Autonomic System And Hypothalamus

The autonomic system regulates glands, smooth muscle and cardiac muscle. It contains sympathetic and parasympathetic components. The sympathetic system as a whole is a catabolic system, expending energy, as in the flight or fight response to danger, e.g. increasing the heart rate and contractility and shunting blood to the muscles and heart. The parasympathetic system is an anabolic system, conserving energy, e.g. in slowing the heart rate and in promoting the digestion and absorption of food. The cell bodies of preganglionic sympathetic fibers lie in the intermediolateral columns of the spinal cord at spinal cord levels T1-L2 (Figs. 45, 46). Those of the parasympathetic system occupy comparable positions at spinal cord levels S2-S4.

TI-L2 SPLANCHNIC

BRANCH

GANGLION

Spinal Cord Sympathetic
Fig. 45 A. The sympathetic nerve routes. B. The parasympathetic nerve routes.

ROOT GANGLION

Sympathetic Nerve Routes

GANGLION

ROOT GANGLION

TI-L2 SPLANCHNIC

GANGLION

BRANCH

GANGLION

Autonomic System
Fig. 46 Schematic view of the autonomic nervous system.

In addition, cranial nerves 3, 7, 9 and 10 have parasympathetic components (3 — pupil and ciliary body constriction; 7 — tearing and salivation; 9 — salivation; 10 — the vagus and its ramifications).

In order to extend all over the body the sympathetic system fibers leave the spinal cord at levels T1-L2, enter the paravertebral ganglion chain and then may travel up or down the chain for considerable distances prior to synapsing (Fig. 46). 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. 46).

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. 45A).

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 noradrenaline, with the exception of certain synapses, as for sweating, that contain acetylcholine (i.e., are cholinergic). In the chart below, note that secretory function in general are stimulated by cholinergic fibers.

Structure

Lacrimal gland Salivary glands Sweat glands

Heart Rate

Force of ventricular contraction

Blood vessels

Lungs

Gastrointestinal tract

GI sphincters

Adrenal medulla

Urinary bladder Sex organs

Sympathetic function

Dilates pupil (mydriasis) No significant effect on ciliary muscle

No significant cffect

No significant effect

Stimulates secretion (cholinergic fibers)

Increases Increases

Dilates or constricts cardiac &

skeletal muscle vessels* Constricts skin and digestive system blood vessels Dilates bronchial tubes

Inhibits motility and secretion contracts

Stimulates secretion of adrenaline (cholinergic fibers)

Ejaculation

Parasympathetic function

Contracts pupil (miosis) Contracts ciliary muscle (accommodation)

Stimulates secretion

Stimulates secretion

No significant effect

Decreases Decreases

No significant effect

Constricts bronchial lubes Stimulates bronchial gland secretion Stimulates motility and secretion relaxes

No significant effect

Contracts Erection**

•Stimulation of beta-2 receptors dilates cardiac and skeletal muscle vessels whereas stimulation of alpha-1 receptors constricts. Most dilation of cardiac and skeletal vessels, though, may be due to nonautonomic, local tissue autoregulatory responses to lack of oxygen. ♦»Parasympathetic = erection, since parasympathetic is a longer word

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 mediated penile and clitoral erection and the sympathetic controls ejaculation.

Proceeding rostrally from the caudal tip of the spinal cord, one first finds a parasympathetic area (S2-0S4), followed by a sympathetic region (T1-L2), then parasympathetic areas (CNs 10, 9, 7, 3) and then successively a sympathetic and parasympathetic area, a strange alternating sequence. The latter two areas are the posterior and anterior parts of the hypothalamus (Fig. 46).

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 vessels.

Many circuits connect the hypothalamus with various areas of the cerebral cortex, brain stem and thalamus. Figure 47 shows the reverberating Papez circuit believed to be involved in the emotional content of conscious thought processes. It provided 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.

Fig. 47 The Papez circuit (shaded areas). The labeled structures as a whole are referred to as the limbic system. The hippocampus, among other things, is involved in the storage of short-term memory. The amygdala is important in the initiation of emotional responses. (Modified from Clark, R.G., Manterand Gatz's Essentials of Clinical Neuroanatomy & Neurophysiology, F.A. Davis Company, Philadelphia, 1975.)

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.

Anterior thalamic nucleus

Gyrus cinguli

Anterior thalamic nucleus

Gyrus cinguli

Papez Circuit

Fig. 47 The Papez circuit (shaded areas). The labeled structures as a whole are referred to as the limbic system. The hippocampus, among other things, is involved in the storage of short-term memory. The amygdala is important in the initiation of emotional responses. (Modified from Clark, R.G., Manterand Gatz's Essentials of Clinical Neuroanatomy & Neurophysiology, F.A. Davis Company, Philadelphia, 1975.)

Hippocampus

Hippocampus

Korsakoff s 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 hypothalamus lies close to the pituitary gland. A disorder of one structure may affect the other. Figure 48 shows the major hypothalamo-pituitary connections. Note that nerve fibers from the paraventricular and supraoptic nuclei connect with the posterior pituitary. These nuclei secrete the hormones 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 luteinizing hormone.

body

Fig. 48 The connections between the hypothalamus and pituitary gland.

Disorders of the autonomic system include:

1. Riley-Day syndrome (familial dysautonomia), a disease associated with degenerative changes in the central nervous system and the peripheral autonomic system. Symptoms include decreased lacrimation, transient skin blotching, attacks of hypertension, episodes of hyperpyrexia and vomiting, impairment of taste discrimination, relative insensitivity to pain, and emotional instability.

Mammillary

Hypothalamus

stem

Fig. 48 The connections between the hypothalamus and pituitary gland.

Paraventricular nucleus

Supraoptic nucleus-

Portal system Anterior lobe

Mammillary stem

2. Adiposogenital syndrome, characterized by obesity and retarded development of secondary sexual characteristics, sometimes is associated with lesions in the hypothalamus.

3. Precocious puberty may result from hypothalamic tumors.

4. The common cold. Temperature elevation is apparently the consequence of some 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.

6. Diabetes insipidus. Vasopressin (antidiuretic hormone) enhances the reuptake of water in the kidney. Interference with its production, as by an invading tumor, leads to diabetes insipidus, characterized by excessive production of urine and excessive thirst (up to 20 liters imbibed daily).

7. Horner's syndrome. Interruption of the cervical sympathetic nerves (or in some cases their central origins in the spinal cord and brain stem) leads to ptosis, miosis and decrease in sweating on the involved side of the face. Sometimes this is the result of a tumor of the apex of the lung (Pan-coast tumor) that interrupts the fibers as they course from the superior cervical ganglion (the most rostral ganglion in the sympathetic chain) to the carotid artery on their way to the orbit.

Surgical procedures are performed that interrupt the sympathetic innervation of the lower extremities, in order to increase circulation of cases of vascular insufficiency.

8. Hirschsprung's megacolon—Congenital absence of parasympathetic ganglion cells in the wall of the colon, resulting in poor colonic motility and a dilated colon.

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