Neuromuscular Physiology II: Action Potentials, Synaptic Transmission, and Neuronal Circuits

Neuromuscular Physiology II

Overview

This study guide covers the fundamental principles of neuromuscular physiology, focusing on the generation and propagation of action potentials, synaptic transmission, neurotransmitter characteristics, and the organization of neuronal circuits. These concepts are essential for understanding how the nervous system communicates and controls muscle activity.

Action Potentials

Definition and Importance

• Action potential is a rapid, transient change in the membrane potential of a neuron or muscle cell, allowing for the transmission of electrical signals along the cell membrane.

• Action potentials are essential for neural communication, muscle contraction, and the functioning of the nervous system.

Steps in an Action Potential

1. Resting Membrane Potential: The neuron maintains a stable, negative internal charge (typically around -70 mV) due to the distribution of ions (mainly Na+ and K+) across the membrane.

2. Depolarization (Threshold Reached): A stimulus causes the membrane potential to become less negative. When the threshold (about -55 mV) is reached, voltage-gated Na+ channels open.

3. Rapid Depolarization: Na+ ions rush into the cell, causing the membrane potential to become positive (up to +30 mV).

4. Repolarization: Voltage-gated Na+ channels close, and voltage-gated K+ channels open. K+ exits the cell, returning the membrane potential toward the resting value.

5. Hyperpolarization: K+ channels may remain open slightly longer, causing the membrane potential to become more negative than the resting potential before stabilizing.

6. Restoration: The Na+/K+ ATPase pump restores the original ion distribution, maintaining the resting membrane potential.

Equation:

$V_m = V_{rest} + \Delta V$

where $V_m$ is the membrane potential, $V_{rest}$ is the resting membrane potential, and $\Delta V$ is the change due to ion movement.

Effects of Ion Imbalances and Poisons

• Na+/K+ Pump Inhibition: Poisons that block this pump prevent restoration of ion gradients, leading to failure of action potential generation over time.

• Hyponatremia: Low extracellular Na+ reduces the driving force for Na+ influx, impairing depolarization and action potential propagation.

Conduction of Action Potentials

Decremental vs. Non-Decremental Conduction

• Decremental conduction: Signal strength decreases as it travels (e.g., graded potentials).

• Action potentials: Non-decremental; the signal maintains its strength along the axon.

Continuous vs. Saltatory Conduction

• Continuous conduction: Occurs in unmyelinated axons; action potentials propagate along every segment of the membrane.

• Saltatory conduction: Occurs in myelinated axons; action potentials "jump" from one node of Ranvier to the next, increasing conduction speed.

Example: Myelinated motor neurons use saltatory conduction to rapidly transmit signals to muscles.

Synaptic Transmission

Events at the Synaptic Knob

• Arrival of an action potential at the synaptic knob opens voltage-gated Ca2+ channels.

• Ca2+ influx triggers vesicle fusion and neurotransmitter release into the synaptic cleft.

• Neurotransmitters bind to receptors on the postsynaptic membrane, initiating a response.

Neurotransmitters

Characteristics of a Neurotransmitter

• Manufactured and stored in the presynaptic neuron.

• Released in response to an action potential.

• Acts on specific receptors on the postsynaptic cell.

• Has a mechanism for deactivation (e.g., enzymatic breakdown, reuptake).

Termination of Neurotransmitter Action

• Enzymatic breakdown: Enzymes degrade neurotransmitters (e.g., acetylcholinesterase for acetylcholine).

• Reuptake: Neurotransmitters are taken back into the presynaptic neuron (e.g., serotonin reuptake).

• Diffusion: Neurotransmitters diffuse away from the synaptic cleft.

Example: Selective serotonin reuptake inhibitors (SSRIs) block serotonin reuptake, prolonging its action.

Neuronal Circuits

Types of Neuronal Circuits

Type	Description	Example
Converging	Multiple inputs converge onto a single neuron	Visual and auditory stimuli integrating in the brain
Diverging	One neuron sends signals to multiple downstream neurons	Motor neuron activating several muscle fibers
Reverberating	Neurons form a feedback loop, sustaining activity	Breathing rhythm generation
Parallel-After-Discharge	Input is transmitted along several parallel pathways to a common output	Complex problem-solving tasks

Summary Table: Action Potential Steps

Step	Membrane Potential	Main Event
Resting	-70 mV	Na+/K+ pump maintains gradients
Threshold	-55 mV	Voltage-gated Na+ channels open
Depolarization	+30 mV	Na+ influx
Repolarization	Returns negative	K+ efflux
Hyperpolarization	Below -70 mV	K+ channels remain open briefly

Additional info: The above notes expand on the provided slides and text, adding definitions, examples, and context for clarity and completeness.