ArticlesPhysiologyNeurophysiology

ArticlesPhysiologyNeurophysiology

Neurophysiology

Author: Christina Loukopoulou, MSc • Reviewer: Dimitrios Mytilinaios, MD, PhD
Last reviewed: September 18, 2024
Reading time: 21 minutes

Neurophysiology is the branch of physiology dedicated to understanding the mechanisms and functions of the nervous system, the body's most intricate and vital network.

It delves into the dynamic interplay between the brain, spinal cord, and peripheral nerves, unraveling the ways in which these structures work in unison to regulate everything from basic physiological processes to advanced cognitive functions.

At its core, neurophysiology examines how neurons, the fundamental units of the nervous system, communicate through electrochemical signals, how they integrate vast amounts of information, and how they generate precise, coordinated responses to both internal and external stimuli.

Key facts about neurophysiology
Neurophysiology	Study of the nervous system, focusing on neuron communication, action potentials, and coordinating the body’s responses to stimuli.
Central Nervous System (CNS)	Comprises the brain and spinal cord; responsible for processing sensory information and generating commands.
Peripheral Nervous System (PNS)	Includes nerves outside the CNS; divided into the somatic (voluntary movements) and autonomic (involuntary functions) systems.
Homeostasis	The nervous system maintains stable internal conditions through feedback mechanisms (e.g., temperature regulation).
Motor control (efferent division)	Involves brain, spinal cord, and peripheral nerves to regulate muscle movement; controlled by the motor cortex, basal ganglia, and cerebellum.
Autonomic responses	Involuntary actions controlled by the autonomic nervous system, including heart rate and digestion; divided into sympathetic ("fight-or-flight") and parasympathetic ("rest-and-digest").
General senses	Includes touch, temperature, pain, pressure, and proprioception; detected by various receptors throughout the body.
Special senses	- Vision: Light detection by eyes; processed in the occipital lobe. - Hearing: Sound waves detected by the ear; processed in the temporal lobe. - Smell and taste: Chemical detection by olfactory receptors and taste buds. - Vestibular sensations: Balance and spatial orientation via the vestibular system in the inner ear.
Neuronal signaling	Neurons communicate via action potentials and synaptic transmission involving neurotransmitters.
Neurotransmitters	Chemical messengers; classified as excitatory (e.g., glutamate) or inhibitory (e.g., GABA), regulating various functions like mood, cognition, and sleep.
Neurological disorders	Includes Alzheimer’s, Parkinson’s, epilepsy, multiple sclerosis, stroke, and migraines, affecting different aspects of brain and nervous system functionality.

Contents

Nervous system
Homeostasis
Motor control
Senses
1. General senses
2. Special senses
Neuronal signaling
Neurotransmitters and their roles
Neurological disorders
Sources
Related articles

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Nervous system

From a structural point of view, the nervous system is divided into two major components: the central nervous system (CNS) and the peripheral nervous system (PNS).

The CNS includes the brain and spinal cord, serving as the main control center. The brain, housed within the skull, is the most complex organ, containing billions of neurons communicating through trillions of synapses. It is responsible for processing sensory information, generating thoughts, and issuing commands. The spinal cord, protected by the vertebral column, is the primary pathway for transmitting signals between the brain and the rest of the body.
The PNS consists of nerves extending from the brain and spinal cord to other body parts, including the limbs and organs. It is divided into the somatic nervous system, which controls voluntary movements and relays sensory information, and the autonomic nervous system (ANS), which regulates involuntary functions like heart rate and digestion. The ANS is further subdivided into the sympathetic and parasympathetic systems, which typically work in opposition. The sympathetic nervous system is primarily responsible for the body's "fight-or-flight" response. Activation of the sympathetic nervous system leads to physiological changes such as an increase in heart rate, dilation of the pupils, and the release of adrenalin/epinephrine. These changes optimize the body for rapid, intense physical activity, enhancing alertness and responsiveness to external stimuli. In contrast, the parasympathetic nervous system governs the "rest-and-digest" functions. It slows the heart rate, reduces blood pressure, and stimulates digestive processes.

Homeostasis

One of the most crucial functions of the nervous system is homeostasis. Homeostasis is the body's ability to maintain a stable internal environment despite changes in external conditions. This balance is crucial for survival and is tightly regulated by the nervous system. The system monitors physiological variables like temperature, pH, and blood pressure and responds appropriately when these variables deviate from normal ranges.

The nervous system uses feedback mechanisms to maintain homeostasis. For instance, in temperature regulation, receptors detect changes in body temperature and send signals to the hypothalamus, the brain's thermoregulatory center. If the body is too hot, the hypothalamus triggers cooling mechanisms like sweating and vasodilation (widening of blood vessels) to release heat. If the body is too cold, it triggers shivering and vasoconstriction (narrowing of blood vessels) to conserve heat. These negative feedback loops are essential for keeping the body functioning within optimal parameters.

Motor control

Motor control refers to the nervous system's ability to regulate and guide muscles and limbs to perform desired movements. It involves the brain, spinal cord, and peripheral nerves.

The spinal cord is a key player in motor control, acting as a major conduit for motor information. It transmits signals between the brain and muscles and contains reflex arcs, which are simple neural circuits that control reflex actions. For instance, the knee-jerk reflex is mediated by a reflex arc in the spinal cord, allowing the leg to extend rapidly in response to a tap on the knee without involving the brain.

Voluntary movements are primarily controlled by the motor cortex in the frontal lobe. When a person decides to move, the motor cortex sends signals through the corticospinal tract to the spinal cord, which then directs the appropriate muscles.

The basal ganglia and cerebellum are crucial for fine-tuning movements. The basal ganglia help initiate movements and regulate their intensity, while the cerebellum monitors ongoing movements, comparing intended actions with actual ones and making adjustments to ensure smooth and accurate motion.
Motor learning, the process of acquiring new motor skills, involves the cerebellum and changes in synaptic strength within neural circuits.

Autonomic responses are involuntary actions regulated by the autonomic nervous system (ANS), which governs the activity of smooth muscles, cardiac muscles, and glands. Unlike voluntary motor control, autonomic responses manage essential bodily functions such as heart rate, digestion, and respiratory rate. The ANS is divided into two components:

The sympathetic nervous system initiates the "fight-or-flight" response, preparing the body for intense physical activity by increasing heart rate, dilating pupils, and redirecting blood to muscles.
The parasympathetic nervous system supports the "rest-and-digest" response, slowing heart rate, facilitating digestion, and conserving energy during relaxed states.

While the somatic nervous system controls voluntary movements, the autonomic nervous system regulates these involuntary responses, ensuring the body maintains homeostasis during both stressful and restful periods.

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Senses

The sensory systems allow the body to perceive and interpret the environment, enabling appropriate reactions and adaptations. Sensory information is collected by specialized receptors and transmitted to the CNS, where it is processed and integrated to form a coherent picture of the world.

General senses

General senses include touch, temperature, pain, pressure, and proprioception (the sense of body position). These senses are detected by receptors located throughout the body. For example:

Mechanoreceptors respond to mechanical forces like pressure and vibration.
Thermoreceptors detect temperature changes.
Nociceptors sense painful stimuli, whether from mechanical damage, extreme temperatures, or chemical irritation.
Proprioceptors provide information about body position and movement, crucial for balance and coordination.

Special senses

Special senses are more complex and involve specialized organs:

Vision is mediated by the eyes, which detect light and convert it into electrical signals. These signals are processed by the visual cortex in the occipital lobe to produce images.
Hearing involves the detection of sound waves by the ear, which are converted into electrical signals by hair cells in the cochlea. These signals are then processed by the auditory cortex in the temporal lobe.
Smell and taste are chemical senses that detect airborne and soluble chemicals, respectively. The olfactory receptors in the nose and taste buds on the tongue send signals to the brain that are interpreted as specific smells and tastes.

Vestibular sensations are crucial for balance and spatial orientation, allowing the body to detect changes in head position and motion. The vestibular system is located within the inner ear and consists of two main components:

The semicircular canals, which detect rotational movements of the head. Each of the three canals is oriented in a different plane (horizontal, anterior, and posterior) and is filled with fluid that moves when the head rotates. This movement bends hair cells in the canals, sending signals to the brain to adjust posture and eye movements.
The otolith organs (the utricle and saccule), which detect linear accelerations and gravitational forces. These structures contain small crystals that shift in response to head movements, activating hair cells that provide information about changes in head position, such as tilting or moving forward.

Signals from the vestibular system are sent to the vestibular nuclei in the brainstem and then integrated with visual and proprioceptive information. This helps maintain balance, coordinate eye movements with head movements (vestibulo-ocular reflex), and adjust posture, making it essential for activities like walking, running, and even standing still.

Neuronal signaling

At the core of neurophysiology, lie all the mechanisms that control how neurons communicate with each other and with other cells in the body. This communication which is also known as neuronal signaling occurs through the transmission of electrical impulses, known as action potentials, along the different parts of a neuron. When an action potential reaches the end of an axon, it triggers the release of neurotransmitters, chemical messengers that cross the synapse, the gap between neurons. These neurotransmitters bind to receptors on the adjacent neuron, leading to the generation of a new electrical signal in that neuron. This intricate process allows the nervous system to rapidly process and respond to information, enabling everything from reflex actions to complex thought processes.

Resting membrane potential

The resting membrane potential is the electrical potential difference across the neuron's membrane when it is not actively transmitting a signal. It is approximately -70 millivolts (mV) inside the cell relative to the outside. This potential is maintained by the distribution of ions, primarily sodium (Na⁺), potassium (K⁺), and chloride (Cl^-), and the activity of the sodium-potassium pump (Na⁺/K⁺ ATPase), which moves 3 Na⁺ ions out of the cell and 2 K⁺ ions into the cell.

Action potentials

Action potentials are rapid, transient changes in membrane potential that propagate along the axon. They are essential for nerve signal transmission. Key phases include:

Depolarization: Triggered when the membrane potential becomes more positive, typically due to the influx of Na⁺ ions through voltage-gated channels.
Repolarization: Following the peak of the action potential, K⁺ channels open, allowing K⁺ ions to exit the cell, restoring the negative membrane potential.
Hyperpolarization: The membrane potential temporarily becomes more negative than the resting potential due to prolonged K⁺ channel activity before returning to the resting state.

Synaptic transmission

Synapses are the junctions where neurons communicate with other neurons or effector cells. There are two main types:

Chemical synapses: Involve the release of neurotransmitters from the presynaptic neuron into the synaptic cleft, where they bind to receptors on the postsynaptic neuron, leading to changes in its membrane potential.
Electrical synapses: Involve direct electrical coupling between neurons through gap junctions, allowing for faster signal transmission.

Synaptic transmission is a fundamental process in neuronal communication, involving the release and reception of neurotransmitters across the synapse. When an action potential reaches the presynaptic terminal of a neuron, it triggers the release of neurotransmitters stored in vesicles. These chemical messengers are released into the synaptic cleft and bind to specific receptors on the postsynaptic membrane. This binding causes ion channels to open, leading to changes in the membrane potential of the postsynaptic neuron. Depending on the type of neurotransmitter and receptor involved, this can result in excitatory postsynaptic potentials (EPSPs), which increase the likelihood of the neuron firing an action potential, or inhibitory postsynaptic potentials (IPSPs), which decrease this likelihood. Over time, synaptic plasticity allows these synapses to strengthen or weaken in response to activity, playing a crucial role in learning, memory formation, and the adaptability of the nervous system.

Neurotransmitters and their roles

Neurotransmitters are the chemical messengers that transmit signals across synapses from one neuron to another. They play a crucial role in regulating a wide range of bodily functions and behaviors, from mood and sleep to heart rate and digestion.

Neurotransmitters can be broadly classified into two categories based on their effects on the postsynaptic neuron:

Excitatory neurotransmitters, such as glutamate, increase the likelihood that the neuron will fire an action potential. Glutamate is the most abundant excitatory neurotransmitter in the brain and is involved in cognitive functions like learning and memory.
Inhibitory neurotransmitters, such as gamma-aminobutyric acid (GABA), decrease the likelihood of an action potential. GABA plays a key role in reducing neuronal excitability and preventing overstimulation, which is crucial for maintaining balance in brain activity.

Neurological disorders

Neurological disorders encompass a wide range of conditions that affect the nervous system, including the brain, spinal cord, and peripheral nerves. These disorders can result from genetic factors, infections, injuries, degenerative processes, or environmental influences. Understanding the neurophysiological basis of these conditions is essential for developing effective treatments and therapies.

Alzheimer's disease: A progressive neurodegenerative disorder characterized by the loss of neurons and synapses, particularly in the hippocampus and cerebral cortex. This leads to memory loss, cognitive decline, and personality changes. The accumulation of amyloid plaques and tau tangles in the brain are hallmark features of Alzheimer's disease. Current treatments focus on managing symptoms and slowing disease progression, but research into disease-modifying therapies continues.
Epilepsy: A disorder characterized by recurrent, unprovoked seizures caused by abnormal electrical activity in the brain. Epilepsy can result from genetic factors, brain injuries, or developmental issues. Treatment often involves anticonvulsant medications that stabilize neuronal activity and prevent seizures.
Multiple sclerosis (MS): An autoimmune disorder where the immune system attacks the myelin sheath, the protective covering of nerve fibers. This leads to disrupted communication between the brain and the rest of the body, causing symptoms like muscle weakness, coordination problems, and fatigue. Treatment focuses on managing symptoms, slowing disease progression, and modifying the immune response.
Stroke: A stroke occurs when the blood supply to part of the brain is interrupted or reduced, depriving brain tissue of oxygen and nutrients. This can lead to cell death and loss of function in the affected area. The effects of a stroke depend on its severity and location, ranging from minor speech or movement difficulties to complete paralysis or death. Rehabilitation and medication are key components of stroke recovery and prevention.
Parkinson's disease: A neurodegenerative disorder characterized by the progressive loss of dopamine-producing neurons in the basal ganglia, leading to motor symptoms such as tremors, rigidity, bradykinesia (slowness of movement), and postural instability. Treatment typically involves medications that increase dopamine levels or mimic its action, as well as physical therapy to manage symptoms.
Migraine: A neurological condition that causes severe, recurring headaches, often accompanied by other symptoms like nausea, vomiting, and sensitivity to light and sound. Migraines are believed to involve abnormal brain activity affecting nerve signals, chemicals, and blood vessels in the brain. Treatment includes medications to relieve symptoms and prevent future attacks.

Sources

All content published on Kenhub is reviewed by medical and anatomy experts. The information we provide is grounded on academic literature and peer-reviewed research. Kenhub does not provide medical advice. You can learn more about our content creation and review standards by reading our content quality guidelines.

Barrett, K. E., Barman, S. M., Yuan, J., & Brooks, H. L. (2019). Ganong’s review of medical physiology (26th ed.). McGraw-Hill Education/Medical.
Gordon Betts, J., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022, April 20). Preface. Anatomy and Physiology 2e; OpenStax. https://openstax.org/books/ana...
Hall, J. E., & Guyton, A. C. (2011). Guyton and Hall Textbook of Medical Physiology. Saunders.
McKinley, M. P., & O’Loughlin, V. (2011). Human Anatomy (3rd ed.). McGraw Hill Higher Education.

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