salt bridges to interchain ionic associations may assist in intermediate filament assembly by utilizing electrostatic interactions to stabilize coiled-coil dimers. The head and tail regions of intermediate filament proteins vary in length and amino acid composition, with greater variations in length occurring in the tail regions. Peripherin, unlike keratin IFs, can self-assemble and exist as homopolymers (see polymer). They can also heteropolymerize, or co-assemble, with other type III proteins or the light neurofilament subunit (NF-L) to form intermediate filament networks. Type III proteins like peripherin can exist in different states within a cell. These states include nonfilamentous particles which combine to firm short IFs, or squiggles. These squiggles come together to form long IFs that make up cytoskeletal networks. Studies of network assembly in spreading fibroblasts and differentiating nerve cells show that particles move along microtubules in a kinesin and dynein-dependent manner, and as spreading continues, the particles polymerize into intermediate filaments. In addition to the main species of peripherin, 57 kDa, two other forms have been identified in mice: Per 61 and Per 56. These two alternatives are both made by alternative splicing. Per 61 is created by introducing a 32 amino acid insertion within coil 2b of the α-helical rod domain of peripherin. Per 56 is made by a receptor on exon 9 of the peripherin gene transcript which induces a frameshift and replacement of a 21 amino acid sequence in the C-terminal found on the dominant 57 form with a new 8 amino acid sequence. The functions of these two alternative forms of peripherin are unknown. Per 57 and 56 are normally co-expressed, whereas Per 61 is not found in normal peripherin expression in adult motor neurons. Tissue distribution Peripherin is widely expressed in the cell body and axons of neurons in the peripheral nervous system. These include small-sized root ganglion neurons, lower motor neurons, sensory and motor neurons of the cranial nerves, and autonomic neurons in ganglia and the enteric nervous system. It is also expressed in the central nervous system in a small set of brainstem and spinal cord neurons that have projections toward peripheral structures. Some of these structures include the hypothalamic magnocellular nuclei, pontine cholinergic nuclei, some cerebellar nuclei, and scattered neurons in the cerebral cortex. They can also be found in the ventral horn neurons and in the cholinergic laterodorsal tegmentum (LDT) and pedunculopontine tegmentum (PPT) nuclei. A comparison of peripherin expression