Neuroanatomy IV: Movement

Everything we do is linked in with our brain, for it is a series of complex brain processes which enable us our very being. Of course the process requires more than the brain for us to move, we must have muscles and bones as working components also. There are three muscle types which should be taken into account: skeletal, cardiac and smooth. If I were to mention to you then that the smooth muscles are the ones coating many organs such as the stomach and that cardiac muscles surrounded and made up the heart, the function of skeletal muscles may then seem rather obvious. These muscles help us to move.

The process of movement occurs when the neurons (leading from the brain) attach to the skeletal muscles. These neurons are known as motor neurons. These come in varying types (alpha, gamma etc – dependent on type of neurotransmitter released. When the neuron is activated, signal is sent down the axon of the neuron and consequently – eventually – reaches the terminal buttons were a synapse is made with the muscle and thus muscle contraction (and thus movement) are possible. Of course, the strength and frequency of neuron firing will impact the level of movement.

The communication process which occurs between the brain and the muscles is a little more complex than this information must be sent back to the brain to adjust ones muscle movement – else we would be in a state of permanent tension caused by muscle contractions. Many of you will likely be aware that for muscles to work their function(s) – one of which is making the bones move and joints work - then they must somehow be connected to the skeleton. This is done through tendons, these tendons are one of two key messenger pathways called proprioceptors - the other is the spindles inside the muscles. These messengers register the level of muscle contraction (and the intensity) and send messages back through the central nervous system to the brain where communication is consequently processed and sent back, changes in muscle tension then happen.

It is the spindles within the muscles which generate action potentials which communicate via the spinal cord (sensory neurons) and synapse with the motor neurons to create muscle contractions. This is then called the monosynaptic pathway – singular pathway. Another form of communication between the brain and the muscles is the polysynaptic reflexes, these then use more than one pathways to communicate with the brain – such as pain. When the stimulus causing reaction it causes firing of several neurons and leads to eventual communication with the brain and the registration of pain.

The cortex of the brain seems the most likely area of processing for movement. Specifically it seems to be related to pre-frontal areas of the cortex. These cortical areas are; posterior parietal association cortex, dorsolateral prefrontal association cortex and secondary motor cortex. Minor neurons (neurons which fire both upon performing a movement and watching someone perform a movement) are found in all three of those areas and are believed to be linked to learning and facial expression.

The primary motor cortex appears to be located in the precentral gyrus (located in the frontal lobe) and explanations of this structure are as follows. The precentral gyrus appears to have areas of its surface (cortex) designated for controlling different body parts (this has been tested through electrical stimulation of these specific areas and watching body reactions) and so each movement then shows signal in that area – if we were to see it on an fMRI scan for example. The more complexity required for moving that area of the body the larger the designated area on the cortex of the precentral gyrus.

Of course, it is then obvious that in cases of paralysis or deviations from regular body movement, it is clear that damage has occurred to the neuronal pathways or they have indeed been severed and consequently due to lack of or no communication with the brain, the area cannot move properly or in some cases at all.

There are four main descending pathways (from brain to body) which should be mentioned when it comes to understanding movement. The first two are known as dorsolateral pathways, the second two as ventromedial pathways.

The dorsolateral corticospinal tract starts in the primary motor cortex and is made up mainly of betz cells (pyramidal neurons), it communicates with the medulla, spinal cord (and white matter) and finally signals to sensory and motor neurons which synapse with muscles. These muscles are specifically those of the wrists, hands, fingers and toes.

The dorsolateral corticorubrospinal tract once again starts in the primary motor cortex, communicates through the red nucleus with the medulla and then the cranial nerves in order to control facial movement. However, this tract divides and some of the neurons continue and communicate with the interneurons of the spinal cord and consequently go on to control the muscles of the arms and legs.

The ventromedial corticospinal tract these communicate via interneuronal circuits and control the torso and any proximal limbs whilst the ventromedial cortico-brainstem-spinal tract communicates from the primary motor cortex to the brain stem and further descends the spinal cord in a bilateral fashion. This then communicates with all spinal cord segments and activates both sides of the torso of the body.

The cerebellum has many a role to play in movement and coordination, specifically it is responsible for timing and processing of movement information.