Motor Pathway

Overview of Motor Pathways

  • The motor pathways are responsible for executing motor functions and are typically represented by the ventral corticospinal pathway (pyramidal pathway).

Central Nervous System (CNS) Structure

  • Components of the CNS:

    • Frontal Lobe

    • Precentral Gyrus (Primary Motor Cortex)

    • Postcentral Gyrus (Primary Sensory Cortex)

    • Parietal Lobe

    • Occipital Lobe

    • Cerebellum

    • Temporal Lobe

    • Brain Stem

    • Spinal Cord

Neuron Types in the CNS

  • All neurons within the CNS are multipolar neurons.

  • No motor or sensory neurons are found in the CNS; rather, the CNS processes sensory information and generates commands for motor function.

Organization of Motor Pathways

  • Motor pathways are characterized as a two neuron network:

    • First Order Neuron:

    • Originates from the primary motor cortex (precentral gyrus).

    • Descends through the spinal cord and synapses on a second neuron.

    • Second Order Neuron:

    • Travels from the spinal cord to the target muscle, innervating it.

Functions of Different Cortex Areas

  • Premotor Cortex:

    • Primarily involved in planning movements.

    • Communicates with the primary motor cortex to execute these plans.

Pyramidal Tracts

  • Ventral Corticospinal Tract:

    • The pathway descends from the cortex to the spinal cord.

    • It crosses over (decussates) at the spinal level and synapses on a second order neuron in the ventral horn of the spinal cord.

    • This pathway controls the contralateral side of the body:

    • The left side of the brain controls motor activity on the right side of the body.

  • Lateral Corticospinal Tract:

    • Descends from the cortex and crosses over at the medullary pyramids in the brainstem before synapsing on the alpha motor neurons of the spinal cord.

Spinal Cord Structure

  • Gray Matter Organization:

    • Dorsal Horn

    • Ventral Lateral Horn

    • Ventral Horn

  • The central canal runs through the spinal cord and is filled with cerebrospinal fluid.

  • Dorsal Root: Carries sensory information into the spinal cord.

  • Ventral Root: Carries motor functions out of the spinal cord.

  • Dorsal Root Ganglia: Contains sensory neuron cell bodies.

  • Spinal nerves emerge from the merging of the dorsal and ventral roots.

Decussation of Motor Neurons

  • Decussation is the crossing of nerve fibers. For motor pathways:

    • In the ventral corticospinal tract, decussation occurs at the spinal cord level.

    • In the lateral corticospinal tract, decussation occurs at the medullary pyramids.

Alpha Motor Neuron

  • The second order neuron (alpha motor neuron):

    • Leaves through the ventral horn to innervate skeletal muscle via mixed nerves.

Rubrospinal Tract

  • Function: Helps maintain muscle tone through subconscious activation.

  • Originates from the red nucleus:

    • The first order neuron descends and decussates above the medullary pyramids.

    • Synapses on an interneuron before ultimately synapsing on the alpha motor neuron.

Summary of Pathways

  • Motor pathways generally consist of two neurons, with an exception for the lateral corticospinal and rubrospinal tracts, which involve an intermediary neuron.

  • Pathways decussate either at the spinal level or at the medullary pyramids.

  • The synapse between the two neurons occurs in the ventral horn of the spinal cord, which contains the cell bodies for both groups of neurons involved in motor function.

Key Points

  • The ventral corticospinal tract primarily engages muscles on the contralateral side of the body due to the crossing of motor signals.

  • Understanding the distinctions in motor pathways is crucial for differentiating how various parts of the brain control voluntary muscle movements and maintain tone in skeletal muscles.