Motoneurons structure

Drugs that modify this reflex arc may modulate excitatory or inhibitory synapses (see Chapter 21). Thus, to reduce the hyperactive stretch reflex, it is desirable to reduce the activity of the la fibers that excite the primary motoneuron or to enhance the activity of the inhibitory internuncial neurons. These structures are shown in greater detail in Figure 27-11. 

Of particular interest is a structure-activity study carried out by Shirahama and co-workers which has revealed an order of depolarizing activity as assayed in the newborn rat spinal motoneuron for the naturally occurring kainoids and synthetic analogues shown in Figure 7.31 (Note Figure 7 refers to the kainoid 1 having a cis relative disposition of the C-3 and C-4 substituents.)... 

The smallest structural unit of skeletal musculature is the striated muscle fiber. It contracts in response to an impulse of its motor nerve. In executing motor programs, the brain sends impulses to the spinal cord. These converge on a-motoneurons in the anterior hom of the spinal medulla. Bundled in motor nerves, efferent axons course to skeletal muscles. Simple reflex contractions to sensory stimuli, conveyed via the dorsal roots to the motoneurons, occur without participation of the brain. Neural circuits that propagate afferent impulses into the spinal cord contain inhibitory intemeurons. These serve to prevent a possible overexcitation of motoneurons (or excessive muscle contractions) due to the constant barrage of sensory stimuli. 

The botulinum toxins are large polypeptide molecules (approximately 150 kDa) which contain Zn ions. They are synthesised as single polypeptides which are cleaved by endogenous protease action to yield the active forms, each of which consists of two polypeptide chains linked by a disulphide bond. The larger of the two chains, denoted the heavy (H) chain, has a mass of approximately 95 kDa and the smaller, the light (L) chain, a mass of approximately 50 kDa. The structure is illustrated in Figure 1. The H chain is composed of two domains of similar mass the C-terminal domain (He) is responsible for the specificity of binding of the toxin to peripheral motoneurones and the N-terminal domain (Hn) for penetration of the toxin into the neuronal cell. In the neuronal cytoplasm, the L chain is released by proteolysis and it is this subunit which is responsible for the disruption of the exocytotic apparatus, which causes blockade of transmitter release. 

---