Hello everyone, welcome to this tutorial on Lower Motor Neuronal Control.
This is really the first of a set of new topics that we will consider, where the
primary focus will be on understanding the way the brain controls voluntary
movement. And we begin by considering the lowest
level of this set of neural elements that run between the cerebral cortex and the
muscle tissue that actually does the movement of the body.
So, this tutorial will unfold in 4 parts. this first part, I'd like to give you a
bit of an overview of the somatic motor system and define for you and talk just a
little bit about motor units. For this first part of our tutorial, I
have two core concepts in the field of neuroscience in mind, again, the
complexity of the brain is always before us, and so I'd like to not miss an
opportunity to remind you that the brain is the body's most complex organ.
And again, most of what we'll be talking about today pertains to the organization
and function of circuits that are genetically determined, and they form the
foundation for the nervous system's ability to execute voluntary movement.
So, I've got some learning objectives that pertain to this first part of the
tutorial. I'd like for you to be able to discuss
the general somatotopic organization of the motor neurons that are found in the
ventral horn of the spinal cord. And I'd like for you to be able to
characterize the motor unit and discuss different types of motor units.
Okay. So, let's get into this by talking in
broad terms about the organization of the human central nervous system.
Now, you've seen this figure before. And I used it to introduce the concepts
of central nervous system, peripheral nervous system, and the general
phenomenon that is so critical for understanding human behavior, and that is
sensory motor integration. Well, now that we have completed our
survey of the sensory systems that take information from the internal and
external environment, process that information along the way between the
spinal chord, the brain stem, the diencephalon and then finally up to the
level of the cerebral cortex. Then that information becomes synthesized
and integrated and circuitry that is distributed across the lobes of the
cerebral hemispheres and runs loops into subcortical structures, such as, the
basal ganglia and even back down through the thalamus.
And through emotional centers including structures like the amygdala, memory
centers, structures like the hippocampus and the parohippocampal gyrus.
Well, ultimately, this process of sensorimotor integration gives rise to
behavior. And it's to the behavioral response that
we now turn. And so, what we're going to be talking
about are components of our motor systems.
And as this chart is attempting to make clear, there are parallel pathways that
allow our motor system to produce behavior.
there's a channel that we'll have a tutorial about in just a little while
talking about the autonomic nervous system, or what we sometimes call the
visceromotor system. But for the next couple of sessions, I
really want to focus in on what we'll call the somatic motor system.
This is usually what we think about when we consider human movement.
We're thinking about the activation of musculoskeletal effector systems and the
production of physical movement of our frame.
And so, this involves the activities of central nervous system stations that send
signals out through motor nerves to striated or to skeletal muscles and that
actually produces the movement. So, we're going to work our way into this
motor system one step at a time. And this figure now really lays out a
framework for understanding this somatic motor system.
And let me highlight a few of the key features here.
First of all we'll work our way from below, and then up through this motor
system. And we begin with this idea that the
skeletal muscles are the effector systems.
And so, if we want to understand the control of the skeletal muscles, we need
to consider the neurons within the central nervous system that motivate
those skeletal muscles and cause their contraction.
These we call lower motor neurons. And so, these lower motor neurons exist
with in a lower motor neural circuitry that is found at every segment of the
spinal cord and for the cranial nerve motor nuclei, also in the brainstem in
association with our cranial nerve motor nuclei.
This circuitry that we find, receives sensory inputs.
And so, one dimension of activity that we will explore today is this idea of local
segmental circuits that perform reflexive adjustments of movement in response to
specific sources of sensory information. So, the local circuitry of the spinal
cord and brain stem mediates reflexes. There are also circuits that span the
segments of the spinal chord, and in some instances, span levels of the brainstem
where we find different kinds of cranial nerve motor nuclei.
These kinds of circuits, we call intersegmental circuits, because they are
required to coordinate the activities across the longitudinal length of the
central nervous system where we find our lower motor neurons.
One example might be swimming or perhaps swaying our arms while we are ambulating
on our two feet. So obviously, in both of those
activities, there's coordination between the movements of our arms mediated via
the cervical enlargement of the spinal cord in the movements of our legs which
are coordinated at the level of the lumbosacral outflow from the spinal cord.
Now, let's move up the nervous system just a bit and consider how is the
outflow from these circuits that govern the lower motor neurons controlled.
Well, they're controlled by a set of neurons that we call upper motor neurons.
And these upper motor neurons exist in the motor cortex and in the brainstem.
Now, I want you to notice, the upper motor neurons themselves do not innervate
skeletal muscle. Rather, they provide descending input
that are governing the way the local circuits work to organize the outflow of
the lower motor neurons. And in some very special instances, there
maybe direct projections, from upper motor neurons to the alpha motor neurons,
that actually do supply innervation to skeletal muscle.
But notice for the most part this arrow is a lot larger than the one providing
direct input to the alpha motor neuron which is to imply that the way our motor
system is organized is that the upper motor neurons largely coordinate the
local circuits that then do the job of organizing the output to skeletal
muscles. Now, when we're thinking of movement, we
are often thinking about voluntary movement, movement that we consider, and
that we actually express through active will.
When we are thinking about voluntary movement, you should be thinking about
activities that are planned, that are coordinated, that are initiated at the
level of the motor cortex. The motor cortex is the posterior part of
the frontal lobe, where our executive functions are integrating sensory, and
emotion, and mnemonic signals, and devising plans for the production of
behavior. And the actual final signal to produce
that behavior is a complicated process that we'll talk about in subsequent
tutorials. For today, I want you to understand that
these are the kinds of activities performed by the motor cortex as we
prepare to execute a voluntary behavior. And then ultimately, of course, that
information is relayed down to our lower motor neurons.
Now, in addition to the motor cortex, there are also upper motor neurons in the
brainstem and these motor neurons are involved in setting the stage for the
execution of voluntary movement. Now much of the voluntary movement that
we make, we can think of it as the expression of skill.
Consider, for example, typing on a keyboard or using a pen or a pencil, or
some other writing implement, perhaps performing dance or music with a musical
instrument. These are all aspects of skill and
primarily, skill is expressed through the activities of our hands, our feet, and
the lower oral regions of our face. Well, in order to be skilled in these
body parts, we need to have proper adjustments of posture and we need to
have the proper amount of tone present within the muscle systems that actually
express that skill. And this is largely where these brainstem
centers come in. So, well, I don't particularly think of
these brainstem centers as expressing voluntary behavior, I do think of them as
being essential for the expression of the skilled voluntary motor acts that we
normally think about when we consider human movement.
So, from these brainstem centers, for example, we have the coordination of
posture as well as the organization of basic movements that are essential for
sustaining goal directed behavior such as locomotion.
and other aspects of sort of rhythmical highly stereotypical activities including
things that we do with our cranial structures, like chewing.
Well, there's another aspect of upper motor neuronal control that we'll talk
about a few tutorials down the line. And this is perhaps somewhat at the
interface between what we might call voluntary and involuntary behavior.
And what I'm thinking of is the expression of emotional behavior.
Emotional expression reflects the coordination of both somatic motor and
visceral motor activities. And this is not so much a function of the
posterior part of our frontal lobe. Rather, it's a function of those parts of
the brain that tend to be present mostly on the ventral and the medial aspects of
the forebrain, involving structures like the orbital aspect of the frontal lobe or
the medial midline aspect of the frontal lobe.
And these structures are interconnected with structures in the temporal lobe
including the amygdala and the hippocampus, where emotion and memory can
impact our motivation for behavior. And these behavioral systems that are
integrated and can produce motor output, not from the posterior frontal lobe where
voluntary goal-oriented behaviors are planned and initiated, but rather through
deeper parts of the ventral forebrain where motivations come from.
Okay. Lastly, as we continue to provide a broad
overview of our motor systems, I want to talk about these structures that we find
over here to the right-hand side of this figure, the basal ganglia and the
cerebellum. Now, the basal ganglia and the cerebellum
are not considered to be upper motor neuronal systems.
you might wonder why, neither the basal ganglia nor the cerebellum connect to
muscle so that might qualify them to be called upper motor neurons.
But, the basal ganglia and the cerebellum they don't project directly to those
circuits that govern the output of our lower motor neurons, which is the usual
way that we define what constitutes an upper motor neuronal system.
Rather, the basal ganglia and the cerebellum, they interact directly with
networks of upper motor neurons. So, notice that the arrows enter this
block diagram at the level of our upper motor neuron systems, okay?
So, we'll see how this works in subsequent tutorials but for now, let me
simply say that the basil ganglia is especially involved in the initiation and
suppression of movement. Whereas, the cerebellum is involved in
the coordination of ongoing movement. We'll see an important role for the basal
ganglia just as movements are getting started.
And now, once the movements are ongoing, then the cerebellum has a critical role
to play in coordinating their ongoing activity.
Okay, well, let's now turn out attention to the organization of our lower motor
neurons,. And we will eventually be talking about
our segmental reflexes that are coordinated at this level of lower motor
neurons in the spinal cord and brainstem.
