Biophysical principles of intraoperative neuromonitoring (IONM)

Our nervous system can be classified in various ways, with the two primary methods being anatomical and functional. Anatomically, the nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the brain and the spinal cord, both of which are entirely encased in bone, providing protection and structural support. On the other hand, the PNS includes all nerve structures that extend from the brain and spinal cord, reaching various parts of the body to relay signals and ensure communication between the CNS and our peripheral organs and tissues. This division not only reflects the structural organization, it also indicates the distinct roles and protections afforded to each component of the nervous system. If classified by function, one can separate between the somatic/animal nervous system and the vegetative/autonomous nervous system.

Understanding the human nervous system reveals a fascinating duality that governs our interaction with the world and the regulation of our internal environment. This intricate system is divided into two primary components: the somatic (or animal) nervous system and the autonomic (or vegetative) nervous system. Each plays a crucial role in maintaining our body functionality, both voluntarily and involuntarily.

The human nervous system

Mastering voluntary actions the somatic nervous system encompasses parts of both the central and peripheral nervous systems. It is our window to the external world, responsible for the conscious perception of stimuli and the execution of voluntary movements. For instance, when you decide to move your arm, this system is absolutely at work.

At the heart of voluntary movement one can find the motor system. Motor impulses are primarily generated in the Gyrus praecentralis, a region in the brain responsible for initiating movement. These impulses travel down the largest motor pathway in the central nervous system, known as the corticospinal tract or pyramidal system. This pathway crosses at the medulla oblongata before transmitting impulses to the anterior horns of the spinal cord, where they connect to the second motoneuron. From here, signals exit the CNS via the anterior roots of the spinal nerves, traveling through peripheral nerves to reach the motor end plate. It is at this junction that the stimulus transmission to the muscles occurs, resulting in movement.

Sustaining vital functions contrasting the somatic system, the autonomic nervous system operates involuntarily. It regulates vital functions essential for survival, such as breathing, circulation, digestion, and water balance. This system ensures our internal organs function smoothly without conscious effort, maintaining homeostasis and responding to internal changes.

Sensory Pathways

Perception and Processing our body's ability to perceive and process stimuli, both from the environment and internally, lies within the sensitive system. One of the critical pathways for sensory information is the posterior cord pathway. This pathway is essential for conveying finely discriminative sensations and proprioception from the periphery to the CNS.

Additionally, the anterolateral system transmits crucial information about pain, temperature, pressure, and touch, ensuring we can react appropriately to various stimuli.

The microscopic heroes of the nervous system, the brain and spinal cord, our central nervous systems powerhouses, are intricately composed of nerve cells, or neurons, and their supportive companions, the glial cells. These neurons are the unsung heroes responsible for processing the vast array of signals that keep us functioning every second of our lives.

The Journey of a Signal

Action Potential Neurons receive information through their branched projections called dendrites, which act like antennas picking up signals from neighboring nerve cells. These signals then travel to the neurons cell body, or soma, where the processing these stimuli happens through a complex and highly sophisticated mechanism. Once processed, the neuron sends these signals down its axon, a long, slender projection, to the terminal buttons.

Here, the signals are transmitted to the dendrites of other nerve cells or directly to an effector organ, such as a muscle cell, initiating a response. To truly appreciate the marvel of neurons, its essential to understand the concept of the action potential, which underlies nerve stimulation. At rest, a nerve cells membrane holds a potential of -70mV. When a stimulus reaches the nerve cell, an action potential is generated at the junction between the neuron and its axon, provided the stimulus surpasses a certain threshold voltage. This phase, known as depolarization, triggers a sequence of events leading to the neuron firing an electrical signal.

Following depolarization, the membrane undergoes repolarization, returning to its resting potential. This cyclical process ensures that the neuron is ready to fire again when the next stimulus arrives. Interestingly, this entire process can also be initiated through an external electrical stimulus, showcasing the adaptability and responsiveness of our nervous system.

The Role of Glial Cells

While neurons often take the spotlight, glial cells play a critical supporting role. They provide structural support, nourishment, and protection to neurons, ensuring the optimal functioning of our nervous system. This symbiotic relationship between neurons and glial cells is vital for maintaining the intricate network of communication within our brain and spinal cord.

By exploring these systems, we gain a deeper appreciation for the complex orchestration that keeps us functioning seamlessly. The somatic system empowers us with voluntary control over our movements, while the autonomic system diligently maintains our internal environment, allowing us to thrive. Understanding these microscopic processes offers us profound insights into how our nervous system operates. The seamless transmission and processing of signals enable us to interact with the world, respond to stimuli, and perform countless voluntary and involuntary actions. Neurons, with their remarkable ability to generate and propagate action potentials, are at the heart of this complex and fascinating system, making them truly the microscopic heroes of our bodies.

In medical technology, the human body in all its complexity should always take center stage. This is particularly evident for intraoperative neuromonitoring (IONM). Since every new item of research and development, may it be software or hardware, accessory products or training methods, serves primarily one overarching goal: Enhancing patient safety.