What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Function of Nerves: How the Nervous System Works

Name: Function of Nerves: How the Nervous System Works
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The function of a nerve is to transmit electrical impulses, called action potentials, from one part of the body to another along the membranes of its axons. A nerve is a bundle of axons wrapped in connective tissue, not a single cell, and every nerve carries one of three functional classes of fiber: sensory (afferent), motor (efferent), or autonomic (sympathetic and parasympathetic).

That definition matters in practice because almost every clinical sign a veterinarian interprets can be traced back to one of those three classes failing. A dog that drags a toe has a motor or proprioceptive problem. A cat that chews its own feet has a sensory problem. A horse with recurrent colic may have an autonomic problem. Knowing which fiber class is involved narrows the differential list before any diagnostic test is run, and it tells you what a nerve conduction study is actually measuring.

What a Nerve Actually Is

Students often picture a nerve as a long wire. The reality is closer to a telephone cable. A single axon belongs to one neuron, and its cell body sits either in the central nervous system (brain and spinal cord) or in a peripheral ganglion. Many axons then travel together inside a connective tissue sheath to form one grossly visible nerve.

In adult mammals, peripheral nerves are bundles of disseminated motor, sensory, and autonomic axons that behave as stable anatomical units [1]. The only normal exceptions to that fixed wiring are the very distal terminal branches in target organs such as skin, which can remodel [1].

Each axon is insulated by myelin, a lipid-rich sheath produced by Schwann cells in the peripheral nervous system. Myelin is not uniform in composition. Sulfatide, a major glycolipid of myelin, exists as thirteen distinct molecular species in adult murine dorsal root ganglia, and these species are already diversified before mature myelin forms [2]. That molecular detail matters because myelin integrity determines conduction speed, and conduction speed determines how fast a reflex can protect a limb from injury.

Nerve trunks are not always pure conduits either. In the adult human superior hypogastric plexus and hypogastric nerve, thousands of neuronal cell bodies sit embedded within the nerve tracts rather than in discrete ganglia, and more than 90 percent of those neurons in one sampled hypogastric nerve were tyrosine hydroxylase-immunoreactive, meaning presumed noradrenergic [3]. So the textbook model of "nerve equals cable" is a useful simplification, not an absolute rule.

The Three Functions of Nerves

Sensory (Afferent) Function

Sensory nerves carry information from the periphery toward the central nervous system. The word afferent means "conducting inward." Their job is to report mechanical, thermal, and chemical conditions in the body so the brain and spinal cord can respond.

The vagus nerve provides a clean example. Vagal sensory neurons located in the nodose ganglia innervate all layers of the stomach wall, where they detect mechanical and chemical signals and inform the brain, driving vago-vagal reflexes [4]. The most abundant terminal structure, the intraganglionic laminar ending, sits in intimate contact with myenteric plexus neurons and likely acts as both a tension sensor and a chemosensor [4].

Sensory fibers are also the ones that fail first in many neuropathies. In dogs with suspected hereditary sensory and autonomic neuropathy, sensory nerve action potential amplitudes, sensory nerve conduction velocity, and cord dorsum potential amplitudes were significantly reduced in one electroclinical pattern, while a second, more severe pattern showed absent sensory responses plus motor abnormalities [5]. That distinction is clinically useful because the two patterns carried very different outcomes [5].

Motor (Efferent) Function

Motor nerves carry commands from the central nervous system to skeletal muscle. The word efferent means "conducting outward." When a motor axon reaches its target, it branches into terminals that contact individual muscle fibers at the neuromuscular junction.

Motor function is quantifiable. In people undergoing neck dissection with preservation of the spinal accessory nerve, compound muscle action potential amplitude fell 45.2 percent from a baseline of 9.15 ± 2.04 mV to a three-week nadir of 5.01 ± 2.47 mV, then partially recovered to 6.26 ± 2.23 mV by three months [6]. That trajectory shows two things students should remember. Motor axons can lose function after mechanical insult without being cut, and they can recover, but recovery is incomplete over that time frame.

Autonomic Function

Autonomic nerves control smooth muscle, cardiac muscle, and glands. They operate below conscious control and split into two opposing divisions.

The parasympathetic division supports digestion and restoration. Vagal cholinergic parasympathetic efferent neurons sit in the dorsal motor nucleus of the vagus in the caudal brainstem, and most of them do not contact gastric effector cells directly. Instead they innervate enteric neurons in the myenteric plexus, which act as muscle motor neurons and pattern generators that regulate motility and secretion programs [4].

The sympathetic division does the opposite in the gut. The sympathetic pathway inhibits digestion by suppressing peristalsis and constricting the pyloric sphincter, while the parasympathetic pathway promotes digestion by stimulating peristaltic activity and relaxing that sphincter [7].

Autonomic fibers are frequently damaged alongside motor and sensory fibers. In Guillain-Barré syndrome, an acute immune-mediated neuropathy with predominant motor involvement, autonomic dysfunction is common and typically features sympathetic hyperactivity, and it is thought to result from conduction failure in autonomic nerve fibers similar to the damage in motor and sensory fibers [8].

Summary Table: Nerve Types at a Glance

Fiber type	Direction of impulse	Primary neurotransmitter	Target tissue	Example
Sensory (afferent)	Periphery to CNS	Glutamate (central terminals), CGRP and substance P (peripheral terminals)	Dorsal horn, brainstem nuclei	Vagal afferents from the stomach wall [4]
Motor (efferent)	CNS to periphery	Acetylcholine	Skeletal muscle at the neuromuscular junction	Spinal accessory nerve to trapezius [6]
Parasympathetic	CNS to periphery	Acetylcholine	Smooth muscle, cardiac muscle, glands, enteric neurons	Vagal efferents to the myenteric plexus [4]
Sympathetic	CNS to periphery	Norepinephrine (noradrenergic)	Smooth muscle, blood vessels, pelvic viscera	Noradrenergic neurons of the hypogastric nerve [3]

The Synapse: Where One Nerve Hands Off to the Next

A synapse is the junction where an action potential in one neuron influences the next cell. The presynaptic terminal releases a chemical neurotransmitter into a narrow gap, and that transmitter binds receptors on the postsynaptic membrane. Transmission across a synapse is slower than conduction along a myelinated axon, and it is also where signals get filtered, amplified, or blocked.

Synaptic behavior is not fixed. In rat brainstem slices, stimulating the solitary tract with bursting patterns of two to four pulses at 150 to 350 Hz significantly reduced the amplitude of the first excitatory postsynaptic current in nucleus tractus solitarius neurons compared with single-pulse stimulation [9]. That finding is directly relevant to vagus nerve stimulation therapy, because it shows the pattern of stimulation changes how much signal actually gets through the first central relay [9].

Synapses also carry specific chemical codes. In a skin-lung reflex, activating auricular TRPV1-positive afferents attenuated allergic airway inflammation, and the immunosuppression depended on the neuropeptide CGRPβ [10]. Silencing those same sensory neurons worsened lung inflammation [10]. This is a somato-visceral reflex, meaning an external sensory input changes the behavior of an internal organ, and it illustrates that sensory nerves do more than report sensation.

The Reflex Arc

A reflex arc is the shortest complete circuit in the nervous system. It contains a receptor, an afferent limb, a central integration point, an efferent limb, and an effector. The classic example is the stretch reflex: a muscle spindle detects stretch, a sensory axon carries that signal to the spinal cord, a motor neuron fires, and the muscle contracts.

Reflexes can also be long and vagally mediated. The vago-vagal reflex uses sensory neurons in the nodose ganglia to detect gastric wall tension and chemical signals, then returns motor output through dorsal motor nucleus efferents to the enteric nervous system [4]. The stomach therefore has a local reflex loop that runs through the brainstem, not just the spinal cord.

Reflexes can even be triggered from the skin to an internal organ. The auricular vagus reflex suppresses airway inflammation through a sensory-motor arc, and the authors describe it as an evolutionarily conserved somato-visceral reflex by which exteroceptive inputs impact visceral inflammation [10]. This is the physiological basis for transcutaneous neuromodulation as a therapeutic strategy [10].

How Nerve Function Is Tested in Practice

Nerve conduction studies are the standard electrophysiological method for assessing peripheral nerve function. They measure how much electrical signal a nerve can carry and how fast it travels. Motor studies record compound muscle action potentials from a muscle after stimulating its nerve. Sensory studies record sensory nerve action potentials from the nerve itself.

Sensory nerve conduction studies can separate disease patterns. In eleven dogs with suspected hereditary sensory and autonomic neuropathy, pattern 1 dogs had recordable but significantly reduced sensory nerve action potential amplitudes, sensory nerve conduction velocity, and cord dorsum potential amplitudes, while pattern 2 dogs had absent sensory responses in most nerves tested plus reduced compound muscle action potential amplitudes or reduced motor conduction velocity, or both [5]. Dogs in pattern 2 were euthanized within twelve months, whereas five of six pattern 1 dogs had a different outcome [5].

Nerve conduction studies also detect subclinical disease. In amyotrophic lateral sclerosis, electrophysiological evidence of sensory neuropathy was found in 20 of 114 patients (17.5 percent) and entrapment neuropathy in 28 of 114 (24.6 percent), and patient-reported symptoms did not correlate with the electrophysiological findings [11]. That mismatch is a recurring theme. What the nerve is doing electrically and what the patient feels are two different measurements.

Autonomic function is assessed separately. Heart rate variability measures beat-to-beat changes that reflect parasympathetic and sympathetic tone. In type 2 diabetes of less than five years' duration, patients had significantly reduced parasympathetic heart rate variability indices, including root mean square of successive differences and high-frequency power, alongside reduced sensory and motor nerve amplitudes, with the strongest correlation between high-frequency power and sural sensory nerve action potential amplitude (r = 0.62) [12]. Autonomic and somatic nerve dysfunction therefore travel together more often than not.

Other modalities add structural information. Nerve ultrasound and skin biopsy can refine diagnostic reasoning in sensory-predominant neuropathy. In one patient with progressive sensory neuropathy, nerve ultrasound showed bilateral reduction of upper-limb nerve cross-sectional area, predominantly in the median and ulnar nerves at the forearm, and skin biopsy showed loss of somatic intraepidermal fibers with relative preservation of autonomic innervation [13]. That combination of findings prompted targeted genetic testing [13].

Comparative Notes: Dog, Cat, Horse, and Human

Dogs develop inherited sensory and autonomic neuropathies that have been characterized electrodiagnostically. The two patterns described above show that the same broad diagnosis can present as either a survivable sensory loss or a rapidly progressive tetraparesis, and comprehensive electrodiagnostic testing helps classify which is which [5].

Cats share the same basic fiber classes and reflex organization as dogs, and the same clinical logic applies. A cat with an absent withdrawal reflex has a sensory or motor limb problem, and a cat with unexplained regurgitation or megaesophagus may have an autonomic or vagal component. Species differences in vagal anatomy and enteric control mean findings from rodent models do not translate directly to cats or dogs, which is why species-specific studies remain necessary [14].

Horses are large enough that nerve length itself becomes a clinical variable. Longer axons mean longer conduction distances and greater cumulative metabolic demand, and equine practitioners rely heavily on reflex testing and gait observation because electrophysiology equipment sized for horses is less widely available than for small animals.

Humans provide the bulk of detailed electrophysiological data, and some of it is directly instructive for veterinary students. The spinal accessory nerve recovery curve after neck dissection, the correlation between cardiac autonomic function and peripheral nerve conduction in early diabetes, and the multimodal workup of idiopathic sensory neuropathy all illustrate general principles of nerve function that apply across species [12][13][6].

Clinical Relevance, Limitations and Common Mistakes

Peripheral nerve injury causes partial or complete loss of motor, sensory, and autonomic function, and there is currently no effective treatment that guarantees rapid and complete functional recovery [15]. That is why prevention, early recognition, and supportive care carry so much weight in veterinary neurology.

Several misconceptions show up repeatedly in student exams and clinical notes.

The first is treating a nerve as a single cell. A nerve is a bundle of axons from many neurons, which is why a partial nerve injury can spare some functions while abolishing others.

The second is assuming that a normal clinical examination rules out nerve disease. In amyotrophic lateral sclerosis, patient-reported symptoms showed no correlation with electrophysiological evidence of sensory involvement [11]. In early type 2 diabetes, autonomic and peripheral neuropathy often remain clinically silent while heart rate variability and nerve conduction studies are already abnormal [12].

The third is assuming that autonomic and somatic nerves fail independently. The correlation between high-frequency heart rate variability power and sural sensory nerve action potential amplitude in early diabetes shows they often decline together [12].

The fourth is assuming that sensory nerves only sense. Sensory neurons release neuropeptides that modulate immune responses, as shown by the CGRPβ-dependent suppression of airway inflammation through the auricular vagus reflex [10].

The fifth is assuming that nerve trunks contain only axons. The human hypogastric nerve contains thousands of neuronal cell bodies, mostly presumed noradrenergic, arranged in clusters embedded within nerve tracts [3].

A related point concerns the broader role of peripheral nerves in organ function. Peripheral nerves regulate pulmonary physiology in a nerve-type-specific and compartment-specific manner through neurotransmitters, neurotrophins, and axon-guidance cues, and dysregulated pulmonary innervation has been implicated in acute lung injury, airway hyperresponsiveness, pulmonary fibrosis, and lung cancer [14]. Nerves are not passive wiring. They are active participants in organ biology.

Peripheral nerve involvement also extends beyond the classic neuropathies. In Parkinson's disease, peripheral sensory, autonomic, and motor pathways including the neuromuscular junction and enteric circuits show structural and functional abnormalities that contribute to pain, orthostatic and visceral dysfunction, gait instability, and weakness [16]. The mechanisms that maintain normal peripheral nerve function include neurotrophic factors, neuromuscular junction integrity, calcium signaling, and mitochondrial homeostasis [16].

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Review

A nerve is a bundle of axons, not a single cell. Each axon belongs to one neuron.
Sensory (afferent) fibers carry impulses toward the central nervous system. Motor (efferent) fibers carry impulses away from it to skeletal muscle.
Autonomic fibers split into parasympathetic (acetylcholine, rest and digest) and sympathetic (norepinephrine, fight or flight) divisions.
The synapse is where chemical transmission occurs, and the pattern of stimulation changes how much signal crosses it [9].
A reflex arc is the shortest complete circuit: receptor, afferent limb, integration point, efferent limb, effector.
Nerve conduction studies measure amplitude and velocity and can detect disease before clinical signs appear [12][11].
Autonomic and somatic nerve dysfunction frequently occur together, so abnormal heart rate variability should prompt a peripheral nerve evaluation [12].

Frequently Asked Questions

What is the main function of a nerve?

The main function of a nerve is to transmit electrical impulses along its axons. Sensory nerves carry impulses toward the central nervous system, motor nerves carry impulses to skeletal muscle, and autonomic nerves carry impulses to smooth muscle, cardiac muscle, and glands.

What is the difference between sensory and motor nerves?

Sensory nerves conduct impulses inward from the periphery to the central nervous system, while motor nerves conduct impulses outward from the central nervous system to skeletal muscle. A single named nerve can contain both types of fiber, which is why nerve injury often produces mixed sensory and motor deficits.

What does the autonomic nervous system do?

The autonomic nervous system controls involuntary functions such as heart rate, digestion, and glandular secretion. Its parasympathetic division promotes digestion and restoration, and its sympathetic division inhibits digestion and prepares the body for activity [7].

How does a reflex arc work?

A reflex arc begins with a receptor detecting a stimulus, continues through an afferent sensory fiber to an integration point in the spinal cord or brainstem, and returns through an efferent fiber to an effector such as a muscle. Vagally mediated reflexes can run through the brainstem rather than the spinal cord [4].

Can nerve damage be detected before symptoms appear?

Yes. Nerve conduction studies and heart rate variability testing can detect abnormalities before clinical signs are obvious. In early type 2 diabetes, reduced parasympathetic heart rate variability and reduced nerve amplitudes were present in patients with less than five years of disease duration [12].

Do all animals have the same nerve types?

All mammals share the same three functional classes of nerve fiber: sensory, motor, and autonomic. The anatomy and the relative importance of each pathway differ between species, and rodent findings do not always translate directly to dogs, cats, or horses [14].