Neurophysiology is a specialized field that investigates the function of the nervous system, which includes the brain, spinal cord, and peripheral nerves. This discipline focuses specifically on the electrical and chemical processes that allow the nervous system to generate, transmit, and integrate information. It operates as a subdiscipline of both physiology, the study of how living organisms function, and neuroscience, the comprehensive study of the nervous system. By examining processes that range from molecular interactions within a single cell to the activity of complex neural circuits, the findings from this research are fundamental to understanding how the nervous system coordinates behavior, sensory perception, and cognitive function.
Cellular Foundations of Neurophysiology
The functional basis of neurophysiology rests upon two primary cell types: neurons and glial cells. Neurons are the principal signaling units, specialized for rapidly transmitting information over long distances within the body. Glial cells provide the essential support structure, modulating the environment and insulating the neurons. Oligodendrocytes and Schwann cells create the myelin sheath that wraps around axons, facilitating faster signal conduction.
The ability of a neuron to signal begins with the membrane potential, an electrical voltage difference maintained across the cell membrane. In a resting state, the inside of the neuron is typically negative relative to the outside, often around -70 millivolts (mV), known as the resting membrane potential. This potential is maintained by the unequal distribution of ions, primarily sodium (Na+), potassium (K+), and chloride (Cl-), actively managed by the sodium-potassium ATPase pump.
Changes to this resting state are expressed as graded potentials, which are small, localized shifts in membrane voltage. These potentials occur primarily in the dendrites and cell body in response to incoming stimuli, such as neurotransmitters binding to receptors. Their amplitude is directly proportional to the strength of the stimulus.
Graded potentials can be either excitatory, moving the cell toward the signaling threshold, or inhibitory, stabilizing the resting state. These local changes dissipate quickly over distance, but they can be summed up in the cell body. The neuron integrates these incoming signals to determine if a full-scale signal should be transmitted down the axon.
Electrical and Chemical Communication
The summation of excitatory graded potentials must reach a specific trigger point, called the threshold potential, to initiate the neuron’s primary long-distance signal. This trigger point fires the action potential, an electrical impulse that acts on the “all-or-nothing” principle. If the threshold is reached, the impulse fires with a fixed, maximal amplitude regardless of how much stronger the stimulus was above the threshold.
The action potential is a rapid, three-phase event. It begins with a massive influx of positive sodium ions through voltage-gated channels, causing a swift depolarization of the membrane. This is followed by a repolarization phase, where voltage-gated potassium channels open to allow positive potassium ions to rush out of the cell, returning the voltage to a negative state. The potassium channels close slowly, leading to a brief hyperpolarization where the membrane potential temporarily dips below the resting level.
This electrical signal propagates rapidly down the axon without losing strength. In many neurons, this speed is enhanced by the insulating myelin sheath, which forces the signal to jump from one unmyelinated gap, called a node of Ranvier, to the next. This process, known as saltatory conduction, allows for conduction velocities that can be up to 100 meters per second.
Once the action potential reaches the axon terminal, it communicates with the next cell via chemical synaptic transmission. The arrival of the electrical impulse opens voltage-gated calcium channels, causing calcium ions to flow into the terminal. This influx of calcium is the immediate trigger for synaptic vesicles, which contain neurotransmitters, to fuse with the presynaptic membrane.
The neurotransmitters are then released into the synaptic cleft and diffuse across to bind with specific receptor proteins on the postsynaptic cell membrane. This binding causes ion channels on the receiving cell to open or close, generating the next graded potential. This conversion from an electrical signal within the neuron to a chemical signal between neurons is the fundamental mechanism for all nervous system information processing.
Key Research and Clinical Techniques
Neurophysiology relies on specialized methods to measure the electrical and chemical activity of the nervous system. One widely used clinical tool is Electroencephalography (EEG), which measures the electrical activity of the brain from the scalp. EEG electrodes detect the collective, summated activity of large populations of cortical neurons, which manifest as characteristic brain waves.
EEG is valuable for measuring the timing of neural events due to its millisecond temporal resolution, though its spatial resolution is limited to broad cortical areas. The patterns of these brain waves, categorized by frequency (e.g., alpha, beta, theta), are analyzed to assess states of consciousness, sleep stages, and general brain health.
For studying the function of individual cells and ion channels, researchers use patch clamping. This method involves pressing a glass micropipette against a cell membrane to isolate a tiny patch containing one or a few ion channels. By creating a tight electrical seal, scientists can precisely control the voltage across that membrane patch.
The ability to control the voltage allows researchers to measure the minute ionic currents flowing through individual channels, which are in the picoampere range. This technique has been instrumental in characterizing the properties of the voltage-gated sodium and potassium channels that govern the action potential. It provides a detailed view of the molecular mechanisms underlying neuronal excitability.
Role in Understanding Neurological Function
The principles of neurophysiology are directly applied to understand and diagnose conditions where nervous system function is disrupted. The field provides a functional framework for interpreting a wide range of neurological and psychiatric disorders. For instance, in epilepsy, the condition is a disorder of excessive and hypersynchronous electrical activity in neuronal networks.
Neurophysiological studies show that seizures result from an imbalance between excitatory and inhibitory synaptic transmission, leading to uncontrolled, rhythmic firing of neuron populations. The primary diagnostic tool, EEG, directly visualizes these functional disruptions, recording the characteristic “spike-and-wave” discharges that define epileptic activity.
Neurophysiology is also central to understanding the relationship between sleep and neurological function. The normal cyclical changes in brain state during sleep create neurophysiological conditions that can either increase or decrease the likelihood of seizures or other involuntary movements. For example, the deep, synchronized firing patterns of non-rapid eye movement (NREM) sleep can activate interictal epileptiform discharges, which are abnormal electrical bursts.
This field provides insight into functional disorders like chronic pain, where the transmission of sensory signals becomes amplified or improperly regulated in the spinal cord and brain. By mapping the functional integrity of neural pathways, neurophysiology helps clinicians determine the functional impact of conditions, providing objective data to guide therapeutic strategies that aim to restore normal electrical and chemical signaling.