Nervous Centers

Classification of Nervous Centers

Clusters of neurons are anatomically and functionally very diverse. They are generally classified according to several key criteria:

1. By localization in the nervous system:
2. Spinal (located in the spinal cord);
3. Brainstem;
4. Cerebellar;
5. Subcortical;
6. Cortical (localized in the cerebral cortex).

1. By function performed:
2. Motor (control movements);
3. Sensory (process information from sense organs).

1. By level of complexity:
2. Simple. Include a minimum of elements. A classic example is the spinal center of the knee-jerk reflex.
3. Complex. Combine vast arrays of cells, for example, the cortical speech center.

1. By regulation sphere:
2. Somatic (control of the musculoskeletal system);
3. Autonomic (regulation of internal organs);
4. Mental (ensuring higher nervous activity).

Basic Properties of Nervous Centers

The work of neuronal ensembles differs radically from the conduction of an impulse along a single nerve fiber. This is due to the presence of multiple chemical synapses.

• One-way conduction. The signal always moves strictly from the receptor to the effector. This is because neurotransmitter-sensitive receptors are located exclusively on the postsynaptic membrane.
• Central delay. Time is required to transmit a chemical signal across each synaptic cleft. The more synapses in the chain, the longer the delay.
• Low functional lability. Centers are incapable of reproducing very high rhythms of excitation due to the prolonged propagation time of the signal through complex neural networks.
• High fatiguability. During prolonged activity, neurotransmitter stores in presynaptic terminals become depleted, leading to synaptic depression and temporary failure of the center.
• Chemical sensitivity. Neurons readily react to endogenous substances and exogenous drugs. Chemical agents can block neurotransmitter synthesis, interfere with its receptor binding, or disrupt reuptake. Reactions can range up to changes in gene expression.

Complex Integrative Processes

Nervous centers do not just transmit a signal; they process, accumulate, and coordinate it with other tasks of the body.

Summation of excitations The center is able to accumulate postsynaptic potentials to reach the threshold level.

• Temporal summation: impulses arrive rapidly one after another through the same synapse.
• Spatial summation: signals arrive simultaneously, but through different synapses from different sources.

Aftereffect and tone Even if the stimulus is removed, the reflex response may continue. This aftereffect occurs due to the circulation of impulses through closed neural loops (reverberation), strong residual depolarization, or powerful polysynaptic potentials. In addition, centers possess tone—background activity in which they constantly send mild stimulating impulses to the periphery without apparent external stimulation.

Dominance and subordination The central nervous system has a strict hierarchy: lower departments are subordinate to higher ones (subordination). With high tonic excitation, a dominant focus forms—a center with extremely high excitability. It suppresses competitors and literally "draws" foreign impulses to itself.

Reciprocity and irradiation Clear movements require reciprocity—mutually coordinated inhibition, when the excitation of the flexor center automatically inhibits the extensor center. If the stimulation is very strong, irradiation (divergence) occurs: the excitation process becomes generalized, goes beyond the boundaries of a single center, and encompasses vast areas of the brain.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of formation and features of EPSPs (excitatory postsynaptic potentials)
• The phenomenon of reverberation: how closed neural circuits are structured
• The nature of background tonic activity of efferent nerves
• The specifics of the action of pharmacological agents on pre- and postsynaptic membranes
• Differences between irradiation and convergence processes in neural networks

Other topics in the "General Physiology of the Central Nervous System" section

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