The ability to move is considered a key feature of what allows us to describe organisms as living; even microscopic organisms like bacteria have the ability to undertake rather primitive movements.
However, what is clear is that with evolution come advances in the complexity of movements that an organism can perform. In an advanced mammal like a human a variety of parts of the nervous system, including the brain are involved in movement control, albeit different elements of movement.
Some movements appear to occur in response to something specific. So, for example, an organism like a slug may be able to move towards your prized cabbages because it can detect some chemical being given off by the cabbage and this is enough to trigger it to move towards them. Conversely, if you find a snail munching on your lettuces, if you pick it off, the body of the snail will recoil into the shell, moving away from the stimulus. In simple animals like snails, all of their movements are simple and predictable, these are known as reflexes.
Humans also use reflexes for some movements where, for example, we don’t want or need conscious thought. If you put your hand onto a damagingly hot surface you may rapidly withdraw it using a reflex action – you do not have to consciously think about moving your hand. This is the human equivalent of the snail shrinking back into its snail and represents an evolutionarily ancient form of movement.
Our understanding of the biology of these reflexes in humans indicates that, in many cases, the brain is not directly involved; the reflex works using sensory neurones in the body, which detect the damaging heat, neurones in the spinal cord which bring information relating to the sense together and then motor neurones which control your muscles to move your hand away.
The brain and movement
As organisms became more complex, they evolved to also have brains to control the increasingly specialised parts of their bodies. This included having regions of the brain that had an input into movement control. These individual areas may have somewhat different functions in relation to movement.
Some are able to refine the reflex responses and enable the body to learn particular patterns of movement behaviour – for example, you have to learn to ride a bicycle by practicing. However, even if you do not then ride a bicycle for several decades, you are still able to pick it up again very quickly. This is because your brain learned the motor sequence and response originally and that information is still encoded in its synapses and circuits, it just needs a bit of refinement.
Other bits of the brain are able to initiate movement in the absence of a specific prompt to do so. For example, when you decide that you want to go to the shop to buy something there does not have to be an immediate external sensory cue for you to do this. We can think of these areas as responding to internally generated cues for activity. In the video of the brain model we met some of the key structures involved in all of these elements of movement control, although, of course, the reality is that there are many other regions beyond these that are involved in human movement.
In the previous video we saw the locations of:
- the cerebellum
- the motor cortex
- the group of structures collectively known as the basal ganglia.
Look up each of these structures, either using an internet search engine or a book, and find out:
1. What element of movement they are responsible for.
2. How human movement is affected if one of these areas is damaged or diseased.
“For the body to make any given gesture, the sequence and duration of each of the basic movements of each body segment involved must be controlled in a very precise manner. One of the cerebellum’s jobs is to provide this control over the timing of the body’s movements. It does so by means of a loop circuit that connects it to the motor cortex and modulates the signals that the motor cortex sends to the motor neurons.
In humans, the cerebellum also plays a role in analyzing the visual signals associated with movement. These signals may come either from the movement of objects within the field of vision or from the sight of the moving body segments themselves. The cerebellum appears to calculate the speed of these movements and adjust the motor commands accordingly. Errors in such calculations largely account for the poor motor control observed in patients who have suffered injuries to the cerebellum.
As regards cognitive impairments, some signs of cerebellar involvement have been found in the areas of language, attention, memory, and emotions. For example, in some autistic children, cognitive delays have been partly attributed to insufficient development of certain parts of the cerebellum.
Lastly, another important property of the cerebellum is its ability to learn and remember, which is based, among other things, on the distinctive cell architecture of the cerebellar cortex.
Cerebellar syndrome is the term used to designate manifestations of damage to the cerebellum, regardless of origin. For example, if a patient with cerebellar syndrome tries to touch an object, the movement of his hand will begin late, then accelerate beyond what is normal. Braking also will be too late, and inefficient, so that his hand ends up missing the object and going past it. This movement then ends with oscillations of the hand and arm.
People with cerebellar syndrome also appear to have some problems in co-ordinating balance and posture. These people have an uncertain gait, spreading their feet more widely apart as they strike the ground. If these people are jostled, the reflexes that compensate for the imbalance overreact, often resulting in oscillations of the entire body. These people also cannot tilt their trunks forward or backward without losing their balance.”
“Motor cortex is the region of the cerebral cortex involved in the planning, control, and execution of voluntary movements.”
“The motor cortex can be divided into several main parts:
- the primary motor cortex is the main contributor to generating neural impulses that pass down to the spinal cord and control the execution of movement. However, some of the other motor cortical fields also play a role in this function.
- the premotor cortex is responsible for some aspects of motor control, possibly including the preparation for movement, the sensory guidance of movement, the spatial guidance of reaching, or the direct control of some movements with an emphasis on control of proximal and trunk muscles of the body.
- the supplementary motor area (or SMA), has many proposed functions including the internally generated planning of movement, the planning of sequences of movement, and the coordination of the two sides of the body such as in bi-manual coordination.
- The posterior parietal cortex is sometimes also considered to be part of the group of motor cortical areas. It is thought to be responsible for transforming multisensory information into motor commands, and to be responsible for some aspects of motor planning, in addition to many other functions that may not be motor related.
- The primary somatosensory cortex, especially the part called area 3a, which lies directly against the motor cortex, is sometimes considered to be functionally part of the motor control circuitry.”
and/or in a bit more detail/a few more words:
“The anatomical region of the brain known as Area 4 was given the name primary motor cortex (symbol: M1) after Penfield showed that focal stimulations in this region elicited highly localized muscle contractions at various locations in the body. This mapping is represented somatotopically on the motor cortex, where the surface area devoted to controlling the movements of each body part varies in direct proportion to the precision of the movements that can be made by that part.
The motor cortex also includes Area 6, which lies rostrally to Area 4 and is divided into the premotor area (or premotor cortex) and the supplementary motor area. The premotor cortex is believed to help regulate posture by dictating an optimal position to the motor cortex for any given movement. The supplementary motor area, for its part, seems to influence the planning and initiation of movements on the basis of past experience. The mere anticipation of a movement triggers neural transmissions in the supplementary motor area.
Besides the frontal cortex, the posterior parietal cortex clearly plays a role in voluntary movements, by assessing the context in which they are being made. The parietal cortex receives somatosensory, proprioreceptive, and visual inputs, then uses them to determine such things as the positions of the body and the target in space. It thereby produces internal models of the movement to be made, prior to the involvement of the premotor and motor cortices.
Within the posterior parietal cortex, two particular areas are distinguished. Area 5 receives information from somatosensory areas 1, 2, and 3 of the cortex. Area 7 further integrates the already highly integrated signals from the visual areas of the cortex, such as MT and V5.
The parietal lobes are themselves closely interconnected with the prefrontal areas, and together these two regions represent the highest level of integration in the motor control hierarchy. It is here that the decisions are made about what action to take. The posterior parietal and prefrontal areas send their axons to Area 6 which, once it has been informed about the kind of action to take, helps to determine the characteristics of the appropriate movement for this purpose.”
“Every part of the body is represented in the primary motor cortex, and these representations are arranged somatotopically — 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.”
seems from this that planning, preparation and control of voluntary movement in any/every part of the body would/could be affected by damage to the motor cortex.
that the areas of the body can be mapped onto the motor cortex suggests that discrete areas of damage might produce effects on only some body movements?
“Basal ganglia are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia is associated with a variety of functions including: control of voluntary motor movements, procedural learning, routine behaviors or “habits” such as bruxism, eye movements, cognition and emotion.
Currently popular theories implicate the basal ganglia primarily in action selection; that is, it helps determine the decision of which of several possible behaviors to execute at any given time. In more specific terms, the basal ganglia’s primary function is likely to control and regulate activities of the motor and premotor cortical areas so that voluntary movements can be performed smoothly. Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The “behavior switching” that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.“
“While their exact motor function is still debated, the basal ganglia clearly
regulate movement. Without information from the basal ganglia, the cortex is unable to properly direct motor control, and the deficits seen in Parkinson’s and Huntington’s disease and related movement disorders become apparent.”
the basal ganglia appear key to what’s happening in Parkinson’s disease.
some useful web sources/resources:
http://www.neuroanatomy.wisc.edu/coursebook/motor2.pdf (PDF document)
motor system more generally, and/or parkinson’s disease more generally: