A-Motoneurons

Both TeNT and BoNTs bind the presynaptic membrane of a-motoneurons, but then TeNT follows a different intracellular trafficking route and this must be determined by yet unidentified specific receptor(s). 

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. 

Figure 7.2. Stractures of synapses, a Some anatomical variations. Left An intemeural synapse connects two nerve cells. Middle A neuromuscular synapse (also called motor endplate) connects the terminal button of a motoneuron to a skeletal muscle cell. Right An automic nerve cell may form several secretory varicosities that release the transmitter in the target tissue, without forming a circtrmscribed contact zone with an individtral target cell, b Electron microcoscopy of a nemomnsctrlar synapse. The synaptic cleft is very narrow, so that diffusion of secreted transmitter to the receptors on the postsynaptic membrane is fast. Ntrmerous vesicles line up close to the presynaptic membrane. Note the striated myofilament in the postsynaptic cell, c Light microscopy. A single nemon branches out and sends terminal buttorrs to multiple muscle fibers.

GABA (y-aminobutyric acid) is derived from glutamate by decarboxylation. It occurs predominantly in inhibitory synapses in the central nervous system, as does glycine. Tetanus toxin acts on glycinergic neurons in the spinal cord, which exert inhibitory regulation on the a-motoneurons. ... 

Within the skeletal muscle. These neurons - the a-motoneurons - are not part of the autonomic but of the somatic system. 

When an action potential traveling down the axon of a motoneuron reaches the myoneural endplate, a process occurs that releases acetylcholine into the synaptic cleft and consequently depolarizes the postsynaptic membrane. A similar process probably occurs at cholinergic synapses in the central nervous system. In 1950 Fatt and Katz discovered a spontaneous subthreshold activity (MEPP) of motor nerve endings and were thereby led to the concept that acetylcholine is released in definite units (quanta) of 10 to 10 molecules. Electron microscopy subsequently revealed characteristic vesicles about 40 nm in diameter, clustered near presynaptic membranes. Subcellular fractionation procedures were devised by Whittaker and de Robertis for the isolation of these vesicles from brain homogenates in sucrose density gradients, and it was soon demonstrated that they were indeed concentrated reservoirs of acetylcholine. The hypothesis that the vesicles discharge the quanta of transmitter became irresistible. 

Hounsgaard, J., H. Hultbom, B. Jespersen O. Kiehn. 1988. Bistability of a-motoneurones in the decerebrate cat and in the acute spinal cat after intravenous 5-hydroxytryptophan. J. Physiol. 405 345-67. 

Channel capacity The maximum rate of information flow through a specific pathway from source to receiver. In the context of the human, a sensorimotor pathway (e.g., afferent sensory nerves, processors, descending cerebrospinal nerve and a-motoneuron) is an example of a channel through which motor control information flows from sensors to actuators (muscle). 

As a result, the la-feedback signal changes, effecting a corresponding change in the excitation of the so-called a-motoneurons. The spindle control loop is active. This process can be described as conditioned feedback. The disturbance is essentially compensated (servo assistance within the 7-loop). 

This excitation is fed back negatively to the a-motoneuron via the so-called Ib-fibers and an inhibitory interneuron I. 

The central nervous system controls the muscles through two sets of coordinated efferent systems, the a- and y-motoneurons [26, 169, 170] (Figure 5.5). The contraction of skeletal muscle is controlled directly by the a-motoneurons alpha drive is used where speed of movement is essential, and... 

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