The Muscular System Anatomy & Physiology

Muscles: the machines of the body Skeletal Muscle Characteristics Voluntary: move due to a conscious decision Striated: striped in appearance Multinucleate: Cells long; more nuclei More mitochondria for energy production 500+ muscles, 40-50% body weight

Skeletal Muscle Extras Soft and fragile yet TOUGH Exerts TREMENDOUS power Power provided by bundles of muscle fibers wrapped in connective tissue Connective tissue coverings allows for transmission of blood and nerves to muscles & provides support

Functional Characteristics of Skeletal Muscle Produces movement: for locomotion, manipulation and responding to the external environment; also for expressing emotions Maintaining posture: works continuously to fight downward pressure of gravity

Functional Characteristics of Skeletal Muscle Stabilizing joints: muscle tendons stabilize and reinforce joints that have poor articulating surfaces Generating heat: heat is a by-product of muscular activity; maintains normal body temperature

Skeletal Muscle Interactions Muscles are arranged in the body to either work together or work in opposition of each other Number 1 Rule of Muscle Activity: MUSCLES ONLY PULL*** THEY NEVER PUSH! In other words, muscle contractions are caused by SHORTENING of fibers

Skeletal Muscle Interactions Insertion: the movable attachment of a muscle Examples: biceps tendon attachment to radial tuberosity Origin: the fixed, immovable point of attachment Examples: biceps tendon attachment to greater tubercle A muscle contraction involves a muscle’s insertion moving toward its origin

Origins and Insertions

4 Functional Groups of Skeletal Muscles 1.) Agonists (prime movers): a muscle that provides the major force for providing a specific movement Example: biceps brachii- prime mover of elbow flexion

4 Functional Groups of Skeletal Muscles 2.) Antagonists: muscles that oppose/reverse a movement; relaxed when the agonist is active; helps regulate action of agonist by providing resistance Can also be agonists Example: the triceps brachii is the antagonist to the biceps during elbow flexion

4 Functional Groups of Skeletal Muscles 3.) Synergists: “work together”; aids agonist by promoting the same movement or reducing unnecessary movements that might occur as the agonist contracts Example: biceps brachii and brachioradialis in elbow flexion

4 Functional Groups of Skeletal Muscles 4.) Fixators: a synergist that immobilizes a bone or a muscle’s origin Example: erector spinae (muscles for posture) All types of muscles work together to provide smooth, coordinated and precise movements. Any one muscle can be in any functional group, dependant on its action

Detailed Skeletal Muscle Deep fascia: sheet/band of dense connective tissue covering muscles Epimysium: connective tissue covering many fascicles, all bound together

Detailed Skeletal Muscle Fascicle: a bundle of fibers wrapped in perimysium Perimysium: connective tissue surrounding a bundle of muscle fibers Endomysium: connective tissue surrounding each muscle fiber Aponeuroses: sheetlike epimysia that connects muscle to each other and to cartilage/bone

Detailed Skeletal Muscle Tendon: cordlike band of epimysia connecting muscles to bones Durable and able to cross bony projections Small in diameter to fit in crowded joint spaces

Nervous Stimulation of a Muscle Cell Skeletal muscle cells possess unique abilities: Excitability: can receive & respond to a stimulus Contractility: they can shorten Extensibility: lengthen or stretch Elasticity: cells return to resting form after contracted or stretched When a skeletal muscle contracts, it is told to do so by the nervous system

Nervous Stimulation of a Muscle Cell Motor Neuron: nerve extending from spinal cord to muscle fiber Motor Unit: one motor neuron and all the skeletal muscle cells it stimulates Neuromuscular Junction: where the end of the nerve and the beginning of the muscle fiber meets Synaptic cleft: space between nerve and muscle fiber

Motor Unit

Nervous Stimulation of a Muscle Cell Action Potential: an electrical current that can cause a muscular contraction Neurotransmitter: a chemical that carries a signal from the nerve to the muscle; continues the nervous stimulation Acetylcholine (ACh): the neurotransmitter specific to muscles

Nervous Stimulation of a Muscle Cell The nervous system sends a neural signal to the motor unit. This causes a release of ACh into the neuromuscular junction, where it binds to receptors on the muscle cell. This causes an action potential to run throughout the muscle cell, initiating a muscle contraction.

Synaptic Cleft

Microscopic Anatomy

Microscopic Anatomy of Skeletal Muscle Muscle cell = muscle fiber Sarcomere: contractile units of myofibrils, aligned end-to-end Myofibril: contractile proteins of muscle cells; lie parallel along length of fiber Thin myofilaments: actin Thick myofilaments: myosin

Microscopic Anatomy of Skeletal Muscle A band: where the myosin filaments are aligned H Zone: less dense portion of A band where thin filaments don’t overlap thick filaments Z line: separates sarcomeres; where actin filaments attach I band:light bands; where the actin filaments are aligned; extends to tips of myosin filaments

Sarcomere

Graded Responses of Skeletal Muscle Contraction A graded response: the degree of muscle shortening is dependant on 1. The speed of muscle stimulation 2. The number of muscle cells stimulated

Speed of Stimulation Muscle Twitch: a single, brief contraction caused by a mistake of the nervous system Summing of Contractions: when nervous impulses are delivered to a muscle at a rapid rate so it can’t relax between impulses; the contractions are summed(added) to create a smooth, strong contraction Tetanus: no relaxation between contractions; smooth and sustained

Number of Cells Stimulated Small, weak contractions: few cells initiated Large, strong contractions: all motor units are active and all muscle cells stimulated

Types of Skeletal Muscle Contractions Isotonic: same tone or tension Myofilaments slide, muscle shortens & movement occurs

Types of Skeletal Muscle Contractions Isometric: same length Muscles DO NOT shorten Myosin myofilaments do not move, tension in the fibers build Muscle tries to move but cannot

Energy for Muscle Contractions There are 3 different energy systems in the body that provide for muscle contractions Definitions: Creatine Phosphate (CP): high energy molecule only in muscles which help replenish energy stores Adenosine Triphoshate (ATP): energy source in the body Glucose: blood sugar Glycogen: storage form of blood sugar

Energy for Muscle Contractions Lactic Acid: a byproduct of an anaerobic pathway; causes muscle burning Aerobic: using oxygen during activity Anaerobic: not using oxygen during activity

Energy Systems There are 3 ways energy is utilized for muscular activity: 1. ATP-PC/Rapid Recovery  Anaerobic, lasting no more than 20 seconds  ATP production rate is rapid  ATP stores broken down rapidly and resupplied by creatine phosphate (transfers a phosphate to ADP)  More creatine stored in muscles than ATP

Energy Systems 2. Lactic Acid/Anaerobic  Breaks down glucose for energy without using oxygen  Muscles are working too fast for oxygen & glucose to be delivered by aerobic respiration  Results in build up of lactic acid in muscles: promotes muscle fatigue and soreness

Lactic Acid/Anaerobic Continued  Muscle fatigue: when a muscle is unable to contract even though it is still being stimulated.  Oxygen Debt: occurs during prolonged muscle activity; can’t get oxygen fast enough for the muscle’s needs; lactic acid builds up leading to fatigue; repaid by breathing deeply until the muscles receive enough oxygen to get rid of lactic acid and make more ATP and creatine phosphate

Energy Systems 3. Aerobic Respiration  During light exercise, ATP stores are regenerated by using oxygen  Can last for hours if glucose supplies are present  Used in endurance activities

Types of Skeletal Muscle Fibers There are 2 different kinds: 1. Slow twitch  For endurance  Speed of contraction is slow  Uses aerobic respiration  Low glycogen stores-more fats used  Slow rate of fatigue

Types of skeletal muscle fibers 2. Fast twitch  Explosive movements for short distances  Speed of contraction very fast  Some aerobic, some anaerobic fibers  High glycogen stores but fatigues quickly The distribution of muscle fibers is not complete until the teenage years. It varies between muscles, within muscles and between people.