Neurons and Action Potential
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MCAT Biology › Neurons and Action Potential
Sarin gas is a potent nerve agent that quickly causes serious physiological effects if ingested, even in very small quantities. It inhibits acetylcholinesterase, an enzyme that degrades acetylcholine. Acetylcholinesterase generally acts at the neuromuscular junction.
Sarin gas may cause which of the following?
Loss of control of respiratory muscles
Dilated pupils
Inhibition of peristalsis
Increased production of white blood cells
Reduced blood pH (acidosis)
Explanation
Acetylcholine is the neurotransmitter that acts at neuromuscular junctions. Acetylcholinesterase degrades acetylcholine at the synaptic cleft, allowing the muscle to relax. If acetylcholinesterase is inhibited, acetylcholine will remain in the synaptic cleft and continuously stimulate the muscle. Breathing requires the ability to contract and relax respiratory muscles. Without rapid administration of an antidote, sarin gas usually results in death from asphyxiation. Acetylcholine causes pupil constriction and gastrointestinal motility. It is not associated with a rapid increase in leukocyte production.
Sarin gas is a potent nerve agent that quickly causes serious physiological effects if ingested, even in very small quantities. It inhibits acetylcholinesterase, an enzyme that degrades acetylcholine. Acetylcholinesterase generally acts at the neuromuscular junction.
Sarin gas may cause which of the following?
Loss of control of respiratory muscles
Dilated pupils
Inhibition of peristalsis
Increased production of white blood cells
Reduced blood pH (acidosis)
Explanation
Acetylcholine is the neurotransmitter that acts at neuromuscular junctions. Acetylcholinesterase degrades acetylcholine at the synaptic cleft, allowing the muscle to relax. If acetylcholinesterase is inhibited, acetylcholine will remain in the synaptic cleft and continuously stimulate the muscle. Breathing requires the ability to contract and relax respiratory muscles. Without rapid administration of an antidote, sarin gas usually results in death from asphyxiation. Acetylcholine causes pupil constriction and gastrointestinal motility. It is not associated with a rapid increase in leukocyte production.
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.
The neurotransmitter released by the axons in the Edinger-Westphal neurons is most likely __________.
acetylcholine
norepinephrine
glutamine
epinephrine
dopamine
Explanation
Acetylcholine is correct. We are told from the passage that the neurons which make up the Edinger-Westphal nucleus are parasympathetic neurons. Therefore, this question is really testing one's knowledge of the neurotransmitter used by parasympathetic neurons. We cannot be expected to know from the question alone which neurotransmitter these neurons use. However, we are supposed to be aware that neurons that are parasympathetic use the neurotransmitter acetylcholine.
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.
What side effect may occur after exposure to a chemical that inhibits the release of acetylcholinesterase?
Repeated stimulation of postsynaptic neurons
An inability to stimulate neurons
An inability to release acetylcholine
Lack of receptors on the postsynaptic neuron
Explanation
Acetylcholinesterase is the enzyme responsible for breaking down acetylcholine, an excitatory neurotransmitter released into the synaptic cleft. If acetylcholine cannot be broken down by this enzyme, the neurotransmitter will continue to attach to the receptors on the postsynaptic cell. This can result in continuous, uncontrolled stimulation of neurons.
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.
The neurotransmitter released by the axons in the Edinger-Westphal neurons is most likely __________.
acetylcholine
norepinephrine
glutamine
epinephrine
dopamine
Explanation
Acetylcholine is correct. We are told from the passage that the neurons which make up the Edinger-Westphal nucleus are parasympathetic neurons. Therefore, this question is really testing one's knowledge of the neurotransmitter used by parasympathetic neurons. We cannot be expected to know from the question alone which neurotransmitter these neurons use. However, we are supposed to be aware that neurons that are parasympathetic use the neurotransmitter acetylcholine.
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.
What side effect may occur after exposure to a chemical that inhibits the release of acetylcholinesterase?
Repeated stimulation of postsynaptic neurons
An inability to stimulate neurons
An inability to release acetylcholine
Lack of receptors on the postsynaptic neuron
Explanation
Acetylcholinesterase is the enzyme responsible for breaking down acetylcholine, an excitatory neurotransmitter released into the synaptic cleft. If acetylcholine cannot be broken down by this enzyme, the neurotransmitter will continue to attach to the receptors on the postsynaptic cell. This can result in continuous, uncontrolled stimulation of neurons.