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Unconditioned Reflexes

For medical students2 min readUpdated 2026-10-10

Unconditioned reflexes are innate, genetically determined reactions of the organism to stimuli. They proceed via pre-formed reflex arcs and occur consistently whenever an adequate stimulus acts on the corresponding receptors.

NatureInnate and genetically determined reactions.
Trigger ConditionAction of an adequate stimulus on a receptor.
Complex FormsIvan Pavlov equated them with instincts.

Main Characteristics and Mechanisms

According to Ivan Pavlov's teachings, unconditioned reflexes are genetically determined and present in the organism from birth. Their realization is carried out on the basis of innate reflex arcs that are anatomically fixed.

The main condition for such a reaction to occur is the action of an adequate stimulus on strictly defined receptors. Under normal conditions, these reflexes occur constantly and unfailingly. A classic example of a basic unconditioned reflex is salivation, which is triggered by the direct contact of food with the receptors of the oral cavity. Furthermore, Pavlov equated the most complex forms of these innate reactions to instincts.

Classification of Complex Unconditioned Reflexes

Complex innate reflexes are not just single responses, but entire complexes of specific behavioral and autonomic reactions. The main groups are typically classified as follows:

Orienting-Exploratory Activity

Among all types of innate reactions, orienting-exploratory activity occupies a special place. As a rule, it manifests in response to any unexpected changes in the external environment.

Ivan Pavlov characterized this reaction to novel stimuli as the 'what-is-it?' reflex. This activity has a pronounced adaptive nature, allowing the organism to quickly assess environmental changes. In addition, the orienting reflex serves as a fundamental basis for the formation of many complex forms of learning.

Components of Complex Reflexes

Any complex unconditioned reflex is realized through a combination of motor (behavioral) and autonomic manifestations.

1. Complex Feeding Reflex Triggered when food acts on the receptors of the gastrointestinal tract or on distant receptors (e.g., olfactory). It includes:

2. Complex Defensive Reflex Occurs in response to threatening stimuli and is accompanied by:

Types of Defensive Reactions

The behavioral component of a defensive reflex can be executed through two fundamentally different scenarios:

  1. Passive defensive reaction. The animal performs a passive action in response to a signal. For example, it may simply move to a safe compartment of an experimental chamber, thereby preventing painful stimulation without engaging in active opposition.
  2. Active defensive reaction. In this case, the animal performs active instrumental actions. An example is pressing a lever or manipulating objects with the goal of completely stopping or physically avoiding the damaging stimulus.

Frequently asked questions

Where are the nerve centers of the main unconditioned reflexes localized?

The centers of various unconditioned reflexes are localized within structures of the spinal cord and brain.

  • Salivatory center — located in the medulla oblongata.
  • Sympathetic centers — the first neurons are located in the lateral horns of the thoracic and upper lumbar segments of the spinal cord.
  • Parasympathetic centers — localized in the sacral segments of the spinal cord.
  • Respiratory centers — inspiratory and expiratory components are located in the medulla oblongata, the apneustic center in the pons, and the pneumotaxic center in the midbrain.
What are the main differences between unconditioned and conditioned reflexes?

The main differences lie in their origin, variability, and conditions of formation.

FeatureUnconditioned ReflexesConditioned Reflexes
OriginInnate, genetically determinedAcquired during individual life
Degree of variabilityRelatively constantQualitatively distinct, highly variable form
Conditions of occurrenceAction of adequate stimuli on corresponding receptorsAssociated with the learning process, pairing of an indifferent and an unconditioned stimulus
Nature of reactionReaction to actual impactAnticipatory reaction reflecting properties of a future event
What mechanisms of unconditioned reflex inhibition exist?

The following mechanisms and types of inhibition of reflex activity are described:

  • Sechenov inhibition — a classic example of descending central inhibitory influence: placing a salt crystal on the area of the optic thalami (thalamic structures of the reticular formation) in a frog causes suppression of the passive defensive reaction (withdrawing the paw in response to acid).

According to electrophysiological nature, they are classified into:

  • Hyperpolarizing inhibition — occurs on postsynaptic membranes under the action of inhibitory neurotransmitters and decreases tissue excitability.
  • Persistent depolarizing inhibition — occurs at high frequencies of excitation; accumulation of the excitatory neurotransmitter prevents the return of the postsynaptic membrane potential to the resting state.
  • Stabilizing inhibition — associated with the action of certain biologically active substances: sodium permeability of membranes is blocked without changing the resting potential, making the generation of excitation impossible.
What did Ivan Pavlov call the 'what-is-it?' reflex?

This is what Ivan Pavlov called the orienting-exploratory reaction of the organism to any novel or unexpected stimuli. It has adaptive significance and forms the basis of learning.

What is the difference between an active and a passive defensive reaction?

In a passive reaction, the animal moves away from the threat (e.g., moving to a safe place), preventing pain. In an active reaction, the animal performs instrumental actions (e.g., pressing a lever) to forcibly terminate the damaging stimulus.

What components are included in the complex feeding reflex?

The complex feeding reflex consists of motor activity, glandular secretory activity, and autonomic changes (readjustment of respiration and cardiac activity).

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