Chapter 6 The Muscular System

The Muscular System Interactions of Skeletal Muscles in the Body Muscles usually work in groups, i.e. perform “group actions” Muscles are usually arranged in antagonistic pairs flexor-extensor abductor-adductor, etc.

600+ Skeletal Muscles 5 Golden Rules of Muscle Activity With a few exceptions, all muscles cross at least one joint The bulk of the muscle lies proximal to the join crossed All muscles have at least two attachments; the origin and the insertion Muscles can only pull; they never push During contraction, the muscle insertion moves toward the orgin

How Skeletal Muscles Produce Movement Muscles exert force on tendons that pull on bones Muscles usually span a joint Muscle contraction changes the angle or position of one bone relative to another Skeletal muscle must be stimulated by a nerve impulse to contract Brachialis flexes forearm

How Skeletal Muscles Produce Movement origin Origin: the attachment of the muscle to the bone that remains stationary Insertion: the attachment of the muscle to the bone that moves Belly: the fleshy part of the muscle between the tendons of origin and/or insertion belly insertion

Interactions of Skeletal Muscles in the Body Prime Mover (agonist): the principle muscle that causes a movement ex: biceps brachii, flexion of forearm Antagonist: the principle muscle that causes the opposite movement ex: triceps brachii, extension of forearm

Antagonists of the Forearm

Antagonists of the Thigh

Antagonists of the Foot

Interactions of Skeletal Muscles in the Body Synergists: muscles that assist the prime mover ex: extensor carpi (wrist) muscles are synergists for the flexor digitorum muscles when you clench your fist Fixators: synergists that stabilize the origin of a prime over ex: several back muscles that stabilize scapula when the deltoid flexes the arm

Functional Roles of Skeletal Muscles Group Actions: most movements need several muscles working together While the prime movers (agonist and synergists) are contracting to provide the desired movement other muscles (antagonists) are relaxing & being stretched out passively agonist and antagonist change roles depending on the action e.g., abduction versus adduction Synergists and Fixators become Agonists and Antagonists in different movements

Naming Skeletal Muscles Location of the muscle Shape of the muscle Relative Size of the muscle Direction/Orientation of the muscle fibers/cells Number of Origins Location of the Attachments Action of the muscle

Muscles Named by Location Epicranius (around cranium) Tibialis anterior (front of tibia) tibialis anterior

Naming Skeletal Muscles Shape: deltoid (triangle) trapezius (trapezoid) serratus (saw-toothed) rhomboideus (rhomboid) orbicularis and sphincters (circular) Rhomboideus major

Muscles Named by Size Psoas minor maximus (largest) minimis (smallest) longus (longest) brevis (short) major (large) minor (small) Psoas major

Muscles Named by Direction of Fibers Rectus (straight)-parallel to long axis Transverse Oblique Rectus abdominis External oblique

Muscles Named for Number of Origins biceps (2) triceps (3) quadriceps (4) Biceps brachii

Muscles Named for Origin and Insertion Sternocleidomastoid originates from sternum and clavicle and inserts on mastoid process of temporal bone insertion origins

Muscles Named for Action Flexor carpi radialis (extensor carpi radialis) –flexes wrist Abductor pollicis brevis (adductor pollicis) –flexes thumb Abductor magnus – abducts thigh Extensor digitorum – extends fingers Adductor magnus

Arrangement of Fascicles Parallel (strap-like), ex: sartorius Fusiform (spindle shaped), ex: biceps femoris

Arrangement of Fascicles Pennate ("feather shaped"), ex: extensor digitorum longus Bipennate, ex: rectus femoris Multipennate, ex: deltoid

Arrangement of Fascicles Convergent, ex: pectoralis major Circular (sphincters), ex: orbicularis oris

Arrangement of Fascicles Range of motion: depends on length of muscle fibers (fascicles); long fibers = large range of motion parallel and fusiform muscles Power: depends on total number of muscle fibers; many fibers = great power convergent, pennate, bipennate, multipennate

Lever Systems and Leverage Lever: i.e. bones, a rigid rod that moves on some fixed point Fulcrum: i.e. joint, a fixed point Resistance: the force opposing movement Effort: the force exerted to achieve action

Levers A lever is acted upon at 2 different points by: resistance or load the force that opposes movement the load or object (bone or tissue) to be moved effort the force exerted to achieve a movement the effort is provided by muscle(s) Motion is produced when the effort exceeds the resistance (isotonic contraction)

Lever Systems and Leverage Leverage: the mechanical advantage gained by a lever Power: muscle tension (effort) farther from joint (fulcrum) produces stronger contraction (opposes greater resistance) Range of motion (ROM): muscle tension (effort) closer to joint (fulcrum) produces greater range of motion.

Mechanical Advantage Load is near fulcrum, effort is far away Only a small effort is required to move an object Allows a heavy object to be moved with a small effort Example: car jack

Mechanical Disadvantage Load is far from the fulcrum, effort is near the fulcrum a large effort is required to move the object allows object to be moved rapidly, a “speed lever” throwing a baseball

Lever Systems and Leverage First-class lever: (EFR) Effort-Fulcrum-Resistance

Leverage Systems and Leverage Second class lever: (FRE) Fulcrum-Resistance-Effort

Leverage Systems and Leverage Third-class lever: (FER) Fulcrum-Effort-Resistance

Skeletal Muscles You will need to know all of these!