hypomyelination

Hypomyelination

(Wobbly Kittens)

Hypomyelination is a disorder of myelin formation. It affects the central nervous system, and is reported in most domestic animals. Although in many cases a genetic basis has been demonstrated, several viruses have been implicated and the possibility of toxic or nutritional factors such as thiamine deficiency, should not be ruled out.

The nervous system contains billions of neurons. These basic building blocks of the nervous system have evolved from primitive neuron effector cells which respond to various stimuli by contracting. Contraction has become the specialised function of muscle cells, while transmission of nerve impulses has become the specialised function of neurons.

NEURONS

A typical spinal motor neuron has 5 -7 processes called dendrites which extend out from the cell body and branch out extensively. It also has along fibrous axon which originates from a somewhat thickened area of the cell body, the axon hillock. A short distance from it's origin, the axon acquires a sheath of myelin, a protein- lipid complex made up of many layers of unit membrane. The myelin sheath envelopes the axon except at its ending and at periodic constrictions about 1mm apart called nodes of Ranvier. The axon ends in a number of synaptic knobs. These knobs contain granules in which the synaptic transmitter secreted by the nerve is stored. Some mammalian neurons and most neurons in the invertebrates are unmyelinated; the axons are invested in Schwann cells, but there has been no rotation of the axon to produce multiple layers of myelin membrane.

Motor neuron with myelinated axon.

Diagrammatic representation of the relation of axons to schwann cell in unmyelinated (left) and Myelinated nerve(right). In the former, the axons are simply buried in the cell. In the latter, the Schwann cell membrane is coiled many times around the axon, forming the multiple layers of membrane that make up myelin.

MYELINATION

Myelinogenesis begins some time after the middle of gestation and continues in the postnatal period for varying times depending on the species. It is more advanced at birth in those species in which the young are able to stand and walk soon after, for it correlates with the overall maturity of the nervous system.

The process requires a complex unfolding of events in order to be successful. In the first place there must be a differentiation of competent myelinating cells in sufficient number, they must migrate too, recognise and contact the target axons appropriately. Secondly it has been clearly shown that the axon itself must send a specific signal to the myelinating cell to initiate it's investment. The diameter of the axon dictates whether or not it is myelinated and how thick the sheath will be. Finally the molecular components of the myelin must be produced and delivered to their correct sites in the membrane.

Myeliation does not occur synchronously throughout the nervous system, but in a distinctly regional sequence. Thus lesions of this type may involve some tracts more than others, or completely spare some tracts. There may be a complete absence of myelin or a reduced quantity, hypomyelination. As would be expected clinical signs are manifested early in life and the very common feature is the onset of a sever generalised tremor syndrome at about three weeks, which may vary in severity. The tremor disappears at rest or during sleep. The gait is erratic and uncoordinated but the young suckle, defaecate, urinate, are bright and grow well.

The deficiency of myelin may in some cases be permanent, while in others it seems that the myelination may be delayed, but eventually proceeds to the extent that clinical deficits resolve by six to eight weeks in kittens, with no evidence of the breakdown of previously formed sheaths.

References:

Review of Medical Psysiology / W.F.Ganong

Pathology of domestic animals/K.V.F.Jubb, P.C. Kennedy, N.C.Palmer