The Frontal lobes - Mahidol University

19/07/54
From cerebral to motor neurons
The motor axons move closer together as they travel down through the cerebral white
matter, and form part of the posterior limb of the internal capsule.
The motor fibers continue down into the brainstem. The bundle of corticospinal axons is
visible as two column‐like structures ("pyramids") on the ventral surface of medulla
oblongata ‐ this is where the name pyramidal tract comes from.
After the decussation, the axons travel down the spinal cord as the lateral corticospinal tract.
Fibers that do not cross over in the medulla oblongata travel down the separate anterior
corticospinal tract, and most of them cross over to the contralateral side in the spinal cord,
shortly before reaching the lower motor neurons.
Lower motor neurons
In the spinal cord, the axons of the upper motor neuron connect (most of them via
interneurons, but to a lesser extent also via direct synapses) with the lower motor neurons,
located in the ventral horn of the spinal cord.
In the brainstem, the lower motor neurons are located in the motor cranial nerve nuclei
(oculomotor, trochlear, motor nucleus of the trigeminal nerve, abducens, facial, accessory,
hypoglossal). The lower motor neuron axons leave the brain stem via motor cranial nerves
and the spinal cord via anterior roots of the spinal nerves respectively, end‐up at the
neuromuscular plate and provide motor innervation for voluntary muscles.
The Principles of Motor Controls of Movements:
1. The central nervous system (CNS) has to choose the right group
of muscles by selecting specific pathways.
2. The CNS must give the right amount of excitatory or inhibitory
inputs (“Command”) to specific motoneuron pools
3. The excitatory and inhibitory commands must be regulated
“Spatially” and “Temporally”.
4. The CNS must regulate the following parameters:
‐ force
‐displacement (distance)
‐ velocity, acceleration or deceleration
The extrapyramidal system is a neural network located in the
brain that is part of the motor system involved in the coordination of movement.
The system is called "extrapyramidal" to distinguish it from the tracts of the motor
cortex that reach their targets by traveling through the "pyramids" of the medulla.
The pyramidal pathways (corticospinal and some corticobulbar tracts) may directly
innervate motor neurons of the spinal cord or brainstem (anterior (ventral) horn
cells or certain cranial nerve nuclei), whereas the extrapyramidal system centers
around the modulation and regulation (indirect control) of anterior (ventral) horn
cells.
Extrapyramidal tracts are chiefly found in the reticular formation of the pons and
medulla, and target neurons in the spinal cord involved in reflexes, locomotion,
complex movements, and postural control. These tracts are in turn modulated by
various parts of the central nervous system, including the nigrostriatal pathway,
the basal ganglia, the cerebellum, the vestibular nuclei, and different sensory
areas of the cerebral cortex. All of these regulatory components can be considered
part of the extrapyramidal system, in that they modulate motor activity without
directly innervating motor neurons.
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