Well, if you knew my son, you'd know that he was a rather stoic young man and
seldom truly expressed his emotion. this was a different setting where we put
him up in a studio to get his picture taken, and the photographer is waving a
stuffed animal in from of him, trying to entertain him, get him to smile.
And so, so what we see here is about as close to a happy expression as we ever
saw in our 18-month old son. and and contrast that to this very forced
facial expression it's quite easy to tell the difference between which expression
came from a force of will, and which perhaps reflects a true experience of
emotion. So why do you think we're so good at
telling the difference between a genuine emotion in a smile and a forced smile?
Well, I think partly the answer is that we understand, whether we articulate in
these terms or not. That there are different neural centers
in the fore-brain that are organizing the expressions in the human face that are
conveying genuine emotion or a contrived volitional motor act.
That is meant to imitate that emotion. Well the genuine emotion that we might
experience and express in the muscles of our face is motivated primarily by the
medial divisions of the premotor cortex that run into the banks of the singular
[INAUDIBLE].
In fact, there's a region. Right in about this region of the human
brain called the cingulate motor area. And this region is active when we express
an emotional smile. However, if we present a cheesy smile, a
pyramidal smile to a camera, that smile is being organized and executed.
By motor circuits in the lateral and inferior part of the premotor and primary
motor cortex. This is a pathway that runs along the
corticobulbar path in parallel with the corticospinal fibers, so we consider this
to be part of our pyramidal motor system. So quite different regions of the
forebrain are responsible for the emotional smile, and what we sometimes
call the parabital smile. Now, if you view the tutorial that I've
prepared to discuss the control of facial expression by upper motor neurons.
You'll realize that there's also and important clincal side, that can be
associated with the connection between these two different parts of the
pre-motor cortex and the lower motor circuits of the facial motor nucleus.
So so please take some time and review that tutorial because testing of facial
motor. Performance is a key part of the
neurological exam. I'd like to give you one more example of
how our volitional motor systems and our emotional motor systems are integrated
and coordinated to produce a desired motor output in this case the example is
human speech. Now, my speech isn't particularly
emotional at the moment. I suppose it could be but, you sort of,
know what you're getting from me at this point in the course.
but nevertheless, we can break down speech into different components, right?
So there is the pure active vocalization That is, just producing the sound itself.
So that requires some coordination, of motor output.
And then there is, of course, what we're interested in and that is the semantic
content of speech. That comes with the production of the
phonemes that are representing the vowels, and syllables, and the structure
of the vocalization that conveys the semantic content.
So it's necessary to integrate these two components of speech in order to actually
produce understandable human language. So how does this happen?
So these different aspects of speech are coordinated by motor output from the
brain stem. So, for example one needs to coordinate
the movements of the mouth mouth opening contractions of the floor of the mouth
the movements of the tongue movements of the pharynx and the larynx in order to.
Coordinate that vocal motor apparatus for the production of the speech sounds that
are necessary for intelligible language, but there's also coordination of
respiratory rhythms and the muscles that force air through the larynx.
So there's additional musculature that must be coordinated with the vocal motor
apparatus found in the oral pharynx and larynx.
And so this all occurs through the activity of lower motor circuits.
These lower motor circuits involve the nucleus ambiguous, the facial nucleus.
The trigeminal motor nucleus, the hypoglossal nucleus and the dorsal motor
nucleus of vagus. So already, you can understand the
complexity of motor output that is required to make human speech.
Well, when we back up now and consider how do upper motor neurons govern these
lower motor neurons we learn a little bit about the organization of parallel
pathways. So, concerning the semantic content of
speech this information is processed in our lateral pre-motor cortex.
In that very special part that we call Broca's area, that is especially
concerned with expressing human speech. Well Broca's area, being part of our
premotor cortex, sends projections to these lower motor neurons.
And also sends connections to our primary motor cortex that interact with these
lower motor circuits and this helps give some structure to the vocal articulation
of speech. But these signals would not be sufficient
in and of themselves to produce speech. In addition to those projections that are
emanating from the lateral part of the pre-motor cortex, we also need to engage
our medial pre-motor regions, as well as structures including the amygdala.
And the hypothalamus that are going to be important for[UNKNOWN] speech with
emotion. And so there relays from these ventral
and medial regions of the fore brain to the structures in the brain stem.
Including cell groups that are part of this periaqueductal grey region of the
mid brain and in turn the periaqueductal gray sends connections to the reticular
formation. Especially parts of the medullary
reticular formation, that then interact with our lower motor circuits.
So the coordination of, of these two limbs is what is necessary to produce
human speech, so of course, ultimately that coordination is derived from
executive centers in the prefrontal cortex.
Well, I think speech is perhaps a surprising example of convergence of
parallel motor commands from very different parts of the brain.
That are responsible for creating the complete expression of human speech.
That has semantic content. We can think of that as a skilled
articulation of our vocal motor apparatus, but also has the necessary
gain. That has the necessary tone, and has.
structure that we interpret as emotion. And these signals are modulated by very
different parts of the pre motor cortex, and even regions of the brain that are
outside of our conventional collection of centers that we call the motor system.
I'm thinking now of the hypothalamus and the amygdala.
That contribute to the expression of emotion in human speech.
Okay, we've finally come to the final part of this rather lengthy tutorial on
upper motor neurons. And what I'd like to talk about next in
our final segment is the clinical picture that we would discover in an individual
who has suffered injury or disease that affected upper motor neurons.