3 Variants of pain theory:

1) gates theory – see above;

2) theory of specificity: there are special receptors (nociceptors), special conductive ways and special zones in cortex. This theory is origined form R.Decart who wanted to answer the question how organism reactred to noceoceptive stimulus;

3) theory of pattern or intensiveness – excessive flow of impulses from different receptors (cutaneous, gustatory, sound and all) alongside without specific ways causes pain.

ANTINOCICEPTIVE SYSTEM INCLUDES:

1. structures influencing on gelatinose substance (its activators):

   • cortex frontal lobe;

   • caudate nucleus (basal ganglion);

   • thalamus nucleus;

   • cerebellum neurons;

   • hypothalamic centers;

   • midbrain central grey near-aqueductal substance;

   • red nucleus;

   • black substance (midbrain);

   • reticular formation nuclei (several);

2. skin mechanoreceptors excitement leads to the fact that impulses part from them (on the way to medulla oblongata) come to gelatinose substance and activate it;

3. structures which synthesize row of substances:

• endogenic opiates (dinorphine, endorphins, encephalines);

• neurotensine (stronger than in opiates);

• oxitocine (weakly);

• vasopressine (weakly);

• serotonin (in medulla oblongata) – strongly;

• adrenaline (at stress, in sportsmen).

Acupuncture causes activation of central grey near-aquedctal substance and, in turn:

1. activation of neurons of II-III gelatinose substance;

2. opioid peptides increasing in blood.

TRACTS

ANTERIOR SPINOTHALAMIC TRACT

Anterior spinothalamic tract is formed by the fibers of second order neurons of the pathway for crude touch (Fig.21).

Situation

This tract is situated in anterior white funiculus near the periphery.

Origin

The fibers of anterior spinothalamic tract arise from the chief sensory cells of posterior gray horn which form the second order neurons of the crude touch pathway. The first order neurons are situated in the posterior nerve root ganglia. These neurons receive the impulses of crude touch sensation from the pressure receptors. The axons of these neurons reach the chief sensory cells.

Course

After taking origin, the fibers ascend in posterior gray horn for 2 or 3 segments in the same side. Then, the fibers cross obliquely in the anterior white commissure and enter the anterior white funiculus. Here, the fibers ascend through other segments of spinal cord, medulla, pons and midbrain and reach thalamus. When the tract ascends through brainstem, the number of fibers is considerably reduced as some of the fibers form the collaterals and reach the reticular formation of brainstem.

Termination

The fibers of anterior spinothalamic tract terminate in the ventral posterolateral nucleus of thalamus. The cells of this thalamic nucleus form the third order neurons of the pathway. The fibers from thalamic nucleus carry the impulses to some sensory areas of cerebral cortex.