Ascending Tracts

Ascending tracts carry sensory signals up the spinal cord. Sensory signals typically travel across three neurons from their origin in the receptors to their destination in the sensory areas of the brain: a first-order neuron that detects a stimulus and transmits a signal to the spinal cord or brain-stem; a second-order neuron that continues as far as a "gateway" called the thalamus at the upper end of the brainstem; and a third-order neuron that carries the signal the rest of the way to the sensory region of the cerebral cortex. The axons of these neurons are called the first-through third-order nerve fibers. Deviations from the pathway described here will be noted for some of the sensory systems to follow.

The major ascending tracts are as follows. The names of most ascending tracts consist of the prefix spino- followed by a root denoting the destination of its fibers in the brain.

• The gracile13 fasciculus (GRAS-el fah-SIC-you-lus) carries signals from the midthoracic and lower parts of the body. Below vertebra T6, it composes the entire dorsal column. At T6, it is joined by the cuneate fasciculus, discussed next. It consists of first-order nerve fibers that travel up the ipsilateral side of the spinal cord and terminate at the gracile nucleus in the medulla oblongata of the brainstem. These fibers carry

10decuss = to cross, form an X 11 contra = opposite

12ipsi = the same + later = side 13gracil = thin, slender

10decuss = to cross, form an X 11 contra = opposite

12ipsi = the same + later = side 13gracil = thin, slender

Table 13.1 Major Spinal Tracts

Tract

Column

Decussation

Functions

Ascending (sensory) Tracts

Gracile fasciculus

Cuneate fasciculus Spinothalamic Dorsal spinocerebellar Ventral spinocerebellar

Dorsal Dorsal

Lateral and ventral

Lateral

In medulla In spinal cord None

In spinal cord

Limb and trunk position and movement, deep touch, visceral pain, vibration, below level T6

Same as gracile fasciculus, from level T6 up Light touch, tickle, itch, temperature, pain, and pressure Feedback from muscles (proprioception) Same as dorsal spinocerebellar

Descending (motor) Tracts

Lateral corticospinal Ventral corticospinal Tectospinal Lateral reticulospinal Medial reticulospinal Vestibulospinal

Lateral Ventral

Lateral and ventral Lateral Ventral Ventral

In medulla None

In midbrain None None None

Fine control of limbs Fine control of limbs

Reflexive head-turning in response to visual and auditory stimuli Balance and posture; regulation of awareness of pain Same as lateral reticulospinal Balance and posture

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Ascending Tracts

Figure 13.4 Tracts of the Spinal Cord. All of the illustrated tracts occur on both sides of the cord, but only the ascending sensory tracts are shown on the left (red), and only the descending motor tracts on the right (green).

If you were told that this cross section is either at level T4 or T10, how could you determine which is correct?

Figure 13.4 Tracts of the Spinal Cord. All of the illustrated tracts occur on both sides of the cord, but only the ascending sensory tracts are shown on the left (red), and only the descending motor tracts on the right (green).

If you were told that this cross section is either at level T4 or T10, how could you determine which is correct?

signals for vibration, visceral pain, deep and discriminative touch (touch whose location one can precisely identify), and especially proprioception14 from the lower limbs and lower trunk. (Proprioception is a nonvisual sense of the position and movements of the body.)

  • The cuneate15 (CUE-nee-ate) fasciculus (fig. 13.5a) joins the gracile fasciculus at the T6 level. It occupies the lateral portion of the dorsal column and forces the gracile fasciculus medially. It carries the same type of sensory signals, originating from level T6 and up (from the upper limb and chest). Its fibers end in the cuneate nucleus on the ipsilateral side of the medulla oblongata. In the medulla, second-order fibers of the gracile and cuneate systems decussate and form the medial lemniscus16 (lem-NIS-cus), a tract of nerve fibers that leads the rest of the way up the brainstem to the thalamus. Third-order fibers go from the thalamus to the cerebral cortex. Because of decussation, the signals carried by the gracile and cuneate fasciculi ultimately go to the contralateral cerebral hemisphere.
  • The spinothalamic (SPY-no-tha-LAM-ic) tract (fig. 13.5b) and some smaller tracts form the anterolateral system, which passes up the anterior

14proprio = one's own + cept = receive, sense

15cune = wedge

16lemniscus = ribbon and lateral columns of the spinal cord. The spinothalamic tract carries signals for pain, temperature, pressure, tickle, itch, and light or crude touch. Light touch is the sensation produced by stroking hairless skin with a feather or cotton wisp, without indenting the skin; crude touch is touch whose location one can only vaguely identify. In this pathway, first-order neurons end in the dorsal horn of the spinal cord near the point of entry. Second-order neurons decussate to the opposite side of the spinal cord and there form the ascending spinothalamic tract. These fibers lead all the way to the thalamus. Third-order neurons continue from there to the cerebral cortex.

• The dorsal and ventral spinocerebellar (SPY-no-SERR-eh-BEL-ur) tracts travel through the lateral column and carry proprioceptive signals from the limbs and trunk to the cerebellum, a large motor control area at the rear of the brain. The first-order neurons of this system originate in the muscles and tendons and end in the dorsal horn of the spinal cord. Second-order neurons send their fibers up the spinocerebellar tracts and end in the cerebellum. Fibers of the dorsal tract travel up the ipsilateral side of the spinal cord. Those of the ventral tract cross over and travel up the contralateral side but then cross back in the brainstem to enter the ipsilateral cerebellum. Both tracts provide the cerebellum with feedback needed to coordinate muscle action, as discussed in chapter 14.

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Midbrain

Gracile -nucleus

Medulla

First-order neuron

Spinal cord

Midbrain

Gracile -nucleus

Medulla

First-order neuron

Spinal cord

Medial Fasciculus Spinal Cord

Third-order neuron

Thalamus

Medial lemniscus

Gracile fasciculus Cuneate fasciculus

Second-order neuron

Cuneate nucleus

Medial lemniscus

' Receptors for body movement, limb positions, fine touch discrimination, and pressure

Somesthetic cortex

) (postcentral gyrus)

Third-order neuron

Thalamus

Medial lemniscus

Second-order neuron

Cuneate nucleus

Medial lemniscus

Gracile fasciculus Cuneate fasciculus

' Receptors for body movement, limb positions, fine touch discrimination, and pressure

Midbrain

Medulla

Spinal cord

First-order neuron

Anterolateral System Thalamus

Somesthetic cortex (postcentral gyrus)

Third-order neuron

Thalamus

Anterolateral system

Second-order neuron

-Spinothalamic tract

Receptors for pain, heat, and cold

Somesthetic cortex (postcentral gyrus)

Third-order neuron

Thalamus

Midbrain

Medulla

Second-order neuron

Spinal cord

First-order neuron

-Spinothalamic tract

Anterolateral system

Receptors for pain, heat, and cold

Figure 13.5 Some Ascending Pathways of the CNS. The spinal cord, medulla, and midbrain are shown in cross section and the cerebrum and thalamus (top) in frontal section. Nerve signals enter the spinal cord at the bottom of the figure and carry somatosensory information up to the cerebral cortex. (a) The cuneate fasciculus and medial lemniscus; (b) the spinothalamic tract.

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Responses

  • ilse
    Where is the ascending tracts in spinal nerves?
    7 years ago
  • berylla
    Which tract carries signals for fine motor movements?
    6 years ago
  • rosario
    What type of signals does the ascending ract?
    7 months ago

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