Physiology of Nerve Impulses and Action Potentials

Neural Pathway Components

• Integrating center: It contains neurons and analyzes information associations brought by sensory neurons to produce a response.
• Association neuron: Connects sensory and motor neurons.
• Motor or efferent neuron: The nerve impulse leads to an effector.
• Effector: This is the target of the nerve impulse (skeletal muscle, smooth muscle, cardiac muscle, or glands).

Membrane Potential Fundamentals

The membrane consists of a positively charged extracellular environment and a negatively charged intracellular environment. The electric potential of the membrane is recorded using microelectrodes.

Resting Potential

The differential distribution of charges on the sides of the membrane determines that the neuron is electrically polarized. In this state (intracellular environment negative and extracellular positive), there is no nerve conduction.

Action Potential

By stimulating the axon of a neuron, a change in the polarity of the membrane occurs, resulting in a positive interior and a negative exterior.

The Nature of Nervous Impulses

The action potential that travels the length of the plasma membrane of the neuron is the nerve impulse. The resting potential can be modified due to stimuli sensed by sensory receptors; this is known as depolarization (increased permeability to sodium).

When this change occurs (interior positive and exterior negative), the potential change happens at the receptive site of the neuron and is called the receptor potential.

The All-or-Nothing Law

If the stimulus is weak, no nerve impulse is generated, even if there is a receptor potential. A specific stimulus intensity threshold is required for this to happen. If the intensity reaches or exceeds the threshold of excitation of a neuron, it triggers a nerve impulse, always of the same magnitude. This means it is not directly proportional to the intensity of the stimulus; this is known as the Law of All or Nothing. When the polarity of the membrane is restored, it is called repolarization.

The nerve impulse is initiated once it reaches the same magnitude; however, it is not more intense simply because the stimulus is stronger.

Factors Influencing Propagation Speed

The speed of propagation of action potentials does not depend on the strength of the stimulus, but rather on the following factors:

• Axon Diameter: A larger diameter results in higher speed.
• Myelin Sheath: The presence or absence of the myelin sheath significantly affects speed.
• Temperature: Nerve cells conduct impulses at slower speeds when they are at lower temperatures.

Types of Nerve Conduction

• Continuous conduction: Each area undergoes progressive depolarization of the membrane surrounding the axon (occurring in neurons that have no myelin sheath).
• Saltatory conduction: The action potential "jumps" from one node of Ranvier to another, making the process much faster.