Motor Output from the Brain and Spinal Cord

Abstract

All skeletal muscles are controlled by activation and inhibition of motor neurons in the spinal cord and brainstem. Motor neurons integrate information from a wide variety of sources in the brain and spinal cord. Simple limb reflexes and basic patterns of limb movements, including locomotion, are coordinated by spinal cord interneurons and sensory neurons. Descending inputs from the reticular formation, the vestibular nuclei and the superior colliculus in the brainstem are important in the control of automatic, axial, postural and locomotor movements. Descending inputs from several areas of the cerebral cortex and from the red nucleus are important in the control of voluntary, distal limb and fine movements. The basal ganglia in the basal forebrain and the cerebellum in the hindbrain are involved in the modification and learning of movements. These structures act indirectly on motor output, via outputs through the thalamus to widespread areas of the cerebral cortex.

Key Concepts

  • All voluntary movements involve motor neurons in the spinal cord and brainstem that cause contraction of skeletal muscles.
  • Neurons in many areas of the brain and spinal cord are active at overlapping times to control voluntary movements.
  • The spinal cord can produce coordinated movements of the limbs, such as locomotion, as well as simple reflexes.
  • The superior colliculus can trigger orienting movements, especially of the eyes and head.
  • Voluntary limb movements involve neurons in several parts of the cerebral cortex, including especially the frontal lobe, which project axons to the spinal cord.
  • Cortical movement control signals are modified by loops to and from the basal ganglia and the cerebellum, going through the thalamus.
  • The basal ganglia modify the speed and amplitude of movements and contribute to learning new movement sequences.
  • The cerebellum adapts movements to altered sensory feedback and may also contribute to a wide variety of other functions, including memory, language and emotion.
  • Patients with movement disorders may in the future be able to control a limb or prosthetic device with ‘their thoughts’, via a brain–machine interface that monitors the activity of some of their cortical neurons.

Keywords: motor neuron; interneuron; spinal cord; locomotion; central pattern generator; motor cortex; cerebellum; basal ganglia; superior colliculus; motor learning

Figure 1. Spinal cord organisation within each spinal segment.
Figure 2. Descending supraspinal pathways into the spinal cord.
Figure 3. The basal ganglia and the cerebellum indirectly modify cerebral cortical motor output (+ indicates an excitatory connection, and − indicates an inhibitory connection).
Figure 4. Prolonged electrical stimulation of a particular location within the frontal lobe of the cerebral cortex causes a monkey's arm to adopt a particular posture (changing hand position is indicated by trails of dots, which converge on the final hand position), regardless of the initial hand position (different trails of dots). (a–f) The results of stimulation of six different locations. Adapted from Graziano et al. ().
Figure 5. A monkey quickly learns to control the rate of action potentials (indicated by the series of black vertical lines in (a) and (b)) of one particular neuron in its motor cortex in a kind of video game (a), in response to an instructed cue (indicated by pink rectangles in (b)). An increase in the smoothed rate of action potentials above a certain threshold value triggers electrical stimulation of an extensor arm muscle, which extends the wrist, and provides the monkey feedback via cursor movement on the computer screen. The monkey correctly increases this neuron's firing rate when instructed to do so and not at other times (c). Moritz et al. (). Reproduced with permission of Springer Nature.
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Ziskind‐Conhaim L, Fetcho JR, Hochman S, MacDermott AB and Stein PSG (2010) Neurons and Networks in the Spinal Cord. Blackwell Publishing: Boston On behalf of the New York Academy of Sciences.

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Berkowitz, Ari(May 2020) Motor Output from the Brain and Spinal Cord. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000189.pub4]