The Muscular System: Ch 8.5-8.8

Control of Muscle Tension Muscle Tone: sustained contraction of individual fibers 1 action potential = muscle twitch Multiple action potentials = enhanced muscle contraction # impulses/second = frequency of stimulation Total tension depends on rate of nerve impulses

Time in milliseconds (msec) Twitch Contraction Contraction period Brief contraction of all muscle fibers in a motor unit Response to a single action potential Three Stages: Latent Period: action potential moving across sarcolemma Contraction Period: repetitive power strokes Relaxation Period: power strokes cease Force of contraction Time in milliseconds (msec) Relaxation period Latent period

Frequency of Stimulation Wave Summation: multiple stimuli arriving before muscle fiber completely relaxes Unfused (incomplete) Tetanus: stimulation = 20-30 times/sec Fused (complete) Tetanus: stimulation = 80-100 times/sec Force of contraction (a) Single twitch (b) Wave summation (c) Unfused tetanus (d) Fused tetanus Time (msec) Action potential

Motor Unit Recruitment The number of contraction motor units increases Motor units fire at different times to prevent muscle fatigue Precise movements requiring little tension involve small motor units Imprecise movements requiring a lot of tension involve large motor units

Cardiac Muscle Only found in the heart Striated and involuntary Fibers are branched, short, and large in diameter Single, centrally located nucleus

Cardiac Muscle Contraction Autorhythmicity: built-in rhythm of the heart Group of cardiac muscle acts as a pacemaker Transmits current via intercalated disks Resting heart rate: ≈ 75 bpm Hormones and neurotransmitters can affect rate of contraction ATP produced mainly by Aerobic CR

Smooth Muscle Found in many internal organs blood vessels Non-striated and involuntary Small in length and diameter Single, centrally located nucleus

Smooth Muscle Thick, Thin, and Intermediate Filaments Dense body Intermediate filament Thick filament Thin filament Relaxed Contracted Thick, Thin, and Intermediate Filaments Thin filaments attach to dense bodies Functionally similar to Z-disc Sliding Filament mechanism generates tension which transfers to intermediate filaments

Two Types of Smooth Muscle Visceral (single unit) Multiunit Muscle Tissue More common Tightly bound together by gap junctions Autorythmic All fibers contract in unison Small arteries/veins and most hollow organs (i.e. stomach, intestines, bladder, etc) Less common Individual fibers w/ own nerve ending Stimulated by nervous system Fibers contract individually Large arteries/veins, airways to the lungs, eye muscles

Smooth Muscle Contraction Very slow and long contraction Calcium ions enter and leave slowly Smooth muscle can shorten an stretch Respond to autonomic nervous system, hormones, and local factors (i.e., pH, O2/CO2, Temp, etc)