Which part of the brain is responsible for planning and initiating movements?

The neural circuits responsible for the control of movement can be divided into four distinct but highly interactive subsystems, each of which makes a unique contribution to motor control (). The first of these subsystems is the local circuitry within the gray matter of the spinal cord and the analogous circuitry in the brainstem. The relevant cells include the lower motor neurons (which send their axons out of the brainstem and spinal cord to innervate the skeletal muscles of the head and body, respectively) and the local circuit neurons (which are the major source of synaptic input to the lower motor neurons). All commands for movement, whether reflexive or voluntary, are ultimately conveyed to the muscles by the activity of the lower motor neurons; thus they comprise, in the words of the great British neurophysiologist Charles Sherrington, the “final common path” for movement. The local circuit neurons receive sensory inputs as well as descending projections from higher centers. Thus, the circuits they form provide much of the coordination between different muscle groups that is essential for organized movement. Even after the spinal cord is disconnected from the brain in an experimental animal such as a cat, appropriate stimulation of local spinal circuits elicits involuntary but highly coordinated movements of the four limbs that resemble walking.

Figure 16.1

Overall organization of neural structures involved in the control of movement. Four systems—local spinal cord and brainstem circuits, descending modulatory pathways, the basal ganglia, and the cerebellum—make essential and distinct contributions (more...)

The second motor subsystem consists of neurons whose cell bodies lie in the brainstem or cerebral cortex. The axons of these higher-order or upper motor neurons descend to synapse with the local circuit neurons or, more rarely, with the lower motor neurons directly. The upper motor neuron pathways that arise in the cortex are essential for the initiation of voluntary movements and for complex temporal sequences of movement. In particular, descending projections from cortical areas in the frontal lobe, including Brodmann's area 4 (the primary motor cortex), the lateral part of area 6 (the lateral premotor cortex), and the medial part of area 6 (the medial premotor cortex) are essential for planning, initiating, and directing temporal sequences of voluntary movements. Upper motor neurons originating in the brainstem are responsible for regulating muscle tone and for orienting the eyes, head, and body with respect to vestibular, somatic, auditory, and visual sensory information. Their contributions are thus critical for basic navigational movements of the body, and in the control of posture.

The third and fourth subsystems are structures (or groups of structures) that have no direct access to either the local circuit neurons or the lower motor neurons; instead, they control movement by regulating the activity of the upper motor neurons. The third and larger of these subsystems, the cerebellum, is located on the dorsal surface of the pons (see Chapter 1). The cerebellum acts via its efferent pathways to the upper motor neurons as a servomechanism, detecting the difference, or “motor error,” between an intended movement and the movement actually performed (see Chapter 19). The cerebellum uses this information about discrepancies to mediate both real-time and long-term reductions in these motor errors (the latter being a form of motor learning). As might be expected from this account, patients with cerebellar damage exhibit persistent errors in movement. The fourth subsystem, embedded in the depths of the forebrain, consists of a group of structures collectively referred to as the basal ganglia (see Chapter 1). The basal ganglia suppress unwanted movements and prepare (or “prime”) upper motor neuron circuits for the initiation of movements. The problems associated with disorders of basal ganglia, such as Parkinson's disease and Huntington's disease, attest to the importance of this complex in the initiation of voluntary movements (see Chapter 18).

Despite much effort, the sequence of events that leads from thought to movement is still poorly understood. The picture is clearest, however, at the level of control of the muscles themselves. It therefore makes sense to begin an account of motor behavior by considering the anatomical and physiological relationships between lower motor neurons and the muscle fibers they innervate.

The brain is key to our existence, but there’s a long way to go before neuroscience can truly capture its staggering capacity. For now though, our Brain Control series explores what we do know about the brain’s command of six central functions: language, mood, memory, vision, personality and motor skills – and what happens when things go wrong.

Having voluntary control over body movements is the only way we can interact with people, objects and our environment. Body movement is not just about controlling arms and legs; it’s also for our head and eyes to visually explore the world, for our facial expressions to show emotion, and for articulation of our lips, tongue and mouth to communicate.

Further reading: What brain regions control our language? And how do we know this?

The devastating effects of the brain losing its ability to control body movements are seen in motor neuron disease – where progressive degeneration and muscle wasting leads to some patients becoming “locked-in”, meaning they can’t move or communicate in any way.

The motor system and primary motor cortex

The brain’s motor system is contained mostly in the frontal lobes. It starts with premotor areas, for planning and coordinating complex movements, and ends with the primary motor cortex, where the final output is sent down the spinal cord to cause contraction and movement of specific muscles.

The primary motor cortex on the left side of the brain controls movement of the right side of the body, and vice-versa, the right motor cortex controls movement of the left side of the body.

Different areas of the primary motor cortex connect to, and control, movement of different parts of the body, forming a kind of body map known as the homunculus.

The size of the area on the homunculus determines the level of fine movement control we have with that part of the body. So, for instance, a large proportion of the motor cortex is devoted to our thumb, fingers, mouth and lips, as they are vital for manipulating objects and speech articulation.

The connection from the primary motor cortex to muscles of the body is so important that any damage leads to an impaired ability to move. If someone suffers a stroke, for instance, that causes damage to the primary motor cortex on one side of their brain, they will develop an impaired ability to move on the opposite side of their body.

Further reading: Some people can’t see, but still think they can: here’s how the brain controls our vision.

If the area of damage is specific to only part of the primary motor cortex, such as the hand area of the homunculus, it will affect movements only of the corresponding part of their body, for example, the hand.

Neuroplasticity and movement rehabilitation

As with other parts of the brain, when neurons of the primary motor cortex are damaged they will never regrow or repair. However, the brain can heal itself and regain some lost function through neuroplasticity. This means undamaged parts can change their connections and remap to other areas of the body to take over function, compensating for damaged parts of the motor cortex.

Neuroplasticity is the fundamental principle in physical rehabilitation, such as physiotherapy for patients following stroke, that allows patients to regain motor function and recover. Through neuroplasticity, the more a particular movement is performed, the stronger the brain pathways for that movement become and the easier it gets to perform that movement in the future.

Let’s look at an example of a stroke patient, Harry, who has problems with movement in his left leg. Harry might have altered patterns of walking due to damage in the leg area of the motor cortex of the right side of his brain. To help Harry regain efficient walking ability, the physiotherapist helps him perform sequences or patterns of walking by practising activation and control of specific muscle groups in his left leg.

Further reading: We’re capable of infinite memory, but where in the brain is it stored, and what parts help retrieve it?

At first, Harry will need lots of concentration to use the correct muscles as his brain is laying down new neural pathways to compensate for the damaged areas. But as this practice is repeated and the new pathways are established and strengthened, correct movement becomes easier without much concentration.

This same principle of neuroplasticity also applies for learning in the healthy brain. Anytime we acquire a new skill such as learning to ride a bike, writing our signature or dancing the tango, it’s our brain’s ability to strengthen or make new connections to adapt and change that allows us to learn.

So if you are a ballet dancer or a gymnast, a swimmer or a soccer player, a watch-maker or micro-surgeon, your brain connections in your motor system will be different depending on the practice and skill you have with fine movement of different parts of your body.

This article was co-written with Zita Arends, who is a physiotherapist in stroke rehabilitation and aged care.

Which part of the brain is responsible for planning movements?

Which part of the brain is associated with planning selecting and initiating muscle movements?

Which part of the brain is responsible for planning and initiating movements quizlet?

What is responsible for initiating movement?