Descending tracts

The white matter is composed of myelinated axons of neurons. These axons are grouped together according to function to form tracts. Neurons transmitting impulses toward the brain in the ascending tracts carry sensory information. Those transmitting impulses away from the brain in the descending tracts carry motor information. 

Figure 7.2 Ascending and descending tracts in white matter of the spinal cord. Tracts are formed of bundles of neuronal axons that transmit similar types of information.

Table 7.1 Ascending and Descending Tracts in White Matter of the Spinal Cord...

Descending tracts. Voluntary movement of skeletal muscles is controlled by two types of descending tracts. Neurons in these tracts terminate on and influence activity of alpha motor neurons in the ventral horn. The two types of tracts include ... 

As with the ascending tracts, descending tracts cross from one side of the CNS to the other. Most of the tracts cross over in the medulla of the brainstem. Therefore, the right side of the brain influences the activity of the alpha motor neurons and thus the skeletal muscles on the left side of the body. The locations of specific descending tracts are illustrated in Figure 7.2 and a summary of their functions is found in Table 7.1. 

If a spinal cord is cross-sectioned, the gray matter appears as a roughly H-shaped area in its middle which is, divided into dorsal (posterior), lateral, and ventral (anterior) horns. The horns are interconnected by a crossbar, the gray commissure. The rest of the spinal cord is the white matter, made up largely of tracts of myelinated nerve fibers (axons). Ascending tracts carry afferent sensory impulses towards the brain, descending tracts transmit motor impulses from the brain to the motor neurons in the ventral or lateral horns of the gray matter. 

The brain itself is almost totally insensitive to pain. The scalp, arteries, muscles, mucous membranes of the sinuses, ear, and the teeth are all pain-sensitive structures. The crucial areas in the creation of headache are the suboccipital and upper cervical areas as well as the scalp. In the upper two cervical segments, the sensory fibers of the first three cervical segments are joined by the descending tracts of cranial nerves V, IX, and X. These three cranial nerves, along with the second cervical nerve, mediate the referral of excessive coimective tissue tension in the cervical area as pain in the cranial vault or cephalgia. 

ALS is a disorder of the motor neurons and the cortical neurons that provide their input. The disorder is characterized by rapidly progressive weakness and muscle atrophy. Most affected patients die of respiratory compromise and pneumonia after 2 to 3 years. There is prominent loss of motor neurons in the spinal cord and brainstem although the oculomotor neurons are spared. Large pyramidal motor neurons in layer V of motor cortex, which are the origin of the descending corticospinal tracts, are also lost. 

Following the development of the motor program, neurons originating in the multimodal motor association areas transmit impulses by way of association tracts to neurons of the primary motor cortex. The primary motor cortex is located in the precentral gyrus, which is the most posterior region of the frontal lobe adjacent to the multimodal motor association areas (see Figure 6.3) this area initiates voluntary contractions of specific skeletal muscles. Neurons whose cell bodies reside here transmit impulses by way of descending projection tracts to the spinal cord, where they innervate the alpha motor neurons (which innervate skeletal muscles). 

The corticospinal tracts originate in the cerebral cortex. Neurons of the primary motor cortex are referred to as pyramidal cells. Most of these neurons axons descend directly to the alpha motor neurons in the spinal cord. In... 

These two types of descending motor tracts do not function in isolation. They are extensively interconnected and cooperate in the control of movement. For example, in order to grasp a doorknob to open a door, there is subconscious positioning of the body to face the door and extend an arm toward the doorknob. 

Fleetwood-Walker S., Hope P., Mitchell R. (1988). Antinociceptive actions of descending dopaminergic tracts on cat and rat dorsal horn somatosensory neurones. J. Physiol, bond. 399, 335-48. 

Chlordecone is well distributed throughout the male reproductive tract of rats and appears in the ejaculate. In rats given a single oral dose of 40 mg/kg chlordecone, the descending order of... 

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