The field of neurophysiology provides insight into how the nervous system works and how its dysfunction can lead to disease. Physiology is the study of how organisms and their parts function. Neurophysiology can be defined as the study of the functioning of the nervous system, which includes the brain, the spinal cord, peripheral nerves, and sensory organs. Neurophysiologists approach the nervous system at multiple levels of organization that include functional systems, circuits, single neurons, and neuronal compartments.
Some researchers focus on how the cellular- and molecular-level functions of individual neurons are regulated. For example, they might study how specific receptors on dendrites contribute to the post synaptic response, which could lead to an action potential. They may also investigate excitation-secretion coupling at the synapse, which means that they are interested in how neuronal excitation and presynaptic machinery affect the release of neurotransmitters.
Neurophysiologists can tackle a range of scientific questions from single molecules in neurons to widespread activity in the brain. They may ask how neurons process information received at their dendritic arbors, using multiple techniques to investigate the axon, dendrites and dendritic spines of a single neuron simultaneously. And some of them look at patterns of activity over the large regions of the nervous system. Circuits are usually studied in the context of a simple behavior or stimulus response that they are thought to control.
Neurophysiology is the subspecialty of neuroscience and physiology that focuses on the physiology and functioning of the nervous system, often using electrophysiological or molecular biological tools.
Patch clamping is one of the most widely used techniques for investigating neurons at the cell and molecular level. Using a little bit of suction, a fine glass capillary electrode is sealed onto the neuron allowing for internal monitoring of whole cell excitability. There are also patch clamp configurations where a small patch of membrane is excised from the cell, thereby providing access to the cytoplasmic side of the plasma membrane for pharmacological manipulation.
Calcium imaging can be used to investigate excitation over the entire neuron. Neurons are loaded with a dye that changes its fluorescence in response to an elevated calcium concentration within the cell. While intracellular calcium has many functions, calcium imaging can be used as an indirect measure of action potentials as shown with this example neuron.
Techniques used to study neural circuits need to be able to monitor many neurons at once. The use of multielectrode arrays with many contacts is one method used to record from multiple neurons simultaneously.
Neural circuitry can also be investigated using optogenetics, in which neurons are modified to express light sensitive ion channels. When exposed to light, these channels open and, depending on their ion selectivity, they can either inhibit or excite the neuron, which provides insight into what role that neuron plays in a particular circuit and the behavioral response governed by that circuit.
To visualize patterns of activity on a broader scale, a variety of techniques are used. Electroencephalography or EEG uses electrodes on the skull to monitor electrical activity across the entire brain.
A second method is electrocorticography or ECoG, which also monitors electrical activity except that the electrodes are placed on the surface of the brain. This method is typically performed in conjunction with a clinical procedure such as those performed in epilepsy patients.
A technique known as functional near-infrared spectroscopy employs infrared light to monitor oxygen use as a correlate of neural activity, which can be monitored during behavioral tasks.
One of the major goals of neurophysiological research is to figure out the cause and treatment of nervous system dysfunction, such as in epilepsy. One approach is to implant electrodes for electrocorticographic recordings of patient brain activity in order to localize the seizure-causing dysfunctional regions of the brain.
An exciting new application for neurophysiology is the development of brain-machine interfaces. In these interfaces, brain activity is monitored while the subject thinks about doing a task, such as moving a cursor on a screen. The activity is fed into the computer as the command signal for the cursor. In essence, this is device control using thought.
Another application of neurophysiology is the investigation of neural circuitry using optogenetics. By assembling a fiber optic cord that connects to a coupler implanted in the brain of a genetically engineered mouse, these researchers can directly visualize the behavioral outcome of stimulating defined neural circuits.