The Human Nervous System

Juergen K Mai, George Paxinos, Juergen K. Mai, George Paxinos

• Publisher: Academic Press
• Year: 2011
• ISBN:
  9780080921303
• Edition: 3
• Topic: Biowissenschaften
• Subtopic: Neurowissenschaft

The previous two editions of the Human Nervous System have been the standard reference for the anatomy of the central and peripheral nervous system of the human. The work has attracted nearly 2, 000 citations, demonstrating that it has a major influence in the field of neuroscience. The 3e is a complete and updated revision, with new chapters covering genes and anatomy, gene expression studies, and glia cells. The book continues to be an excellent companion to the Atlas of the Human Brain, and a common nomenclature throughout the book is enforced. Physiological data, functional concepts, and correlates to the neuroanatomy of the major model systems (rat and mouse) as well as brain function round out the new edition.

• Adopts standard nomenclature following the new scheme by Paxinos, Watson, and Puelles and aligned with the Mai et al. Atlas of the Human Brain (new edition in 2007)
• Full color throughout with many new and significantly enhanced illustrations
• Provides essential reference information for users in conjunction with brain atlases for the identification of brain structures, the connectivity between different areas, and to evaluate data collected in anatomical, physiological, pharmacological, behavioral, and imaging studies

VI

Systems

Chapter 29 Lower Brainstem Regulation of Visceral, Cardiovascular, and Respiratory Function

Chapter 30 Somatosensory System

Chapter 31 Trigeminal Sensory System

Chapter 32 Pain System

Chapter 33 Gustatory System

Chapter 34 The Olfactory System

Chapter 35 The Vestibular System

Chapter 36 Auditory System

Chapter 37 Visual System

Chapter 38 The Emotional Systems

Chapter 39 Cerebral Vascular System

Chapter 29

Lower Brainstem Regulation of Visceral, Cardiovascular, and Respiratory Function

William W. Blessing¹ and Eduardo E. Benarroch²

¹ Departments of Physiology and Medicine, Centre for Neuroscience, Flinders University, Adelaide, SA, Australia

² Department of Neurology, Mayo Clinic, Rochester, MN, USA

Outline

Introduction

Classification of Brainstem Neuronal Groups

Cardiovascular Function

Introduction

Excitatory Presympathetic Neurons in the Rostral Ventrolateral Medulla

Inhibitory Vasomotor Neurons Present in the Caudal Ventrolateral Medulla

Medullary Neural Circuitry Mediating Baroreceptor-Vasomotor and Cardiomotor Reflexes

Medullary Cardiovascular Circuitry and Hypertension in Humans

Angiotensin II Receptors in the Medulla and Human Hypertension

Parasympathetic Preganglionic Motoneurons Regulating Cerebral Vasculature

Medullary Raphe Neurons and Sympathetic Control of the Cutaneous Circulation and Brown Adipose Tissue in Thermoregulatory Control

Respiratory Function

Neural Circuitry Controlling Respiration – Responses to Hypoxia and Hypercarbia

Salivation, Swallowing and Gastrointestinal Function, Nausea, and Vomiting

Neural Circuitry Controlling Salivation

Neural Circuitry Controlling Swallowing

Neural Circuitry Controlling Gastric and Intestinal Function

Lower Brainstem Regulation of Vomiting

Lower Brainstem Regulation of Pituitary Vasopressin and ACTH Secretion

Lower Brainstem Regulation of Pelvic Viscera

Involvement of Putative Brainstem Autonomic and Respiratory Neurons in Human Neurodegenerative Disease

Acknowledgments

There is little direct functional or neuroanatomical information concerning the actual connectivity and actual function of brainstem circuitry in humans. Fortunately, homeostatic functions subserved by brainstem control structures are similar in humans and experimental animals and the general neuroanatomical arrangement is also similar in humans and experimental animals. Modern studies in animals have elucidated the brainstem neurotransmitter pathways controlling cardiovascular, respiratory gastrointestinal, and genitourinary functions. The results of animal studies can be used as a framework for describing the corresponding human neurotransmitter pathways. Clues can also be obtained from neuron-specific neuropathological changes occurring in neurological disorders such as Parkinson’s disease and multiple system atrophy.

Introduction

In this chapter we outline the functional organization of lower brainstem neurons regulating various aspects of homeostatic function in humans. This is a somewhat speculative task since we still have little direct functional or neuroanatomical information concerning the actual connectivity and actual function of brainstem circuitry in humans. The scant information available for the non-human primate brain is summarized in Blessing and Gai (1997). Studies correlating clinical dysfunction with the site of brainstem lesions have not been especially helpful in defining which brainstem nuclei control particular homeostatic cardiovascular/visceral/respiratory functions. Unilateral lower brainstem lesions may not cause clinically obvious dysfunction, and bilateral lesions may be fatal. Damage to fibers of passage may confound the clinicopathological interpretation of any particular lesion. Modern imaging procedures still yield low resolution, artifact-prone information concerning the brainstem. Fortunately large portions of the brainstem are neuroanatomically similar in humans and experimental animals, and the homeostatic functions subserved by brainstem control structures are also similar in humans and experimental animals.

The organization of the central nervous system can be conceived as a series of increasingly complex control loops, analogous to the loops of a computer program. The lowest order loop is the sensory-motor reflex arc. Higher order “if … then” loops refine and specify the activity of lower order loops. Brainstem neuronal circuitry is capable of organizing quite complex responses, including those traditionally thought to be integrated in the forebrain. Such complexity in lower brainstem function is in agreement with Hughlings Jackson’s concept of multiple representations of the same function at different levels of the nervous system. Neilsen and Sedgwick (1949) describe an anencephalic infant who proved to have no neural tissue above the midbrain when autopsied after 85 days’ survival. “The patient startled in the presence of loud noises. If we handled the patient roughly he cried weakly, but otherwise like any other infant, and when we coddled him he showed contentment and settled down in our arms. When a finger was placed into his mouth he sucked vigorously. He would sleep after feeding and awaken when hungry, expressing his hunger by crying.”

The traditional practice of delineating relatively independent “autonomic” and “somatic” nervous systems, differentially regulating bodily functions from separate central control regions, has hindered efforts to understand how the brain controls bodily homeostasis. Interactions with the external environment (e.g. catching and eating prey) are coordinated with control of the internal environment (e.g. digesting and absorbing prey). Natural selection has molded functionally integrated organisms, with integrated neural control systems, not separate nervous systems (Blessing, 1997a, b).

For each of the functions to be discussed in this chapter, the first step will involve a brief summary of our understanding derived from studies on experimental animals. Many of the primary references are cited in Blessing (1997b). We will also present evidence that degeneration in specific brainstem groups, homologous to those defined in experimental animals, may be important in autonomic, respiratory, and endocrine manifestations of multiple system atrophy (MSA), Parkinson’s disease (PD), diffuse Lewy body disease (DLB), sudden infant death syndrome (SIDS), and central congenital hypoventilation syndrome (CCHS).

Classification of Brainstem Neuronal Groups

As well as containing control loops of middle order complexity, the brainstem contains lowest order sensory-motor reflex arcs functioning as “the spinal cord for the head.” Thus, for example, the fifth cranial (trigeminal) nerve has sensory and motor nuclei within the brainstem that are organized in a fashion similar to the various spinal segmental levels. It is helpful to keep in mind Cajal’s original observation that motor control systems descending from the cerebral hemispheres synapse principally on interneurons and premotor neurons rather than on the lower motor neurons themselves. Cranial lower motoneurons do not have axonal collateral projections to other cranial nerve nuclei. Integrated function is mediated via premotor neurons with complex projections to different cranial nerve nuclei. These premotor neurons receive inputs from interneurons in loops of increasing complexity. Swallowing, for example, involves coordinated contraction of muscles of the face and lips, jaw, pharynx, tongue and neck muscles, as well as the upper esophagus. Swallowing thus depends on motoneurons distributed through trigeminal, facial, glossopharyngeal, vagal, hypoglossal, and accessory nuclei. The relevant interneurons and premotor neurons are still poorly anatomically defined even in experimental animals, so that “swallowing centre,” “pontine gaze centre,” and “micturition centre” still refer to functional concepts as well as neuroanatomical realities. As these centers are neuroanatomically characterized they tend to be “subtracted” from the region of the brain previously described as the “reticular formation,” sometimes (erroneously) conceived as a non-specific net-like structure.

Brainstem neuronal groups can be classified as follows, commencing with the motor end of the lowest order loop.

1. Somatic motoneurons with efferent axons distributed via the motor cranial nerves to innervate striated muscle in the head and neck.

2. Somatic premotoneurons that function to coordinate the different striated muscles involved in integrated activities such as eye movements or swallowing.

3. Parasympathetic preganglionic motoneurons with axons exiting in cranial nerves III, VII, IX, and X. The “preganglionic”epithet reflects the historical definition of autonomic neurons with respect to the peripheral ganglia.

4. Presympathetic motoneurons, with axons descending to innervate sympathetic preganglionic neurons in the thoracic and upper lumbar spinal cord.

5. Preparasympathetic neurons, with axons descending to innervate parasympathetic preganglionic neurons in the sacral spinal cord, especially those concerned with genitourinary and bowel eliminative functions. Other brainstem preparasympathetic motoneurons have intra-brainstem axons innervating cranial parasympathetic preganglionic motoneurons in the brainstem.

6. Pre-phrenic and pre-thoracic inspiratory and expiratory motoneurons.

7. Interneurons with increasing orders of complexity, capable of producing the patterned somatic and visceral effector responses.

8. Secondary sensory (afferent) neurons in the various dorsally and dorsolaterally situated afferent nuclei. These nuclei include those with inputs from somatic afferents (e.g. principal sensory nucleus of the trigeminal nerve and spinal nucleus of the trigeminal nerve) and visceral afferents (e.g. nucleus of the tractus solitarius). Projection targets of secondary sensory neurons include brainstem interneurons and premotor neurons, and motoneurons. Some interneurons (e.g. those in the pontine parabrachial nuclei) are probably closer to the afferent than to the efferent side of the brainstem neuronal organization. Other interneurons, for example those in the retrotrapezoid nucleus, are now thought to directly sense the carbon dioxide/acidity level of the arterial blood.

Cardiovascular Function

Introduction

The different bodily tissues receive a blood supply appropriate to their involvement in the various activities that constitute the individual’s daily life. During ingestion of food, for example, the salivary glands require a greatly increased blood supply so that sufficient saliva can be secreted. During sex...