MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

SECTION 3. ANATOMICAL AND PHYSIOLOGICAL ASPECTS OF BODY FUNCTION SELF-REGULATION

NERVOUS MECHANISMS OF PHYSIOLOGICAL REGULATION

Excitation and Inhibition in the Central Nervous System

The morphofunctional unit of the Central Nervous system is the neurocyte (nerve Cell). It is the neurocytes that perform the complex Functions of the central nervous system. Nerve Cells are interconnected via synaptic contacts. The number of synapses is enormous; on the soma of a pyramidal neurocyte in the Cerebral Cortex, it can reach several thousand. Synapses are formed between neurocyte structures (central synapses), as well as between the neurocyte axon and effectors (Muscle or glandular cells) - peripheral synapses. A synapse consists of a synaptic button (the terminal of a neurocyte process), a presynaptic membrane, a subsynaptic membrane (the region of the membrane on the body of another neurocyte), a synaptic cleft between these membranes, and synaptic vesicles containing a neurotransmitter inside the synaptic button (Fig. 3.1).

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Fig. 3.1. Diagram of neurotransmitter release and processes occurring in a hypothetical central synapse:

1 - axon; 2 - microtubules; 3 - synaptic vesicle; 4 - synaptic cleft; 5 - dendrite; 6 - neurotransmitter receptor;

7 - subsynaptic membrane; 8 - presynaptic membrane; 9 - Mitochondria.

An Action Potential propagating along a nerve fiber reaches the synaptic button and promotes the release of a neurotransmitter from it. According to their functional effects, Neurotransmitters are classified as excitatory and inhibitory. Excitatory neurotransmitters include catecholamines - adrenaline, noradrenaline, dopamine, serotonin - as well as acetylcholine, substance P, and others.

Inhibitory neurotransmitters include gamma-aminobutyric acid (GABA) and Glycine.

The neurotransmitter

is released into the synaptic cleft and binds to chemoreceptors (protein substances) on the subsynaptic membrane. Neurotransmitters alter the ionic permeability of the subsynaptic membrane, causing excitation under METABOLISM/18.html">The Influence of an excitatory neurotransmitter, and inhibition under the influence of an inhibitory one. Excitation can propagate further, which is associated with excitatory synapses and excitatory neurotransmitters. Inhibition is non-propagating, which is due to the existence of specific inhibitory synapses and inhibitory neurotransmitters; it manifests as the suppression of another excitation. Examples of inhibitory neurocytes in the Spinal Cord are Renshaw interneurons, and in the Brain - Purkinje Cells of the cerebellar cortex.

The mosaic pattern and dynamics of excitation and inhibition processes underlie the integrative and Regulatory Functions of the central nervous system.

Synapses (central - between Neurons in the brain, and peripheral - between a nerve and a muscle or a gland) conduct excitation in only one direction - from the nerve to the muscle or gland. Excitation passes through the synapse unilaterally: from the presynaptic membrane to the subsynaptic membrane.

Another feature of excitation conduction is a synaptic conduction delay of 2-3 ms.

The initial excitatory impulses arriving at the synapse increase its excitability, thereby facilitating the subsequent transmission of excitation through it. This phenomenon is known as facilitation.