Neurophysiology is a branch of biology and neuroscience that focuses on the functions of the nervous system. This field provides understanding into how individuals think, feel, move, and perceive the world, investigating the mechanisms behind generating and transmitting electrical signals.
The Neuron: Basic Unit of Function
Neurons are specialized cells that serve as the fundamental units of communication within the nervous system. Neurons consist of three main parts: the cell body (soma), dendrites, and an axon. The cell body contains the nucleus, serving as the neuron’s control center.
Dendrites are branches that extend from the cell body and receive incoming electrical signals from other neurons. These signals can either excite or inhibit the neuron, influencing whether it will generate its own signal. The axon is a projection that transmits electrical signals away from the cell body to other neurons or target cells, such as muscles or glands.
Neurons communicate using electrical signals known as action potentials. An action potential is a rapid, temporary shift in the neuron’s membrane potential, caused by the flow of charged ions in and out of the cell. This electrical impulse is generated at the axon hillock and then travels down the axon. The myelin sheath, a fatty layer covering many axons, helps to speed up this signal transmission.
Neural Communication and Circuits
Neurons communicate with each other at specialized junctions called synapses. At a chemical synapse, a gap called the synaptic cleft separates the transmitting neuron (presynaptic neuron) from the receiving neuron (postsynaptic neuron). When an action potential reaches the end of the presynaptic axon, it triggers the release of chemical messengers called neurotransmitters into this synaptic cleft.
Neurotransmitters then diffuse across the synaptic cleft and bind to receptor molecules on the postsynaptic neuron’s membrane. This binding action causes changes in the postsynaptic neuron, either exciting it to generate its own action potential or inhibiting it from firing. Excess neurotransmitters are reabsorbed by the presynaptic neuron or broken down by enzymes to terminate the signal.
These individual synaptic connections form complex neural circuits. Neural circuits are responsible for processing information and generating specific responses. For example, in a simple reflex arc, sensory neurons connect to interneurons, which then connect to motor neurons, allowing a rapid, automatic response to a stimulus. The combined balance of excitatory and inhibitory inputs determines whether a neuron will fire an action potential within a circuit.
Mapping Brain Function and Behavior
Neurophysiology explores how different parts of the nervous system contribute to specific functions and behaviors. The cerebral cortex, the brain’s outermost layer, processes complex cognitive tasks, emotions, and various sensory inputs, while also executing voluntary movements. This structure is divided into motor, sensory, and association areas, each with specialized roles.
Sensory areas of the cerebral cortex process information from our senses. For instance, the primary somatosensory cortex in the parietal lobe handles touch, temperature, and pain. The occipital lobe contains the primary visual cortex, which processes visual information. Auditory information is processed in the temporal lobe, which plays a role in short-term memory and learned emotional responses.
Motor areas, located in the frontal lobe, control voluntary movements. The primary motor cortex directly orchestrates skeletal muscle movements. Adjacent to this is the premotor cortex, which plans and coordinates complex movements. Beyond these primary areas, association areas integrate sensory and motor information, supporting higher cognitive functions such as learning, memory, reasoning, and emotions. These functions involve coordinated activity across multiple neural circuits rather than isolated brain regions.
Neurophysiology in Health and Disease
Neurophysiology provides insight into both normal nervous system function and the mechanisms underlying neurological and psychiatric disorders. Disruptions in neurotransmitter systems, for example, can significantly impact brain function. An imbalance in glutamate, an excitatory neurotransmitter, is associated with conditions such as Alzheimer’s disease, dementia, Parkinson’s disease, and seizures. Similarly, imbalances in gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter, can contribute to anxiety, irritability, sleep disturbances, and depression.
In Parkinson’s disease, the loss of dopamine-producing neurons leads to motor symptoms like tremors and slowness of movement. Depression is a common non-motor symptom in Parkinson’s disease and may be linked to dysfunctions in serotonergic and noradrenergic systems, as well as dopaminergic pathways. Epilepsy, characterized by hypersynchronization of neuronal activity, involves abnormal electrical patterns in the brain. Neurophysiology aims to identify these underlying functional impairments to develop targeted treatments and improve patient outcomes.
Studying the Nervous System: Key Techniques
Neurophysiologists employ various techniques to study the nervous system. Electroencephalography (EEG) is a non-invasive method that measures the electrical activity generated by groups of neurons in the brain. EEG is particularly useful for detecting rapid changes in brain activity and identifying abnormal electrical patterns. While it offers excellent temporal resolution, its spatial resolution is limited, making it difficult to pinpoint the exact location of activity deep within the brain.
Functional Magnetic Resonance Imaging (fMRI) is another non-invasive technique that detects brain activity by measuring changes in blood flow and oxygenation. When a brain region becomes more active, it requires more oxygenated blood, leading to a detectable signal. fMRI provides detailed spatial maps of active brain regions, making it suitable for identifying areas involved in specific tasks or cognitive processes. However, fMRI has a slower temporal resolution compared to EEG, as blood flow changes are slower than electrical signals.
Electromyography (EMG) is used to assess the electrical activity of muscles and the nerves that control them. Surface electrodes are placed on the skin over the muscles to record the electrical impulses generated during muscle contraction. EMG helps in diagnosing nerve and muscle disorders. It can identify abnormal muscle activity or assess muscle fatigue.