CHAPTER 12

The Nervous system

The nervous system is the most complex system of the human body. It is generally considered by doctors to be the most diffcult to understand, examine, diagnose and treat. Because of its complexity, it is the most poorly understood system and one of the most difficult to explain. Despite the relatively little that is known about the nervous system, the knowledge that is available to medical science could fill several medical encyclopedias.

The brain is the central control center of all the functions of the body [ See Figure 12A ]. Without your even thinking about it, the brain controls your rate of breathing, blood pressure, body temperature and hundreds of other unconscious processes such as digestion of food and elimination of waste.

Divisions of the Brain:
The brain, while clearly the most complex electrical wiring system ever designed, is often subdivided in several ways for simplicity. For example, the left and right sides, the sympathetic and parasympathetic systems, and higher and lower areas, are distinguished. The higher areas are further divided into localized areas which have specific functions. These divisions will be discussed briefly. It may help to refer to [ Figure 12A ] as you read.

1. Right and left sides: Generally, the right side of the brain controls the left side of the body, and vice versa. There are connections between the two sides of the brain which allow for coordination of the left and right sides of the body. In addition, the left side of the brain, in most people, stores and executes language functions, while the right side usually controls creative and artistic thought.

2. Sympathetic and parasympathetic systems: The sympathetic system, in essence, speeds up bodily functions. When activated, it increases heart rate, respirations, and dilates the pupils (among other things). It is activated in situations of fear or pain. The parasympathetic system works the opposite way and does things like slow the heart rate and respirations. These two systems are never completely "on" or "off" but always attempt to balance each other to maintain the body at a steady state under normal conditions.

3. Higher and lower divisions: The more complex a function is, the higher in the brain it is controlled. For example, mathematic calculations occur in the cerebrum which is the most complex (highest) part of the brain. The more simple functions, which are generally automatic and reflexive, are controlled from lower brain centers. An example here would be the breathing center which is located in the brain stem. The simplest functions are controlled at the level of the spinal cord, as in the case of the knee-jerk reflex.

4. Finally, the cerebrum is divided into areas with specific functions: The frontal areas integrate complex thoughts and ideas and affect personality, while the temporal areas sense and interpret sound and control emotions. The occipital areas receive and interpret visual information from the eyes, while the parietal areas interpret sensation and motor (muscle) output.

Examination of the Nervous System:
The goal of examiliing the nervous system is to see if there are any neurological problems which could signal disease. Because the various nerve cells in the brain are arranged into specialized areas, and because they are interconnected in specific ways, a doctor can use the information obtaiüed in a neurological exam to determine where in the nervous system a certain abnormality may be located. So, doing a neurological exam on a patient who complains of weakness, dizziness, or loss of muscle function is much like a detective searching for clues. The answers found by the doctor enable him or her to predict where in the nervous system the problem is located. For example, if a patient complains of weakness in the hand, the problem could be in the nerves which run down the arm, or in the spinal cord, or in the lower or upper part of the brain. By doing a careful neurological exam the doctor should be able to assemble all the findings (clues) and determine where in the nervous system the problem originates.

Most doctors divide their examination of the nervous system into three parts: 1) the examination of the patient's mental status and level of consciousness; 2) the examination of the nerves in the head; and, 3) the examination of the nerves in the rest of the body.

1. Mental Status and Level of Consciousness
   The first step in examining the nervous system is ascertaining the level of consciousness of the patient. Are they awake and alert or are they sleepy, lethargic, obtunded (almost unresponsive), or even comatose (not responsive at all)? If a patient is not awake and alert, this could be an important clue to many potentially serious conditions such as a brain tumor, meningitis (infection of the membrane around the brain), head injury, stroke, or metabolic imbalances in the body.
   
   The second step is making sure that the higher functions of the brain are intact. These would include such complex functions as memory, speech, personal identity, and a general orientation to the situation at hand. To test these functions doctors ask several simple questions: What is your name?, Where are we now?, What is today's date?, and Why are you here? A patient who can answer all of these correctly is said to be fully oriented. A person who has difficulty with one or more of these questions may have had a brain injury or may have an underlying problem with higher brain functions such as mental retardation or dementia.

2. Nerves in the Head (Cranial Nerves)
   There are twelve pairs of cranial nerves. The nerves which branch out directly from under the brain are called cranial nerves. They control most functions of the head and some in the neck and lower body organs. Doctors frequently examine cranial nerves with simple tests to determine that they are all intact and functioning correctly.
   
   The functions of the twelve pairs of cranial nerves are diverse. They receive complex input signals from the senses and provide output signals which control a myriad of body functions. The cranial nerves are responsible for sensing five sensory inputs: smell, taste, sight, sound, and touch. The cranial nerves also control movements like chewing, eye focussing, swallowing, and gagging. They also control very complicated reflexes such as the reflex that makes you blink your eyes when dangerous objects are approaching. Doctors have both simple and complex ways of testing each pair of cranial nerves. The tests doctors do when examining the cranial nerves enable them to determine where the problem is in the nerves or brain. Also, the tests help them diagnose the disease which is involved. For a more detailed description of how doctors examine the cranial nerves.

3. Nerves in the Body (spinal nerves):
   At the base of the brain, a large collection of nerve cells leaves the skull together, forming the spinal cord. It is through the spinal cord that most of the body's sensory information from the body (input) travels to the brain. The spinal cord also carries the nerves for motor commands (movement of muscles) from the brain to the rest of the body (output).
   
   The spinal cord is the conduit through which the entire body is controlled from the neck down. [ Figure 12B ] Exiting from the spinal cord are 31 pairs of nerves. They leave the spinal cord at different levels all the way from the neck to the tailbone. The structure of the spinal cord is arranged in a manner that allows for disease or injury of a specific part of it to produce predictable neurological symptoms. It is the doctor's job, by examining the patient's body, to correlate the symptoms with physical findings and thus diagnose the disease process responsible for a problem.
   
   

   

   Types of sensory input that the brain receives from the trunk and limbs of the body and how they are tested:
   1. Pain - a sensation which is unpleasant. This is tested by lightly pressing on an area of skin with a sharp object.

   2. Temperature - distinguishing between hot and cold. This is tested by having a patient distinguish between hot and cold objects.

   3. Pressure - a sensation of pressing down (such as the sensation of pressure on your feet while you are standing).

   4. Simple touch - the sensation that you are being touched by something but not being able to define its size or shape. An example would be the sensation of being touched by a cotton swab.

   5. Position sense - The sense of where a limb is positioned in relation to the rest of body. When catching a ball, for instance, your brain needs to know exactly where your hand is (without your having to look at it) so that it can control the movements necessary to get it directly in front of the ball. There are sensors in each muscle and joint that convey signals to the brain about exactly where each body part is located.

   6. Fine touch - consists of being touched and being able to determine the size and shape of the object which is touching you. Doctors frequently use two pins placed a few millimeters apart and gently touch the patient with both at the same time. They ask the patient if they are being touched in one place or two. The closest distance between two pins at which the patient can recognize two sensations is an estimate of the fine touch capabilities of that area of skin. Different parts of the body have different abilities to perceive fine touch. The back, for example, is not as accurate as the tips of the fingers.

   7. Vibratory sense - the feeling that something is vibrating. Doctors usually use the tuning fork for this test. While the fork vibrates they place it on one of the patient's joints and ask what is felt. The patient should describe vibration. Next, keeping the tuning fork in position, the doctor asks the patient to say as soon as the vibratory sensation stops. In general, the patient with normal vibratory nerve pathways should feel the vibration until it stops, or until the examiner with normal vibratory sense can no longer feel it.

   Testing reflexes (so what's the hammer for?):
   Surely, almost everyone has had a doctor hammer on his or her arm or knee to "test the reflexes". If the reflex being tested is a true reflex, the brain does not control the movement, it happens automatically. When the doctor hammers on the tendon in front of the knee, which causes contraction of the quadriceps muscle, the message of muscle shortening travels to the spinal cord and immediately returns as an impulse through the nerves, making the lower leg jerk out forward.

   Self demonstration:
   You can demonstrate the knee reflex at home quite easily. Have someone sit on the edge of a table or bed with their legs dangling over the edge. They should be completely relaxed. Find their patellar tendon by feeling for the kneecap and the top of the shin bone. In the gap between these two bones, the tendon crosses the joint space of the knee. Now gently tap the tendon with a soft but firm object (a rubber spatula from the kitchen would be fine). Note the patellar reflex. Next, to exaggerate the effect of the output of the brain and spinal cord on the knee reflex, ask your subject to grab one hand in the other and try to pull his or her hands apart with all their strength. This extra motor output from the brain will exaggerate the knee reflex in such a way that it should be slightly more prominent if you test it while the subject is doing this hand-to-hand maneuver.

   Abnormally brisk or dull reflexes indicate different problems. For example, if only one reflex or pair of reflexes are affected, it could mean that specific sensors, nerves, or muscles are diseased. If all the reflexes are abnormal, the whole spinal column or the brain could be diseased.

   Each muscle in the body has a given strength depending on its use. Muscles that oppose gravity and move the entire body's weight are large and strong. Muscles that control fine intricate movements are smaller and weaker, such as the muscles of the hand. Doctors often test the strength of muscles or muscle groups by having a patient overcome forces applied against them. For example, the strength of the deltoid muscle (shoulder) could be tested by having the patient hold his or her arms straight out to the side, parallel to the ground, while the doctor tries to push the arms downwards toward the ground. The strength of the calf muscles may be tested by having the patient walk on his or her tip toes.

   Testing the plantar response:
   There is an additional reflex which should be tested for in any complete neurological exam. It is called the plantar response or Babinski test. This test is performed by stroking the sole of the foot with the edge of a rigid object (such as the handle of the reflex hammer) and observing the response of the big toe. Stroke the foot firmly, beginning near the heel and proceeding upwards along the outer edge of the foot and finally curving over towards the big toe. (Try this at home by using a key on the bottom of a volunteer's foot.) This stimulus is interpreted by the nervous system as a potentially dangerous one. The big toe curls downward to protect the sole of the foot by curving the bony structures of the foot upwards into an arch. Imagine stepping on a pin. As a reflex, your toes curl downward to prevent deep penetration of the pin and injury. This is a response similar to the plantar response. (This normal reflex is also controlled by the nerves in the brain and spine, in a way similar to the control of the knee jerk reflex, i.e. increased output increases the response, while decreased output inhibits the response.) In fact, with some diseases of the brain or spinal cord, the reflex is absent and the toe curls upward instead. Doctors call this a positive plantar response or Babinski sign and it indicates that there may be disease in the nervous system between the brain and the nerve tracts that send messages down the spine to control muscles. These problems may arise as a consequence of bleeding or tumors in the brain or spinal cord, or multiple sclerosis. The problems may also be due to metabolic imbalances such as: low glucose (sugar) in the blood, or low oxygen in the blood.

   Chapter 13