The Sensorimotor System and Human Reflexes

Summary

Professor Dave's lesson delves into the sensorimotor system, detailing how the brain sends signals to the body for movement. The hierarchy starts from the association cortex, moving through secondary and primary motor cortices, to the brain stem and muscles. Key areas include the posterior parietal and dorsolateral prefrontal association cortices, which integrate sensory data to direct motor actions. The primary motor cortex, somatotopically mapped, controls muscle movements. The cerebellum and basal ganglia play crucial roles in motor learning and movement facilitation. The script also explains muscle-spindle feedback and reflexes like the stretch reflex and withdrawal reflex, illustrating the brain's communication with muscles.

Takeaways

• 🧠 The sensorimotor system is responsible for motor output, controlling how the brain sends signals to the body.
• 📈 It has a hierarchical structure, with signals starting from the association cortex and moving down to muscles.
• 🔄 The system exhibits functional segregation, with different levels performing distinct functions.
• 👀 The posterior parietal association cortex integrates information from various sensory systems and sends it to motor areas.
• 🤔 The dorsolateral prefrontal association cortex processes information from the posterior parietal cortex and influences motor planning.
• 🏃‍♂️ The secondary motor cortex, including the supplementary motor area and premotor cortex, programs patterned movements.
• 🤲 The primary motor cortex, located in the precentral gyrus, is where many sensorimotor signals converge and direct muscle actions.
• 🧭 It is somatotopic, with body parts mapped to specific cortical regions, as depicted by the motor homunculus.
• 🌐 The cerebellum plays a role in motor learning and precise timing, integrating information from various motor-related areas.
• 🔁 The basal ganglia facilitate wanted movements and inhibit unwanted ones, influencing motor output.
• 🛤️ Descending motor pathways include four main tracts that originate in the cerebral cortex and innervate motor units in muscles.
• 🔁 The muscle-spindle feedback circuit allows for communication between muscles and the brain, involving receptors like Golgi tendon organs and muscle spindles.

Q & A

What is the sensorimotor system?

The sensorimotor system is responsible for how the brain sends signals out to the body to tell it what to do, which is known as motor output.

How is the sensorimotor system organized?

The sensorimotor system is hierarchically organized, with signals typically beginning in the association cortex and moving through secondary and primary motor cortices, brain stem motor nuclei, and finally to muscles.

What is functional segregation in the context of the sensorimotor system?

Functional segregation refers to each level of the sensorimotor system performing a different function, similar to the sensory systems, but with information flowing down instead of up.

What are the two areas of the sensorimotor association cortex?

The two areas of the sensorimotor association cortex are the posterior parietal association cortex and the dorsolateral prefrontal association cortex.

What does the posterior parietal association cortex receive information from?

The posterior parietal association cortex receives information from the visual, auditory, and somatosensory systems.

What is the role of the dorsolateral prefrontal association cortex in the sensorimotor system?

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.

What is the function of the secondary motor cortex?

The secondary motor cortex is involved in the programming of patterned movement upon being given instructions by the dorsolateral prefrontal cortex.

Where is the primary motor cortex located and what is its main function?

The primary motor cortex is located in the precentral gyrus of the frontal lobe and is the main area from which signals leave the brain to tell the muscles what to do.

What is the significance of the motor homunculus in the primary motor cortex?

The motor homunculus represents the somatotopic mapping of the body in the primary motor cortex, showing that specific regions of the cortex correspond with specific regions of the human body, with areas like the hands and facial features having a larger representation.

What are the roles of the cerebellum and basal ganglia in the sensorimotor system?

The cerebellum is involved in motor learning and precise timing, while the basal ganglia facilitate wanted movements and inhibit unwanted movements.

How many main paths do signals descend along in the descending motor pathways?

Signals descend along four main paths in the descending motor pathways: the dorsolateral corticospinal tract, the dorsolateral corticorubrospinal tract, the ventromedial corticospinal tract, and the ventromedial cortico-brainstem-spinal tract.

What are the two types of receptors found within skeletal muscles?

The two types of receptors found within skeletal muscles are Golgi tendon organs and muscle spindles.

How does the stretch reflex work?

The stretch reflex occurs when the spindles of a muscle stretch, sending a signal up afferent neurons to the spinal cord, which then sends a signal down motor neurons to cause the muscle to contract.

What is the withdrawal reflex and how does it work?

The withdrawal reflex is an automatic response to a harmful stimulus, like touching something hot, where sensory neurons excite spinal interneurons that either excite or inhibit motor neurons to rapidly move the limb away from danger.

What is reciprocal innervation?

Reciprocal innervation is the strategy of combining the excitation of certain motor neurons with the inhibition of others, as seen in reflexes like the withdrawal reflex.

Outlines

00:00

🧠 Understanding the Sensorimotor System

Professor Dave introduces the sensorimotor system, which is responsible for motor output, the process by which the brain sends signals to the body. The system is hierarchical and parallel, starting from the association cortex and moving through various brain regions down to the muscles. It exhibits functional segregation, with each level performing a distinct function. The posterior parietal association cortex integrates sensory information and sends it to motor areas, while the dorsolateral prefrontal association cortex coordinates with motor cortex and the frontal eye field. The secondary motor cortex is involved in patterned movement programming, and the primary motor cortex is where many sensorimotor signals converge, controlling muscle actions through a somatotopic mapping. The cerebellum and basal ganglia, while not part of the direct pathways, play crucial roles in motor learning and facilitating desired movements, respectively.

05:02

🚶‍♂️ Descending Motor Pathways and Muscle-Spindle Feedback

This section discusses the descending motor pathways, which are the routes signals take from the brain to the muscles. There are four main paths: the dorsolateral corticospinal tract, the dorsolateral corticorubrospinal tract, the ventromedial corticospinal tract, and the ventromedial cortico-brainstem-spinal tract. These pathways facilitate muscle control and movement. The paragraph also explains the muscle-spindle feedback circuit, which is the communication system between muscles and the brain. It involves Golgi tendon organs and muscle spindles, which respond to changes in muscle tension and length, respectively. The stretch reflex and withdrawal reflex are highlighted as examples of how this feedback circuit works, with the latter involving reciprocal innervation to produce rapid, protective movements.

Keywords

💡Sensorimotor system

The sensorimotor system is the part of the nervous system that connects sensory input to motor output, allowing the brain to control voluntary movements. In the video, the sensorimotor system is described as having a hierarchical structure, starting from the association cortex and extending to the muscles, highlighting its role in processing sensory information and initiating movement.

💡Motor output

Motor output refers to the process by which the brain sends signals to the muscles to initiate movement. It is a key concept in the video, as it illustrates the brain's role in controlling the body's actions. The script explains that motor output begins in the association cortex and travels through various brain regions before reaching the muscles.

💡Hierarchy

Hierarchy, in the context of the sensorimotor system, refers to the structured levels through which signals pass from the brain to the muscles. The video script describes how motor signals typically begin in higher cortical areas and descend through several stages, including the primary motor cortex and brain stem, before reaching the muscles.

💡Functional segregation

Functional segregation is the concept that different parts of the brain have specialized functions. The video script mentions that each level of the sensorimotor system performs a distinct function, similar to how sensory systems process information. This segregation allows for the efficient processing of sensory input and the coordination of motor responses.

💡Association cortex

The association cortex is a part of the brain involved in higher cognitive functions, such as integrating sensory information and planning movements. In the video, it is highlighted that the association cortex plays a crucial role in the sensorimotor system by receiving sensory input and sending signals to motor areas.

💡Primary motor cortex

The primary motor cortex is a region of the brain that is directly responsible for controlling voluntary movements. As explained in the video, it is located in the precentral gyrus of the frontal lobe and is somatotopic, meaning that different areas of the cortex correspond to different parts of the body.

💡Cerebellum

The cerebellum is a region of the brain that plays a key role in motor control, particularly in coordinating movement and maintaining balance. The video script explains that the cerebellum receives information from motor cortices and integrates this with feedback from the somatosensory system to aid in motor learning.

💡Basal ganglia

The basal ganglia are a group of nuclei in the brain that are involved in the control of voluntary movements. The video script describes how the basal ganglia facilitate wanted movements and inhibit unwanted ones, playing a critical role in the execution of motor actions.

💡Motor homunculus

The motor homunculus is a representation of the body's parts mapped onto the primary motor cortex, with each body part proportionally represented according to the amount of cortex dedicated to its movement. The video uses the motor homunculus to illustrate the somatotopic organization of the primary motor cortex.

💡Muscle-spindle feedback circuit

The muscle-spindle feedback circuit is a sensory mechanism within muscles that provides information about muscle length and tension to the central nervous system. The video script explains how this circuit is crucial for reflexes such as the stretch reflex, where the stretching of muscle spindles sends signals that cause muscle contraction.

💡Reciprocal innervation

Reciprocal innervation is a strategy used by the nervous system to control muscle movement by simultaneously exciting some neurons and inhibiting others. The video script uses the example of the withdrawal reflex to illustrate how this mechanism allows for rapid and coordinated muscle responses.

Highlights

Introduction to motor output and the sensorimotor system.

The hierarchical organization of the sensorimotor system.

Functional segregation within the sensorimotor system.

The role of the sensorimotor association cortex in integrating sensory information.

The dorsolateral prefrontal association cortex's influence on motor cortex and eye movement.

The secondary motor cortex's role in programming patterned movement.

The primary motor cortex as the main area for sensorimotor signal convergence.

The somatotopic organization of the primary motor cortex.

The importance of the motor homunculus in mapping body regions to the cortex.

The cerebellum's role in motor learning and precise timing.

The basal ganglia's facilitation of wanted movements and inhibition of unwanted movements.

The four main descending motor pathways and their functions.

The journey of signals from the cerebral cortex to motor units.

The muscle-spindle feedback circuit for muscle and brain communication.

The function of Golgi tendon organs and muscle spindles in sensing muscle tension and length.

The stretch reflex as a classic example of the muscle-spindle feedback circuit in action.

The withdrawal reflex and its rapid response mechanism.

Reciprocal innervation as a strategy for combining neuron excitation and inhibition.

The central nervous system's collaboration with the muscular system for motion production.

Transcripts

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.