Overview of the Nervous System
Vocabulary that will help you understand this section
What is the nervous system? What does it do? and, How does it do it? Of these three seemingly simple questions, the first two are reasonable and straightforward, but the third is much more difficult to answer. Neuroscientists have some ideas about what the nervous system does, but because there are so many levels of understanding, and because neuroscience research is unfolding information with such rapidity, our ability to describe how the nervous system processes information is constantly changing. Aptly described as the final frontier of biology, neuroscience research is beginning to reveal some of the mysteries of the brain, the spinal cord, the specialized sense organs, and the complicated network of nerves that together make up the nervous system.
The nervous system keeps us aware of our environment and allows us to react appropriately to what is going on around us. For example, if threatened, we use our senses- sight, smell, hearing - to assess the level of danger and to run away, hide, or confront the source of danger. But, the simple idea that the nervous system enables us to interact with our environment by sensing and responding to it does not fully capture some of the more interesting aspects of the system. After all, we think, compose music, express love, experience sadness, do mathematics and even study the brain. These are all nervous system activities that help to make us what we are. How we are able to perform these activities remains one of the deeper mysteries of the nervous system. The best computer that we know of pales in comparison to what we can do.
Over the past thirty years, neuroscientists have made tremendous inroads into the investigations of the nervous system, and are beginning to answer some of the more complex questions. Through the action of our nervous system we can carry out, sometimes with little effort, a wide variety of tasks that thus far have proven difficult for machines.
Information about the environment is acquired through sensory cells that are specialized to respond to a particular external stimulus. In all cases, the sensory cell generates an electrical signal in response to the stimulus via a process called sensory transduction. The electrical signals are forwarded to the spinal cord or brain where they are processed and sometimes returned to effector organs such as muscle or glands to produce the appropriate response.
The basic signaling unit of the nervous system is the nerve cell, or neuron, which comes in many different shapes, sizes and chemical content.
The general layout of neurons is fairly constant. Information is received on structures known as dendrites and passed on via an axon which ends on a dendrite of the next cell in line. The electrical signal in axons is a brief voltage change called an action potential, or nerve impulse, which can travel long distances, sometimes at high speeds, without changing size or shape. When an action potential arrives at the end of the axon, it leads to the release of a stored chemical substance called a chemical neurotransmitter which has one of two effects on the next cell in line. It can either act to increase the probability that the next cell will fire an action potential, in which case, the process is said to be excitatory, or it can act to decrease the probability that the next cell would fire an action potential, in which case, the process is said to be inhibitory. The chemical neurotransmitter used by a particular neuron is often used to identify that class of neuron.
One interesting aspect of the action potential is that it has roughly the same shape in nerves of different modalities. Therefore, information about the stimulus is not encoded in the shape or form of the action potential but rather is conveyed by the site of origin. Interruption of the action potential will prevent information from reaching its destination and the consequence will depend on where the interruption occurs. Scientists know quite a lot about the basis of the action potential and a number of pharmacological agents exist that will reversibly block the propagation of the action potential. When applied near an aching tooth, agents such as Novocain will suppress action potentials carrying information in pain fibers and will lead to pain suppression. However, when applied to a nerve carrying information to a muscle, Novocain will lead to paralysis.
The place where an axon contacts a dendrite or effector organ, such as a muscle or gland, is called a synapse. As mentioned above, there are excitatory and inhibitory synapses depending on the effect that the chemical transmitter has on the next cell in line - the postsynaptic cell. In some cases, the number of synapses between two neurons and the strength of an individual synapse is dependent on the level of usage between the two cells. The strengthening of synaptic activity through use is thought to be one mechanism underlying learning and memory. The changes in synaptic strength or function in response to use is generally referred to as "plasticity," which in general declines with age. The final form of the nervous system appears to be influenced by the very activity that it generates. In a very young nervous system, these connections seem to come in an over-abundance for the actual need. If you use a particular pathway, it becomes solidified and stronger, and establishes itself. Pathways that are not used fall away, sometimes permanently.
This is very important for children, because there are windows of opportunity that are open early in their life. Because they are not open later, children have to take advantage of them early in life. Although these pathways do not completely close, they are not as wide open in adulthood. For example, children acquire language faster than adults because they learn a lot faster than adults.
The nervous system, in order to work properly, has to be connected properly. There are connections and maps that allow the system to work in a very automatic way. The specificity of connection is quite a wonder when one considers that we develop from a single fertilized egg that undergoes multiple divisions to produce many cells that migrate and establish numerous connections among themselves. A dramatic example of the specificity of connections is the tendon jerk reflex that occurs when a doctor uses a rubber mallet to hit a tendon on the knee, thereby causing an extension of the lower leg around the knee. For the reflex to work, specialized muscle cells, called stretch receptors, send signals to the spinal cord where excitation is passed to neurons that innervate the very muscles in which the stretch receptors reside. Exactly how the appropriate connections are established is not known.
As mentioned earlier, the establishment of connections, particularly in certain parts of the nervous systems, can be modified by the level of electrical activity in those parts, and this property called plasticity, is higher in younger systems. However, one emerging concept is that even older nervous systems exhibit levels of plasticity, though reduced, when compared to that in the young system. The idea then is that the brain forever changes and every experience leaves a little imprint on our nervous system.