior: Extend thigh, flex leg (Hamstring group: Biceps femoris, Semitendinosus, Semimembranosus).

Muscles of the Leg

Produce movements at ankle, intertarsal joints, and toes.
Anterior compartment: Tibialis anterior, Extensor digitorum longus, Extensor hallucis longus (dorsiflexion and toe extension).
Posterior compartment: Soleus, Tibialis posterior, Gastrocnemius (plantar flexion).

Abductors and Lateral Rotators of the Thigh

Gluteus maximus, Gluteus medius, Gluteus minimus, Piriformis, Obturator externus, Obturator internus, Gemellus (abduction and rotation of thigh).

Quadriceps Group

Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius (powerful knee extension).

Example: Muscle Naming

Extensor carpi radialis longus: Extends wrist, located on radius, long muscle.

Additional info: The above notes synthesize and expand upon the provided slides and handwritten notes, filling in academic context for muscle structure, function, and terminology.