Nerve roots

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14 Nerve roots

1 During embryonic development, some immature neurons send out ventral roots while others project along the marginal zone to form fiber tracts. Sensory fibers enter the dorsal horn from the neural crest.

2 In the mature vertebral canal, note disparities of levels, e.g. a collapsed T11 vertebra would crush cord segment L1.

3 Recall that ventral roots S2–4 of cauda equina contain preganglionic parasympathetic fibers vital for bladder and bowel control and that the corresponding posterior roots contain visceral afferents vital for reflexes.

4 Do remember the extradural venous plexus and the harm it can do.

5 It is clinically important to appreciate that a sense of numbness/tingling in the fingers in later life may result from compression of posterior nerve roots!

6 For the commonest and lowest two levels of disk prolapse, the next spinal nerve is the one likely to be caught.

7 Make certain that you understand the anatomy of lumbar puncture (spinal tap).

Development of the Spinal Cord

Cellular differentiation

The neural tube of the embryo consists of a pseudostratified epithelium surrounding the neural canal (Figure 14.1A). Dorsal to the sulcus limitans, the epithelium forms the alar plate; ventral to the sulcus it forms the basal plate.

Figure 14.1 (A–C) Cellular differentiation in the embryonic spinal cord.

The neuroepithelium contains germinal cells which synthesize DNA before retracting to the innermost, ventricular zone, where they divide. The daughter nuclei move outward, synthesize fresh DNA, then retreat and divide again. After several such cycles, postmitotic cells round up in the intermediate zone. Some of the postmitotic cells are immature neurons; the rest are glioblasts which, after further division, become astrocytes or oligodendrocytes. Some of the glioblasts form an ependymal lining for the neural canal.

The microglial cells of the CNS are derived from basophil cells of the blood.

Enlargement of the intermediate zone of the alar plate creates the dorsal horn of gray matter. The dorsal horn receives central processes of dorsal root ganglion cells (Figure 14.1B). As explained in Chapter 1, the ganglion cells derive from the neural crest.

Partial occlusion of the neural canal by the developing dorsal gray horn gives rise to the dorsal median septum and to the definitive central canal of the cord (Figure 14.1C).

Enlargement of the intermediate zone of the basal plate creates the ventral gray horn and the ventral median fissure (Figure 14.1C). Axons emerge from the ventral horn and form the ventral nerve roots.

In the outermost, marginal zone of the cord, axons run to and from spinal cord and brain.

Neural cord

The neural tube reaches caudally only to the level of the second lumbar mesodermal somites. Following closure of the posterior neuropore, the ectoderm and mesoderm at the level of the more caudal lumbar and sacral somites blend to form a neural cord. This ribbon of cells becomes canalized and links up with the neural tube; it forms the lower end of the spinal cord.

Ascent of the spinal cord (Figure 14.2)

The spinal cord occupies the full length of the vertebral canal until the end of the 12th postconceptual week. The sixth to eighth are marked by regression of the caudal end of the neural tube, to become a neuroglial thread, the filum terminale.

Figure 14.2 (A, B) Regression of coccygeal segments of spinal cord creates the filum terminale. (C, D) Ascent of spinal cord. (Note: Recent evidence indicates that, as represented here, the number of embryonic coccygeal vertebrae does not exceed three or four.)

After the 12th week, the vertebral column grows rapidly and drags the cord upward. The tip of the cord is at second or third lumbar level at the time of birth. The adult level (first or second lumbar) is attained 3 weeks later.

In consequence of greater ascent of the lower part of the cord compared to the upper part, the spinal nerve roots show an increasing disparity between their segmental levels of attachment to the cord and the corresponding vertebral levels (Figure 14.3).

Figure 14.3 Segmental and vertebral levels compared. Spinal nerves 1–7 emerge above the corresponding vertebrae; the remaining spinal nerves emerge below.

Neural arches

During the fifth week, the mesenchymal vertebrae surrounding the notochord give rise to neural arches for protection of the spinal cord (Figure 14.4). The arches are initially bifid (split). Later, they fuse in the midline and form the vertebral spines.

Figure 14.4 Normal, bifid stage of neural arch development in an embryo of 8 weeks.

Conditions where the two halves of the neural arches have failed to unite are collectively known as spina bifida (Clinical Panel 14.1).

Clinical Panel 14.1 Spina bifida

Among the more common congenital malformations of the CNS are several conditions included under the general heading spina bifida. The ‘bifid’ effect is produced by failure of union of the two halves of the neural arches, usually in the lumbosacral region (Figure CP 14.1.1)

Spina bifida occulta (A) is usually symptom-free, being detected incidentally in lumbosacral radiographs.

In spina bifida cystica, a meningeal cyst protrudes through the vertebral defect. In 10% of these cases, the cyst is a meningocele containing no nervous elements (B). In 90%, unfortunately, the cyst is a meningomyelocele, containing either spinal cord or cauda equina (C); the lower limbs, bladder, and rectum are paralyzed, as in the case illustrated in Figure CP 14.1.2, and meningitis is likely to supervene sooner or later. To make matters worse, an Arnold–Chiari malformation (Ch. 4) is almost always present as well.

The most severe form of spina bifida is myelocele (D), where the neural folds have remained open and CSF leaks on to the surrounding skin. The clinical outlook is very poor.

Figure CP 14.1.1 Varieties of spina bifida. (A) Spina bifida occulta. (B) Meningocele. (C) Meningomyelocele. (D) Myelocele.

Figure CP 14.1.2 Lumbar meningomyelocele (from a photograph). The ‘frog leg’ posture is characteristic of combined femoral and sciatic nerve paralysis, with preservation of hip flexion by the iliopsoas.

Adult Anatomy

The spinal cord and nerve roots are sheathed by pia mater and float in cerebrospinal fluid contained in the subarachnoid space. The pial denticulate ligament pierces the arachnoid and anchors the cord to the dura mater on each side. Outside the dura is the extradural (epidural) venous plexus (Figure 14.5), which harvests the vertebral red marrow and empties into the segmental veins (deep cervical, intercostal, lumbar, sacral). These veins are without valves, and reflux of blood from the territory of segmental veins is a notorious cause of cancer spread from prostate, lung, breast, and thyroid gland. For example, nerve root compression from collapse of an invaded vertebra may be the presenting sign of cancer in one of these organs.

Figure 14.5 Relationships of the sixth cervical spinal nerve.

The respective anterior and posterior nerve roots join at the intervertebral foramina, where the posterior root ganglia are located (Figure 14.5). The arachnoid mater blends with the perineurium of the spinal nerve, and the dura mater blends with the epineurium. The nerve roots carry extensions of the subarachnoid space into the intervertebral foramina.

Below cord level, nerve roots seeking the lower lumbar and sacral intervertebral foramina constitute the cauda equina (‘horse’s tail’). The cauda equina floats in the lumbar subarachnoid cistern (Figures 14.6–14.8), which reaches to the level of the second sacral vertebra. At its upper end, the cauda comprises nerve roots L3–S5 of both sides – a total of 32 roots (excluding the insignificant coccygeal roots).

Figure 14.6 The cauda equina in the lumbar cistern. Contributions to the femoral and sciatic nerves are shown on the right side.

Figure 14.7 Midline sagittal section of embalmed cadaver displaying thoracic, lumbar, and sacral spinal cord and cauda equina. Arrow indicates most frequent intervertebral disk to prolapse.

(Reproduced, with permission, from Atlas of Human Sectional Anatomy (2003) (Liu, S. et al., eds). Jinan: Shantung Press of Science and Technology.)

Figure 14.8 Sagittal MRI scans of the vertebral canal, weighted so as to enhance cerebrospinal fluid. (A) Brainstem, cerebellum, and cervical spinal cord are outlined. (B) Lumbosacral spinal cord and cauda are outlined.

(From a series kindly provided by Professor J. Paul Finn, Director, Magnetic Resonance Research, Department of Radiology, David Geffen School of Medicine at UCLA, California.)

In the center of the cauda equina is the unimportant filum terminale, which pierces the meninges to become attached to the coccyx.

Distribution of Spinal Nerves

Each spinal nerve gives off a recurrent branch which provides mechanoreceptors and pain receptors for the dura mater, posterior longitudinal ligament, and intervertebral disc. The synovial facet joints between successive articular processes are each supplied by the nearest three spinal nerves. Pain caused by injury or disease of any of the above structures is referred to the cutaneous territory of the corresponding posterior rami (Figure 14.9).

Figure 14.9 Cutaneous distribution of posterior rami of spinal nerves.

Segmental sensory distribution: the dermatomes

A dermatome is the strip of skin supplied by an individual spinal nerve. The dermatomes are orderly in the embryo (Figure 14.10) but they are distorted by outgrowth of the limbs (Figure 14.11). Spinal nerves C5–T1 are drawn into the upper limb, so that C4 dermatome abuts T2 at the level of the sternal angle. Nerves L2–S2 are drawn into the lower limb, so that L2 abuts S3 dermatome over the buttock. Maps like those in Figure 14.11 fail to portray overlap in the cutaneous distribution of successive dorsal nerve roots. On the trunk, e.g., the skin over an intercostal space is supplied by the nerves immediately above and below in addition to the proper nerve.

Figure 14.10 Embryonic dermatome pattern.

Figure 14.11 Adult dermatome pattern.

Segmental motor distribution

In the limbs, the individual muscles are supplied by more than one spinal nerve because of interchange in the brachial and lumbosacral plexuses. The segmental supply of the limbs is expressed in terms of movements in Figure 14.12.

Figure 14.12 Segmental control of limb movements.

(Adapted from Last, R.J. (1973) Anatomy: Regional and Applied, 5th edn. Edinburgh: Churchill Livingstone; and Rosse, C. and Clawson, D.K. (1980) The Musculoskeletal System in Health and Disease. Hagerstown: Harper & Row.)

Segmental sensory inputs and segmental motor outputs are combined during execution of withdrawal or avoidance reflexes (Box 14.1). (The prevalent term, flexor reflex, is too limited; e.g. a stimulus applied to the lateral surface of a limb may elicit adduction.)

Box 14.1 Lower limb withdrawal reflex

Figure Box 14.1.1 depicts a lower limb withdrawal reflex with crossed extensor thrust. (A) The right foot is about to enter the stance phase of locomotion. (B) Contact with a sharp object initiates a withdrawal reflex, together with the crossed extensor response required to support the entire body weight.

Sequence of events

1 Plantar nociceptors send impulse trains along tibial–sciatic afferent fibers (a) having parent posterior root ganglion somas within the L5–S1 intervertebral foramen. The impulses ascend cauda equina (b) and enter segment L5 of spinal cord. Some impulses are despatched up and down Lissauer’s tract (c) to activate segments L2–L4 and S1.

2 In all five segments, primary nociceptive afferents excite flexor reflex internuncials in the base of the posterior horn (2a). Several internuncials may be interposed, in series, between entering afferents and target motor neurons. Axons of medially placed internuncials cross the midline in the gray commissure, allowing impulse trains to activate contralateral internuncials (2b).

3 On the stimulated side, α and γ motor neurons in cord segments L3–S1 contract iliopsoas (a), hamstrings (b), and ankle dorsiflexors (d). At the same time (not shown here), ipsilateral 1a inhibitory internuncials are recruited to silence the antigravity motor neurons.

4 On the contralateral side, α and γ motor neurons in cord segments L2–L5 contract gluteus maximus (not visible here) and quadriceps femoris (c).

Note: Not shown in the figure are lateral spinothalamic tract relay neurons (see Ch. 15). These neurons receive inputs from nociceptive afferent fibers in Lissauer’s tract and they relay impulses to brain sites able to decode the location and nature of the initial stimulus.

Figure Box 14.1.1 Withdrawal reflex. MN, motor neuron.

Nerve root compression syndromes

Nerve root compression within the vertebral canal is most frequent where the spine is most mobile, namely at lower cervical and lower lumbar levels (Clinical Panel 14.2). The effects of root compression may be expressed in five different ways:

1 Pain perceived in the muscles supplied by the corresponding spinal nerve(s).

2 Paresthesia (numbness or tingling) along the respective dermatome(s).

3 Cutaneous sensory loss – more likely if two successive dermatomes are involved, because of overlap.

4 Motor weakness.

5 Loss of a tendon reflex if the segmental level is appropriate (Table 14.1).

Clinical Panel 14.2 Nerve root compression

Cervical roots

The intervertebral disks and synovial joints of the neck are subject to degenerative disease (cervical spondylosis) in 50% of 50-year-olds and in 70% of 70-year-olds. Although any or all of the joints may deteriorate, problems are most frequent in relation to vertebra C6, which provides the fulcrum for flexion/extension movements of the neck. Spinal nerve C6 (above) or C7 (below) may be pinched by extruded disk material or by bony outgrowths (osteophytes) beside the synovial joints (Figure CP 14.2.1). Sensory, motor, and reflex disturbances may result in accordance with the data in Figures 14.10 and 14.11 and Table 14.1.

Lumbosacral roots

The term lumbar spinal stenosis signifies narrowing of the lumbar vertebral canal due to encroachment by a prolapsing intervertebral disk or by bony osteophytes. Fully 95% of all disk prolapses occur immediately above or below the last lumbar vertebra. The typical herniation is posterolateral, with compression of the nerve roots passing to the next intervertebral foramen (Figure CP 14.2.2).

Symptoms include backache caused by rupture of the annulus fibrosus, and pain in the buttock/thigh/leg caused by pressure on posterior root fibers contributing to the sciatic nerve. The pain is increased by stretching the affected root, e.g. by having the straightened leg raised by the examiner.

An L4–L5 disc prolapse produces pain/paresthesia over the L5 dermatome. Motor weakness may be detected during dorsiflexion of the great toe (later, of all toes and of the ankle), and during eversion of the foot. Abduction of the hip may also be weak; this movement is tested with the patient lying on one side.

With an L5–S1 prolapse (the commonest of all) (Figure CP 14.2.3), symptoms are felt in the back of the leg/sole of foot (S1 dermatome). Plantar flexion may be weak and the ankle jerk reduced or absent.

Figure CP 14.2.1 Spondylosis on the right side of vertebra C7. Osteophytes are pinching C7 spinal nerve trunk.

Figure CP 14.2.2 Nerves compressed (arrows) by posterolateral prolapse of the two lowest intervertebral disks.

Figure CP 14.2.3 Sagittal MRI revealing a prolapsed L5/S1 intervertebral disk pressing against the cauda equina (arrow).

(Kindly provided by Professor Robert D. Zimmerman, Department of Radiology, Weil Medical College of Cornell University, New York.)

Table 14.1 Segmental levels of tendon reflexes

Segmental level	Reflex
C5, 6	Biceps
	Brachioradialis (‘supinator reflex’)
C7	Triceps
L3, 4	Knee jerk
S1	Ankle jerk

Note: Peripheral nerve entrapment syndromes are considered in Chapter 12.

Lumbar puncture (spinal tap)

The procedure involved in removing a sample of cerebrospinal fluid from the lumbar cistern is described in Clinical Panel 4.2. This procedure should not be performed if there is any reason to suspect the presence of raised intracranial pressure. (It is performed occasionally when there is some uncertainty, but only under immediate neurosurgical cover.)