Preganglionic neurons system

Finally, the preganglionic neuron may travel to the adrenal medulla and synapse directly with this glandular tissue. The cells of the adrenal medulla have the same embryonic origin as neural tissue and, in fact, function as modified postganglionic neurons. Instead of the release of neurotransmitter directly at the synapse with an effector tissue, the secretory products of the adrenal medulla are picked up by the blood and travel throughout the body to all of the effector tissues of the sympathetic system. 

Figure 9.1 The autonomic nervous system and its effector organs. The efferent pathways of this system consist of two neurons that transmit impulses from the CNS to the effector tissue, preganglionic neuron (solid line), and postganglionic neuron (dashed line). As illustrated, most tissues receive nervous input from both divisions of the ANS the sympathetic and the parasympathetic.

Figure 9.2 Autonomic nerve pathways. All preganglionic neurons release acetylcholine (Ach), which binds to nicotinic receptors (N) on the postganglionic neurons. All postganglionic neurons in the parasympathetic system and some sympathetic postganglionic neurons innervating sweat glands release Ach that binds to muscarinic (M) receptors on the cells of the effector tissue. The remaining postganglionic neurons of the sympathetic system release norepinephrine (NE), which binds to alpha (a) or beta (P) receptors on cells of the effector tissue. The cells of the adrenal medulla, which are modified postganglionic neurons in the sympathetic system, release epinephrine (EPI) and NE into the circulation.

Anatomy of the parasympathetic system. The cell bodies of parasympathetic preganglionic neurons are located in the brainstem and the sacral spinal... 

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. 

The preganglionic neurons of the sympathetic nervous system have their cell bodies in the thoracic and lumbar regions of the spinal cord, termed the thoracolumbar division. The preganglionic neurons of the parasympathetic division have their cell bodies in the brainstem and in the sacral region of the spinal cord, termed the craniosacral division. The cranial part of the parasympathetic nervous system innervates structures in the head, neck, thorax, and abdomen (e.g., the stomach, part of the intestines, and pancreas). The cranial parasympathetic fibers leave the CNS in the oculomotor, facial, glos-... 

The parasympathetic system is exclusively a cholinergic system. The neurotransmitter acetylcholine is produced at both pre- and post-ganglionic nerve cells. Acetylcholine is the neurotransmitter at the preganglionic neurons of the sympathetic system also, but the postganglionic sympathetic stimulation is by norepinephrine. 

The release of epinephrine and norepinephrine from the adrenal medulla is controlled by the sympathetic division of the autonomic nervous system. As discussed in Chapter 18, sympathetic cholinergic preganglionic neurons directly innervate this gland. An increase in sympathetic activity causes increased firing in these neurons, which in turn stimulates the release of epinephrine and norepinephrine from the adrenal medulla. 

Presynaptic H3 heteroreceptors, when activated, inhibit the release of transmitter from several peripheral cholinergic neuron systems, probably preganglionic as well as postganglionic parasympathetic (Table 3). The receptors may be activated by histamine secreted from mast cells. 

Ganglionic transmission Acetylcholine transmits both sympathetic and parasympathetic impulses from the "preganglionic" neurons in the brain and spinal cord to nicotinic ganglionic (N ) receptors on "postganglionic" neurons of the autonomic nervous system. This occurs in sympathetic ganglia, which are located along the spinal cord, and in parasympa-... 

The two most common neurotransmitters released by neurons of the ANS are acetylcholine (Ach) and norepinephrine (NE). Several distinguishing features of these neurotransmitters are summarized in Table 9.3. Nerve fibers that release acetylcholine are referred to as cholinergic fibers and include all preganglionic fibers of the ANS — sympathetic and parasympathetic systems all postganglionic fibers of the parasympathetic system and sympathetic postganglionic... 

The basis for the antihypertensive activity of the ganglionic blockers lies in their ability to block transmission through autonomic ganglia (Fig. 20.2C). This action, which results in a decrease in the number of impulses passing down the postganglionic sympathetic (and parasympathetic) nerves, decreases vascular tone, cardiac output, and blood pressure. These drugs prevent the interaction of acetylcholine (the transmitter of the preganglionic autonomic nerves) with the nicotinic receptors on postsynaptic neuronal membranes of both the sympathetic and parasympathetic nervous systems. 

Parasympathetic neurons The parasympathetic preganglionic fibers arise from the cranial and sacral areas of the spinal cord and synapse in ganglia near or on the effector organs. In both the sympathetic and parasympathetic systems, postganglionic fibers extend from the ganglia to effector organs. 

The preganglionic fibers terminating in the adrenal medulla, the autonomic ganglia (both parasympathetic and sympathetic), and the postganglionic fibers of the parasympathetic division use acetylcholine as a neurotransmitter (Figure 4.2). Cholinergic neurons innervate voluntary muscles of the somatic system and are also found in the CNS. 

Usually excitatory, except for some parasympathetic nerve endings where it is inhibitory (such as the effect on the heart by the vagus nerve). Secreted by many neurons, including those in the motor area of the brain, basal ganglia, skeletal muscle motor neurons, all preganglionic autonomic nervous system neurons, all postganglionic parasympathetic neurons, and some postganglionic sympathetic neurons. 

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