USABO - USA Biology Olympiad Neurobiology and Sensory Systems Questions and Answers — Questions and Answers
Question 1: A single neuron receives multiple excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) simultaneously at various locations on its dendrites and soma. Which of the following statements best describes how the neuron integrates these signals to determine whether to fire an action potential?
- The integrated sum of all EPSPs and IPSPs at the axon hillock must depolarize the membrane to the threshold potential. (Correct answer)
- An action potential is fired if any single EPSP is strong enough to exceed the threshold potential on its own.
- The neuron fires if the total number of incoming EPSPs is greater than the total number of incoming IPSPs, regardless of their magnitude.
- Only EPSPs that arrive at the presynaptic terminal can trigger an action potential in the postsynaptic neuron.
Correct answer: The integrated sum of all EPSPs and IPSPs at the axon hillock must depolarize the membrane to the threshold potential.
Neuron firing is determined by the process of summation at the axon hillock. This region integrates all incoming signals, both spatially (from different locations) and temporally (at different times). If the net effect of all EPSPs and IPSPs causes the membrane potential at the axon hillock to reach the threshold of excitation (typically around -55mV), voltage-gated sodium channels open, and an action potential is initiated. The other options are incorrect as they oversimplify this integrative process.
Question 2: During the repolarization phase of an action potential, which of the following events is primarily responsible for the rapid decrease in membrane potential from its peak positive value back towards the resting potential?
- The influx of sodium (Na+) ions through voltage-gated channels.
- The efflux of potassium (K+) ions through voltage-gated channels. (Correct answer)
- The influx of chloride (Cl-) ions, causing hyperpolarization.
- The closing of all ion channels and activity of the Na+/K+ pump.
Correct answer: The efflux of potassium (K+) ions through voltage-gated channels.
Repolarization is driven by two main events: the inactivation of voltage-gated Na+ channels (which stops depolarization) and the opening of voltage-gated K+ channels. The opening of these K+ channels allows positively charged potassium ions to flow out of the cell, down their electrochemical gradient, making the inside of the membrane more negative and returning the potential towards its resting state.
Question 3: In the process of auditory transduction within the cochlea, the bending of stereocilia on hair cells is the critical mechanical event that opens ion channels. This bending is directly caused by the relative movement between which two structures?
- The oval window and the round window
- Reissner's membrane and the basilar membrane
- The basilar membrane and the tectorial membrane (Correct answer)
- The tympanic membrane and the stapes
Correct answer: The basilar membrane and the tectorial membrane
Sound waves transmitted to the cochlear fluid cause the basilar membrane to vibrate. The organ of Corti, which contains the hair cells, sits on the basilar membrane. The stereocilia of these hair cells are in contact with the overlying, relatively stationary tectorial membrane. As the basilar membrane moves up and down, the stereocilia are bent against the tectorial membrane, which opens mechanosensitive ion channels and initiates a neural signal.
Question 4: When a photon of light strikes a photoreceptor (rod or cone) in the human retina, it initiates a signaling cascade. Which of the following correctly describes the state of a rod cell's membrane potential and its rate of glutamate release while in complete darkness?
- Hyperpolarized; low glutamate release
- Hyperpolarized; high glutamate release
- Depolarized; low glutamate release
- Depolarized; high glutamate release (Correct answer)
Correct answer: Depolarized; high glutamate release
In a counterintuitive mechanism, photoreceptor cells are depolarized in the dark. This is due to a constant influx of Na+ and Ca2+ ions through cGMP-gated channels, known as the 'dark current'. This depolarized state (~ -40mV) causes the continuous release of the neurotransmitter glutamate. When light strikes, it activates a G-protein cascade that breaks down cGMP, closing these channels, hyperpolarizing the cell, and reducing glutamate release.
Question 5: A patient is diagnosed with a demyelinating disease, where the myelin sheaths surrounding axons in the central nervous system are progressively damaged. Which of the following is the most direct physiological consequence of this loss of myelin on nerve impulse transmission?
- The refractory period of the action potential is eliminated.
- The amplitude of the action potential at each node of Ranvier is significantly increased.
- The resting membrane potential of the axon becomes unstable and hyperpolarized.
- The speed of saltatory conduction is decreased, potentially leading to transmission failure. (Correct answer)
Correct answer: The speed of saltatory conduction is decreased, potentially leading to transmission failure.
Myelin acts as an electrical insulator, allowing the action potential to 'jump' from one node of Ranvier to the next in a process called saltatory conduction. This dramatically increases the speed of nerve impulse transmission. When myelin is lost, the electrical current dissipates through the now-exposed axonal membrane, slowing down or completely halting the propagation of the action potential as it may no longer be strong enough to depolarize the next node to threshold.
Question 6: The olfactory system possesses several unique characteristics when compared to other major sensory systems like vision or audition. Which of the following is a key distinguishing feature of the primary olfactory pathway?
- Each olfactory receptor neuron expresses a wide variety of different odorant receptor proteins.
- It is the only major sensory system that does not first relay information through the thalamus before projecting to the primary sensory cortex. (Correct answer)
- Sensory signals are first processed in the cerebellum for fine-tuning before reaching conscious awareness.
- The sensory receptors are remarkably stable and are not replaced throughout an individual's lifetime.
Correct answer: It is the only major sensory system that does not first relay information through the thalamus before projecting to the primary sensory cortex.
A hallmark of the olfactory pathway is its direct connection to the cerebral cortex. While information from vision, hearing, taste, and touch is routed through the thalamus (the brain's primary sensory relay station) before reaching the relevant primary cortical area, axons from the olfactory bulb project directly to the primary olfactory cortex (piriform cortex) and parts of the limbic system. This direct connection is thought to contribute to the strong link between smell and memory/emotion.
USABO - USA Biology Olympiad Neurobiology and Sensory Systems Questions and Answers
A single neuron receives multiple excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) simultaneously at various locations on its dendrites and soma.
Which of the following statements best describes how the neuron integrates these signals to determine whether to fire an action potential?