the sarcolemma is the component of the neuromuscular (myoneural) junction that is considered the presynaptic membrane.
the sarcolemma is the component of the neuromuscular (myoneural) junction that is considered the presynaptic membrane.
which component of the neuromuscular (myoneural) junction is considered the presynaptic membrane?
the sarcolemma
release of acetylcholine vesicles from the presynaptic terminal occurs only after adequate depolarization of the efferent nerve. once a motor nerve action potential reaches the presynaptic nerve terminal it causes an increase in intracellular calcium concentration by causing an increase in ion conductance through voltage gated calcium channels. this increase in calcium concentration allows the acetylcholine vesicles to fuse with the plasma membrane at the presynaptic membrane, in a process called exocytosis, thus releasing acetylcholine into the synapse. once acetylcholine is present in the synapse it is able to bind to nicotinic acetylcholine receptors increasing conductance of certain cations, sodium and potassium in the postsynaptic membrane and producing an excitatory end т и ироооurrent. as cations flow into the postsynaptic cell, this causes a depolarization, as the membrane voltage increases above normal resting potential. if the signal is of sufficient magnitude, than an action potential will be generated post synaptically. the action potential will propagate through the sarcolemma to the interior of the muscle fibers eventually leading to an increase in intracellular calcium levels and subsequently initiating the process of excitation–contraction coupling. once coupling begins it allows the sarcomeres of the muscles to shorten, thus leading to the contraction of the muscle.
neuromuscular junction disease
the release of neurotransmitter is accomplished by the fusion of neurotransmitter vesicles to the presynaptic membrane. although the details of this mechanism are still being studied there is a consensus on some details of the process. synaptic vesicle fusion with the presynaptic membrane is known to require a local increase of calcium from as few as a single, closely associated calcium channels and the formation of highly stable snare complexes. one prevailing model of synaptic vesicle fusion is that snare complex formation is catalyzed by the proteins of the active zone such as munc18, munc13, and rim. the formation of this complex is thought to "prime" the vesicle to be ready for vesicle fusion and release of neurotransmitter (see below: releasable pool). after the vesicle is primed then complexin binds to the snare complex this is called "superprimed". the vesicles that are superprimed are within the readily releasable pool (see below) and are ready to be rapidly released. the arrival of an action potential opens voltage gated calcium channels near the snare/complexin complex. calcium then binds to changes the conformation of synaptotagmin. this change in conformation of allows synaptotagmin to then dislodge complexin, bind to the snare complex, and bind to the target membrane. when synaptotagmin binds to both the snare complex and the membrane this induces a mechanical force on the membrane so that it causes the vesicle membrane and presynaptic membrane to fuse. this fusion opens a membrane pore that releases the neurotransmitter. the pore increases in size until the entire vesicle membrane is indistinguishable from the presynaptic membrane.
active zone