Skeletal Muscle Physiology

Muscular System Functions Body movement (Locomotion) Maintenance of posture Respiration Diaphragm and intercostal contractions Communication (Verbal and Facial) Constriction of organs and vessels Peristalsis of intestinal tract Vasoconstriction of b.v. and other structures (pupils) Heart beat Production of body heat (Thermogenesis)

Properties of Muscle Excitability: capacity of muscle to respond to a stimulus Contractility: ability of a muscle to shorten and generate pulling force Extensibility: muscle can be stretched back to its original length Elasticity: ability of muscle to recoil to original resting length after stretched

Types of Muscle Skeletal Smooth Cardiac Attached to bones Makes up 40% of body weight Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement Voluntary in action; controlled by somatic motor neurons Smooth In the walls of hollow organs, blood vessels, eye, glands, uterus, skin Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, In some locations, autorhythmic Controlled involuntarily by endocrine and autonomic nervous systems Cardiac Heart: major source of movement of blood Autorhythmic

Connective Tissue Sheaths Connective Tissue of a Muscle Epimysium. Dense regular c.t. surrounding entire muscle Separates muscle from surrounding tissues and organs Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle Endomysium. Loose connective tissue that surrounds individual muscle fibers Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

Nerve and Blood Vessel Supply Motor neurons stimulate muscle fibers to contract Capillary beds surround muscle fibers Muscles require large amts of energy Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

Muscle Tissue Types

Skeletal Muscle Long cylindrical cells Many nuclei per cell Striated Voluntary Rapid contractions Skeletal muscle attaches to our skeleton. *The muscle cells a long and cylindrical. *Each muscle cell has many nuclei. *Skeletal muscle tissue is striated. It has tiny bands that run across the muscle cells. *Skeletal muscle is voluntary. We can move them when we want to. *Skeletal muscle is capable of rapid contractions. It is the most rapid of the muscle types.

Basic Features of a Skeletal Muscle Muscle attachments Most skeletal muscles run from one bone to another One bone will move – other bone remains fixed Origin – less movable attach- ment Insertion – more movable attach- ment

Basic Features of a Skeletal Muscle Muscle attachments (continued) Muscles attach to origins and insertions by connective tissue Fleshy attachments – connective tissue fibers are short Indirect attachments – connective tissue forms a tendon or aponeurosis Bone markings present where tendons meet bones Tubercles, trochanters, and crests

Skeletal Muscle Structure Composed of muscle cells (fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, and multinucleated Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length Develop from myoblasts; numbers remain constant Striated appearance Nuclei are peripherally located

Muscle Attachments

Antagonistic Muscles

Microanatomy of Skeletal Muscle In this unit we will primarily study skeletal muscle. Each muscle cell is called a muscle fiber. Within each muscle fiber are many myofibrils.

Parts of a Muscle

Motor Unit: The Nerve-Muscle Functional Unit A motor unit is a motor neuron and all the muscle fibers it supplies The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500) Muscles that control fine movements (fingers, eyes) have small motor units Large weight-bearing muscles (thighs, hips) have large motor units

Motor Unit: The Nerve-Muscle Functional Unit Muscle fibers from a motor unit are spread throughout the muscle Not confined to one fascicle (bundle of skeletal muscle fibers surrounded by perimysium) Therefore, contraction of a single motor unit causes weak contraction of the entire muscle Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

Motor Unit All the muscle cells controlled by one nerve cell A motor unit is all the muscle cells controlled by one nerve cell. This diagram represents two motor units. Motor unit one illustrates two muscle cells controlled by one nerve cell. When the nerve sends a message it will cause both muscle cells to contract. Motor unit two has three muscle cells innervated by one nerve cell.

Power Output: The Most Physiologically Relevant Marker of Performance Power = work / time = force x distance / time = force x velocity Peak power obtained at intermediate loads and intermediate velocities. Figure from Berne and Levy, Physiology Mosby—Year Book, Inc., 1993.

Three Potential Actions During Muscle Contraction: Biceps muscle shortens during contraction shortening Isometric-muscle does not change its length isometric Most likely to cause muscle injury lengthening Biceps muscle lengthens during contraction

Recall The Motor Unit: motor neuron and the muscle fibers it innervates Spinal cord The smallest amount of muscle that can be activated voluntarily. Gradation of force in skeletal muscle is coordinated largely by the nervous system. Recruitment of motor units is the most important means of controlling muscle tension. Since all fibers in the motor unit contract simultaneously, pressures for gene expression (e.g. frequency of stimulation, load) are identical in all fibers of a motor unit. To increase force: Recruit more M.U.s Increase freq. (force –frequency)

Increased use: strength training Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns. Long term gains almost solely the result of hypertrophy i.e. increased size.

Performance Declines with Aging --despite maintenance of physical activity 100 80 Performance (% of peak) 60 40 Shotput/Discus Marathon 20 Basketball (rebounds/game) 10 20 30 40 50 60 Age (years) D.H. Moore (1975) Nature 253:264-265. NBA Register, 1992-1993 Edition

Number of motor units declines during aging AGE-ASSOCIATED ATROPHY DUE TO BOTH… Individual fiber atrophy (which may be at least partially preventable and reversible through exercise). Loss of fibers (which as yet appears irreversible). Campbell et al., (1973) J Neurol Neurosurg Psych 36:74-182.

Motor unit remodeling with aging Central nervous system Muscle Motor neuron loss AGING Fewer motor units More fibers/motor unit

Mean Motor Unit Forces: FF motor units get smaller in old age and decrease in number S motor units get bigger with no change in number Decreased rate of force generation and POWER!! 225 200 175 Adult Old 150 125 Maximum Isometric Force (mN) 100 75 50 25 FF FI FR S Motor Unit Classification Kadhiresan et al., (1996) J Physiol 493:543-552.

Refresher Course in Muscle Physiology 4/11/2003 Muscle injury may play a role in the development of atrophy with aging. Muscles in old animals are more susceptible to contraction- induced injury than those in young or adult animals. Muscles in old animals show delayed and impaired recovery following contraction-induced injury. By way of introduction, I’ll briefly reiterate some points that John made in the previous talk… Increased susceptibility Decreased ability to recover and prolonged deficits Because muscles are injured repeatedly throughout life, these two observations, along with others provide circumstantial evidence that muscle injury plays a role in the development of atrophy and weakness with aging. Muscles of animals of all ages, except perhaps the oldest-old, can continue to adapt to the habitual level of activity. Following severe injury, muscles in old animals display prolonged, possibly irreversible, structural and functional deficits. EB2003--Susan Brooks

Disorders of Muscle Tissue Muscle tissues experience few disorders Heart muscle is the exception Skeletal muscle – remarkably resistant to infection Smooth muscle – problems stem from external irritants

Disorders of Muscle Tissue Muscular dystrophy – a group of inherited muscle destroying disease Affected muscles enlarge with fat and connective tissue Muscles degenerate Types of muscular dystrophy Duchenne muscular dystrophy Myotonic dystrophy

Disorders of Muscle Tissue Myofascial pain syndrome – pain is caused by tightened bands of muscle fibers Fibromyalgia – a mysterious chronic-pain syndrome Affects mostly women Symptoms – fatigue, sleep abnormalities, severe musculoskeletal pain, and headache

Aerobic Respiration Needs oxygen for respiration Glucose + Oxygen  Carbon Dioxide + Water + Energy Energy=ATP

ATP ATP or adenosine triphosphate is the form of energy that muscles and all cells of the body use. *The chemical bond between the last two phosphates has just the right amount of energy to unhook myosin heads and energize them for another contraction. Pulling of the end phosphate from ATP will release the energy. ADP and a single phosphate will be left over. New ATP can be regenerated by reconnecting the phosphate with the ADP with energy from our food.

Anaerobic Respiration Without oxygen for respiration Glucose  Lactic Acid + Energy Incomplete breakdown of glucose 5% of energy released by aerobic respiration Lactic Acid-produces an oxygen debt because oxygen is needed to oxidize lactic acid (liver) REST

Muscle Fatigue Lack of oxygen causes ATP deficit Lactic acid builds up from anaerobic respiration Muscle fatigue is often due to a lack of oxygen that causes ATP deficit. Lactic acid builds up from anaerobic respiration in the absence of oxygen. Lactic acid fatigues the muscle.

Muscle Fatigue

Muscle Atrophy Weakening and shrinking of a muscle May be caused Immobilization Loss of neural stimulation Muscle atrophy is a weakening and shrinking of a muscle. It can be caused by immobilization or loss of neural stimulation.

Muscle Hypertrophy Enlargement of a muscle More capillaries More mitochondria Caused by Strenuous exercise Steroid hormones Hypertrophy is the enlargement of a muscle. Hypertrophied muscles have more capillaries and more mitochondria to help them generate more energy. Strenuous exercise and steroid hormones can induce muscle hypertrophy. Since men produce more steroid hormones than women, they usually have more hypertrophied muscles.

Steroid Hormones Stimulate muscle growth and hypertrophy Steroid hormones such as testosterone stimulate muscle growth and hypertrophy.

Muscle Tonus Tightness of a muscle Some fibers always contracted Muscle tonus or muscle tone refers to the tightness of a muscle. In a muscle some fibers are always contracted to add tension or tone to the muscle.

Tetany Sustained contraction of a muscle Result of a rapid succession of nerve impulses Tetany is a sustained contraction of a muscle. It results from a rapid succession of nerve impulses delivered to the muscle.

Tetanus This slide illustrates how a muscle can go into a sustained contraction by rapid neural stimulation. In number four the muscle is in a complete sustained contraction or tetanus.

Refractory Period Brief period of time in which muscle cells will not respond to a stimulus The refractory period is a brief time in which muscle cells will not respond to stimulus.

Refractory The area to the left of the red line is the refractory period for the muscle contraction. If the muscle is stimulated at any time to the left of the line, it will not respond. However, stimulating the muscle to the right of the red line will produce a second contraction on top of the first contraction. Repeated stimulations can result in tetany.

Refractory Periods Skeletal Muscle Cardiac Muscle Cardiac muscle tissue has a longer refractory period than skeletal muscle. This prevents the heart from going into tetany. Cardiac Muscle

Isometric Contraction Produces no movement Used in Standing Sitting Posture Isometric contractions produce no movement. They are used in standing, sitting and maintaining our posture. For example, when you are standing muscles in your back and abdomen pull against each other to keep you upright. They do not produce movement, but enable you to stand.

Isotonic Contraction Produces movement Used in Walking Moving any part of the body Isotonic contractions are the types that produce movement. Isotonic contractions are used in walking and moving any part of the body.  

Muscle Spindle

Muscle Spindle Responses

Alpha / Gamma Coactivation

Golgi Tendon Organs

Developmental Aspects: Regeneration Cardiac and skeletal muscle become amitotic, but can lengthen and thicken Myoblast-like satellite cells show very limited regenerative ability Cardiac cells lack satellite cells Smooth muscle has good regenerative ability There is a biological basis for greater strength in men than in women Women’s skeletal muscle makes up 36% of their body mass Men’s skeletal muscle makes up 42% of their body mass

Developmental Aspects: Male and Female These differences are due primarily to the male sex hormone testosterone With more muscle mass, men are generally stronger than women Body strength per unit muscle mass, however, is the same in both sexes

Developmental Aspects: Age Related With age, connective tissue increases and muscle fibers decrease Muscles become stringier and more sinewy By age 80, 50% of muscle mass is lost (sarcopenia) Decreased density of capillaries in muscle Reduced stamina Increased recovery time Regular exercise reverses sarcopenia