It’s Professor Dave, let’s check your reflexes.
We just went over the basics regarding how sensory information gets to your brain, so
before we dig into specific aspects of brain function, let’s first complete the circuit,
and learn a bit more about how the brain sends signals out to the body to tell it what to do.
This is called motor output, and the system in control of this is called the sensorimotor system.
Let’s get a closer look now.
The first thing to understand about the sensorimotor system is its hierarchy.
Just the way sense perception involves signals getting shuttled to a primary cortex, then
a secondary cortex and then an association cortex, motor output typically begins in the
association cortex, and then move through a secondary motor cortex, primary motor cortex,
brain stem motor nuclei, all the way to a muscle.
Not all signals make it through all of these locations, some make it to muscles without
hitting every stop, and certain bodily functions rely on signals bypassing certain stops to
initiate a rapid response.
Nevertheless, we can say that the sensorimotor system is hierarchically organized, and in
parallel fashion.
We can also say that this system exhibits functional segregation.
Each level that we just mentioned performs a different function.
This makes it very similar to the sensory systems, but instead of information flowing
up, information flows down.
Of course the two work in close conjunction, as sensory feedback largely directs motor
output in most cases.
Let’s start at the top with the sensorimotor association cortex.
This has two areas, the posterior parietal association cortex, and the dorsolateral prefrontal
association cortex.
These are in turn each made of a few areas with different functions.
The posterior parietal association cortex receives information from the visual, auditory,
and somatosensory systems.
This information is integrated and an output is sent to areas of the motor cortex.
The dorsolateral prefrontal association cortex receives information from the posterior parietal
cortex, and sends information to the primary and secondary motor cortex, as well as the
frontal eye field.
Moving on to the secondary motor cortex, this area receives information from the two association
areas we just mentioned, and sends information largely to the primary motor cortex.
It consists of the supplementary motor area and the premotor cortex, although these have
substructures that are still being researched.
The secondary motor cortex is involved in the programming of patterned movement upon
being given instructions by the dorsolateral prefrontal cortex.
Next up is the primary motor cortex.
This sits in the precentral gyrus of the frontal lobe, and it is where many of the sensorimotor
signals converge.
It is also the main area from which signals leave the brain to tell the muscles what to do.
Just like the somatosensory cortex, which we learned about in the previous tutorial,
the primary motor cortex is somatotopic, meaning that specific regions of the cortex correspond
with specific regions of the human body.
We can look at the motor homunculus to see how these regions are mapped, and again we
see that the hands get a lot of real estate, as do the facial features.
Apart from these regions that we’ve just discussed, let’s briefly mention two others,
the cerebellum and the basal ganglia.
These are not part of the pathways we outlined, but they are still important sensorimotor
structures.
The cerebellum contains a disproportionately large number of neurons, actually the majority
of the neurons in the brain.
This receives information from the primary and secondary motor cortexes, signals from
brain stem motor nuclei, and feedback from motor responses through the somatosensory system.
It integrates the information from these sources and, and is thus believed to play a role in
motor learning, when developing skills that require precise timing.
The basal ganglia are not as dense but they are quite complex.
They are part of loops that receive cortical input and transmit it through the thalamus
and back to the cortex.
We believe this plays a role in motor output by facilitating wanted movements and inhibiting
unwanted movements.
So that’s a brief outline of what we call descending motor pathways.
Signals descend along four main paths, two of which move through the dorsolateral region
of the spinal cord, and two of which move through the ventromedial region of the spinal cord.
These first two are the dorsolateral corticospinal tract and the dorsolateral corticorubrospinal
tract, while the other two are the ventromedial corticospinal tract and the ventromedial cortico-brainstem-spinal tract.
These all originate in the cerebral cortex, but have different functions.
Information travels all the way to motor units, which are comprised of a single neuron and
all of the skeletal muscle fibers that it innervates, and that’s where the body obeys
the brain.
Feel free to check out my tutorials on muscle types, as well as the mechanism of muscle
contraction, if you are interested in learning more about that part before moving forward here.
Otherwise, let’s now briefly discuss the muscle-spindle feedback circuit.
This is the way the muscles and the brain communicate.
Within the skeletal muscles we can find two kinds of receptors.
These are Golgi tendon organs, and muscle spindles.
The first of these are embedded in the tendons, which as we recall are the things that connect
the muscle to a bone, and these respond to changes in muscle tension.
Muscle spindles are embedded in the muscle tissue, and they respond to changes in muscle length.
Information travels from these receptors to the central nervous system via extrafusal
and intrafusal motor neurons, and spindle afferent neurons, depending on their origin.
We can use this knowledge to understand certain reflexes of the human body.
First let’s examine the stretch reflex.
This is the classic reflex involved when the doctor taps on your knee with a mallet, and
your leg extends.
What happens is that the tap causes spindles of the thigh muscle to stretch, which sends
a signal up the afferent neurons to the spinal cord.
This sends another signal down the motor neurons that causes the thigh muscle to contract,
and your leg extends.
There is also the withdrawal reflex.
This one is more like when you touch something hot and suddenly pull your hand away without
even having to think about it.
The stimulus causes sensory neurons to fire, and this excites spinal interneurons which
do two things.
Excitatory spinal interneurons excite motor neurons in the bicep, and inhibitory spinal
interneurons inhibit motor neurons in the tricep.
This simultaneous action causes a rapid jerking motion in the arm, the fastest way to get
you out of danger.
This strategy of combining the excitation of certain neurons with the inhibition of
others is called reciprocal innervation, and it is very common.
So we now have a basic understanding of how the central nervous system works with the
muscular system in order to produce motion, and of course any motion more involved than
a simple reflex is going to be much more complicated than what we have outlined here.
But before we go deeper with all that, let’s take a look at some other topics.