1. The Muscular System: Ch 8.5-8.8

2. 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

3. 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

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

5. 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

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

7. 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

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

9. 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

10. 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

12. 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)