Muscles &Muscle Tissue Chapter 9
Function of Muscles • Movement • Posture • Stabilization of Joints • Thermogenesis (heat production)
Functional Characteristics of Muscle • Excitability (irritability) • Can receive and respond to stimuli. • Stimuli can include nerve impulses, stretch, hormones or changes in the chemical environment. • Contractility – the ability to shorten with increasing tension. • Extensibility – the ability to stretch. • Elasticity – the ability to snap back (recoil) to their resting length after being stretched.
Three types of muscle Skeletal Smooth Cardiac
Characteristics of Skeletal Muscle • Striated • Multinucleate (it is a syncytium) • Voluntary • Parallel fibers
A thin myofilament Made of actin, troponin and tropomyosin
Muscle Contraction:the Sliding Filament Theory • Muscle contraction requires: • Stimulus – the generation of an action potential. • Crossbridge formation – interaction between the thick and thin myofilaments. This is triggered by Ca++ ions released from the sarcoplasmic reticulum. • Energy – ATP to energize the myosin molecules.
T- tubules supply the stimulus,Sarcoplasmic Reticulum supplies the Ca++,Mitochondria supply the ATP.
Excitation-Contraction coupling • Stimulus or excitation is required for muscles to contract. • In skeletal muscle, the stimulus is from a motor neuron. • The stimulus is in the form of an action potential. • This action potential starts at the neuromuscular junction (NMJ).
The actual synapse acetylcholine
A Synapse Synaptic vesicles
Graded Muscle Responses • The “twitch” describes just one fiber, it really isn’t how muscles normally work. • Muscles contract smoothly and respond to different levels of demand. • This is accomplished through “graded responses”. There are two ways muscle responses are “graded”
Wave Summation • Wave summation is accomplished by repeated stimuli. • As the rate of stimulus delivery increases, there is less and less time for the fiber to relax between stimuli. • If the stimuli are rapid enough, the muscle fiber will contract completely and remain contracted until the stimulus stops of the muscle fatigues. • This is called tetanus (or tetany).
Recruitment • Since action potentials are “all-or-none” responses, when a fiber is stimulated to tetany, it exerts maximum tension. • To respond to stronger stimuli and thus increase the amount of tension, muscles will recruit more motor units until they reach maximal stimulus and all the motor units are recruited • This continues until they either accomplish their task or fatigue.
Types of contractions • Isometric – means “same length”. Force is developed without measurable movement. • Isotonic – means “same tension” movement is achieved with force or “tension” remaining constant.
Eccentric Contraction This refers to the tension that is applied on a muscle as it lengthens. Example: When you lower the dumbbell you just lifted, you don’t do so by allowing the muscle to just relax. Think about the consequences of that! Eccentric contractions are about 50% more forceful than concentric ones.
Fiber Types • Fast Glycolytic (fast twitch a or type IIB) • few mitochondria, adapted for fast, powerful contraction, large diameter, little myoglobin (white fibers), few capillaries, fatigues rapidly • Fast Oxidative (fast twitch b or type IIA) • many mitochondria, fast contraction, intermediate diameter, lots of myoglobin & capillaries (pink fibers), moderately fatigue resistant • Slow Oxidative (slow twitch or type I) • many mitochondria, slow contraction, smallest in diameter, don’t hypertrophy, lots of myoglobin & capillaries, very fatigue resistant, red fibers