Sympathetic Anatomy

Cell bodies of presynaptic sympathetic efferent neurons are found in the paired lateral horns of the spinal cord, an area identifiable between the T1 and L2 or L3 vertebrae. The axons, or nerve fibers of these cells, exit the interior of the spinal cord through ventral rootlets, which coalesce to form the larger ventral roots and eventually become the ventral rami. sympathetic fibers almost immediately divert into white rami communicantes (Fig. 1), branching from these spinal nerves, which connect them to paired columns of parasympathetic ganglia called the sympathetic trunks located on either side of the spinal cord. Vertebrae from T1 to S5 have corresponding pairs of ganglia; all are interconnected with both ascending and descending nerve fibers, forming the complex columnlike structures (1).

Presynaptic Sympathetic Fiber Pathway

Fig. 1. Pathways of sympathetic motor fibers. The three potential paths of travel taken by presynaptic sympathetic motor fibers can be seen above. Preganglionic fibers traveling to the heart and other areas of the thoracic cavity synapse either immediately on reaching the sympathetic trunk or traverse to other levels to synapse. Complete passage through the paired trunks also occurs with prevertebral ganglia. Modified from ref. 1.

Fig. 1. Pathways of sympathetic motor fibers. The three potential paths of travel taken by presynaptic sympathetic motor fibers can be seen above. Preganglionic fibers traveling to the heart and other areas of the thoracic cavity synapse either immediately on reaching the sympathetic trunk or traverse to other levels to synapse. Complete passage through the paired trunks also occurs with prevertebral ganglia. Modified from ref. 1.

Preganglionic sympathetic neurons synapse within the ganglia of the sympathetic trunk. The 10- to 20-nm separation distance (2) between presynaptic and postsynaptic cells is called the synaptic cleft, in which neurotransmitter is released from synaptic vesicles. Acetylcholine is the neurotransmitter released from preganglionic neurons in both the sympathetic and parasympathetic branches of the autonomic nervous system. This compound binds to receptors on postsynaptic cell membranes, causing localized depolarizations of these cells, which may subsequently initiate action potentials that propagate down their axons. Postganglionic neurotransmitters vary between the two branches. In the sympathetic nervous system, norepinephrine is the primary postsynaptic neurotransmitter released. Such junctions can also be activated by epinephrine, and both can often be associated with cotransmitters such as dopamine (2) and histamine (3). Both norepinephrine and epi-nephrine play important roles during sympathetic stimulation of the heart, as is discussed in this chapter.

Three primary paths of travel are commonly identified for presynaptic (also referred to as preganglionic) nerve fibers on reaching the sympathetic trunk. A preganglionic fiber can immediately synapse on the cell body of a postganglionic fiber at the level of the trunk on which the fiber entered. Pregangli-onic fibers can also follow a route that traverses through the sympathetic trunk, then either ascends or descends to synapse within a higher or lower level ganglion. A third, but less common, path of travel for presynaptic neurons involves passing through the sympathetic trunk completely, then synapsing within a prevertebral ganglion in close proximity to the viscera to be innervated. In general, presynaptic fibers traveling to the head, neck, thoracic cavity, and limbs will follow one of the first two courses. Innervation of organs and glands located in the abdominopelvic cavity follow the third path through prevertebral ganglia (Fig. 1).

A variation of the second path occurs primarily with innervation of sweat glands, hair follicles, and peripheral arteries. Presynaptic nerves that arrive at the paired sympathetic ganglion traverse through the white rami communicantes. Instead of immediately continuing to peripheral regions of the body after synapsing, the postsynaptic neurons next travel through

Sympathetic Innervation Human Heart

Fig. 2. Autonomic innervation of the heart. Vagal innervation of the right atrium can be observed. The area where many axons congregate just prior to innervation of the heart is depicted as the cardiac plexus. Sympathetic fibers branching from an arbitrary vertebral level of the paired sympathetic trunks is also illustrated. AV, atrioventricular; SA, sinoatrial. Modified from ref. 4. © 2001 by Frederic H. Martini. Reprinted by permission of Pearson Education, Inc.

Fig. 2. Autonomic innervation of the heart. Vagal innervation of the right atrium can be observed. The area where many axons congregate just prior to innervation of the heart is depicted as the cardiac plexus. Sympathetic fibers branching from an arbitrary vertebral level of the paired sympathetic trunks is also illustrated. AV, atrioventricular; SA, sinoatrial. Modified from ref. 4. © 2001 by Frederic H. Martini. Reprinted by permission of Pearson Education, Inc.

gray (unmyelinated) rami (Fig. 1) and exit along large bundles of nerve fibers called primary rami. From the primary rami, smaller nerve branches bifurcate and act to control the vascula-ture (vasodilation and vasoconstriction), hair follicle stimulation, and sweating. If the nerves are destined for the head, their cell bodies are located in the superior cervical ganglion, and their axons follow the path of the carotid arteries to their respective destinations. The muscles of the eye are also innervated by this collection of sympathetic neurons.

Nerve fibers traveling from the central nervous system to a destination elsewhere in the body are termed efferent. Afferent nerves carry information from various locations in the body to the central nervous system. Frequently, these respective paths of travel occur in parallel, with their fibers bunched closely together to form larger nerve branches. The main nerve branches controlling the sympathetic behavior of the heart and lungs are the cardiopulmonary splanchnic nerves, which consist of both efferent and afferent fibers. Efferent nerves navigate a route originating from the ganglia in the upper cervical region (superior, middle, and inferior cervical ganglia) and the upper thoracic (T1 to T5) levels of the sympathetic trunk. The inferior, middle, and superior cardiac nerves in turn originate from corresponding cervical ganglia and approach the base of the heart before branching into smaller nerves and distributing themselves throughout much of the myocardium and vasculature. The cardiac plexus can be considered an imaginary grouping of the nerve bundles traveling to and from the heart (Fig. 2).

Incoming postsynaptic sympathetic neurons that innervate the human heart are highly concentrated around and near the aortic arch. Some of this innervation occurs throughout the aortic arch itself, as well as at the base of the ascending portion of the vessel. Many branches from these nerves continue down the aorta or under the arch to the pulmonary trunk, where they again diverge and track with the pulmonary arteries. Still more neuronal bifurcations have been identified that then extend to reach other areas of the heart, including both atria and the right and left ventricles. Sympathetic innervation of the sinoatrial and atrioventricular nodes is important for control of heart rate, but has not been distinguished in greater concentration at these areas relative to elsewhere in the atria (5,6). Many nerves have been identified epicardially, often following the path of the coronary arteries and veins (5,7). In general, sympathetic innervation is more highly concentrated in the ventricles than in the atria (8). Within the ventricles, a higher distribution is observed toward the base of the heart as opposed to the apex, with nerves in the epicardium at a slightly greater concentration than in the endocardium. This latter tendency is also evident in the atria (8).

3. ADRENAL MEDULLA

The sympathetic nervous system also controls the hormonal secretions of the paired suprarenal (adrenal) glands in the abdomen, which are components of the endocrine system. Specifically, preganglionic fibers, with their cell bodies located in the lower thoracic (T10-T12) segments of the spinal cord, travel to the adrenal medulla by the abdominopelvic splanchnic nerves. It is in the medulla, or central portions of the suprarenal glands, that norepinephrine and epinephrine are released into the bloodstream (1). The release of these catecholamines into the blood is considered a postsynaptic response initiated from this type of sympathetic activation. Specifically, the cortex surrounding the medulla portions of the adrenal glands are responsible for producing multiple steroid hormones. As blood drains from the highly vascularized cortex to the medulla, the aforementioned hormones can be used to convert norepinephrine to epineph-rine. The respective mechanisms of action for these two similarly structured catecholamines are discussed in Section 9.

4. PARASYMPATHETIC ANATOMY

The parasympathetic (craniosacral) nervous system branches from four paired cranial nerves and the lower sacral segment of the spinal cord (S2-S4). The vagus nerve (cranial nerve X) is the main effector for cardiac functions controlled by input from the parasympathetic branch of the autonomic nervous system (Fig. 2). Efferent fibers of the vagus nerves originate in the medulla oblongata and weave through the neck alongside the carotid arteries to the thoracic and abdominopelvic cavities, bifurcating many times along the way to innervate an assortment of organs. Specifically, the efferent fibers of the cranial parasympathetic branch communicate with blood vessels of the head and other viscera; the sacral portion of the spinal cord innervates viscera of the lower abdominopelvic cavity, like the urinary bladder and colon, as well as their respective blood vessels.

Unlike the short sympathetic preganglionic fibers, the para-sympathetic division of the autonomic nervous system gener ally has very long preganglionic fibers and short postsynaptic fibers. Hence, the parasympathetic ganglia are often located very proximal to, or actually within, the target organ. As discussed, acetylcholine is the primary neurotransmitter at both preganglionic and postganglionic junctions.

Within the heart, the majority of the parasympathetic ganglia are located near the sinoatrial node and within the conduction tissue surrounding the atrioventricular node (5). Consequently, the right and left vagus nerves envelope a large and overlapping portion of the atria, in which short postsynaptic fibers from both branches act on the conduction centers of the heart (Fig. 2). However, the endings of the right vagus primarily innervate the sinoatrial nodal region, and many projections from the left are typically observed at the atrioventricular node (5). In fact, high concentrations of vagal innervation situated within a localized region of epicardial fat near the atrioventricular node have been described for the human heart, and it is hypothesized that nerves located within this "pad" have little effect on behavior of the sinoatrial node (9). Thus, it is likely that each region is controlled independent of the other. Parasympathetic junctions are also observed in the ventricles, but only at one-half to one-sixth as frequently as sympathetic innervation (8). Nevertheless, nerves of the parasympathetic division of the autonomic nervous system outnumber those of the sympathetic division in the atria by 30-60% (8). Interestingly, although sympathetic innervation has been described to occur at approximately an equal distribution between the endocardial and epicardial surfaces of the heart, vagal nerve endings are reportedly located at almost twice the density (1.7 to 1) within the myocardium when compared with their epicardial distribution (8).

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • TEODROS
    What neurotransmitters are released by the preganglionic fibers of the sympathetic nervous system?
    5 years ago
  • maximilian
    Which nerve invenerate the ventricles of the heart para or sympathetic?
    5 years ago
  • Oskar Alexander
    Which presynaptic neuron travels through white rami?
    5 years ago
  • kati rautavaara
    What type of nerve fibers are primarily found in the ventricles?
    4 years ago

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