Action Potentials and Synapse Biology
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MCAT Biology › 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.
What allows for the dorsal column pathway to be faster than the pain pathway?
Myelination
Stronger action potential
Weaker action potential
Longer distance
Shorter distance
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.
The heart contains autorhythmic cells, which can generate an action potential on their own. These cells then spread the action potential throughout the heart, resulting in a contraction. Which of the following mechanisms is an explanation for why these cells can spontaneously generate action potentials?
Specialized channels allow sodium to enter the cell, which leads to depolarization
Specialized channels allow sodium to exit the cell, which leads to depolarization
These cells have no resting potential
These cells do not have sodium-potassium pumps, which allows for quicker depolarization
Explanation
Remember that an action potential starts with the diffusion of sodium into the cell. As more sodium enters the cell, more voltage gated sodium channels open up. This leads to depolarization of the cell. With a steady diffusion of sodium into the cell, the threshold stimulus will eventually be attained, and an action potential will be generated. It is the steady diffusion of sodium into the autorhythmic cells which results in an action potential.
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?
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 drive potassium out of the cell
The sodium-potassium pump is required propagate an action potential
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
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.
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?
There is little myelination in the pain pathway
There is excess myelination in the pain pathway
The action potential amplitude was too high
The action potential amplitude is too low
None of these
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.
What feature makes the axon hillock the location for initiation of action potentials?
There is a very high density of voltage-gated sodium channels
The nerve membrane is the thinnest at this region of a neuron
Voltage-gated potassium channels are absent at this location
Sodium-potassium pumps are absent at this location
None of these
Explanation
For an action potential to occur, voltage-gated sodium channels must open to cause a sharp depolarization (increase) in the membrane potential. Pairing that information with knowledge that action potentials originate at the axon hillock, no other answer choice makes sense. It is only logical, then, that a high density of voltage-gated channels be present at the location where action potentials are first initiated.
Which of the following refers to the process by which action potentials jump from one node of Ranvier to another?
Saltatory conduction
Potential distribution
Sodium-potassium pump
Threshold stimulus
Diffusion
Explanation
The answer is saltatory conduction. Saltatory conduction is the term used to define the process of action potential jumping described in the question. The other possbilities, while involved in the nervous system and its function, do not adaquately describe the process in question.
Saltatory conduction of action potentials requires which of the following?
Myelin
Thinner axon
Chemical synapse
Electrical synapse
None of these
Explanation
Saltatory conduction is a process that propagates an action potential more quickly down the length of an axon in a "leapfrog" manner. This propagation occurs in the gaps between myelin on an axon, called nodes of Ranvier. Without myelin, these nodes would not exist, and the rate at which an action potential is transmitted would decrease. People suffering with multiple sclerosis (MS) have myelin degradation, and thus have decreased motor and other neurological processes.
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 __________.
the axons are myelinated by oligodendrocytes
the axons are myelinated by Schwann cells
a lower density of sodium channels are found in the axons leaving the optic disc
a higher 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
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.
The transmission of electrical signals from one neuron to another __________.
is slower via chemical synapses than electrical synpases
is slower via electrical synapses than chemical synapses
is bi-directional in chemical synapses
is uni-directional in electrical synapses
involves saltatory conduction across the synapse
Explanation
Electrical synapses transmit signals faster than chemical synapses due to the physical connection of neural cells through gap junctions. Chemical synapses are slower due to the action potential needing to arrive in the terminal bud, causing calcium channels to open. This causes neurotransmitter vesicles to fuse to the presynaptic membrane, releasing neurotransmitters to diffuse across the synaptic cleft.
Electrical synapses can allow bi-directional transmission of signals, but chemical synapses cannot. Saltatory conduction involves action potential propagation along the axon via the nodes of Ranvier, and is not involved in the synapse.
What mediates the docking and fusion of synaptic vesicles?
Binding of V- and T-snares
Binding of acetylcholine molecules to nicotinic receptors
Binding of calcium to T-snares
Binding of calcium to G-proteins in the vesicle membrane
Binding of MAO to norepinephrine
Explanation
During the docking and fusion of synaptic vesicles, the increased levels of calcium in the synaptic terminal will lead to calcium ions binding to synaptotagmin, which facilitates the binding of V- and T-snares to initiate fusion. None of the other answer choices make sense with respect to vesicle fusion at the presynaptic terminal.