Neuroscience: A Journey Through the Brain

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The Major Systems: Motor Systems

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The control of movement can be achieved at one of two levels: the spinal cord, or the brain.

Spinal Control of Movement (back to top)

Ever heard the expression "Running like a chicken with its head cut off"? Complex patterns of movement can be generated in the spinal cord, without descending input from higher brain centers.

Deep within skeletal muscles are receptors called muscle spindles. Sensory Ia axons wrap themselves around this spindle, and go on to form synapses with motor neurons and interneurons in the ventral horns of the spinal cord. This sensory input to the spinal cord allows feedback from muscles to modify the motor signals sent to them. 

For example, the myotatic reflex occurs when a muscle contracts after being pulled on. If the sensory neurons are cut, this reflex does not occur.

The muscle fibers inside the spindles are called intrafusal fibers, while the rest of the muscle fibers (which form the bulk of the muscle in the body) are called extrafusal fibers. Intrafusal fibers receive input from gamma motor neurons in the spinal cord. Activity in gamma motor neurons forms part of another feedback control loop to control contraction of muscles.

Also in skeletal muscles are Golgi tendon organs, which send activity back to the spinal cord via Ib sensory neurons. These sensory neurons form connections with inhibitory interneurons in the spinal cord to control reverse myotatic reflexes

For example, this reflex causes muscles to relax at a certain level of tension in the contractors, preventing overload of the muscle. Interneurons also generate flexor reflex and the crossed extensor reflex. The flexor reflex  is used to withdraw your limb from an aversive stimulus. The crossed extensor reflex is used to compensate for the extra load imposed by limb withdrawal on anti-gravity muscles, such as those in the leg.

Within the spinal cord are also contained circuits that give rise to rhythmic motor activity - central pattern generators. The circuit is not clearly established yet, but may involve steady input to two excitatory interneurons that connect to motor neurons controlling both flexion and extension.

Brain Control of Movement (back to top)

The central motor system is arranged as a hierarchy, normally split into three levels.

Descending Spinal Tracts (back to brain control) (back to top)

The brain needs to communicate with the spinal cord in order to control movement, and does so via two separate pathways. 

Tract Location of origination Function
Vestibulospinal Vestibular nuclei of the medulla which relays sensory information to the inner ear (vestibular apparatus). Keep the head balanced as the body moves, and turn the head towards new stimuli.
Tectospinal Superior colliculus of the midbrain, which receives direct input from the retina
Pontine reticulospinal  Pontine reticular formation Enhances the anti-gravity reflexes in the spinal cord
Medullary reticulospinal  Medullary reticular formation Liberate anti-gravity muscles from reflex control

Planning of Movement by the Cerebral Cortex (back to brain control) (back to top)

There are several areas in the brain involved in planning and instruction of voluntary movement. These include:

The Basal Ganglia (back to brain control) (back to top)

The basal ganglia form the major inputs to the ventral lateral nucleus of the thalamus, which in turn provides major inputs to area 6, comprised of the PMA and SMA, The basal ganglia is a collection of subcortical nuclei including the following:

  • caudate nucleus and putamen (called the striatum)

  • globus pallidus

  • subthalamus

  • substantia nigra

The pathways between the cortex and the basal ganglia can be illustrated as to the right:

This pathway illustrates the connections between the Cerebral Cortex, the Basal Ganglia, and all of the Descending Spinal Tracts involved in the control of movement by the brain.

(back to top)

Created and Maintained by: Melissa Davies
Last Updated: April 10, 2002 09:10 AM