General Functions of the Nervous System
The nervous system (systema nervosum) acts as a universal coordinator. It not only ensures the synchronized function of all organ systems but also enables the body to adapt appropriately to various stimuli.
Globally, its functions are divided into three sequential steps:
- Perception of internal and external stimuli.
- Analysis, synthesis, and storage of received information.
- Transmission of integrated signals to effector organs (muscles or glands).
Neuron: Structure and Impulse Direction
The foundation of nervous tissue is the neuron, which functions with the essential metabolic and structural support of neuroglia.
Every neuron has a distinct anatomical structure:
- Cell body (corpus / soma) — the central metabolic region.
- Dendrites (dendritae) — short, highly branched receptive processes.
- Axon (axon, or neuritum) — the single long efferent process.
Neurons are organized into chains along which action potentials propagate. Signals always travel in a strictly unidirectional pathway: they are received by dendrites, conducted to the neuronal cell body, and then propagated along the axon toward the next target cell or tissue.
Interneuronal Contacts: Synapses
For an impulse to pass from one neuron to another, it requires a specialized junction called a synapse (synapsis). Signal transmission here is primarily chemical, mediated by signaling molecules known as neurotransmitters.
Key features of synaptic transmission include:
- Transmission is always accompanied by a brief temporal delay.
- A single neuron is capable of forming extensive arborizations — numbering in the thousands of synapses.
- Plastic changes in the formation and elimination of synaptic contacts form the cellular basis of learning and memory.
Classification of Neurons and the Reflex Arc
Based on their functional role, neurons are classified into three main groups:
- Afferent (sensory) neurons — neurones afferentes.
- Interneurons (association) neurons — neurones associativi.
- Efferent (motor) neurons — neurones efferentes.
These cells combine to form reflex arcs. The simplest reflex arc consists of three elements: a signal travels from an afferent neuron to an interneuron, and then to an efferent neuron. More complex reflex arcs incorporate numerous interneurons organized into extensive neuronal networks.
Nerves and Types of Nerve Fibers
Macroscopic nerves are essentially bundles of neuronal processes wrapped in connective tissue sheaths. The fibers within them are divided into two main functional types.
Afferent fibers:
- Formed by the central processes (or dendritic projections) of sensory neurons.
- Terminate in specialized receptive structures — receptors.
- The cell bodies of these neurons reside outside the central nervous system in sensory ganglia (such as dorsal root ganglia or cranial nerve sensory ganglia).
Efferent fibers:
- Formed by the axons of motor neurons.
- The cell bodies of these neurons are located directly within the gray matter of the brain and spinal cord.
- These fibers are referred to as motor or centrifugal fibers, conveying commands from the CNS to effectors (muscles or glands).
The vast majority of peripheral nerves are mixed nerves, containing both afferent and efferent fibers.
Division of the Nervous System
Anatomically and functionally, the nervous system is divided into several major divisions.
Based on anatomical and topographical principles:
- Central nervous system (CNS) (systema nervosum centrale): comprises the brain and spinal cord.
- Peripheral nervous system (PNS) (systema nervosum periphericum): encompasses cranial and spinal nerves, along with ganglia and plexuses.
Functionally, the nervous system is divided into the somatic nervous system (systema nervosum somaticum) and the autonomic nervous system (systema nervosum autonomicum). The autonomic nervous system is further subdivided into sympathetic and parasympathetic divisions, which exert largely antagonistic effects on visceral organs.