Neural Control of Feeding and Swallowing

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Eating and drinking are basic pleasures in life that most of us take for granted, yet the ease with which we perform these tasks belies their complex neurologic system of control. Recent studies of human swallowing have begun to unravel some of these complexities, evolving our understanding and thus ultimately helping to generate novel therapies for the treatment of swallowing problems after cerebral injury, such as stroke. This article provides a general overview of current knowledge of the neural control mechanisms that underlie the coordination of mastication, oral transport, swallowing, and respiration in humans.

Section snippets

Oral feeding

The ability of organisms to maintain adequate energy stores is critical for survival and achieved through feeding. In mammals, this encompasses many factors, including the sight, smell, taste, texture, and temperature of food, all of which involve complex and interrelated neural circuits and responses that have been developed and honed through individual experiences. Some of these homeostatic and hedonistic mechanisms are beyond the remit of this article; feeding is discussed beginning with

The masticatory sequence

In humans, the processing of solid ingested material, regardless of size and texture, occurs in a stereotyped manner as categorized by Hiiemae and Palmer [1], [2]. This process is termed the masticatory sequence and is composed of four main components.

  • 1.

    Stage I transport, during which food is ingested as a bite-sized portion and positioned on the occlusal surface (between the teeth) for further breakdown if necessary.

  • 2.

    Processing, during which time trituration occurs. The number of processing

The neurophysiology of mastication

Mastication itself is an intermittent, rhythmic process during which the masticatory muscles—tongue, cheeks, lips, palate, and jaw muscles (masseters, temporalis, lateral and medial pterygoids)—coordinate to mechanically break down solid food by way of the teeth into smaller fragments that are mixed with saliva to form a cohesive bolus that can be easily swallowed. Mastication is not a prerequisite of ingestion, because liquid boluses do not require this preswallow step; however, it is the

The role of sensory feedback in mastication

Peripheral information from receptor systems in the dentures, tongue, cheek, jaw elevator muscles, and temporomandibular joint are also of importance [4], [5], [6]. Although they are not necessary during generation of the basic masticatory rhythm, they monitor the progress of chewing and modify commands sent to the appropriate effector muscles by significantly influencing the pattern generator directly and indirectly by descending motor pathways or through jaw reflexes. Degenerative diseases,

Integration of the chemical senses

As mentioned earlier, normal feeding depends on the integration of several systems that include the chemical senses of smell (olfactory) and taste (gustatory). Together they give rise to flavor perception and are accounted for by three sensory systems: olfactory (Fig. 1), gustatory (Fig. 2), and trigeminal chemosensory. Olfactory information can influence feeding behavior, social interactions, and reproduction in many animals. The gustatory system provides information about the quality,

Smell

The olfactory epithelium (approximately 10 cm2 in an average 70-kg male) contains sensory receptors that detect and transduce information about the identity, concentration, and quality of a wide range of chemical stimuli termed odorants. This information is projected directly to neurons in the olfactory bulb by way of cranial nerve I (olfactory), which in turn projects to the pyriform cortex (olfactory cortex) in the temporal lobe (see Fig. 1) and other brain regions, such as the amygdala,

Taste

The human taste system (see Fig. 2) acts in concert with the olfactory and trigeminal chemoreceptive systems to indicate whether or not food should be ingested. Once in the mouth, the chemical constituents of the food interact with (peripheral) taste receptor cells throughout the oral cavity, in lingual and extralingual locations, providing information about the identity (sweet, sour, salty, bitter, and umami), concentration, and pleasant or unpleasant quality [18], [19], [20]. This information

Swallowing

For most people, swallowing or deglutition is a normal and effortless task, but despite its ease, it is a complex and dynamic sensorimotor activity involving 26 pairs of muscles and five cranial nerves. This complexity emerges as a consequence of the common shared pathway between the respiratory and gastrointestinal tracts and has arisen to avoid the threat of food or liquid entering the airway. Swallowing thus enables the safe delivery of ingested food, as a bolus, from the mouth to the

The neurophysiology of swallowing

The central neural control of swallowing is described as being “multidimensional in nature” [31] recruiting at all levels of the nervous system, and hypothesized to be organized in a hierarchical manner as shown in Fig. 3[32]. The brainstem swallowing center, which contains the central pattern generator, is at the core of the system and represents the first level of control. Rostral to this and representing a second level of swallowing control are the subcortical structures, such as the basal

Brainstem control of swallowing

The brainstem swallowing center is located in the upper medullary and pontine areas of the brain and is bilaterally distributed within the reticular formation. This network of neurons is made up of three functional components: an afferent component, an efferent component, and a complex organizing system of interneurons known as the central pattern generator. Although the cortex is recognized to be responsible for the initiation of swallowing [31], the central pattern generator organizes the

The role of the cerebral cortex in swallowing

The importance of the cortex in the control of human swallowing has been recognized for more than a century, dysphagia being first reported by Henry Charlton Bastian (1898) in a patient following hemispheric stroke [55]. Much of our understanding of the central brain control of swallowing has come from invasive animal studies artificially stimulating cortical swallowing areas. In anesthetized animals, electrical microstimulation of either cortical hemisphere is capable of inducing full swallow

Integration with respiration

In addition, for individuals to feed orally in a normal manner, several processes must occur simultaneously, thereby requiring the integration of swallowing with several other functions, particularly respiration. Respiration, like swallowing and mastication, is controlled by many of the same neural areas, including the brainstem and cerebral cortex [94]. During a normal swallow, respiration must be temporarily halted, a process termed deglutition apnea. This process is centrally generated and

Summary

The study of the neurologic control mechanisms that underlie the coordination of mastication, oral transport, swallowing, and respiration in humans is an exciting area of research with numerous unanswered questions. In this article we have provided an overview of current knowledge in the field and propose that significant advances in our understanding will still emerge with the application of modern imaging techniques, such as TMS, PET, fMRI, and MEG. As we continue to unravel more and more of

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