Central Nervous System
Parasympathetic Nervous System (PNS)
Fig 1 below shows preganglionic nerves emerging from the brainstem via cranial nerves III, VII, IX and X and from sacral segments S2 to S4 of the spinal cord.
The parasympathetic ganglia associated with cranial nerves lll, Vll, lX and X and the sacral nerves are shown as circular structures.
In many cases the ganglia associated with the vagus and the sacral segments of the parasympathetic nervous system are within the target organ. Thus, for example, the ganglion for the branch of the vagus nerve (cranial nerve X) which supplies the heart is buried within the heart muscle. The postganglionic nerves are therefore short.
Most of the parasympathetic motor output occurs via the vagus nerve.
Further details of the structure of the autonomic nervous system will be discussed in the Anatomy module of this programme.
Fig 1 The parasympathetic nervous system. Preganglionic nerves emerging from the brainstem with cranial nerves III, VII, IX and X and from sacral segments S2 to S4
Motor action of PNS
In contrast to the sympathetic NS which, under stress conditions, is activated as a ‘package’, the motor functions of the PNS are activated in a discrete, organ-specific manner.
Nevertheless, a situation in which some parasympathetically-mediated responses are activated concurrently can be identified as the “Resting and digesting scenario”.
The characteristics of the ‘resting and digesting’ scenario are: Increased parasympathetically-driven contraction of smooth muscle in the gastrointestinal tract (increased motility), together with increased secretion of digestive juices including hydrochloric acid. Heart rate decreases to low end of normal range. Accommodation of the eyes for close vision. Pupils of the eyes are constricted.
In some circumstances, the activity of an organ is determined by the balance between the two branches of the autonomic NS.A denervated heart, with no autonomic nerve supply, has a typical rate of about 110 beats/minute. Under resting conditions, the parasympathetic NS predominates and slows heart rate to the 50 – 100 beats/minute normal range. During exercise the sympathetic NS predominates and heart rate, in an adult, may rise to about 180 beats/minute. In children the maximum heart rate may be even faster. The patient with a non-functional autonomic NS, for example a poorly controlled type 1 diabetic, may present with a resting heart rate of about 110 beats/minute, the same as a denervated heart.