Preganglionic axons

In the autonomic ganglia, the varicosities in the terminal branches of the preganglionic axons come into close contact primarily with the dendrites of the ganglionic cells and make synaptic connection with them. 

Following synthesis, the ACh is taken up and stored in synaptic vesicles (Fig. 8-5). The ACh is protected from metabolic degradation in storage until it is released into the synaptic cleft after stimulation of the preganglionic axon. The synaptic vesicles ordinar-... 

Figure 2.12 From voltage-clamp to current-clamp micro-electrode recordings of synaptic current (/, lower trace) and synaptic potential with superimposed action potential (V, upper trace) from a neuron in an isolated rat superior cervical sympathetic ganglion following a single stimulus (S) applied to the preganglionic nerve trunk. The interval between the stimulus and the postsynaptic response includes the conduction time along the unmyelinated axons of the preganglionic nerve trunk. (SJ Marsh and DA Brown, unpublished)...

The efferent pathways of the ANS consist of two neurons that transmit impulses from the CNS to the effector tissue. The preganglionic neuron originates in the CNS with its cell body in the lateral horn of the gray matter of the spinal cord or in the brainstem. The axon of this neuron travels to an autonomic ganglion located outside the CNS, where it synapses with a postganglionic neuron. This neuron innervates the effector tissue. 

Anatomical differences between the peripheral somatic and autonomic nervous systems have led to their classification as separate divisions of the nervous system. These differences are shown in Figure 9.1. The axon of a somatic motor neuron leaves the CNS and travels without interruption to the innervated effector cell. In contrast, two neurons are required to connect the CNS and a visceral effector cell of the autonomic nervous system. The first neuron in this sequence is called the preganglionic neuron. The second neuron, whose cell body is within the ganglion, travels to the visceral effector cell it is called the postganglionic neuron. 

Additionally, functional nicotinic receptors have been found on the preganglionic terminals of embryonic chick ganglia (195) and on the axon terminals of postganglionic superior cervical ganglion neurons in culture, where it was found that receptors on terminals differ in agonist sensitivity from the somatic ones (196). 

The synaptic transmission mentioned previously is the mechanism by which a nerve impulse is sent across the synaptic cleft (Fig. 8-2). Unlike the electrical mechanism in die axon, it is a chemical process. The preganglionic fibers of both systems release ACh at their endings when stimulated it is the neurotransmitter (neurohormone) at this point. The chemical transmitter replaces cable transmission here. ACh, previously synthesized in the nerve... 

Some of the ACh molecules successfully diffuse across the cleft and bind to a special area on the surface membrane of the dendrite of the adjacent cells that can be referred to as a chemoreceptor.4 The binding of the ACh with the receptor changes the membrane s permeability, which causes depolarization and finally a new action potential. As this newly generated impulse (which is actually a continuation of the preganglionic impulse) passes down the axon and reaches the terminal, it stimulates the release of another chemical mediator (or the same). It in turn may combine with receptors in the next cell—either another nerve cell or one in the organ or tissue innervated by the fiber. Such chemical transmission at the synapse is believed to be a general phenomenon in mammals. An alternate mechanism of total electrical impulse transmission where pre- and postsynaptic membranes touch is, of course, conceivable and has been found to exist in lower life forms, such as the crayfish. 

Schafer, T., Scwab, M.E. and Thoenen, H. (1983) Increased formation of preganglionic synapses and axons due to a retrograde trans-synaptic action and NGF in rat sympathetic nervous system. 7. Neurosci. 3 1501-1510. 

The cells that give rise to the preganglionic fibers of this division lie mainly in the intermediolat-eral columns of the spinal cord and extend from the first thoracic to the second or third lumbar segment. The axons from these cells are carried in the anterior (ventral) nerve roots and synapse with neurons lying in sympathetic ganglia outside the cerebrospinal axis. Sympathetic ganglia are found in three locations paravertebral, prevertebral, and terminal. 

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