Ventricular and Cerebrospinal Fluid System

The meninges, ventricular system, subarachnoid space, and cerebrospinal fluid (CSF) constitute a functionally unique system that has an important role in maintaining a stable environment within which the central nervous system can function. The membranes that constitute the meninges serve as supportive and protective structures for neural tissue. The CSF itself provides a cushioning effect during rapid movement of the head and mechanical buoyancy to the brain. In addition to providing a pathway for the removal of brain metabolites, it functions as a chemical reservoir that protects the local environment of the brain from changes that may occur in the blood, thus ensuring the brain’s continued undisturbed performance. The CSF system is present at the supratentorial, posterior fossa, and spinal levels. Because of this extensive anatomical distribution and function, pathologic alterations of the CSF system can occur in many neurologic disorders.

Consciousness System

Consciousness is the state of awareness of self and the environment and involves 2 main functions: arousal (level of consciousness) and awareness (content of consciousness). These functions involve a group of brainstem and forebrain areas that form the consciousness system and include the reticular formation of the brainstem, hypothalamus, basal forebrain, thalamus, and cerebral cortex. The aims of this chapter are to describe the anatomy of the consciousness system, physiologic basis of cortical operations, regulation of wakefulness and sleep, mechanism of attention, and pathologic states of altered consciousness.

Anatomical Localization

The diagnosis of neurologic disorders is a skill that requires the application of basic scientific information to a clinical problem. As knowledge about the nervous system increases, more complicated neurologic problems can be solved in more sophisticated ways; however, the basic approach to the solution of all neurologic problems remains unchanged.

Clinical Problems

Clinical cases are presented with questions and answers to allow the reader to assess knowledge and identify areas for additional study. A small number of questions in this chapter address details of pharmacology that are not included in the text of previous chapters. We have included these questions because they are representative of clerkship shelf, residency in-training, or American Board of Psychiatry and Neurology examinations.

Supratentorial Level

The supratentorial level consists of 2 main anatomical regions: the diencephalon and the telencephalon. The anatomy, physiology, and clinical correlations of lesions affecting the diencephalon and visual pathways are described in another chapter. The telencephalon forms the cerebral hemispheres, which consist of the cerebral cortex, basal ganglia, and subcortical white matter tracts that interconnect areas of the cerebral cortex with one another and with the basal ganglia, thalamus, brainstem, and spinal cord. The medial portion of the cerebral hemispheres includes the amygdala, hippocampal formation, and cingulate cortex. These areas are involved in emotional and memory processing. The olfactory system is intimately related to these structures. The lateral and inferior aspects of the cerebral hemispheres include most of the frontal, insular, parietal, temporal, and occipital lobes. Neurons distributed in several cortical areas interact, forming functional networks that control different cognitive functions.

Diencephalon

The supratentorial level includes all structures located within the skull and above the tentorium cerebelli. These structures develop from the embryonic prosencephalon and, therefore, include derivatives of the diencephalon and telencephalon. The visual system, a derivative of the diencephalon, provides input to the cerebral cortex for image formation and to subcortical structures to trigger the light reflex and entrain the circadian rhythm. This chapter discusses the anatomy, physiology, and clinical correlates of the diencephalic components of systems at the supratentorial level.

Spinal Level

The spinal level includes the vertebral column and its contents. The spinal canal within the vertebral column is the passage formed by the vertebrae. It extends from the foramen magnum of the skull through the sacrum of the spinal column and contains the spinal cord, nerve roots, spinal nerves, meninges, and vascular supply of the spinal cord. Five of the major systems are represented in the spinal canal: the sensory, motor, autonomic, vascular, and cerebrospinal fluid systems. The vascular and cerebrospinal fluid structures are the support systems of the spinal cord. Diseases of the spinal canal involve 1 or more of these systems and produce patterns of disease distinctive to this level. The anatomical and physiologic characteristics of the spinal cord and spinal nerves that permit the identification and localization of diseases in the spinal canal are presented in this chapter.

Vascular System

The blood vessels to an organ provide it with a relatively constant supply of oxygen and other nutrients and a means for removal of metabolic waste. Failure to meet these vital requirements results in disease in that organ. Because of the unique structure and organization of the nervous system, localized abnormalities in its blood supply may produce devastating alterations in neural function. This chapter describes the normal anatomy and physiology of the vascular supply to neural tissue and the clinical manifestations of pathologic processes affecting this system.

Motor System

All bodily movements, including those of internal organs, are the result of muscle contraction, which is under neural control. The muscles of the limbs, trunk, neck, and eyes are derived from somites. The muscles involved in facial expression, mastication, phonation, and swallowing are derived from the branchial arches. Somatic and limbic motor pathways arising from the cerebral cortex and brainstem control the activity of the motor neurons innervating all these muscles. The motor system, like the sensory system, includes a complex network of structures and pathways at all levels of the nervous system. This network mediates many types of motor activity. An understanding of its organization and the integration of the motor system with the sensory system is necessary for accurate localization and diagnosis of neurologic disease.

Neurochemistry

Communication between neurons occurs primarily at the level of synapses. The most common form of communication in the nervous system is through chemical synapses, which consist of presynaptic and postsynaptic components separated by a synaptic cleft. The presynaptic terminals contain synaptic vesicles, which are involved in the storage and release of neurotransmitters by the process of exocytosis. Complex mechanisms control the synthesis, vesicular storage, and release of neurotransmitters and regulate the availability of neurotransmitter at the level of the synaptic cleft. The effects of the neurochemical transmitter on its target are mediated by neurotransmitter receptors. Specific neurotransmitter systems are responsible for fast neuronal excitation or inhibition, while other neurotransmitter systems regulate the excitability of neurons in the nervous system. Abnormalities in neurochemical transmission are responsible for many disorders, including acute neuronal death, seizures, neurodegenerative disorders, and psychiatric diseases. Most importantly, neurochemical systems provide the target for pharmacologic treatment of these disorders. The aims of this chapter are to review the basis of neurochemical transmission and the distribution, biochemistry, and function of specific neurotransmitter systems.