Nervous system

The nervous system coordinates sensing, internal processing and responses through networks of neurons and supporting cells. Its organization connects rapid electrical signals with chemical communication. The central nervous system and peripheral nervous system describe major anatomical divisions.

Myelin is a multilayered wrapping of glial cell membrane around segments of an axon. Its lipid-rich layers increase electrical insulation and reduce effective membrane capacitance, helping local electric current reach the next node of Ranvier. Action potentials are regenerated at these gaps, producing saltatory conduction.^[ref]

A Schwann cell makes one myelinated segment in the peripheral nervous system; an oligodendrocyte can supply segments on several axons in the central nervous system. The thickness and spacing of myelin affect signal timing. In a mouse experiment, altered visual experience changed myelin segment length and optic nerve conduction, linking experience to the physical geometry of neural transmission.^[ref]

Myelination forms layers of myelin around segments of an axon. Oligodendrocytes produce these sheaths in the central nervous system, while Schwann cells do so in the peripheral nervous system. Gaps, each called a node of Ranvier, allow action potentials to be regenerated between insulated segments, supporting saltatory conduction.

The arrangement affects when signals reach their targets, not just whether an axon is present. In mice, altered visual experience shortened segments wrapped in myelin along the optic nerve pathway and was associated with slower conduction along the optic nerve. The observations connected environmental input, myelin sheath geometry and a measured physiological consequence.^[ref] This experimental result does not imply that more myelin always produces a uniformly faster or better-performing nervous system; segment geometry and the timing required by a circuit matter.

The peripheral nervous system comprises neural structures outside the brain and spinal cord, including many nerves and ganglia. It carries sensory input and motor or autonomic output. Peripheral location does not imply that every fiber has the same signaling direction.

A spinal nerve is a paired peripheral nerve formed by the union of a sensory dorsal root and a motor ventral root near the spinal cord. Humans usually have 31 pairs. Because sensory and motor fibres travel together after the roots unite, a spinal nerve is mixed even though its two roots have different principal signaling directions.^[ref]

The resulting spinal nerve divides into posterior and anterior spinal nerve rami, both of which remain mixed. A dorsal root and a posterior ramus are therefore different structures: one joins the spinal cord, while the other distributes fibres after root union. Anterior spinal nerve rami can reorganize into a nerve plexus, so one named limb nerve can contain fibres from several spinal levels.^[ref]

A ventral root is a bundle of outgoing motor axons leaving the anterior side of the spinal cord. It contains somatic motor fibres and, at appropriate spinal levels, autonomic efferent fibres. It joins a sensory dorsal root to form a mixed spinal nerve.^[ref]

For example, an action potential destined for a limb muscle can leave the spinal cord through a ventral root, then continue through a mixed spinal nerve and an anterior spinal nerve ramus. The directional separation near the spinal cord does not mean every later branch is motor-only. A ventral root is also distinct from the anterior branch formed after the roots unite.

A spinal nerve ramus is a branch formed after a spinal nerve has been assembled from its dorsal root and ventral root. Both posterior and anterior spinal nerve rami carry mixed sensory and motor fibres. Posterior branches principally supply the back; anterior branches supply the anterolateral trunk and, through nerve networks, the limbs.^[ref]

A root and a ramus therefore describe different positions in the branching pathway. Fibres have already mixed before the spinal nerve rami separate. Many anterior branches contribute to a nerve plexus, redistributing axons from several spinal levels into named peripheral nerves. The branch points route existing fibres rather than forming new synapses.

A dorsal root is the bundle of sensory axons entering the posterior side of the spinal cord at a spinal level. The associated dorsal root ganglion contains the soma of many of these sensory neurons. Their peripheral processes receive information from the body, while their central processes enter through the dorsal root.^[ref]

The dorsal root joins the ventral root to form a mixed spinal nerve. This organization separates sensory entry from motor exit close to the spinal cord while allowing both directions to share a peripheral route. The dorsal root ganglion houses cell bodies; it is not normally a chain of sensory-to-sensory synapses.

The pudendal nerve is a mixed nerve of the sacral plexus, usually receiving fibers associated with S2–S4 spinal nerves. It carries sensory information from much of the perineum and motor signals to skeletal muscles involved in continence, including the external anal sphincter and external urethral sphincter. Its route includes the pudendal canal.

A description of anatomy is a pattern with variation, not a guarantee that every individual has identical branching. Dissection of 28 cadavers documented additional branches and interconnections near the external anal sphincter in some specimens.^[ref] The study also illustrates the difference between a skeletal muscle being structurally intact and its nerve supply being intact: coordinated contraction depends on both the skeletal muscle and the arriving motor signals.

A dorsal root ganglion is a collection of sensory neuron cell bodies on a dorsal root associated with a spinal nerve. These neurons carry information from peripheral tissues toward the spinal cord, including signals relevant to touch, temperature, pain and proprioception.

The sensory axon typically branches into a peripheral process and a central process; the ganglion is not simply a relay in which every incoming signal must cross a synapse onto another neuron. Its neurons also comprise multiple functional populations. Single-cell RNA sequencing of mouse dorsal root ganglion neurons separated molecularly distinct sensory types without assigning them beforehand solely from a few selected markers.^[ref] Anatomical location, molecular profile and stimulus response provide complementary descriptions of a sensory neuron.

A bundle of axons in the peripheral nervous system, enclosed by supporting connective tissue. A nerve can carry sensory fibres, motor fibres, or both; it is distinct from an individual neuron.^[ref]

The sciatic nerve is a large mixed nerve connecting the lumbosacral plexus with the lower limb. It passes through the posterior hip region into the thigh and contains the components that continue as the tibial nerve and common fibular nerve. These pathways carry motor signals to skeletal muscles and sensory information from substantial regions of the leg and foot.

Its course relative to the piriformis muscle varies among individuals. A retrospective magnetic resonance imaging study compared common and variant anatomical arrangements and did not find a statistically significant difference in the recorded diagnoses involving pain.^[ref] An unusual anatomical arrangement therefore does not by itself establish that the arrangement is causing symptoms; structure and physiological consequence require separate evidence.

The common fibular nerve is a major branch of the sciatic nerve that supplies pathways to the lateral and anterior leg and part of the foot. Its course near the fibular neck gives nerve location practical importance when relating injury to motor and sensory changes.

The tibial nerve is a major branch of the sciatic nerve carrying fibers toward posterior-leg and plantar structures. Its motor and sensory components have different targets. A lesion’s level determines which downstream branches are affected.

The femoral nerve arises from the lumbar plexus and supplies anterior-thigh motor pathways and sensory routes to part of the lower limb. Its relation to quadriceps femoris links it particularly to knee extension.

A nerve carrying afferent information from sensory structures towards the central nervous system.

An afferent pathway in the vagus nerve, including pathways that carry information from the gastrointestinal tract into brainstem circuits.

A nerve injury damages a nerve or its constituent fibers. Conduction failure, disruption of axons and loss of supporting structure represent different severities. Functional recovery depends on the nature and location of damage rather than the nerve’s name alone.

Interruption or loss of a structure's nerve supply. Its physiological consequences can include direct motor or secretory changes and indirect effects through altered perfusion.

The brachial plexus redistributes fibers from lower cervical and upper thoracic spinal pathways into nerves supplying the upper limb. A terminal nerve therefore contains fibers from several spinal levels. Plexus position and branch organization help localize patterns of injury.

The posterior-cord nerve supplying much of the extensor apparatus of the upper limb.

The posterior interosseous nerve is the continuation of the deep motor pathway of the radial nerve in the forearm. It supplies many extensor muscles. Loss of these movements can occur without the cutaneous sensory loss expected from a more proximal radial-nerve lesion.

The axillary nerve is a branch of the brachial plexus supplying the deltoid muscle, teres minor and a region of shoulder skin. Its course near the shoulder joint connects local injury with a particular combination of motor and sensory deficits.

The vestibular system senses head movement and orientation relative to gravity. Signals from the inner ear interact with visual and bodily information to support gaze and posture. A sensory signal about acceleration is distinct from an unconstrained measurement of absolute position.

Motion sickness is a physiological syndrome associated with movement-related sensory conditions, often including nausea. Disagreement among expected and received visual and vestibular signals is an important explanatory framework. Physical motion and perceived motion need not match exactly.

A reflex is a relatively organized response elicited through a neural pathway by an input. Sensory processing, interneuronal connections and output pathways contribute to the response. Its occurrence without deliberate initiation does not mean that its strength cannot be modified by context.

The gag reflex is a pharyngeal response to stimulation of sensitive regions of the pharynx. Afferent pathways prominently involve the glossopharyngeal nerve, with motor output involving the vagus nerve. The observed response depends on both sensory input and motor execution.

A nerve plexus is an interconnecting arrangement of nerve fibers or branches. It can redistribute fibers among outgoing nerves, so one peripheral branch need not correspond to a single spinal level. A plexus is an organization of pathways rather than a separate neuron type.

The lumbosacral plexus is the connected system of lumbar plexus and sacral plexus pathways formed from anterior spinal nerve rami of lower spinal nerves. It redistributes sensory and motor axons into peripheral nerves supplying the pelvis and lower limb. Its connecting lumbosacral trunk usually carries contributions from L4 and L5 into the sacral plexus.^[ref]

For example, the femoral nerve draws mainly on upper lumbar levels, while the sciatic nerve combines lower lumbar and sacral contributions. A named peripheral nerve consequently does not correspond one-to-one with a spinal nerve. The nerve plexus reroutes existing axons; its intersections do not imply that the fibres form synapses.

The sacral plexus is a network of anterior spinal nerve rami, usually incorporating L4–S4 spinal contributions, lying against the anterior surface of the piriformis muscle. It distributes sensory and motor fibres to much of the pelvis and lower limb. Major branches include the sciatic nerve and the pudendal nerve.^[ref]

Its mixed origins explain an otherwise puzzling arrangement: the sciatic nerve carries fibres associated with several spinal nerves, and fibres from one spinal level can leave through different peripheral branches. A nerve plexus is therefore a redistribution network, rather than a new set of neurons inserted into each pathway.

The lumbar plexus is a network of upper lumbar spinal pathways supplying parts of the abdominal wall and lower limb. Its branches include the femoral nerve and obturator nerve. Shared spinal origins and separate peripheral destinations help explain overlapping injury patterns.

A chiefly L2–L4 pathway through the obturator canal to the medial thigh; supplies adductor muscles and a variable cutaneous territory.

The oesophageal plexus is a neural network around the oesophagus, receiving vagal and sympathetic contributions. It links distributed autonomic pathways to the wall of the digestive passage. A network’s position should be distinguished from the origins of its fibers.

The pulmonary plexus is a network of autonomic and sensory pathways associated with the roots of the lungs and airways. Vagal and sympathetic contributions connect central regulation with local airway and vascular targets.

The cardiac plexus is a network of autonomic and sensory pathways associated with the heart. Sympathetic and vagal contributions influence cardiac function through different mechanisms. Sharing a plexus does not make every fiber’s effect identical.

The pharyngeal plexus is a network of nerve branches supplying the pharynx. It includes motor, sensory and autonomic contributions. Its organization helps coordinate a shared passage used in swallowing and vocal activity.

A ganglion is a collection of neuronal cell bodies outside the central nervous system in the usual neuroanatomical usage. Sensory and autonomic ganglia differ in organization: the presence of cell bodies does not imply that every passing fiber synapses there.

The otic ganglion is a parasympathetic relay receiving fibers associated with the glossopharyngeal nerve through the lesser petrosal nerve. Postganglionic fibers reach the parotid gland using the auriculotemporal nerve.^[ref]

The submandibular ganglion is a parasympathetic relay associated with the lingual nerve. Preganglionic fibers arrive through the chorda tympani; output contributes to control of the submandibular gland and sublingual gland. Sharing a route does not imply that all lingual-nerve fibers synapse there.

A small ganglion between the optic nerve and lateral rectus muscle. Preganglionic fibres of the parasympathetic nervous system relay here; sensory and already postganglionic fibres of the sympathetic nervous system traverse it without another relay.^[ref]

The short ciliary nerves connect the ciliary ganglion region with the eye. They carry postganglionic parasympathetic fibers together with sensory and sympathetic fibers; these components differ in origin and synaptic history.^[ref]

A bundle of axons connecting the central nervous system with peripheral structures. This anatomical use is distinct from the mathematical nerve of a category.

Peripheral neuropathy is dysfunction or damage of peripheral nerve pathways. Sensory, motor and autonomic components can be affected in different combinations. Distribution across nerves and fiber types is therefore part of its description, rather than one symptom defining every case.

Motor supply to the respiratory diaphragm, arising chiefly from cervical spinal segments C3–C5.

A cranial nerve is one of the traditionally numbered neural pathways associated with the brain and head. The group contains sensory, motor and mixed pathways. Their developmental and central connections are not identical merely because they share the cranial-nerve designation.

The optic nerve carries retinal ganglion cell axons from the retina toward central visual pathways. It is a central nervous-system pathway despite its traditional classification as a cranial nerve. Its fibers encode distributed retinal output rather than a literal picture traveling along a cable.

The facial nerve is a mixed cranial nerve with motor pathways for facial expression and additional taste and parasympathetic components. Different branches carry different subsets of fibers, so a local lesion need not affect every facial-nerve function equally.

The superior salivatory nucleus is a pontine source of preganglionic parasympathetic pathways associated with the facial nerve. Outputs contribute to control of lacrimal and salivary secretion through peripheral relays. Its name does not limit every output to one salivary gland.

The geniculate ganglion contains sensory neuronal cell bodies associated with the facial nerve. It lies at a bend in the nerve’s course. Sensory cell bodies and neighboring parasympathetic fibers should not be mistaken for the same functional component.

A mixed gustatory and secretomotor branch that crosses the middle ear and joins the lingual nerve.

The trigeminal nerve is a cranial nerve providing major sensory pathways from the face and motor pathways to muscles of mastication. Its three main divisions distribute different territories. Sensation and jaw movement can therefore be affected differently by a lesion.

The auriculotemporal nerve is a branch of the mandibular division of the trigeminal nerve. It carries sensation from temporal and auricular regions and conveys postganglionic secretory fibers from the otic ganglion toward the parotid gland.

The lingual nerve carries general sensation from the anterior tongue through the mandibular division of the trigeminal nerve. Fibers joining through the chorda tympani add taste and parasympathetic components. A shared peripheral route therefore contains pathways with different central origins.

The sensory long buccal branch of the mandibular division, distinct from the motor buccal branches of the facial nerve.

The ophthalmic nerve is the first main division of the trigeminal nerve. It supplies sensory pathways from the forehead, upper orbital region and parts of the eye and nose. Its sensory role differs from the motor control of eye movement by other cranial nerves.

The nasociliary nerve is a branch of the ophthalmic nerve carrying sensory pathways from orbital and nasal structures. Its branches include the long ciliary nerves and ethmoidal pathways. Anatomical branching helps connect a sensory territory to a central route.

The posterior ethmoidal nerve is a branch of the nasociliary nerve supplying sensory pathways from posterior ethmoidal and nearby sinus regions. Its distribution differs from the more anterior territory of the anterior ethmoidal nerve.

The anterior ethmoidal nerve is a branch of the nasociliary nerve supplying parts of the nasal cavity and external nose. Its course passes between orbital, cranial and nasal regions, illustrating why the apparent surface territory does not describe the whole pathway.

The long ciliary nerves are branches associated with the nasociliary nerve reaching the eye without passing through the ciliary ganglion. They carry sensory pathways and sympathetic fibers. Their route differs from that of the short ciliary nerves.^[ref]

The lacrimal nerve is a branch of the ophthalmic nerve supplying sensation near the lacrimal gland and lateral upper eyelid. Secretory fibers can join its peripheral route from a different central origin. A nerve’s name does not imply one uniform fiber type.

The frontal nerve is a branch of the ophthalmic nerve within the orbit (anatomy). Its terminal branches include the supraorbital nerve and supratrochlear nerve, supplying sensation to forehead and adjacent scalp regions.

The supratrochlear nerve is a terminal branch of the frontal nerve supplying sensation near the medial forehead and upper eyelid. Its smaller medial territory complements the more lateral distribution of the supraorbital nerve.

The supraorbital nerve is a terminal branch of the frontal nerve supplying forehead and anterior scalp sensation. Its exit near the upper orbital margin provides an anatomical landmark connecting a surface sensory field to a deeper nerve course.

The accessory nerve is the eleventh cranial nerve, chiefly associated with motor supply to sternocleidomastoid muscle and trapezius. Its spinal-root course illustrates that the traditional cranial-nerve grouping does not mean every component originates within the brain.

The glossopharyngeal nerve is a mixed cranial nerve with sensory, motor and parasympathetic components. Its pathways connect the posterior tongue, pharynx, carotid sinus and parotid gland with central circuits. Different components travel through distinct branches and relays.

The carotid sinus nerve is a sensory branch associated with the glossopharyngeal nerve, conveying information from carotid pressure- and chemical-sensing structures. These inputs contribute to regulation of circulation and breathing; the sensed variables are distinct from the resulting reflex outputs.

The lesser petrosal nerve carries preganglionic parasympathetic fibers associated with the glossopharyngeal nerve toward the otic ganglion. The eventual secretory pathway continues toward the parotid gland through other peripheral branches.

The vagus nerve is the tenth cranial nerve, carrying sensory, branchial motor and parasympathetic pathways. Its branches connect the brainstem with pharyngeal, laryngeal and many thoracic and abdominal targets. Different functions use different fiber populations, so selective branch interruption need not remove every vagal influence. The historical name pneumogastric denotes the same nerve.

Vagotomy is interruption of the vagus nerve or selected branches. Its physiological effects depend on which fibers and destinations are interrupted. Loss of a vagal influence does not prove that all regulation of the target has disappeared.^[ref]

The recurrent laryngeal nerve is a branch of the vagus nerve whose course loops around major thoracic vessels before returning toward the larynx. It supplies most intrinsic laryngeal motor pathways. The long recurrent course connects neck function with vulnerability along a thoracic route.

The superior laryngeal nerve is a branch of the vagus nerve with internal and external divisions. Sensory supply above the vocal folds and motor supply to the cricothyroid muscle travel through different branches, linking anatomy to distinct functions.

The inferior vagal ganglion, also called the nodose ganglion, contains sensory neuronal cell bodies associated with many visceral vagal pathways. It is distinct from the superior vagal ganglion in location, developmental origin and the sensory populations represented.

The superior vagal ganglion, also called the jugular ganglion, contains sensory neuronal cell bodies near the skull exit of the vagus nerve. It should be distinguished from the more inferior nodose population and from central vagal motor nuclei.

The abducens nerve is the sixth cranial nerve, supplying the lateral rectus muscle that turns the eye outward. Its motor pathway arises in the abducens nucleus. Eye movement and sensation from the eye are carried through different pathways.

The trochlear nerve is the fourth cranial nerve, supplying the superior oblique muscle of the eye. Its fibers cross before their dorsal exit from the brainstem, an unusual arrangement among cranial motor pathways.

The oculomotor nerve is the third cranial nerve. It supplies most extraocular motor pathways and carries parasympathetic fibers toward the ciliary ganglion. Eye movement, eyelid elevation and pupillary regulation therefore involve distinguishable components.

The division of the nervous system associated with bodily sensation and control of skeletal muscle. Facial electromyography records activity of skeletal muscle; it is not itself an autonomic recording.

The part of the nervous system regulating organs, glands and related internal functions. It includes the sympathetic nervous system and parasympathetic nervous system. Its activity supports bodily regulation during emotion, exercise and many nonemotional states; no single recording is a direct readout of a private feeling.

An autonomic nerve carries fibers involved in regulation of internal organs, glands or vascular targets through the autonomic nervous system. A named peripheral nerve can contain autonomic fibers together with sensory or somatic motor components. The fiber’s function and its anatomical route are different classifications.

A division of the autonomic nervous system contributing to regulation of organs, including control of the heart rate. Changes in cardiac activity can reflect interacting sympathetic and parasympathetic influences rather than a single scalar quantity called activation.

Adjective referring to the parasympathetic nervous system.

The neural network associated with the digestive tract, contributing to local coordination of motility and secretion.