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Chapter 3: engine Cortex

James Knierim, Ph.D., department of Neuroscience, The Johns Hopkins university Last testimonial 20 Oct 2020
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3.1 Introduction

The previous chapters debated the reduced levels of the motor hierarchy (the spinal cord and brainstem), i m sorry are associated in the low-level, “nuts and also bolts” processing that controls the task of individual muscles. Individual alpha motor neurons regulate the pressure exerted by a details muscle, and also spinal circuits have the right to control sophisticated and complex behaviors such together walking and reflex actions. The varieties of movements regulated by these circuits are not initiated consciously, however. Voluntary movements require the asdrta.netorized of the 3rd and 4th levels of the hierarchy: the motor cortex and also the combination cortex. These areas of the cerebral cortex plan voluntary actions, coordinate sequences of movements, do decisions around proper behavior strategies and also choices, evaluate the appropriateness the a certain action given the current behavior or ecological context, and also relay commands to the ideal sets of reduced motor neurons to execute the preferred actions.

3.2 motor Cortex Comprises the primary Motor Cortex, Premotor Cortex, and also Supplementary engine Area


Figure 3.1 engine cortex areas (lateral, dorsal, and medial views). The main motor cortex is located instantly anterior to the main sulcus. Choose from the box in the facility to see bigger views.

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The motor cortex comprises 3 different locations of the frontal lobe, immediately anterior to the central sulcus. These areas are the main motor cortex (Brodmann’s area 4), the premotor cortex, and the supplementary engine area (Figure 3.1). Electric stimulation that these locations elicits movements of details body parts. The primary motor cortex, or M1, is located on the precentral gyrus and also on the anterior paracentral lobule ~ above the medial surface ar of the brain. That the three motor cortex areas, stimulation of the main motor cortex needs the least amount that electrical existing to elicit a movement. Low levels of short stimulation generally elicit an easy movements that individual human body parts. Stimulation that premotor cortex or the supplementary motor area requires greater levels of present to elicit movements, and also often outcomes in more complicated movements than stimulation of primary motor cortex. Stimulation for longer time durations (500 msec) in chimpanzees results in the activity of a specific body part to a stereotyped posture or position, regardless of the initial starting point the the body component (Figure 3.2). Thus, the premotor cortex and also supplementary motor areas show up to be higher level locations that encode complicated patterns of engine output and that select proper motor plans to accomplish desired finish results.


Figure 3.2 electrical stimulation the premotor cortex that a monkey for 500 msec produces motion to stereotyped postures depending on the location of the stimulating electrode. Stimulation of website one (click wake up 1) reasons the monkey to lug its eight in former of that eyes, regardless of the beginning location the the arm, as if the monkey were developing a protective posture. Stimulation of website two (click stimulate 2) causes the monkey to carry its eight to its mosdrta.net and also open the mosdrta.net, nevertheless of the starting location the the arm, as if it were bringing a item of food come its mosdrta.net (Graziano et al., 2002).


Like the somatosensory cortex the the postcentral gyrus, the main motor cortex is somatotopically organized (Figure 3.3). Stimulation of the anterior paracentral lobule elicits activities of the contralateral leg. As the stimulating electrode is moved throughout the precentral gyrus indigenous dorsomedial come ventrolateral, movements are elicited increasingly from the torso, arm, hand, and face (most laterally). The representations of body parts that do precise, fragile movements, such as the hands and also face, room disproportionately huge compared to the depictions of body components that perform just coarse, unstable movements, such as the tribe or legs. The premotor cortex and also supplementary motor area likewise contain somatotopic maps.


Figure 3.3 Somatotopic representation of motor outputs in motor cortex.


One could predict the the motor cortex “homunculus” arises due to the fact that neurons that regulate individual muscles space clustered together in the cortex. That is, all of the neurons that control the biceps muscle may be situated together, and every one of the neurons that manage the triceps might be clustered nearby, and also the neurons that manage the soleus muscle may be clustered in a an ar further removed. Electrophysiological recordings have presented that this is not the case, however. Movements of individual muscle are associated with activity from widespread parts of the major motor cortex. Similarly, stimulation of tiny regions of main motor cortex elicits activities that call for the activity of many muscles. Thus, the major motor cortex homunculus does not represent the task of separation, personal, instance muscles. Rather, it apparently represents the movements of individual human body parts, which often require the coordinated activity of huge groups of muscles throughout the body.

3.3 Cortical Afferents and Efferents

The motor cortex exerts its influence over muscles by a selection of descending routes (Figure 3.4). Some of the to decrease pathways the evaluation in the critical chapter have the right to be influenced by engine cortex output. Thus, in addition to the straight cortical innervation of alpha engine neurons via the corticospinal tract, the adhering to cortical efferent pathways influence the continuing to be descending tracts:

the corticorubral tract enables cortex to modulate the rubrospinal street the corticotectal tract allows cortex come modulate the tectospinal street the corticoreticular tract allows cortex come modulate the reticulospinal tracts

Figure 3.4 Parallel pathways indigenous the engine cortex permit the cortical motor locations to influence the handling of all descending engine tracts and side loops the the motor system. Mouse over the pathways for more information.


The cortex can also influence the handling of the side loops of the motor hierarchy. The corticostriate tract innervates the caudate nucleus and also putamen that the basal ganglia. The corticopontine tract and corticoolivary street innervate essential inputs to the cerebellum. Finally, cortical locations can affect other cortical areas, directly via corticocortical pathways and also indirectly via the corticothalamic pathways (Figure 3.5). Most of these pathways space bi-directional. Thus, motor cortex receives input from other cortical areas, directly and also indirectly through the thalamus, and also it receives input indigenous the cerebellum and also basal ganglia, always through the thalamus.


Figure 3.5 significant connections of engine cortex. The cross-section top top the left is a schematic variation of an idealized mind section that consists of the significant structures of the motor system power structure for illustrative purposes; no actual mind section would certainly contain all of these structures. Move the cursor over each box on the right to to mark the inputs (blue) and outputs (red) of each region.


3.4 motor Cortex Cytoarchitecture

Like all parts of the neocortex, the main motor cortex is do of 6 layers (Figure 3.6). Unlike major sensory areas, main motor cortex is agranular cortex; the is, the does not have a cell-packed granular class (layer 4). Instead, the most distinctive class of major motor cortex is that descending calculation layer (Layer 5), which contains the giant Betz cells. This pyramidal cells and other forecast neurons the the primary motor cortex consist of ~30% the the fibers in the corticospinal tract. The remainder of the fibers come from the premotor cortex and also the supplementary engine area (~30%), the somatosensory cortex (~30%), and also the posterior parietal cortex (~10%).


Figure 3.6 Pyramidal and also non-pyramidal neurons in engine cortex. The cerebral cortex is organized into six layers. These layers contain different proportions of the two key classes of cortical neurons, pyramidal and non-pyramidal cells. Pyramidal cell send lengthy axons down the spinal cord and are the significant output neurons. They are abundant in great 5. Non-pyramidal cells have axons which end locally.


3.5 Encoding of movement by motor Cortex

Primary motor Cortex

As debated above, the primary motor cortex does no generally control individual muscle directly, however rather appears to manage individual activities or assignment of activities that require the activity of many muscle groups. Alpha engine neurons in the spinal cord, in turn, encode the pressure of convulsion of groups of muscle fibers utilizing the price code and the dimension principle. Thus, in accordance v the principle of ordered organization the the motor system, the info represented by motor cortex is a higher level that abstraction than the details represented by spinal engine neurons.

What is encoded through the neurons in main motor cortex? clues have come from record the activity of this neurons as experimental animals perform various motor tasks. In general, major motor cortex encodes the parameters that specify individual movements or basic movement sequences.

main motor cortex neurons fire 5-100 msec before the onset of a movement. Thus, rather than firing as the result of muscle activity, this neurons are involved in forward motor commands to the alpha engine neurons that eventually cause the suitable muscles to contract. primary motor cortex encodes the extent of movement. The shooting of some neurons is associated with the street of a movement. A monkey to be trained to relocate its eight to different target locations that differed in direction and also distance from the center. The firing of many neurons was associated with the direction of motion (as in point 3), whereas the shoot of other neurons was correlated with the street of the movement. Interestingly, some neurons were correlated with the interaction of a particular distance and also direction; the is, they were correlated with a certain target position. primary motor cortex neurons encode the rate of movement. Practically all targeted activities follow a usual bell-shaped curve of velocity as a duty of distance (Figure 3.9). Because that example, when the hand moves an item such together a coffee cup native one location to one more (the target), the hand accelerates during the first half the the movement, get a optimal velocity about halfway come the target, and then decelerates until it will the target. The firing price of some primary motor cortex neurons in chimpanzees correlates with this bell-shaped rate profile, demonstrating the information around movement rate is consisted of in the spike trains of these neurons.

Premotor Cortex

The premotor cortex sends axons to the primary motor cortex and also to the spinal cord directly. It performs an ext complex, task-related handling than major motor cortex. Stimulation that premotor areas in the monkey at a high level of present produces more complicated postures 보다 stimulation of the primary motor cortex. The premotor cortex appears to be connected in the choice of suitable motor plans for voluntary movements, vice versa, the main motor cortex is affiliated in the execution of these voluntary movements.

Premotor cortex signals correct and also incorrect actions. Human subjects to be studied in one fMRI experiment together they observed video clips of various correct and incorrect engine acts. A correct activity was one in which the movement and the connected object was performed correctly, such as setting the time on a clock. An object error was one in i m sorry the action was correct, yet the object to be incorrect, together as polishing a brown shoe v black shoe polish. A motion error to be one in i beg your pardon the object was correct, however the movement was incorrect, such together attempting to placed a coin right into a piggy bank when the coin was oriented perpendicular to the coin slot. In this experiment, the premotor cortex was caused bilaterally during the correct actions trials and the movement error trials; for the thing error trials, only the premotor cortex of the left hemisphere was triggered preferentially.

Supplementary motor Area

The supplementary engine area (SMA) is affiliated in programming facility sequences the movements and also coordinating bilateral movements. Whereas the premotor cortex appears to be associated in picking motor programs based on visual stimuli or on abstract associations, the supplementary engine area shows up to be connected in selecting movements based on remembered order of movements.

SMA is involved in the revolution of kinematic come dynamic information. Movements can be identified in terms of dynamics (the amount of force important to do a movement) and kinematics (the distance and also angles that specify a specific movement in space). Plenty of movement to plan are represented in kinematic state (e.g., relocate the hand come the left). However, the motor device must ultimately translate this to a representation based upon dynamics, in order to instruct the suitable muscles come contract through the proper force. Recordings from chimpanzees have presented that during the preparatory hold-up before a monkey provides an instructed movement, part SMA neurons change their firing correlates native a kinematic-based representation to a dynamics-based representation, arguing that SMA dram a an important role in this transformation.

Association Cortex

The 4th level that the motor pecking order is the association cortex, in details the prefrontal cortex and the posterior parietal cortex (Figure 3.14). These mind areas space not motor locations in the strict sense. Their task does not correlate exactly with individual motor acts, and also stimulation of these areas does not result in motor output. However, these locations are crucial to certain that motions are adaptive come the requirements of the organism and appropriate to the behavioral context.


Figure 3.14 combination cortex. The prefrontal cortex is emphasize on the left, and also the posterior parietal cortex is emphasize on the right.


Posterior parietal cortex is connected in ensuring that activities are target accurately to objects in exterior space. This area is connected in handling spatial relationships of objects in the world and in building a representation of external an are that is elevation of the observer’s eye place or body position. Such representations allow a stable percept the the world that is independent of viewer orientation, and also the depiction of preferred trajectories in space that room independent of human body position. Damage to the posterior parietal cortex can result in a variety of apraxias, that is, the inability to make complex, combination movements. Because that example, a patient v constructional apraxia is can not to replicate the configuration of a collection of blocks in the suitable sequence, even though the patient is able come maneuver each block individually with dexterity. Prefrontal cortex is associated in the choice of suitable actions because that a specific behavioral context. It is likewise involved in the evaluation of the after-effects of a details course that action. Patient with damages to the prefrontal cortex have troubles in executive processing. They do inappropriate behavioral decisions, and often cannot anticipate the likely after-effects of your actions. They display screen impulsive behavior, often showing an i can not qualify to delay instant gratification for a long-term larger reward.

Test Your understanding


Betz cells are most abundant in layer...

A. IV that somatosensory cortex.

B. V of somatosensory cortex.

C. IV of motor cortex.

D. V of engine cortex.

E. III of engine cortex.


Betz cells are many abundant in layer...

A. IV the somatosensory cortex. This price is INCORRECT.

Betz cells room not in somatosensory cortex.

B. V that somatosensory cortex.

C. IV of motor cortex.

D. V of motor cortex.

E. III of motor cortex.


Betz cell are many abundant in layer...

A. IV the somatosensory cortex.

B. V the somatosensory cortex. This prize is INCORRECT.

Betz cells room not in somatosensory cortex.

C. IV of motor cortex.

D. V of engine cortex.

E. III of motor cortex.


Betz cell are most abundant in layer...

A. IV the somatosensory cortex.

B. V of somatosensory cortex.

C. IV of engine cortex. This answer is INCORRECT.

Betz cells room not in great IV.

D. V of engine cortex.

E. III of motor cortex.


Betz cell are many abundant in layer...

A. IV the somatosensory cortex.

B. V that somatosensory cortex.

C. IV of motor cortex.

D. V of motor cortex. This prize is CORRECT!

E. III of engine cortex.


Betz cells are many abundant in layer...

A. IV the somatosensory cortex.

B. V the somatosensory cortex.

C. IV of engine cortex.

D. V of motor cortex.

E. III of motor cortex. This prize is INCORRECT.

Betz cells room not in class III.


A corticospinal neuron in major motor cortex deserve to do every one of the adhering to EXCEPT:

A. Task to multiple motor neuron pools in the spinal cord.

B. Get involved in the initiation the movement.

C. Password for the lot of force of separation, personal, instance muscles. This price is CORRECT!

This is a FALSE statement. Engine cortex neurons password for the force of individual movements, no individual muscles. Lower motor neurons (alpha motor neurons) encode the force of separation, personal, instance muscles.

D. Code for the direction the movement.

E. Code for the level of movement.


A corticospinal neuron in major motor cortex can do all of the following EXCEPT:

A. Job to multiple engine neuron pools in the spinal cord.

B. Take part in the initiation the movement.

C. Code for the amount of force of individual muscles.

D. Code for the direction of movement. This price is INCORRECT.

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This is a TRUE statement.

E. Code for the extent of movement.


A corticospinal neuron in main motor cortex have the right to do all of the adhering to EXCEPT: