Physiology: Neurophysiology

Neurophysiology is the branch of physiology that explains how the nervous system works as a living, communicating network. It connects molecular events at synapses to whole-body functions such as sensation, movement, posture, and rapid protective reflexes. At its core are a few recurring themes: neurons convert stimuli into electrical signals, synapses relay and transform those signals, and circuits integrate information to produce coordinated outputs.

Understanding neurophysiology is less about memorizing parts of the brain and more about tracing how information flows. A pinprick becomes a patterned volley of impulses; that pattern is filtered and interpreted; then motor pathways translate the result into action. This article focuses on four foundational areas: synaptic transmission, sensory processing, motor control, and reflexes.

The electrical language of neurons

Neurons communicate primarily through changes in membrane potential. At rest, a neuron maintains a voltage difference across its membrane, largely set by ion gradients and selective permeability. When inputs depolarize the membrane past threshold, voltage-gated ion channels open and generate an action potential, a brief, regenerative spike that propagates along the axon.

Action potentials are not just “on” or “off” events. Timing and frequency matter. Many sensory systems encode stimulus intensity through firing rate and recruitment of additional neurons. Timing also carries information, especially in circuits that depend on synchrony and precise sequencing.

Propagation speed depends on axon diameter and myelination. Myelin increases conduction velocity by enabling saltatory conduction, in which action potentials “jump” between nodes of Ranvier. This improves speed and energy efficiency, which is crucial for long pathways involved in motor control and somatosensation.

Synaptic transmission: where signals are converted and computed

Most neural processing happens at synapses. Synaptic transmission comes in two major forms: electrical and chemical.

Chemical synapses and neurotransmitters

At chemical synapses, an arriving action potential triggers neurotransmitter release. Calcium influx into the presynaptic terminal leads to vesicle fusion and transmitter exocytosis into the synaptic cleft. Neurotransmitters then bind postsynaptic receptors, changing ion conductance and shifting the postsynaptic membrane potential.

The postsynaptic response may be:

• Excitatory postsynaptic potentials (EPSPs), which depolarize the neuron and increase the chance of firing.
• Inhibitory postsynaptic potentials (IPSPs), which hyperpolarize the neuron or stabilize the membrane, reducing the likelihood of firing.

In practice, neurons are constantly receiving many small EPSPs and IPSPs. Whether an action potential occurs depends on integration across space (many synapses across dendrites and soma) and time (rapid sequences of inputs). This integration is why synapses are often described as computational elements, not just relays.

Electrical synapses and fast synchronization

Electrical synapses use gap junctions that directly connect cytoplasm between cells. They support extremely fast signaling and can synchronize activity across groups of neurons. While less common than chemical synapses in many human brain regions, they play important roles in rhythm generation and rapid coordination in certain circuits.

Synaptic plasticity: change as a feature, not a bug

Neurophysiology also includes the mechanisms that change synaptic strength. Long-term potentiation and long-term depression are commonly cited examples of how repeated activity can strengthen or weaken synapses. These forms of plasticity provide a physiological basis for learning and adaptation, and they influence how sensory and motor circuits refine performance over time.

Sensory processing: from stimulus to perception

Sensory processing begins with transduction, the conversion of physical energy into neural signals. Specialized receptors are tuned to particular stimulus types: mechanoreceptors for touch and vibration, photoreceptors for light, chemoreceptors for taste and smell, and so on.

Coding strategies in sensory systems

Sensory systems use several encoding strategies:

• Modality (labeled lines): Different receptor types and pathways signal different sensory qualities.
• Intensity: Often represented by firing rate and the number of recruited afferent fibers.
• Timing: Precise spike timing can carry information about rapid stimulus changes.
• Receptive fields: Each neuron responds to stimulation in a particular region; receptive field size influences spatial resolution.

Adaptation is another key feature. Many receptors decrease their response during sustained stimulation, emphasizing changes rather than steady-state conditions. This is why you stop noticing clothing on your skin after a few minutes, yet immediately detect a new pressure point.

Integration and filtering

Sensory signals are filtered at multiple stages. Spinal and brainstem circuits can gate input before it reaches higher centers. Attention, context, and descending pathways can modulate sensory transmission, influencing what reaches conscious perception and how strongly it is experienced.

Pain processing offers a clear example of integration. Nociceptive signals can be amplified or dampened by spinal interneurons and descending control systems. The same injury can feel different depending on stress, expectation, or competing stimuli, reflecting genuine physiological modulation rather than “imagined” sensation.

Motor control: turning plans into coordinated movement

Motor control involves multiple levels of organization: spinal circuits generate basic patterns, brainstem pathways regulate posture and tone, and cortical areas support voluntary, goal-directed movement. Smooth movement requires both command and correction.

Motor units and force production

The fundamental output element of the motor system is the motor unit, a motor neuron and the muscle fibers it innervates. Force is controlled by:

• Recruitment: activating more motor units, typically from smaller to larger units.
• Rate coding: increasing the firing rate of active motor neurons to produce greater muscle tension.

These principles explain how the same muscle can produce delicate movements or powerful contractions depending on neural drive.

Feedback and feedforward control

Movement is guided by sensory feedback from muscle spindles, Golgi tendon organs, joint receptors, vision, and vestibular inputs. Feedback allows correction, but it has delays. For rapid actions, the nervous system relies on feedforward control, using internal predictions about the body and environment. Skilled movements, such as catching a ball, combine both: a predictive initial command and continuous correction as new information arrives.

Coordination across circuits

Motor pathways do not act in isolation. Interneurons in the spinal cord coordinate agonist and antagonist muscles, while higher centers select actions, suppress unwanted movements, and adjust strategies based on outcomes. This layered architecture supports both stability and flexibility: stable posture while standing, and quick reconfiguration when stepping onto uneven ground.

Reflexes: fast, automatic, and clinically informative

Reflexes are rapid, stereotyped responses to specific stimuli. They protect the body and stabilize function without requiring conscious deliberation. Importantly, reflexes are not rigid. They are modulated by context and descending control.

The stretch reflex

The classic example is the stretch (myotatic) reflex. When a muscle is stretched, muscle spindle afferents increase firing and excite motor neurons that contract the same muscle. This helps maintain muscle length and contributes to postural stability. Reciprocal inhibition often accompanies this, reducing activity in antagonist muscles to allow efficient movement.

Withdrawal reflexes and protective responses

Noxious stimuli can trigger withdrawal reflexes that rapidly pull a limb away. These involve polysynaptic pathways and coordinated activation across multiple muscles. Crossed extensor responses can support the body by extending the opposite limb, illustrating how reflex circuits integrate information across the spinal cord.

Why reflexes matter in neurophysiology

Reflexes reveal the functional status of sensory afferents, synapses, interneurons, and motor neurons. In clinical contexts, altered reflexes can indicate changes in excitability or disruption in pathways. Even without diving into diagnostic rules, the principle is straightforward: because reflex arcs are relatively well-mapped, their behavior provides a window into nervous system function.

Bringing it together: integration as the central principle

Neurophysiology is unified by integration. Synaptic transmission determines how signals are combined and transformed. Sensory processing converts external events into patterns the nervous system can interpret. Motor control turns those patterns into purposeful actions. Reflexes provide rapid, automatic stability and protection, while remaining adjustable to the larger goals of the organism.

Seen this way, the nervous system is not a set of isolated modules. It is a dynamic, adaptive network that continuously balances speed with accuracy, stability with flexibility, and automatic responses with voluntary control. That balance is what makes sensation meaningful and movement effective.