Intramembranous particles at the nodes of Ranvier of the cat spinal cord: a morphometric study.

Size and distribution of intramembranous particles at nodes of Ranvier of the cat spinal cord were investigated by the freeze-etching technique and compared with those at the internodal axon. The particles are larger (up to 20 nm) at the nodal than at the internodal segment (up to 13 nm), and these large particles are more densely packed in the nodal (400 per sq micrometer) than in the internodal E (external) face (4 per sq micrometer). The nodal E face reveals a much denser overall population (1200--1300 per sq micrometer) of particles than the internodal E face (100--200 per sq micrometer), while at the P (protoplasmic) face the particle density is similar in nodal and internodal segments (1200--1600 per sq micrometer). It is suggested that the large nodal particles may be related to the mechanism of nerve excitation.