生物物理学 II Handout No. 5①②

Figure 14-6 Neurotransmitteris released in fixed increments, or quanta. Each quantumof transmitter produces a unit postsynaptic potential of fixed amplitude.
The amplitude of the postsynaptic
potential evoked by nerve stimulation
is equal to the unit amplitude multiplied by the number of quanta oftransmitter released.
A. Intracellular recordings from a
muscle fiber at the endplate show thepostsynaptic change in potential
when eight consecutive stimuli of thesame size are applied to the motor
nerve. To reduce transmitter output
and to keep the end-plate potentials
small, the tissue was bathed in aCa2+-deficient (and Mg2+-rich)solution. The postsynaptic responses
to the stimulus vary. Two presynaptic
impulses elicit no postsynaptic
response (failures); two produce unit
potentials; and the others produce
responses that are approximately two
to four times the amplitude of the unit
potential. Note that the spontaneous
miniature end-plate potentials (S) are
the same size as the unit potential.
(Adapted from Liley 1956.)
B. After many end-plate potentials were recorded, the number of responses at
each amplitude was counted and then plotted in the histogram shown here. The
distribution of responses falls into a number of peaks. The first peak, at 0 mV,represents failures. The first peak of responses, at 0.4 mV, represents the unit
potential, the smallest elicited response. This unit response is the same
amplitude as the spontaneous miniature-end-plate potentials linsetl. The otherpeaks in the histogram occur at amplitudes that are integral multiples of theamplitude of the unit potential. The red line shows a theoretical distribution
composed of the sum of several Gaussian functions fitted to the data of thehistogram. In this distribution each peak is slightly spread out, reflecting the factthat the amount of transmitter in each quantum, and hence the amplitude of thepostsynaptic response, varies randomly about the peak. The number of events
under each peak divided by the total number of events in the histogram is theprobability that the presynaptic terminal releases the corresponding number ofquanta. This probability follows a Poisson distribution (see Box 14-1 ). The
distribution of amplitudes of the spontaneous miniature potentials, shown in theinset, is also fit by a Gaussian curve. (Adapted from Boyd and Martin 1956.)③Figure 14-8 The events of exocytosis at the presynaptic terminal
are revealed by electron microscopy. The images on the left are
freeze-fracture electron micrographs of the cytoplasmic half (P face) of thepresynaptic membrane (compare Figure 14-7). Thin-section electron micrographs ofthe presynaptic membrane are shown on the right. (Adapted from Alberts et al.
1989.)
A. Parallel rows of intramembranous particles arrayed on either side of an active
zone may be the voltage-gated Ca2+ channels essential for transmitter release.
B. Synaptic vesicles begin fusing with the plasma membrane within 5 ms after thestimulus. Fusion is complete within another 2 ms. Each opening in the plasma
membrane represents the fusion of one synaptic vesicle. In thin-section electron
micrographs, vesicle fusion events are observed in cross section as W -shaped
structures.
C. Membrane retrieval becomes apparent as coated pits form within about 10 safter fusion of the vesicles with the presynaptic membrane. After another 10 s thecoated pits begin to pinch off by endocytosis to form coated vesicles. Thesevesicles include the original membrane proteins of the synaptic vesicle and also
contain molecules captured from the external medium. The vesicles are recycled at
the terminals or are transported to the cell body, where the membrane constituentsare degraded or recycled.

FIGURE 11.24 Release and Retrieval of
Vesicle Membrane monitored by changes
in membrane capacitance. Increases incell capacitance measured with the wholecell patch pipette recording techniqueoccur in a stepwise fashion reflecting thefusion of individual vesicles with the plasma membrane. Corresponding decreases
in capacitance are seen during vesicle
retrieval. The recordings are from a rat
mast cell, which has particularly large
secretory vesicles (800 nm in diameter).
(After Fernandez, Neher, and Gomperts,1984.)