Chapter 18, Problem 5CR

Summary Introduction

To explain:

The pathways used in the sympathetic division.

Introduction:

The sympathetic division may stimulate a single effector or many effectors, at the same time. For example, when smooth muscle controls the diameter of the pupil of the eye, a single effector is involved. During an emergency or stressed situation, many effectors are involved which respond together. This process is called mass activation.

</!--SUMMARY-INTRODUCTION-->

Explanation:

Following are the four basic pathways used in the sympathetic division:

• The spinal nerve pathway:

The spinal nerve pathway continues from the spinal cord to effectors of the skin of the neck, torso, and limbs. In this pathway, a preganglionic neuron synapses with a ganglionic neuron in a sympathetic trunk ganglion. The postganglionic axon continues through a gray ramus that is the same level as the ganglionic neuron. Then it enters the spinal nerve and continues to its target organ.

• The postganglionic sympathetic nerve pathway:

The postganglionic sympathetic nerve pathway continues from the spinal cord to the internal organs of the thoracic cavity (the esophagus, heart, lungs, and thoracic blood vessels), effectors of the skin of the head and neck, neck viscera, also the superior tarsal and dilator papillae muscles in the eye. In this pathway, the preganglionic neuron synapses with a ganglionic neuron in a sympathetic trunk ganglion. The postganglionic axon extends away from the sympathetic trunk ganglion and projects directly to the effector organ.

• Splanchnic nerve pathway:

The splanchnic nerve pathway continues from the spinal cord to the abdominal and pelvic organs like stomach, small intestine, kidney. In this pathway, the preganglionic axons pass through the sympathetic trunk ganglia without synapses and continue to the prevertebral ganglia. There, the preganglionic axon synapses with a ganglionic neuron. The postganglionic axon then projects to the effector organs.

• The adrenal medulla pathway:

In the adrenal medulla pathway, the internal region of the adrenal gland, called adrenal medulla is directly innervated by preganglionic sympathetic axons. The axons of the preganglionic neuron continue through both the sympathetic trunk and prevertebral ganglia and then synapse on neurosecretory cells within the adrenal medulla. The stimulation of these cells causes the secretion of epinephrine and norepinephrine into the blood. These hormones prolong the effect of sympathetic stimulation.