The Nervous System

A large portion of our lecture included information on the neuron and how neurons communicate with eachother. A neuron is the fundamental cell of the nervous system and is a transmitter of electro-chemical signals. The neuron moves charged particles or ions from the soma down the axon to the synaptic terminal where it is then turned into a complete chemical signal called a neurotransmitter (protein). Neurons have several functions including to relay sensory information, to receive sensory information, to help maintain homeostasis and to regulate glandular secretions.

During lecture we drew a detailed neuron, which helped to understand how a neuron operates. A neuron contains a cell body//soma//perikaryon, which contains a nucleus and Nissl bodies (dense ribosomes) on which neurotransmitters are produced. Off of the body (in a multipolar neuron) are extensions called dendrites which have receptors embedded in their membranes so that neurotransmitters from other neurons can bind and communicate. This binding creates a change in the membranes permeability and Na+ is allowed to enter. There is also another extension from the body, called an axon, and it is usually longer than the dendrites. There is a region in front of the axon, in the cell body, in which there are no granules called the Axon Hillock. It is here that the Na+ will build up and upon reaching a critical density will diffuse down the axon, where it is moved by synaptic vesicles into synaptic bulbs//synaptic terminals. A massive influx of Na+ causes the permeability to change and Ca+ enters. This rush of Ca+ causes the vesicle to burst allowing the neurotransmitters to move out and make contact with another neurotransmitter. Those neurotransmitters not used will re-enter the synaptic bulbs and be transported back to the cell body where it will be recycled. A neuron can bind with a different neurotransmitter than it produces, but it cannot produce more than one kind of neurotransmitter.

We also discussed the three structural types of neurons; unipolar, bipolar, and multi polar as well as the three types of potential for neurons. Resting potential is an inactive state in which the Na+/K+/ATP pumps in the membrane produce a -70mv across the bilayer by pumping out 3 Na+ and pumping in 2 K+ simultaneously. The second potential known as Graded or local potential is the one that I understand the least. From what I do understand the neuron reaches this potential when the charge is between -70mv and -54 mv. Once the neuron reaches a -55 mv it hits a threshold and moves into Action Potential. In this state, due to the huge change in charge, there is an influx of Na+.

In lecture we briefly discussed the structure of the nervous system, which includes the CNS, which includes the brain and spinal chord, and the PNS, which includes the nerves that travel toward and away from the CNS. These can be broke down into cranial and spinal nerves. The Autonomic system also belongs to the PNS. Along with neurons there are four types of accessory cells called Neuroglia which are part of the nervous tissue as well. Astrocytes work as a screen and stop mutagens from killing neurons, which cannot undergo mitosis. Oligodendrites produce the myelin sheath which is wrapped around Schwann cells that are attached to the axon. The myelin allows the nerve impulse to move quicker and more efficiently. Microglia are the smallest and are an immune system cell that prevents the brain from becoming infected. And lastly there are the Ependymal cells which are the producers of CFS in our brain.

The part of the lecture that I connected with was the part about the astrocytes. These are known to be the most common cells identified in childhood tumors and a close friend's son has just been diagnosed with a brain tumor. He has had several MRI's since developing atypical episodes in which he stops breathing for 1 1/2 minutes and beyond. During these episodes he can respond to stimulation such as pinching but only when severe. Neurologists believed that he had an atypical seizure disorder but after 6 months of no change in his development as well as no changes on several MRIs, they have come to the conclusion that he has an early stage brain tumor. They were able to see a small area in his brain that contained an abnormal gelatinous material and they expect that this will slowly harden and optimistically they may be able to remove it at that time. As far as I know, they do not know the type of cells that make up the tumor, and after listening to your lecture, I am curious to see if they are in fact Astrocytes.