Neural Centers

Classification of Neural Centers

Neuron clusters are anatomically and functionally diverse. They are typically 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:
2. Motor (control movements);
3. Sensory (process information from sensory organs).

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

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

Basic Properties of Neural Centers

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

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

Complex Integrative Processes

Neural centers do not merely transmit signals; they process, accumulate, and coordinate them with other bodily tasks.

Summation of Excitation The center can accumulate postsynaptic potentials to reach the threshold level.

• Temporal summation: impulses rapidly arrive 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. Additionally, centers exhibit tone — a 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 divisions are subordinate to higher ones (subordination). When tonic excitation is high, a dominance forms — a focus with extremely high excitability. It suppresses competitors and literally "attracts" impulses from other sources.

Reciprocity and Irradiation Precise movements require reciprocity — coordinated reciprocal inhibition, where excitation of the flexor center automatically inhibits the extensor center. If the stimulus is very strong, irradiation (divergence) occurs: the process of excitation generalizes, extends beyond a single center, and encompasses vast areas of the brain.