MCAT Biology Review
Chapter 4: The Nervous System
1. Resting membrane potential depends on:
1. differential distribution of ions across the axon membrane.
2. the opening of voltage-gated calcium channels.
3. active transport of ions across the membrane.
1. I only
2. I and II only
3. I and III only
4. II and III only
2. All of the following are associated with the myelin sheath EXCEPT:
1. faster conduction of nerve impulses.
2. nodes of Ranvier forming gaps along the axon.
3. increased magnitude of the potential difference during an action potential.
4. saltatory conduction of action potentials.
3. Which of the following is true with regard to the action potential?
1. All hyperpolarized stimuli will be carried to the axon terminal without a decrease in size.
2. The size of the action potential is proportional to the size of the stimulus that produced it.
3. Increasing the intensity of the depolarization increases the size of the impulse.
4. Once an action potential is triggered, an impulse of a given magnitude and speed is produced.
4. Which of the following correctly describes a difference between nerves and tracts?
1. Nerves are seen in the central nervous system; tracts are seen in the peripheral nervous system.
2. Nerves have cell bodies in nuclei; tracts have cell bodies in ganglia.
3. Nerves may carry more than one type of information; tracts can only carry one type of information.
4. Nerves contain only one neuron; tracts contain many neurons.
5. Which of the following accurately describes sensory neurons?
1. Sensory neurons are afferent and enter the spinal cord on the dorsal side.
2. Sensory neurons are efferent and enter the spinal cord on the dorsal side.
3. Sensory neurons are afferent and enter the spinal cord on the ventral side.
4. Sensory neurons are efferent and enter the spinal cord on the ventral side.
6. When a sensory neuron receives a stimulus that brings it to threshold, it will do all of the following EXCEPT:
1. become depolarized.
2. transduce the stimulus to an action potential.
3. inhibit the spread of the action potential to other sensory neurons.
4. cause the release of neurotransmitters onto cells in the central nervous system.
7. When the potential across the axon membrane is more negative than the normal resting potential, the neuron is said to be in a state of:
8. Which of the following statements concerning the somatic division of the peripheral nervous system is INCORRECT?
1. Its pathways innervate skeletal muscle.
2. Its pathways are usually voluntary.
3. Some of its pathways are referred to as reflex arcs.
4. Its pathways always involve more than two neurons.
9. Which of the following is a function of the parasympathetic nervous system?
1. Increasing blood sugar during periods of stress
2. Dilating the pupils to enhance vision
3. Increasing oxygen delivery to muscles
4. Decreasing heart rate and blood pressure
10.Which of the following neurotransmitters is used in the ganglia of both the sympathetic and parasympathetic nervous systems?
11.In which neural structure are ribosomes primarily located?
3. Axon hillock
12.An autoimmune disease attacks the voltage-gated calcium channels in the nerve terminal. What is a likely symptom of this condition?
1. Spastic paralysis (inability to relax the muscles)
2. Flaccid paralysis (inability to contract the muscles)
3. Inability to reuptake neurotransmitters once released
4. Retrograde flow of action potentials
13.A neuron only fires an action potential if multiple presynaptic cells release neurotransmitter onto the dendrites of the neuron. This is an example of:
1. saltatory conduction.
3. a feedback loop.
4. inhibitory transmission.
14.A disease results in the death of Schwann cells. Which portion of the nervous system is NOT likely to be affected?
1. Central nervous system
2. Somatic nervous system
3. Autonomic nervous system
4. Parasympathetic nervous system
15.A surgeon accidentally clips a dorsal root ganglion during a spinal surgery. What is a likely consequence of this error?
1. Loss of motor function at that level
2. Loss of sensation at that level
3. Loss of cognitive function
1. I only
2. II only
3. I and II only
4. I, II, and III
Answers and Explanations
The polarization of the neuron at rest is the result of an uneven distribution of ions between the inside and outside of the cell. This difference is achieved through the active pumping of ions into and out of the neuron (using the Na+/K+ ATPase). Voltage-gated calcium channels are important in the nerve terminal, where the influx of calcium triggers the fusion of vesicles containing neurotransmitter with the membrane, but not in maintaining resting membrane potential.
Myelin is a white lipid-containing material surrounding the axons of many neurons in the central and peripheral nervous systems. It is arranged on the axon discontinuously; the gaps between the segments of myelin are called nodes of Ranvier, eliminating choice (B). Myelin increases the conduction velocity by insulating segments of the axon so that the membrane is permeable to ions only at the nodes of Ranvier, eliminating choice (A). The action potential jumps from node to node, a process known as saltatory conduction, eliminating choice (D). Action potentials are often described as being “all-or-nothing”; the magnitude of the potential difference in an action potential is constant, regardless of the intensity of the stimulus. Thus, myelin does not affect the magnitude of the potential difference in an action potential, makingchoice (C) the correct answer.
As in the previous question, the action potential is often described as an all-or-nothing response. This means that, whenever the threshold membrane potential is reached, an action potential with a consistent size and duration is produced. Neuronal information is coded by the frequency and number of action potentials, not the size of the action potential, eliminating choices (B) and (C) and making choice (D) the correct answer. Hyperpolarizing (inhibitory) signals are not transmitted to the nerve terminal, eliminating choice (A).
Nerves are collections of neurons in the peripheral nervous system and may contain multiple types of information (sensory or motor); they contain cell bodies in ganglia. Tracts are collections of neurons in the central nervous system and contain only one type of information; they contain cell bodies in nuclei.
Sensory neurons are considered afferent (carrying signals from the periphery to the central nervous system) and enter the spinal cord on the dorsal side. Motor neurons are considered efferent (carrying signals from the central nervous system to the periphery) and exit the spinal cord on the ventral side.
When a sensory neuron receives a signal that is strong enough to bring it to threshold, one can assume that the receptor becomes depolarized, allowing it to transduce the stimulus to an action potential. The action potential will then be carried by sensory neurons to the central nervous system, where the cell will release neurotransmitters. Therefore, among the given choices, the only incorrect statement is found in choice (C). If a receptor is stimulated, it will promote the spread of the action potential to postsynaptic sensory neurons in the spinal cord, which can send the signal toward the brain.
When the potential across the axon membrane is more negative than the normal resting potential, the neuron is referred to as hyperpolarized. Hyperpolarization occurs right after an action potential and is caused by excess potassium exiting the neuron.
The somatic division of the peripheral nervous system innervates skeletal muscles and is responsible for voluntary movement. Some of the pathways in this part of the nervous system are reflex arcs, which are reflexive responses to certain stimuli that involve only a sensory and a motor neuron. These neurons synapse in the spinal cord and do not require signaling from the brain. The pathways of the somatic division can involve two, three, or more neurons, depending on the type of signal. The correct answer therefore is choice (D).
The parasympathetic nervous system governs the “rest-and-digest” response. The parasympathetic nervous system slows the heart rate, decreases blood pressure, promotes blood flow to the GI tract, and constricts the pupils, among other functions. The sympathetic nervous system governs the fight-or-flight response, including increased heart rate and blood pressure, decreased blood flow to the digestive tract, and increased blood flow the muscles. Choice (D) is the only answer choice that represents a function of the parasympathetic nervous system.
Acetylcholine is the neurotransmitter released by the preganglionic neuron in both the sympathetic and parasympathetic nervous systems. The postganglionic neuron in the sympathetic nervous system usually releases norepinephrine, while the postganglionic neuron in the parasympathetic nervous system releases acetylcholine.
Neurons contain very specialized structures, including dendrites, axons, and the axon hillock. However, neurons are still cells and must carry out cellular functions including protein synthesis. The cell body or soma contains the nucleus, endoplasmic reticulum, and ribosomes.
First, consider the function of voltage-gated calcium channels. When the nerve terminal depolarizes, voltage-gated calcium channels open, allowing for influx of calcium. This influx of calcium triggers fusion of the synaptic vesicles containing neurotransmitters with the membrane of the neuron at the nerve terminal. This allows for exocytosis of the neurotransmitters into the synapse. If a disease blocked the influx of calcium, there would be no release of neurotransmitters. A lack of neurotransmitters means that the neuron cannot send signals. Thus, any symptoms resulting from this disease would be due to an inability of neurons to communicate. If neurons cannot communicate, flaccid paralysis may be one of the results.
Some neurons require multiple instances of excitatory transmission to be brought to threshold. These excitatory signals may be close to each other in time (temporal) or in space (spatial); either way, this pattern of excitation is termed summation.
Schwann cells are responsible for myelination of cells in the peripheral nervous system. Thus, the central nervous system is unlikely to be affected. The peripheral nervous system includes the somatic nervous system and the autonomic nervous system. The autonomic nervous system is composed of both the parasympathetic and sympathetic nervous systems. Thus, choice (A) is the right answer.
The dorsal root ganglion contains cell bodies of sensory neurons only. If a dorsal root ganglion is disrupted at a certain level, there will be a loss of sensation at that level.