XIII - Operational Aspects of the Nervous System
This is the integrated functioning of the nervous system. In this unit nervous control of movement, memory, sleep, and brain waves will be considered.
A. Organizational pattern - All nervus system activities, from the simplest to the most complex, require five components. These are as follow.
1. Receptor - This is a nerve ending or specialized organ which senses the environment, either internal or external.
2. Sensory or afferent neuron - This conducts the impulse from the receptor to the integrator.
3. Integrator - This is usually in the brain or cord, and it makes possible a meaningful, directed response. In its simplest form, an integrator can be a single synapse.
4. Motor or efferent neuron - This conducts the impulse from the integrator to the effector.
5. Effector - This is a muscle or gland.
B. Control of movement - We are only capable of contracting muscles and secreting glands. It is the sum total of these activities which constitutes human behavior. It is the nervous system which regulates these activities. Movement can be of two types.
1. Reflexive - This movement is not under conscious control. It is predictable and stereotyped, but it is ultimately the basis of all movement.
2. Voluntary - This is conscious or willed movement. Much of what we consider voluntary is actually reflexive
C. Spinal reflexes - These represent the simplest of movement control systems. It is at the level of the spinal cord that the five components of nervous action are most easily seen. Spinal reflexes may be protective when initiated by peripheral receptors, or form part of voluntary movement when initiated by the motor areas of the cerebral cortex.
1. Monosynaptic reflex - This is the simplest of the spinal reflexes requiring only a single synapse for integration. It is found in the stretch reflex, where the stretching of a muscle initiates its own contraction.
2. Polysynaptic reflex - The simplest of these is the three neuron reflex. In addition to the components found in the monosynaptic reflex, an additional neuron is added in between the sensory and the motor neuron. A familiar example of this reflex is the patellar tendon reflex (knee jerk).
3. Reciprocal innervation - Inhibition is as important as excitation. For example, skeletal muscles exist in antagonistic groups. When one group is contracting it is imperative that the opposite group be relaxed. This is insured by having inhibitory interconnecting neurons establishing an IPSP on the motor neurons supplying those muscles. The motor neurons supplying the contracting groups have EPSP's established on them. In reciprocal innervation, whenever one group of muscles is excited its antagonist is at the same time inhibited.
4. Crossed extensor reflex - This is a more complex reflex in that it involves both sides of the body. When the appendage (arm or leg) is flexed or withdrawn and the other is automatically extended. Note that when the flexor on one side is stimulated, the flexor on the opposite side is inhibited and the extensor.
5. Influences of higher centers - The previous examples of spinal reflexes can be activated by impulses coming from the brain. For example, when we walk, impulses from the voluntary regions of the brain activate the crossed extensor reflex which is absolutely essential for normal walking.
D. Voluntary movement - This is initiated by the primary motor area in the frontal lobe of the brain. The impulses initiated here eventually get to the skeletal muscles via spinal tracts and peripheral nerves. The major tracts involved in movement are as follows.
1. Corticospinal (pyramidal) tracts - About a third of these tracts originate in the primary motor area, the remainder arise in other parts of the brain.
a. The tracts descend to the medulla where about 90% of the fibers cross over the opposite side of the nervous system. These fibers from the lateral corticospinal tracts.
b. The remaining ten percent do not cross over until they reach their level of termination in the cord. They form the ventral corticospinal tracts.
c. Note that the tracts all cross over: therefore the left side of the brain controls movement on the right side of the body.
d. The fibers of the tract normally synapse with an interconnecting neuron in the cord which in turn synapses with a motor neuron running out to a muscle. Sometimes the first fiber synapses with a second fiber which runs directly out to the muscle. Therefore there is a minimum of two and a maximum of three neurons in a complete pathway.
2. Extracorticospinal (extrapyramidal) tracts - These include all of the other motor fibers exclusive of the corticospinal tracts. All cross over. They arise and interconnect with regions of the brain important in coordinating motor activity such as the basal ganglia, reticular formation, and cerebellum. Impulses on these tracts do not initiate movement but rather modify it so that it is smooth and coordinated. These tracts are very complex, multisynaptic, and have both excitatory and inhibitory fibers. The three major ones are the rubrospinal, vestibulospinal, and reticulospinal.
E. Memory and learning - These two items are part of the higher order functions of the nervous system. Learning does not apparently require a nervous system as complex as ours as many simpler forms of animal life are capable of modifying their behavior which is what learning is. However no other life form that we know of has the ability to learn and retain as much as we do. It is not possible to localize learning or memory. They appear to be whole brain functions. Obviously learning is of little use without memory, and intelligence is a function of how rapidly we learn and how we are able to use what we learn in problem solving, organization, and abstract reasoning. There are two major categories of memory.
2. Long term memory - Short term memory can be converted into long term memory by repetition (an hour of repetitions seems to be effective). Conversion can be much quicker if the stimulus is very pleasant or unpleasant, that is, if it evokes a strong emotional response. There are two types of long term memory.
a. Secondary memory - These memories will disappear with time and may be difficult to recall.
b. Tertiary memory - These are memories that are always with us. This type of memory includes our names and other constantly recalled information.
Long term memories are stored in the cerebral cortex. The limbic system, especially the hippocampus and amygdaloid body, is also very important in the formation of long term memory. The mechanisms of long term memory probably involve the following.
a. Synaptic potentiation - This results from increased release of neurotransmitter in presynaptic pathway that is repeatedly stimulated. The effect is to increase the amount of excitatory neurotransmitter released and to maintain continuous EPSPs.
b. Formation of additional synapses - When a neuron is repeatedly stimulated it responds by sprouting additional contacts with the postsynaptic neuron, creating additional synapses. This means that activity by the presynaptic neuron will have greater effect.
These processes result in the production of a memory engram. It seems to require about one hour for conversion of short term memory into a memory engram. Stimuli which have a great emotional impact may be converted instantaneously. Stimuli with less impact must be repeated many times in order to be stored.
F. Sleep - This is a loss of consciousness that occurs on a daily basis. One cause seems to be neural fatigue and during sleep the cortex is less active. Consciousness is maintained by the reticular activating system. Inactivation of the RAS results in sleep. Sleep consists of two different levels.
1. NREM (nonrapid eye movement) sleep - This is divided into four stages that range from the transition from wakefulness to sleep (stage 1) to deep sleep (stage 4).
2. REM (rapid eye movement) sleep - This occurs usually 50 to 90 minutes after falling asleep. It resembles stage one NREM, but there are differences in physiological functions such as respiration and blood pressure. It is during REM that most dreaming occurs.
There is a cycling between NREM and REM during the sleep period. The first REM period last from 5 to 10 minutes and is followed by another long (90 minutes) NREM period. Eventually the REM periods become longer until the last one, which lasts about 50 minutes. In a typical eight hour sleep period, REM periods total 90 to 120 minutes. Neuronal activity is greater during REM and as we age the percentage of time that we spend in REM decreases.
G. Brain waves and the Electroencephalograph - It is possible to assess total brain functioning by studying the electrical activity of the brain. By placing electrodes around the head it is possible to record this activity just as electrodes on the chest can measure the electrical activity of the heart (ECG). Normally functioning brains show four distinct electrical patterns or waves.
1. Alpha - These are seen at rest when the eyes are closed. They disappear during sleep and when an individual concentrates on a task.
2. Beta waves - These are characteristic of a person under stress or psychological tension.
3. Theta waves - Found in children an very frustrated adults. They may also indicate pathology such as tumors.
4. Delta waves - These are characteristic of infants, persons of all ages when at sleep, and also of certain pathologies.
Many states now use brain activity as a measure of death. A person who does not show normal brain wave patterns over extended periods of time can be declared neurologically dead even though the heart and other organs may be functioning. This is sometimes referred to as a "flat EEG," although usually there is some type of electrical activity and therefore flat lines really are not seen on the graph.
H. Aging of the CNS - Like other parts of the body, the brain changes with age. These changes, which begin around 30, include the following.
1. A reduction in brain size and weight, largely due to a reduction in the volume of the cerebral cortex.
2. A reduction in neurons occurs.
3. There is a decrease in blood flow.
4. There are changes involving the organization of the brain. Dendritic branching declines and the numbers of synapses decline.
5. There are chemical changes in the cells themselves.
As a result of these changes, neural processing becomes less efficient and memory consolidation can become more difficult, especially for secondary memories. As a result older people may have more difficulty recalling recent events than those of the distant past. Reaction times become slower, motor control decreases, and all of the senses become less acute. In spite of all of this, most elderly people (85%) have no difficulty functioning in society.
I. Pathologies of the CNS
1. Cerebral palsy - A group of motor disorders caused to motor area during fetal life, birth, or infancy. Women who have rubella during pregnancy may have children who suffer from this disorder. Principal symptoms of this disease is impaired motor activity. About 70% of the victims appear to be retarded but this may be due to an inability to speak and communicate properly.
2. Multiple sclerosis - This diseases results in progressive destruction of the myelin sheaths of the CNS. The first symptoms appear between the ages of 20 and 40. Loss of myelin results in a neural short circuiting. Individuals lose motor abilities and eventually become bedridden. Death usually occurs in 7 to 30 years after the first symptoms appear.
3. Epilepsy - A disorder characterized by short, recurrent, periodic attacks of motor, sensory, or psychological malfunction. Attacks or seizures are initiated by abnormal and irregular discharges of millions of neurons in the brain. There are different kinds depending upon the region of the brain affected.
a. Grand mal - This is initiated by bursts that travel throughout the motor area and spread into conscious areas. There is a loss of consciousness, spasms, and sometimes loss of urinary and bowel control are characteristic.
b. Petit mal - Generally electrical discharges are restricted to one or several small areas. Persons lose contact with the environment for about five to thirty seconds. Individuals appear to be day dreaming.
c. Psychomotor epilepsy - This form is frequently confused with psychosis. Outbursts occur in the temporal lobe causing a loss of touch with reality.
Many forms of epilepsy can be treated with drugs which make the neurons less excitable.
4. Stroke (CVA - Cerebrovascular accident) - These represent loss of blood circulation to an area of the brain and subsequent neural death. The most common cause is blockage of a blood vessel by a blood clot.