3a. Muscle Contraction

Motor Unit

Functional unit of contractility; comprised of spinal nerves & cord, nerve fiber (axon), and the muscle fiber being innervated

Motor Neuron Pool

All motor neurons innervating a muscle; 1:1 muscle fiber to motor neuron ratio in mature animals, each neuron innervates multiple fibers

Slow twitch fibers

AKA: Type I

Fatigue resistant; high myoglobin count > red coloration, high mitochondria concentration, low glycogen concentration

Intermediate (Fast) Twitch

AKA Type IIa

Fatigue resistant, fewer myoglobin > pink coloration, highest mitochondrial concentration, high glycogen concentration

Fast Twitch

AKA Type IIb

Non-fatigue resistant, no myoglobin > white coloration, fewer mitochondria, high glycogen concentration

Which types of muscle fibers undergo oxidative metabolism?

Slow (I) and Intermediate (IIa) Twitch fibers

What type of metabolism do Fast Twitch muscle fibers undergo?

Glycolytic metabolism

What neurotransmitter is released during muscle contraction

Acetylcholine (ACh); increases membrane permeability to Na ions

As the AP spreads across the surface of the sarcolemma

AChE breaks down ACh, to begin repolarizing the cell

What causes the depolarization of muscle cells?

Opening of Ca-ion voltage gated channel

Where is Ca stored when not in use

The sarcoplasmic reticulum; when released induce contraction of sarcomere

Termination of Muscle Contraction

1. Na-Ca exchange, and Ca-ion pump out Ca ions

2. Ca pump sequesters Ca within sarcoplasmic reticulum

3. Ca is bound in the SR via calreticulin & calsequestin

What is the relationship between muscle size and duration of twitch responses?

The larger the muscle in mass, the longer the duration of the twitch response

Laten period

Time between peak contraction and start of relaxation

How many muscle cells needed to generate a twitch response?

1; single muscle cell produces single twitchT

Tetany

Sustained muscle contraction, due to uniform frequencies

Unfused tetany

Rapid, repetitive contractions with little relaxation between; due to reception of high, unfused frequencies

Fused tetany

Sustained contraction with little relaxation between; due to rapid, repetitive, fused frequencies

Spatial (recruitment) Summation

Multiple, presynaptic neurons release enough ACh to generate an AP

Temporal Summation

Singular neuron releases enough ACh to generate an AP

Isometric contraction

Constant length of muscle maintained, tension placed on fibers increased during activity

Isotonic contraction

Decrease in length when active; increase in upward velocity when baring constant weight

Slow waves

Rhythmic, spontaneous depolarizations (5–15 mV) of gastrointestinal smooth muscle cells that set the maximum frequency of contraction, known as the basic electrical rhythm (BER)

Generation of Slow Waves

1. Voltage-gated Ca-ion channels open

2. AP spike

3. Ca-ion influx, rise in ICM conc.

4. Ca-dependent K-channels open

5. Slow hyperpolarization

6. Ca-ion channels close, decreasing ICM conc.

7. Ca-dependent K-channes close

Cross Bridge cycling in Smooth Muscle

fundamental, repeating process of actin and myosin filaments interacting within muscle cells to produce contraction, using ATP for energy

Summary of Comparisons