The rule of 4 of the brainstem: a simplified method for understanding brainstem anatomy and brainstem vascular
syndromes for the non-neurologist.
The Rule Of 4 & The Brainstem
The rule of 4 is a simple method developed to help �students of neurology� to remember the anatomy of the brainstem and thus the features of the various brainstem vascular syndromes. As medical students, we are taught detailed anatomy of the brainstem containing a bewildering number of structures with curious names such as superior colliculi, inferior olives, various cranial nerve nuclei and the median longitudinal fasciculus. In reality when we do a neurological examination we test for only a few of these structures. The rule of 4 recognizes this and only describes the parts of the brainstem that we actually examine when doing a neurological examination. The blood supply of the brainstem is such that there are paramedian branches and long circumferential branches (the anterior inferior cerebellar artery (AICA), the posterior inferior cerebellar artery (PICA) and the superior cerebellar artery (SCA). Occlusion of the paramedian branches results in medial (or paramedian) brainstem syndromes and occlusion of the circumferential branches results in lateral brainstem syndromes. Occasionally lateral brainstem syndromes are seen in unilateral vertebral occlusion. This paper describes a simple technique to aid in the understanding of brainstem vascular syndromes.
Any attempt to over simplify things runs the risk of upsetting those who like detail and I apologize in advance to the anatomists among us, but for more than 15 years this simple concept has helped numerous students and residents understand, often for the first time, brainstem anatomy and the associated clinical syndromes that result.
In The Rule Of 4 There Are 4 Rules:
There are 4 structures in the �midline� beginning with M.
There are 4 structures to the side beginning with S.
There are 4 cranial nerves in the medulla, 4 in the pons and 4 above the pons (2 in the midbrain).
The 4 motor nuclei that are in the midline are those that divide equally into 12 except for 1 and 2, that is 3, 4, 6 and 12 (5, 7, 9 and 11 are in the lateral brainstem).
If you can remember these rules and know how to examine the nervous system, in particular the cranial nerves, then you will be able to diagnose brainstem vascular syndromes with ease.
Figure 1 shows a cross-section of the brainstem, in this case at the level of the medulla, but the concept of 4 lateral and 4 medial structures also applies to the pons, only the 4 medial structures relate to midbrain vascular syndromes.
The 4 Medial Structures & The Associated Deficit Are:
The Motor pathway (or corticospinal tract): contra lateral weakness of the arm and leg.
The Medial Lemniscus: contra lateral loss of vibration and proprioception in the arm and leg.
The Medial longitudinal fasciculus: ipsilateral inter- nuclear ophthalmoplegia (failure of adduction of the ipsilateral eye towards the nose and nystagmus in the opposite eye as it looks laterally).
The Motor nucleus and nerve: ipsilateral loss of the cranial nerve that is affected (3, 4, 6 or 12).
The 4 Lateral Structures & The Associated Deficit Are:
The Spinocerebellar pathways: ipsilateral ataxia of the arm and leg.
The Spinothalamic pathway: contra lateral alteration of pain and temperature affecting the arm, leg and rarely the trunk.
The Sensory nucleus of the 5th: ipsilateral alteration of pain and temperature on the face in the distribution of the 5th cranial nerve (this nucleus is a long vertical structure that extends in the lateral aspect of the pons down into the medulla).
The Sympathetic pathway: ipsilateral Horner�s syndrome, that is partial ptosis and a small pupil (miosis)
These pathways pass through the entire length of the brainstem and can be likened to �meridians of longitude� whereas the various cranial nerves can be regarded as �parallels of latitude�. If you establish where the meridians of longitude and parallels of latitude intersect then you have established the site of the lesion.
Figure 2 shows the ventral aspect of the brainstem.
The 4 Cranial Nerves In The Medulla Are:
9 Glossopharyngeal: ipsilateral loss of pharyngeal sensation. 10 Vagus: ipsilateral palatal weakness. 11 Spinal accessory: ipsilateral weakness of the trapezius and sternocleidomastoid muscles. 12 Hypoglossal: ipsilateral weakness of the tongue.
The 12th cranial nerve is the motor nerve in the midline of the medulla. Although the 9th, 10th and 11th cranial nerves have motor components, they do not divide evenly into 12 (using our rule) and are thus not the medial motor nerves.
The 4 Cranial Nerves In The Pons Are:
5 Trigeminal: ipsilateral alteration of pain, temperature and light touch on the face back as far as the anterior two-thirds of the scalp and sparing the angle of the jaw. 6 Abducent: ipsilateral weakness of abduction (lateral movement) of the eye. 7 Facial: ipsilateral facial weakness. 8 Auditory: ipsilateral deafness.
The 6th cranial nerve is the motor nerve in the pons.
The 7th is a motor nerve but it also carries pathways of taste, and using the rule of 4 it does not divide equally in to 12 and thus it is not a motor nerve that is in the midline. The vestibular portion of the 8th nerve is not included in order to keep the concept simple and to avoid confusion. Nausea and vomiting and vertigo are often more common with involvement of the vestibular connections in the lateral medulla.
The 4 Cranial Nerves Above The Pons Are:
4 Olfactory: not in midbrain. 5 Optic: not in midbrain. 6 Oculomotor: impaired adduction, supraduction and infraduction of the ipsilateral eye with or without a dilated pupil. The eye is turned out and slightly down. 7 Trochlear: eye unable to look down when the eye is looking in towards the nose.
The 3rd and 4th cranial nerves are the motor nerves in the midbrain.
Thus a medial brainstem syndrome will consist of the 4 M�s and the relevant motor cranial nerve, and a lateral brainstem syndrome will consist of the 4 S�sand either the 9�11th cranial nerve if in the medulla, or the 5th, 7th and 8th cranial nerve if in the pons.
MEDIAL (PARAMEDIAN) BRAINSTEM SYNDROMES
Let us assume that the patient you are examining has a brainstem stroke. If you find upper motor neurone signs in the arm and the leg on one side then you know the patient has a medial brainstem syndrome because the motor pathways is paramedian and crosses at the level of the foramen magnum (decussation of the pyramids). The involvement of the motor pathway is the �meridian of longitude�. So far the lesion could be anywhere in the medial aspect of the brainstem, although if the face is also affected it has to be above the mid pons, the level where the 7th nerve nucleus is.
The motor cranial nerve �the parallels of latitude� indicates whether the lesion is in the medulla (12th), pons (6th) or midbrain (3rd). Remember the cranial nerve palsy will be ipsilateral to the side of the lesion and the hemiparesis will be contralateral. If the medial lemniscus is also affected then you will find a contra lateral loss of vibration and proprioception in the arm and leg (the same side affected by the hemiparesis) as the posterior columns also cross at or just above the level of the foramen magnum. The median longitudinal fasciculus (MLF) is usually not affected when there is a hemiparesis as the MLF is further back in the brainstem.
The MLF can be affected in isolation �a lacunar infarct� and this results in an ipsilateral internuclear ophthalmoplegia, with failure of adduction (movement towards the nose) of the ipsilateral eye and leading eye nystagmus on looking laterally to the opposite side of the lesion in the contra lateral eye. If the patient had involvement of the left MLF then, on being asked to look to the left, the eye movements would be normal, but on looking to the right the left eye would not go past the midline, while there would be nystagmus in the right eye as it looked to the right.
Figure 3 shows the clinical features of the medial brainstem syndromes.
LATERAL BRAINSTEM SYNDROMES
Once again we are assuming that the patient you are seeing has a brainstem problem, most likely a vascular lesion. The 4 S�s or �meridians of longitude� will indicate that you are dealing with a lateral brainstem problem and the cranial nerves or �parallels of latitude� will indicate whether the problem is in the lateral medulla or lateral pons.
A lateral brainstem infarct will result in ipsilateral ataxia of the arm and leg as a result of involvement of the Spinocerebellar pathways, contralateral alteration of pain and temperature sensation as a result of involvement of the Spinothalamic pathway, ipsilateral loss of pain and temperature sensation affecting the face within the distribution of the Sensory nucleus of the trigeminal nerve (light touch may also be affected with involvement of the spinothalamic pathway and/or sensory nucleus of the trigeminal nerve). An ipsilateral Horner�s syndrome with partial ptosis and a small pupil (miosis) is because of involvement of the Sympathetic pathway. The power tone and the reflexes should all be normal. So far all we have done is localize the problem to the lateral aspect of the brainstem; by adding the relevant 3 cranial nerves in the medulla or the pons we can localize the lesion to this region of the brain.
The lower 4 cranial nerves are in the medulla and the 12th nerve is in the midline so that 9th, 10th and 11th nerves will be in the lateral aspect of the medulla. When these are affected, the result is dysarthria and dysphagia with an ipsilateral impairment of the gag reflex and the palate will pull up to the opposite side; occasionally there may be weakness of the ipsilateral trapezius and/or sternocleidomastoid muscle. This is the lateral medullary syndrome usually resulting from occlusion of the ipsilateral vertebral or posterior inferior cerebellar arteries.
The 4 cranial nerves in the pons are: 5th, 6th, 7th and 8th. The 6th nerve is the motor nerve in the midline, the 5th, 7th and 8th are in the lateral aspect of the pons, and when these are affected there will be ipsilateral facial weakness, weakness of the ipsilateral masseter and pterygoid muscles (muscles that open and close the mouth) and occasionally ipsilateral deafness. A tumour such as an acoustic neuroma in the cerebello-pontine angle will result in ipsilateral deafness, facial weakness and impairment of facial sensation; there may also be ipsilateral limb ataxia if it compresses the ipsilateral cerebellum or brainstem. The sympathetic pathway is usually too deep to be affected.
If there are signs of both a lateral and a medial (paramedian) brainstem syndrome, then one needs to consider a basilar artery problem, possibly an occlusion.
In summary, if one can remember that there are 4 pathways in the midline commencing with the letter M, 4 pathways in the lateral aspect of the brainstem commencing with the letter S, the lower 4 cranial nerves are in the medulla, the middle 4 cranial nerves in the pons and the first 4 cranial nerves above the pons with the 3rd and 4th in the midbrain, and that the 4 motor nerves that are in the midline are the 4 that divide evenly into 12 except for 1 and 2, that is 3, 4, 6 and 12, then it will be possible to diagnose brainstem vascular syndromes with pinpoint accuracy.
P. GATES
The Geelong Hospital, Barwon Health, Geelong, Victoria, Australia
El Paso, TX. Chiropractor, Dr. Alexander Jimenez continues the discussion on the anatomy of nerve fibers, receptors, spinal tracts and brain pathway/s. As the spinal nerve nears the spinal cord, it splits into the dorsal and ventral roots. The dorsal root only contains the axons of sensory neurons. While the ventral roots contain only the axons of motor neurons. Some of the branches synapse with local neurons in the dorsal root ganglion, posterior (dorsal) horn, and even the anterior (ventral) horn, at the spine where they enter.
Other branches travel short distances up or down the spine to interact with neurons at other levels of the spinal cord. A branch can also turn into the posterior (dorsal) column white matter to connect with the brain. Spinal nerve systems that connect to the brain are contralateral, in that the right side of the body is connected to the left side of the brain and the left side of the body is connected to the right side of the brain.
Cranial nerves convey specific sense information from the head and neck directly to the brain. Whereas spinal information is contralateral, cranial nerve systems are for the most part�ipsilateral, meaning that a cranial nerve on the right side of the head is connected to the right side of the brain. Some cranial nerves contain only sensory axons. Other cranial nerves have both sensory and motor axons, including the trigeminal, facial and glossopharyngeal. General senses of somatosensation for the face travel through the trigeminal system.
PATHWAYS
THE POSTERIOR COLUMN� MEDIAL LEMNISCUS SYSTEM CONVEYS INFORMATION ABOUT TOUCH AND LIMB POSITION
POSTERIOR COLUMN MEDIAL LEMNISCAL PATHWAY
The term posterior column refers to the entire contents of a posterior funiculus, exclusive of its share of the propriospinal tract. The posterior columns consist mainly of ascending collaterals of large myelinated primary afferents carrying impulses from various kinds of mechanoreceptors (although substantial numbers of second-order fibers and unmyelinated fibers are also included). This has traditionally been considered the major pathway by which information from low-threshold cutaneous, joint, and muscle receptors reaches the cerebral cortex.
2-Minute Neuroscience: Touch & The Dorsal Columns-Medial Lemniscus
DAMAGE TO THE POSTERIOR COLUMN�MEDIAL LEMNISCUS SYSTEM CAUSES IMPAIRMENT OF PROPRIOCEPTION AND DISCRIMINATIVE TACTILE FUNCTIONS
�As might be expected from the types of afferents contained in the posterior columns, this pathway carries information important for the conscious appreciation of touch, pressure, and vibration and of joint position and movement. However, because input from cutaneous receptors also reaches the cortex by other routes, damage to the posterior columns causes impairment, but not abolition, of tactile perception. Complex discrimination tasks are more severely affected than is the simple detection of stimuli. Other functions, such as proprioception and kinesthesia, are classically considered to be totally lost after posterior column destruction. The result is a distinctive type of ataxia (incoordination of movement); the brain is unable to direct motor activity properly without sensory feedback about the current position of parts of the body. This ataxia is particularly pronounced when the patient�s eyes are closed, preventing visual compensation.�
Given the role of the posterior column, the patient should be screened for any abnormalities regarding their sense of fine touch, vibration, barognosis, graphesthesia, stereognosis, kinaesthesia, two-point discrimination and conscious proprioception:
A common way of testing for fine touch is to ask the patient to recognize common objects placed within a cloth using their touch.
Vibration sense can be tested using a low pitched C128 tuning fork placed along a bony prominence of the desired corresponding spinal level(s) to be tested.
Barognosis refers to the ability to determine the approximate weight of an object.
Graphesthesia refers to the ability to recognize writing on the skin by touch. The practitioner can draw out a letter on the patients skin as a way of testing.
Kinaesthesia refers to ones own sense of body motion (excluding equilibrium which is controlled in part by the inner ear) and is commonly tested using the subject�s ability to detect an externally imposed passive movement, or the ability to reposition a joint to a predetermined position.
Proprioception is often assessed using the Rombergs test. This examination is based on the notion that a person requires at least two of the three following senses to maintain balance while standing: proprioception; vestibular function and vision. A patient who has a defect within their proprioceptive mechanism can still maintain balance by using vestibular function and vision. In the Romberg test, the patient is stood up and asked to close their eyes. A loss of balance is interpreted as a positive Romberg sign.
THE SPINOTHALAMIC TRACT CONVEYS INFORMATION ABOUT PAIN AND TEMPERATURE
A GOOD BRAIN CAN MODULATE PAIN
SPINOTHALAMIC TRACT
Pain is a complex sensation, in that a noxious stimulus leads not only to the perception of where it occurred but also to things such as a rapid increase in level of attention, emotional reactions, autonomic responses, and a greater likelihood that the event and its circumstances will be remembered. Corresponding to this complexity, multiple pathways convey nociceptive information rostrally from the spinal cord. One of them (the spinothalamic tract) is analogous to the posterior column�medial lemniscus pathway.
SPINOTHALAMIC TRACTS
Two main parts of the Spinothalamic Tract (STT)
Lateral Spinothalamic Tract
Transmission of pain and temperature
Anterior Spinothalamic Tract
Transmission of crude touch and firm pressure
DAMAGE TO THE ANTEROLATERAL SYSTEM CAUSES DIMINUTION OF PAIN AND TEMPERATURE SENSATIONS
Examination:
Given the role of the spinothalamic tract, the patient should be screened for any abnormalities regarding their sense of touch, pain, temperature, and pressure sensation.
Screening for such abnormalities is commonly done using gentle pin pricks and cotton wool, to contrast between sharp and soft, following cutaneous sensory nerve root distributions. Hot and cold discrimination can be ascertained using the cold metal arm of a tuning fork, and a warm palm or heated object.
2 Minute Neuroscience: Pain & The Anterolateral System
HAUSER ET AL. FIBROMYALGIA, 2015
�Pain processing and its modulation: Activation of peripheral pain receptors (also called nociceptors) by noxious stimuli generates signals that travel to the dorsal horn of the spinal cord via the dorsal root ganglion. From the dorsal horn, the signals are carried along the ascending pain pathway or the spinothalamic tract to the thalamus and the cortex. Pain can be controlled by nociception- inhibiting and nociception-facilitating neurons. Descending signals originating in the supraspinal centers can modulate activity in the dorsal horn by controlling spinal pain transmission. CNS, central nervous system.�
SPINAL INFORMATION REACHES THE CEREBELLUM BOTH DIRECTLY AND INDIRECTLY
The spinal cord is an important source of information used by the cerebellum in the coordination of movement. This information reaches the cerebellar cortex and nuclei both directly, by way of spinocerebellar tracts, and indirectly, by way of relays in brainstem nuclei. A number of spinocerebellar tracts have been described, some representing the upper extremity and others the lower extremity. Only three have been well characterized.
Ascending Tracts | Spinocerebellar Tract
DESCENDING PATHWAYS INFLUENCE THE ACTIVITY OF LOWER MOTOR NEURONS
El Paso, TX. Chiropractor, Dr. Alexander Jimenez discusses the anatomy of nerve fibers, receptors, spinal tracts and brain pathways. Regions of the Central Nervous System (CNS) coordinate various somatic processes using sensory inputs and motor outputs of peripheral nerves. Important areas of the CNS that play a role in somatic processes are separated in the spinal cord brain stem. Sensory pathways that carry peripheral sensations to the brain are referred to an ascending pathway, or tract. Various sensory modalities follow specific pathways through the CNS. Somatosensory stimuli activate receptors in the skin, muscles, tendons, and joints throughout the entire body. The somatosensory pathways are divided into two separate systems based on the location of the receptor neurons. Somatosensory stimuli from below the neck run along the sensory pathways of the spinal cord, and the somatosensory stimuli from the head and neck travel through cranial nerves.
ANATOMY OF RECEPTORS, NERVE FIBERS, SPINAL CORD TRACTS AND BRAINSTEM PATHWAYS
RECEPTORS AND RECEPTOR BASED THERAPY
NEURONS NEED THREE THINGS TO SURVIVE!
FUNCTIONAL NEUROLOGY KEY CONCEPTS
The cell needs three things to survive.
Oxygen, glucose and stimulation.
Stimulation = Chiropractic, exercise, etc.
Stimulation leads to neuronal growth
Neuronal growth leads to plasticity
Subluxations alter the frequency of firing of neurons
Activation of one side will stimulate ipsilateral cerebellum and contralateral cortex (usually)
Proper stimulation CAN reduce pain.
CHIROPRACTIC IS RECEPTOR-BASED THERAPY
INTRODUCTION
The ongoing activity and output of the CNS are greatly influenced, and sometimes more or less determined, by incoming sensory information.
The basis of this incoming sensory information is an array of sensory receptors, cells that detect various stimuli and produce receptor potentials in response, often with astonishing effectiveness.
The health of the neuron, however, plays a huge role in how neurons can produce receptor potentials, the endurance of the neuron and the ability to create plasticity.
�Neurons that fire together, wire together.� Hebbian Theory
TYPES OF RECEPTORS
Chemoreceptors
Smell, taste, interoceptors
Thermoreceptors
Temperature
Mechanoreceptors
Cutaneous receptors for touch, auditory, vestibular, proprioceptors
Nociceptors
Pain
PARTS OF RECEPTORS
Although their morphologies vary widely, all receptors have three general parts:
1. Receptive Area 2. Area Rich In Mitochondria
Health of the neurons within the receptors will determine its response to stimulation
3. Synaptic Area To Pass Messages To The CNS
RECEPTIVE FIELDS
These are particular areas in the periphery where application of an adequate stimulus causes the receptors to respond.
Neurons in successive levels of sensory pathways (second- order neurons, thalamic and cortical neurons-also have receptive fields, although they may be considerably more elaborate than those of the receptors.
TRANSDUCTION
Sensory receptors use ionotropic and metabotropic mechanisms to produce receptor potentials
Sensory receptors transduce some physical stimulus into an electrical signal � a receptor potential � that the nervous system can understand.
Sensory receptors are similar to postsynaptic membranes as their adequate stimuli are analogous to neurotransmitters.
THE DIAMETER OF A NERVE FIBER IS CORRELATED WITH ITS FUNCTION
BIGGER = FASTER
Larger fibers conduct action potentials faster than do smaller fibers.
A? fibers are the largest and most rapidly conducting myelinated fibers.
The slowest conducting fibers of the body are the C fibers
RECEPTORS IN MUSCLES AND JOINTS DETECT MUSCLE STATUS AND LIMB POSITION
MUSCLE SPINDLES
Muscle spindles (Fig. 9-14) are long, thin stretch receptors scattered throughout virtually every striated muscle in the body.
These muscle spindles sense muscle length and proprioception (�one�s own� perception).
They are quite simple in principle, consisting of a few small muscle fibers with a capsule surrounding the middle third of the fibers.
These fibers are called intrafusal muscle fibers (fusus is Latin for �spindle,� so intrafusal means �inside the spindle�), incontrast to the ordinary extrafusal muscle fibers (�outside the spindle�).
The ends of the intrafusal fibers are attached to extrafusal fibers, so whenever the muscle is stretched, the intrafusal fibers are also stretched.
The central region of each intrafusal fiber has few myofilaments and is noncontractile, but it does have one or more sensory endings applied to it.
When the muscle is stretched, the central part of the intrafusal fiber is stretched, mechanically sensitive channels are distorted, the resulting receptor potential spreads to a nearby trigger zone, and a train of impulses ensues at each sensory ending.
GOLGI TENDON ORGANS
Golgi tendon organs are spindle-shaped receptors found at the�junctions between muscles and tendons. They are similar to Ruffini endings in their basic organization, consisting of interwoven collagen bundles surrounded by a thin capsule (Fig. 9-16).
Large sensory fibers enter the capsule and branch into fine processes that are inserted among the collagen bundles. Tension on the capsule along its long axis squeezes these fine processes, and the resulting distortion stimulates them.
If tension is generated in a tendon by making its attached muscle contract, tendon organs are found to be much more�sensitive and can actually respond to the contraction of just a few muscle fibers.
Thus Golgi tendon organs very specifically monitor the tension generated by muscle contraction and come into play whe
n fine adjustments in muscle tension need to be made (e.g., when handling a raw egg).
�
Thus the mode of action of Golgi tendon organs is quite different from that of muscle spindles (Fig. 9-17). If a muscle�contracts isometrically, tension is generated across its tendons, and the tendon organs signal this; however, the muscle spindles signal nothing because muscle length has not changed (assuming that the activity of the gamma motor neurons remains unchanged).
In contrast, a relaxed muscle can be stretched easily, and the muscle spindles fire; the tendon organs, however, experience little tension and remain silent. A muscle, by virtue of these two types of receptors, can have its length and tension monitored simultaneously.
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LATERAL BRAINSTEM SYNDROMES
The lower 4 cranial nerves are in the medulla and the 12th nerve is in the midline so that 9th, 10th and 11th nerves will be in the lateral aspect of the medulla. When these are affected, the result is dysarthria and dysphagia with an ipsilateral impairment of the gag reflex and the palate will pull up to the opposite side; occasionally there may be weakness of the ipsilateral trapezius and/or sternocleidomastoid muscle. This is the lateral medullary syndrome usually resulting from occlusion of the ipsilateral vertebral or posterior inferior cerebellar arteries.
The term posterior column refers to the entire contents of a posterior funiculus, exclusive of its share of the propriospinal tract. The posterior columns consist mainly of ascending collaterals of large myelinated primary afferents carrying impulses from various kinds of mechanoreceptors (although substantial numbers of second-order fibers and unmyelinated fibers are also included). This has traditionally been considered the major pathway by which information from low-threshold cutaneous, joint, and muscle receptors reaches the cerebral cortex.
The cell needs three things to survive.
The health of the neuron, however, plays a huge role in how neurons can produce receptor potentials, the endurance of the neuron and the ability to create plasticity.
Chemoreceptors
Although their morphologies vary widely, all receptors have three general parts:
Sensory receptors use ionotropic and metabotropic mechanisms to produce receptor potentials
Larger fibers conduct action potentials faster than do smaller fibers.
junctions between muscles and tendons. They are similar to Ruffini endings in their basic organization, consisting of interwoven collagen bundles surrounded by a thin capsule (Fig. 9-16).
sensitive and can actually respond to the contraction of just a few muscle fibers.
