Material Basis: The Reflex Arc
Every reflex is executed via a specialized neural pathway—the reflex arc. The classical chain comprises five sequential components:
- Receptor — perceives the initial stimulus.
- Afferent fiber — the process of a sensory neuron that transmits the signal to the central nervous system.
- Central neurons — structures within the spinal cord or brain (interneurons).
- Efferent fiber — the axon of a motor neuron (motor neuron).
- Effector — the working organ executing the command (contracting muscle or secreting gland).
The execution of a response always begins with the excitation of a receptor by a stimulus. Next, the signal travels along the sensory fiber, switches to interneurons, reaches the motor neuron, and travels via its axon to the muscle, causing contraction.
Classification and Interaction of Reflexes
Organism responses are classified by numerous criteria. Based on center localization, they are divided into spinal, bulbar, brainstem, and cortical reflexes. By effector, they are muscular or secretory, and by regulated organs, somatic or autonomic.
Functional significance allows reflexes to be divided into flexor and extensor, static and statokinetic, single and chain, as well as tendon and myotatic. From a biological standpoint, protective (defensive) and orienting reactions are the most critical.
When multiple reflexes are triggered simultaneously, they interact. Allied reflexes facilitate each other, enhancing the overall response. Antagonistic reactions, conversely, exert inhibitory influences on each other.
Conduction Characteristics of Excitation
Reaction speed directly depends on the number of synapses in the arc.
- Monosynaptic arcs: provide the fastest responses. Their receptors are muscle spindles (proprioceptors). The signal from stretch receptors travels along the afferent neuron and is transmitted directly to the efferent motor neuron in the anterior horns of the spinal cord. Interneurons are absent, ensuring minimal reaction time.
- Polysynaptic arcs: are triggered by exteroceptors (e.g., in response to an external pain or tactile stimulus). The signal from the sensory fiber first switches to several interneurons and is only then transmitted to the motor neuron. Due to synaptic delay at each interneuron, the reflex time is noticeably longer.
There are also polysegmental arcs, in which excitation spreads via axon collateral branches to multiple spinal cord segments, activating efferent neurons at different levels.
Autonomic Functions and Spinal Shock
The spinal cord controls vital autonomic processes: defecation, urination, sweating, erection, ejaculation, and vascular tone regulation. Sympathetic centers lie in the lateral horns of the thoracic and upper lumbar segments, while parasympathetic centers lie in the sacral segments.
Transection of neural pathways between the spinal cord and medulla oblongata causes a severe physiological condition—spinal shock. Such a "spinal animal" completely loses independent breathing due to the loss of connection with the brainstem respiratory center. Arterial hypotension develops, vascular tone drops, and the ability to maintain body temperature is lost (poikilothermia). All complex forms of goal-directed activity (feeding, defensive, sexual) completely disappear.
Clinical Examination Methods
Evaluating reflexes allows a physician to determine the level of central nervous system lesions. For example, transection below the lumbar segments results in paralysis of the anal and urinary sphincters.
To test cutaneous abdominal reflexes, a stroke stimulus is applied to the abdomen. The upper abdominal reflex (closure level $T_7 – T_8$) is elicited 3–4 fingers above the umbilicus.
Periosteal reflexes are tested using a neurological hammer. Striking the styloid process elicits the brachioradialis reflex—elbow flexion and forearm pronation ($C_5 – C_8$ arc). Striking the medial edge of the scapula elicits the scapulohumeral response ($C_3 – C_5$ level).
Mucosal reflexes, such as the corneal reflex (eyelid closure when touching the cornea with a wisp of cotton) or pharyngeal reflex (gag reflex when touching the pharyngeal wall with a spatula), are mediated by cranial nerve sensory and motor fibers.