Motor signals are transmitted directly from the cortex to the spinal cord through the corticospinal tract and indirectly through multiple accessory pathways that involve the basal ganglia, cerebellum, and various nuclei of the brain stem. In general, the direct pathways are concerned more with discrete and detailed movements, especially of the distal segments of the limbs, particularly the hands and fingers.
Corticospinal (Pyramidal) Tract
The most important output pathway from the motor cortex is the corticospinal tract, also called the pyramidal tract, shown in Figure 1. The corticospinal tract originates about 30 percent from the primary motor cortex, 30 percent from the premotor and supplementary motor areas, and 40 percent from the somatosensory areas posterior to the central sulcus.
Fig1. Corticospinal (pyramidal) tract. (Modified from Ranson SW, Clark SL: Anatomy of the Nervous System. Philadelphia: WB Saunders, 1959.)
After leaving the cortex, it passes through the posterior limb of the internal capsule (between the caudate nucleus and the putamen of the basal ganglia) and then downward through the brain stem, forming the pyramids of the medulla. The majority of the pyramidal fibers then cross in the lower medulla to the opposite side and descend into the lateral corticospinal tracts of the cord, finally terminating principally on the interneurons in the intermediate regions of the cord gray matter; a few terminate on sensory relay neurons in the dorsal horn, and a very few terminate directly on the anterior motor neurons that cause muscle contraction.
A few of the fibers do not cross to the opposite side in the medulla but pass ipsilaterally down the cord in the ventral corticospinal tracts. Many, if not most, of these fibers eventually cross to the opposite side of the cord either in the neck or in the upper thoracic region. These fibers may be concerned with control of bilateral postural movements by the supplementary motor cortex.
The most impressive fibers in the pyramidal tract are a population of large myelinated fibers with a mean diameter of 16 micrometers. These fibers originate from giant pyramidal cells, called Betz cells, that are found only in the primary motor cortex. The Betz cells are about 60 micrometers in diameter, and their fibers transmit nerve impulses to the spinal cord at a velocity of about 70 m/ sec, the most rapid rate of transmission of any signals from the brain to the cord. There are about 34,000 of these large Betz cell fibers in each corticospinal tract. The total number of fibers in each corticospinal tract is more than 1 million, so these large fibers represent only 3 percent of the total. The other 97 percent are mainly fibers smaller than 4 micrometers in diameter that conduct background tonic signals to the motor areas of the cord.
Other Fiber Pathways From the Motor Cortex. The motor cortex gives rise to large numbers of additional, mainly small, fibers that go to deep regions in the cerebrum and brain stem, including the following:
1. The axons from the giant Betz cells send short col laterals back to the cortex. These collaterals are believed to inhibit adjacent regions of the cortex when the Betz cells discharge, thereby “sharpening” the boundaries of the excitatory signal.
2. A large number of fibers pass from the motor cortex into the caudate nucleus and putamen. From there, additional pathways extend into the brain stem and spinal cord, as discussed in the next chapter, mainly to control body postural muscle contractions.
3. A moderate number of motor fibers pass to red nuclei of the midbrain. From these nuclei, additional fibers pass down the cord through the rubrospinal tract.
4. A moderate number of motor fibers deviate into the reticular substance and vestibular nuclei of the brain stem; from there, signals go to the cord by way of reticulospinal and vestibulospinal tracts, and others go to the cerebellum by way of reticulocerebellar and vestibulocerebellar tracts.
5. A tremendous number of motor fibers synapse in the pontile nuclei, which give rise to the pontocerebellar fibers, carrying signals into the cerebellar hemispheres.
6. Collaterals also terminate in the inferior olivary nuclei, and from there, secondary olivocerebellar fibers transmit signals to multiple areas of the cerebellum.
Thus, the basal ganglia, brain stem, and cerebellum all receive strong motor signals from the corticospinal system every time a signal is transmitted down the spinal cord to cause a motor activity.
Incoming Sensory Fiber Pathways to the Motor Cortex
The functions of the motor cortex are controlled mainly by nerve signals from the somatosensory system but also, to some degree, from other sensory systems such as hearing and vision. Once the sensory information is received, the motor cortex operates in association with the basal ganglia and cerebellum to excite an appropriate course of motor action. The more important incoming fiber pathways to the motor cortex are the following:
1. Subcortical fibers from adjacent regions of the cerebral cortex, especially from (a) the somatosensory areas of the parietal cortex, (b) the adjacent areas of the frontal cortex anterior to the motor cortex, and (c) the visual and auditory cortices.
2. Subcortical fibers that arrive through the corpus callosum from the opposite cerebral hemisphere. These fibers connect corresponding areas of the cortices in the two sides of the brain.
3. Somatosensory fibers that arrive directly from the ventrobasal complex of the thalamus. These fibers relay mainly cutaneous tactile signals and joint and muscle signals from the peripheral body.
4. Tracts from the ventrolateral and ventroanterior nuclei of the thalamus, which in turn receive signals from the cerebellum and basal ganglia. These tracts provide signals that are necessary for coordination among the motor control functions of the motor cortex, basal ganglia, and cerebellum.
5. Fibers from the intralaminar nuclei of the thalamus. These fibers control the general level of excitability of the motor cortex in the same way they control the general level of excitability of most other regions of the cerebral cortex.
