Peripheral nervous system: nerves and ganglia

The peripheral nervous system includes all neural structures outside the brain and spinal cord. Its purpose is to deliver information from receptors to the central nervous system and transmit commands to muscles, glands, and internal organs. [1]

From a practical standpoint, it's more convenient to think of the peripheral nervous system as divided into two major "branches." The first branch is responsible for conscious sensations and voluntary movements (the somatic part). The second branch regulates the functioning of internal organs "in automatic mode" (the autonomic part), including the sympathetic, parasympathetic, and enteric divisions. [2]

The peripheral nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves, as well as nodes (ganglia) where the cell bodies of neurons are located, and nerve endings in tissues. The spinal nerves are mixed: they carry sensory, motor, and some autonomic impulses. [3]

The ganglia in this system are not "mere thickenings." Sensory ganglia (e.g., dorsal root ganglia) contain the cell bodies of primary sensory neurons, and autonomic ganglia are "switching stations" between neurons of the central nervous system and organs. [4]

Table 1. Main components of the peripheral nervous system

Component	Examples	Leading role
Cranial nerves	12 pairs	The connection of the brain with the structures of the head and neck, part of the autonomic functions
Spinal nerves	31 pairs	Connection of the spinal cord with the trunk and limbs, mixed fibers
Sensory ganglia	Dorsal root ganglia	Transmission of pain, temperature, touch, and proprioception to the central nervous system
Autonomic ganglia	Sympathetic trunks, parasympathetic ganglia	Switching of autonomous pathways to organs
Nerve endings	Receptors and effector endings	Input (sensitivity) and output (movement, secretion)

The information in the table summarizes the data on the structure of the somatic and autonomic parts of the peripheral nervous system. [5]

Peripheral nerve as an "organ": bundles, sheaths, nutrition and blood-nerve barrier

A peripheral nerve is not structured as a single cable, but as a set of bundles (fascicles) containing axons. This is important because injury or compression damages not only the "wire" but also its sheaths, vessels, and the microenvironment, which are crucial for recovery. [6]

Connective tissue sheaths provide a framework and protection. The endoneurium surrounds individual fibers, the perineurium "packs" the fibers into bundles and maintains intrafascicular pressure, and the epineurium covers the nerve externally and unites several bundles into a single trunk. [7]

A key modern focus is the barrier function. Tight junctions between perineurium cells and endoneurial capillaries form a blood-nerve barrier, which limits the penetration of molecules and cells from the blood into the endoneurium. This barrier protects axons but also influences the course of inflammatory and immune neuropathies and drug penetration. [8]

The nerve's blood supply is provided by the vasa nervorum, which branch from nearby arteries and are particularly abundant near joints. Their tortuosity is considered an adaptation to the nerve's movement during flexion and extension, but diabetes and other conditions can impair the microcirculation of these vessels, increasing the nerve's vulnerability. [9]

Table 2. Nerve sheaths and their functions

Layer	What surrounds	Main functions
Epineurium	The entire nerve trunk and several bundles	Mechanical protection, "frame", vascular guide
Perineurium	A separate bundle of fibers	Tensile strength, pressure support, participation in the blood-nerve barrier
Endoneurium	A single nerve fiber	Support and microenvironment for axons and Schwann cells
Endoneurial capillaries	Inside the bundle	Together with the perineurium they participate in the barrier function

The information in the table reflects the current description of the fascial membranes and the blood-nerve barrier. [10]

Nerve fibers: myelin, conduction velocity, afferent and efferent pathways

Peripheral nerves contain different types of fibers. Some fibers are covered with myelin, which is formed by Schwann cells, while others are unmyelinated. Myelin allows signals to be transmitted more quickly and efficiently, and damage to it alters the symptomatic presentation and conduction test results. [11]

A simplified rule applies: the thicker the fiber and the better its myelination, the higher the conduction velocity. Therefore, fast fibers are usually associated with proprioception and fine tactile sensitivity, while the slowest fibers often carry information about pain and temperature. [12]

Signals can also be described by direction. Afferent fibers carry information from receptors to the central nervous system, while efferent fibers conduct impulses to muscles and glands. Within the spinal nerves, these streams are "collected together," which is why many peripheral nerves are mixed. [13]

Differences in fiber types underlie neurophysiological methods. In demyelinating processes, conduction velocity initially decreases and latency increases, while in axonal lesions, response amplitude often decreases. This logic is widely used in the electrophysiological diagnosis of peripheral neuropathies. [14]

Table 3. Fiber types by myelin and conduction velocity

Fiber type	Myelin	Typical function	Typical conduction velocity
Large myelinated	Eat	Proprioception, vibration, fine touch, rapid motor signals	Tens or more meters per second
Thin myelinated	Yes, the layer is thinner.	"Fast" pain, cold, some skin sensitivity	Units and tens of meters per second
Unmyelinated	No	"Slow" pain, warmth, part of the autonomic influences	Fractions and units of meters per second
Preganglionic autonomic	Eat	Signal from the central nervous system to the autonomic ganglion	Units and tens of meters per second
Postganglionic autonomic	Usually no	Signal from the autonomic ganglion to the organ	Fractions and units of meters per second

The information in the table is consistent with modern descriptions of myelin and the physiology of impulse conduction in the peripheral nervous system. [15]

Ganglia and the autonomic nervous system: sensory ganglia, sympathetic, parasympathetic and enteric divisions

Sensory ganglia are located near the spinal cord (dorsal root ganglia) or cranial nerves. They contain the cell bodies of primary sensory neurons, which receive signals from peripheral receptors and transmit them to the central nervous system. This is of particular importance for pain: dorsal root ganglia are involved in the development of chronic pain and are considered a therapeutic target in certain approaches. [16]

The autonomic nervous system controls involuntary functions: heart rate, vascular tone, respiration, digestion, and other processes. Its classic architecture consists of two neurons per organ: a preganglionic neuron extends from the central nervous system to the ganglion, and a postganglionic neuron extends from the ganglion to the organ. [17]

The sympathetic division typically mobilizes resources during stress, increasing the body's readiness for exertion, while the parasympathetic division primarily maintains a "recovery and digestion" mode. This functional opposition simplifies the picture, but it helps to understand why an imbalance of autonomic influences manifests itself, for example, in changes in heart rate, sweating, and intestinal motility. [18]

The enteric division of the autonomic nervous system is organized as a network of neurons in the wall of the gastrointestinal tract and is capable of coordinating a significant portion of digestive functions locally. It is closely linked to sympathetic and parasympathetic influences, so autonomic disorders often manifest as gastrointestinal symptoms. [19]

Table 4. Types of ganglia of the peripheral nervous system

Ganglion type	Where is it located?	What neurons does it contain?	What does it do?
Dorsal root ganglion	In the intervertebral foramen	Cell bodies of primary sensory neurons	Transmission of pain, temperature, touch, proprioception
Sensory ganglia of the cranial nerves	At the brainstem and along the cranial nerves	Cell bodies of sensory neurons	Sensory signals from the head and neck
Sympathetic ganglia	Paravertebral and prevertebral	Postganglionic sympathetic neurons	Regulation of blood vessels, heart, sweat glands, etc.
Parasympathetic ganglia	Closer to the organ or in the organ	Postganglionic parasympathetic neurons	Regulation of "restorative" functions
Enteric ganglia	In the intestinal wall	Local area network neurons	Coordination of motility and secretion of the gastrointestinal tract

The information in the table corresponds to modern descriptions of sensory ganglia and the autonomic nervous system. [20]

Table 5. Divisions of the autonomic nervous system: key differences

Department	Common task	Where are the ganglia located?	Peculiarity of the path
Sympathetic	Rapid mobilization, stress response	Near the spine and in the abdominal cavity	Often a long postganglionic region
Parasympathetic	Recovery, digestion, resource conservation	Near an organ or inside an organ	Often a long preganglionic region
Enteral	Local regulation of the intestine	In the wall of the gastrointestinal tract	Capable of operating autonomously, but regulated externally

The information in the table summarizes the anatomical and functional differences of the 3 divisions of the autonomic nervous system. [21]

Why it's important in the clinic: Nerve injury, regeneration, and examination

Peripheral nerves are vulnerable to trauma, compression, and metabolic stress. Clinically, injury classifications are widely used to help predict recovery: in mild cases, conduction is temporarily blocked; in more severe cases, the axon is damaged and Wallerian degeneration begins distally to the injury site. [22]

Regeneration in the peripheral nervous system is generally more feasible than in the central nervous system, but it depends on the integrity of the "guide tubes" of the sheaths and the response of Schwann cells. In Wallerian degeneration, Schwann cells switch to regeneration support mode and form structures that help axons re-grow toward their target. [23]

As a rough guideline, the rate of axon regrowth in the clinic is often given as about 1 mm per day under favorable conditions, but the actual rate and final outcome depend greatly on the distance to the muscle or skin, the degree of damage, age, concomitant diseases and the quality of surgical tactics in case of ruptures. [24]

Evaluation of suspected peripheral nervous system damage relies on a combination of clinical examination and neurophysiology. Nerve conduction velocity testing and electromyography help differentiate demyelinating from axonal processes, assess the severity and extent of damage, and monitor recovery. [25]

Table 6. Peripheral nerve injuries and expected recovery logic

Type of damage	What suffers	Wallerian degeneration	Typical prognosis without surgery
Conductor block	Temporary dysfunction without axon rupture	Usually no	Often recovery is complete
Axon damage with intact sheaths	Axon, but the frame is preserved	Yes	Recovery is possible, but slower.
Complete rupture of the nerve	Axon and sheaths	Yes	Without surgical restoration, function usually does not return.

The classification logic and relationship to Wallerian degeneration are described in current reviews of peripheral nerve injury.[26]

Table 7. Methods of examination of the peripheral nervous system

Method	What does it show?	When it is especially useful
Neurological examination	Distribution of weakness, sensitivity, reflexes	The first step is to determine the site of the lesion
Nerve conduction velocity testing	Speed, latency, and response amplitudes	Demyelination versus axonopathy, focal compressions
Electromyography	Signs of denervation and reinnervation in muscles	Assessment of severity and duration, monitoring of recovery
Ultrasound examination of the nerve	Size, structure, compression, tumors of the membranes	Compression neuropathies, trauma, local changes
Magnetic resonance imaging as indicated	Deep structures, roots, plexuses	Lesions of plexuses, roots, space-occupying processes

The diagnostic capabilities of electrophysiology and the principles of study planning are described in detail in modern guidelines. [27]