Author Topic: Know your brain: Motor cortex  (Read 118 times)

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Know your brain: Motor cortex
« on: July 06, 2019, 10:53:07 AM »
source: https://neuroscientificallychallenged.com/blog/know-your-brain-motor-cortex

I deem this to be one of the most significant and early constructs of your mind despite being controlled by your frontal lobe Wiki! Therefopre, I assert that it (the Frontal Lobe is integral in memory processing in determining the why we do what we do. This article is on how we do what we do.

Know your brain: Motor cortexWiki

Where the fuck is the motor cortex ?



MOTOR CORTEX (IN RED)

The motor cortex is found in the frontal lobe, spreading across an area of cortex situated just anterior to a large sulcus known as the central sulcus, which runs down the side of the cerebral hemispheres.

The motor cortex is often divided into two major regions: the primary motor cortex, which is found in a gyrus known as the precentral gyrusWiki that is positioned just in front of the central sulcus, and the nonprimary motor cortexWiki, which is anterior to the primary motor cortex and contains two prominent regions known as the premotor cortex and supplementary motor cortex.

What the fuck is the motor cortex and what does it do ?

In 1870 physicians Gustav Theodor Fritsch and Eduard Hitzig, using awake dogs as their subjects, electrically stimulated the area of the brain we now know as the motor cortex and found that the stimulation caused the dogs to move involuntarily.

Additionally, they found that stimulating the motor cortex in different locations caused different muscles to move. This experiment led to the identification of the motor cortex as the primary area of our brain involved with planning and executing voluntary movements.

There are several distinct regions within the motor cortex. The area found to be the most sensitive to electrical stimulation--in that it requires the least amount of stimulation to produce a corresponding muscle movement--is the primary motor cortex.

The primary motor cortex is arranged such that different parts of the region are associated with motor control of different parts of the body, a topographic organization that is similar--although less precise--than that seen in the somatosensory cortex.



WATCH THIS 2-MINUTE NEUROSCIENCE VIDEO TO LEARN MORE ABOUT THE MOTOR CORTEX.

The primary motor cortex contains large neurons with triangular-shaped cell bodies that are called pyramidal neuronsWiki; these are the primary output cells of the motor cortex.

The axons of pyramidal cells leave the motor cortex carrying information about a desired movement and enter one of the tracts of the pyramidal system, which includes the corticospinal and corticobulbar tracts.

Both tracts carry information about voluntary movement down from the cortex; the corticospinal tract carries such information to the spinal cord to initiate movements of the body, while the corticobulbar tract carries motor information to the brain stemWiki to stimulate cranial nerve nuclei and cause movements of the head, neck, and face. Pyramidal neurons of the motor cortex are also known as upper motor neurons. They form connections with neurons called lower motor neurons, which directly innervate skeletal muscle to cause movement.

Other areas of the motor cortex, known collectively as the nonprimary motor cortexWiki, are found anterior to the primary motor cortex and also appear to play important roles in movement.

Despite their name, the nonprimary motor areas shouldn't be viewed as taking a secondary role to the primary motor cortex. Instead, the nonprimary motor areas are just involved in different aspects of movement, such as the planning of movements and the selection of actions based on environmental context.

The nonprimary motor cortex is often divided into two main regions: the supplementary motor cortex and the premotor cortexWiki.

The exact functions of these areas are not very well understood. It is thought that the supplementary motor cortex may be important to the execution of sequences of movement, the attainment of motor skills, and the executive control of movement, which can involve things like making decisions to switch to different movements based on incoming sensory information.

The premotor cortex makes a large contribution (~30%) to the neurons that will enter the corticospinal tractWiki, but it seems to be more active than the primary motor cortex during the planning of--rather than the execution of--movements.

Neurons in the premotor cortex also appear to be involved with incorporating sensory cues (e.g. the location of an object to be grasped) into a movement to ensure it is executed properly, as well as with the selection of actions based on behavioral context (e.g. picking up a cup to move it from the table vs. picking up a cup to take a drink from it).

There are also populations of neurons, sometimes called mirror neurons, in the premotor cortex that are activated when observing someone else carry out a movement; these cells may be involved in helping us to understand and/or imitate the actions of others.



Imagine if one could manipulate this area ... don't reach for that drug just yet and drop that gun !

see https://brainconnection.brainhq.com/2013/03/05/the-anatomy-of-movement/ for an article on The Anatomy of Movement

Almost all of behavior involves motor function, from talking to gesturing to walking. But even a simple movement like reaching out to pick up a glass of water can be a complex motor task to study.

EDIT: I believe that memory is critical to and (re)built from these motor function experiences and therefore is integral to the process.

Not only does your brain have to figure out which muscles to contract and in which order to steer your hand to the glass, it also has to estimate the force needed to pick up the glass.

Other factors, like how much water is in the glass and what material the glass is made from, also influence the brains calculations.

Not surprisingly, there are many anatomical regions which are involved in motor function.

The primary motor cortex, or M1, is one of the principal brain areas involved in motor function.

M1 is located in the frontal lobe of the brain, along a bump called the precentral gyrus (figure 1a).

The role of the primary motor cortex is to generate neural impulses that control the execution of movement. Signals from M1 cross the body's midline to activate skeletal muscles on the opposite side of the body, meaning that the left hemisphere of the brain controls the right side of the body, and the right hemisphere controls the left side of the body.

Every part of the body is represented in the primary motor cortex, and these representations are arranged somatotopicallyWiki — the foot is next to the leg which is next to the trunk which is next to the arm and the hand.

The amount of brain matter devoted to any particular body part represents the amount of control that the primary motor cortex has over that body part.

For example, a lot of cortical space is required to control the complex movements of the hand and fingers, and these body parts have larger representations in M1 than the trunk or legs, whose muscle patterns are relatively simple. This disproportionate map of the body in the motor cortex is called the motor homunculus (figure 1b).



Figure 1a: Principal cortical domains of the motor system. The primary motor cortex (M1) lies along the precentral gyrus, and generates the signals that control the execution of movement. Secondary motor areas are involved in motor planning. The plane of section is elaborated in figure 1b.



Figure 1b: The motor homunculus in primary motor cortex. A figurative representation of the body map encoded in primary motor cortex.

The section corresponds to the plane indicated in figure 1a.

Body parts with complex repertories of fine movement, like the hand, require more cortical space in M1, while body parts with relatively simpler movements, like the hip, require less cortical space
.

Other regions of the cortex involved in motor function are called the secondary motor cortices.

These regions include the posterior parietal cortexWiki, the premotor cortex, and the supplementary motor area (SMA).

The posterior parietalWiki cortex is involved in transforming visual information into motor commands. For example, the posterior parietal cortex would be involved in determining how to steer the arm to a glass of water based on where the glass is located in space.

The posterior parietal areas send this information on to the premotor cortex and the supplementary motor area.

The premotor cortex lies just in front of (anterior to) the primary motor cortex.

It is involved in the sensory guidance of movement, and controls the more proximal muscles and trunk muscles of the body.

In our example, the premotor cortex would help to orient the body before reaching for the glass of water. The supplementary motor area lies above, or medial to, the premotor area, also in front of the primary motor cortex.

It is involved in the planning of complex movements and in coordinating two-handed movements. The supplementary motor area and the premotor regions both send information to the primary motor cortex as well as to brainstem motor regions.

Neurons in M1, SMA and premotor cortex give rise to the fibers of the corticospinal tractWiki. EDIT: The corticospinal tract is a white matter motor pathway starting at the cerebral cortex that terminates on lower motor neurons and interneurons in the spinal cord, controlling movements of the limbs and trunk.



The corticospinal tract is the only direct pathway from the cortex to the spine and is composed of over a million fibers. These fibers descend through the brainstem where the majority of them cross over to the opposite side of the body.

After crossing, the fibers continue to descend through the spine, terminating at the appropriate spinal levels.

The corticospinal tract is the main pathway for control of voluntary movement in humans. There are other motor pathways which originate from subcorticalWiki groups of motor neurons (nuclei). These pathways control posture and balance, coarse movements of the proximal muscles, and coordinate head, neck and eye movements in response to visual targets.

Subcortical pathways can modify voluntary movement through interneuronal circuits in the spine and through projections to cortical motor regions.

The spinal cord is comprised of both white and gray matter (EDIT: like the brain but opposite in function - cool, right ?!).

The white matter consists of nerve fibers traveling through the spine. It is white because the nerve fibers are insulated with myelin for faster conduction of signals. Like many other large fiber bundles, the corticospinal tract courses through the lateral white matter of the spine.

The inside of the spinal cord contains gray matter, composed of the cell bodies of cells including motor neurons and interneurons. In a cross-section of the spinal cord, the shape of the gray matter resembles a butterfly.

Fibers in the corticospinal tract synapse onto motor neurons and interneurons in the ventral horn of the spine.

Fibers coming from hand regions in the cortex end on motor neurons higher up in the spine (in the cervical levels) than fibers from the leg regions which terminate in the lumbar levels. The lower levels of the spine therefore have much less white matter than the higher levels.

Within the ventral hornWiki, motor neurons projecting to distal muscles are located more laterally than neurons controlling the proximal muscles.

Neurons projecting to the trunk muscles are located the most medially.

Furthermore, neurons of extensors (muscles that increase the joint angle such as the triceps muscle) are found near the edge of the gray matter, but the flexors (muscles which decrease the joint angle such as the biceps muscle) are more interior.

It is important to note that a single motor neuron in the spine can receive thousands of inputs from the cortical motor regions, the subcortical motor regions and also through interneurons in the spine. These interneurons receive input from the same regions, and allow complex circuits to develop.

Cortical control of skeletal muscles.

Signals generated in the primary motor cortex travel down the corticospinal tract (green) through the spinal white matter to synapse on interneurons and motor neurons in the spinal cords ventral horn. Ventral horn neurons in turn send their axons (blue) out through the ventral roots to innervate individual muscle fibers. In this example, a signal from M1 travels through the corticospinal tract and exits the spine around the sixth cervical level. A peripheral motor neuron relays the signal out to the arm to activate a group of myofibrils in the bicep, causing that muscle to contract.

Collectively, the ventral horn motor neuron, its axon, and the myofibrilsWiki that it innervates are called a single motor unit.

Each motor neuron in the spine is part of a functional unit called the motor unit (figure 2 EDIT: missing).

The motor unit is composed of the motor neuron, its axon and the muscle fibers it innervates. Smaller motor neurons typically innervate smaller muscle fibers. Motor neurons can innervate any number of muscle fibers, but each fiber is only innervated by one motor neuron. When the motor neuron fires, all of its muscle fibers contract.

The size of the motor units and the number of fibers that are innervated contribute to the force of the muscle contraction.

There are two types of motor neurons in the spine, alpha and gamma motor neurons. The alpha motor neurons innervate muscle fibers that contribute to force production. The gamma motor neurons innervate fibers within the muscle spindle.

The muscle spindle is a structure inside the muscle that measures the length, or stretch, of the muscle. The role of the musclespindle in reflexes such as the knee jerk reflex will be reviewed in the Motor Systems Physiology section of this NeuroSeries.

The golgi tendonWiki organ is also a stretch receptor, but it is located in the tendons that connect the muscle to the skeleton. It provides information to the motor centers about the force of the muscle contraction. Information from muscle spindles, golgi tendon organs and other sensory organs are directed to the cerebellum. The cerebellum is a small grooved structure located in the back of the brain beneath the occipital lobe.

This motor region is specifically involved when learning a new sport or dance step or instrument.

The cerebellum is involved in the timing and coordination of motor programs. The actual motor programs are generated in the basal ganglia. The basal ganglia are several subcortical regions that are involved in organizing motor programs for complex movements.

Damage to these regions result in spontaneous, inappropriate movements.


The basal ganglia send output to other subcortical brain regions and the cortex.

Through the interaction of many anatomical motor regions, everyday movements seem effortless and more complex movements can be learned.



« Last Edit: July 06, 2019, 11:04:49 AM by Chip »
Over 90% of all computer problems can be traced back to the interface between the keyboard and the chair !

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