A convulsion is an episode of involuntary muscle contractions that can produce visible stiffening or jerking. It describes a motor manifestation, whereas an epileptic seizure is defined by its abnormal neural activity.

Not every seizure produces a convulsion. Some primarily change sensation or awareness, so a definition based only on visible shaking would miss important seizure types.^[ref]

Pain attributed to a nerve distribution or nerve disorder.

Parkinsonism is a clinical pattern centered on slowness of movement with additional features such as rigidity or rest tremor. Parkinson's disease is one cause, but medications and other neurological disorders can produce related patterns.

Separating syndrome from cause prevents circular reasoning: observing slowed movement does not by itself identify one underlying molecular mechanism or one specific disease.

Ataxia is impaired coordination of movement. It can affect gait, limb targeting, posture or articulation of speech, and is distinct from inability to move because of elementary weakness.

cerebellum dysfunction can cause ataxia, but impaired sensory feedback or vestibular function can also disrupt coordination. Comparing sensory conditions and different tasks helps identify which contribution is failing.

Spinal cord disease is dysfunction of the spinal cord caused by processes such as injury, compression, inflammation or impaired blood supply. A lesion can affect local circuits as well as pathways carrying signals to and from more distant levels.

A combination of a sensory level, altered reflexes and weakness can therefore be more informative than any one finding. The distribution follows neural organization rather than simply the outline of a single peripheral nerve.

Paralysis is loss of voluntary movement in a muscle or group of muscles from impaired motor function. Its distribution can range from a local nerve territory to a whole side of the body or both lower limbs.

The term describes an impairment, rather than a unique cause. Damage to a central pathway, peripheral nerve or neuromuscular system can affect movement through different mechanisms, while pseudoparalysis can resemble motor incapacity for another reason.

A neurological examination systematically assesses functions of the nervous system, including mental state, cranial nerves, strength, sensation, coordination and reflexes. Patterns across tasks can localize dysfunction more effectively than a single isolated score.

For example, apparent inability to perform a learned action can reflect elementary weakness, impaired comprehension or apraxia. Comparing the relevant component functions is essential to interpreting the failed action.

A head injury is trauma affecting structures of the head. Injury to the scalp or skull and injury to the brain are related but distinct possibilities; a visible external wound is not required for traumatic brain injury.

Concussion is one possible neurological consequence. The same observed confusion can also have other contributors, so its interpretation depends on the injury history and the wider examination.^[ref]

A neurological deficit is loss or impairment of a function attributable to the nervous system, such as weakness, altered sensation or impaired language. Its distribution can constrain the location of a lesion, while its time course helps characterize the process.

For example, hemiplegia describes one-sided paralysis, whereas a sensory level describes a boundary in sensory findings. Neither observation alone identifies the complete underlying disease.

A headache is pain perceived in the head or upper neck. It can be a primary neurological disorder, as in migraine, or a symptom arising from another process. The pain location alone therefore does not specify its mechanism.

The distinction matters experimentally as well as clinically: studies should define the headache phenotype being measured rather than combine every episode of head pain as if it were one biological condition.

Wernicke encephalopathy is a neurological syndrome caused by severe deficiency of thiamine, vitamin B1. Its manifestations can include altered mental state, abnormalities of eye movement and ataxia; the complete classical combination need not be present in every affected person.

The mechanism concerns impaired nutrient-dependent cellular function, not simply the presence of alcohol in the body. Alcohol-related malnutrition is one setting, but other causes of severe thiamine deficiency can also produce the syndrome.^[ref]

The round-trip time measures the elapsed time for an exchange to travel to a peer and for a response to return. Feedback-based sending can be limited by this delay as well as by link capacity.

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.

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

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.

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.

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 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.

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.

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.