Each discrete sensory record of a particular gesture or series of gestures is called a sensory engram, and once the feeling of it is firmly established as a clear, recallable memory, this engram works something like the templates which produce different stitching patterns in a sewing machine. When a person wishes to accomplish some act, the first recalls the sensory engram associated with past repetitions of the act. That is, he begins by remembering how it felt to do it. The motor systems are then set into motion to reproduce the remembered sequence of sensations laid down in the engram.
The translation of an engram into motion involves the entire collection of sensory and motor apparata that we have examined up to this point – the skin and joint receptors, the deep tissue pressure receptors, the Golgi tendon organs, the stretch receptors of the spindles, the reflex of internuncial circuitry of the spinal cord, the stereotyped sequences of reflex responses organized in the brainstem, the gamma motor system, the alpha motor system the cerebellum, and the successively more conscious centers of choice and voluntary sequential integration – the thalamus, the hypothalamus, and the cortical regions. The sensory cortex has memorized the feel of a gesture or a series, and each time it is recalled for the purpose of repeating the action the proprioceptive feedback of all the body parts is comparted against that memory for each step of the intended repetition, and cerebellar corrections are made automatically and unconsciously. And of course, this translation of desire into movement must all take place within the context of the overall levels of tonus, the characteristic postures, the chronic limitations, and prevailing state of mind of the actor. Any template can only function within the range of possibilities inherent in the sewing machine as it presently stands.
We do not know just what an engram is physiologically, nor where it is located anatomically. The sensory cortex appears to be crucial to their organization, but not exclusively so. Some theorists hold that each memory 'bit' is specific to a single cell or reverberating loop of cells. Others maintain that memory is stored in a more holographic manner, such that all of the cells of the sensory system participate in some way in all sensory memories.Whatever their actural form of storage, their existence and their control of motor behaviour cannot be doubted. The 'learning' of a new motor skill is the process of establishing a new series of sensory engrams, and the ability to repeat that skill depends absolutely upon the preservation of the intact sensory engram.
Motor elements merely supply the motion; it is the sensory side of the nervous system which establishes the control of that motion.
The impulses carrying the information contained in sensory engrams are fed out to the muscles through various routes contained within the direct corticospinal path and the multineuronal path in the spinal cord. These pathways are themselves organizational features of the nervous system, each coloring in a specific way the translation of sensory memory into muscular activity. Some of them make direct uninterrupted connections from the cortex to the skeletal motor units (the direct corticospinal path); these have the advantage of producing almost instantaneous effects upon the muscle cells from cortical activities. Some synapse onto many internuncial reflex circuits on their way down the spinal cord (the multineuronal path), and these have the advantage of utilizing fixed, inherited reflex movements in order to build up more complicated sequences. Still others pass through the brainstem and all the basal ganglia, and hence can use the reptilian brain's entire vocabulary of stereotyped gestures and fixations in order to carry out still more complex commands.
– taken from Deane juan, Job's Body