SKELETAL MUSCLE - STRUCTURE AND FUNCTION

Muscle Actions

1. Prime mover - a muscle whose contraction is primarily responsible for a particular movement. Most movements are the result of the contraction of more than one muscle, and frequently a single muscle contributes to the production of two or more movements. 2. Antagonist - muscles that oppose one another upon contraction - biceps and triceps. Antagonists are located on opposite sides of a joint. 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.

Criteria Used To Name Muscles

1. Shape - deltoid (triangular), trapezius, rhomboid, latissimus (wide) 2. Action - various muscle names include the terms flexor, extensor, adductor, or pronator. 3. Location - tibialis anterior, intercostals, pectoralis major 4. Divisions - triceps brachii, quadriceps femoris 5. Size relationships - gluteus maximus, gluteus medius, gluteus minimus. Several names include the terms "brevis" (short), and "longus" (long). 6. Direction of fibers - transversus (across), rectus (straight)

Skeletal Muscle Shapes

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 4. Longitudinal (strap) - fasciculi run parallel to the long axis of the muscle - sartorius, rectus abdominus 5. Radiate - fibers fan out from a single attachment - pectoralis major

A decrease in skeletal muscle mass of _ to _ percent per decade after the age of __ years, is a universal consequence of aging.

A decrease in skeletal muscle mass of 3 to 8 percent per decade after the age of 30 years, is a universal consequence of aging.

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. When the axon is close to the muscle it separates many times (bifurcates) 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 two to three 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. 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.

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. The two major contractile proteins of the sarcomere are actin (thin filament) and myosin (thick filament). Myosin has cross-bridges extending from its thick central core. The most prominent theory of muscle contraction is 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.

Physical characteristics of old age

A significant decline in skeletal muscle mass, strength, and power Decreased Physical Activity Decreased Food Intake

Functional Properties of Skeletal Muscle Tissue

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.

Age-related decrease in skeletal muscle mass has been reported even among healthy, physically active subjects, and the rate of muscle loss has been estimated to range from __-__% per year past the age of ___.

Age-related decrease in skeletal muscle mass has been reported even among healthy, physically active subjects, and the rate of muscle loss has been estimated to range from 1-2% per year past the age of 50.

fiber twitch

All of the muscle fibers in a given motor unit will have identical contractile and metabolic properties. 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. 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.

Muscle Length-Tension Relationship

An isolated muscle can exert its maximal force or tension while in a resting stretched position. As the muscle shortens, less tension can be exerted. When a muscle is stretched or shortened, the length of each individual sarcomere also 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. Longer or shorter than this optimal length, the number of cross-bridge formations is reduced and thus active force production is reduced. Muscles must 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, thus causing the leg extensor muscles to exert much higher forces at any angular velocity of the knee in the concentric phase. The enhancement of performance in this "stretch-shortening" cycle can be attributed to restitution of elastic energy and stretch reflex potentiation of muscle.

Blood Supply

Because muscle tissue can increase its metabolic rate 100x, it requires an abundant blood supply. On average, there are about 3-4 capillaries surrounding each muscle fiber of a sedentary person. Training can induce capillary angiogenesis, resulting in up to 7 capillaries per muscle fiber. 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.

Fasciculi

Fasciculi 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).

Sarcopenia

Greek word meaning "poverty of flesh" Defined as the age-related loss of skeletal muscle mass, strength, and function

Skeletal muscle as an endocrine organ (Functions of skeleton)

In recent years, research has shown that skeletal muscles, particularly contracting skeletal muscles, produce a variety of signalling protein molecules called "myokines" that are secreted into the circulation and have a wide range of beneficial, hormone-like effects throughout the body. There is cross-talk between skeletal muscle and other organs. Myokines may be at least partially responsible for the many beneficial effects that exercise has on the body. Regular exercise reduces the risk for numerous chronic diseases.

Angle of Muscle Pull

In the intact human body, the muscles act on the bones about the joints to form a lever system. 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. In summary, three factors that affect the expression of strength by a muscle are: 1. Speed of shortening. 2. The initial length of the muscle fibers. 3. The angle of pull of the muscle on the bony skeleton.

Sarcopenia markedly enhances:

Loss of physical functioning The risk of disability - up to 4.5 times higher in older persons with sarcopenia than in those with normal muscle tissue and strength More dependency towards other people More medical-care costs

Types of Motor Units

Motor units can be classified in two ways on the basis of speed of contraction (slow twitch & fast twitch), and in 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" (often referred to as type IIb - found in rodents)

Muscle biopsies

Muscle biopsies (micro sampling) are to determine fiber type proportions in humans.

Sarcopenia is caused by various factors:

Neural Apoptosis - neuronal atrophy in areas of the brain responsible for motor control. There is reduced peripheral nerve regeneration and reduced neuromuscular junction remodeling resulting in loss of motor units and subsequent atrophy of muscle fibers. These processes are slowed down by exercise programs. Imbalance of muscle protein synthesis/degradation Decline in mitochondrial function in skeletal muscle Physical inactivity - "If you don't use it, you lose it!" Hormonal imbalance - decreased levels of testosterone, estrogen, growth hormone, etc Food intake - inadequate protein intake, Vitamin D intake, etc.

Not all muscles insert on bone.

Not all muscles insert on bone. Most of the muscles controlling facial expression originate from bone and insert in the skin.

Connective Tissue

The three layers of connective tissue, endomysium, perimysium, and epimysium, surrounding muscle fibers, bundles of muscle fibers (fasciculi), and whole muscle, respectively. They serve in part to maintain intramuscular pressure and, thereby, augment force production.

Force Control of a Motor Unit.

There are two methods by which a muscle can control force production: 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. 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.

Muscle Force-Velocity and Power-Velocity Relations

When maximally activated, the faster a muscle shortens the less force it produces. When the velocity of shortening is zero, this is defined as an isometric contraction (i.e., no change in muscle length). If the velocity is negative, then the muscle is lengthening when it is activated. This is an eccentric contraction. An example of an eccentric contraction is when you slowly lower the barbell after doing a biceps curl. You are still using your biceps, but your muscle is lengthening. In this condition it is possible for the biceps to generate more force than at the opposite speed while shortening. This eccentric type contraction is what is targeted when performing the training technique of plyometrics. Remember, the curve defining the force-velocity relation is based on maximal activation. The vast majority of muscle activations we use are of the submaximal variety and are located well underneath the maximum curve.

Can the age-related loss in muscle mass, and decrease in muscle strength and power be slowed or reversed by exercise training?

YES! However, you can't stop the aging process.

Tendons

are extensions of connective tissue membranes beyond the end of the muscle.

Stabilize joints (Functions of skeleton)

as muscles pull on the bones to cause movement, they stabilize and strengthen the joints of the skeleton

Skeletal muscle

attaches to and moves the skeleton. The contractile molecules are very organized giving skeletal muscle a striated pattern, hence the name, "striated muscle". It is under voluntary control. Skeletal muscle comprises about 36 percent of the total body weight in women and 42 percent in men. 75 percent of skeletal muscle is water, 20 percent is protein, and the remainder consists of inorganic salts, pigments, fats, and carbohydrates.

Smooth muscle

found in the walls of hollow organs and blood vessels. The contractile molecules are not aligned in a set pattern like skeletal muscle, hence the name smooth muscle. This muscle is under involuntary control.

Origin

less movable end of a muscle, usually proximal.

Insertion

more movable end of a muscle, usually distal.

Generate heat to maintain body temperature (Functions of skeleton)

muscles generate heat as they contract. Since skeletal muscle accounts for at least 40% of body mass, this heat is vitally important in maintaining normal body temperature.

Produce skeletal movement (Functions of skeleton)

skeletal muscle contractions pull on tendons attached to the bones and move the bones of the skeleton. Skeletal muscles are responsible for all locomotion and manipulation.

Guard entrances and exits within the body (Functions of skeleton)

skeletal muscle sphincters encircled the openings of the urinary and digestive tracts, and provide voluntary control over swallowing, defecation, and urination.

Maintain posture and body position (Functions of skeleton)

tension in skeletal muscles maintains body posture against the effects of gravitational forces.

Cardiac muscle

the contractile tissue of the heart wall. This muscle has characteristics of both smooth muscle and skeletal muscle. The contractile molecules are organized very much like skeletal muscle. Like smooth muscle, it is also under involuntary control.

Breathing (Functions of skeleton)

the diaphragm is a skeletal muscle

Muscle cells (fibers)

the only cells in the body that have the property of contractility, which allows them to shorten and develop tension.

Support soft tissues (Functions of skeleton)

the skeletal muscles in the abdominal wall support the weight of the visceral organs and protect internal tissues from injury.

Size principle of motor unit recruitment -

there is a recruitment order of motor units as exercise intensity increases. 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, until all the motor units of all types are activated.

Provide nutrient reserves (Functions of skeleton)

when the diet contains inadequate calories, muscle protein can be broken down to provide an energy source.

Belly

widest portion of a muscle, between its origin and insertion.