Classification of the nervous system
To fully describe the nervous system, two classification principles are applied. The anatomical approach divides the system into two divisions depending on the location of structures, while the functional approach divides it according to the nature of the regulated processes.
Anatomical subdivision:
- Central nervous system (CNS): includes the spinal cord and brain. Clusters of neuron cell bodies here form gray matter (cortex and nuclei). Clusters of nerve fibers form white matter — pathways or tracts. Within a single tract, fibers of only one modality (either afferent or efferent) typically run.
- Peripheral nervous system (PNS): unites all nerve structures outside the CNS. Clusters of neuron cell bodies form nerve nodes (ganglia) here. Bundles of nerve fibers surrounded by connective tissue form nerve trunks (nerves). Unlike white matter tracts, peripheral nerves are often mixed, containing sensory as well as motor, and myelinated as well as unmyelinated fibers.
Functional subdivision:
- Somatic nervous system: ensures interaction with the external environment and motor reactions (innervates structures originating from somites — skin, skeletal muscles).
- Autonomic nervous system: is responsible for maintaining the constancy of the body's internal environment.
Sensory (afferent) link 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 and reside in spinal ganglia (thickenings of the dorsal roots of the spinal cord) or in sensory ganglia along the course of cranial nerves.
The dendrites of these cells originate from receptors in the tissues of the "soma" and run to the neuron cell body as part of mixed peripheral nerves. The axons head toward the spinal cord and enter it via the dorsal roots (radix dorsalis).
The further fate of the axon depends on the level at which the reflex is integrated:
- 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 does not enter the gray matter, but instead enters the posterior funiculi of the white matter and ascends to the nuclei of the medulla oblongata.
Associative and effector links
After a signal is perceived, it must be processed and transmitted to the effector organ.
Intercalary (associative) link connects the sensory neuron with the motor neuron. The golden rule: the cell bodies of absolutely all associative neurons lie exclusively in the gray matter of the CNS. The dendrites of intercalary cells are short and do not extend beyond the boundaries of the gray matter. Axons transmit the impulse either to motor neurons or to subsequent intercalary cells (including ascending to subcortical structures or the cortex).
Effector (motor) link implements the reaction itself — 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 cranial nerve nuclei in the brainstem. A distinguishing feature of motor neurons is that signals from intercalary cells arrive not only at their dendrites, but also in vast numbers directly onto the cell body via axosomatic synapses.
Axons of motor neurons exit 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 end plate. This completes the three-neuron arc of the unconditioned somatic reflex.
Briefly: answers to topical questions
To which morphological type do motor neurons and interneurons belong?
Motor neurons (efferent neurons) of the anterior horns of the spinal cord belong to the multipolar morphological type. The provided sources do not contain explicit indications of the classification morphological type of interneurons (associative neurons).
- Motor neurons are multipolar cells whose dendrites receive impulses from numerous different afferent and efferent systems.
Where are the cell bodies of effector neurons of the autonomic nervous system located?
The cell bodies of effector neurons of the autonomic nervous system are always located exclusively outside the central nervous system. They are localized in peripheral autonomic ganglia.
From here, the axons of these neurons run to various target effector organs (such as myocytes, blood vessels, and glandular cells) as part of slow unmyelinated postganglionic fibers.
Which glial cells form the myelin sheath in the peripheral nervous system?
In the peripheral nervous system, the sheath of nerve fibers is formed bylemmocytes.
Synonymous names for these cells are given in the sources:
- lemmocytes;
- neurolemmocytes;
- Schwann cells.
Sense in the back, run in the front: sensory roots of the spinal cord are always dorsal (radix dorsalis), while motor roots are ventral (radix ventralis).
Class and exam questions
What is the difference between white matter and gray matter?
Gray matter consists of clusters of neuron cell bodies in the central nervous system (nuclei and cortex). White matter consists of nerve fibers (processes) that form conduction tracts.
Does a reflex arc always contain three neurons?
No. An associative (intercalary) neuron may be absent. In such cases, the sensory neuron forms a synapse directly with the effector neuron.
Where are the cell bodies of sensory neurons of the somatic arc located?
They always lie outside the spinal cord or brain — in peripheral sensory ganglia (spinal ganglia or cranial nerve ganglia).
What else to study on this topic
- Organization and types of white matter pathways in the spinal cord
- Histological structure of a peripheral nerve and its connective tissue sheaths
- Specific structural features and links of the autonomic reflex arc (compared to the somatic one)
- Ultrastructure of the motor end plate (neuromuscular junction)
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "The Nervous System and Somatic Reflex Arc" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in "Nervous system: nerves, ganglia, spinal cord"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.