Skeletal Muscle Anatomy and Physiology (PDF)

Anatomy and Physiology – Open Stax Course Learning Outcome 5. Explain skeletal muscle function including cross-bridge cycling, motor unit types, physiological control of force generation, and the mechanical influences affecting force output. (I) (LE, IC) Learning Goal Apply a foundational knowledg...

Anatomy and Physiology – Open Stax Course Learning Outcome 5. Explain skeletal muscle function including cross-bridge cycling, motor unit types, physiological control of force generation, and the mechanical influences affecting force output. (I) (LE, IC) Learning Goal Apply a foundational knowledge of the structure of skeletal muscle to its functional roles. Learning Outcomes - Relate the structural similarities and differences between skeletal, smooth and cardiac muscle to their functions. - Describe the functional roles of skeletal muscle. - List the criteria used to name skeletal muscles, provide an example of each. - Briefly describe the different types of muscle architecture and their functional differences. - Differentiate between the types of muscle actions and use them appropriately when describing joint movements. - Describe the structure of skeletal muscle, relate the components to their function. - Draw a sarcomere including how it changes during contraction based on the sliding filament theory. - Define motor unit and describe the functional characteristics of fast and slow motor units. - Describe the force control of a motor unit, including the size principle. - Describe the mechanical factors which influence muscle force production, sketch the length/tension, force/velocity and power/velocity curves. I. Myology II. Overview of Skeletal Muscle III. Structure of Skeletal Muscle IV. Function of Skeletal Muscle Anatomy and Physiology – Open Stax  Myology Myology – the scientific study of muscle Muscle cells (fibers) are the only cells in the body that have the property of contractility, which allows them to shorten and develop tension. Anatomy and Physiology – Open Stax Three Muscle Types Skeletal Muscle Smooth Muscle Cardiac Muscle Anatomy and Physiology – Open Stax  1. Skeletal Muscle Attaches to and moves the skeleton. The contractile molecules are very organized giving skeletal muscle a striated (striped) pattern - “striated muscle”. It is under voluntary control. Anatomy and Physiology – Open Stax  1. Skeletal Muscle Skeletal muscle comprises about 36% of the total body weight in women and 42% in men. 75 percent of skeletal muscle is water 20% is protein remainder consists of – inorganic salts (K+, Cl-), – pigments – fats – carbohydrates. mypilates.co.za Anatomy and Physiology – Open Stax Anatomy and Physiology – Open Stax Skeletal Muscle Anatomy and Movement Analysis Need to; Identify and name (proper spelling) muscles listed in lab manual. Know muscle functions for movement Analyze exercises – identifying joint movements and muscles involved  2. Smooth muscle Found in the walls of hollow organs and blood vessels. contractile molecules are not aligned, creating smooth appearance Smooth muscle is under involuntary control. Connections between cells allows them to contract together – syncytium Anatomy and Physiology – Open Stax  3. Cardiac muscle The contractile tissue of the heart wall. has characteristics of both smooth muscle and skeletal muscle. Like skeletal muscle. – contractile molecules are organized in striations Like smooth muscle – under involuntary control – Connections between cells allow them to contract together. syncytium - facilitating ejection of blood. Anatomy and Physiology – Open Stax I. Myology II. Overview of Skeletal Muscle III. Structure of Skeletal Muscle IV. Function of Skeletal Muscle  II. Overview of Skeletal Muscle A. Functions of Skeletal Muscle 1. Produce skeletal movement 2. Maintain posture and body position 3. Breathing – the diaphragm is a skeletal muscle 4. Stabilize joints 5. Support soft tissues – e.g support weight of organs 6. Generate heat to maintain body temperature 7. Guard entrances and exits within the body 8. Chewing food and talking 9. Provide nutrient reserves – muscle protein for energy 10. Skeletal muscle as an endocrine organ - Signalling proteins (myokines) have hormone like effects mypilates.co.za  II. Overview of Skeletal Muscle B. Criteria Used To Name Muscles 1. Shape 2. Action 3. Location 4. Division 5. Size relationships 6. Directions of the fibers. Anatomy and Physiology – Open Stax  B. Criteria Used To Name Muscles 1. Shape – deltoid (triangular) trapezius rhomboid latissimus (wide)  B. Criteria Used To Name Muscles 2. Action - various muscle names include the terms flexor, extensor, adductor, or pronator. e.g. - Adductor Magnus Adduction Anatomy and Physiology – Open Stax  B. Criteria Used To Name Muscles 3. Location tibialis anterior Intercostals pectoralis major cost (ribs) Pect (chest) B. Criteria Used To Name Muscles 4. Divisions biceps brachii (bi – two) triceps brachii (tri – three) quadriceps femoris (quad – four) Anatomy and Physiology – Open Stax B. Criteria Used To Name Muscles 5. Size relationships - gluteus maximus, gluteus medius, gluteus minimus. Several names include the terms "brevis" (short), and "longus" (long).  B. Criteria Used To Name Muscles 6. Direction of fibers – transverse (across) rectus (straight) Transverse Abdominis Rectus Abdominus Anatomy and Physiology – Open Stax  C. Skeletal Muscle Architecture Fasciculi (bundle of muscle fibres) may run parallel to long axis of muscle (greater range of motion, less strength) or insert diagonally into a tendon running the length of the muscle (smaller range of motion, greater strength).  C. Skeletal Muscle Architecture 1. Unipennate - all fasciculi insert on one side of a tendon - semimembranosus 2. Bipennate - fasciculi insert on both sides of tendon - rectus femoris 3. Multipennate - convergence of several tendons – deltoid  C. Skeletal Muscle Architecture 4. Longitudinal (strap) - fasciculi run parallel to the long axis of the muscle - sartorius, rectus abdominus 5. Radiate (convergent) - fibers fan out from a single attachment - pectoralis major Anatomy and Physiology – Open Stax  D. Muscle Actions Most movements require the cooperative action of several muscles functioning as a group. 1. Prime mover - a muscle whose contraction is primarily responsible for a particular movement. 2. Antagonist - muscles that oppose one another upon contraction - biceps brachii vs. triceps brachii; quadriceps vs. hamstrings - Antagonists are located on opposite sides of a joint. Biceps and Triceps are Antagonists D. Muscle Actions Movements are usually the result of the contraction of more than one muscle, and frequently a single muscle contributes to the production of two or more movements. 3. Fixators/stabilizers - muscles that immobilize a bone or joint near the origin of the prime mover so that the prime mover can act more efficiently. - more muscle stabilizers are required using free weights compared to using weight machines I. Myology II. Overview of Skeletal Muscle III. Structure of Skeletal Muscle IV. Function of Skeletal Muscle Anatomy and Physiology – Open Stax III. Structure of Skeletal Muscle A. Gross Anatomy Origin - less movable end of a muscle, usually proximal. Insertion - more movable end of a muscle, usually distal. Belly - widest portion of a muscle, between its origin and insertion. III. Structure of Skeletal Muscle Not all muscles insert on bone. Most of the muscles controlling facial expression originate from bone and insert in the skin. Anatomy and Physiology – Open Stax  III. Structure of Skeletal Muscle B. Connective Tissue The three layers of connective tissue surround; serve in part to maintain intramuscular pressure thereby augmenting force production. whole muscle – epimysium bundles of muscle fibers (fasciculi) – perimysium muscle fibers – endomysium Anatomy and Physiology – Open Stax III. Structure of Skeletal Muscle B. Connective Tissue Tendons are extensions of connective tissue membranes beyond the end of the muscle. Tendons transmit the force of contractile tissue to bone.  III. Structure of Skeletal Muscle B. Connective Tissue Tendons are much stronger than muscle and as such can receive force from a large number of muscle fibers (with a much larger total cross-sectional area than the tendon) and insert on to a small area of a bone such as a tuberosity*. * Tuberosity is a large, roughened process on the bone. (e.g. deltoid tuberosity)  III. Structure of Skeletal Muscle C. Microanatomy of Skeletal Muscle A muscle cell is a muscle fiber. Within each muscle cell/fiber are many myofibrils. Each myofibril consists of a large array of contractile proteins arranged repeatedly in series. This gives skeletal muscle its striated pattern of light and dark areas or bands. Each repeated array of contractile proteins is called a sarcomere. Anatomy and Physiology – Open Stax III. Structure of Skeletal Muscle C. Microanatomy of Skeletal Muscle The two major contractile proteins of the sarcomere are actin (thin filament) myosin (thick filament). Anatomy and Physiology – Open Stax  III. Structure of Skeletal Muscle The Sarcomere, from Z line to Z line, is the functional unit of muscle Contracted sarcomere Anatomy and Physiology – Open Stax III. Structure of Skeletal Muscle C. Microanatomy of Skeletal Muscle Myosin has cross- bridges extending from Actin its thick central core. Actin has binding sites for myosin Myosin The most prominent theory of muscle contraction is the sliding filament theory. Anatomy and Physiology – Open Stax  III. Structure of Skeletal Muscle The Sliding Filament Theory This theory suggests that, when the muscle is activated, the protruding cross-bridges on myosin attach to actin and, with the aid of ATP, the cross- bridge microstructure can “rotate” thus causing the thin actin filament to “slide” over myosin. This causes the sarcomere to shorten. Relaxed sarcomere Contracted sarcomere Anatomy and Physiology – Open Stax III. Structure of Skeletal Muscle D. Blood Supply about 3-4 capillaries surrounding each muscle fiber of a sedentary person. up to 7 capillaries per muscle fiber with training (angiogenesis) III. Structure of Skeletal Muscle D. Blood Supply Muscles require a good supply of blood for continued force generation. When muscle force increases, so does intramuscular pressure. The increase in intramuscular pressure can exceed that of blood pressure and restrict blood flow within the muscle In general, this can start to occur at about 15-20 % of maximum muscle force and completely halt blood flow at about 50 % of maximum force. Rhythmic contraction and relaxation during exercise like running and cycling facilitate the maintenance of blood flow. I. Myology II. Overview of Skeletal Muscle III. Structure of Skeletal Muscle IV. Function of Skeletal Muscle V. Sarcopenia IV. Function of Skeletal Muscle A. The Motor Unit B. Types of Motor Units C. Force Control of a Motor Unit. D. Mechanical Factors Which Influence Muscle Force Production IV. Function of Skeletal Muscle Activation of a muscle fiber can, in the extreme, produce either maximum force or maximum velocity. In between these extremes, a muscle fiber creates a combination of force and movement. A. The Motor Unit A motor unit is the functional unit of a muscle. It consists of a motor neuron and all the muscle fibers that motor neuron innervates. The cell body of a motor neuron is located in the spinal cord. The axon of that motor neuron extends from the spinal cord to the target muscle which may be a few millimeters away or a few feet away. A. The Motor Unit When the axon is close to the muscle it separates (bifurcates) many times to innervate all the muscle fibers of that particular motor unit. For an average motor unit the motor neuron will innervate about 200 muscles fibers. The range is from 2-3 muscle fibers per motor unit for muscles capable of very fine movements to 2000 fibers per motor unit for large muscles that perform only gross movements. A. The Motor Unit If the cell body of the motor neuron receives a strong enough stimulus, an action potential is generated. This action potential travels along the axon and all its bifurcations to stimulate each and every muscle fiber in that particular motor unit. This is known as the all-or-none law. Anatomy and Physiology – Open Stax  B. Types of Motor Units Motor units can be classified in – two ways on the basis of speed of contraction slow twitch & fast twitch – three ways on the basis of metabolic characteristics: Slow twitch oxidative (SO) – also called “type I” Fast twitch oxidative-glycolytic (FOG) – also called “type IIa” Fast twitch glycolytic (FG) – also called “type IIx” Structural and Functional Characteristics of Skeletal Muscle Motor Units Type I Type IIa Type IIx B. Types of Motor Units Use the muscle biopsy technique to determine fiber type proportions in humans.  Muscle Fiber Types http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#EXCITE Type IIb fibers are found in rodents, fibers with similar characteristics in humans are Type IIx B. Types of Motor Units Men, women, and children generally have 45 to 55% slow twitch fibers in their arm and leg muscles. However, there can be considerable variation in fiber type distribution from muscle to muscle and from person to person. All of the muscle fibers in a given motor unit will have identical contractile and metabolic properties.  B. Types of Motor Units Endurance athletes have higher than average proportions of slow twitch muscle fibers in the muscles used in their sport, while power athletes have high proportions of fast twitch muscle fibers. B. Types of Motor Units Slow twitch fibers and fast twitch fibers can’t be inter- converted by physical training. However, FOG and FG fibers can be inter-converted by physical training. Type IIb fibers are found in rodents, fibers with similar characteristics in humans are Type IIx B. Types of Motor Units Current research indicates that genetic factors largely determine a person’s predominant muscle fiber distribution. C. Force Control of a Motor Unit 1. Multiple Motor Unit Summation. A large muscle may contain up to 2,000 motor units. A skeletal muscle can increase force production by activating more motor units. A low force requires the activation of a small number of motor units while a higher force requirement progressively enlists more motor units.  C. Force Control of a Motor Unit 2. Frequency or Wave Summation. If a single action potential travels down a motor neuron axon, the motor unit response is a twitch. If many action potentials travel down the axon at a rate faster than the twitch response time of the motor unit, then the mechanical force response will summate. At high motor neuron firing rates (i.e., many action potentials in a short period of time) this effect can generate on average about 5x the force of a single twitch. tetanus. C. Force Control of a Motor Unit When all motor units are activated, and they are all activated at a high firing rate, then the muscle is maximally activated. C. Force Control of a Motor Unit Size principle of motor unit recruitment – As the muscle force requirement increases, motor units with progressively larger axons are recruited. – Slow twitch motor units with the lowest activation threshold are selectively recruited during light to moderate effort. – More rapid, powerful movements progressively activate FOG motor units and then FG motor units. IV. Function of Skeletal Muscle A. The Motor Unit B. Types of Motor Units C. Force Control of a Motor Unit. D. Mechanical Factors Which Influence Muscle Force Production Anatomy and Physiology – Open Stax D1. Muscle Length-Tension Relationship When a muscle is stretched or shortened, the length of each individual sarcomere increases or decreases, respectively. Within the range of sarcomere lengths there is an optimal length at which provides for the greatest possible number of cross- bridge formations.  D1. Muscle Length-Tension Relationship Longer or shorter than this optimal length, the number of cross-bridge formations is reduced and thus active force production is reduced. Anatomy and Physiology – Open Stax D1. Muscle Length-Tension Relationship Muscles can be stretched from their resting length for most effective action. – Example - flexing the knee, hip, and ankle joints before jumping. In a movement such as vertical jumping, – preparatory counter movement shifts the force-velocity curve to the right, – causing the leg extensor muscles to exert much higher forces at any angular velocity of the knee in the concentric phase. D1. Muscle Length-Tension Relationship The enhancement of performance in this "stretch-shortening cycle” can be attributed to restitution of elastic energy (passive tension) and stretch reflex potentiation of muscle. Advantage for sprint and distance athletes Reduces force needed from muscle and reduces velocity of contraction which increases force development with contraction.  D2. Muscle Force-Velocity Relationship When the velocity is positive, then the muscle is shortening. Concentric contraction If the velocity is negative, then the muscle is lengthening Eccentric contraction. When the velocity of shortening is zero, there is no change in muscle length. Isometric contraction Anatomy and Physiology – Open Stax D2. Muscle Force-Velocity Relationship When maximally activated, the faster a muscle shortens the less force it produces. The faster it lengthens in the eccentric phase, the more force it produces – likely due to passive elements (Titin) D2. Muscle Power-Velocity Relationship Power is not an independent factor influencing force, rather power is the product of force x velocity. Magali Harvey – Photo by Dean Mouhtaropoulos / Getty ImagesProvince 2014 D2. Muscle Power-Velocity Relationship Maximum power output occurs at: - approximately one half of maximum velocity and one third of maximum concentric force. Power = Force x Velocity D3. Angle of Muscle Pull In the intact human body, the muscles act on the bones about the joints to form a lever system. Optimal position of the muscle and bone together determines the maximum amount of tension that can be developed during a single contraction.  D3. Angle of Muscle Pull When a muscle is pulling at an angle of 90 degrees to a bone, all of the muscle contractile force is acting to rotate the bone around the joint. At angles greater than 90 degrees, the magnitude of the rotational component of the muscle pull force decreases while the magnitude of the stabilizing component of the force increases. D. Mechanical Factors Which Influence Muscle Force Production. In summary, three factors that affect the expression of strength by a muscle are: 1. The initial length of the muscle fibers. 2. Speed of shortening. 3. The angle of pull of the muscle on the bony skeleton.