Classification of the Nervous System
Two classification principles are used to comprehensively describe the nervous system. The anatomical approach divides the system based on the location of its structures into two divisions, while the functional approach categorizes it by the nature of the regulated processes.
Anatomical Subdivision:
- Central Nervous System (CNS): Includes the spinal cord and brain. Aggregations of neuronal cell bodies here form gray matter (cortex and nuclei). Aggregations of nerve fibers form white matter—projection, association, and commissural pathways or tracts. Typically, fibers of only one modality (either afferent or efferent) run within a single tract.
- Peripheral Nervous System (PNS): Combines all neural structures outside the CNS. Aggregations of neuronal cell bodies form ganglia. Bundles of nerve fibers surrounded by connective tissue form nerve trunks (nerves). Unlike white matter tracts, peripheral nerves are often mixed, containing both sensory and motor fibers, as well as myelinated and unmyelinated fibers.
Functional Subdivision:
- Somatic Nervous System: Provides interaction with the external environment and motor reactions (innervates structures derived from somites—skin, skeletal muscles).
- Autonomic Nervous System: Responsible for maintaining the constancy of the internal environment (homeostasis) of the body.
Sensory (Afferent) Component of the Arc
Any reflex begins with the perception of a stimulus. In the somatic reflex arc, this is provided by a sensory neuron. Morphologically, these are pseudounipolar neurons whose cell bodies are strictly located outside the CNS in the spinal sensory ganglia (dorsal root ganglia) or sensory ganglia along the course of cranial nerves.
The dendrites of these cells originate from receptors in the tissues of the 'soma' and travel to the neuron's cell body as part of mixed peripheral nerves. The axons project toward the spinal cord and enter it via the dorsal roots (radix dorsalis).
The subsequent pathway of the axon depends on the level of reflex integration:
- Segmental level: The axon forms a synapse on an interneuron in the same or an adjacent segment of the spinal cord.
- Conduction level: The axon bypasses the gray matter, enters the posterior funiculi of the white matter, and ascends to the nuclei of the medulla oblongata.
Associative and Effector Components
After the signal is perceived, it must be processed and transmitted to the effector organ.
The Interoceptive (Association) Component connects the sensory neuron to the motor neuron. A fundamental rule: the cell bodies of absolutely all associative neurons lie exclusively within the gray matter of the CNS. The dendrites of interneurons are short and do not extend beyond the gray matter. Axons transmit impulses either to motor neurons or to subsequent interneurons (including ascending projections to subcortical structures or the cortex).
The Effector (Motor) Component executes the actual response—the contraction of a skeletal muscle. It is formed by motor neurons whose cell bodies lie in the anterior horns of the spinal cord (cornu anterius) or in the motor cranial nerve nuclei in the brainstem. A distinguishing feature of motor neurons is that signals from interneurons arrive not only at their dendrites but also massively and directly onto the cell body via axosomatic synapses.
The axons of motor neurons emerge from the spinal cord as part of the ventral roots (radix ventralis), join the mixed peripheral nerve, and reach the skeletal muscle, where they form a specialized synapse—the motor endplate (neuromuscular junction). This completes the three-neuron arc of an unconditioned somatic reflex.