Descending Tracts

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The descending fiber tracts that originate in the brain consist of two major groups: the corticospinal, or pyramidal tracts, and the extrapyramidal tracts (table 8.5). The pyramidal tracts descend directly, without synaptic interruption, from the cerebral cortex to the spinal cord. The cell bodies that contribute fibers to these pyramidal tracts are located primarily in the precentral gyrus (also called the motor cortex). Other areas of the cerebral cortex, however, also contribute to these tracts.

From 80% to 90% of the corticospinal fibers decussate in the pyramids of the medulla oblongata (hence the name "pyramidal tracts") and descend as the lateral corticospinal tracts. The remaining uncrossed fibers form the anterior corticospinal tracts, which decussate in the spinal cord. Because of the crossing over of fibers, the right cerebral hemisphere controls the musculature on the left side of the body (fig. 8.21), whereas the left hemisphere controls the right musculature. The corticospinal tracts are primarily concerned with the control of fine movements that require dexterity.

Table 8.4 Principal Ascending Tracts of Spinal Cord

Tract

Origin

Termination

Function

Anterior spinothalamic Lateral spinothalamic

Posterior horn on one side of cord but crosses to opposite side Posterior horn on one side of cord but crosses to opposite side

Thalamus, then cerebral cortex Thalamus, then cerebral cortex

Conducts sensory impulses for crude touch and pressure Conducts pain and temperature impulses that are interpreted within cerebral cortex

Fasciculus gracilis and fasciculus cuneatus

Peripheral afferent neurons; ascends on ipsilateral side of spinal cord but crosses over in medulla

Nucleus gracilis and nucleus cuneatus of medulla; eventually thalamus, then cerebral cortex

Conducts sensory impulses from skin, muscles, tendons, and joints, which are interpreted as sensations of fine touch, precise pressures, and body movements

Posterior spinocerebellar

Posterior horn; does not cross over

Cerebellum

Conducts sensory impulses from one side of body to same side of cerebellum; necessary for coordinated muscular contractions

Anterior spinocerebellar

Posterior horn; some fibers cross, others do not

Cerebellum

Conducts sensory impulses from both sides of body to cerebellum; necessary for coordinated muscular contractions

3rd Order Sensory Neuron

Fasciculus cuneatus

(axons of first-order sensory neurons)

Lateral spinothalamic tract

(axons of second-order neurons)

Joint stretch receptor (proprioceptor)

Fasciculus cuneatus

(axons of first-order sensory neurons)

Joint stretch receptor (proprioceptor)

Spinal cord

Fasciculus gracilis

(axons of first-order sensory neurons)

(axons of first-sensory neuro l *

Touch receptor

(axons of first-sensory neuro l *

Touch receptor

First Order Neuron Crude Touch

Pain receptor

Axons of first-order neurons (not part of spinothalamic tract)

Figure 8.20 Ascending tracts carrying sensory information. This information is delivered by third-order neurons to the cerebral cortex. (a) Medial lemniscal tract; (b) lateral spinothalamic tract.

Lateral spinothalamic tract

(axons of second-order neurons)

Pain receptor

Axons of first-order neurons (not part of spinothalamic tract)

Temperature receptor

Figure 8.20 Ascending tracts carrying sensory information. This information is delivered by third-order neurons to the cerebral cortex. (a) Medial lemniscal tract; (b) lateral spinothalamic tract.

Table 8.5 Descending Motor Tracts to Spinal Interneurons and Motor Neurons

Tract

Category

Origin

Crossed/Uncrossed

Lateral corticospinal

Pyramidal

Cerebral cortex

Crossed

Anterior corticospinal

Pyramidal

Cerebral cortex

Uncrossed

Rubrospinal

Extrapyramidal

Red nucleus (midbrain)

Crossed

Tectospinal

Extrapyramidal

Superior colliculus (midbrain)

Crossed

Vestibulospinal

Extrapyramidal

Vestibular nuclei (medulla oblongata)

Uncrossed

Reticulospinal

Extrapyramidal

Reticular formation (medulla and pons)

Crossed

Clinical Investigation Clue

Remember that Frank was paralyzed on the right side of his body.

■ Damage to which descending motor tract would account for Frank's paralysis?

The remaining descending tracts are extrapyramidal motor tracts, which originate in the midbrain and brain stem regions (table 8.5). If the pyramidal tracts of an experimental animal are cut, electrical stimulation of the cerebral cortex, cerebellum, and basal nuclei can still produce movements. The descending fibers that produce these movements must, by definition, be extrapyramidal motor tracts. The regions of the cerebral cortex, basal nuclei, and cerebellum that participate in this motor control have

The Central Nervous System 211

Primary motor area of cerebra cortex

Internal capsule

Thalamus

Thalamus

Primary motor area of cerebra cortex

Internal capsule

Descending Tracts

Cervical spinal cord

Lumbar spinal cord

Cervical spinal cord

Lumbar spinal cord

■ Figure 8.21 Descending corticospinal (pyramidal) motor tracts. These tracts contain axons that pass from the precentral gyrus of the cerebral cortex down the spinal cord to make synapses with spinal interneurons and lower motor neurons.

numerous synaptic interconnections, and they can influence movement only indirectly by means of stimulation or inhibition of the nuclei that give rise to the extrapyramidal tracts. Notice that this motor control differs from that exerted by the neurons of the precentral gyrus, which send fibers directly down to the spinal cord in the pyramidal tracts.

The reticulospinal tracts are the major descending pathways of the extrapyramidal system. These tracts originate in the reticular formation of the brain stem, which receives either stimulatory or inhibitory input from the cerebrum and the cerebellum. There are no descending tracts from the cerebellum; the cerebellum can influence motor activity only indirectly by its effect on the vestibular nuclei, red nucleus, and basal nuclei (which send axons to the reticular formation). These nuclei, in turn, send axons down the spinal cord via the vestibulospinal tracts, rubrospinal tracts, and reticulospinal tracts, respectively (fig. 8.22). Neural control of skeletal muscle is explained in more detail in chapter 12.

Cerebral cortex

Thalamus

Cerebellum

Basal nuclei

Red nucleus

Brain stem reticular formation a.

Vestibular nucleus

Lower motor neurons a

■ Figure 8.22 The higher motor neuron control of skeletal muscles. The pyramidal (corticospinal) tracts are shown in pink and the extrapyramidal tracts are shown in black

The corticospinal tracts appear to be particularly important in voluntary movements that require complex interactions between sensory input and the motor cortex. Speech, for example, is impaired when the corticospinal tracts are damaged in the thoracic region of the spinal cord, whereas involuntary breathing continues. Damage to the pyramidal motor system can be detected clinically by the presence of Babinski's reflex, in which stimulation of the sole of the foot causes extension of the great toe upward and fanning of the other toes. (Normally, in adults, such stimulation causes the plantar reflex, a downward flexion, or curling, of the toes.) Babinski's reflex is normally present in infants because neural control is not yet fully developed.

Test Yourself Before You Continue

1. Explain why each cerebral hemisphere receives sensory input from and directs motor output to the contralateral side of the body.

2. List the tracts of the pyramidal motor system and describe the function of the pyramidal system.

3. List the tracts of the extrapyramidal system and explain how this system differs from the pyramidal motor system.

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