Also called Striated Muscle due to transverse bands or striations.
Major Functional Characteristics of Skeletal Muscle
Contractility: ability to shorten with force.
Excitability: capacity to respond to a stimulus.
Extensibility: ability to be stretched to their normal resting length.
Elasticity: ability to recoil to their original resting length.
Skeletal Muscle Structure
Connective Tissue Coverings
Epimysium/Muscular fascia: connective tissue sheath that surrounds a skeletal muscle.
Muscle fasciculi: numerous visible bundles that make up the muscle.
Perimysium: loose connective tissue that surrounds the muscle fasciculi.
Muscle fibers: several muscle cells that compose a fasciculus.
Endomysium: loose connective tissue that surrounds a muscle fiber.
Muscle Fiber Structure
Sarcolemma: cell membrane of the muscle fiber.
Transverse tubules (T tubules): tube-like invaginations which occur at regular intervals along the muscle fiber.
Sarcoplasmic reticulum: highly organized smooth endoplasmic reticulum with a relatively high concentration of Ca2+ (for muscle contraction).
Sarcoplasm: cytoplasm of a muscle fiber.
Myofibrils: threadlike structures composed of:
Actin Myofilaments (thin filaments; purple).
Myosin Myofilaments (thick filaments; green).
Sarcomere: highly ordered, repeating units of actin + myosin myofilaments; joined end to end to form the myofibril.
Actin and Myosin Myofilaments
Troponin molecules: binding sites for Ca2+; attached at specific intervals along the actin myofilaments.
Tropomyosin filaments: cover the attachment sites on the actin myofilaments; located along the groove between the twisted strands of actin myofilaments.
Myosin heads: resemble golf club heads;
Bind to attachment sites.
Bend and straighten.
Break down ATP.
Sarcomere
Basic structural and functional unit of skeletal muscle.
Z disk: network of protein fibers forming an attachment site for actin myofilaments.
I band: consists of actin myofilaments; spans each Z disk.
A band: darker, central region that extends the length of the myosin myofilaments.
H zone: second light zone that consists of myosin myofilaments.
M line: dark-staining bands.
The arrangement of actin and myosin filaments in sarcomeres gives the myofibrils a banded appearance.
The alternating I bands and A bands of the sarcomeres are responsible for the striations in the skeletal muscle fibers.
Excitability of Muscle Fibers
Resting membrane potential: cell membranes have a negative charge on the inside relative to a positive charge outside; occurs because there is an uneven distribution of ions.
Concentration of K+ inside the cell membrane > outside the cell membrane.
Concentration of Na+ outside the cell membrane > inside the cell membrane.
Different types of Ion Channels
Nongated/Leak channels: always open.
Chemically gated channels: closed until a chemical binds them and stimulates them to open.
Depolarization: the inside of the cell membrane comes more positive than the outside of the cell; Na+ ions move into cells.
Repolarization: the change back to the resting membrane potential; K+ ions move out of cells.
Action Potentials: the rapid depolarization and repolarization of the cell membrane; results in muscle contraction.
Nerve Supply
Motor neurons: specialized nerve cells that stimulate muscles to contract.
Neuromuscular junction: a branch that forms a junction with a muscle fiber.
Synapse: cell-to-cell junction between a nerve cell and another nerve cell/effector cell.
Motor unit: a single motor neuron and all the skeletal muscle fibers it innervates.
Presynaptic terminal: enlarged axon terminal.
Synaptic cleft: the space between the presynaptic terminal and the muscle fiber membrane.
Postsynaptic membrane: the muscle fiber membrane.
Synaptic vesicles: presynaptic terminal that contains small vesicles.
Acetylcholine (ACh): neurotransmitter contained in the vesicles; a molecule released by a presynaptic nerve cell that stimulates/inhibits a postsynaptic cell.
Acetylcholinesterase: an enzyme that rapidly breaks down the acetylcholine in the synaptic cleft between the neuron and the muscle fiber.
Muscle Contraction
Sliding filament model: sliding of actin myofilaments past myosin myofilaments during contraction.
Cross-bridges: myosin heads attach to the myosin attachment sites on the actin myofilaments.
Muscle Twitch, Summation, Tetanus, Recruitment
Muscle Twitch: contraction of a muscle fiber in response to a stimulus.
Lag/Latent Phase: time between the application of a stimulus and the beginning of contraction.
Contraction Phase: time during which the muscle contracts.
Relaxation Phase: time during which the muscle relaxes.
Summation: the force of contraction of an individual muscle fiber is increased by rapidly stimulating them.
Tetanus: convulsive tension; a sustained contraction that occurs when the frequency of stimulus is so rapid that no relaxation occurs. Caused by Ca2+ build up in the myofibrils.
Recruitment: the number of muscle fibers contracting is increased by the increasing number of motor units stimulated + muscle contracts with more force.
Stimulus frequency: number of times a motor neuron is stimulated per second.
Energy Requirement for Muscle Contraction
Aerobic Respiration: requires O<em>2; breaks down glucose to produce ATP, CO</em>2, H2O.
Anaerobic respiration: doesn’t require O2; breaks down glucose to yield ATP and lactic acid.
Creatine phosphate: high-energy molecule that can be stored in muscle fibers.
Fatigue
A state of reduced work capacity.
Muscular Fatigue
When muscle fibers use ATP faster than they are produced; when the effectiveness of Ca2+ to stimulate actin + myosin is reduced.
Physiological contracture: muscles may become incapable of either contracting or relaxing.
Psychological fatigue: involves the CNS; an individual perceives that continued muscle contraction is impossible.
Type of Muscle Contractions
Isometric contractions: equal distance; length of the muscle does not change; the amount of tension increases during the contraction process.
Isotonic contraction: equal tension; the amount of tension produced by the muscle is constant during contraction; length of the muscle decreases.
Concentric contractions: isotonic; muscle tension increases as the muscle shortens.
Eccentric contractions: isotonic; tension is maintained in a muscle; the opposing resistance causes the muscle to lengthen.
Muscle Tone
Constant tension produced by body muscles over long periods of time.
Responsible for keeping the back and legs straight, the head in an upright position, and the abdomen from bulging.
Slow-Twitch and Fast-Twitch Fibers
Classification of Muscle Fiber
Slow Twitch: contains type I myosin; contracts slowly and resistant to fatigue respiration
Fast Twitch
Type IIa – intermediate speed; more fatigue resistant than type IIb.
Type IIb – contract 10x faster than type I.
Myoglobin: stores oxygen temporarily.
Hypertrophy: enlarging of muscle fibers.
Satellite cells: undifferentiated cells just below the endomysium.
Autorhythmicity: resulting periodic spontaneous contraction of smooth muscle.
Intercalated disks: specialized structures that facilitate action potential conduction between cells.
Skeletal Muscle Anatomy
General Principles
Tendon: muscle connected to a bone.
Aponeuroses: broad, sheet like tendons.
Retinaculum: a band of connective tissue that holds down the tendons at each wrist and ankle.
Origin: head; most stationary end of the muscle.
Insertion: end of the muscle attached to the bone undergoing the greatest movement.
Belly: part of the muscle between the origin & the insertion.
Agonist: muscle that accomplishes a certain movement.
Antagonist: muscle acting in opposition to an agonist.
Synergists: a group of muscles working together to produce a movement.
Prime Mover: muscle that plays the major role in accomplishing desired movement.
Fixators: muscles that hold one bone in place relative to the body.
Nomenclature
According to Location: temporalis, frontalis, pectoralis, brachialis.
According to Origin and Insertion: sterno (sternum), cleido (clavicle), mastoid (mastoid process), brachio (arm), radialis (radius).
According to Number of Origin: biceps (2), triceps, quadriceps.
According to Function: flexor (flexion).
According to Size: maximus (largest), minimus (smallest), vastus (large).
According to Shape: deltoid (triangle), orbicularis (circular).
Orientation of Fasciculi: rectus (straight).
Muscles of the Head and Neck
Facial Expression
Occipitofrontalis: raises the eyebrows.
Orbicularis oculi: encircle the eyes, tightly close the eyelids, and causes crow’s feet wrinkles.
Orbicularis oris: encircles the mouth.
Buccinator: kissing muscles; pucker the mouth; flattens the cheeks in whistling/blowing.
Zygomaticus: elevate the upper lip and corner of the mouth.
Levator labii superioris: sneering; elevates one side of the upper lip.
Depressor anguli oris: frowning & pouting; depresses the corner of the mouth.
Mastication (for chewing)
Temporalis: fan-shaped muscle
Masseter: seen & felt on the side of the head.
Pterygoid (paired): protraction, excursion, elevation of mandible.
Tongue and Swallowing Muscles
Intrinsic muscles: located within the tongue and change its shape.
Extrinsic muscles: attached to and move the tongue.
Hyoid muscles: hold the hyoid bone; elevate the larynx.
Pharyngeal elevators: elevate the pharynx.
Pharyngeal constrictors: constrict the pharynx from superior to inferior (forcing food into the esophagus); also open the auditory tube.
Neck Muscles
Sternocleidomastoid: prime mover of the lateral muscle group; rotates the head; flexes the neck or extends the head; prayer muscle.
Torticollis: wryneck; injury to the sternocleidomastoid.
Deep neck muscles : flexes/extends head and neck
Trapezius: extends and laterally flexes neck.
Trunk Muscles
Muscles Moving the Vertebral Column
Erector spinae: responsible for keeping the back straight and the body erect.
Deep back muscles: responsible for several movements of the vertebral column.
External intercostal: elevate the ribs during inspiration.
Internal intercostal: contract during forced expiration, depressing the ribs.
Diaphragm: major movement produced in the thorax during quiet breathing.
Scalenes: inspiration and rib elevation.
Abdominal Wall Muscles
Linea alba: tendinous area of the abdominal wall that consists of white connective tissue.
Rectus abdominis: located on each side of the linea alba.
Tendinous intersections: causes the abdominal wall of a lean, well-muscled person to appear segmented.
External/Internal abdominal oblique & Transversus abdominis: flex and rotate the vertebral column or compress the abdominal contents.
Pelvic Floor and Perineal Muscles
Pelvic Floor: pelvic diaphragm.
Levator ani: muscle that forms the pelvic floor.
Perineum: associated with the male/female reproductive structures.
Perineum bulbospongiosus: constricts the urethra; erects the penis, clitoris.
Perineum ischiocavernosus: compresses the base of penis/clitoris.
Perineum external anal sphincter: keeps the orifice of the anal canal closed.
Upper Limb Muscles
Scapular Movements
Attach the scapula to the thorax and move the scapula.
Acts as fixators to hold the scapula firmly in position when the muscle of the arm contract.
Move the scapula into different positions.
Trapezius.
Levator scapulae.
Rhomboids.
Serratus anterior.
Pectoralis minor.
Arm Movements
Pectoralis major: adducts the arm and flexes the shoulder; extend the shoulder from a flexed position.
Latissimus dorsi: swimmer’s muscle; medially rotates and adducts the arm and powerfully extends the shoulder.
Rotator cuff muscles: attached the humerus to the scapula and forms a cuff/cap over the proximal humerus.
Deltoid: attaches the humerus to the scapula and clavicle; major abductor of the upper limb.
Forearm Movements
Triceps brachii: primary extensor of the elbow.
Biceps brachii & brachialis: primary flexors of the elbow.
Brachioradialis: posterior forearm muscle; helps flex the elbow.
Supination and Pronation
Supinator: supination of the forearm or turning the flexed forearm so that the palm is up.
Pronator: pronation, turning other forearm so that the palm is down.
Wrist and Finger Movements
Retinaculum: fibrous connective tissue that covers the flexor & extensor tendons and holds them in place around the wrist.
Flexor carpi: flexes the wrist.
Extensor carpi: extends the wrist.
Flexor digitorum: flexor of the digits/fingers.
Extensor digitorum: extension of the fingers.
Intrinsic hand muscles: 19 muscles located within the hand.
Interossei: responsible for abduction and adduction of the fingers.
Tennis elbow: inflammation and pain due to forceful, repeated contraction of the wrist extensor muscles.
Palmus longus: tightens palm skin.
Lower Limb Muscles
Thigh Movements
Iliopsoas: flexes the hip
Tensor fasciae latae: helps steady the femur on the tibia when a person is standing.
Gluteus maximus: extends the hip; adducts and laterally rottes the thigh; contributes most of the mass.
Gluteus medius: extends the hip when the thigh is flexed at a 45o angle; common site for injections in the buttocks (sciatic nerve lies deep to the g. maximus).
Leg Movements
Quadriceps femoris: primary extensors of the knee.
Sartorius: tailor’s muscle; longest muscle in the body; flexes the hip and knee; rotates the thigh laterally for sitting cross legged.
Hamstring muscles: flexing the knee.
Adductor muscles: adducting the thigh.
Ankle and Toe Movements
Gastrocnemius & Soleus: form the bulge of the calf.
Calcaneal tendon: Achilles tendon; flexors and are involved in plantar flexion of the foot.
Fibularis muscles: primary everters of the foot; aid in plantar flexion.
Intrinsic foot muscles: flex, extend, abduct, and adduct the toes