This diagram presents an overview of the nervous system with a focus on the physiology of neurons and how they transmit signals. Here's a detailed look at each part of the diagram:

The functional unit is the neuron:

• Structure: Neurons consist of a cell body (soma), dendrites, an axon, Schwann cells, and axon terminals.

• Function: Neurons transmit electrical signals throughout the body. Dendrites receive signals, the cell body processes them, and the axon transmits the signals to other neurons or effectors.

Resting Potential:

• Ionic Balance: Typically, a neuron at rest maintains a potential difference across its membrane due to the ionic balance, with more potassium ions (K+) inside and more sodium ions (Na+) outside. This is maintained by the Na+/K+ pump, which pumps 3 Na+ out for every 2 K+ pumped in.

Action Potential:

• Stimulus and Depolarization: When a stimulus acts on a neuron, it can cause the depolarization of the membrane, where Na+ channels open, allowing Na+ to rush into the cell, making the inside more positive.

• Graph Representation: The graph depicts the stages of an action potential, showing the rapid rise (depolarization), peak, and fall (repolarization) of the membrane potential, followed by a slight dip below the resting potential (hyperpolarization).

Impulse Propagation:

• Depolarization and Repolarization: Following the initial depolarization at one spot, the adjacent areas of the membrane depolarize, and this process continues down the axon. Then, repolarization occurs as K+ rushes out of the axon.

• Wave of Depolarization: This process creates a wave of depolarization that travels along the nerve axon, known as an action potential or nerve impulse.

The Synapse:

• Synaptic Transmission: At the synaptic knob, voltage-gated calcium channels open, allowing Ca2+ to enter the cell, which causes vesicles containing neurotransmitters to fuse with the presynaptic membrane and release their contents into the synaptic cleft.

• Neurotransmitter Action: The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane, leading to depolarization if the signal is excitatory, or hyperpolarization if inhibitory.

Diagram:

• The bottom portion of the image illustrates the divisions of the nervous system:

  • Central Nervous System (CNS): Comprising the brain and spinal cord.

  • Peripheral Nervous System (PNS): All nerves outside the CNS, which can be sensory (afferent) or motor (efferent).

    • Somatic Nervous System: Controls voluntary movements.

    • Autonomic Nervous System: Regulates involuntary functions and has two subdivisions:

      • Sympathetic: Often referred to as the "fight or flight" system.

      • Parasympathetic: Known as the "rest and digest" system.

Understanding these concepts is crucial for the MCAT as it tests knowledge of physiological systems, including the nervous system. This content often ties into questions about cell biology, neurophysiology, and pharmacology, as it is vital to understand how neurons communicate and how this communication can be influenced by various drugs.

________

Here's a detailed framework for understanding the nervous system as relevant to the MCAT Biology section:

1. Neuronal Structure and Function:

• Neuron Components:

  • Cell body (soma): Contains the nucleus and organelles.

  • Dendrites: Receive signals from other neurons.

  • Axon: Conducts electrical impulses away from the cell body.

  • Myelin Sheath (Schwann cells): Insulates the axon, speeding up impulse transmission.

  • Axon Terminals: Release neurotransmitters to communicate with other cells.

• Key Point: The neuron is the fundamental unit of the nervous system, specialized for rapid signal transmission.

2. Resting Membrane Potential:

• Ionic Pump Dynamics:

  • Na+/K+ pump: Actively transports 3 Na+ out and 2 K+ in, establishing a negative resting membrane potential.

• Importance: The resting potential is crucial for the creation of action potentials, which are necessary for neuronal communication.

3. Action Potential Mechanics:

• Depolarization: A stimulus triggers Na+ channels to open, allowing Na+ ions to flood into the neuron, reducing the membrane's polarity.

• Repolarization: K+ channels open, K+ exits the neuron, and the membrane potential is restored.

• Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential.

• Graph Interpretation: Understand the stages of an action potential as visualized on a voltage-time graph.

4. Propagation of the Action Potential:

• Process: The action potential propagates along the axon as a wave of depolarization, followed by repolarization.

• Saltatory Conduction: In myelinated neurons, the impulse jumps from node to node (Nodes of Ranvier), increasing speed.

5. Synaptic Transmission:

• Synaptic Knob Events:

  • Voltage-gated Ca2+ channels open in response to an arriving action potential.

  • Influx of Ca2+ facilitates the fusion of neurotransmitter vesicles with the presynaptic membrane.

• Synaptic Cleft Role: Neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic neuron.

• Outcome: Depending on the neurotransmitter and receptor, the postsynaptic neuron may be excited or inhibited.

6. Nervous System Organization:

• Central Nervous System (CNS): Brain and spinal cord, responsible for processing information.

• Peripheral Nervous System (PNS): All neural elements outside the CNS.

  • Sensory (Afferent) Division: Transmits sensory information to the CNS.

  • Motor (Efferent) Division: Transmits motor commands from the CNS to muscles and glands.

    • Somatic Nervous System: Controls voluntary muscular systems and conscious perception.

    • Autonomic Nervous System: Regulates involuntary functions.

      • Sympathetic Nervous System: Activates the body's "fight or flight" response.

      • Parasympathetic Nervous System: Controls the "rest and digest" activities.

7. Clinical Relevance:

• Drug Actions: Many medications act by altering synaptic transmission, affecting neurotransmitter release, receptor binding, or ionic channel activity.

• Diseases: Neurological and psychiatric disorders can often be traced to dysfunctions within neuronal communication pathways.

For MCAT preparation, it's important to integrate this framework with practice questions and to understand how alterations in these processes can lead to various pathologies, how drugs can modify synaptic transmission, and how different parts of the nervous system can be selectively activated or inhibited.

________

Creating original practice questions and solutions can be a good way to illustrate and reinforce the concepts of the nervous system relevant to the MCAT. I can provide you with examples in the style of MCAT questions. Here are practice questions along with detailed explanations:

Question 1: Resting Membrane Potential

Which of the following correctly describes the changes in ion concentrations during the establishment of the resting membrane potential of a neuron?

A. Na+ concentration is higher inside the neuron, and K+ concentration is higher outside. B. Na+ concentration is higher outside the neuron, and K+ concentration is higher inside. C. Both Na+ and K+ concentrations are higher inside the neuron. D. Both Na+ and K+ concentrations are higher outside the neuron.

Solution:

The correct answer is B. The Na+/K+ pump actively transports 3 Na+ ions out of the neuron and 2 K+ ions into the neuron against their concentration gradients. This activity contributes to a higher concentration of Na+ outside the neuron and a higher concentration of K+ inside the neuron, which is essential for maintaining the negative resting membrane potential.

Question 2: Action Potential

During an action potential, the rapid depolarization phase (upstroke) is primarily due to the movement of which ion?

A. Na+ flowing out of the cell. B. Na+ flowing into the cell. C. K+ flowing out of the cell. D. K+ flowing into the cell.

Solution:

The correct answer is B. The upstroke of the action potential is due to the rapid opening of voltage-gated Na+ channels, which allows Na+ ions to flow into the cell along their electrochemical gradient. This influx of Na+ ions causes the rapid depolarization phase of the action potential.

Question 3: Synaptic Transmission

A neuroscientist applies a drug that blocks voltage-gated calcium channels at the synaptic knob. What is the likely outcome on neurotransmitter release?

A. Neurotransmitter release will be enhanced. B. Neurotransmitter release will be unaffected. C. Neurotransmitter release will be inhibited. D. The drug will depolarize the synaptic knob.

Solution:

The correct answer is C. Voltage-gated calcium channels are essential for the release of neurotransmitters. When action potentials reach the synaptic knob, these channels open and allow Ca2+ to enter the neuron, which is crucial for triggering the fusion of neurotransmitter vesicles with the presynaptic membrane and the subsequent release of neurotransmitters. Blocking these channels will inhibit neurotransmitter release.

Question 4: Nervous System Organization

Which part of the nervous system is responsible for increasing the heart rate during times of stress?

A. Somatic nervous system. B. Parasympathetic nervous system. C. Central nervous system. D. Sympathetic nervous system.

Solution:

The correct answer is D. The sympathetic nervous system is part of the autonomic nervous system and is responsible for the body's 'fight or flight' response, which includes increasing the heart rate during times of stress.

Question 5: Neurotransmitter Action

An inhibitory neurotransmitter opens Cl- channels on the postsynaptic neuron. What is the expected effect on the postsynaptic membrane potential?

A. Depolarization. B. Hyperpolarization. C. No change in membrane potential. D. A temporary overshoot of the action potential.

Solution:

The correct answer is B. The opening of Cl- channels will allow Cl- ions to enter the neuron, which are negatively charged. The influx of negative ions will make the inside of the cell more negative, resulting in hyperpolarization of the postsynaptic membrane and thus inhibiting the possibility of generating an action potential.

These questions are designed to test understanding of the nervous system at a level appropriate for the MCAT. Reviewing and understanding these concepts is vital for success on the exam.

________

Here's a second set of practice questions to help you understand the nervous system concepts commonly tested on the MCAT, along with detailed explanations for each.

Question 1: Ionic Gradients

A researcher adds a toxin to a neuronal culture that specifically inhibits the function of the Na+/K+ ATPase. Assuming the toxin takes effect immediately, which of the following will occur first within the affected neurons?

A. Hyperpolarization of the membrane potential B. Depolarization of the membrane potential C. Immediate neuronal death D. No change in the action potential frequency

Solution:

The correct answer is B. The Na+/K+ ATPase is responsible for maintaining the resting membrane potential by pumping 3 Na+ ions out and 2 K+ ions into the neuron. If this pump is inhibited, Na+ ions will begin to accumulate inside the neuron, reducing the electrical gradient (less negative inside), leading to depolarization.

Question 2: Synaptic Communication

If a neuron were to release a neurotransmitter that opens Na+ channels on the postsynaptic cell, what would be the immediate effect on the postsynaptic neuron?

A. The neuron would hyperpolarize. B. The neuron would experience an inhibitory postsynaptic potential (IPSP). C. The neuron would depolarize. D. The neuron would stop producing action potentials.

Solution:

The correct answer is C. Opening Na+ channels would allow Na+ ions to enter the postsynaptic neuron, making the interior more positive and thus depolarizing the membrane. This depolarization could contribute to reaching the threshold for an action potential if sufficient in magnitude, representing an excitatory postsynaptic potential (EPSP).

Question 3: Neuron Action Potential Propagation

Which feature of myelinated axons most directly allows for faster propagation of action potentials compared to unmyelinated axons?

A. Increased diameter of the axon B. Decreased capacitance of the axonal membrane C. More frequent opening of Na+ channels D. Shorter length of the axon

Solution:

The correct answer is B. Myelin sheaths provide insulation that reduces the capacitance of the axonal membrane. This insulation allows the action potential to 'jump' from node to node (saltatory conduction) rather than having to propagate smoothly along the entire length of the axon, significantly speeding up the transmission.

Question 4: Autonomic Nervous System

Which of the following physiological responses is most likely to be induced by activation of the parasympathetic nervous system?

A. Dilation of the pupils B. Increased digestion C. Accelerated heart rate D. Conversion of glycogen to glucose

Solution:

The correct answer is B. The parasympathetic nervous system is responsible for 'rest and digest' functions. Activation of the parasympathetic system promotes digestion by increasing gut motility and digestive enzyme production, among other effects.

Question 5: Neurotransmitter Release

During the process of neurotransmitter release, which of the following ions is essential for the vesicles to fuse with the presynaptic membrane?

A. Na+ B. K+ C. Ca2+ D. Cl-

Solution:

The correct answer is C. Calcium ions (Ca2+) play a critical role in neurotransmitter release. When an action potential reaches the synaptic terminal, it triggers the opening of voltage-gated Ca2+ channels. The influx of Ca2+ causes synaptic vesicles to fuse with the membrane and release their neurotransmitter contents into the synaptic cleft.

These questions illustrate key concepts in neural communication and function, closely mirroring the depth and style of MCAT questions. Understanding the principles behind each answer choice is essential for mastery of the material.

________

To consolidate MCAT Biology knowledge of the Human Nervous System into long-term memory, students should regularly revisit and reflect upon key questions that reinforce core concepts. Here's a list of major questions that can facilitate this process:

1. Structure and Function of Neurons:

• How does the structure of a neuron relate to its function in signal transmission?

• What are the roles of dendrites, axons, and the myelin sheath in neuronal function?

2. Resting Membrane Potential:

• What ion channels and transporters are involved in establishing the resting membrane potential?

• How do the concentrations of Na+ and K+ differ inside and outside of a neuron at rest?

3. Action Potential:

• What are the stages of an action potential, and which ions are involved in each stage?

• How does the refractory period ensure the unidirectional flow of an action potential along an axon?

4. Synaptic Transmission:

• How do neurotransmitters cross the synaptic cleft and what effect do they have on the postsynaptic neuron?

• What is the difference between an excitatory and an inhibitory neurotransmitter?

5. Neurotransmitter Receptors:

• How do ligand-gated and voltage-gated ion channels differ in function?

• What is the significance of receptor specificity in pharmacology?

6. Nervous System Divisions:

• How is the nervous system divided anatomically and functionally?

• What are the roles of the central and peripheral nervous systems?

7. Somatic vs. Autonomic Nervous System:

• How do the somatic and autonomic nervous systems differ in terms of control and response?

• Within the autonomic nervous system, how do sympathetic and parasympathetic responses differ?

8. Neural Integration:

• What is the summation, and how do temporal and spatial summation contribute to neuronal firing?

• How does the nervous system integrate multiple synaptic inputs to produce a coordinated response?

9. Neuroplasticity:

• What is meant by neuroplasticity, and how does it relate to learning and memory?

• How can the concept of neuroplasticity be applied to recovery from neurological injuries?

10. Pathophysiology:

• How can alterations in normal neuronal functions lead to neurological disorders?

• What mechanisms might be targeted by pharmaceuticals to treat disorders of the nervous system?

11. Sensory Systems:

• How are different types of sensory information transmitted to the brain?

• What pathways do sensory signals follow from their point of origin to their processing centers in the brain?

12. Motor Control:

• How does the nervous system initiate and regulate voluntary movement?

• What is the role of motor cortex, basal ganglia, and cerebellum in movement?

13. Higher-Order Functions:

• What role does the prefrontal cortex play in behavior and decision making?

• How do different brain regions interact to produce complex behaviors and cognitive functions?

14. Homeostasis and the Nervous System:

• How does the nervous system regulate physiological homeostasis?

• What are the neural mechanisms behind the regulation of body temperature, hunger, and thirst?

Regularly revisiting these questions and actively recalling the answers can help students to deepen their understanding and aid in the retention of this complex material. It's not just about memorizing facts, but also about understanding how these facts interrelate and apply to various biological contexts.