Types of spinal neurons

The smallest neurons (soma diameters 5–20 µm) are propriospinal, being entirely contained within the cord. Some are confined within a single segment; others span two or more segments by way of the neighboring propriospinal tract. Many of the smallest neurons participate in spinal reflexes. Others are intermediate cell stations interposed between fiber tracts descending from the brain and motor neurons projecting to the locomotor apparatus. Others again are so placed as to influence sensory transmission from lower to higher levels of the CNS.

Medium-sized neurons (soma diameters 20–50 µm) are found in all parts of the gray matter except the substantia gelatinosa. Most are relay (projection) cells receiving inputs from posterior root afferents and projecting their axons to the brain. The projections are in the form of tracts, a tract being defined as a functionally homogeneous group of fibers. As will be seen, the term ‘tract’ is often used loosely because many projections originally thought to be ‘pure’ contain more than one functional class of fiber.

The largest neurons of all are the alpha motor neurons (soma 50–100 µm) for the supply of skeletal muscles. Scattered among them are small, gamma motor neurons supplying muscle spindles. In the medial part of the anterior horn are Renshaw cells, which exert tonic inhibition upon alpha motor neurons.

Spinal reflex arcs originating in muscle spindles and tendon organs have been described in Chapter 10, and the withdrawal reflex in Chapter 14.

In thick sections of the spinal cord, the nerve cells exhibit a laminar (layered) arrangement. True lamination is confined to the posterior horn (Figure 15.3), but 10 laminae of Rexed have been defined in the gray matter as a whole in order to correlate findings from animal research in different laboratories.

Figure 15.3 Laminae (I–X) and named cell groups at mid-thoracic level.

Spinal ganglia

The spinal or posterior (dorsal) root ganglia are located in the intervertebral foramina, where the anterior and posterior roots come together to form the spinal nerves. Thoracic ganglia contain about 50,0000 unipolar neurons, and those serving the limbs contain about 100,000. The individual ganglion cells are invested with modified Schwann cells called satellite cells (Figure 15.4). The common stem axon of each cell bifurcates, sending a centrifugal process into one or other ramus of the spinal nerve (or into the recurrent branch) and a centripetal (‘center-seeking’) process into the spinal cord. Following stimulation of the peripheral sensory receptors, trains of nerve impulses traverse the point of bifurcation without interruption, although the cell body is also depolarized. The initial segment of the stem axon does not normally generate impulses but it may do so if the adjacent part of the posterior root is compressed, e.g. by a prolapsed intervertebral disc.

Figure 15.4 Posterior root ganglion. In bottom of figure, note T-shaped bifurcation of stem fibers.

Traditionally, the centripetal axons of all spinal ganglion cells have been thought to enter posterior nerve roots. It is now known that many visceral afferents (in particular) enter the cord by way of ventral roots and work their way to the posterior gray horn (Ch. 13). This feature accounts for the frequent failure of posterior rhizotomy (surgical section of posterior roots) to relieve pain originating from intra-abdominal cancer.

Central terminations of posterior root afferents (Figure 15.5)

In the dorsal root entry zone close to the surface of the cord, the afferent fibers become segregated into medial and lateral streams. The medial stream comprises medium and large fibers which divide within the posterior funiculus into ascending and descending branches. The branches swing into the posterior gray horn and synapse in laminae II, III, and IV. The largest ascending fibers run all the way to the posterior column nuclei (gracilis/cuneatus) in the medulla oblongata. These long fibers form the bulk of the gracile and cuneate fasciculi.

Figure 15.5 Targets of primary afferent neurons in the posterior gray horn.

The lateral stream comprises small (Aδ and C) fibers which, upon entry, divide into short ascending and descending branches within the posterolateral tract of Lissauer. They synapse upon neurons in the marginal zone (lamina I) and in the substantia gelatinosa (lamina II); some fibers synapse upon dendrites of cells belonging to laminae III–V.

Categories of sensation

In accordance with the flowchart in Figure 15.6, neurologists speak of two kinds of sensation, conscious and nonconscious (unconscious). Conscious sensations are perceived at the level of the cerebral cortex. Nonconscious sensations are not perceived; they have reference to the cerebellum (see later).

Figure 15.6 Categories of sensation. ^aExteroceptors can be categorized as telereceptors receiving from a distance (retina and cochlea) and somatic receptors on the body surface (touch, pain, etc.)

^bEnteroceptors (Gr. enteron, ‘gut’) are strictly a subdivision of interoceptors, a term signifying all of the viscera. In pathological states they may produce conscious visceral/viscerosomatic sensations.

Sensory testing

Routine assessment of somatic exteroceptive sensation includes tests for:

In alert and cooperative patients, active and passive tests of conscious proprioception can be performed. Active tests examine the patient’s ability to execute set-piece activities with the eyes closed:

Passive tests of conscious proprioception include:

Posterior (dorsal) column–medial lemniscal pathway

The first-order afferents include the largest somas in the posterior root ganglia. Their peripheral processes collectively receive information from the largest sensory receptors: Meissner’s and Pacinian corpuscles, Ruffini endings and Merkel cell–neurite complexes, neuromuscular spindles, and Golgi tendon organs. The centripetal processes from cells supplying the lower limb and lower trunk give branches to the gray matter before ascending as the gracile fasciculus (fasciculus gracilis) to reach the gracile nucleus in the medulla oblongata (Figure 15.8). The corresponding collaterals from the upper limb and upper trunk run in the cuneate fasciculus (fasciculus cuneatus) to reach the cuneate nucleus.

Figure 15.8 The posterior column–medial lemniscal pathway. FC, fasciculus cuneatus; FG, fasciculus gracilis; NC, nucleus cuneatus; NG, nucleus gracilis; VPL, VPM, ventral posterior lateral, ventral posterior medial nuclei of thalamus.

The second-order afferents commence in the posterior column nuclei, namely the nucleus gracilis and nucleus cuneatus. They pass ventrally in the tegmentum of the medulla oblongata before intersecting their opposite numbers in the great sensory decussation. Having crossed the midline, the fibers turn rostrally in the medial lemniscus.

The medial lemniscus diverges from the midline as it ascends through the tegmentum of the pons and midbrain. It terminates in the lateral part of the ventral posterior nucleus of the thalamus (ventral posterolateral nucleus).

Terminating in the medial part of the same nucleus (ventral posteromedial nucleus) is the trigeminal lemniscus, which serves the head region.

The third-order afferents project from the thalamus to the somatic sensory cortex (see Ch. 27 for details).

Functions

The chief functions of the posterior column–medial lemniscal pathway are those of conscious proprioception and discriminative touch. Together, these attributes provide the parietal lobe with an instantaneous body image so that we are constantly aware of the position of body parts both at rest and during movement. Without this informational background, the execution of movements is severely impaired.

In humans, disturbance of posterior column function is most often observed in association with demyelinating diseases such as multiple sclerosis. The classic symptom is known as sensory ataxia. This term signifies a movement disorder resulting from sensory impairment, in contrast to cerebellar ataxia, in which a movement disorder results from a lesion within the motor system. The patient with a severe sensory ataxia can stand unsupported only with the feet well apart and with the gaze directed downward to include the feet. The gait is broad-based, with a stamping action that maximizes any conscious proprioceptive function that remains (Figure 15.9).

Figure 15.9 The ‘stamp and stick’ gait of sensory ataxia.

Sensory testing in posterior column disease reveals severe swaying when the patient stands with the feet together and the eyes closed. This is Romberg’s sign. (Inability to ‘toe the line’ with the eyes closed is tandem Romberg’s sign.) The finger-to-nose and/or heel-to-knee test may reveal loss of kinesthetic sense. Joint sense and vibration sense may also be impaired.

Note: Romberg’s sign may be elicited in patients suffering from vestibular disorders (Ch. 19). In cerebellar disorders, there may be instability of station whether the eyes are open or closed (Ch. 25).

Tactile, painful, and thermal sensations are preserved, but there is impairment of tactile discrimination. The patient has difficulty in discriminating between single and paired stimuli applied to the skin (two-point discrimination test); in identifying numbers traced on to the skin by the examiner’s finger; and in distinguishing between objects of similar shape but of different textures.

A difficulty in assigning specific functional deficits to posterior column disease is the rarity of pathology affecting the posterior funiculi alone. In particular, the posterior part of the lateral funiculus is likely to be involved as well. Postmortem findings from patients having different degrees of pathology in the posterior and lateral funiculi suggest that kinesthetic sense from the lower limb may be mediated in part by fibers that leave the gracile fasciculus at thoracic level and relay rostrally in the posterior part of the lateral funiculus.

Spinothalamic pathway

The spinothalamic pathway consists of second-order sensory neurons projecting from laminae I–II, IV–V of the posterior gray horn to the contralateral thalamus (Figure 15.10). The cells of origin receive excitatory and inhibitory synapses from neurons of the substantia gelatinosa; these have important ‘gating’ (modulatory) effects on sensory transmission, as explained in Chapter 24.

Figure 15.10 The spinothalamic pathway. ASTT, anterior spinothalamic tract; LSTT, lateral spinothalamic tract; VP, ventral posterior nucleus of thalamus.

The axons of the spinothalamic pathway cross the midline in the anterior commissure at all segmental levels. Having crossed, they run upward in the anterolateral part of the cord. This anterolateral pathway (as it is usually called) is divisible into an anterior spinothalamic tract located in the anterior funiculus and a lateral spinothalamic tract located in the lateral funiculus. The two tracts merge in the brainstem as the spinal lemniscus. The spinal lemniscus is joined by trigeminal afferents from the head region, and it accompanies the medial lemniscus to the ventral posterior nucleus of the thalamus, terminating immediately behind the medial lemniscus. Third-order sensory neurons project from the thalamus to the somatic sensory cortex (Ch. 27).

Functions

The functions of the spinothalamic pathway have been elucidated by the procedure known as cordotomy, whereby the spinothalamic pathway is interrupted on one or both sides for the relief of intractable pain. For a percutaneous cordotomy, the patient is sedated and a needle is passed between the atlas and the axis, into the subarachnoid space. Under radiological guidance, the needle tip is advanced into the anterolateral region of the cord. A stimulating electrode is passed through the needle. If the placement is correct, a mild current will elicit paresthesia (tingling) on the opposite side of the body. The anterolateral pathway is then destroyed electrolytically. Afterwards, the patient is insensitive to pinprick, heat, or cold applied to the opposite side (Figure 15.11). Sensitivity to touch is reduced. The effect commences several segments below the level of the procedure because of the oblique passage of spinothalamic fibers across the white commissure.

Figure 15.11 Usual extent of analgesia (shaded) following cordotomy at C1–C2 segmental level.

Cordotomy is sometimes performed for patients terminally ill with cancer. It is not used for benign conditions because the analgesic (pain-relieving) effect wears off after about a year. This functional recovery may be due to nociceptive transmission either in uncrossed fibers of the spinoreticular system (see later) or in C-fiber collaterals sent to the posterior column nuclei by some axons of the lateral root entry stream.

The internal anatomy of the human spinothalamic pathway has been worked out from postoperative sensory testing and is shown in Figure 15.12. The picture is one of modality segregation. The lateral spinothalamic tract mediates noxious and thermal sensations separately, and the anterior spinothalamic tract mediates touch. The lateral tract is somatotopically arranged, the neck being represented at the front and the leg at the back. The anterior tract is also somatotopic.

Figure 15.12 Sensory modalities in spinothalamic pathway at upper cervical level.

A rare but classical condition illustrating dissociated sensory loss is illustrated in Clinical Panel 15.1.

Clinical Panel 15.1 Syringomyelia

Syringomyelia is a disorder of uncertain etiology, characterized by development of a syrinx (fusiform cyst) in or beside the central canal, usually in the cervical region (Figure CP 15.1.1). Initial symptoms arise from obliteration of spinothalamic fibers decussating in the white commissure.

The early clinical picture is one of dissociated sensory loss: sensitivity is lost to painful and thermal stimuli whereas sensitivity to touch is retained because the posterior column–medial lemniscal pathway is preserved.Typically, the patient develops ulcers on the fingers arising from painless cuts and burns. The joints of the elbow, wrist, and hand may become disorganized over time, or even dislocated, owing to loss of warning sensation from stretched joint capsules. Progressive expansion of the syrinx may compromise conduction in the long ascending and descending pathways.

Figure CP 15.1.1 Syringomyelia. Shading shows distribution of analgesia.

Spinoreticular tracts

The spinoreticular tracts are the phylogenetically oldest somatosensory pathways. The reticular formation of the brainstem has scant regard for the midline, being often bilaterally distributed in terms of its ascending and descending connections. Spinoreticular fibers originate in laminae V–VII and accompany the spinothalamic pathway as far as the brainstem (Figure 15.13). Postmortem studies of nerve fiber degeneration following cordotomy procedures indicate that at least half of the spinoreticular fibers may be uncrossed. Accurate estimations based on axonal degeneration are difficult because some spinothalamic fibers give off collaterals to the reticular formation as they pass by.

Figure 15.13 Ascending pathways at upper cervical level. ASCT, anterior spinocerebellar tract; ASTT, anterior spinothalamic tract; CF, cuneate fasciculus; GF, gracile fasciculus; LSTT, lateral spinothalamic tract; PLT, posterolateral tract; PSCT, posterior spinocerebellar tract; RSCT, rostral spinocerebellar tract; SOT, spino-olivary tract; SRT, spinoreticular tract; ST, spinotectal tract.

The spinoreticular tracts terminate at all levels of the brainstem and they are not somatotopically arranged. Impulse traffic is continued rostrally to the thalamus in the ascending reticular activating system (Ch. 24). Briefly, the spinoreticular system has two interrelated functions:

In summary, the phylogenetically old, ‘paleospinothalamic’ pathways through the reticular formation are concerned with the arousal and affective (emotional) aspects of somatic sensory stimuli. In contrast, the direct, ‘neospinothalamic’ pathway is analytical, encoding information about modality, intensity, and location.

Spinocerebellar pathways

Four fiber tracts run from the spinal cord to the cerebellum. They are:

The first two are principally concerned with nonconscious proprioception. The second two report continuously about the state of play among the internuncial neurons of the spinal cord.

Nonconscious proprioception

Nonconscious proprioception is served by the posterior spinocerebellar tract for the lower limb and lower trunk, and by the cuneocerebellar tract for the upper limb and upper trunk. Both are uncrossed, in keeping with the known control by each cerebellar hemisphere of its own side of the body.

The posterior spinocerebellar tract originates in the posterior thoracic nucleus (formerly, dorsal nucleus or Clarke’s column) in lamina VII at the base of the posterior gray horn (Figure 15.3). The nucleus extends from T1 through L1 segmental levels and the primary afferents from the lower limb enter the gracile fasciculus to reach it (Figure 15.14). It receives primary afferents of all kinds from the muscles and joints, including an intense input from muscle spindle primaries. It also receives collaterals from cutaneous sensory neurons. The fibers of the posterior spinocerebellar tract are the largest in the entire CNS, measuring 20 µm in external diameter. Very fast conduction is required to keep the cerebellum informed about ongoing movements. The tract ascends close to the surface of the cord (Figure 15.14) and enters the inferior cerebellar peduncle.

Figure 15.14 Functional anatomy of a spindle primary afferent from the lower limb. (1) Stretch reflex; (2) Ia internuncial serving reciprocal inhibition; (3) nonconscious proprioception; (4) kinesthesia.

The cuneocerebellar tract comes from the accessory cuneate nucleus, which lies above and outside the cuneate nucleus. The primary afferent inputs are of the same nature as those for the posterior thoracic nucleus; they reach it through the cuneate fasciculus. The cuneocerebellar tract enters the inferior cerebellar peduncle.