Motor Pathways - Anatomy & Physiology

Spinal Cord Tracts

Introduction

Motor pathways carry signals from the brain to the skeletal muscle and smooth muscle such as those contained in glands. The system consists of upper and lower motor neurones. The information provided below is primarily focussed on the motor pathways that coordinate skeletal muscle movement, i.e. motor pathways related to voluntary control of skeletal muscles.

Motor Pathways

The connections between the motor cortex in the forebrain and motor neurons within the spinal cord are made up of two pyramidal tracts; the Pyramidal System and the Extrapyramidal System. These motor pathways are transmitted via ventral horns within the spine. Upper motor neurones are located mainly within the neopallium of the cerebellum. They govern the excitation or inhibition of lower motor neurones.

Stimulation of the motor neurons is as a result of activity within the cerebral cortex and/or thalamus. There is a balance between the excitatory and inhibitory inputs from these parts of the brain. The thalamus effectively acts as a relay station from the cerebrum and cerebellum before neurons lead into the motor pathways of the spine.

Pyramidal System (Corticospinal)

Pyramidal System © Sarah Hamilton

The pyramidal tract produces fine movements associated with skill, e.g. writing and playing a musical instrument. This tract is composed of direct connections that contain no synapses within the brain stem. Nerve fibres of the pyramidal system originate in the cerebral cortex and then pass to the thalamus, and medulla oblongata. Therefore the neurons pass directly through the ventral aspect of the medulla oblongata in a pyramidal shape providing the reason for the name of this tract. Some fibres decussate in the medulla oblongata, whilst others remain ipsilateral until they leave the spinal cord where they cross the midline of the body. The result of this is that all fibres cross the midline at some point, so that the left side of the brain controls the right side of the body and vice versa.

Motor tracts within the pyramidal system are fundamental in the control and coordination of muscle groups that require concentration and concious thought to control. This is particularly true for movements of the hands and fingers and therefore these pyramidal tracts are most developed in primates, although may exist in other species such as the cat, where they aid in grooming. It is thought that the evolution of pyramidal tracts is closely associated with the development of concious and fine movements. In most domestic species that lack the ability to undertake fine movements the pyramidal tracts are primarily involved in control of the jaw, lips and aspects of the face.

Extrapyramidal System

Extrapyramidal System © Sarah Hamilton

The extrapyramidal system represents part of the motor pathways that has synapses within the brain stem which is in contrast to those of the pyramidal system. The extrapyramidal tract is distinguishable from the pyramidal system as tracts do not run within the pyramids of the medulla oblongata and instead run outside. These synapses within the brain stem make it possible for the for motor neurone signals to be influenced as they enter the ventral horn of the spinal cord. The cerebellum utilises this influence to ensure that movements are smooth and coordinated.

It is thought that the extrapyramidal system represents an earlier evolutionary development than the pyramidal system and therefore is more often a feature of animals less able to perform complex movements. It is the most important system in domestic species.

• More complex than the pyramidal system as involves various multisynaptic pathways that relay within several nuclei in the brain.
  • Nuclei are dispersed from the telencephalon to the medulla oblongata and may be visible grossly.
  • Parts of it take origin from the motor cortex, others from the tectum and reticular formation.
  • Neurones then relay within nuclei which include:
    • Red nucleus
    • Substantia nigra
    • Caudate nucleus
    • Subthalamic nuclei
    • Olive in the medulla oblongata
    • Reticular formation
  • Neurones that relay in the red nucleus and reticular formation relay directly onto LMNs, others relay to other nuclei.
• The main descending motor tracts from these two nuclei are:
  • Rubrospinal tract - important in carnivores and ungulates. Modulates pattern generators in the spinal cord.
  • Reticospinal tract
  • Vestibulospinal tract
  • Tectospinal tract

Links

• Pathology of the Nervous System