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Stretching the muscles of the buccal mass of Aplysia evoked proprioceptive reflexes. These consisted of a direct reflex in which the stretched muscle contracted and a crossed reflex in which the contralateral homolog of the stretched muscle contracted as well. Both reflexes were accompanied by corresponding changes in neural activity in the buccal nerves. The muscle contraction and efferent neural activity were abolished by blocking synaptic transmission in the buccal ganglia. Blocking neuromuscular transmission blocked the contractions but not the stretch-induced afferent neural activity. Proprioceptive responses were obtained from isolated buccal nerve-muscle preparations. Both tonic on- and on-off responses were observed. These responses persisted after blocking synaptic transmission at the muscle, indicating that they were due to afferent fibers rather than peripheral interneurons. Proprioceptive neurons with centrally located cell bodies were found. These included previously identified neurons B4 and B5 as well as small cells. Proprioceptive neurons responded to muscle stretch with peripherally initiated axonal spikes that conducted into the central nervous system (CNS) and preceded their somatic spikes. These responses persisted after blocking synaptic transmission in the CNS. Several motor neurons were found. When intracellularly stimulated, these evoked contractions of their target muscle even after blocking synaptic transmission in the CNS. The motor neurons responded synaptically to stretching the ipsilateral muscle. Some responded to stretching of the contralateral homologous muscle as well. The motor neurons differed in their axonal projections, with some projecting only ipsilaterally, others bilaterally. The majority of motor neurons were inhibited by muscle stretch due to inhibitory monosynaptic input from the proprioceptive cells B4 and B5. The stretch reflex occurred when the motor neurons fired due to postinhibitory rebound. The synaptic organization of the reflex was considered.

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