MCAT Biology : Action Potentials and Synapse Biology

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #31 : Action Potentials And Synapse Biology

The optic nerve is formed from the axons of all retinal ganglion cells. The optic nerves from each eye join at the optic chiasm and eventually enter either the left or right optic tract. The optic tract projects to three subcortical areas. One is the lateral geniculate nucleus, which is responsible for processing visual information. One is the pretectal area, which produces pupillary reflexes based on information from the retina. Finally, the superior colliculus uses the information from the retina to generate eye movement.

When light is shone upon one eye, it causes constriction of the pupil in both eyes. Constriction of the eye in which the light is shone is the direct response while constriction of the other is known as the consensual response. The pupillary reflexes are mediated through retinal ganglion neurons that project to the pretectal area which lies anterior to the superior colliculus. The cells in the pretectal area project bilaterally to preganglionic parasympathetic neurons in the Edinger-Westphal nucleus. This is also known as the accessory oculomotor nucleus. The preganglionic parasympathetic neurons in the Edinger-Westphal nucleus send axons through the oculomotor nerve to innervate the ciliary ganglion. The ciliary ganglion's postganglionic neuron innervates the smooth muscle of the pupillary sphincter.

It has been determined that the frequency of action potentials increases dramatically in axons once they have left the optic nerve. The most likely explanation for this increase is __________.

Possible Answers:

the axons are myelinated by oligodendrocytes

a lower density of sodium channels are found in the axons leaving the optic disc

these axons are made up of more thickly myelinated "A" class nerve fibers

a higher density of sodium channels are found in the axons leaving the optic disc

the axons are myelinated by Schwann cells

Correct answer:

the axons are myelinated by oligodendrocytes

Explanation:

The axons are myelinated by oligodendrocytes. This question calls on our knowledge of the nervous system outside of what is stated in the passage. We are looking for the most likely explanation for the increase in the frequency of the action potential. Myelinated nerves have the ability to increase the frequency of action potential conduction. Therefore, we can narrow the options down to either myelinated by Schwann cells or oligodendrocytes. The question then becomes: Which cells are responsible for the myelination? In both cases, glial cells are responsible for laying down the myelin sheath. In the central nervous system (CNS), these cells are called oligodendrocytes, while in the peripheral nervous system they are called Schwann cells. Since we are talking about nerves located in the CNS, the correct answer is oligodendrocytes.

Example Question #31 : Action Potentials And Synapse Biology

The brain is a very delicate structure with little room to move around. Surrounding the brain and the spinal cord are three protective layers in addition to the skull and the vertebral column. Directly surrounding the brain and spinal cord is the pia mater. Following the pia mater is the arachnoid mater. Between the pia mater and the arachnoid mater is the sub-arachnoid space where the cerebrospinal fluid circulates. Finally, the protective layer is the dura mater is loosely attached to the arachnoid mater but is strongly associated with the skull bone.

Depending on the type of injury, a certain type of vein and/or artery are more susceptible to injury. For example, the meningeal artery and vein run through the foramen spinosum and travel between the two layers making up the dura mater. As the artery and the vein are traveling in between the dura mater, there is a vulnerable region at the temple. A strike to the temple region could rupture these vessels and result in a epidural hematoma. 

Traveling from the cerebral cortex to the venous dural sinus (located at certain regions between the two layers of the dura mater) is the cerebral vein. When an injury results in the dura mater shifting away from the arachnoid mater, the cerebral vein could rupture and lead to a subdural hematoma.

A hematoma in the brain is a life-threatening condition. The brain needs constant supply of blood for nutrients, oxygen for metabolism and energy. Among other things, the brain uses energy to drive the sodium-potassium pump. What is the relationship between the pump and an action potential?  

Possible Answers:

The sodium-potassium pump is required to maintain the resting potential, which is about 

The sodium-potassium pump is required drive sodium into the cell 

The sodium-potassium pump is required to maintain the resting potential, which is about 

The sodium-potassium pump is required to maintain the resting potential, which is about 

The sodium-potassium pump is required to drive potassium out of the cell

The sodium-potassium pump is required propagate an action potential 

Correct answer:

The sodium-potassium pump is required to maintain the resting potential, which is about 

Explanation:

The sodium-potassium pump is required to maintain the resting potential of the cell. The pump pushes three sodium ions out of the cell for every two potassium ions into the cell. This odd number allows for the cell to stay in a negative resting potential.

Example Question #41 : Nervous System And Nervous Tissue

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

What allows for the dorsal column pathway to be faster than the pain pathway? 

Possible Answers:

Shorter distance 

Weaker action potential

Myelination 

Stronger action potential

Longer distance

Correct answer:

Myelination 

Explanation:

Fine touch, vibration, proprioception and 2 points discrimination all utilizes the dorsal column pathway. The upper region utilizes the cuneate fasciculus region in the spinal cord while the lower region depends on the gracile fasciculus. According to the passage, these sensations are part of the rapid pathway whereas other sensations such as pain is not as fast. The dorsal column pathway is heavily myelinated while the pain pathway is not as myelinated. Action potential is an all-or-nothing event and the amplitude is fixed.  

Example Question #31 : Action Potentials And Synapse Biology

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

Why is the pain of stepping on a nail not felt immediately?

Possible Answers:

The action potential amplitude is too low

The action potential amplitude was too high

There is little myelination in the pain pathway

There is excess myelination in the pain pathway

None of these

Correct answer:

There is little myelination in the pain pathway

Explanation:

Myelination allows for the action potential to travel at a faster rate via saltatory conduction of. The low amount of myelination in the pain pathway delays the pain signal to the brain. Note that myelination can increase the conduction speed of an action potential by 5-50 orders of magnitude.

Example Question #102 : Systems Biology And Tissue Types

The central nervous system consists of the brain and the spinal cord. In general, tracts allow for the brain to communicate up and down with the spinal cord. The commissures allow for the two hemispheres of the brain to communicate with each other. One of the most important commissures is the corpus callosum. The association fibers allow for the anterior regions of the brain to communicate with the posterior regions. One of the evolved routes from the spinal cord to the brain is via the dorsal column pathway. This route allows for fine touch, vibration, proprioception and 2 points discrimination. This pathway is much faster than the pain route. From the lower limbs, the signal ascends to the brain via a region called the gracile fasciculus. From the upper limbs, the signal ascends via the cuneate fasciculus region in the spinal cord.

One of the most utilized neurotransmitters is acetylcholine. Which of the following methods will increase the amount of the neurotransmitters in the synaptic cleft? 

I. Increasing the action potential frequency

II. Decrease the calcium concentration surrounding the neuron

III. Inhibit acetylcholine esterase

Possible Answers:

II only

I and III

III only

I only

II and III 

Correct answer:

I and III

Explanation:

The presynaptic neuron require an action potential in order to open the voltage gated calcium channel. The opening of this calcium channel will allow the influx of calcium and trigger the release of vesicles with the neurotransmitter inside. The exocytosis of acetylcholine from the presynaptic cleft will then bind to the receptor on the postsynaptic cleft. Inhibiting acetylcholinesterase will prevent the breakdown of the neurotransmitter and allow for it to bind to the receptor longer.  

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