In this model, spinal motor neurons integrate synaptic activity and, when a threshold is reached, they fire an action potential. The firing of this action potential is followed by a period of hyperpolarization or refraction to further stimulus in the neuron. This early "integrate and fire" model was then extrapolated to other areas of the nervous system, including the cortex and central nervous system, which strongly influenced the development of theories relating to neuron and nervous system function.
Early in the 1970’s, studies started to emerge that revealed the existence of neurons that operated under much more complex intrinsic firing properties. The functional output of these neurons and neuron systems could not be explained by the existing model of the "integrate and fire" hypothesis.
Since the discoveries of these complex firing patterns, many other forms of neural interaction and modulation have also been discovered. We now know that, in addition to complex firing patterns, neurons also interact via a variety of forms of chemical synaptic transmission, electrical coupling through gap junctions, interactions through electric and magnetic fields, and can be modulated by neurohormones and neuromodulators, such as dopamine and serotonin.
With this fundamental change in our understanding of neuron function came new understanding of the functional interconnectivity of neuron systems, new methods of investigation and new functional approaches to treatment of nervous system dysfunction.
Through the course of this series on Functional Neurology, I will attempt to explore and explain the fundamental concepts and theories that comprise this exciting clinical approach to diagnosis and treatment. Please understand from the beginning that Functional Neurology is not a technique. Virtually all existing chiropractic techniques can be utilized in the application of the concepts of the functional neurological approach to the patient.
In the next few segments of this series on Functional Neurology, I will introduce some of the fundamental concepts and definitions that will be referred to frequently in subsequent segments, so we are "speaking the same language."
This series will be successful if it awakens a desire in you to discover the challenge that defines the human nervous system.
The first concept that we will consider is that of the central integrative state of a single neuron and then, via extrapolation, the central integrative state of a collection of neurons forming a system or circuit.
Central Integrative State of a Neuron (CIS)
The central integrative state of a neuron (CIS) is the total integrated input received by the neuron at any given moment and the probability arising from that state of integration that the neuron will produce an action potential based on the degree of polarization and the firing requirements of the neuron to produce an action potential at one or more of its axons.
The physical state of polarization existing in the cell, at any given moment, is determined by the temporal and spatial summation of all the excitatory and inhibitory stimuli it has processed at that moment. The complexity of this process can be put into perspective when you consider that a single motor neuron may have up to 10,000 individual synapses, and certain neurons in the cerebellum 100,000 different synapses firing at any given moment.
The firing requirements of the neuron are usually genetically determined, but environmentally established, and can demand the occurrence of complex arrays of stimulatory patterns before a neuron will discharge an action potential. Some examples of different stimulus patterns that exist in neurons are "and/or" gated neurons. "And" pattern neurons only fire an action potential if two or more specific conditions are met. "Or" pattern neurons only fire an action potential when one or the other specific conditions are present. These types of neurons have been demonstrated in the association motor cortex in man.
The neuron may be in a state of relative depolarization, which implies the membrane potential of the cell has shifted toward the firing threshold of the neuron. This generally implies that the neuron has become more positive on the inside relative to the outside and the potential difference across the membrane has become smaller. Alternatively, the neuron may be in a state of relative hyperpolorization, which implies the membrane potential of the cell has moved away from the firing threshold. This implies that the inside of the cell has become more negative in relation to the outside environment and the potential difference across the membrane has become greater.
The membrane potential is established and maintained across the membrane of the neuron by the flux of ions, usually sodium (Na) and potassium (K). The movement of these ions across the membrane of the neuron is determined by changes in the permeability or ease at which each ion can move through selective channels in the membrane.
The firing threshold of the neuron is the membrane potential that triggers the activation of voltage gated channels, which are usually concentrated in the area of the neuron known as the axon hillock or activation zone that allow the influx of Na into the cell, resulting in the generation of an action potential in one or more of the neuron’s axons.
Central Integrative State of a Functional Unit of Neurons
The concept of the CIS described above, in relation to a single neuron, can be loosely extrapolated to a functional group of neurons. Thus, the central integrative state of a functional unit or group of neurons can be defined as the total integrated input received by the group of neurons at any given moment and the probability that the group of neurons will produce action potential output based on the state of polarization and the firing requirements of the group.
The concept of the central integrative state can be used to estimate the status of a variety of variables concerning the neuron or neuron system, such as:
• the probability that any given stimulus to a neuron or neuron system will result in the activation of the neuron, or neuron system;
• the state of prooncogene activation and protein production in the system;
• the rate and duration that the system will respond to an appropriate stimulus.
Next Segment, we will introduce the concepts of trans-neural degeneration, diaschisis and neural plasticity.