1 Anatomy and Physiology of Phonation

Summary

This chapter describes the anatomy and physiology of key laryngeal structures involved in phonation (i.e., vocal fold vibration). A description of the cartilaginous framework, connective tissues, intrinsic and extrinsic laryngeal muscles, and details of neural innervation are provided and physiological principles underlying vocal fold oscillation are explained. Because the process of phonation is a combination of respiratory and laryngeal function, a basic description of inspiratory and expiratory activity and control will also be provided. Finally, the relationship between normal anatomy and physiology to disordered conditions is presented.

Keywords: larynx, phonation, respiration, voice, dysphonia

1.1 Learning Objectives

At the end of this chapter, learners will be able to

• Identify the cartilaginous, soft tissue, and neurological substrates of the larynx.

• Compare and contrast the roles of intrinsic and extrinsic laryngeal muscles.

• Discuss the aerodynamic and muscular forces underlying vocal fold vibration during phonation.

• Compare and contrast muscular and neurological control during adjustments of vocal fundamental frequency and intensity.

• Identify key musculature involved in inspiration and expiration and describe the necessary coordination between respiratory and laryngeal function to produce phonation.

• Describe the key characteristics of the sound wave produced at the vocal fold level and how supraglottal function affects and transforms this sound.

1.2 Introduction

Phonation is primarily the result of aerodynamic forces acting on the inherently elastic tissue of the vocal folds, setting them into vibration and creating acoustic energy which we call "voice." The characteristics of this vibration (e.g., the frequency of vibration) may be modified by muscular forces which influence the effective mass and tension of the vibrating folds. We are constantly modifying the aerodynamic and muscular forces underlying voice production to result in the wide variations of pitch and loudness produced in typical speech or in other aspects of voice function such as singing.

When laryngeal structure or physiology is impaired, the result may be a negative impact on laryngeal function, the efficiency of phonation, communication effectiveness, and subsequent perceptual characteristics of voice quality. The negative impacts on phonation and voice quality are perceptually labeled as dysphonia, which many speech-language pathologists will encounter during clinical practice. The purpose of this chapter is to provide an overview of the anatomy and physiology underlying normal, healthy phonation as a foundation to facilitate advanced understanding of impairments that result in dysphonia and their assessment, diagnosis, and treatment, which subsequent chapters will cover.

1.3 Evolution and Biological Roles of the Larynx

Voice production is considered an "overlaid" or nonbiological function of the larynx, taking a backseat to respiration, airway protection, and the generation of lung pressure to fixate the thorax during physical activity. According to Hirose, the larynx evolved as a simple muscular sphincter atop the primitive lung that would protect against entry of water or food. 1 This sphincter evolved into a more complicated valve capable of abduction (separation) or adduction (combination) at various levels within the larynx. Eventually, in mammals this protective structure also became used as a type of flutter valve that would vibrate during the controlled expiration.

During passive and active respiration, the glottis (the space between the two vocal folds) acts as an air valve. At rest, the two vocal folds lie in an abducted (away from midline) position creating an open glottis and a continuous passageway from the lungs to the oral cavity. During inspiratory cycles, the glottis will widen and then return to a resting open position during expiration. Deep, forceful inhalations will be accompanied by an even wider glottis. The vocal folds can be adjusted to an adducted (toward midline) position, so that the glottis will close forcefully to allow for the buildup of subglottal air pressure, which is required for coughing and clearing material that inadvertently falls into or irritates the larynx, trachea, or lungs. By closing the glottis in this manner, an individual can also fixate the thorax to direct muscular effort to the limbs for lifting and exercise. In a similar manner, glottal closure allows for the generation of abdominal pressures for bodily functions such as defecation.

Phonation for speech is an intentional (voluntary) behavior, but a number of involuntary reflexes mediated by the nervous system can override phonation, interrupting ongoing voice production (e.g., causing a voice break) or preventing a speaker from initiating phonation (e.g., think about food/liquid "going down the wrong pipe" when swallowing-you will experience strong laryngeal closure and/or cough, but not be able to produce voice until the stimulus is cleared). Peripheral sensory receptors located throughout the laryngeal surfaces can elicit adductor responses and cough subsequent to noxious stimuli. Central nervous system (CNS) pattern generators also mediate tonic and phasic activity in the 10th cranial nerve, called the vagus nerve, during respiration and swallowing.

The laryngeal adductor reflex is a response triggered by activation of the sensory division of the vagus nerve, which elicits activity within a brainstem nucleus called the solitary tract nucleus (STN-a.k.a., nucleus tractus solitaries; ? Fig. 1.1) in the medulla. The STN forms a reflex loop with the motor nucleus of the vagus nerve, the nucleus ambiguus (NA). Upon activation, the STN relays excitatory signals to the NA which then stimulates activity in the laryngeal adductor muscles via firing of alpha motor neurons. This reflex serves to forcefully close the glottis to protect the lower airway. At its most extreme, the laryngeal adductor reflex can present as laryngospasm, which is a prolonged tonic contraction of the laryngeal adductor muscles.

Fig. 1.1 Locations of sensory and motor nuclei of the vagus nerve in the brainstem.

(From LaPointe L. Atlas of Neuroanatomy for Communication Science and Disorders, 1st ed. New York: Thieme Publishers; 2011.)

The production of cough can be under voluntary or involuntary control by the nervous system. Involuntary cough is triggered by stimulation of the laryngeal sensory receptors and typically serves to eliminate foreign particles or adverse sensations throughout the larynx or in the subglottic spaces. 2 The physiology of cough can be grouped into three phases: an inspiratory phase involving recruitment of the inspiratory muscles (diaphragm and intercostals), a compressive phase involving recruitment of the expiratory muscles along with strong medial compression at the glottis via recruitment of the laryngeal adductors (and subsequent buildup of subglottal pressure), and an expulsive phase via recruitment of additional activity in the abdominal muscles resulting in high pressure air pulses being sent through the glottis. 2 The pressure pulses flowing through the glottis help to clear the airway but also result in phonation, producing the familiar perception of a cough sound. Conscious suppression of cough is possible through cortical mechanisms; however, the neural circuitry which allows for this voluntary override of the cough reflex is not well understood. 3 Voice therapy focusing on conscious control and patterning of the respiratory muscles along with semiocclusion (narrowing) in the oral cavity during exhalation are used as a behavioral voice therapy modality to treat conditions of chronic cough. 4

It is clear that the basic biological functions of the larynx generally have precedence over the overlaid development of behavioral voice function. In fact, the body's need to breathe or protect the trachea and lungs can interrupt phonation, which many readers of this book will have experienced when trying to speak while swallowing or after an exhausting physical exercise.

1.4 Respiratory Function

Phonation is built upon a foundation of respiration. Respiratory drive provides the power source for phonation. Speakers breathe during speech using a combination of diaphragmatic and thoracic muscular activity. Primary use of the diaphragm (? Fig. 1.2) for speech breathing is considered the most efficient method as its contraction is not resisted by bone, whereas the thoracic muscles must expand the rib cage to influence lung volume. During passive rest breathing, brainstem pattern generators control respiratory cycles at an unconscious level. However, for speech, pyramidal (voluntary) pathways in the CNS engage the lower motor neurons (LMN) of specific spinal nerves that form the phrenic nerve (nerves C3, C4, and C5) which innervates the diaphragm. Spinal intercostal...