Muscular System Notes

Muscular System Functions (M2RPC3)

• Movement of the body.
• Maintenance of posture.
• Respiration.
• Production of body heat.
• Communication.
• Constriction of organs and vessels.
• Contraction of the heart.

Characteristics of Skeletal Muscle

• Constitutes approximately 40% of body weight.
• Muscles are attached to the skeletal system.
• 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 $Ca^{2+}$ (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 $Ca^{2+}$; 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.
  1. Concentration of $K^+$ inside the cell membrane > outside the cell membrane.
  2. 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.
  1. Lag/Latent Phase: time between the application of a stimulus and the beginning of contraction.
  2. Contraction Phase: time during which the muscle contracts.
  3. 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 $Ca^{2+}$ 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, $CO</em>2$, $H_2O$.
• Anaerobic respiration: doesn’t require $O_2$; 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 $Ca^{2+}$ 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

1. Slow Twitch: contains type I myosin; contracts slowly and resistant to fatigue respiration
2. 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

1. According to Location: temporalis, frontalis, pectoralis, brachialis.
2. According to Origin and Insertion: sterno (sternum), cleido (clavicle), mastoid (mastoid process), brachio (arm), radialis (radius).
3. According to Number of Origin: biceps (2), triceps, quadriceps.
4. According to Function: flexor (flexion).
5. According to Size: maximus (largest), minimus (smallest), vastus (large).
6. According to Shape: deltoid (triangle), orbicularis (circular).
7. 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 $45^o$ 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
• Extensor digitorum longus: extends 4 lateral toes, everts foot.
• Extensor halluces longus: extends great toe; inverts foot.
• Tibialis anterior: inverts foot.
• Fibularis tertius: everts foot.
• Flexor digitorum longus: flexes 4 lateral toes, inverts foot.
• Flexor halluces longus: flexes great toe, inverts foot.
• Tibialis posterior: inverts foot.
• Fibularis brevis: everts foot.
• Fibularis longus: everts foot.

Effects of Aging on Skeletal Muscle

• Decreased muscle mass.
• Slower reaction time.
• Reduced stamina.
• Increased recovery time.

Types of Muscular Tissue

1. Skeletal (striated voluntary)
   • Large, long, cylindrical cells.
   • Multinucleated.
   • Attached to bones.
   • Responsible for body movement.
2. Cardiac (striated involuntary)
   • Cylindrical cells.
   • Branched and connected to one another by intercalated disks.
   • Single nucleated.
   • Found in the heart.
   • Pumps the blood.
3. Smooth (nonstriated involuntary)
   • End tapered cells.
   • Single nucleated.
   • Found in hollow organs: stomach, intestine; skin, eyes.
   • Regulates size of organs, forces fluid through tubes, controls the amount of light entering the eye, produces ‘goose bumps’.