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

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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 muscle strength, and connective tissue reinforcement.
Muscle inactivity: Leads to atrophy (degeneration and loss of mass), which can begin almost immediately and result in a decline of muscle strength by 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 increases force.
Degree of muscle stretch: Muscle fibers with sarcomeres at 80–120% of their normal 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 recruited, greater force
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 stretch		Muscle fibers with sarcomeres at 80–120% resting length generate more force

Muscle Group Functions

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.

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).

Function and Contraction

Longitudinal layer: Contraction shortens organ.
Circular layer: Contraction constricts lumen of organ.
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.
Can maintain moderate contraction for prolonged periods with little energy cost.

Regulation of Contraction

Controlled by nerves, hormones, or local chemical changes.
Neural regulation: Neurotransmitter binding causes graded or action potentials; effects can be stimulatory or inhibitory depending on organ.

Origin and Insertion in Muscle Contraction

Definitions

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).
Most skeletal muscles form two or more attachment points, often to bones via tendons.

Muscle Naming Criteria

Location: Bone or body region associated with muscle.
Shape: Deltoid (triangle), trapezius (trapezoid).
Size: Maximus (largest), minimus (smallest), longus (long).
Direction of fibers: Rectus (straight), transverse, oblique.
Number of origins: Biceps (two), triceps (three).
Location of attachments: Named according to origin and insertion.
Action: Flexor, extensor, adductor.

Fascicle Arrangement

Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).
Convergent: 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 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.
Levers can operate at mechanical advantage (effort farther from fulcrum) or disadvantage (effort nearer to fulcrum).

Major Muscles: Points of Attachment and Function

Muscles of Facial Expression

Insert into skin, not bone; important for nonverbal communication.
Two groups: Muscles of the scalp and 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 Back

Sternocleidomastoid: Divides neck into anterior and posterior triangles.
Pharyngeal muscles: Propel food during swallowing.
Anterior neck muscles: Move head.
Intrinsic back muscles: Extend trunk and maintain posture (Erector Spinae group: Iliocostalis, Longissimus, Spinalis; Quadratus Lumborum).

Muscles of Respiration

Inspiration: Diaphragm, External intercostals.
Expiration: Relaxation of inspiratory muscles and contraction of internal intercostals and abdominal muscles.

Muscles of the Abdominal Wall

Four paired muscles: Rectus abdominis, External obliques, Internal obliques, Transversus abdominis.
Actions: Lateral flexion, trunk rotation, and assistance in functions like urination, defecation, childbirth, coughing, and vomiting.

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 fix the shoulder girdle and increase arm movement range.
Prime movers of arm: Pectoralis major, Latissimus dorsi, Deltoid.
Rotator cuff muscles (synergists and fixators): 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

Thigh flexors: Iliopsoas (iliacus and psoas major), Tensor fasciae latae, Rectus femoris.
Thigh extensors: Hamstring muscles (Biceps femoris, Semitendinosus, Semimembranosus).
Adductors: Adductor magnus, Adductor longus, Adductor brevis, Pectineus, Gracilis.
Quadriceps group: Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius.

Muscles of the Gluteal Region

Abductors and lateral rotators: Gluteus maximus, Gluteus medius, Gluteus minimus, Piriformis, Obturator externus, Obturator internus, Gemellus.

Muscles of the Leg

Anterior compartment: Tibialis anterior, Extensor digitorum longus, Extensor hallucis longus (dorsiflexion and toe extension).
Posterior compartment: Soleus, Tibialis posterior, Gastrocnemius (plantar flexion).

Key Equations

Force of muscle contraction:
Mechanical advantage of a lever:

Example

Biceps brachii: Origin at scapula, insertion at radius; flexes forearm at elbow.
Rectus abdominis: Origin at pubic crest, insertion at xiphoid process and costal cartilages; flexes vertebral column.

Additional info: Muscle tables and diagrams referenced in the notes are based on standard anatomical sources and may be expanded for clarity.