Functional neuroanatomy and physiology of the masticatory system
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lecture for Post Grad. students (Courtesy Management of Temporomandibular Disorders and Occlusion by Jeffrey P Okeson)
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Functional neuroanatomy and physiology of the masticatory system
- 1. Functional Neuro-Anatomy And Physiology Of The Masticatory System
- 2. Introduction Masticatory system: Nerves Muscles Three sections: Neuro-anatomy and function Physiologic activities Oro-facial pain 1/5/2018 2
- 3. Anatomy And Function Of Neuromuscular System Neurologic structures Neuron Sensory receptor Brain and brainstem Muscles Motor unit The muscle Muscle sensory receptor 1/5/2018 3
- 4. Anatomy And Function Of Neuromuscular System Neuro-muscular function Function of sensory receptors Reflex action Reciprocal innervation Regulation of muscle activity Influence from higher centers 1/5/2018 4
- 7. Sensory Receptors Neurologic structures or organs located in all body Various types Exteroceptors Nociceptors Proprioceptors Interoceptors 1/5/2018 7
- 9. The Brainstem and Brain Spinal tract nucleus Reticular formation Thalamus Hypothalamus Limbic structures Cortex 1/5/2018 9
- 11. Reticular Formation Concentrations of cell or nuclei Pathway towards higher centers Monitoring impulses that enter brainstem Controls the overall activity of the brain important influence on pain and other sensory input 1/5/2018 11
- 12. Thalamus Located in the very center of the brain Made up of numerous nuclei Interrupt impulses 1/5/2018 12
- 13. Hypothalamus Small structure in the middle of the base of the brain Major center of the brain for controlling internal body functions Stimulation excites the sympathetic nervous system increased level of emotional stress can stimulate the hypothalamus to up regulate the sympathetic nervous system and greatly influence nociceptive impulses entering the brain 1/5/2018 13
- 14. Limbic Structures limbic => “border” Border structures of the cerebrum and the diencephalon Control our emotional and behavioral activities Responsible for specific behaviors such as anger, rage, and docility Control emotions such as depression, anxiety, fear, and paranoia pain/pleasure center 1/5/2018 14
- 15. Cortex Made up predominantly of gray matter Associated with the thinking process and memory storage Acquisition of skills Different regions have different functions 1/5/2018 15
- 18. Motor Unit Variation in innervation Fewer muscle fibers per motor neuron => more precise movement Inferior lateral pterygoid muscle has a relatively low muscle fiber– motor neuron ratio Masseter has greater number of motor fibers per motor neuron 1/5/2018 18
- 21. Muscle Function Three potential functions Isotonic Contraction: When a large number of motor units in the muscle are stimulated, contraction or an overall shortening of the muscle occurs Isometric Contraction: When a proper number of motor units contract in opposition to a given force, the resultant function of the muscle is to hold or stabilize the jaw 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, precise muscle lengthening can occur that allows smooth and deliberate movement 1/5/2018 21
- 22. Muscle Function Eccentric contraction Often injurious Forced lengthening of a muscle at the same time that it is contracting 1/5/2018 22
- 23. Muscle Sensory Receptors Four major types Muscle spindles Golgi tendon organs Pacinian corpuscles Nociceptors 1/5/2018 23
- 25. Muscle Spindles Monitor tension within the skeletal muscles Fibers parallel Within each spindle, the nuclei of the intrafusal fibers are arranged in two distinct fashions: Chainlike (nuclear-chain type) Clumped (nuclear-bag type) 1/5/2018 25
- 31. Nociceptors Sensory receptors stimulated by injury Throughout most of the tissues Several general types 1/5/2018 31
- 33. Function Of Sensory Receptors Passive stretching of muscle => spindles inform the CNS of activity Active muscle contraction monitored by both the Golgi tendon organs and the muscle spindles Movement of the joints and tendons stimulates pacinian corpuscles 1/5/2018 33
- 34. Reflex Action A reflex action is the response resulting from a stimulus that passes as an impulse along an afferent neuron to a posterior nerve root or its cranial equivalent, from which it is then transmitted to an efferent neuron leading back to the skeletal muscle Reflex action may be monosynaptic or polysynaptic Two general reflex actions are important in the masticatory system myotatic reflex nociceptive reflex 1/5/2018 34
- 37. Myotatic (Stretch) Reflex Occurs without a specific response from cortex Important in determining the resting position of the jaw Principal determinant of muscle tonus in elevator muscles Protects masticatory system from sudden stretching of a muscle Maintains stability of the musculoskeletal system with muscle tonicity 1/5/2018 37
- 39. Nociceptive (Flexor) Reflex Protects the teeth and supportive structures from potential damage due to sudden and unusually heavy functional forces 1/5/2018 39
- 40. Reciprocal Innervation Control of antagonistic muscles is of vital importance in reflex activity Antagonism in muscle activity Jaw opening The neurologic controlling mechanism for these antagonistic groups is known as reciprocal innervation Enables smooth and exact control 1/5/2018 40
- 41. Influence From Higher Centers Within the brainstem is a pool of neurons that control rhythmic muscle activities known as the central pattern generator (CPG) Example: During the process of chewing the CPG initiates contraction of the supra- and infrahyoid muscles at the precise time the elevator muscles are told to relax. This allows the mouth to open and accept food. Next, the CPG initiates contraction of the elevator muscles while relaxing the supra- and infrahyoid muscles, producing closure of the mouth onto the food. 1/5/2018 41
- 42. Influence From Higher Centers Increased emotional stress => the limbic structures and hypothalamic/pituitary/adrenal axis (HPA) => gamma efferent system => contraction of the intrafusal fibers Increase in muscle tonus More sensitive to external stimuli Greater risk of muscle fatigue Increase in interarticular pressure of the TMJ 1/5/2018 42
- 43. Influence From Higher Centers Increased gamma efferent activity may also increase the amount of irrelevant muscle activity Reticular formation involved in the HPA pathway Often these activities assume the role of nervous habits, such as biting on the fingernails or on pencils, clenching the teeth, or bruxism 1/5/2018 43
- 44. MAJOR FUNCTIONS OF MASTICATORY SYSTEM 1/5/2018 44
- 45. Functions Mastication Swallowing Speech 1/5/2018 45
- 47. Mastication Act of chewing food Initial stage of digestion generally automatic and practically involuntary 1/5/2018 47
- 48. The Chewing Stroke Rhythmic and well-controlled separation and closure Under the control of the CPG (brainstem) 1/5/2018 48
- 51. Anterior Movement Early stages Incising of food Forward movement Alignment and position of opposing incisors Later stages Crushing of bolus Concentrated on posterior teeth Little anterior movement 1/5/2018 51
- 53. The Chewing Stroke (Incisor) 1/5/2018 53
- 54. The Chewing Stroke (Molar) 1/5/2018 54
- 55. The Chewing Stroke (Condyle) 1/5/2018 55
- 58. Lateral Movement Relates to the stage of mastication Food introduced in mouth: lateral movement is great Varies according to the consistency Hardness of food also has an effect on the number of chewing strokes 1/5/2018 58
- 59. Lateral Movement Although mastication can occur bilaterally, about 78% of observed subjects have a preferred side where the majority of chewing occurs Side with the greatest number of tooth contacts during lateral glide People who seem to have no side preference simply alternate their chewing from one side to the other 1/5/2018 59
- 60. Tooth Contacts During Chewing When food introduced Increases when bolus is broken down Minimal force Two types: Gliding Single Average contact time during mastication 194ms 1/5/2018 60
- 61. Tooth Contacts During Chewing influence or even dictate the initial opening and final grinding phase tall cusps and deep fossae promote a predominantly vertical chewing stroke Flattened or worn teeth encourage a broader chewing stroke 1/5/2018 61
- 62. Tooth Contacts During Chewing 1/5/2018 62
- 63. Tooth Contacts During Chewing Effect of TMJ Normal individuals masticate with chewing strokes that are well rounded, more repeated, and with definite borders Person with TMJ pain, less repeated pattern, strokes are much shorter and slower and have an irregular pathway Relate to the altered functional movement of the condyle around which the pain is centered 1/5/2018 63
- 64. Forces Of Mastication Maximal biting force varies Males can bite with more force than can females Greatest maximal biting force reported is 975 lb. (443 kg) More on molar than that of incisor Increase with age up to adolescence Study by Gibbs and colleagues reports that grinding phase of closure stroke averaged 58.7 lb. on posterior teeth With tougher foods, chewing occurs predominantly on first molar and second premolar areas 1/5/2018 64
- 65. Role Of Soft Tissues In Mastication Mastication could not be performed without the aid of adjacent soft tissue Role of lips Guide and control intake Sealing off the oral cavity Tongue Maneuvering of food initiates the breaking up process by pressing it against the hard palate Push food onto the occlusal surfaces of teeth After eating, tongue sweeps the teeth to remove any food residue that has been trapped in the oral cavity 1/5/2018 65
- 67. Swallowing Series of coordinated muscular contractions Food from oral cavity through the esophagus to the stomach Consists of voluntary, involuntary, and reflex muscular activity decision to swallow depends degree of fineness of food intensity of taste extracted degree of lubrication of bolus 1/5/2018 67
- 68. Stabilization Of Mandible Mandible must be fixed so that contraction of suprahyoid and infrahyoid muscles can control proper movement of hyoid bone needed for swallowing Adult swallow => teeth for mandibular stability (somatic swallow) Absence of teeth => mandible is braced by placing tongue forward and between the dental arches or gum pads (infantile or visceral swallow) 1/5/2018 68
- 69. Infantile Swallow Normal transition from infantile swallow to adult swallow does not occur lack of tooth support because of poor tooth position or arch relationship Discomfort occurs during tooth contact because of caries or tooth sensitivity Over retention => labial displacement of anterior teeth by tongue muscle => anterior open bite 1/5/2018 69
- 70. Adult Swallow Average tooth contact => about 683 ms Three times longer than during mastication Force applied to teeth => approximately 66.5 lb. which is 7.8 lb. more than force applied during mastication 1/5/2018 70
- 76. Frequency of Swallowing Swallowing cycle occurs 590 times during a 24-hour period: 146 cycles during eating, 394 cycles between meals while awake, and 50 cycles during sleep Lower levels of salivary flow during sleep result in less need to swallow 1/5/2018 76
- 78. Speech 3rd major function occurs when a volume of air is forced from lungs by diaphragm through larynx and oral cavity Controlled contraction and relaxation of the vocal cords precise form assumed by the mouth determines the resonance and exact articulation of sound Inspiration of air is relatively quick and taken at the end of a sentence or pause Expiration is prolonged, allowing a series of syllables, words, or phrases to be uttered 1/5/2018 78
- 79. Articulation of Sound Varying relationships of lips and tongue to the palate and teeth Sounds formed by lips are the letters “M,” “B,” and “P.” Teeth are important in saying the “S” sound Tongue and palate are especially important in forming “D” sound Many sounds can also be formed by using a combination of anatomic structures. Example, the tongue touches the maxillary incisors to form the “Th” sound 1/5/2018 79
- 80. Articulation of Sound Lower lip touches incisal edges of the maxillary teeth to form the “F” and “V” posterior portion of the tongue rises to touch the soft palate for “K” and “G” 1/5/2018 80
- 82. Considerations of Orofacial Pain Types Acute Chronic Dull achy that can significantly decrease the individual’s ability to function Affects quality of life 1/5/2018 82
- 83. 1/5/2018 83
- 84. Pain Modulation Degree of pain relates more closely to the patient’s perceived threat of the injury and the amount of attention given to the injury Pain modulation means that impulses arising from a noxious stimulus can be altered before they reach the cortex for recognition Occur as the primary neuron synapses with the interneuron or as the input ascends to complex brainstem and cortex May have excitatory or inhibitory effect 1/5/2018 84
- 85. Pain Modulation Mechanisms by which pain can be modulated Non-painful cutaneous stimulation system Intermittent painful stimulation system psychological modulating system 1/5/2018 85
- 86. Non-painful Cutaneous Stimulation System It has been postulated that if the larger fibers are stimulated at the same time as the smaller ones, the larger fibers will have precedence and mask the input to the CNS from the smaller ones, described as the gate control theory For the effect to be great, the stimulation of the large fibers must be constant and below a painful level The effect is immediate and usually vanishes after the large- fiber stimulus is removed 1/5/2018 86
- 87. Non-painful Cutaneous Stimulation System Noxious input that reaches the spinal cord can also be altered at virtually every synapse in the ascending pathway to the cortex Descending inhibitory system = modulate this input so as not to be perceived by the cortex as pain Transcutaneous electrical nerve stimulation (TENS) = Constant sub threshold impulses in larger nerves near the site of an injury or other lesion block the input from smaller nerves, preventing painful stimuli from reaching the brain 1/5/2018 87
- 88. Intermittent painful stimulation system Stimulation of areas of the body that have high concentrations of nociceptors and low electrical impedance Reduce pain felt at a distant site due to the release of endorphins Two basic types Enkephalins Beta-endorphins Enkephalins appear to be released in the cerebrospinal fluid and therefore act quickly and locally to reduce pain Beta-endorphins are released into the bloodstream like hormones by the hypophysis cerebri are slower-acting but their effect lasts longer 1/5/2018 88
- 89. Psychological Modulating System Not well understood Certain psychological states affect pain Emotional stress can be strongly correlated with levels of pain Patients who devote a great amount of attention to their pain are likely to suffer more Prior conditioning and experience also affect the degree of pain felt 1/5/2018 89
- 90. Types Of Pain When a patient describes a pain whose site and source are in the same location, is it is referred to as a primary pain When patient feels the pain is not where the pain is coming from. These are called heterotopic pains. There are generally three types of heterotopic pains Central pain Projected pain Referred pain 1/5/2018 90
- 91. Types Of Pain Referred pain clinical rules The most frequent occurrence of referred pain is within a single nerve root, passing from one branch to another (e.g., a mandibular molar referring pain to a maxillary molar) Sometimes referred pain can be felt outside the nerve responsible for it. When this occurs, it generally moves cephalad (upward, toward the head) and not caudal In the trigeminal area, referred pain rarely crosses the midline unless it originates at the midline. For example, pain in the right TMJ will not cross over to the left side of the face 1/5/2018 91
- 92. The Central Excitatory Effect Certain input into the CNS, such as deep pain, can create an excitatory effect on other unassociated interneurons. This phenomenon is called central excitatory effect Neurons carrying nociceptive input into the CNS can excite other interneurons in one of two possible ways 1/5/2018 92
- 93. The Central Excitatory Effect Afferent input is constant and prolonged, it continuously bombards the interneuron, resulting in an accumulation of neurotransmitter substance at the synapses. If this accumulation becomes great, the neurotransmitter substance can spill over to an adjacent interneuron, causing it also to become excited. From there, the impulses go to the brain centrally, and the brain perceives nociception as being transmitted by both neurons. 1/5/2018 93
- 94. The Central Excitatory Effect Single interneuron may itself be one of many neurons that converge to synapse with the next ascending interneuron. As this convergence nears the brainstem and cortex, it can become increasingly difficult for the cortex to evaluate the precise location of the input. 1/5/2018 94
- 95. Clinical Manifestations Of The Central Excitatory Effect When afferent interneurons are involved, referred pain is often reported Wholly dependent on the original source of pain Local provocation of the source increases the pain both at the source and often also at the site A local anesthetic blockade of the site does not affect the pain felt, since this is not the origin of the pain A local anesthetic blockade of the source reduces both the source and the site of referred pain 1/5/2018 95
- 96. Clinical Manifestations Of The Central Excitatory Effect Another type of pain sensation that can be experienced when afferent interneurons are stimulated is secondary hyperalgesia Secondary hyperalgesia/allodynia is present when there is increased sensitivity of tissues without a local cause. A common location for secondary hyperalgesia/allodynia is the scalp. Patients who experience constant deep pain will commonly report that their “hair hurts.” local anesthetic blocking at the source may not immediately arrest the symptoms. Instead, may linger for some time(12 to 24 hours) after the blockade is administered 1/5/2018 96
Notes de l'éditeur
- This chapter is divided into three sections basic neuroanatomy and function of the neuromuscular system basic physiologic activities of mastication, swallowing, and speech concepts and mechanisms that are necessary to understand orofacial pain
- The neuron: basic structural unit. composed of a mass of protoplasm n protoplasmic processes. cell bodies located in the spinal cord are found in the gray substance. Cell bodies found outside the CNS are grouped together in ganglia. capable of transferring electrical and chemical impulses. May b afferent or efferent or interneurons
- Sensory receptors r neurologic structures or organs located in all body tissues dat provide information to CNS by way of afferent neurons. receptors in peripheral tissues such as the skin n oral mucosa r exteroceptors receptors specific for discomfort and pain r nociceptors. located throughout the body Proprioceptors provide information regarding the position n movement
- first-order neurons (primary afferents) carry input into dorsal horn to synapse with second-order neurons. The second-order neuron then crosses over and ascends on to higher centers. Small interneurons connect the primary afferent neuron with primary motor (efferent) neuron, allowing reflex arc activity. dorsal root ganglion contains the cell bodies of the primary afferent neurons
- second-order neurons, these neurons carry them to the higher centers for interpretation and evaluation numerous centers in the brainstem and brain
- Through out body 1st order neuron synapse in dorsal horn but afferent fibers from face and oral cavity donot enter through spinal nerves. trigeminal nerve entering the brainstem at the level of the pons. primary afferent neuron enters the brainstem to synapse with a second-order neuron in trigeminal spinal tract nucleus (STN of V). spinal tract nucleus is divided into three regions; subnucleus oralis (sno), subnucleus interpolaris (sni), and subnucleus caudalis (snc). also composed of the motor nucleus of V (MN of V) and the main sensory nucleus of V (SN of V). cell bodies of the trigeminal nerve are located in the gasserian ganglion (GG). Once of second-order neuron receives the input, it is carried on to the thalamus (Th) for interpretation.
- The reticular formation plays an extremely important role in monitoring impulses that enter the brainstem. Controls the overall activity of the brain by either enhancing the impulses to the brain or inhibiting the impulses
- major center of the brain for controlling internal body functions, such as body temperature, hunger, and thirst Stimulation of the hypothalamus excites the sympathetic nervous system throughout the body, increasing the overall level of activity of many internal parts of the body
- basic component of the neuromuscular system which consists of a number of muscle fibers 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 initiates depolarization of the muscle fibers
- The number of muscle fibers innervated by one motor neuron varies greatly according to the function of the motor unit may innervate only 2 or 3 muscle fibers, as in the ciliary muscles (which precisely control lens of the eye) Inferior lateral pterygoid muscle has a relatively low muscle fiber– motor neuron ratio and therefore is capable of the fine adjustments in length needed to adapt to horizontal changes in the mandibular position Masseter has greater number of motor fibers per motor neuron, which corresponds to its more gross functions of providing the force necessary during mastication
- To understand effect muscles have on each other and their bony attachments, basic skeletal relationships of the head and neck must be observed. skull is supported in position by the cervical spine. It is not centrally located or balanced over the cervical spine. it is overbalanced to the anterior and quickly fall forward
- Muscles are needed to overcome this imbalance 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, sternocleidomastoid, splenius capitis, and longus capitis 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).
- Isotonic Contraction: occurs in the masseter when the mandible is elevated, forcing the teeth through a bolus of food Isometric Contraction: occurs in the masseter when an object is held between the teeth Controlled relaxation: This type of controlled relaxation is observed in the masseter when the mouth opens to accept a new bolus of food during mastication.
- An example of eccentric contraction occurs with the tissue damage associated during an extension-flexion injury (whiplash injury)
- muscle spindles, which are specialized receptor organs found in the muscle tissues; Golgi tendon organs, located in the tendons; Pacinian corpuscles, located in tendons, joints, periosteum, fascia, and subcutaneous tissues; nociceptors, found throughout all the tissues of the masticatory system
- Skeletal muscles consist of two types of muscle fibers. The first is extrafusal fiber, which is contractile and makes up the bulk of the muscle; the other is intrafusal fiber, which is only minutely contractile. A bundle of intrafusal muscle fibers bound by a connective tissue sheath is called a muscle spindle
- Two types of afferent nerves, larger fibers conduct impulses at a higher speed and have lower thresholds (central region) and end in the poles of the spindle (away from the central region) are the smaller group (II, A-beta) Fiber stretch is monitored at the nuclear-chain and nuclear-bag regions Afferent of muscles of mastication go to CNS trigeminal nucleus
- intrafusal fibers receive efferent innervation by way of fusimotor nerve fibers, originate in the CNS; when stimulated, they cause contraction of the intrafusal fibers. When the intrafusal fibers contract, the nuclear chain and nuclear-bag areas are stretched, which is registered as if the entire muscle were stretched, and afferent activity is initiated Thus there are two manners in which the afferent fibers of the muscle spindles can be stimulated: generalized stretching of the entire muscle (extrafusal fibers) and contraction of the intrafusal fibers by way of the gamma efferents. The muscle spindles can register only the stretch; they cannot differentiate between these two activities.
- extrafusal muscle fibers receive innervation by way of the alpha efferent motor neurons, cell bodies in the trigeminal motor nucleus. Stimulation of these neurons therefore causes the group of extrafusal muscle fibers (the motor unit) to contract
- Located in the muscle tendon they are more sensitive and active in reflex regulation during normal function. They primarily monitor tension occur in series with the extrafusal muscle fibers Afferent fibers enter near the middle of the organ and spread out over the extent of the fibers. Tension on the tendon stimulates the receptors in the Golgi tendon organ. Therefore contraction of the muscle also stimulates the organ
- large oval organs made up of concentric lamellae of connective tissue perception of movement and firm pressure At the center of each corpuscle is a core containing the termination of a nerve fiber. These corpuscles are found in the tendons, joints, periosteum, tendinous insertions, fascia, and subcutaneous tissue. Pressure applied to such tissues deforms the organ and stimulates the nerve fiber
- Several general types exist: some respond exclusively to noxious mechanical and thermal stimuli; others respond to a wide range of stimuli, from tactile sensations to noxious injury; still others are low-threshold receptors specific for light touch, pressure, or facial hair movement. The last type is sometimes called a mechanoreceptor.
- only monosynaptic jaw reflex. The myotatic reflex is activated by a sudden application of downward force to the chin with a small rubber hammer. This results in contraction of the elevator muscles (masseter), preventing further stretching and often causing elevation of the mandible into occlusion.
- Sudden stretching of the muscle spindle increases the afferent output from the spindle. The afferent impulses pass into the brainstem by way of the trigeminal mesencephalic nucleus. There the afferent fibers synapse in the trigeminal motor nucleus with the alpha efferent motor neurons, leading directly back to the extrafusal fibers of the elevator muscle, which was stretched. The reflex information sent to the extrafusal fibers is to contract. Note the presence of the gamma efferent fibers. Stimulation of these can cause contraction of the intrafusal fibers of the spindle and thus sensitize the spindle to a sudden stretch
- To prevent dislocation, the elevator muscles (and other muscles) are maintained in a mild state of contraction called muscle tonus. This property of the elevator muscles counteracts the effect of gravity on the mandible and maintains the articular surfaces of the joint in constant contact
- polysynaptic reflex to noxious stimuli and is therefore considered to be protective The nociceptive reflex is activated by unexpectedly biting on a hard object. The noxious stimulus is initiated when the tooth and periodontal ligament is stressed. Afferent nerve fibers carry the impulse to the trigeminal spinal tract nucleus. The afferent neurons stimulate both excitatory and inhibitory interneurons. The interneurons synapse with the efferent neurons in the trigeminal motor nucleus. Inhibitory interneurons synapse with efferent fibers leading to the elevator muscles. The message carried is to discontinue contraction. The excitatory interneurons synapse with the efferent neurons that innervate the jaw, depressing muscles. The message sent is to contract, which brings the teeth away from the noxious stimulus
- For the mandible to be elevated by the temporal, medial pterygoid, or masseter muscles, the suprahyoid muscles must relax and lengthen. Likewise for the mandible to be depressed, the suprahyoids must contract while the elevators relax and lengthen. Helps in regulation of muscle activity
- For the CPG to be most efficient, it must receive constant sensory input from the masticatory structures. Therefore, the tongue, lips, teeth, and periodontal ligaments are constantly feeding back information that allows the CPG to determine the most appropriate and efficient chewing stroke. Once an efficient chewing pattern that minimizes damage to any structure is found, it is learned and repeated. This learned pattern is called a muscle engram
- In the absence of any significant emotional state, the action is usually predictable and accomplishes the task efficiently. However, when the individual is experiencing higher levels of emotion, such as fear, anxiety, frustration, or anger, the following major modifications of muscle activity can occur:
- Mastication is defined as the act of chewing food. It represents the initial stage of digestion, when the food is broken down into small particles for ease of swallowing It is a functional activity that is generally automatic and practically involuntary; yet when desired, it can be readily brought under voluntary control
- Mastication is made up of rhythmic and well-controlled separation and closure of the maxillary and mandibular teeth
- (mandible is traced in frontal plane) complete chewing stroke has a movement pattern described as tear-shaped. It can be divided into an opening phase and a closing phase. incisal edges of the teeth are about 16 to 18 mm apart. It then moves laterally 5 to 6 mm from the midline as the closing movement begins closing movement has been further subdivided into crushing phase (trap food) n grinding phase (mandible is guided by the occlusal surfaces of teeth back to intercuspal position, which causes cuspal inclines of teeth to pass across each other, permitting shearing and grinding of bolus of food)
- As teeth approach each other, lateral displacement is lessened, when teeth are only 3 mm apart, jaw occupies position only 3 to 4 mm lateral to starting position of the chewing stroke. At this point the teeth are so positioned that buccal cusps of mandibular teeth are almost directly under buccal cusps of the maxillary teeth on the side to which the mandible has been shifted
- In the early stages, incising of food is often necessary. During incising, the mandible moves forward a significant distance, depending on the alignment and position of the opposing incisors
- sagittal plane (working side) During opening phase mandible moves slightly anteriorly During the closing phase, it follows a slightly posterior pathway, ending in an anterior movement back to the maximum intercuspal position amount of anterior movement depends on contact pattern of anterior teeth and stage of mastication
- The movement of the mandibular first molar in the sagittal plane during a typical chewing stroke varies according to the side on which the person is chewing Working side: first molar moves in a pathway similar to that of the incisor In other words, the molar moves slightly forward during the opening phase and closes on a slightly posterior pathway, moving anteriorly during the final closure as the tooth intercuspates
- The condyle on the right side also follows this pathway, closing in a slightly posterior position with a final anterior movement into intercuspation
- left mandibular first molar drops almost vertically, with little anterior or posterior movement until the complete opening phase has occurred. Upon closure, the mandible moves slightly anteriorly and the tooth returns almost directly to intercuspation The condyle on the left side also follows a pathway similar to that of the molar. There is no final anterior movement into the intercuspal position in the pathway of either the molar or the condyle
- The harder the food, the more chewing strokes needed. It is interesting, that in some subjects the number of chewing strokes does not change with varying textures of food. This might suggest that for some subjects the CPG is less influenced by sensory input and more by muscle engrams.
- Chewing on one side leads to unequal loading of the temporomandibular joints. Under normal conditions, this does not create any problem owing to the stabilizing effect of the superior lateral pterygoids on the discs.
- In the final stages of mastication, just prior to swallowing, contact occurs during every stroke but forces to the teeth are minimal. Two types of contacts have been identified: gliding, which occurs as the cuspal inclines pass by each other during the opening and grinding phases of mastication, and single, which occurs in the maximum intercuspal position
- When the posterior teeth contact in undesirable lateral movement, the malocclusion produces an irregular and less repeatable chewing stroke
- These slower, irregular but repeatable pathways appear to relate to the altered functional movement of the condyle around which the pain is centered
- Mastication could not be performed without the aid of adjacent soft tissue structures. As food is introduced into the mouth, the lips guide and control intake while also sealing off the oral cavity. The lips are especially necessary when liquid is being introduced.
- decision to swallow depends on several factors: the degree of fineness of the food, the intensity of thetaste extracted, and the degree of lubrication of the bolus. During swallowing the lips are closed, sealing the oral cavity. The teeth are brought up into their maximum intercuspal position, thus stabilizing the mandible.
- Stabilization of the mandible is an important part of swallowing. The normal adult swallow utilizing the teeth for mandibular stability has been called the somatic swallow. When teeth are not present, as in the infant, the mandible must be braced by other means. In the infantile swallow,or visceral swallow, mandible is braced by placing tongue forward and between the dental arches or gum pads. This type of swallow occurs until the posterior teeth erupt
- On occasion, the normal transition from infantile swallow to adult swallow does not occur. This may be due to:…. Over retention of infantile swallow can result in labial displacement of the anterior teeth by the powerful tongue muscle. This may present clinically as an anterior open bite
- is voluntary and begins with selective parting of masticated food. separation is performed mostly by tongue. bolus is placed on the dorsum of the tongue and pressed lightly against the hard palate. The tip of the tongue rests on the hard palate just behind the incisors. The lips are sealed and the teeth are brought together. The presence of the bolus on the mucosa of the palate initiates a reflex wave of contraction in the tongue that presses the bolus backward. As the bolus reaches the back of the tongue, it is transferred to the pharynx.
- Once the bolus has reached the pharynx, a peristaltic wave caused by contraction of the pharyngeal constrictor muscles carries it down to the esophagus. The soft palate rises to touch the posterior pharyngeal wall, sealing off the nasal passages. The epiglottis blocks the pharyngeal airway to the trachea and keeps the food in the esophagus. During this stage of swallowing, the pharyngeal muscular activity opens the pharyngeal orifices of the Eustachian tubes, which are normally closed. It is estimated that these first two stages of swallowing together last about 1 second.
- The third stage of swallowing consists of passing the bolus through the length of the esophagus and into the stomach. Peristaltic waves take 6 to 7 s to carry the bolus through the esophagus.
- As the bolus approaches the cardiac sphincter, the sphincter relaxes and lets it enter the stomach. In the upper section of the esophagus, the muscles are mainly voluntary and can be used to return food to the mouth when necessary for more complete mastication. In the lower section, the muscles are entirely involuntary.
- Acute pain provides protection from environment challenges (the nociceptive reflex) Some pains last far longer than normal healing time and therefore no longer have protective value termed chronic
- approximately 45% of human sensory cortex is dedicated to the face, mouth, and oral structures This degree of sensory dedication suggests that these structures have significant meaning to the individual.
- degree of pain and suffering does not correlate well with the amount of tissue damage. Instead, the degree of pain relates more closely to the patient’s perceived threat of the injury and amount of attention given to the injury Pain modulation means that impulses arising from a noxious stimulus, which are primarily carried by afferent neurons from the nociceptors, can be altered before they reach the cortex for recognition
- Transcutaneous electrical nerve stimulation (TENS) is an example of the non painful cutaneous stimulation system masking a painful sensation
- pain-modulation system can be evoked by the stimulation of areas of the body that have high concentrations of nociceptors and low electrical impedance
- The first is central pain. When a tumor or other disturbance is present in the CNS, the pain is often felt not in the CNS but in peripheral structures projected pain: neurologic disturbances cause painful sensations to shoot down the peripheral distributions of the same nerve root that is involved in the disturbance referred pain. sensations are felt not in the involved nerve but in other branches of that nerve or even in an entirely different nerve
- Diagnostic blocking of the painful areas can be extremely valuable in providing information that can help to differentiate the site of pain from the source of pain.