We have already seen how afferent axons from sensory receptors enter the spinal column through the dorsal root and motor neurons exit through the ventral root. The afferent and efferent axons are grouped into spinal nerves before separating to go their separate ways. Once outside the spinal column, these axons and nerves are part of the peripheral nervous system, or PNS. We also have seen how efferent axons connect to muscle fibers via the neuromuscular junction (NMJ) to initiate muscle contraction. So not very much is left to add, unless we go into more detail.
As far as output from the nervous system is concerned, there are two parts:
- the somatic motor system, which controls conscious perception and voluntary motor response, and
- the autonomic nervous system (ANS), which is responsible for control, such as the glands, heart and involuntary muscles .
There are three sorts of movement, controlled by different neural circuits:
- involuntary movements which regulate internal functions and homeostasis;
- conscious voluntary movements;;
- reflexive movements, automatic reactions which nevertheless may become conscious.
The third type, reflexive movement, is a fast, protective mechanism, which generally occurs before the brain is informed of the situation. Instead of the sensory neurons’ contacting the brain to obtain a command, these signals pass directly to interneurons in the spinal column and from there back out to the muscles, thereby avoiding the delay which would result from contacting the brain to obtain a response. This is of course how the classic knee-jerk reaction works. The interneurons are linked together into networks. The signal may also be passed to the brain to become conscious.
The anterior cingulate cortex (ACC) compares plans to how things are really happening in order to correct for errors or trouble due to unforeseen circumstances. It also monitors progress towards goals. It is connected to the lateral prefrontal cortex, giving it access to working memory.
We will not look any deeper on the subject of motor control. So let’s go study how learning and memory take place.