MUSCLE CONTRACTIONPART 4

• For skeletal muscles to contract:• The fiber must be stimulated by a nerve ending

• An action potential must be generated along the sarcolemma

• The action potential must be propagated along the sarcolemma

• Intracellular calcium must rise to trigger contraction

• THE NEUROMUSCULAR JUNCTION• a connection between an axon

terminal and a muscle fiber where stimulation of the muscle cell to contract occurs.

• consists of the plasma membrane of the motor neuron axon terminal, the synaptic cleft, and the motor endplate.

• The motor endplate • part of the sarcolemma where

chemically regulated ion channels that respond to neural stimulation are found.

• Junctional folds increase the surface area at the motor endplate.

• A nerve impulse causes the release of acetylcholine • binds to receptors on the motor end plate, triggering a series of electrical events on the

sarcolemma.

• An action potential opens voltage regulated Ca++ channels in the axon terminal.

• Ca++ influx into the axon stimulates fusion of synaptic vesicles with the axon terminal plasma membrane and the release of neurotransmitter (Ach) in the synaptic cleft.

• Ach diffuses across the synaptic cleft, binds to receptors on the motor endplate, and opens chemically-regulated ion channels in the sarcolemma.

• Ach is broken down by acetylcholine esterase, which terminates stimulation of the sarcolemma

• When acetylcholine binding with receptors opens chemically-regulated ion channels in the sarcolemma Na+ ions enter the cell faster than K+ ions exit, which makes the membrane potential slightly less negative (depolarizes the membrane) This is an end plate potential.

• Positively charged ions move across the inside of the sarcolemma into more negative areas - this is a wave of depolarization. The depolarization can be measured (just like a resting membrane potential) and is referred to as a graded local potential, or in this specific case, an endplate potential.

• Generation of an action potential across the sarcolemma occurs in response to the wave of depolarization reaching a voltage regulated Na+

channel with sufficient strength to open it.

• The degree of depolarization required to open a voltage regulated Na+ channel is called threshold (typically 15 - 20 mV above the resting membrane potential).

• The influx of Na+ through voltage regulated channels opens voltage regulated K+ channels.

• As K+ leaves the cell it becomes repolarized and can be stimulated again.

• EXCITATION-CONTRACTION COUPLING

EXCITATION-CONTRACTION COUPLING

• the sequence of events by which an action potential on the sarcolemma results in the sliding of the myofilaments.

• Ionic calcium in muscle contraction is kept at almost undetectable levels within the cell through the regulatory action of intracellular proteins.

• Muscle fiber contraction follows exposure of the myosin binding sites, and follows a series of events.

• Contraction of a Skeletal Muscle• A motor

unit consists of a motor neuron and all the muscle fibers it innervates. It is smaller in muscles that exhibit fine control.

• The muscle twitch is the response of a muscle to a single action potential on its motor neuron. Note the latent period, the period of contraction, and the period of relaxation on the myogram.

• Skeletal Muscle Contraction• Contraction of a skeletal muscle as a whole depends upon the contraction of

individual skeletal muscle cells.

• An individual skeletal muscle cell will either contract or not contract if it is stimulated.

• This is referred to as the “all or none” response. However, because each muscle consists of a number of individual muscle cells, the contraction of whole muscles can vary.

• The different degree of contraction that can occur in a whole muscle results in graded responses to different degrees of stimuli.

• Graded responses are achieved in two ways:1. Changing the frequency of stimulation

2. Changing the number of muscle cells stimulated to contract

• GRADED MUSCLE RESPONSES• There are three kinds of graded muscle responses:

• wave summation, multiple motor unit summation (recruitment), and treppe.

• Wave summation is generated by increasing the frequency of the stimulus.

• Multiple motor unit summation or recruitment is generated by increasing the strength of the stimulus (increasing the number of motor neurons firing).

• Treppe is its own thing - the response occurs with the frequency and strength of stimulus held constant.

• Muscle Response to Increased Frequency of Stimulation: Wave Summation

• Muscle Response to Stronger Stimuli: Multiple Motor Unit Summation (Recruitment)

• Recruitment of Motor Neurons: The Size Principle

• Tetanus• Contractions of skeletal muscles result from the impulses delivered to them by

nerves. The impulses normally are normally delivered at a high frequency and results in the phenomenon called tetanus

• If only a single impulse or stimulus is delivered to a muscle a contraction occurs and is quickly followed by relaxation of the muscle. This is called a muscle twitch.

• If many impulses or stimuli are delivered to the muscle the muscle contracts but does not have time to relax before it contracts again This is called tetanus.

• If the frequency of stimulation permits the muscle to relax to an even slight degree between contractions, the tetanus is unfused or incomplete.

• If the frequency is so high that relaxation does not occur during contraction, the tetanus is fused or complete.

• Our ability to produce smooth and sustained movements when we use our muscles is the result of tetanus.

•Characteristics of Whole Muscle Contraction• Muscle tone

• is the phenomenon of muscles exhibiting slight contraction, even when at rest, which keeps muscles firm, healthy, and ready to respond

• Isometric & Isotonic Contractions• Isometric

• When muscle does not shorten during contraction

• Isometric Contractions: Tension but no shortening of the muscleoccurs. Energy is still used!

• Isometric contractions generate force without changing the length of the muscle

• Isometric• Example: contractions that serve to keep the body fixed in position as

in

• 1. maintaining posture,

• 2. maintaining balance,

• 3. fixing a proximal joint so a distal joint may move,

• 4. Maintaining muscle tone.

• Isotonic• When muscles shorten during contraction but tension

on the muscle remains constant throughout the contraction

• Tension produced and overall shortening of the muscleas a load is moved through the range of motion of the joint.

• Isotonic contractions serve to bring about movement or change in body position. Example = flexion, extension, adduction, abduction, etc.

• Isotonic contractions generate force by changing the length of the muscle and can be concentric contractions or eccentric contractions.

• A concentric contraction causes muscles to shorten, thereby generating force. • Concentric (Of a motion),

in the direction of contraction of a muscle. (E.g., extension of the lower arm via the elbow joint while contracting the triceps and other elbow extensor muscles

• Eccentric contractions cause muscles to elongate in response to a greater opposing force. • Eccentric --Against or in the opposite

direction of contraction of a muscle. • (E.g., flexion of the lower arm

(bending of the elbow joint) by an external force while contracting the triceps and other elbow extensor muscles to control that movement.

• Isotonic contractions • result in movement

occurring at the joint and a change in the length of muscles (the force remains constant).• Concentric isotonic

contractions –• The muscle shortens as

it moves the load

• Eccentric isotonic contractions• The muscle lengthens as

it resists the load

• Isometric contractions • result in increases in muscle

tension, but no lengthening or shortening of the muscle occurs.

Energetics of Muscle Contraction• Sources of Energy for Muscle Contraction

• Energy is needed for • Cross-bridge pulling actin

• To pump calcium from the sarcoplasm to the sarcoplasmic reticulum after contraction

• Pumping sodium-potassium

• Concentration of ATP in the muscle fiber sufficient to maintain contraction for only 1 to 2 seconds

• ATP is split to form ADP which transfers the energy from the ATP to the contracting machinery

• ADP is rephosphorylated to form new ATP

• Three sources of energy for rephosphorylation• Phosphocreatine- similar to ATP

• Glycolysis of glycogen stored in muscle• Breakdown to pyruvic acid and lactic acid

• Can occur without oxygen (anaerobic)

• Oxidative metabolism • More than 95% of all energy used by the muscles for sustained, long-term contraction is

derived by this mechanism

• Muscle Metabolism• Muscles contain very little stored ATP, and consumed ATP is replenished

rapidly through phosphorylation by creatine phosphate, glycolysis and anaerobic respiration, and aerobic respiration.

• Muscles will function aerobically as long as there is adequate oxygen, but when exercise demands exceed the ability of muscle metabolism to keep up with ATP demand, metabolism converts to anaerobic glycolysis.

• Muscle fatigue is the physiological inability to contract due to the shortage of available ATP.

• Oxygen debt is the extra oxygen needed to replenish oxygen reserves, glycogen stores, ATP and creatine phosphate reserves, as well as conversion of lactic acid to pyruvic acid glucose after vigorous muscle activity.

• Heat production during muscle activity is considerable. It requires release of excess heat through homeostatic mechanisms such as sweating and radiation from the skin.

Force of Muscle Contraction

As the number of muscle fibers stimulated increases, force of

contraction increases.

Large muscle fibers generate more force than smaller muscle

fibers.

As the rate of stimulation increases, contractions sum up,

ultimately producing tetanus and generating more force.

There is an optimal length-tension relationship when the

muscle is slightly stretched and there is slight overlap

between the myofibrils.

• Velocity and Duration of Contraction• There are three muscle fiber types: slow oxidative fibers, fast oxidative fibers,

and fast glycolytic fibers.

• Muscle fiber type is a genetically determined trait, with varying percentages of each fiber type in every muscle, determined by specific function of a given muscle.

• As load increases, the slower the velocity and shorter the duration of contraction.

• Recruitment of additional motor units increases velocity and duration of contraction.

• Adaptations to Exercise

• Aerobic, or endurance, exercise promotes an increase in capillary penetration, the number of mitochondria, and increased synthesis of myoglobin, leading to more efficient metabolism, but no hypertrophy.

• Resistance exercise, such as weight lifting or isometric exercise, promotes an increase in the number of mitochondria, myofilaments and myofibrils, and glycogen storage, leading to hypertrophied cells.

• Fast vs Slow Muscle Fibers• Every muscle of the body is composed of a mixture of fast and slow muscle

fibers• Slow Fibers (Type 1, Red Muscle)

• Smaller than fast fibers

• Have more extensive blood vessel system and more capillaries to supply extra amounts of oxygen

• Have great numbers of mitochondria to support high levels of oxidative metabolism

• Respond more slowly but with prolonged contraction

• Contain large amounts of myoglobin, an iron containing protein. It combines with oxygen and stores it until needed (greatly speeds oxygen transport to the mitochondria. Gives the muscle a red appearance

• Example: soleus

• Fast Fibers (Type 2, white muscle)• Muscles that react rapidly• Large for great strength of contraction• Extensive sarcoplasmic reticulum for rapid release of calcium ions to initiate

contraction• Presence of large amounts of glycolytic enzymes for rapid release of energy

by the glycolytic process• Have less extensive blood supply because oxidative metabolism is of

secondary importance• Have fewer mitochondria.• Deficient in myoglobin gives it white appearance• Example: anterior tibialis