Neurophysiology is the study of the function of the nervous system, investigating the mechanisms by which it operates to control the body and process information. This field seeks to understand how the nervous system—including the brain, spinal cord, and all extending nerves—generates and transmits signals. Neurophysiology explores the dynamic electrical and chemical processes that allow communication between cells. The work of neurophysiologists helps reveal how we sense, move, think, and feel, providing a foundation for understanding both healthy function and neurological disorders.
Defining the Scope of Neurophysiology
The discipline of neurophysiology broadly examines how the nervous system works at every scale, from molecular interactions to the behavior of the entire organism. The nervous system is anatomically divided into the Central Nervous System (CNS), composed of the brain and spinal cord, and the Peripheral Nervous System (PNS), which includes all nerves outside the CNS. Neurophysiology investigates the function of both of these interconnected divisions.
This exploration operates across multiple levels of complexity, beginning with individual proteins and ion channels within cell membranes. It then scales up to examine how networks of cells interact to form microcircuits and how these circuits combine to produce system-level behaviors, such as reflexes and sleep-wake cycles. Techniques like electroencephalography (EEG) and nerve conduction studies are employed to measure the bioelectrical activity that defines nervous system function. These methods provide insight into conditions like epilepsy and nerve dysfunction.
The Neuron as the Fundamental Unit
The neuron is the primary functional cell that enables all nervous system processes, specialized for electrical and chemical communication. Each neuron possesses a distinct structure that facilitates its role in receiving, integrating, and transmitting information. The central part of the cell is the soma, or cell body, which contains the nucleus and the machinery necessary for cell maintenance.
Extending from the soma are the dendrites, which are branched structures that function as the main receiving antennas for signals from other neurons. These dendrites collect incoming information, which is then integrated in the soma before a signal is sent outward. The axon is a single, long projection that transmits the electrical signal away from the cell body toward target cells, such as other neurons, muscles, or glands.
These communication lines are supported by glial cells, which provide insulation, metabolic support, and protection to the neurons. Certain glial cells form the myelin sheath, a fatty layer that wraps around the axon to speed up signal transmission. The coordinated action of neurons, facilitated by these support cells, forms the physical basis for all nervous system activity.
Electrical Signaling and the Action Potential
The language of the nervous system is electrical, and the fundamental unit of this communication is the action potential, often called a nerve impulse. This is a rapid, all-or-nothing change in the voltage across a neuron’s membrane, allowing signals to travel long distances down the axon. Before firing, the neuron maintains a negative charge inside relative to the outside, known as the resting membrane potential, typically around -70 millivolts (mV). This resting state is maintained by the unequal distribution of ions, regulated by the sodium-potassium pump.
The action potential is initiated when the neuron receives enough excitatory input to raise its membrane potential past a threshold, usually near -55 mV. Once this threshold is reached, voltage-gated sodium channels open, causing a rapid rush of positively charged \(\text{Na}^{+}\) ions into the cell. This influx of positive charge is called depolarization, and it quickly reverses the membrane potential, making the inside of the cell briefly positive, often reaching a peak of around +30 mV.
Almost immediately after this peak, the cell begins repolarization, where the voltage-gated sodium channels become inactivated, halting the \(\text{Na}^{+}\) influx. Simultaneously, voltage-gated potassium channels open, allowing \(\text{K}^{+}\) ions to flow rapidly out of the cell, carrying positive charge away. This efflux returns the membrane potential to a negative value.
The \(\text{K}^{+}\) channels are slow to close, causing a brief period of hyperpolarization where the membrane potential temporarily dips below the resting potential. This makes the neuron less likely to fire again immediately. This controlled sequence of ion movement ensures the electrical signal is propagated along the axon until it reaches the terminal, preparing to communicate with the next cell.
Synaptic Transmission (Chemical Communication)
Once the electrical action potential reaches the end of the axon, it must bridge the gap to the next neuron via chemical communication at a specialized junction called the synapse. The action potential terminates at the presynaptic terminal, triggering the conversion of the electrical signal into a chemical one. This conversion begins when the change in voltage opens voltage-gated calcium ion (\(\text{Ca}^{2+}\)) channels in the terminal membrane.
The resulting rush of \(\text{Ca}^{2+}\) ions into the terminal triggers the release of neurotransmitters. These chemical messengers are stored inside synaptic vesicles, which fuse with the presynaptic membrane and empty their contents into the synaptic cleft. The neurotransmitter molecules rapidly diffuse across this cleft to bind to specific receptor proteins located on the postsynaptic neuron’s membrane.
The binding of the neurotransmitter causes a change in the postsynaptic cell, making it more or less likely to fire its own action potential. Excitatory neurotransmitters, such as glutamate, cause depolarization, pushing the voltage closer to the firing threshold. Conversely, inhibitory neurotransmitters, like GABA, cause hyperpolarization, suppressing the generation of an action potential. The final step involves clearing the neurotransmitter from the synaptic cleft, which prevents continuous stimulation and ensures the system is ready for the next signal.
Neurophysiology in Functional Systems
The cellular mechanisms of electrical and chemical signaling form the basis for all observable functions of the nervous system. The rapid nature of the action potential and synaptic transmission allows for the immediate encoding and processing of sensory information. When a sensory receptor detects a stimulus, this information is converted into a train of action potentials transmitted along sensory nerves to the CNS. The brain interprets the frequency and pattern of these electrical spikes to perceive the world.
These signals are also fundamental to the motor system, which controls movement. Motor intent originates in the cerebral cortex and is translated into commands sent to spinal motor neurons. The final output signal, delivered via a motor neuron’s axon to a muscle fiber, is a release of neurotransmitters that triggers muscle contraction. This complex pathway is modulated by sensory feedback and coordination centers like the cerebellum to ensure movement is smooth and accurate.
Even the simplest behaviors are governed by these principles, such as the reflex arc, where a sensory input directly activates a motor neuron at the spinal cord level. This allows for extremely fast, protective responses, like quickly pulling a hand away from a hot surface. The study of these systems provides a comprehensive view of how the nervous system integrates cellular events to govern thought, sensation, and action.