PSYCH 454 Lecture Notes - Lecture 18: Alpha Motor Neuron, Excitatory Postsynaptic Potential, Neuromuscular Junction
24 May 2018
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Lecture 18, part 2
Alpha Motor Neurons
●2 classes of LMN
○Alpha α motor neurons
○Gamma γ motor neurons (more on these later)
●α motor neurons: cause force generation in your muscle (contraction)
○Alpha motor neuron + all fibers it innervates → called “motor unit”
○Ex: reflecting amount of dexterity needed to control that body part
■Innervates many fibers in motor unit controlling legs
■Innervates only a few fibers in motor unit controlling fingers → offers
fine control
○All alpha motor neurons innervate a muscle called “motor neuron pool”
●Neuromuscular junction NMJ = specialized synapse where alpha motor neurons of
skeletal muscle and nerves meet
●Alpha motor neuron→ released acetylcholine transmitter @ NMJ
○AP in alpha MN leads to acetylcholine release
○aCh causes excitatory postsynaptic potential EPSP in muscle fiber (also called
“endplate potential”)
■Muscle fibers enclosed by excitable cell membrane → called
“sarcolemma”
■EPSP is large enough to cause post-synaptic AP
○Post-synaptic AP causes muscle twitch
■Twitch: fast sequence of contraction and relaxation
Graded Muscle Contraction: from muscle twitch to sustained contraction
●Fast (or slow) motor units: rapidly (or slowly) contracting fibers that fatigue rapidly (or
slowly)
●2 ways to increase muscle contraction by firing rate of individual LMN...
○Increase firing rate of MN
○Recruit more motor units → greater contraction at muscle
●High firing rate causes sustained muscle contraction
○Summation of twitches that cause sustained AP
●Or recruit more motor units
○Smallest motor units recruited first → finer control
○Largest motor units recruited last → based on need; more heavier contraction
for larger weights
Inputs to Alpha Motor Neurons (LMN)
●Synaptic inputs in ventral horn control alpha motor
neurons
●3 inputs to alpha motor neurons
Document Summary
Motor neurons : cause force generation in your muscle (contraction) Gamma motor neurons (more on these later) Alpha motor neuron + all fibers it innervates called motor unit . Ex: reflecting amount of dexterity needed to control that body part. Innervates many fibers in motor unit controlling legs. Innervates only a few fibers in motor unit controlling fingers offers fine control. All alpha motor neurons innervate a muscle called motor neuron pool . Neuromuscular junction nmj = specialized synapse where alpha motor neurons of. Alpha motor neuron released acetylcholine transmitter @ nmj skeletal muscle and nerves meet. Ap in alpha mn leads to acetylcholine release. Ach causes excitatory postsynaptic potential epsp in muscle fiber (also called. Muscle fibers enclosed by excitable cell membrane called. Epsp is large enough to cause post-synaptic ap. Twitch : fast sequence of contraction and relaxation. Graded muscle contraction: from muscle twitch to sustained contraction.
