Wednesday, January 21, 2009 Lecture

Our lecture consisted primarily of information based on the nervous system, but also dealt with a brief description of embryology, or the study of the development of a fertilized egg. After labeling the various parts of a typical neuron we discussed the significance of myelin. Myelin ultimately speeds up communication along a neuron since messages can "jump" across the myelin sheath (saltatory conduction). In the Central Nervous System, glial called oligodendrocytes myelinate several axons at once. This is why a damaged nerve cannot regenerate in the CNS since oligodendrocytes cover more than one axon, they do not have the ability to regenerate a single axon. The Schwann Cells are responsible for myelinating the cranial and spinal nerves in the Peripheral Nervous System. Each Schwann Cell covers one axon so if a nerve is damaged in the PNS, Schwann Cells have the ability to regenerate that particular nerve, and subsequently one may eventually regain feeling/mobility of the damaged body part.
Although there are 50 to 60 different types of neurotransmitters, one neuron can only produce one neurotransmitter--although it can be stimulated by more than one type of neurotransmitter. The axon hillock is a region where the cell body tapers into the axon and is a critical region where "voltage-regulated ion channels" exist. Voltage-Regulated Ion Channels are protein channels that open or close because of ionic concentration changes. In contrast, Chemical-Regulated Ion Channels, such as those embedded in the dendtritic tree, react to the binding of specific chemicals or neurotransmitters. We went on to discuss how ion channels play a significant role in Action Potentials and how depolarization and repolarization affect the transmission of neurotransmitters. Calcium, as we previously know plays not only a critical role in muscle contraction, but also in moving synaptic vesicles to the synaptic bulbs for the release of the neurotransmitter. Calcium in the extracellular fluid is regulated by astrocytes--a type of ganglion. Our brief overview of embryology covered the primary germ layers in a blastocyst: the endoderm, mesoderm, and ectoderm (which gives rise to the skin and nervous systems). We also covered the primary and secondary brain vesicles that grow within 3 and 6 weeks, respectively.
Primary Brain Vesicles Secondary Brain Vesicles
Prosencephalon---------------->Telencephalon (cerebral hemispheres)
Diencephalon: thalamus, hypothalamus, epithalamus
Mesencephalon----------------->(doesn't further differentiate)
mid-brain, corpora quadrigemina
Rhombencephalon-------------->Metencephalon: cerebellum, pons
Myencephalon: medulla oblongata

The information dealing with the regeneration of nerve cells in the PNS was a fascinating topic for me. I've always wondered why those who suffer from paralysis can never regain full mobility even though there are hundreds of other medical miracles that occur on a daily basis. The difference between the Schwann Cells and Oligodendrocytes, however, was something very interesting and very new to me. I did some casual research on the topic of paralysis. From the information, I gathered that paralysis is most often caused by damage to the spinal cord (which is part of the CNS, and is myelinated by oligodendrocytes, which lack the ability to regenerate). Paralysis is not only caused by trauma, but can also occur in stroke victims. My grandmother, who suffered a stroke about 5 years ago, was lucky to not lose any of her mobility, but interestingly has lost her sense of taste and smell.