Lecture 9: motor units and neuromuscular junctions

What does a motor unit consist of?

alpha motor neuron and the muscles fibers innervated by it

Where are the cell bodies of motor neurons located?

in the CNS within the ventral horn of gray matter of spinal cord

What do the axonal branches of the motor neurons terminate as?

neuromuscular junction

If it is a muscle required for greater dexterity, will you have a bigger or smaller motor unit?

smaller

What are three basic patterns of innervation?

focal, distributed, and myoseptal innervation

Which pattern of innervation is the most common?

focal

What is focal innervation?

one synapse per muscle fiber

What is distributed innervation?

2-5 or more synapses per muscle fiber (birds)

What is myoseptal innervation?

pairs of synapses near the origin and insertion of the muscle (reptiles)

What neurotransmitter is at the NMJ?

ACh

What happens at the NMJ?

1. Ca released from voltage change from depolarization

2. vesicles of ACh fuse with presynaptic membrane and are released into synaptic cleft

3. ACh binds to the nicotinic ligand channels to allow Na to pump in and K to pump out resulting in depolarization (EPP)

4. AP propagates along sarcolemma through the t-tubules triggering release of Ca

What is acetylcholinesterase?

enzyme in the synaptic cleft that breaks down acetylcholine

Why is acetylcholinesterase important?

prevents excessive or uncontrolled contractions

What are the presynaptic components?

1. axon terminal in unmyelinated

2. there is a depression in the synaptic cleft where axon terminal sits

3. vesicles containing ACh wait at the presynaptic cleft

What are synaptic cleft components?

1. numerous secondary folds to increase SA on the synaptic cleft

2. AChE fill the primary and secondary clefts to break down ACh (hydrolyze)

What are the postsynaptic components?

1. nicotinic ACh receptors on the crests of the junctional folds

What can help regenerate clusters of the the ligand gated channels during nerve damage to muscle?

GH

What are activated by muscle but inactivated more slowly than ACh?

nicotine or carbamylcholine

What can antagonists of ACh lead to?

paralysis

How many molecules of ACh bind to the ligand gated Na-K channels?

2

What are characteristics of type I fibers?

slow twitch and high fatigue resistance

What are characteristics of type IIa fibers?

fast twitch and fatigues resistance

What are characteristics of type IIx fibers?

fast twitch and lowest fatigue resistance

True or false: depending on muscle type, there will be different mix of type I and II fibers

true

1 multiple choice option

Which types of fibers have higher peak powers?

type IIx then type IIa then type I

What makes muscles red?

myoglobin

What does myoglobin do?

stores oxygen in muscle cells

What happens when muscle uses more O2?

more CO2, which means more ventilation and higher cardiac output and this depletion of O2 creates a gradient to bring more O2 to the area

What does it mean if there is a slower slope for contraction?

its a slow twitch fiber

What is unfused tetanic force?

muscle relaxes a little bit before contracting again

What are the characteristics of slow twitch?

-contraction times of: 60-70 msec

-low tetanic fusion frequency

-fatigue resistant

-high oxidative capacity

What are the characteristics of fast twitch?

-contraction times of: 30-40 msec

-high tetanic fusion frequency

-more easily fatigued (maintain tetanic contractions for short period of time)

What is the size principle for motor units?

recruitment in order of weakest to strongest

Why are weaker motor units recruited first?

the depolarizing voltage rises to threshold more quickly in the weaker motor units

What determines twitch contraction time?

rate of cross bridge recycling and myosin ATPase activity

Which fibers have greater myosin ATPase activity?

type II

What fiber type has the highest glycogen storage?

type IIx

Which muscle is redder?

muscle with more type I

Differences in fatigue resistance between fiber types is dependent upon what?

mitochondria content

Which fibers are more dependent on aerobic metabolism?

slow twitch type I (have high mitocondria)

Which of the type II fibers has higher mitochondria content?

type IIa

Which fiber has the lowest mitochondria content?

type IIb