One of the most striking demographic trends over the past century has been a dramatic extension of life expectancy, which has almost doubled from 44 years in 1890 to >80 years today. An unfortunate accompaniment of this change is an increase in age-related cognitive change, perhaps the most common and feared result of aging. Age-related cognitive change includes both “normal” cognitive aging and several neurodegenerative diseases causing dementia.

Anxiety is a common feeling or emotion experienced by most people. In general, anxiety disorders have the highest prevalence of all psychiatric conditions, with a lifetime prevalence of approximately 25%. Phobias (specific phobia and social anxiety disorder/phobia) are the most common mental disorders in the United States. At least 5% to 10% of the population and possibly as much as 25% of the population may suffer from some type of a phobic disorder (eg, fear of needles, animals, heights). Prevalence estimates of anxiety disorders are generally higher in developed countries than in developing countries. Many anxiety disorders are more prevalent in women than in men, with a ratio of 2:1, except obsessive-compulsive disorder and social anxiety disorder, in which the female-to-male ratio is closer to 1:1. Most anxiety disorders start in childhood.

The sense of hearing allows us to capture sound energy from the environment and informs us about the identity of the sound emitter and its location. Complex sounds are also an essential means of complex communication for biological organisms, reaching their pinnacle in human language. Sound consists of a sequence of air pressure pulses created by vibrating objects such as vocal cords. Vibrations cause compression and rarefaction of the air, which result in the characteristics of frequency, the number of cycles per second, amplitude, and sound intensity, which is usually measured in decibels, a log scale. The sensitivity of the auditory system is very close to the absolute threshold created by random movement of air molecules. Sound reception begins with mechanical modification and transduction in the outer and middle ear and neural coding in the inner ear at the cochlea. Relays in the brainstem and thalamus pass the encoded auditory input to the auditory cortex in the mid-superior temporal lobe, where higher auditory processing allows us to understand language and appreciate music.

The nervous system consists of the central, peripheral, autonomic, and enteric systems. This chapter will discuss the autonomic and enteric nervous systems (ENS). The autonomic nervous system receives inputs from receptors in glands and cardiac and smooth muscle and sends motor commands to those areas. The enteric nervous system is the nervous system of digestion. Previously, the ENS was considered part of the autonomic nervous system, but it is now generally treated separately, occasionally being referred to as the “second brain.”

Smell and taste are called chemical senses because molecules activate the receptors in these systems rather than energy, such as photons in vision and vibrations in hearing. Smell and taste have important gateway functions in approach/withdrawal behaviors. The smell given off by a ripe pineapple induces us to consider eating it, and the ripe, sweet taste prompts consumption of the fruit. However, we are repulsed by the smells and tastes of many bitter and sour substances. Those chemical qualities are often (but not always) associated with spoilage or toxicity. Taste receptors, found mostly on the tongue, exist for sweet, sour, salt, bitter, and, according to some researchers, umami, the savory taste induced by the additive monosodium glutamate (MSG). Taste is also strongly influenced by smell because mastication of food also activates olfactory receptors in the nose. The combination of smell and taste gives rise to the complex perception of flavor. The message encoded by receptors for taste relays through several subcortical structures before reaching the thalamus and then the cortex. Olfaction is the exception among the senses in that neurons in the olfactory bulb project directly to cortex in a pathway whose activation is largely subconscious. However, there is an olfactory projection from cortex back to the thalamus, and then back to cortex. This projection reaches, among other loci, the orbitofrontal cortex, where smell and taste inputs are combined to mediate complex olfactory perceptions such as flavor.

Almost 1 in 5 children either currently or at some point during their life will have a severe mental disorder. Mental and substance use disorders are the leading cause of disability in children and youth worldwide, and depression is the number 1 cause of loss of disability-adjusted life-years. These conditions can affect children’s development, educational attainment, and potential to live fulfilling and productive lives.

The result of billions of years of evolution, the innate biological clock is a nearly ubiquitous feature of life on Earth. The conservation of its basic function—the maintenance of a stable relationship between the organism’s internal physiologic processes and the environmental light cycle—across such a wide swath of species speaks volumes about its importance. This timekeeping mechanism is not simply a response to changing light, but rather an innate clock that responds slowly and predictably to changes in the architecture of daily light cycles. Behaviorally, this clock allows organisms to predict daily changes in the environment and to react accordingly. Physiologically, the clock serves as a master regulator of many processes, providing a temporal pattern for internal organization and output. In humans and other mammals, the master circadian clock is located in the suprachiasmatic nuclei of the hypothalamus—a pair of densely packed nuclei receiving direct light input from specialized cells in the retina and other indirect timing and physiologic information from a slew of other areas in the nervous system and body. In this chapter, we will explore the basic properties of circadian and seasonal rhythms, the more complex molecular constituents of the clock, and finally, their impact on human health.

Life began on earth about a billion years after its formation 4.5 billion years ago. This life consisted of unicellular prokaryotes such as bacteria. Roughly a billion and a half years later, eukaryotes arose, cells with nuclei. About a billion and a half years after that, complex animals arose during the Cambrian explosion 500 million years ago, giving rise in a few million years to primitive vertebrates. Mammals arose about 200 million years ago, and primates about 60 million years ago, after the Cretaceous dinosaur extinction. Several hominid lines arose in the last 5 million years, with humans, Homo sapiens, showing up a few hundred thousand years ago.

Development of the human nervous system involves the generation of the central nervous system (CNS) and peripheral nervous system (PNS) and occurs during embryonic, fetal, and postnatal periods. The study of nervous system development allows a better understanding of the structural organization of the adult nervous system and helps in comprehending the basis of congenital disorders that affect brain function and cause cognitive disorders. Key prenatal stages in neural development include gastrulation, neural induction (NI), neurulation, neurogenesis, gliogenesis, neural migration, and synaptogenesis. Through these prenatal stages, the gross structures of the brain, spinal cord, and nerves are formed; most neuronal and some glial populations are generated; some synapses are formed; and development is governed predominantly by hardwired intrinsic genetic programs that control gene expression and cell–cell interactions. At birth, the mass of the human brain is approximately 350 to 400 g. Postnatally, additional glial cell populations develop, myelination rapidly takes place, dendrites become highly branched, and many more synapses are established. From birth through adolescence, synapses are stabilized and pruned by activity-dependent processes for the construction of neural circuits. This enables experience-driven neuronal activity to influence the wiring of the brain. Postnatally, the human brain nearly quadruples its mass to approximately 1300 to 1400 g in adults.