If only the left semispinalis cervicis contracts on a patient, what specific action does this generate

First you have to take into account the muscular limits to understand the phenomenon that would occur, these limits are:

Origin: Spinous processes of vertebrae T3-T6

Insertion: transverse processes of C1-C3 vertebrae.

This is why the patient, having the muscle in the cervical area and only contracting the one on the left side, would generate a painful neck torsion and paralysis that shortens the length of the neck towards the left side.

It can also give other symptoms and signs as head can give headache to the suboccipital, occipital, frontal, ocular and ear regions. The splenium muscle of the neck can give the occipital region and posterior part of the orbit a headache.

Explanation:

Many times this is due to cervicitis or tension headache due to dental malocclusion, stress, malposition or also due to some disease that generates random muscle hypertonicity of different muscles.

activation of phospholipase c (plc) indirectly increases intracellular ions through inositol triphosphate (ip3).