In order to extend all over the body, the sympathetic system fibers leave the spinal cord at cord levels T1-L2, enter the paravertebral ganglion chain and then may travel up or down the chain for considerable distances prior to synapsing (Fig. 9-19). The sympathetic chain stretches from the foramen magnum to the coccyx and supplies the far reaches of the body with post-ganglionic sympathetic fibers. Parasympathetic fibers reach widespread areas via the vagus (Fig. 9-19).
Fig. 9-19. Schematic view of the autonomic nervous system.
Both parasympathetic and sympathetic systems contain two neurons between the spinal cord and periphery. The first synapse is cholinergic (containing acetylcholine). For the sympathetic system, this synapse is either in the paravertebral chain of sympathetic ganglia or farther away in the prevertebral ganglion plexuses (Fig. 9-18).
Parasympathetic synapses typically lie very close to or within the viscera. The final synapse of the parasympathetic system contains acetylcholine, whereas the final synapse of the sympathetic system contains norepinephrine, with the exception of certain synapses, as for sweating, that contain acetylcholine (i.e. are cholinergic). In the chart below, note that secretory actions in general are stimulated by cholinergic fibers.
Fig. 9-20. Sympathetic and parasympathetic functions.
In extreme fear, both systems may act simultaneously, producing involuntary emptying of the bladder and rectum (parasympathetic) along with a generalized sympathetic response. In more pleasant circumstances, namely in sexual arousal, the parasympathetic system mediates penile and clitoral erection and the sympathetic controls ejaculation. (You can remember the difference, as "parasympathetic" is a longer word.)
the posterior and anterior parts of the hypothalamus (Fig. 9-19).
The hypothalamus, a structure about the size of a thumbnail, is the master control for the autonomic system. Stimulation or lesions result not in isolated-smooth muscle, cardiac muscle or glandular effects but in organized actions involving these systems, e.g. in the fear or rage reaction of the flight or fight response, in increased and decreased appetite, altered sexual functioning, and control of body temperature. For instance, stimulation of the posterior hypothalamus may result in conservation of body heat and an increase in body temperature owing to constriction of cutaneous blood vesels.
Many circuits connect the hypothalamus with various areas of the cerebral cortex, brain stem and thalamus.
Fig. 9-21. The reverberating Papez circuit (modified from Manter and Gatz's ESSENTIALS OF CLINICAL NEUROANATOMY AND NEUROPHYSIOLOGY, by S. Gilman and S. Winans Newman, 7th Edition, 1987), believed to be involved in the emotional content of conscious thought processes. It provides intercommunication between hippocampus, hypothalamus, thalamus and cerebral cortex. Note the input of the olfactory system, which also plays a role in emotion. This is evident if you have ever seen two dogs sniffing one another or noted the prominence of the perfume industry in major department stores.
In Wernicke's syndrome, which occurs in patients who are alcoholic and undernourished, there is paralysis of eye movements, ataxic gait and disturbances in the state of consciousness associated with hemorrhages in the hypothalamus and other regions. KorsakofFs syndrome also occurs in alcoholic patients and consists of memory loss, confusion and confabulation associated with lesions in the mammillary bodies and associated areas. (The mammillary bodies are considered a part of the hypothalamus.)
Proceeding rostrally from the caudal tip of the spinal cord, one first finds a parasympathetic area (S2-S4), followed by a sympathetic region (T1-L2), then parasympathetic areas (CNs 10, 9, 7, 3) and then successively a sympathetic and a parasympathetic area, a strange alternating sequence. The latter two areas are
Fig. 9-22. The major connections between hypothalamus and pituitary gland. The hypothalamus lies close to the pituitary gland. A disorder of one structure may affect the other. Note that nerve fibers from the paraventricular and supraooptic nuclei connect with the posterior pituitary. These nuclei secrete oxytocin
and antidiuretic hormone (vasopressin). These hormones are synthesized and transported in neurons and then released at the ends of the nerve terminals in the posterior pituitary.
The anterior pituitary contains no neuronal connections. Instead, releasing factors are produced in the hypothalamus and are released into the portal circulation and then transported to the anterior pituitary where they stimulate cells in the anterior pituitary to secrete various hormones, including adrenocorticotrophic hormone, thyrotrophic hormone, somatotrophic hormone, follicle stimulating hormone and luteininzing hormone. These are described in greater detail in the Endocrine chapter.
In a sense, the pituitary hormones are involved in autonomic-type functions, but on a more prolonged time scale. For instance, thyroid hormone is catabolic; growth hormone is anabolic; FSH, LH, and oxytocin relate to sexual functioning, and antidiuretic hormone relates to blood pressure.
Disorders of the autonomic system include:
1) Riley-Day syndrome (familial dysautono-mia), a disease associated with degenerative changes in the central nervous system and the peripheral autonomic system. Symptoms include decreased lacrima-tion, transient skin blotching, attacks of hypertension, episodes of hyperpyrexia and vomiting, impairment of taste discrimination, relative insensitivity to pain, and emotional instability.
2) Adiposogenital syndrome, characterized by obesity and retarded development of secondary sexual characteristics, sometimes is associated with lesions in the hypothalmus. ,
3) Precocious puberty may result from hypothalamic tumors.
4) The common cold. Temperature elevation is apparently the consequence of an influence on hypothalamic functioning.
5) Tumors of the pituitary may have a destructive effect on the pituitary gland and hypothalamus by direct extension, e.g. in chromophobe adenoma and craniopharyngioma, which generally are nonsecretory tumors. If the tumor contains functioning glandular tissue, e.g. acidophilic or basophilic adenoma, there may be the opposite effect of hypersecretion of pituitary hormones.
Dilates pupil (mydriasis)
Contracts pupil (miosis)
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