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MCAT Biology › Glia
Which of the following describes the difference between oligodendrocytes and Schwann cells?
Oligodendrocytes myelinate the central nervous system, while Schwann cells myelinate the peripheral nervous system
Oligodendrocytes myelinate the peripheral nervous system, while Schwann cells myelinate the central nervous system
Oligodendrocytes and Schwann cell myelinate different structures of the neuron
Both cell types insulate the axons of cells, but use different insulating materials
Schwann cells can mylinate multiple axons, while oligodendrocytes can only myelinate one axon per cell
Explanation
Both oligodendrocytes and Schwann cells are responsible for myelinating axons in order to increase the transmission rate of signals between neurons. The primary difference is their location. Oligodendrocytes myelinate the central nervous system, while Schwann cells myelinate the peripheral nervous system. Oligodendrocytes are also capable of myelinating multiple axons, while Schwann cells can only myelinate one axon per cell.
Which of the following glial cells produces cerebrospinal fluid?
Ependymal cells
Microglia
Astrocytes
Satellite cells
Oligodendrocytes
Explanation
Ependymal cells, which line cerebral cavities and the central canal, produce cerebrospinal fluid (CSF). CSF is a colorless liquid that protects the brain and spinal cord against injury.
Microglia protect the central nervous system from microbes and debris of injured cells. Astrocytes, located in the central nervous system, provide structural support for neurons in addition to many other tasks involving nutrient and ion regulation. Oligodrendrocytes produce the myelin sheath around the axons of the central nervous system. Satellite cells are not found in the central nervous system, and therefore cannot produce CSF.
What is the major difference between Schwann cells and oligodendrocytes?
Oligodendrocytes myelinate central nervous system axons, whereas Schwann cells myelinate peripheral nervous system axons
Schwann cells are the most common glial cell type
Oligodendrocytes myelinate axons and dendrites, whereas Schwann cells only myelinate axons
Action potentials propagate faster down axons myelinated by Schwann cells
Schwann cells myelinate axons, while oligodendocytes myelinate dendrites
Explanation
Both oligodendrocytes and Schwann cells produce and maintain myelin along axons. Oligodendrocytes are the most common glial cell type, and provide myelination for all neural axons in the central nervous system. Schwann cells provide myelination for neurons in the peripheral nervous system.
The cells that form myelin sheaths around axons outside of the central nervous system are __________.
Schwann cells
dendrites
microglia
oligodendrocytes
ganglia
Explanation
It is important to read this question carefully, as it asks for the cells that form myelin sheaths around axons outside of the central nervous system. In essence, it is asking about the peripheral nervous system.
Schwann cells and oligodendrocytes both form myelin sheaths (a white, fatty substance) around axons, which act as electrical insulation and increase the speed of action potential conduction. Schwann cells perform this function for neurons in the peripheral nervous system (outside of the central nervous system), while oligodendrocytes myelinate neurons in the central nervous system.
In humans, nerve impulses are transmitted with the coordinated action of sodium and potassium ion channels. These channels open in a specific sequence, to allow for membrane potential changes to take place in a directional manner along the length of an axon.
Figure 1 depicts a single phospholipid layer of a cell membrane, and three transmembrane channels important to action potential propagation.
Action potential propagation down a membrane, as depicted in the figure, is typically very slow. Which of the following cell types is responsible for speeding nerve propagation by insulating peripheral axons?
Schwann cells
Oligodendrocytes
Astrocytes
Ependymal cells
Monocytes
Explanation
Schwann cells insulate peripheral axons with myelin. Oligodendrocytes serve a similar purpose, but do so in the central nervous system, and not the peripheral system.
Excessive stimulation of the ulnar nerve results in the sensation known as the "funny bone," and can cause discomfort in the elbow and forearm. What cells are responsible for the myelination of the ulnar nerve?
Schwann cells
Oligodendrocytes
Dendritic cells
Astrocytes
Explanation
There are two types of cells responsible for the myelination of axons throughout the nervous system. Oligodendrocytes myelinate the axons of neurons in the central nervous system. Schwann cells myelinate neurons in the peripheral nervous system. This question is referring to a nerve in the elbow, which would be in the peripheral nervous system.
Astrocytes are a type of glial cell primarily found in the blood-brain barrier. Dendritic cells play a role in the adaptive immune response, and are not considered part of the nervous system.
Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear.
Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:
PrPC + PrPRes → PrPRes + PrPRes
The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.
This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:
Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:
Rabbit 1 and 2: injected with normal saline on days 1 and 2
The above trials serve as controls.
Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2
The above trials use unmodified brain matter.
Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2
The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.
Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2
The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.
Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2
The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.
A scientist shows that PrPC in normal nervous cells helps speed nervous transmission. What other structures help speed nervous transmission?
Schwann cells
Astrocytes
Microglia
Ependymal cells
All glial cells
Explanation
Schwann cells act as insulators on nervous tissue to help propagate nervous transmission via saltatory conduction. This speeds transmission and makes axonal signal propagation much more rapid.
Diseases which destroy the myelin sheath of neurons cause __________.
a decrease in the speed of nerve conduction
cancer
an increase in the speed of nerve conduction
no change
decreased pain perception
Explanation
Myelin sheathes function in insulating the neuron, and allows for much faster propogation of the action potential due to saltatory conduction. Loss of the myelin sheath would slow conduction of signals down the neural axon.
Pain perception is conducted to free nerve endings known as nociceptors, which are not myelinated.
Which of the following structures creates a myelin sheath for axons in the central nervous system?
Oligodendrocytes
Schwann cells
Nodes of Ranvier
Axon hillock
Explanation
Oligodendrocytes and Schwann cells both create myelin sheaths for axons, however, oligodendrocytes function in the central nervous system and Schwann cells function in the peripheral nervous system.
Where might one find Schwann cells?
Trigeminal nerve
Optic nerve
Grey matter
White matter
Broca's area
Explanation
Schwann cells are used to myelinate single axons in the peripheral nervous system. Oligodendrocytes are the counterpart of Schwann cells that exist in the central nervous system, but are capable of myelinating multiple axons at once.
To answer this question, we must select an answer that is part of the peripheral nervous system. Grey matter and white matter only exist in the central nervous system (brain and spinal cord) and can be eliminated. Broca's area is located in the frontal lobe of the cerebrum and will contain oligodendrocytes instead of Schwann cells. The optic nerve and trigeminal nerve are two of the cranial nerves, C2 and CV respectively. The optic nerve is the only cranial nerve to be myelinated by oligodendrocytes. The trigeminal nerve will be myelinated by Schwann cells.