TMD UNDERSTANDING THE GREAT IMPOSTER

Temporomandibular Joint Dysfunction

Understanding the Great Imposter

Relationship between occlusal proprioception and the neurologic effect

Understanding the relationship between occlusal proprioception and the neurologic effect at higher levels is essential in the understanding of TMJD symptoms that these symptoms are usually not in the joint itself, but of neuromusculature nature. Primarily in the masseter, temporal, medial and lateral pterygoid and digastric muscles. Evidence suggests that the basis of symptomatology is a disturbance of proprioception at a mid brain level.

Proprioception and the Brain Stem Reticular Formation

Located in the central part of the brain stem, the reticular formation is a tiny network of nerves the size of man’s little finger. All the major nerve trunks in the body have tendril like branches to the reticular formation. Afferent (carry nerve impulses from receptors or sense organs towards the central nervous system) sensory signals from all parts of the body go to the cerebral cortex by direct pathways ascended through the brain stem; however, they send collateral nerves (side branches) to the reticular formations. The cerebral cortex is the site of perception, thought and ability to respond to a stimulus with anything more than a simple reflex reaction. But directly stimulating the cortex will not awaken the brain. The reticular formation, also know as the reticular activating center (RAC) simply has one action to arouse the brain.

The Trigeminal Nerve

The Trigeminal Nerve (the fifth cranial nerve) is the largest of the cranial nerve, it contains 60% of the total nerve tissue of the twelve cranial nerves. It gives origin to three major divisions:

1. The Ophthalmic V1       
2. The Maxillary V2
3. The Mandibular V3  

It is the sensory nerve of the face, most of the scalp, the teeth, the mouth, nasal cavity, and the temporomandibular joint and it carries proprioceptive impulses from the masticatory muscles. It's motor branches supply the muscles of mastication and the other muscles such as the mylohyoid, anterior belly of the digastric, tensor veli palatini and tensor tympani. (An understanding of the TVP and tensor tympani you may refer back to my previous Blog TMD and the EAR and EYE Connection). 

There are many parasympathetic and sympathetic nerve fibers from the other cranial nerves that join branches of the trigeminal nerve. Its influence on the central nervous system is accordingly disproportionate to that of the other cranial nerves. The proprioceptive stimulation of occlusion is the dominant sensory input into the trigeminal system.

The trigeminal nerve is uniquely associated with the ascending activating reticular system. The primary sensory trigeminal fibers terminate in the reticular formation just medial to the spinal trigeminal nucleus. The spinal afferents from all levels terminate in the spinal and sensory nuclei of the trigeminal nerve. In voluntary movements the sensory nerves conduct impulses from the muscle spindle to a sensory area in the brain. Motor nerves then conduct impulses from the motor area to the masticatory muscles. Both nerve systems branch into the reticular activating system. RAS sends down efferent, (carry nerve impulses away from the central nervous system to effectors such as muscles or glands), impulses that either facilitate or inhibit the response. The reflex movement sensory impulses are transmitted immediately to motor nerves in spinal cord. One nerve activates the muscle and maintains its tone and the other nerve sensitizes the muscle spindle. Both voluntary and reflex mechanisms are under reticular activating system control. 

 

  

All evidence suggests that the reticular activating system has a most important role in regulating all motor activities in the body. It can modify voluntary muscle movements (controlled by the brain) or the reflex movements (controlled by the spinal cord). The importance of this voluntary and reflex arc on muscles of the face and jaw is seen in TMJD patients.

The fact that the RAS can act on the spinal cord reflexes distinguishes even further the role of noxious proprioceptive occlusal contacts (such as grinding or interferences due to mal-occlusion) as pathogenic sources to muscles. The reflex apparatus has two functions:

1. First, it generates automatic muscle movements. When noxious stimuli (i.e. occlusal prematurities) arrive at the spinal cord, they are instantaneously passed on to an adjacent motor nerve and travel back to the affected part of the body to jerk it away from the noxious stimuli. This nociceptive  (A nociceptor is a sensory receptor that responds to potentially damaging stimuli by sending nerve signals to the spinal cord and brain) reflex is designed to protect the body part from injury. The avoidance of the prematurity may protect the tooth from a noxious contact but significant accommodation of muscles and joints is often required.
2. The second function of the reflex system is to keep the muscles ready for action by maintaining muscle “tone”. This muscle tone postures the body part close to the area where function will occur. The muscle is in a state of partial contraction in anticipation of the work to be done. The muscle spindle regulates the resting tone. When muscle contracts, it squeezes the spindle; when the muscle relaxes the pressure on the spindle loosens. Change from normal tone causes the spindle to send signals via the sensory nerve to the spinal cord. The signal then excites a motor nerve to correct the contraction or relaxation of the muscle. This feedback system automatically maintains proper muscle tone.
   
   The tone of the muscle is adjusted to the functional demands on the muscle by nerve impulses which regulate the sensitivity of the spindle. Tactile sensibility for the trigeminal nerve is mediated by the sensory nucleus. Pain and thermal sensibility is mediated by the spinal nucleus or the trigeminal nerve. Under normal circumstances the reticular formation exerts a restraining influence on impulses conducted by the trigeminal as well as by the spinal nerves. It is obvious that if the tonic activity of the reticular formation is disturbed by noxious proprioceptive interferences the restraining influence of the reticular activating system on sensory impulses conducted by the trigeminal nerve will be significantly affected.

It is important to have a thorough understanding of the afferent and efferent neuromuscular connection between occlusal proprioception, the trigeminal nerve, reticular activating formation, cerebral cortex and skeletal muscles. Many of the dysfunctional conditions suffered by patients with pathologic occlusion is explainable when the neuromuscular mechanism is understood.


References:
1.  Chan C.A.: "Diagnostic Principles" Level 2 Occlusion Connections, Las Vegas, NV.
2.  Jankelson, R., "Neuromuscular Dental Diagnosis and Treatment", published by Ishiyaku EuroAmerica, ST Louis, MI. 2nd edition 2005