2functional neuroanatomy and physiology
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CHAPTER 2 Functional Neuroanatomy and Physiology
of the Masticatory System
You cannot successfully treat dysfunction unless you understand function. JPO
The function of the masticatory system is complex. Discriminatory contraction of the various head and neck muscles is necessary to move the mandible precisely and allow effective functioning. A highly refined neurologic control system regulates and coordinates the activities of the entire masticatory system. It consists primarily of nerves and muscles; hence the term neuromuscular system. A basic understanding of the anatomy and function of the neuromuscular system is essential to understanding the influence that tooth contacts, as well as other conditions, have on mandibular movement.
This chapter is divided into three sections. The first section reviews in detail the basic neuroanatomy and function of the neuromuscular system. The second describes the basic physiologic activities of mastication, swallowing, and speech. The third section reviews important concepts and mechanisms that are necessary to understand orofacial pain. Grasping the concepts in these three sections should greatly enhance the clinician's ability to understand a patient's complaint and provide effective therapy.
ANATOMY AND FUNCTION OF THE
For purposes of discussion, the neuromuscular system is divided into two major components: (1) the neurologic structures and (2) the muscles. The anatomy and function of each of these components is reviewed separately, although in many instances it is difficult to separate function. With an understanding of these components, basic neuromuscular function can be reviewed.
The basic component of the neuromuscular system is the motor unit, which consists of a number of muscle fibers that are innervated by one motor neuron. Each neuron joins with the muscle fiber at a motor endplate. When the neuron is activated, the motor endplate is stimulated to release small amounts of acetylcholine, which initiate depolarization of the muscle fibers. Depolarization causes the muscle fibers to shorten or contract.
The number of muscle fibers innervated by one motor neuron varies greatly according to the function of the motor unit. The fewer the muscle fibers per motor neuron, the more precise the movement. A single motor neuron may innervate only two or three muscle fibers, as in the ciliary muscles (which precisely control the lens of the eye). Conversely, one motor neuron may innervate hundreds of muscle fibers, as in any large muscle (e.g., the rectus femoris in the leg). A similar variation exists in the number of muscle fibers per motor neuron within the muscles of mastication. The inferior lateral pterygoid muscle has a relatively low muscle fiber/motor neuron ratio; therefore it is capable of the fine adjustments in length needed to adapt to horizontal changes in the mandibular position. In contrast, the masseter has a greater number of motor fibers per motor neuron, which corresponds to its more gross function of providing the force necessary during mastication.
Hundreds to thousands of motor units along with blood vessels and nerves are bundled together by connective tissue and fascia to make up a muscle. The major muscles that control movement of the masticatory system were described in Chapter 1. To understand the effect these muscles have on each other and their bony attachments, one must observe the basic skeletal relationships of the head and neck. The skull is supported in position by the cervical spine. However, the skull is not centrally located or balanced over the cervical spine. In fact, if a dry skull were placed in its correct position on the cervical spine, it would be over balanced to the anterior and quickly fall forward. Any balance becomes even more remote when the position of the mandible hanging below the anterior portion of the skull is considered. It can be easily seen that a balance of the skeletal components of the head and neck does not exist. Muscles are necessary to overcome this weight and mass imbalance. If the head is to be maintained in an upright position so that one can see forward, muscles that attach the posterior aspect of the skull to the cervical spine and shoulder region must contract. Some of the muscles that serve this function are the trapezius, sternocleidomastoideus, splenius capitis, and longus capitis. It is possible, however, for these muscles to overcontract and direct the line of vision too far upward. To counteract this action, an antagonistic group of muscles exists in the anterior region of the head: the masseter (joining the mandible to the skull), the suprahyoids (joining the mandible to the hyoid bone), and the infrahyoids (joining the hyoid bone to the sternum and clavicle). When these muscles contract, the head is lowered. Thus a balance of muscular forces exists that maintains the head in a desired position (Fig. 2-1). These muscles, plus others, also maintain proper side-to-side positioning and rotation of the head.
The motor unit can carry out only one action: contraction or shortening. The entire muscle, however, has three potential functions:
1. When a large number of motor units in the muscle are stimulated, contraction or an overall shortening of the muscle occurs. This type of shortening under a constant load is called isotonic contraction. Isotonic contraction occurs in the masseter when the mandible is elevated, forcing the teeth through a bolus of food.
2. When a proper number of motor units contract opposing a given force, the resultant function of the muscle is to hold or stabilize the jaw. This contraction without shortening is called isometric contraction, and it occurs in the masseter when an object is held between the teeth (e.g., a pipe or pencil). 3. A muscle can also function through controlled relaxation. When stimulation of the motor unit is discontinued, the fibers of the motor unit relax and return to their normal length. By control of this decrease in motor unit stimulation, a precise muscle lengthening can occur that allows smooth and deliberate movement. This type of controlled relaxation is observed in the masseter when the mouth opens to accept a new bolus of food during mastication.
Using these three functions, the muscles of the head and neck maintain a constant desirable head position. A balance exists between the muscles that function to raise the head and those that function to depress it. During even the slightest movement of the head, each muscle functions in harmony with others to carry out the desired movement. If the head is turned to the right, certain muscles must shorten (isotonic contraction), others must relax (controlled relaxation), and still others must stabilize or hold certain relationships (isometric contraction). A highly sophisticated control system is necessary to coordinate this finely tuned muscle balance.
These three types of muscle activities are present during routine function of the head and neck. Another type of muscle activity, eccentric contraction, can occur during certain conditions. This type of contraction is often injurious to the muscle tissue. Eccentric contraction refers to the lengthening of a muscle at the same time that it is contracting. An example of eccentric contraction occurs with the tissue damage associated during an extension-flexion injury (whiplash injury). At the precise moment of a motor vehicle accident, the cervical muscles contract to support the head and resist movement. If, however, the impact is great, the sudden change in the inertia of the head causes it to move while the muscles contract trying to support it. The result is a sudden lengthening of the muscles while they are contracting. This type of sudden lengthening of muscles while contracting often results in injury and is discussed in later sections of the chapter devoted to muscle pain. Fig. 2-1
Precise and complex balance of the head and neck muscles must exist to maintain proper head position and function. A, Muscle system. B, Each of the major muscles acts like an elastic band. The tension provided must precisely contribute to the balance that maintains the desired head position. If one elastic band breaks, the balance of the entire system is disrupted and the head position altered.
The basic structural unit of the nervous system is the neuron. The neuron is composed of a mass of protoplasm termed the nerve cell body and protoplasmic processes from the nerve cell body called dendrites and axons. The nerve cell bodies located in the spinal cord are found in the gray substance of the central nervous system (CNS). Cell bodies found outside the CNS are grouped together in ganglia. The axon (from the Greek word axon, meaning axle or axis) is the central core that forms the essential conducting part of a neuron and is an extension of cytoplasm from a nerve cell. Many neurons group together to form a nerve fiber. These neurons are capable of transferring electrical and chemical impulses along their axes, enabling information to pass both in and out of the
CNS. Depending on their location and function, neurons are designated by different terms. An afferent neuron conducts the nervous impulse toward the CNS, where as an efferent neuron conducts it peripherally. Internuncial neurons, or interneurons, lie wholly within the CNS. Sensory or receptor neurons, afferent in type, receive and convey impulses from receptor organs. The first sensory neuron is called the primary or first-order neuron. Second - and third-order sensory neurons are internuncial. Motor or efferent neurons convey nervous impulses to produce muscular or secretory effects.