Patterns of innervation and control

Most structures under autonomic control are innervated by sympathetic nerves and parasympathetic nerves. The relative influence of sympathetic and parasympathetic nerves depends on the organ under consideration.

In many organs that receive dual innervation, the influence of sympathetic nerves opposes that of parasympathetic nerves. For example, in the heart, sympathetic nerves increase heart rate, whereas parasympathetic nerves slow heart rate (Fig. 13–1).

In some organs that receive nerves from both divisions of the autonomic nervous system, the effects of sympathetic and parasympathetic nerves are complementary, rather than opposite. For example, in the male reproductive system, erection is regulated by parasympathetic nerves while ejaculation is controlled by sympathetic nerves. If attempts at reproduction are to succeed, cooperative interaction of both systems is needed.

A few structures under autonomic control receive innervation from only one division. The principal example is blood vessels, which are innervated exclusively by sympathetic nerves.

In summary, there are three basic patterns of autonomic innervation and regulation:

Feedback regulation

Feedback regulation is a process that allows a system to adjust itself by responding to incoming information. Practically all physiologic processes are regulated at least in part by feedback control.

Figure 13–2 depicts a feedback loop typical of those used by the autonomic nervous system. The main elements of this loop are (1) a sensor, (2) an effector, and (3) neurons connecting the sensor to the effector. The purpose of the sensor is to monitor the status of a physiologic process. Information picked up by the sensor is sent to the central nervous system (spinal cord and brain), where it is integrated with other relevant information. Signals (instructions for change) are then sent from the central nervous system along nerves of the autonomic system to the effector. In response to these instructions, the effector makes appropriate adjustments in the process. The entire procedure is called a reflex.

Autonomic tone

The term autonomic tone refers to the steady, day-to-day influence exerted by the autonomic nervous system on a particular organ or organ system. Autonomic tone provides a basal level of control over which reflex regulation is superimposed.

When an organ is innervated by both divisions of the autonomic nervous system, one division—either sympathetic or parasympathetic—provides most of the basal control, thereby obviating conflicting instruction. Recall that, when an organ receives nerves from both divisions of the autonomic nervous system, those nerves frequently exert opposing influences. If both divisions were to send impulses simultaneously, the resultant conflicting instructions would be counterproductive (like running heating and air conditioning simultaneously). By having only one division of the autonomic nervous system provide the basal control to an organ, conflicting signals are avoided.

The branch of the autonomic nervous system that controls organ function most of the time is said to provide the predominant tone to that organ. In most organs, the parasympathetic nervous system provides the predominant tone. The vascular system, which is regulated almost exclusively by the sympathetic nervous system, is the principal exception.

Parasympathetic nervous system

Pharmacologically relevant aspects of parasympathetic anatomy are shown in Figure 13–3. Note that there are two neurons in the pathway leading from the spinal cord to organs innervated by parasympathetic nerves. The junction (synapse) between these two neurons occurs within a structure called a ganglion. (A ganglion is simply a lump created by a group of nerve cell bodies.) Not surprisingly, the neurons that go from the spinal cord to the parasympathetic ganglia are called preganglionic neurons, whereas the neurons that go from the ganglia to effector organs are called postganglionic neurons. The anatomy of the parasympathetic nervous system offers two general sites at which drugs can act: (1) the synapses between preganglionic neurons and postganglionic neurons and (2) the junctions between postganglionic neurons and their effector organs.

Sympathetic nervous system

Pharmacologically relevant aspects of sympathetic nervous system anatomy are illustrated in Figure 13–3. As you can see, these features are nearly identical to those of the parasympathetic nervous system. Like the parasympathetic nervous system, the sympathetic nervous system employs two neurons in the pathways leading from the spinal cord to organs under its control. As with the parasympathetic nervous system, the junctions between those neurons are located in ganglia. Neurons leading from the spinal cord to the sympathetic ganglia are termed preganglionic neurons, and neurons leading from ganglia to effector organs are termed postganglionic neurons.

The medulla of the adrenal gland is a feature of the sympathetic nervous system that requires comment. Although not a neuron per se, the adrenal medulla can be looked on as the functional equivalent of a postganglionic neuron of the sympathetic nervous system. (The adrenal medulla influences the body by releasing epinephrine into the bloodstream, which then produces effects much like those that occur in response to stimulation of postganglionic sympathetic nerves.) Because the adrenal medulla is similar in function to a postganglionic neuron, it is appropriate to refer to the nerve leading from the spinal cord to the adrenal as preganglionic, even though there is no ganglion, as such, in this pathway.

As with the parasympathetic nervous system, drugs that affect the sympathetic nervous system have two general sites of action: (1) the synapses between preganglionic and postganglionic neurons (including the adrenal medulla), and (2) the junctions between postganglionic neurons and their effector organs.

Somatic motor system

Pharmacologically relevant anatomy of the somatic motor system is depicted in Figure 13–3. Note that there is only one neuron in the pathway from the spinal cord to the muscles innervated by somatic motor nerves. Because this pathway contains only one neuron, peripherally acting drugs that affect somatic motor system function have only one site of action: the neuromuscular junction (ie, the junction between the somatic motor nerve and the muscle).

Primary receptor types: cholinergic receptors and adrenergic receptors

There are two basic categories of receptors associated with the peripheral nervous system: cholinergic receptors and adrenergic receptors. Cholinergic receptors are defined as receptors that mediate responses to acetylcholine. These receptors mediate responses at all junctions where acetylcholine is the transmitter. Adrenergic receptors are defined as receptors that mediate responses to epinephrine (adrenaline) and norepinephrine. These receptors mediate responses at all junctions where norepinephrine or epinephrine is the transmitter.

Subtypes of cholinergic and adrenergic receptors

Not all cholinergic receptors are the same; likewise, not all adrenergic receptors are the same. For each of these two major receptor classes there are receptor subtypes. There are three major subtypes of cholinergic receptors, referred to as nicotinic_N, nicotinic_M, and muscarinic.* And there are four major subtypes of adrenergic receptors, referred to as alpha₁, alpha₂, beta₁, and beta₂.

In addition to the four major subtypes of adrenergic receptors, there is another adrenergic receptor type, referred to as the dopamine receptor. Although dopamine receptors are classified as adrenergic, these receptors do not respond to epinephrine or norepinephrine. Rather, they respond only to dopamine, a neurotransmitter found primarily in the central nervous system.