Carvell, PhD, PTUniversity
LOCAL POTENTIALS I: SYNAPTIC POTENTIALS-
BC&P Fig 5.7p. 106
The Neuromuscular Junction (NMJ) is a
special excitatory synapse.
Unlike most excitatory synapses in the CNS, the connection between an Alpha Motor
Neuron in the spinal cord and
the target muscle fibers in the periphery provides a ‘guaranteed’ transmission of a
signal to move. A Nerve Action Potential
(NAP) produces a Muscle Action Potential (MAP) by action of a chemical messenger
passed between the nerve and muscle.
Under normal physiological conditions, the MAP always results in a contraction of the
innervated, unfatigued muscle fibers
(motor unit contraction). The chemical messenger at the NMJ is Acetylcholine (ACh). Ach is
stored in synaptic vesicles in the
axon terminal. An NAP causes the vesicles to bind to the axon terminal and release Ach into
the synaptic cleft. ACh receptors
localized to the NMJ on the muscle membrane bind the neurotransmitter. This
chemically-gated process opens ion channels to allow Na+and K+ions to pass through the ion pore. This results in a large
depolarization that triggers the MAP.
SYNAPSES PROVIDE CHEMICALLY-GATED CONNECTIVITY
BETWEEN EXCITABLE CELLS (SYNAPTIC POTENTIALS)
PLAY THE MOVIE
shows activation of an Alpha Motor Neuron, initiating
an AP that travel down the Alpha Motor Axon to
three ‘representative’ muscle fibers of the motor unit.
Muscle fibers contract after a Muscle Action Potential
discharges due to synaptic depolarization of each
muscle fiber at the motor end-plate region (NMJ).