Components of the autonomic reflex arc
The autonomic reflex arc functions according to the classical reflex principle, but has a complex, multi-component structure.
- Afferent (sensory) component. Begins with interoreceptors (chemo-, mechano-, thermo-, or nociceptors) in internal organs. The signal from them travels via sensory fibers to the cells of spinal or peripheral ganglia. A significant portion of the sensory pathways runs within the vagus (n. vagus) and pelvic (n. pelvicus) nerves. It is important to note that the afferent component is often shared by both autonomic and somatic reflexes.
- Second (association) component. Represented by interneurons. Their cell bodies are localized in the lateral horns of the spinal cord, as well as in the nuclei of the medulla oblongata and midbrain. The main task of this component is to relay the impulse to effector cells.
- Third (efferent) component. Features a two-neuron structure. The central neuron sends a preganglionic fiber from the CNS to the autonomic ganglion. There resides the ganglionic neuron, whose postganglionic fiber goes directly to the effector (the organ's smooth muscle or gland).
Differences between the autonomic and somatic reflex arcs
The key difference between somatic and autonomic arcs lies in the localization of neuron cell bodies and the structure of the pathway to the effector organ.
- In somatic arcs, the cell bodies of motor (efferent) neurons are located in the anterior horns of the spinal cord. The axon of such an alpha motor neuron goes directly to the skeletal muscle without peripheral relay stations.
- In autonomic arcs, the cell bodies of the first efferent cells lie in the lateral horns. The pathway to the internal organ is invariably interrupted in a peripheral autonomic ganglion (paravertebral, prevertebral, or intramural).
Classification of reflexes by level of interaction
Depending on which systems are involved in the response, four groups of autonomic reflexes are distinguished:
- Viscero-visceral. Stimulation of one internal organ alters the function of another. A classic example is the Goltz reflex: mechanical stimulation of the mesentery causes a decrease in heart rate (bradycardia).
- Viscero-somatic. Pathological impulses from an internal organ activate spinal motor neurons. This is how protective tension of the anterior abdominal wall muscles (défense musculaire) arises. This same mechanism, associated with the convergence of signals on spinothalamic tract neurons, underlies referred pain.
- Viscero-cutaneous. Internal problems of the organism are reflected on the skin by altered sensitivity or erythema. They appear in the areas of skin (Head's zones) innervated by the same spinal cord segment that receives signals from the diseased organ.
- Cutaneo-visceral (somato-visceral). Tactile stimulation of the body surface affects the activity of internal organs. This principle is actively used in reflexology and superficial massage.
Briefly: answers to topical questions
What neurotransmitters are released in the synapses of autonomic ganglia?
In the provided materials for the parasympathetic nervous system, it is indicated that acetylcholine is the neurotransmitter in preganglionic fibers.
Features of transmission in autonomic ganglia:
- the neurotransmitter from a single cell simultaneously excites the postsynaptic membranes of several cells via diffusion;
- in sympathetic ganglia, a single preganglionic fiber excites about 20 ganglionic neurons.
For acetylcholine, it is noted that it is degraded by acetylcholinesterase.
To avoid confusing the efferent pathways: Somatic arc is Swift (one neuron from the anterior horns goes straight to the muscle). Autonomic arc has a transfer station (the pathway from the lateral horns必 interrupts at the ganglion).
Class and exam questions
Where does the efferent pathway of the autonomic reflex synapse?
It is two-neuron: the cell body of the first neuron is in the CNS (e.g., in the lateral horns of the spinal cord), and switching to the second neuron occurs in the periphery — within an autonomic ganglion.
What are Head's zones?
These are localized areas of skin where referred pain and altered tactile sensitivity occur during diseases of internal organs. This is a classic manifestation of the viscero-cutaneous reflex.
Why are functional tests of the ANS evaluated in clinical practice?
Tests such as the Aschner oculocardiac reflex, skin dermographism, or the orthostatic reaction allow a physician to clinically assess the baseline tone and reactivity of the patient's autonomic nervous system.
What else to study on this topic
- The mechanism of formation of a simple somatic (proprioceptive) monosynaptic arc.
- Features of complex polysynaptic somatic protective reflexes.
- Anatomy of the cutaneo-autonomic reflex (regulation of vascular tone and sweating).
- The role of the vagus nerve as the main sensory collector of the parasympathetic system.
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