AP Lab

1. Structural and Functional Subdivisions of the Peripheral Nervous System (PNS)

The PNS is divided into two main parts:

Somatic Nervous System (SNS): Controls voluntary movements and sensory information. It involves sensory neurons that send signals from sensory organs to the central nervous system (CNS) and motor neurons that send commands from the CNS to skeletal muscles.

Autonomic Nervous System (ANS): Regulates involuntary body functions such as heart rate, digestion, and respiratory rate. It is further divided into:

Sympathetic Nervous System: Prepares the body for "fight or flight" responses.

Parasympathetic Nervous System: Helps the body rest and digest (the "rest and digest" system).

Enteric Nervous System: Manages the functions of the gastrointestinal system.

2. Resting Membrane Potential

Resting membrane potential refers to the electrical charge difference across the neuron’s membrane when the neuron is not transmitting a signal. It is typically around -70mV.

Establishment: It is mainly created by the unequal distribution of ions across the membrane, primarily sodium (Na⁺) and potassium (K⁺). The membrane is more permeable to K⁺ than Na⁺, leading to a net negative charge inside the cell relative to the outside.

Maintenance: The Na⁺/K⁺ pump actively transports 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell, against their concentration gradients, which helps maintain the resting membrane potential.

3. Concentration Gradients for Sodium and Potassium

Sodium (Na⁺): There is a higher concentration of Na⁺ outside the cell than inside.

Potassium (K⁺): There is a higher concentration of K⁺ inside the cell than outside.

These concentration gradients are maintained by the Na⁺/K⁺ pump, which consumes ATP to move Na⁺ out and K⁺ in.

4. Phases of an Action Potential in Neurons

An action potential consists of several phases:

Resting state: The neuron is at its resting membrane potential of about -70mV.

Depolarization: Sodium channels open, allowing Na⁺ to rush into the cell, making the inside more positive.

Repolarization: Potassium channels open, allowing K⁺ to flow out of the cell, which restores the negative internal charge.

Hyperpolarization: The membrane potential becomes more negative than the resting potential before stabilizing back to -70mV.

5. Ion Channel Movement During Different Phases

Depolarization: Sodium (Na⁺) ions move into the neuron through voltage-gated sodium channels.

Repolarization: Potassium (K⁺) ions move out of the neuron through voltage-gated potassium channels.

6. All or None Principle for Action Potentials

The "all or none" principle means that once the threshold is reached, an action potential will fire at full strength. If the threshold is not reached, no action potential will be generated. The action potential's size and speed do not vary with the stimulus strength; only the frequency of action potentials changes with stronger stimuli.

7. Refractory Period of Action Potentials

Absolute refractory period: During this phase, no new action potential can be initiated, no matter the strength of the stimulus. This is due to the inactivation of sodium channels.

Relative refractory period: A stronger-than-usual stimulus is required to generate another action potential. During this period, some sodium channels are reset, but the membrane potential is still hyperpolarized.

8. Factors That Increase Action Potential Conduction

Two main factors:

Myelination: Myelin sheaths insulate the axon, allowing for faster conduction through saltatory conduction (action potentials jump from node to node).

Axon Diameter: Larger axon diameters provide less resistance to ion flow and thus faster conduction of action potentials.

9. Action Potential Propagation in a Myelinated Axon

In myelinated axons, action potentials propagate faster due to saltatory conduction, where the action potential "jumps" from one node of Ranvier (gaps in myelin) to the next. This speeds up transmission as the depolarization is only required at the nodes, rather than along the entire length of the axon.

10. Compound Action Potential Recorded in the Adductor Pollicis Muscle

A compound action potential (CAP) is the sum of the electrical potentials from many individual nerve fibers within a nerve. In the adductor pollicis muscle, the compound action potential can be recorded to assess the function of the median nerve, which innervates this muscle. The amplitude of the CAP reflects the number of axons that are actively conducting signals.

11. Axon Diameter and Conduction Velocity

Research has shown that larger diameter axons conduct action potentials faster due to reduced resistance to the movement of ions within the axon. This relationship means that axons involved in fast, critical functions (like motor neurons) tend to be thicker. The larger the axon diameter, the faster the action potential travels.