**4.4 The motor neurons**

The motor signals are transmitted via efferent nerves that can be thought of as cables containing various fibers and motor neurons to the muscles and glands. There are three different kinds of motor neurons that are important in the swallowing process (**Figure 10**) [5, 25, 29].


The sum of all the above signals executes pre-programmed cooperation between the 148 muscles that are involved in the transport of each food bite from the mouth down to the stomach.


**Figure 10.** *Three types of motor neurons.*

The efferent nerves send signals via the three different motor neuron fiber types to the muscles and glands.

#### **4.5 Understanding the three neurological phases**

#### *4.5.1 Oral phase*

As we have said earlier, the oral phase is consciously controlled (voluntary) and is managed by the brain's cortex region [5, 25]. But when the bolus has passed the anterior palatal arch towards the pharynx, the swallow reflex takes over and this is controlled by the brain stem – no longer consciously or voluntarily controlled.

#### *4.5.2 Pharyngeal phase*

The tongue's movement backward and upwards transports the food towards the pharynx. When the bolus reaches the anterior palatal arch and the smooth palate, the reflexive phase of the swallow starts [5, 6] causing the larynx to rise, As the tongue begins its return movement forward, the epiglottis seals the airway and the food passes into the pharynx. The pharyngeal phase takes between 0.5 and 1 second.

In this phase, the coordination between breathing and swallowing is crucial to avoid food 'going down the wrong way' [5, 25, 30, 31]. Breathing and swallowing are guided by different centers in the brain stem, however, all the muscles that are active in these two functions are controlled from the same concentrated grouping of specialized nerves nucleus in the brain stem. This allows the swallowing center to take control of breathing during a crucial phase in the act of swallowing. When the 1st and 2nd Swallowing Centers signal that swallowing is underway, the body breathes in. During the subsequent exhalation, the food portion is driven to the back of the tongue and the exhalation stops as the bolus crosses the airway. Breathing ceases for 2 seconds about twice as long as it takes for the bolus to pass the pharynx then breathing is resumed with a continued exhalation.

#### *4.5.3 Esophageal phase*

The esophagus' longitudinal musculature forms a stiff pipe, the UES relaxes and opens to allow the passage of the bolus into the esophagus. The sphincter to the stomach – LES - opens to enable the entrance of the food.

The muscle function and the downwards transport of the bolus are controlled by the Vagus CN X and a branch of the Glossopharyngeal CN IX. Together these nerve pathways build a local network in the esophagus' *Plexus Pharyngeus* [25].

Both types of muscle: voluntary skeletal striated muscles and involuntary smooth muscles are present in the esophagus. The voluntary musculature is the same type as we have, for example, in our arms and legs: so-called skeletal striated muscles which are attached to the skeleton or tissue, and that are voluntarily controlled. The smooth musculature cannot be controlled voluntarily but is instead controlled by the autonomic nervous system: functioning unconsciously and involuntarily. These muscles are stimulated via the GVE motor neurons (**Figure 10**) in the brain stem which sends signals to the involuntary musculature.

The esophagus' upper third consists of skeletal striated muscles, the middle third is a mixture of skeletal striated muscles and smooth musculature, and the bottom third is solely smooth muscle.
