Anatomy and Physiology Chapter 6: Muscle System

Muscle Functions

• Initiate and resist movement

• Generate heat

Skeletal muscle

• Striated (parallel), multinucleate cells attached to the skeleton

• Helps with strength and mobility

Types of Muscle movement

• Synergistic: muscles working together to create the same movement

  • Anterior deltoids and triceps help move chest

• Antagonistic: muscles opposing each other

  • Biceps flex forearms, triceps extend forearms

Muscle Ends

• Origin: end of muscle attached to bone that doesn’t move during contraction

• Insertion: end of muscle attached to bone across a joint

• Example: origins not moving on top of shoulder, but bicep insertions connect and move near elbow

Fascicles

• Bundle of muscle cells

Fascia

• Sheath of connective tissue that surrounds fascicles

Myofibrils

• Long, cylindrical, parallel structures in a muscle cell

• Comprise the interior

• Several myofibrils make up a fascicle, several fascia sheaths comprise a muscle

Types of protein filaments

• Myosin: contractile protein- thicker filaments

• Actin: contractile protein- thinner filaments

• Sarcomere (measure from Z-line to Z-line) contracts when the filaments slide closer together

Muscle Contraction Process

1. Electrical impulse arrives at the neuromuscular junction, where motor neurons and muscle cells meet

2. Acetylcholine is released, diffusing across the space and binding to the muscle cell membrane

3. Acetylcholine binding generates an electrical impulse that travels across all parts of the cell, past the T tubules and into the sarcoplasmic reticulum

   1. Sarcoplasmic reticulum: stores calcium ions

   2. T tubules: carry electrical impulse

4. Calcium ions in the sarcoplasmic reticulum are released

5. Calcium binds to troponin and tropomyosin, shifting the protein complex, initiating the sliding filament mechanism, where myosin heads make contact with actin filaments, contracting the muscle

Muscle energy

• Stored ATP: stored in small quantities, used at first quickly

• Stored creatine phosphate: stored three to five times more than ATP, converted quickly to make ATP

• Stored glycogen: stored in varying quantities, primarily used in the first 3-5 minutes of exercise

  • Yields two ATP without oxygen in anaerobic metabolism

  • 36 with aerobic metabolism (yields most ATP of any energy source this way)

Types of muscle contractions

• Isotonic contractions: muscles and skeleton move

  • ie. lifting something

• Isometric contractions: muscles contract, skeleton does not

  • Pushing against an object too heavy to move

Motor unit

Neuron and all muscle cells it controls

• All or nothing principle: muscle cells never contract on their own, under complete control of the motor neuron

Muscle tension

Force exerted by contracting muscles

Twitch

Complete cycle of contraction and relaxation in a muscle cell

• Slow-twitch fibers: break down ATP and contract slowly

  • Endurance

• Fast-twitch fibers: contract quicker

  • Short bursts of energy

  • Use ATP, but also creatine phosphate and anaerobic metabolis

Recruitment

• Increasing of a muscle’s force by activating additional motor units

Cardiac muscle

• Involuntary

• Striated

• Have gap junctions at intercalated discs that allow electrical impulses to travel

• Contract without nerve signals, but respond to nerve activity

• Contracts slower than skeletal muscle, but faster than smooth muscle

Smooth muscle

• Involuntary

• Not striated

• Myosin and actin are not in sarcomeres

• Gap junctions allow cells to activate each other, coordinating contraction

• Contract without nerve signals, but respond to nerve activity

• Contracts more slowly than skeletal and cardiac muscle

• Found in blood vessel walls, digestive tract, uterus, and ureter

Muscular dystrophy

• Means abnormal growth, several subtypes

• Duchenne: a single defective gene causes a lack of a muscle cell protein

  • Lack of dystrophin protein causes an overflow of calcium into muscle cells, causing damage

Tetanus

• Puncture wound to a muscle leads to a bacterial infection

• Overproduction of muscle-stimulating toxin, causes lasting contractions

  • Especially affects jaw and neck muscles, can cause respiratory failure

Cramps

• Dehydration and ion imbalances during exercise

Pulled muscles

Excessive stretching of a muscle can cause fibers to tear apart, causing internal bleeding and swelling

Fasciitis

• Inflammation of fascia, caused by a strain or tear

Masseter muscle group

• Located near cheek

• Closes jaw

Orbicularis oris

• Located near the front of mouth

• Closes lips

Pectoralis major

• Top of chest

• Draws arm forward and toward the body

Serratus anterior

• Below the pec, near the upper abs

• Helps raise arm and push

Biceps brachii

• Upper arm

• Bends forearms at elbow

Rectus abdominus

• Ab muscles

• Compresses abdomen

External oblique

• Next to abs, above hips

• Helps lateral rotation of the trunk (side-to-side)

Abductor longus

• Upper thigh

• Flexes thigh and rotates it laterally

Sartorius

• Extends from upper thigh to knee

• Bends lower leg at the knee

Quadriceps group

• Located just above the knee

• Extends leg at the knee

Tibialis anterior

• Front of leg, below the knee

• Flexes foot toward the knee

Daltoid

• Upper shoulder

• Raises arm

Trapezius

• Middle of upper back

• Draws head back

Triceps brachii

• Back of upper arm

• Straightens forearm at elbow

Latissimus dorsi

• Lower back

• Draws arm backward and toward body

Gluteus Maximus

• Extends thigh

Hamstring group

• Bends knee

• Back of upper leg

Gastrocnemius

• Back of lower leg

• Bends foot away from knee

Label

1. Tendon

2. Muscle

3. Muscle cell

4. Nuclei

5. Fascia

6. Myofibril

7. Sarcomere