Parasympathetic nervous system pathways

Stimulation of the parasympathetic nervous system modifies the organ functions by two main pathways. Firstly, the acetylcholine released from parasympathetic nerves can activate muscarinic receptors which are present in gland cells (sweat glands), smooth muscles and heart. The... 

Neurologic pathways in the sympathetic nervous system originate from the thoracic (T1 to T12) and the upper lumbar segments (LI and L2) of the spinal cord. Neurologic pathways in the parasympathetic nervous system originate from cranial nerves III, VII, IX, and X, from the brainstem, and the sacral segments S2, S3, and S4 from the spinal cord. This is why the parasympathetic nervous system is also known as the craniosacral division of the autonomic nervous system. 

Because of its motor, i.e., activating effect on vascular smooth muscle and its inhibitory effect on intestinal smooth muscle, the sympathetic nervous system has been cast into the role of the component of the nervous system that executes control of visceral function in times of physical emergency for the organism. The phrase fight or flight has been often used to describe the circumstances in which the adrenergic transmitters of the sympathetic system are dominant over the cholinergic parasympathetic system. This concept is perhaps oversimplified but it has the utility of a first approximation of how the two components of the ANS interact in the periphery. Sensory inputs which lead to increased blood pressure, for example, activate the sympathetic pathways. 

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.

The autonomic nervous system exerts the primary control on heart rate. Because the sympathetic and parasympathetic systems have antagonistic effects on the heart, heart rate at any given moment results from the balance or sum of their inputs. The SA node, which is the pacemaker of the heart that determines the rate of spontaneous depolarization, and the AV node are innervated by the sympathetic and parasympathetic systems. The specialized ventricular conduction pathway and ventricular muscle are innervated by the sympathetic system only. 

CNS—brain and spinal cord—and two separate pathways within the peripheral nervous system (PNS) for two-way communication with the peripheral organs. The PNS subdivisions are the somatic and autonomic nervous systems (Figure 11.2). The latter is further divided into sympathetic and parasympathetic divisions (Figure 11.3). 

Lesions of the retina, optic nerve, chiasm, and optic tract do not cause anisocoria. A lesion in the midbrain produces a subtle and transient anisocoria. However, most neurologic causes of anisocoria involve lesions in the efferent pupillary pathway. These defects arise due to asymmetric disruptions of the parasympathetic or sympathetic nervous systems that innervate the iris.The presence of anisocoria may help to limit a lesion to this pathway but does not localize the lesion s location within that pathway. 

A. Anatomic Aspects of the ANS The motor (efferent) portion of the ANS is the major pathway for information transmission from the central nervous system (CNS) to the involuntary effector tissues (smooth muscle, cardiac muscle, and exocrine glands Figure 6-1). The enteric nervous system (ENS) is a semiautonomous part of the ANS, with specific functions for the control of the gastrointestinal tract. The ENS consists of the myenteric plexus (plexus of Auerbach) and the submucous plexus (plexus of Meissner) and includes inputs from the parasympathetic and sympathetic nervous systems. 

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