Material basis: the reflex arc
Any 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 into the central nervous system.
- Central neurons — structures within the spinal cord or brain (interneurons).
- Efferent fiber — the axon of a motor neuron (motoneuron).
- Effector — the working organ that executes the command (a contracting muscle or a 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 is transmitted via its axon to the muscle, causing it to contract.
Classification and interaction of reflexes
The organism's responses are classified according to multiple criteria. Based on the localization of centers, reflexes are categorized as spinal, bulbar, brainstem, and cortical. Based on the effector, they are muscular or secretory, and based on the 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, the most critical are protective (defensive) and orienting responses.
When multiple reflexes are triggered simultaneously, they interact. Allied reflexes facilitate each other, reinforcing the overall response. Antagonistic reactions, conversely, exert inhibitory influences on one another.
Features of excitation propagation
Reaction speed depends directly 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 via the afferent neuron and is transmitted directly to the efferent motoneuron 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 is first relayed to several interneurons and only then transmitted to the motoneuron. Due to synaptic delay at each interneuron, the time of such a reflex is noticeably longer.
There are also polysegmental arcs, in which excitation diverges via axon branches (collaterals) to multiple spinal cord segments, activating efferent neurons at various 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 results in a severe physiological state called spinal shock. Such a "spinal animal" completely loses independent breathing due to the disruption of connection with the brainstem respiratory center. Arterial hypotension develops, vascular tone decreases, and the ability to maintain body temperature is lost (poikilothermia). All complex forms of goal-directed activity (feeding, defensive, sexual) completely disappear.
Clinical examination methodology
Evaluating reflexes allows the physician to determine the level of central nervous system lesion. For instance, transection below the lumbar segments results in paralysis of the anal and urinary sphincters.
To test superficial abdominal reflexes, tactile stroking is applied to the abdomen. The upper abdominal reflex (segmental 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 — flexion at the elbow and pronation of the forearm (arc $C_5 – C_8$). A tap on the medial border of the scapula elicits the scapulohumeral response (level $C_3 – C_5$).
Mucosal reflexes, such as the corneal reflex (eyelid closure when touching the cornea with a wisp of cotton) or pharyngeal reflex (gagging when touching the pharyngeal wall with a tongue depressor), are mediated by sensory and motor fibers of cranial nerves.
Briefly: answers to topical questions
What is the physiological mechanism of synaptic delay?
The physiological mechanism of synaptic delay is due to the time required for neurotransmitter diffusion across the synaptic cleft from the presynaptic to the postsynaptic membrane. In chemical synapses, electrical transmission is impossible due to the high electrical resistance of the wide synaptic cleft (about 20 nm). The signal is transmitted exclusively via neurotransmitter release, which makes this process more prolonged. The response on the postsynaptic membrane occurs with a delay, which in a chemical synapse is 0.2–0.5 ms.
Through which spinal cord pathways is proprioceptive sensitivity transmitted to the cortex?
Proprioceptive sensitivity is conducted to the cortex via the ascending pathways of the posterior funiculi of the spinal cord:
- fasciculus gracilis (fasciculus gracilis) — located medially;
- fasciculus cuneatus (fasciculus cuneatus) — located laterally.
These pathways conduct conscious proprioceptive (muscle-joint) and tactile sensitivity. Their fibers are axons of sensory neurons from dorsal root ganglia and ascend without synapsing in the spinal cord and without decussating until the medulla oblongata, where they terminate in the nucleus gracilis and nucleus cuneatus.
Which receptors, in addition to muscle spindles, participate in the spinal regulation of muscle tone?
In addition to muscle spindles, receptors of the joint capsule and tendon organs participate in the spinal regulation of muscle tone. These receptors perform the following functions:
- Golgi tendon organs — respond to strong muscle tension (active contraction or stretch) and trigger autogenic inhibition of $\alpha$-motoneurons, leading to muscle relaxation;
- Joint capsule receptors — are excited by stretching of joint capsules (upon displacement by an angle from 2–3° to 30°), encoding information about the angle of limb rotation in the joint.
To memorize the segments of abdominal reflexes, use the umbilicus as a landmark (T9-T10 level). The upper abdominal reflex corresponds to T7-T8 (stroking 3–4 fingers above the umbilicus), and the lower corresponds to T11-T12 (3–4 fingers below).
Class and exam questions
What is reflex time?
This is the time interval from the application of a stimulus to the receptor until the onset of the working organ's (effector's) response.
What distinguishes a monosynaptic reflex arc?
It lacks interneurons. The signal from the proprioceptor (muscle spindle) is transmitted directly to the motor neuron, making the response as fast as possible.
Why is the reaction time longer in a polysynaptic reflex?
A polysynaptic arc involves multiple interneurons. A synaptic delay occurs at each of them, which increases the total latent period of the reaction.
What happens if the spinal cord is damaged below the lumbar region?
Paralysis of the anal and urinary sphincters will occur, as the centers regulating these functions are located in the sacral segments.
What else to study on this topic
- Mechanisms of decerebrate rigidity and tonic brainstem reflexes
- Detailed anatomy of the muscle spindle as a proprioceptor
- Methods for testing mucosal reflexes (corneal, pharyngeal, palatal)
- Principles of excitation propagation via collaterals in polysegmental arcs
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