Cytology of the Nervous System
The neuron is the elemental unit of function in the central nervous system (CNS). The uniqueness of neurons among the many cells of the body is due to their ability to maintain large electrochemical potential (voltage) differences across cell membranes, a process termed irritability, and to signal each other with a common code based on fluctuations in the value of these electrochemical potentials, a process termed conductility. The code is passed between cells through a process of synaptic (electrochemical) transmission, thus the third property of neurons is their secretory capability. The shape and distribution of the neurons processes -axons and dendrites - directly influences the manner whereby their synapses occur and, consequently, influences the processing of information in the nervous system. The ability to constantly change these synaptic connections, termed plasticity, in this complex myriad of central nervous system underlies the fantastic potential for learning and memory in the human brain.
| Irritability | Ability to detect sensory stimui and form a neural code |
| Conductility | Ability to conduct an impulse to targeted sites |
| Secretion | Ability to produce and release neurotransmitters |
| Plasticity | Ability to change morphology and functional connections |
Neurons do not exist in isolation or in direct contact with each other; instead they are completely surround and separate from the environment by neuroglial (nerve glue) cells. These small spidery, support cells form a dense plexus into which neurons and blood vessels are embedded. However, neuroglial (or glial cells) do more than simply provide physical support and isolation for neurons, they are important participants in the metabolic processes of the nervous system and are capable of developing membrane potentials and some even undergo depolarization events similar to neurons; other glial cells function as extensions of the immune system within the brain and spinal cord. In this way, glial cells play a major role in defining homeostatic balance in the nervous system.
Abnormal changes in neuronal morphology can occur in numerous pathological conditions. These morphological alterations can greatly influence their functional properties and can lead to degeneration and death of neuronal elements. An important casual factor in many neuronal diseases can be traced to the interaction between the neurons and the surrounding glial cells. This lecture will explore some of the structural and physiological properties of neurons and their glial cells. In addition, we will examine some of the diseases that alter neuronal structure and activity.