Video: Cortical and brainstem control of motor functions

When was the last time I watered that plant? Probably too long ago.

Seems easy, right? However, even this simple everyday task actually engaged several regions of our brain and spinal cord. These areas work in harmony to plan, execute, and refine movement. So let's explore together how our central nervous system makes us move.

When we decide to move, the processes in our nervous system can be broadly split into three phases. One is motor planning. Several brain areas, including the secondary motor areas, the sensory cortices, the basal ganglia, and the cerebellum, cooperate to create an appropriate motor plan. Another one is movement execution. Neurons transmit motor commands from the motor cortex to our spinal cord and then muscles. This makes them contract to create movement.

And finally, movement monitoring. This is mainly the responsibility of the cerebellum, which checks if we are moving as intended and corrects the motor commands if necessary. These processes often occur simultaneously as we plan, execute, and monitor different movements at the same time.

Now that we've seen an overview of the process, let's take a look at each step, starting with movement planning and initiation.

When we decide to move, the cortical areas responsible for self-initiated voluntary tasks, like the prefrontal cortex, communicate our intention to move to the secondary motor cortices in the frontal cortex. For most movements, the premotor and the supplementary motor areas sketch out an initial motor plan.

The exact role of these two areas is not fully known yet. Researchers believe that the premotor area is mainly responsible for planning movements based on sensory stimuli from the environment. In contrast, the supplementary motor area helps to plan learned motor sequences, bilateral actions, and body fixation.

Some cortical areas are even more specialized. For instance, the frontal eye field plans eye movements, and Broca's area is involved in speech production. The secondary motor cortices communicate with several brain regions to refine the motor plan.

To define the most appropriate motor plan, the secondary motor cortices communicate with the sensory cortex, especially the posterior parietal cortex, to gather information about the current body position and the environment. The secondary motor cortices also communicate with the lateral portions of the cerebellar lobes, the cerebrocerebellum. This region fine-tunes aspects of the motor plan, like the planning and timing of sequential movements, and sends information back to the secondary motor cortices.

The secondary motor cortices also communicate with the basal ganglia, or basal nuclei, which further revise the motor program and send it back to the motor cortex. The basal ganglia help modulate posture, muscle tone, reflexes, and other automated skilled movements. Input from the basal ganglia is especially important for movement initiation and suppression. But how is this achieved?

The basal ganglia include the caudate, the putamen, and the globus pallidus. Other structures important for the basal ganglia loop are the subthalamic nucleus and the substantia nigra. The basal ganglia project to the motor nuclei of the thalamus, which regulate the flow of information to the motor cortex. This pathway is particularly important to choose which movements are executed and which are suppressed.

At rest, when no movement is intended, the globus pallidus is active. This keeps the motor nuclei of the thalamus inhibited. Since the motor information is not communicated to the motor cortex, movement is suppressed.

Upon receiving motor information from the secondary motor cortices, the basal ganglia select which movement should start through different pathways. The direct pathway stops inhibiting the thalamic regions required for the intended movement. Because of this, the thalamus gives the green light to the motor cortex to start the intended movement. At the same time, the indirect and hyperdirect pathways stimulate different regions of the globus pallidus, further inhibiting the thalamus and the motor cortex. This ensures that movements not part of the motor plan are avoided.

There's much more to say about the anatomy and pathways of the basal ganglia. If you want to know more, check out our basal ganglia video and the illustrations in the study unit.

Now that the brain has finalized the motor plan, it's time to move.

The motor commands are relayed to the muscles by descending pathways that consist of two or more motor neurons. The upper motor neurons originate from the motor cortex and from the brainstem and carry the motor information to motor neurons located at specific levels of the spinal cord or brainstem. These lower motor neurons travel in the peripheral nervous system to synapse with muscle fibers and are therefore able to make the muscles contract.

The descending pathways are classified into the pyramidal tracts, which travel through the pyramids in the medulla, and the extrapyramidal tracts. Importantly, these tracts contribute to different motor functions.

The pyramidal tracts are primarily responsible for voluntary movements. These tracts originate from the cerebral cortex, especially the primary motor cortex located in the precentral gyrus of the frontal lobe. The movements of different body regions are represented in specific areas of the primary motor cortex, creating a somatotopic representation known as the motor homunculus. Skilled motor actions, like phonation and manual dexterity, have the largest representation.

The motor neurons responsible for head and neck movements originate from the lateral region of the primary motor cortex and descend as the corticobulbar pathway, also called the corticonuclear pathway. These upper motor neurons synapse with mixed and motor cranial nerve nuclei in the brainstem and innervate muscles involved in functions like phonation, facial expressions, and head motion.

Motor neurons that originate from the other regions of the primary motor cortex instead join the corticospinal tract, which innervates the muscles below the neck. An important feature of this pathway is that about 80 percent of the axons of the upper motor neuron decussate in the pyramids of the medulla, forming the lateral corticospinal tract. This tract travels in the contralateral spinal cord to synapse with the lower motor neuron in the anterior horn of the spinal cord. The lateral corticospinal tract mainly innervates limb muscles and is especially important for skilled movements.

The neurons that don't decussate at the pyramids travel instead in the ipsilateral spinal cord and decussate when they reach the spinal level of the lower motor neuron. These neurons form the anterior corticospinal tract, and mainly innervate axial muscles.

The tracts that don't travel through the pyramids are called extrapyramidal. These pathways originate from the brainstem and regulate mainly involuntary muscle activity like breathing, posture, and balance. Several structures can modulate the activation of motor neurons in the extrapyramidal pathways, especially the cortex, the basal ganglia loop, and the cerebellum.

There are four main extrapyramidal tracts. The rubrospinal tract originates from the red nucleus in the midbrain and travels in the lateral funiculus of the spinal cord. The function of this tract is unclear, but it is thought it may help to regulate the tone of the upper limb muscles and modulate movement based on inputs from the cerebellum.

The tectospinal tract originates from the superior colliculus of the midbrain and travels in the ventral funiculus of the spinal cord. It modulates head and eye movements in response to visual and auditory stimuli.

The reticulospinal tracts are made up of neurons from the pontine and medullary portions of the reticular formation, and descend as the anterior and lateral reticulospinal tract respectively. These tracts regulate a wide variety of functions including posture, locomotion, reflexes, and autonomic functions.

The vestibulospinal tract originates from the vestibular nuclei in the pons and medulla oblongata. The lateral vestibulospinal tract travels in the ventral region of the spinal cord. The medial vestibulospinal tract instead descends in the medial longitudinal fasciculus and mainly projects to the neck muscles. Together, these tracts help to integrate vestibular information into the motor function, and are therefore important for balance and posture.

So, we've planned and executed our movement and we're finally watering our plant! Time for the central nervous system to rest? Not quite. What if other factors introduce unexpected challenges? To account for this, our nervous system continuously checks if the movement performed is in line with the motor plan and corrects it when necessary. Let's talk now about movement monitoring.

The most important structure in this context is the cerebellum. Functionally, the cerebellum can be split into three regions. The lateral regions of the cerebellar lobes are the cerebrocerebellum and are mainly involved with motor planning as we discussed earlier. The vermis and the medial regions of the cerebellar lobes constitutes the spinocerebellum, which monitors intended and actual movement to revise the motor output. And the flocculonodular lobe and the nearby regions of the vermis are part of the vestibulocerebellum. This region mainly contributes to dynamic equilibrium.

To monitor movement execution, the cerebellum receives afferent information from different structures: motor commands from the descending pathways, equilibrium information from the vestibular system, and sensory information from peripheral receptors, like proprioceptive information through the spinocerebellar tract.

The cerebellum compares the descending motor commands with the sensory feedback. If there is a mismatch between how we intend to move and the actual movement, the cerebellum corrects movement via projections to the descending pathways, ensuring that the movement occurs as planned. This is what happens when everything works as it should. But what happens when one of these structures is damaged?

When the corticospinal tracts are lesioned, the transmission of motor commands from the brain to the muscles is interrupted. The specific motor deficit depends on the location of the lesion.

Lesions of the lower motor neuron, for instance due to a complete nerve injury, result in flaccid paralysis. This means that the muscle is unable to contract because it receives no motor commands. Reflexes are also abolished or greatly diminished.

Lesions of the upper motor neuron, for example due to stroke or spinal cord injuries, result in spastic paralysis. The muscle is still unable to contract on command, but the lack of descending modulation of the brain makes the spinal reflexes hyperexcitable, increasing the muscle tone.

Importantly, the location of the lesion also determines which side of the body is affected. Lesions of the corticospinal tract above the medulla result in mostly contralateral motor deficits. Instead, lesions after the decussation, for instance in the case of a unilateral spinal cord injury, result in motor deficits localized to the same side of the body.

Degeneration of the basal ganglia and associated structures often results in a wide variety of symptoms, including difficulties in initiating voluntary movements, tremor, and rigidity. One example is Parkinson's disease, where a deficit in dopamine due to degeneration of the substantia nigra results in reduced ability to initiate movements and tremor at rest.

Damage to the cerebellum also influences different aspects of movement. Movements often become slow and inaccurate, and are accompanied by tremor. Balance deficits are also observed, especially following damage to the vestibulocerebellum.

And that concludes our tutorial on the cortical and brainstem control of movement. Next time you perform a simple task like watering a plant, take a moment to appreciate the unbelievable coordination behind it.

Be sure to check out the quiz and other learning materials in the study unit on this topic. See you next time!