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)