motor unit =
motor neuron and all the muscle fibres it innervates
steps leading up to the stage where ACh is released into neuromuscular junction
- AP arrives at axon terminal
- Ca2+ influx
- vesicles containing ACh fuse with membrane
Activation of ACh receptors
- binding of ACh to the receptors on the muscle end-plate
- opening of ligand-gated cation channels
- predominantly Na+ moves into the muscle cell, depolarising it.
why are the effects of ACh only short-lived? (less than 1ms)
acetylcholinesterase rapidly breaks it down.
conditions for muscle AP to be triggered
if enough ligand (ACh) gated channels are opened, the end-plate potential reaches threshold. Voltage-gated Na+ channels open and AP is triggered.
How does AP reach all the necessary fibres?
AP propagates along the sarcolemma into the t-tubule system
end-plate potential does not always result in an action potential TRUE/FALSE
FALSE. The end-plate potential will ALWAYS result in an action potential, in skeletal muscle
RMP in skeletal muscle
-90mV (very low permeability to Na+)
what happens as the AP propagates through the t-tubule?
because they come into close contact with the sarcoplasmic reticulum, the voltage-gated Ca2+ channels in the SR open and Ca2+ floods into the cytosol
when does contraction end?
when Ca2+ is actively pumped back into the SR via Ca2+ATPase pumps and Ca2+ levels fall
when calcium levels fall, what happens in the sarcomere?
Troponin moves back into position, holding tropomyosin over the myosin-binding site on actin.
- used as an ATP store for very quick bursts of activity (less than 15s)
- Donates P to ADP = ATP
- anaerobic process - mainly in type 2 muscles
- good for short, high workloads (mainly type 2 muscles)
- fast but inefficient
- build up of lactic acid (H+ limits duration to 30-40s)
- 2 ATP net
- important in postrural muscles and endurance exercise (Type 1 muscles)
- efficient but slow
- requires oxygen (good blood supply)
- max 300W
Type I muscle
- slow but efficient aerobic cells
- darker red due to more mitochondria and myoglobin
- needs a good blood supply
Type II muscle
- develop large forces
- fatigue rapidly - fast glycolytic
- larger than type I (think about weightlifters)
- pale appearance
- not many mitochondria
- don't need so much blood
regulation of force in muscles depends on...
1. the number of motor units recruited 2. rate of stimulation of individual motor units
single stimulus leads to...
a single twitch (contraction + relaxation)
time for AP vs. time for twitch
twitch is always longer (100ms) than an AP (2ms)
low stimulation frequency leads to...
unfused tetanus (temporal summation)
If another stimulus is applied, before the muscle relaxes completely, then more tension results
the result of high stimulation frequency, where the tensions summate. there is NO RELAXATION between stimuli, until maximum tension is reached
the bacteria Clostridium tetani causes
all muscles of the body reach tetani at the same time, until you can't breathe.
- another way of increasing muscle tension is to recruit more motor units. The more you recruit the higher the tension
- motoneurons are recruited in order of size i.e. smaller first, then medium, then large
moving from a low intensity (casual jog) to a higher intensity (hill sprint), type _ muscles are recruited before type _. At ____ intensities the forces of the multiple motor units are _______ to create an overall higher force.
moving from a low intensity (casual jog) to a higher intensity (hill sprint), type _1_ muscles are recruited before type _2_. At _high_ intensities the forces of the multiple motor units are _summating_ to create an overall higher force.
X = type I muscle fibre (smaller diameter, darker)
Y = type II (larger diameter, pale)
label the following
1 = sarcoplasmic reticulum
2 = t-tubules
3 = sarcolemma
tetani can occur because...
AP time is shorter than contraction time