Anatomy of the Reflex Arc
The reflex arc is the structural and functional foundation of any reflex response, no matter how complex. For a specific response to a stimulus to occur, the nerve impulse must travel a precise pathway. The arc comprises five elements:
- Receptor — sensory nerve endings that first perceive the stimulus.
- Afferent pathway — sensory nerve fibers transmitting the signal from the periphery to the central nervous system.
- Nerve center — a region of the CNS where integration and processing of information take place.
- Efferent pathway — motor nerve fibers 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 if all five links are intact. If the chain is interrupted at any stage, the response will not occur.
Classification of Reflex Responses
All reflex activity is broadly divided into two main categories.
- Unconditioned reflexes — innate, genetically determined responses. They remain relatively constant throughout life and are triggered when adequate stimuli act upon the appropriate receptors. Complex unconditioned responses include feeding, defensive, reproductive, and orienting-exploratory reflexes.
- Conditioned reflexes — responses acquired during an individual's lifetime. They are highly variable and directly linked to learning. Their formation requires reinforcement — the interaction of the newly forming arc with the arc of a strong unconditioned reflex that satisfies vital physiological needs.
The Neuron as the Foundation of the Nervous System
The activity of the central nervous system is rooted in processes occurring within neurons. A neuron consists of a cell body (soma), numerous dendritic branches (which bear a high density of synapses), and a single axon that may give off collateral branches.
According to the principles of neuronal organization formulated by Santiago Ramón y Cajal, the cell represents a morphological, genetic, and functional unit. A fundamental law is the law of dynamic polarization: excitation propagates through a neuron in strictly one direction (from dendrites to axon). There is also strict trophic interdependence: if the cell body of a neuron is destroyed, the axon degenerates, and the ligation of an axon leads to the death of the entire cell.
Functionally, neurons are divided into:
- Afferent (sensory) neurons: transmit information from sensory organs.
- Interneurons: connect cells entirely within the CNS.
- Efferent (motor) neurons: carry commands to muscles or glands.
Signal Generation and Transmission
The mechanisms of neuronal excitation are explained by two complementary theories.
The electrical theory relies on membrane heterogeneity. Integrator synapses (accounting for 75% of the surface area) are located on dendrites, which possess voltage-gated conductivity via calcium channels. Synaptic excitation creates a potential difference between the cell body and the axon hillock (the origin of the axon). The hillock is positively charged and devoid of synapses. When the potential difference rapidly increases, spike activity (action potential) is generated specifically at this site.
The chemical theory attributes excitation to the release of neurotransmitters:
- Excitatory neurotransmitters (acetylcholine, norepinephrine, glutamate) cause depolarization of the postsynaptic membrane, generating an excitatory postsynaptic potential (EPSP).
- Inhibitory neurotransmitters (GABA, glycine) generate inhibitory postsynaptic potentials (IPSP).
Neuromodulators — such as oligopeptides (opioids, substance P, angiotensin II, vasopressin, oxytocin) — can modulate transmission efficiency.
Integrative Activity according to P.K. Anokhin
A neuron does not merely relay a signal. Its integrative activity involves three stages:
- Convergence of diverse excitatory inputs onto the cell's synapses.
- Generation of an integrative state within the cytoplasm (information processing).
- Formation of a specific impulse pattern along the axon.
This process is accompanied by the expression of specific proteins by the neuronal genome, which establishes a distinct membrane response. Consequently, the neuron selectively responds only to those excitations that contribute to satisfying the initial physiological need of the organism.