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The Hypothalamus

One of the most rostral cell groups to influence visceral function, and the one that has direct input to all other visceral nuclei in the neuraxis, is the hypothalamus. In addition to its role in regulating visceromotor functions, the hypothalamus, through a variety of circuits, influences circadian rhythms, neurohormones, reproductive functions, general homeostasis, and behavior. Overview The hypothalamus is the part of the diencephalon involved in…

Visceral Motor Pathways

The primary function of the visceral motor system is the regulation of cardiovascular, respiratory, digestive, urinary, integumentary, and reproductive organs. These organs are the main effectors of homeostasis, the maintenance of a stable internal environment against perturbing influences, both external and internal. In general, visceral motor neurons innervate smooth and cardiac muscles and glandular epithelium or structures made up of combinations of these tissues. Overview The…

Visual Motor Systems

All animals use their sensory organs to scan the environment in search of information. Often these organs are actively oriented toward relevant targets. This orienting behavior is exhibited by creatures from honey bees to humans. Movement of the eyes, for example, allows closer inspection of the visual environment. Moreover, human eyes have a fovea, a small portion of the central retina that has exquisite visual sensitivity.…

The Cerebellum

As indicated by its relative size (about 10% of the weight of the central nervous system), the cerebellum is essential to central nervous system function. However, it executes these responsibilities in unique ways. First, it receives extensive sensory input, but it is not involved in sensory discrimination or interpretation. Second, although it profoundly influences motor function, lesions of relatively large portions of the cerebellar cortex do…

The Basal Nuclei

Voluntary movement is essential to the well-being of living animals. Such behaviors are accomplished by signals that direct the actions of individual muscles. Although these signals originate in the cerebral cortex, they are modulated by a variety of subcortical structures. One such group of structures is the basal nuclei and their functionally associated cell groups. Classically, motor systems have been divided into pyramidal and extrapyramidal on…

Motor System II: Corticofugal Systems and the Control of Movement

Brushing our teeth seems like a simple voluntary movement. The neural basis for this action is, in fact, richly complex. For example, muscles in the upper limb are used cooperatively with jaw muscles, whereas neck and back muscles provide postural support. Sensory feedback from the teeth and gums is linked to muscle afferents conveying tension and proprioceptive signals from the forearm and hand. This sensory input…

Motor System I: Peripheral Sensory, Brainstem, and Spinal Influence on Anterior Horn Neurons

Spinal anterior horn motor neurons whose axons innervate skeletal muscles are called lower motor neurons. These cells activate skeletal muscles to produce characteristic movements of a body part. The activity of these motor neurons is influenced by two sources. First, peripheral sensory input arrives via posterior roots and is transmitted to anterior horn motor neurons and interneurons. Second, extensive descending projections from the cerebral cortex and…

Olfaction and Taste

The olfactory and taste systems sample the rich chemical environment that surrounds us. Information provided by these systems is intimately associated with the enjoyment of foods and beverages. When we refer to the taste of food, what we mean is a complex sensory experience correctly called flavor. Flavor perception results from a combination of the olfactory, taste, and somatosensory cues present in foods and beverages. Olfaction…

The Vestibular System

Humans have the ability to control posture and movements of the body and eyes relative to the external environment. The vestibular system mediates these motor activities through a network of receptors and neural elements. This system integrates peripheral sensory information from vestibular, somatosensory, visceromotor, and visual receptors as well as motor information from the cerebellum and cerebral cortex. Central processing of these inputs occurs rapidly, with…

The Auditory System

Hearing is one of the most important senses. In combination with vision and the ability to speak, it contributes, in a significant way, to the quality of life. In our daily routine, we unconsciously sort out meaningful sounds from background noise, localize the source of sounds, and react (many times in a reflex mode) to unexpected sounds. About 12% of people in the general population experience…

The Visual System

Vision is the sensory modality that perhaps captures the imagination more than any other. Phrases such as “He is the apple of my eye,” or “Her eyes flashed with anger,” or “I see what you mean” are common in the language and literature of most cultures and date back thousands of years. Furthermore, because of the way the nervous system converts optical images into neural signals…

Viscerosensory Pathways

The somatosensory system conveys information from sensory receptors in the skin, joints, and skeletal muscles that allows one to perceive and respond to stimuli arising either from the external environment or from the position of or the movement of the body. Functioning in parallel with somatosensory pathways are fibers that convey information about the status of visceral organs. This input allows the body to maintain homeostasis…

The Somatosensory System II: Nociception, Thermal Sense, and Touch

One crucial role of the somatosensory system is to supply the brain with information related to insults that cause tissue damage. These signals ascend the neuraxis in a fiber bundle called the anterolateral system (ALS). Anyone who has used a hammer or hot skillet has had experience with this system. Hit your thumb with a hammer and, if you are lucky, only high-threshold mechanoreceptors that signal…

The Somatosensory System I: Tactile Discrimination and Position Sense

When you reach into your pocket to determine the types of coins present, you are gathering information through the activation of specialized receptors of the somatosensory system. Specifically, the size of a coin is determined by noting the joint angles when the coin is held between the forefinger and thumb. “Heads and tails” may be identified with the use of slowly adapting receptors sensitive to stimuli…

The Telencephalon

The telencephalon is the largest part of the human brain, constituting about 85% of total brain weight, and is that portion in which all modalities are represented. Various sensory inputs (such as vision and hearing) are localized in some areas, whereas motor functions are represented in other regions: Both are modulated by subcortical nuclei. The telencephalon contains circuits that interrelate regions that have specific functions, such…

The Diencephalon

Although it is considered by some investigators to be part of the brainstem, the diencephalon is treated here as a portion of the forebrain. The diencephalon includes the dorsal thalamus, hypothalamus, ventral thalamus, and epithalamus, and it is situated between the telencephalon and the brainstem. In general, the diencephalon is the main processing center for information destined to reach the cerebral cortex from all ascending sensory…

A Synopsis of Cranial Nerves of the Brainstem

Although the brainstem is small, comprising only about 2.6% of total brain weight, the size of this structure belies its importance. First, all ascending and descending tracts linking the spinal cord and the forebrain traverse the brainstem. Second, there are important ascending fibers (e.g., spinoreticular, spinoperiaqueductal gray) and descending fibers (e.g., rubrospinal, vestibulospinal, reticulospinal) that interconnect the brainstem with the spinal cord. These tracts are essential…

The Midbrain

The mesencephalon, or midbrain, is the most rostral portion of the brainstem. It gives rise to cranial nerves III and IV, conducts ascending and descending tracts, and contains nuclei that are essential to motor function. Caudally the midbrain is continuous with the pons, and rostrally it joins the diencephalon. The cerebral aqueduct, the cavity of the midbrain, is continuous rostrally with the third ventricle and caudally…

The Pons and Cerebellum

The metencephalon consists of the pons and cerebellum. The pons is the middle segment of the brainstem, the caudal part being the medulla and the rostral portion being the midbrain. Although comprising only about 1.3% of the brain by weight, the pons has many important functions. The motor and sensory nuclei and the exit points of cranial nerves V to VIII are associated with the pons.…

The Medulla Oblongata

The medulla oblongata, or myelencephalon, is the most caudal segment of the brainstem. It extends from the level of the foramen magnum to the pons-medulla junction. The cavity of the medulla consists of a narrow, caudal part, which is the continuation of the central canal of the cervical spinal cord, and a flared, rostral portion, which is the medullary part of the fourth ventricle. The modest…