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
Twitch Contraction
- 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
ceaseFo
rce
of c
ontr
actio
nTime in milliseconds
(msec)
Contraction period
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
Forc
e of
con
trac
tion
(a) Single twitch (b) Wave summation (c) Unfused tetanus (d) Fused tetanus
Time (msec)
Action potentialAction 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
- Thin filaments attach to dense bodies
- Functionally similar to Z-disc
- Sliding Filament mechanism generates tension which transfers to intermediate filaments
Dense body
Intermediate filament
Thick filament
Thin filament
Relaxed Contracted
Two Types of Smooth Muscle
Visceral (single unit)- 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)
Multiunit Muscle Tissue- 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)