Sensory nerve endings with connective tissue sheaths are called encapsulated endings. Many encapsulated endings are mechanoreceptors located in the skin and joint capsules (Krause’s end bulb, Ruffini’s corpuscles, Meissner’s corpuscles, and Pacinian corpuscles) and are described in Chapter 15, Integumentary System (page 501). Muscle spindles are encapsulated sensory endings located in skeletal muscle; they are described in Chapter 11, Muscle Tissue (page 325). Functionally related Golgi tendon organs are encapsulated tension receptors found at musculotendinous junctions.
Although the ANS was introduced early in this chapter, it is useful here to describe some of the salient features of its organization and distribution. The ANS is classified into three divisions:
The ANS controls and regulates the body’s internal environment.
The ANS is the portion of the PNS that conducts involuntary impulses to smooth muscle, cardiac muscle, and glandular epithelium. These effectors are the functional units in the organs that respond to regulation by nerve tissue. The term visceral is sometimes used to characterize the ANS and its neurons, which are referred to as visceral motor (efferent) neurons. However, visceral motor neurons are frequently accompanied by visceral sensory (afferent) neurons that transmit pain and reflexes from visceral effectors (i.e., blood vessels, mucous membrane, and glands) to the CNS. These pseudounipolar neurons have the same arrangement as other sensory neurons—that is, their cell bodies are located in sensory ganglia, and they possess long peripheral and central axons, as described above.
The main organizational difference between the efferent flow of impulses to skeletal muscle (somatic effectors) and the efferent flow to smooth muscle, cardiac muscle, and glandular epithelium (visceral effectors) is that one neuron conveys the impulses from the CNS to the somatic effector (Fig. 12.24a), whereas a chain of two neurons conveys the impulses from the CNS to the visceral effectors (Fig. 12.24b). Thus, there is a synaptic station in an autonomic ganglion outside the CNS, where a presynaptic neuron makes contact with postsynaptic neurons. Each presynaptic neuron synapses with several post-synaptic neurons.
Sympathetic and Parasympathetic Divisions of the Autonomic Nervous System
The presynaptic neurons of the sympathetic division are located in the thoracic and upper lumbar portions of the spinal cord.
The presynaptic neurons send axons from the thoracic and upper lumbar spinal cord to the vertebral and paravertebral ganglia. The paravertebral ganglia in the sympathetic trunk contain the cell bodies of the postsynaptic effector neurons of the sympathetic division (Figs. 12.24b and 12.25).
The presynaptic neurons of the parasympathetic division are located in the brain stem and sacral spinal cord.
The presynaptic parasympathetic neurons send axons from the brain stem—that is, the midbrain, pons, and medulla, and the sacral segments of the spinal cord (S2 through S4)— to visceral ganglia. The ganglia in or near the wall of abdominal and pelvic organs and the visceral motor ganglia of cranial nerves III, VII, IX, and X contain cell bodies of the postsynaptic effector neurons of the parasympathetic division (Figs. 12.24c and 12.25).
The sympathetic and parasympathetic divisions of the ANS often supply the same organs. In these cases, the actions of the two are usually antagonistic. For example, sympathetic stimulation increases the rate of cardiac muscle contractions, whereas parasympathetic stimulation reduces the rate.