Boyan G S
Molecular Neurobiology Group, Research School of Biological Sciences, Australian National University, Canberra City.
J Neurosci. 1988 Aug;8(8):2748-57. doi: 10.1523/JNEUROSCI.08-08-02748.1988.
The paired cerci located at the tip of the locust abdomen bear a large number of wind-sensitive filiform hairs, each of which sends an axon via the cercal nerve into the terminal ganglion of the CNS. The filiform afferents fire bursts of action potentials when their hairs are displaced by wind or mechanical stimuli. Filiform axon terminals in the CNS are depolarized concomitantly with the discharge of another type of unit (a primary afferent-depolarizing, or PAD, unit) recorded in the cercal nerve. The instantaneous spike frequency of PAD unit discharges matches the evoked depolarization very closely, and during such depolarizations spike amplitudes in the filiform afferent terminals are reduced by up to 55%. Depolarizing current pulses injected into the axonal terminals of an identified filiform afferent evoke spikes that are blocked by the PAD unit, probably via an intercalated interneuron. The PAD unit makes a monosynaptic connection with only one of the 4 giant interneurons (GIN 2) on each side of the terminal ganglion, and indirect connections with 2 others. Depolarizing current pulses injected into the neuropilar segments of GINs evoke fewer spikes when the PAD unit is active, consistent with the PAD unit's mediation of conductance changes in postsynaptic cells. Iontophoretic injection of Lucifer yellow shows the PAD unit to be an afferent with axon terminals overlapping those of filiform afferents and posteriorly directed branches of interneurons such as GIN 2 in the CNS. Passive movements of a cercus, monitored with a position transducer, show that the PAD unit fires discrete bursts during cercal displacement. The PAD unit most probably has its soma and dendrites in tissue spanning the cercal base. By responding to cercal movements sufficient to also activate filiform hairs, and by mediating conductance changes in both the presynaptic terminals of filiform afferents and the postsynaptic membranes of interneurons, the PAD unit desensitizes a pathway to movement-generated afferent input, and ensures that the locust remains sensitive to external wind stimuli.
位于蝗虫腹部末端的一对尾须上长有大量对风敏感的丝状毛，每根丝状毛都通过尾须神经向中枢神经系统的终神经节发送一条轴突。当它们的毛被风或机械刺激移位时，丝状传入神经会发放动作电位脉冲。中枢神经系统中丝状轴突终末伴随着在尾须神经中记录到的另一种类型的单元（初级传入去极化，或PAD，单元）的放电而发生去极化。PAD单元放电的瞬时尖峰频率与诱发的去极化非常紧密地匹配，并且在这种去极化期间，丝状传入终末的尖峰幅度降低多达55%。注入已识别的丝状传入神经轴突终末的去极化电流脉冲诱发的尖峰被PAD单元阻断，可能是通过一个插入的中间神经元。PAD单元仅与终神经节每一侧的4个巨型中间神经元（GIN 2）中的一个形成单突触连接，并与另外2个形成间接连接。当PAD单元活跃时，注入GINs神经纤维节段的去极化电流脉冲诱发的尖峰较少，这与PAD单元介导突触后细胞电导变化一致。离子电泳注射路西法黄显示，PAD单元是一种传入神经，其轴突终末与丝状传入神经以及中枢神经系统中如GIN 2等中间神经元的向后分支的轴突终末重叠。用位置传感器监测尾须的被动运动表明，PAD单元在尾须移位期间发放离散的脉冲。PAD单元很可能在跨越尾须基部的组织中有其胞体和树突。通过对足以激活丝状毛的尾须运动做出反应，并通过介导丝状传入神经的突触前终末和中间神经元的突触后膜的电导变化，PAD单元使一条通向运动产生的传入输入的通路脱敏，并确保蝗虫对外部风刺激保持敏感。
