Descending Pathways

The influence exerted by the various brain regions on posture and movement is via descending pathways to the motor neurons and the interneurons that affect these neurons. The pathways are of two types: the corticospinal pathways, which, as their name implies, originate in the cerebral cortex; and a second group we shall call the brainstem pathways, which originate in the brainstem.

Fibers from both types of descending pathways end at synapses on alpha and gamma motor neurons or on interneurons that affect the alpha motor neurons either directly or via still other interneurons. Sometimes, as mentioned earlier, these are the same interneurons that function in local reflex arcs, thereby ensuring that the descending signals are fully integrated with local information before the activity of the motor neurons is altered. In other cases, the interneurons are part of neural networks involved in posture or locomotion. The ultimate effect of the descending pathways on the alpha motor neurons may be excitatory or inhibitory.

Importantly, some of the descending fibers affect afferent systems. They do this via (1) presynaptic synapses on the terminals of afferent neurons as these fibers enter the central nervous system, or (2) synapses on interneurons in the ascending pathways. The overall effect of this descending input to afferent systems is to limit their influence on either the local or brain motor control areas, thereby altering the importance of a particular bit of afferent information or sharpening its focus. This descending (motor) control over ascending (sensory) information provides another example to show that there is clearly no real functional separation of the motor and sensory systems.

Corticospinal Pathway The nerve fibers of the cor-ticospinal pathways, as mentioned before, have their cell bodies in sensorimotor cortex and terminate in the spinal cord. The corticospinal pathways are also called the pyramidal tracts, or pyramidal system (perhaps because of their shape as they pass along the surface of the medulla oblongata or because they were formerly thought to arise solely from the pyramidal cells of the motor cortex). In the medulla oblongata near the junction of the spinal cord and brainstem, most of the corticospinal fibers cross the spinal cord to descend on the opposite side (Figure 12-12). Thus, the skeletal muscles on the left side of the body are controlled largely by neurons in the right half of the brain, and vice versa.

Corticospinal pathway

Basal ganglia

Sensorimotor cortex

Corticospinal pathway

Basal ganglia

Sensorimotor cortex

Brainstem pathway

Spinal Cord Crossed Pathways

Thalamus

Brainstem Cerebellum

Spinal cord

Crossover of corticospinal pathway

Spinal cord

FIGURE 12-12

The corticospinal and brainstem pathways. Most of the corticospinal fibers cross in the brainstem to descend in the opposite side of the spinal cord, but the brainstem pathways are mostly uncrossed. Arrows indicate direction of action-potential propagation.

Adapted from Gardner.

Brainstem pathway

Thalamus

Brainstem Cerebellum

Spinal cord

Crossover of corticospinal pathway

Spinal cord

FIGURE 12-12

The corticospinal and brainstem pathways. Most of the corticospinal fibers cross in the brainstem to descend in the opposite side of the spinal cord, but the brainstem pathways are mostly uncrossed. Arrows indicate direction of action-potential propagation.

Adapted from Gardner.

As the corticospinal fibers descend through the brain from the cerebral cortex, they are accompanied by fibers of the corticobulbar pathway ("bulbar" means "pertaining to the brainstem"), a pathway that begins in the sensorimotor cortex and ends in the brainstem. The corticobulbar fibers control, directly or indirectly via interneurons, the motor neurons that innervate muscles of the eye, face, tongue, and throat. These fibers are the main source of control for voluntary movement of the muscles of the head and neck, whereas the corticospinal fibers serve this function for the muscles of the rest of the body. For convenience, we shall henceforth include the corticobulbar pathway in the general term "corticospinal pathways."

Convergence and divergence are hallmarks of the corticospinal pathway. For example, a great deal of convergence from different neuronal sources impinges on neurons of the sensorimotor cortex, which is not surprising when one considers the many factors that can affect motor behavior. As for the descending pathways, neurons from wide areas of the sensorimotor

PART TWO Biological Control Systems

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART TWO Biological Control Systems cortex converge onto single motor neurons at the local level so that multiple brain areas usually control single muscles. Also, axons of single corticospinal neurons diverge markedly to synapse with a number of different motor neuron populations at various levels of the spinal cord, thereby ensuring that the motor cortex can influence many different components of a movement.

This seeming "blurriness" of control appears counterintuitive when we think of the delicacy with which we can move a fingertip and when we learn that the corticospinal pathways have their greatest influence on rapid, fine movements of the distal extremities, such as those made when an object is manipulated by the fingers. After damage to the corticospinal pathways, all movements are slower and weaker, individual finger movements are absent, and it is difficult to release a grip.

Brainstem Pathways Axons from neurons in the brainstem also form pathways that descend into the spinal cord to influence motor neurons. These pathways are sometimes referred to as the extrapyramidal system, or indirect pathways, to distinguish them from the corticospinal (pyramidal) pathways. However, no general term is widely accepted for these pathways, and for convenience we shall refer to them collectively as the brainstem pathways.

Axons of some of the brainstem pathways cross from their side of origin in the brainstem to affect muscles on the opposite side of the body, but most remain uncrossed. In the spinal cord the fibers of the brain-stem pathways descend as distinct clusters, named according to their sites of origin. For example, the vestibulospinal pathway descends to the spinal cord from the vestibular nuclei in the brainstem, whereas the reticulospinal pathway descends from neurons in the brainstem reticular formation.

The brainstem pathways are especially important in the control of upright posture, balance, and walking.

Concluding Comments on the Descending Pathways As stated above, the corticospinal neurons generally have their greatest influence over motor neurons that control muscles involved in fine, isolated movements, particularly those of the fingers and hands. The brainstem descending pathways, in contrast, are more involved with coordination of the large muscle groups used in the maintenance of upright posture, in locomotion, and in head and body movements when turning toward a specific stimulus.

There is, however, much interaction between the descending pathways. For example, some fibers of the corticospinal pathway end on interneurons that play important roles in posture, whereas fibers of the brain-stem descending pathways sometimes end directly on the alpha motor neurons to control discrete muscle movements. Because of this redundancy, loss of function resulting from damage to one system may be compensated for by the remaining system, although the compensation is generally not complete.

The distinctions between the corticospinal and brainstem descending pathways are not clear-cut. All movements, whether automatic or voluntary, require the continuous coordinated interaction of both types of pathways.

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Responses

  • Ralf
    Where do spinothalamic tract crossover?
    8 years ago
  • goytiom
    Why are brain pathways crossed?
    8 years ago
  • J Daecher
    Where do the ascending and descending pathways typically cross over?
    8 years ago
  • ilda trentini
    What are the pathways to the brain for posture?
    8 years ago
  • trina
    How are descending tracts involved with movement?
    8 years ago
  • Gundobad
    How brain control descending pathway?
    8 years ago
  • KENNETH
    How many neurons in decending pathways?
    7 years ago
  • MATHIAS
    Where do descending pathway originate?
    7 years ago
  • What mediates signals related to walking, posture and balance in the descneding pathway?
    6 months ago

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