Anatomy of the reflex arc
A reflex arc is the structural and functional basis of any reflex reaction, even the most complex one. For a specific response to a stimulus to occur, the nerve impulse must follow a strictly defined route. The arc includes five elements:
- Receptor — sensory nerve endings that are the first to perceive the stimulus.
- Afferent pathway — sensory nerve transmitting the signal from the periphery to the centers.
- Nerve center — area of the CNS where information is switched and processed.
- Efferent pathway — motor nerve carrying the impulse to the periphery.
- Effector — the target organ. This can be muscle tissue (responding with contraction) or secretory glandular tissue (responding with secretion).
A critically important rule: a reflex is possible only with the integrity of all five links. If the circuit is broken at any stage, the reaction will not manifest.
Classification of reflex reactions
All reflex activity is broadly divided into two categories.
- Unconditioned reflexes — innate, genetically determined reactions. They are relatively constant throughout life and are triggered by adequate stimuli acting on the corresponding receptors. Complex unconditioned reactions include alimentary, defensive, sexual, and orienting-exploratory reflexes.
- Conditioned reflexes — reactions acquired during an individual's lifetime. They are extremely labile and directly related to learning. Their formation requires reinforcement — the interaction of the newly forming arc with the arc of a powerful unconditioned reflex that satisfies vital needs.
The neuron as the basis of the nervous system
The activity of the central nervous system is based on processes occurring within neurons. A neuron consists of a cell body (soma), numerous dendrite branches (bearing a large number of synapses), and a single axon, which may give off collateral branches.
According to the principles of neuronal organization formulated by S. Ramón y Cajal, the cell represents a morphological, genetic, and functional unit. The most important law is the law of dynamic polarization: excitation propagates through the neuron in strictly one direction (from dendrites to axon). There is also strict trophic unity: removal of the neuron body leads to the death of the axon, and ligation of the axon leads to the death of the entire cell.
According to their function, neurons are divided into:
- Afferent: convey information from sense organs.
- Interneurons (internuncial neurons): connect cells within the CNS.
- Efferent: carry commands to muscles or glands.
Signal generation and transmission
The mechanisms of neuronal excitation are explained by two complementary theories.
Electrical theory relies on membrane heterogeneity. Integrative synapses (occupying 75% of the area) lie on dendrites, which possess voltage-gated conductance via calcium channels. Synaptic excitation creates a potential difference between the cell body and the axon hillock (the site of axon origin). The hillock is positively charged and devoid of synapses. When the potential difference increases rapidly, spike activity (action potential) arises precisely here.
Chemical theory links excitation to the release of neurotransmitters:
- Excitatory neurotransmitters (acetylcholine, norepinephrine, glutamate) cause depolarization of the postsynaptic membrane, forming an excitatory postsynaptic potential (EPSP).
- Inhibitory neurotransmitters (GABA, glycine) generate inhibitory potentials (IPSP).
Transmission efficiency can be modulated by neuromodulators — oligopeptides (opioids, substance P, angiotensin II, vasopressin, oxytocin).
Integrative activity according to P.K. Anokhin
A neuron does not merely transmit a signal. Integrative activity includes three stages:
- Convergence of diverse excitations at the cell's synapses.
- Creation of an integrative state within the cytoplasm (information processing).
- Generation of specific impulse activity in the axon.
This process is accompanied by the expression of specific proteins by the neuronal genome, which shapes the specific membrane response. As a result, the neuron selectively responds only to those excitations that contribute to satisfying the organism's initial need.
Briefly: answers to topical questions
By what criteria are reflexes classified in normal physiology besides their origin (conditioned/unconditioned)?
In normal physiology, reflexes are classified according to several criteria. Criteria for reflex classification:
- By the localization of centers — spinal, bulbar, brainstem, cortical.
- By relation to regulated organs — somatic, autonomic.
- By effector — muscular, secretory.
- By central component structure — monosynaptic, polysynaptic.
- By functional significance — flexor and extensor, static and statokinetic, single and chain, tendon and myotatic.
- By biological significance — protective (defensive), orienting.
A classification based on the interaction of the somatic and autonomic nervous systems is also distinguished.
What types of central inhibition exist in reflex arcs?
Central inhibition is classified according to its electrophysiological nature and localization. Based on electrophysiological nature:
- Hyperpolarizing inhibition — reduction of tissue excitability due to hyperpolarization.
- Persistent depolarizing inhibition — prevention of membrane potential return to the resting state due to high-frequency excitation.
- Stabilization inhibition — blockade of membrane sodium permeability without altering the resting potential.
According to localization, they are divided into:
- Presynaptic inhibition — occurs in an axo-axonic synapse, blocking impulse propagation before reaching the main cell.
- Postsynaptic inhibition — occurs in an axo-somatic synapse, direct inhibition of the postsynaptic neuron.
Reciprocal (conjugate) inhibition is also mentioned.
To easily recall the 5 links of the reflex arc, use the phrase: "Reflex Absolutely Completely Effective Effector" (Receptor, Afferent pathway, Center, Efferent pathway, Effector).
Class and exam questions
What happens if one of the components of the reflex arc is damaged?
The reflex will completely disappear. The main condition for the realization of any reflex reaction is the absolute integrity of all five components of the arc.
Where exactly in the neuron does the nerve impulse originate?
Action potential generation occurs in the initial segment — the axon hillock. This area is positively charged and lacks synapses.
How are conditioned and unconditioned reflexes interrelated?
The development of a conditioned reflex is based on reinforcement—the interaction of a new (conditioned) excitation arc with the arc of a potent unconditioned reflex that satisfies vital needs.
What else to study on this topic
- Functions of neurons: receptor, coding, transport, and trophic.
- The role of nerve growth factor (NGF) in the trophic support and development of synaptic connections.
- Characteristics of fast and slow mechanisms of axonal transport.
- NMDA receptors and the specific action of glutamate in the CNS.
- Syncytial-type nervous system structure in lower invertebrates.
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