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Guided Study: Functional Divisions and Physiology of the Nervous System

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Q1. Provide a breakdown of the functional divisions of the nervous system. Briefly describe each division. (Do not include parasympathetic and sympathetic divisions of the autonomic nervous system.)

Background

Topic: Organization of the Nervous System

This question tests your understanding of how the nervous system is structurally and functionally divided, and the roles of each division.

Key Terms:

  • Central Nervous System (CNS): Brain and spinal cord; processes information and coordinates activity.

  • Peripheral Nervous System (PNS): All neural tissue outside the CNS; connects CNS to limbs and organs.

  • Somatic Nervous System: Controls voluntary movements via skeletal muscles.

  • Autonomic Nervous System (ANS): Regulates involuntary functions (do not include sympathetic/parasympathetic here).

  • Enteric Nervous System: Sometimes considered part of the ANS; controls the gastrointestinal tract.

Step-by-Step Guidance

  1. Start by identifying the two main anatomical divisions: CNS and PNS. Briefly describe the main function of each.

  2. Within the PNS, distinguish between the somatic and autonomic divisions. Describe what each controls (voluntary vs. involuntary actions).

  3. Mention the enteric nervous system as a specialized network within the PNS, and its role in digestive function.

  4. For each division, provide a concise description of its primary function and components.

Try solving on your own before revealing the answer!

Q2. Predict the effect of the poison ouabain, which blocks Na+/K+ pumps, on resting and action potentials in neurons. (Hint: what would happen to the sodium and potassium ion gradients in the presence of this poison?)

Background

Topic: Neuronal Membrane Potentials and Ion Gradients

This question examines your understanding of how the Na+/K+ ATPase pump maintains ion gradients essential for resting and action potentials.

Key Terms and Concepts:

  • Na+/K+ Pump: Actively transports 3 Na+ out and 2 K+ into the neuron, maintaining gradients.

  • Resting Membrane Potential: The voltage difference across the membrane at rest, typically around -70 mV.

  • Action Potential: Rapid change in membrane potential due to ion movement.

Step-by-Step Guidance

  1. Recall the role of the Na+/K+ pump in maintaining high extracellular Na+ and high intracellular K+ concentrations.

  2. Consider what happens to these gradients if the pump is blocked by ouabain (i.e., no active transport of Na+ and K+).

  3. Think about how the loss of these gradients would affect the resting membrane potential over time.

  4. Predict how the ability to generate action potentials would change as the gradients diminish.

Try solving on your own before revealing the answer!

Q3. How do local (graded) potentials and action potentials differ?

Background

Topic: Types of Electrical Signals in Neurons

This question tests your ability to compare and contrast the properties of graded (local) potentials and action potentials.

Key Terms:

  • Graded (Local) Potentials: Small, variable changes in membrane potential that decay with distance.

  • Action Potentials: Large, all-or-none electrical impulses that propagate without decrement.

Step-by-Step Guidance

  1. List the main characteristics of graded potentials (e.g., amplitude, decrement, summation).

  2. List the main characteristics of action potentials (e.g., threshold, all-or-none, propagation).

  3. Compare how each is initiated and where they occur in the neuron.

  4. Discuss how each signal type contributes to neuronal communication.

Try solving on your own before revealing the answer!

Q4. Explain what is going on with Na+ and K+ ions and their voltage-gated channel proteins at A, B, and C on the figure below.

Background

Topic: Phases of the Action Potential

This question asks you to interpret the roles of sodium and potassium ions and their channels during different phases of the action potential, as labeled on a typical action potential graph.

Key Terms:

  • Voltage-Gated Na+ Channels: Open rapidly during depolarization.

  • Voltage-Gated K+ Channels: Open more slowly, responsible for repolarization.

  • Depolarization, Repolarization, Hyperpolarization: Key phases of the action potential.

Step-by-Step Guidance

  1. Identify what typically happens at each labeled point (A, B, C) on an action potential graph (e.g., resting, rising, falling phases).

  2. For each point, describe the state (open/closed/inactivated) of Na+ and K+ voltage-gated channels.

  3. Explain the direction of Na+ and K+ ion movement at each phase.

  4. Relate these channel states to the changes in membrane potential at each point.

Try solving on your own before revealing the answer!

Q5a. How could the propagation of an action potential be referred to as a positive feedback loop?

Background

Topic: Feedback Mechanisms in Physiology

This question tests your understanding of how positive feedback operates during the rising phase of the action potential.

Key Terms:

  • Positive Feedback: A process where an initial change is amplified by its own effects.

  • Voltage-Gated Na+ Channels: Their opening leads to further depolarization.

Step-by-Step Guidance

  1. Recall what triggers the opening of voltage-gated Na+ channels during depolarization.

  2. Describe how the influx of Na+ affects the membrane potential.

  3. Explain how this change leads to the opening of more Na+ channels, reinforcing the process.

  4. Connect this sequence to the definition of a positive feedback loop.

Try solving on your own before revealing the answer!

Q5b. What would happen if depolarization at the trigger zone led to a negative feedback loop instead?

Background

Topic: Feedback Mechanisms in Neuronal Signaling

This question asks you to predict the outcome if a negative feedback mechanism, rather than positive, controlled action potential initiation.

Key Terms:

  • Negative Feedback: A process that counteracts an initial change, maintaining homeostasis.

Step-by-Step Guidance

  1. Define negative feedback and how it typically functions in physiology.

  2. Consider what would happen to the action potential if depolarization triggered mechanisms that reduced further depolarization.

  3. Predict the effect on the ability of the neuron to reach threshold and propagate an action potential.

  4. Relate this to the importance of positive feedback in action potential generation.

Try solving on your own before revealing the answer!

Q6. The disease multiple sclerosis causes a degeneration of myelin. Explain the effect this would have on the conduction speed of affected neurons.

Background

Topic: Myelination and Nerve Conduction

This question tests your understanding of how myelin affects the speed of action potential propagation along axons.

Key Terms:

  • Myelin Sheath: Insulating layer around axons that increases conduction speed.

  • Saltatory Conduction: Action potentials jump between nodes of Ranvier in myelinated axons.

  • Multiple Sclerosis: Demyelinating disease affecting the CNS.

Step-by-Step Guidance

  1. Recall the function of myelin in facilitating rapid action potential conduction.

  2. Describe what happens to action potential propagation when myelin is lost.

  3. Explain how this affects the speed and reliability of neuronal signaling.

  4. Relate these changes to the symptoms seen in multiple sclerosis.

Try solving on your own before revealing the answer!

Q7. Cholinergic synapses (those involving the neurotransmitter acetylcholine) operate in the way shown in Figure 11.20 of your textbook. In a condition called hypocalcemia, the levels of Ca2+ in the extracellular fluid is lower than normal. Explain how this condition would affect activity at a cholinergic synapse.

Background

Topic: Synaptic Transmission and Calcium's Role

This question tests your understanding of how calcium ions are involved in neurotransmitter release at synapses.

Key Terms:

  • Cholinergic Synapse: Synapse that uses acetylcholine as its neurotransmitter.

  • Hypocalcemia: Lower than normal extracellular calcium levels.

  • Neurotransmitter Release: Dependent on Ca2+ influx into the presynaptic terminal.

Step-by-Step Guidance

  1. Recall the role of Ca2+ in triggering vesicle fusion and neurotransmitter release at the synapse.

  2. Consider how lower extracellular Ca2+ would affect the amount of neurotransmitter released.

  3. Predict the effect on postsynaptic cell activation.

  4. Relate this to possible physiological consequences of hypocalcemia.

Try solving on your own before revealing the answer!

Q8a. One EPSP depolarizes the trigger zone of an axon from a resting membrane potential of -70 mV to -60 mV, and threshold is at -55 mV. Will an action potential be generated?

Background

Topic: Synaptic Potentials and Action Potential Threshold

This question tests your understanding of excitatory postsynaptic potentials (EPSPs) and the threshold for action potential initiation.

Key Terms:

  • EPSP (Excitatory Postsynaptic Potential): A depolarizing graded potential.

  • Threshold: The membrane potential at which an action potential is triggered.

Step-by-Step Guidance

  1. Identify the starting (resting) membrane potential and the value after the EPSP.

  2. Compare the depolarized value to the threshold value.

  3. Determine whether the depolarization is sufficient to reach threshold for action potential initiation.

Try solving on your own before revealing the answer!

Q8b. Given the situation in Part (a), if a second, identical EPSP occurs immediately after the first, will an action potential be triggered?

Background

Topic: Temporal Summation of Synaptic Potentials

This question tests your understanding of how multiple EPSPs can summate over time to reach threshold.

Key Terms:

  • Temporal Summation: Successive EPSPs add together if they occur close in time.

Step-by-Step Guidance

  1. Recall the value of the membrane potential after the first EPSP.

  2. Consider the effect of a second, identical EPSP occurring immediately after the first.

  3. Determine whether the combined depolarization would reach or exceed threshold.

Try solving on your own before revealing the answer!

Q8c. If the two EPSPs from Part (b) occurred at the same time, would an action potential be triggered? What form of summation would occur in this case?

Background

Topic: Spatial Summation of Synaptic Potentials

This question tests your understanding of spatial summation, where simultaneous EPSPs from different synapses combine.

Key Terms:

  • Spatial Summation: EPSPs from different locations add together if they occur simultaneously.

Step-by-Step Guidance

  1. Recall the effect of two EPSPs occurring at the same time on the membrane potential.

  2. Determine whether the combined effect would reach threshold for action potential initiation.

  3. Identify the type of summation occurring in this scenario.

Try solving on your own before revealing the answer!

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