Do impulses travel through parts of the brain in a specific order when coordinating a voluntary movement?
Answer by Subhadeep Dutta Gupta:
I will start my answer with the easiest and fundamental question of neuroscience: Why do we and other animals have brains? Not all species on our planet have brains.
Now you all may reason that we have a brain to perceive the world or to think. NO!!!
We have a brain for one reason and one reason only, and that’s to produce adaptable and complex movements. There is no other reason to have a brain. Movement is the only way we have of affecting the world around you (apart from sweating and farting).
Voluntary movements are initiated to accomplish a specific goal- picking up a ball, putting on the brakes when the traffic light turn red.
Step 1: Visualizing and perceiving the target object:
Take for example: You want to lift a ball. The photic stimuli from the ball will reach the occipital (visual) cortex via thalamus (lateral and medial geniculate body).
Step 2: From the visual cortex to the parietal cortex:
Two Stream Hypothesis by Milner and Goodale:
The ventral stream originates in primary visual cortex and extends along the ventral surface into the temporal cortex; the dorsal stream also arises in primary visual cortex, but continues along the dorsal surface into parietal cortex. The ventral stream (or “vision-for-perception” pathway) is believed to mainly subserve recognition and discrimination of visual shapes and objects, whereas the dorsal stream (or “vision-for-action” pathway) has been primarily associated with visually guided reaching and grasping based on the moment-to-moment analysis of the spatial location, shape, and orientation of objects.
Dorsal Stream- Vision for Action System: Dorsal stream delivers information directly to the motor system for immediate reaching, grasping or eye movements.
Ventral Stream- Vision for Perception System: Ventral stream is dedicated to a role in movement planning based on memory of an object and its relationship to other items.
These two streams are bidirectionally interconnected with each other.
Step 3: From parietal cortex to the motor cortex:
Figure: Four functionally discrete areas in the intraparietal sulcus project to areas in the premotor cortex. The medial intraparietal cortex(MIP) represents targets of arm movements and projects to the arm-control area of F2 in the dorsal premotor area (PMd). The lateral intraparietal cortex(LIP) represents targets of eye movements and projects to the frontal eye feld (FEF). The anterior intra parietal cortex (AIP) represents targets for grasping and projects to the hand-control area of F5 in the ventral premotor area (PMv). The ventral intra parietal cortex (VIP) represents the face and projects to the face-control area of F4 in the ventral premotor area.
Vision interacts with the supplementary and premotor systems to prepare the hands for action. When we pick up a pencil our fingers are separated from our thumb by the width of a pencil; when we pick up a drink our fingers are separated from our thumb by the width of the glass. The visual system helps to adjust the grip width before our hand arrives at the object. Similarly, when we insert a letter into a mail slot our hand is aligned to place the letter in the slot. If the slot is tilted, our hand tilts to match the orientation.
Patients with lesions of the parietal cortex cannot adjust their grip width or wrist angle using visual information alone, even though they can verbally describe the size of the object or the orientation of the slot.
The anterior intra parietal cortex has neurons that signal the size, depth, and orientation of objects that can be grasped. Neurons in this area respond to stimuli that could be the targets for a grasping movement, and also respond when the animal makes the movement. Similarly, neurons in the medial intra parietal cortex represent the targets for reaching movements and project to the frontal area that generates the premotor signal for these movements.
The posterior parietal 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.
Step 4: Motor cortex and initiation of movement:
Figure: Organization of the motor cortex
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 brain stem motor regions.
Step 5: Ventral Stream and its role in movement:
On the other hand, the ventral stream follows the following circuitry:
The occipital cortex has strong connections to the medial and inferior temporal lobe (which stores long term memories) and the limbic system (which controls emotions). This pathway provides emotional valence to the movement. From the limbic system, fibres project to prefrontal cortex and terminate to motor cortex as shown below:
Figure: Connections from prefrontal cortex to motor cortex. Prefrontal cortex receives information from limbic, medial and inferior temporal cortices.
Figure: Circuitry connecting visual cortex, parietal cortex, temporal cortex and premotor cortex.
Step 6: Flow of final motor information from the motor cortex to the cerebellum and spinal cord:
Neurons in primary motor cortex, supplementary motor cortex and premotor cortex give rise to the fibers of the corticospinal tract. 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 brain stem 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 subcortical 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.
Figure: Pyramidal (Corticospinal) and Extrapyramidal tracts facilitating voluntary movements
Step 7: Basal Ganglia: Role in gating the initiation of voluntary movement:
The basal ganglia is involved in the enabling of practiced motor acts and in gating the initiation of voluntary movements by modulating motor programs stored in the motor cortex and elsewhere in the motor hierarchy.
Figure: The basal ganglia and motor cortex form a processing loop whereby the basal ganglia enables the proper motor program stored in motor cortex circuits via the direct pathway and inhibits competing motor programs via the indirect pathway. The proper motor programs are selected based on the desired motor output relayed from cortex.
Thus, voluntary movements are not initiated in the basal ganglia (they are initiated in the cortex); however, proper functioning of the basal ganglia appears to be necessary in order for the motor cortex to relay the appropriate motor commands to the lower levels of the hierarchy.
Step 8: Cerebellum and ‘fine tuning’ of voluntary movement:
Most movements are composed of a number of different muscle groups acting together in a temporally coordinated fashion. One major function of the cerebellum is to coordinate the timing and force of these different muscle groups to produce fluid limb or body movements. The cerebellum modifies the motor commands of the descending pathways to make movements more adaptive and accurate. It receives extensive sensory input, and it appears to use this input to guide movements in both a feedback and feedforward control manner.
The cerebellum creates a blue print of the best possible way the cerebral cortex can initiate an accurate and precise movement. Once the cerebellum creates this blue print, it sends it to the thalamus and from there the signal is sent to the cortex and a coordinated stimulus is sent to a specific location on the body.
Figure: A model of feedback control system
Thus, there is a specific sequence of impulses in the brain in mediating voluntary movement.
- Principles of Neural Science, 5th Edition
- The Anatomy of Movement by Susan Schwerin