Pain Sensation from the Body:
The Lateral Spinothalamic Tract
The perception of a pinprick is made by first traveling up the arm carried by the Nerve as it enters the Spinal Cord.
Where a second Neuron inside the Spinal Cord picks up the discharge from the first, crosses to the opposite side of the Spinal Cord, and turns upwards to the Brain.
This Ascending Nerve Fiber Bundle is the Lateral SpinoThalamic Tract. Any damage to this bundle causes Loss of Pain Sensation on the Opposite Side Of The Body Below The Lesion.
The Lateral Spinothalamic Tract is actually only a very tiny part of the Pain Pathways from the Spinal Cord to the Brain.
This is the newest part of the Pain Pathways, which transmits accurate information rapidily to the Partietal Cortex of the Brain.
Here information is processed that allows the person to make a rapid and precise response to painful events.
This precise portion of the Pain Pathways dominates the clinical situation.
Damage to the ''Lateral Spinothalamic Tract'' on the left side of the Spinal Cord would cause Loss Of Pain Perception in the right hand and the entire right side of the body below the level of damage.
If there were only one lesion, pain perception on the left side of the body would be normal, even though the lesion is on the left.
Most MSers have experienced numbness ''from here on down'', during the course of their illness, and were then unable to feel a pinprick in the numb area.
The cause is damage to the region of the Lateral Spinothalamic Tract on the opposite side of the Spinal Cord, specifically at the Spinal Cord level where sensation returns to normal.
By knowing this limited piece of Anatomy, we can locate this one lesion precisely and use the information as a start to establish whether there is more than one lesion in the entire Central Nervous System - a point of great importance in the diagnosis of Multiple Sclerosis.
The great majority of nerve fibers that accompany the Lateral Spinothalamic Tract are much older in evolutionary terms.
They connect with BrainStem structures and with older parts of the Brain and may account for the complex Emotional responses that accompany some painful experiences.
Because they travel as a more diffuse system, it is impossible to estimate the degree of damage to this older system in the Neurological Examination.
Sense Of Position & Movement
The Axon which carries Position and Movement Sensation from the finger joint, travels up the arm and enters the Spinal Cord accompanied by similar Axons from other joints and the skin.
Is the finger bent? How much? Is it moving? If so, in what direction, and how fast?
The thicker Axon carrying this precise information is also a newer system in evolutionary terms.
It transmits most of the precise information from limbs to Brain, and has what amounts to a private line to take the information up the fiber bundle at the Posterior part of the Spinal Cord without crossing to the other side and without connections to other Neurons.
As the column enters the Medulla, it finds a second Neuron that refines the message and sends it on to the Brain, with one intervening connection on the way.
When this highly specific message reaches the Cortex of the Parietal Lobe, it contacts columns of Cortical Neurons that have nothing to do but listen to the particular message of this Spinal Sensory Tract, refine it, and send it on to many other specific sites in adjacent pieces of Cortex.
The total Sensory message, arrives partly in the precise terms of evolutionally New Pathways and partly in the more diffuse terms of Older Pathways.
The mixture of Axons that carry a particular message varies according to ongoing activity of the Brain.
The frequency of firing in any one set of Axons helps to transmit Urgency Of Information.
In the Sensory System alone, over a million fibers go to the lower BrainStem for distribution to many parts of the Brain.
Considering the complexity available in the Sensory System, it is a small wonder that we can distinguish many degrees of sharpness, intensity, and location for sensaations all over the body.
No wonder our individual responses to a particular stimulus may be quite varied on one occasion from another!
A MS Plaque in the Right Posterior Column that carries Position Information causes Loss Of Position Sensitivity in the right hand and body below that lesion.
Thus, with only a pin, and after wiggling a few joints or pieces of skin up and down, the examiner can locate not only the level of the lesion, but also can distinguish between plaques on either the left or the right side of the Anterior Spinal Cord that interrupt Pain, and Plaques on either side of the Posterior Spinal Cord that interrupt Position Sensation.
That's not bad, considering that the diameter of the Spinal Cord is much less than a dime for the greater part of its length.
Like body sensation, Vision travels in tightly packed bundles that carry the most detailed information the Nervous System ever recieves from the outside world. Most MSers have experienced loss of Vision.
Those who have not usually have plaques in the Visual System any way, which may be detected by detailed examination. MS plaques often occur in the Optic Nerve, where they are easy to detect.
The great majority of nerve fibers in the Optic Nerve serve the few degrees of Central Vision where detailed discrimination occurs.
A plaque in the Optic Nerve causes loss of Visual Acuity or actual loss of Vision (a Scotoma) in that part of the Vision Field. The complaint is Blurred Vision, inability to read, or a Blind Spot in the Center of Vision in one Eye along with pain when moving the Eye.
A Central Scotoma occurs because of the predominance of central field fibers in the nerve, not because there is a special tendency for plaques to collect in one or another part of the nerve.
The diagnosis is Optic Neuritis, because MS plaques almost never destroy the entire Optic Nerve.
Visual Acuity eventually returns to normal or almost normal; remaining Optic Nerve fibers still come mostly from the Central Field and serve Visual Acuity as well as they can.
Damage to the Optic Nerves can be detected during a Neurological Examination.
The Optic Nerve ends where it can be seen inside the Eyeball with the Ophthalmoscope, and at that point it is called the Optic Nerve Head.
If the Nerve Head becomes pale through DeMyelination and loss of Blood Vessels on the surface, it is called Optic Atrophy, a very common finding during the examination of MSers.
Even without visible Optic Atrophy, electronic testing, can reveal evidence of DeMyelination by showing Slowed Conduction through the Visual Pathways.
We shall return to a discussion of Visual Evoked Potentials in the next chapter.
MSers almost never have trouble with hearing. Unlike vision, body sensation and some motor tracts, hearing travels through the BrainStem diffusely, rather than in a single, tightly packed bundle of fibers.
As they enter the BrainStem, the Axons that carry hearing from the inner ear separate immediately and travel up both sides of the BrainStem in several pathways on either side.
Eventually they reach the Temporal Lobe Cortex where hearing becomes conscious.
A single lesion in the Brain or BrainStem would have to be very large to cause deftness, and in that case, hearing loss would be the least of the person's trouble.
We have had a glimpse of the Sensory correlations that occur in the Parietal Lobe. Consider now the similar correlations that occur in the Occipital Lobe for Vision, and in the Temporal Lobe for Hearing.
All Three Primary Sensory Areas of the Brain are further connected to the adjacent Association Cortex.
The Association Cortex is then interconnected among all three areas and to Frontal and other parts of the Brain.
- Body Sensation
Eventually they are connected to the Left Temporal Lobe, where associations are transferred into language by processes we only dimly understand.
All this mass of activity on both sides of the Brain occurs constantly, rapidly, and accurately before any motor response is constructed.
In addition to direct Sensory-Motor connections, the influence of language is paramount in determining the motor response.
Portions of the Brain that control Emotions also add or subtract their component, determining whether the woman in our example simply accepts the pinprick and reports it to the doctor or retaliates and leaves!
Part of her unique response is determined by her previous experiences in life, part is determined by the individual situation she is in, and part by the unique ''wiring'' she has had since birth that made her slightly different from all other babies, even in the nursery.
Let us now look at some of the concentrated Motor Tracts that allow her to express herself once she has taken it all in through her Sensory Systems.