Central Components of the Somatosensory System · 39	2

When a heavy object is lifted, the tone normally present in the quadriceps muscle no longer suffices to keep the body erect. Buckling at the knees can be prevented only by an immediate increase in quadriceps tone, which occurs as a result of tonic intrinsic reflexes induced by the stretching of the muscle and of the muscle spindles within it. This feedback mechanism or servomechanism enables automatic adaptation of the tension in a muscle to the load that is placed upon it. Thus, whenever an individual stands, walks, or lifts, action potentials are constantly being relayed back and forth to ensure the maintenance of the correct amount of muscle tension.

Central Components of the Somatosensory System

Having traced the path of afferent impulses from the periphery to the spinal cord in the preceding sections, we will now proceed to discuss their further course within the central nervous system.

Root entry zone and posterior horn. Individual somatosensory fibers enter the spinal cord at the dorsal root entry zone (DREZ; also called the Redlich­Obersteiner zone) and then give off numerous collaterals that make synaptic contact with other neurons within the cord. Fibers subserving different sensory modalities occupy different positions in the spinal cord (Fig. 2.15). It is important to note that the myelin sheaths of all afferent fibers become considerably thinner as the fibers traverse the root entry zone and enter the posterior horn. The type of myelin changes from peripheral to central, and the myelinating cells are no longer Schwann cells, but rather oligodendrocytes.

The afferent fiber pathways of the spinal cord subserving individual somatosensory modalities (Fig. 2.16) will now be described individually.

Posterior and Anterior Spinocerebellar Tracts

Some of the afferent impulses arising in organs of the musculoskeletal system (the muscles, tendons, and joints) travel by way of the spinocerebellar tracts to the organ of balance and coordination, the cerebellum. There are two such tracts on each side, one anterior and one posterior (Fig. 2.16a).

Posterior spinocerebellar tract. Rapidly conducting Ia fibers from the muscle spindles and tendon organs divide into numerous collaterals after entering the spinal cord. Some of these collateral fibers make synaptic contact directly onto the large α motor neurons of the anterior horn (monosynaptic reflex arc,

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Crossed anterio spinocerebellar

tract

Posterior spinocerebellar tract

Anterior spinocerebellar tract

Lateral spinothalamic tract

Spinotectal tract

Spino-olivary tract

Lateral spinothalamic tract (pain, temperature)

Posterior columns

Fasciculus Fasciculus cuneatus gracilis (of Burdach) (of Goll)

Anterior spinothalamic tract

Posterior

columns

Motor fiber

Anterior spinocerebellar tract

Posterior spinocerebellar tract

Proprioception (unconscious)

Muscle spindle and tendon organ (to the cerebellum and forebrain)

Proprioception, vibration, touch, pressure, discrimination

(to the thalamus and cerebral cortex)

Touch, pressure

Pain, temperature

Medial bundle

Lateral bundle

Fig. 2.15 Position of fibers of different somatosensory modalities in the posterior root and root entry zone, and their further course in the spinal cord

System Somatosensory 2 · 40 2

Central Components of the Somatosensory System · 41 2

Figs. 2.15 and 2.11). Other collateral fibers arising at thoracic, lumbar, and sacral levels terminate in a column-shaped nucleus occupying the base of the posterior horn at levels C8­L2, which is variously named the intermediolateral cell column, thoracic nucleus, Clarke’s column, and Stilling’s nucleus. The postsynaptic second neurons with cell bodies lying in this nucleus are the origin of the posterior spinocerebellar tract, whose fibers are among the most rapidly conducting of any in the body. The posterior spinocerebellar tract ascends the spinal cord ipsilaterally in the posterior portion of the lateral funiculus and then travels by way of the inferior cerebellar peduncle to the cerebellar vermis (p. 253; Figs. 2.16a and 2.17). Afferent fibers arising at cervical levels (i.e., above the level of the intermediolateral cell column) travel in the fasciculus cuneatus to make a synapse onto their corresponding second neurons in the accessory cuneate nucleus of the medulla (Fig. 2.17), whose output fibers ascend to the cerebellum.

Anterior spinocerebellar tract. Other afferent Ia fibers entering the spinal cord form synapses with funicular neurons in the posterior horns and in the central portion of the spinal gray matter (Figs. 2.15, 2.16a, and 2.17). These second neurons, which are found as low as the lower lumbar segments, are the cells of origin of the anterior spinocerebellar tract, which ascends the spinal cord both ipsilaterally and contralaterally to terminate in the cerebellum. In contrast to the posterior spinocerebellar tract, the anterior spinocerebellar tract traverses the floor of the fourth ventricle to the midbrain and then turns in a posterior direction to reach the cerebellar vermis by way of the superior cerebellar peduncle and the superior medullary velum. The cerebellum receives afferent proprioceptive input from all regions of the body; its polysynaptic efferent output, in turn, influences muscle tone and the coordinated action of the agonist and antagonist muscles (synergistic muscles) that participate in standing, walking, and all other movements. Thus, in addition to the lower regulatory circuits in the spinal cord itself, which were described in earlier sections, this higher functional circuit for the regulation of movement involves other, nonpyramidal pathways and both α and γ motor neurons. All of these processes occur unconsciously.

Posterior Columns

We can feel the position of our limbs and sense the degree of muscle tension in them. We can feel the weight of the body resting on our soles (i.e., we “feel the ground under our feet”). We can also perceive motion in the joints. Thus, at least some proprioceptive impulses must reach consciousness. Such impulses are derived from receptors in muscles, tendons, fasciae, joint capsules, and

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242 · 2 Somatosensory System

Vermis

	Via superior	3rd neuron
	medullary velum

Anterior spinocerebellar tract,

2nd neuron

	Posterior spinocere-	2nd neuron
	bellar tract
	2nd neuron
	Thoracic nucleus	Fasciculus
		gracilis
	(Clarke’s column,
	Stilling’s nucleus)	Fasciculus
		cuneatus
	1st neuron

Thalamus

Medial lemniscus

Nucleus gracilis and nucleus cuneatus

1st neuron

aa BildlegendeUnconsciouseinfŸgenproprioception===

3rd neuron

		Thalamus
	Anterior spino-	2nd neuron
	thalamic tract

c Coarse touch and pressure ===perception c Bildlegende einfŸgen

bb BildlegendePosition, vibration,einfŸgentouch,===discrimination

3rd neuron

Thalamus

2nd neuron

Lateral spinothalamic tract

Substantia gelatinosa

1st neuron

1st neuron

dPain, temperature (also===tickle, itch,

dBildlegende einfŸgen

sexual sensations)

Fig. 2.16 Major fiber tracts of the spinal cord and the sensory modalities that they subserve. a The anterior and posterior spinocerebellar tracts. b The posterior funiculus (posterior columns). c The anterior spinothalamic tract. d The lateral spinothalamic tract.

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Central Components of the Somatosensory System · 43	2

3rd neuron	Paleo-
	cerebellum

Lateral spinothalamic tract

Medial lemniscus

2nd neuron

Posterior spino-	Proprioceptive, homonymous
cerebellar tract
Anterior spino-
cerebellar tract

Dorsal external arcuate fibers

Nucleus gracilis and nucleus cuneatus

Accessory cuneate nucleus

	Posterior spino-		Proprioception
	cerebellar
			(muscle spindles,
	tract
			Golgi organs, joint
			bodies, etc.)
	Anterior		Position sense,
	spino-
			vibration, pressure,
	cerebellar
			discrimination,
	tract
			touch
	Anterior		(cutaneous receptors,
			muscle and tendon
	spino-
		1st neuron	receptors, Vater-Pacini
	thalamic
			corpuscles)
	tract
			Pressure,touch
			(peritrichial nerve
			endings and various
			cutaneous receptors)
			Pain, temperature
			(free nerve endings,
			Krause and Ruffini
			corpuscles?)

Fig. 2.17 Spinal cord with major ascending pathways and their further course to target structures in the cerebrum and cerebellum (schematic drawing)

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244 · 2 Somatosensory System

connective tissue (Vater­Pacini and Golgi­Mazzoni corpuscles), as well as cutaneous receptors. The afferent fibers conveying them are the distal processes of pseudounipolar neurons in the spinal ganglia. The central processes of these cells, in turn, ascend the spinal cord and terminate in the posterior column nuclei of the lower medulla (Figs. 2.16b and 2.17).

Central continuation of posterior column pathways. In the posterior funiculus of the spinal cord, the afferent fibers derived from the lower limbs occupy the most medial position. The afferent fibers from the upper limbs join the cord at cervical levels and lie more laterally, so that the posterior funiculus here consists of two columns (on either side): the medial fasciculus gracilis (column of Goll), and the lateral fasciculus cuneatus (column of Burdach). The fibers in these columns terminate in the correspondingly named nuclei in the lower medulla, i.e., the nucleus gracilis and the nucleus cuneatus, respectively. These posterior column nuclei contain the second neurons, which project their axons to the thalamus (bulbothalamic tract). All of the bulbothalamic fibers cross the midline to the other side as they ascend, forming the so-called medial lemniscus (Figs. 2.16b and 2.17). These fibers traverse the medulla, pons, and midbrain and terminate in the ventral posterolateral nucleus of the thalamus (VPL, Fig. 6.4, p. 266). Here they make synaptic contact with the third neurons, which, in turn, give off the thalamocortical tract; this tract ascends by way of the internal capsule (posterior to the pyramidal tract) and through the corona radiata to the primary somatosensory cortex in the postcentral gyrus. The somatotopic organization of the posterior column pathway is preserved all the way up from the spinal cord to the cerebral cortex (Fig. 2.19a). The somatotopic projection on the postcentral gyrus resembles a person standing on his head—an inverted “homunculus” (Fig. 9.19, p. 374).

Posterior column lesions. The posterior columns mainly transmit impulses arising in the proprioceptors and cutaneous receptors. If they are dysfunctional, the individual can no longer feel the position of his or her limbs; nor can he or she recognize an object laid in the hand by the sense of touch alone or identify a number or letter drawn by the examiner’s finger in the palm of the hand. Spatial discrimination between two stimuli delivered simultaneously at different sites on the body is no longer possible. As the sense of pressure is also disturbed, the floor is no longer securely felt under the feet; as a result, both stance and gait are impaired (gait ataxia), particularly in the dark or with the eyes closed. These signs of posterior column disease are most pronounced when the posterior columns themselves are affected, but they can also be seen in lesions of the posterior column nuclei, the medial lemniscus, the thalamus, and the postcentral gyrus.

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To the posterior column nuclei

Central Components of the Somatosensory System · 45	2

Fig. 2.18 Posterior funiculus, containing the posterior columns: fasciculus gracilis (medial, afferent fibers from lower limb) and fasciculus cuneatus (lateral, afferent fibers from upper limb)

Fasciculus cuneatus, from upper limb

Fasciculus gracilis, from lower limb

The clinical signs of a posterior column lesion are, therefore, the following:

Loss of the sense of position and movement (kinesthetic sense): the patient cannot state the position of his or her limbs without looking.

Astereognosis: the patient cannot recognize and name objects by their shape and weight using the sense of touch alone.

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Postcentral gyrus

		Arm	Shoulder	Head
	Forearm			Neck
Hand				Trunk
Finger		V		Hip
	IV
III				Thigh
II

	Thumb	Leg
	Eye
	Face	Toes,
	Upper lip	genitalia
	Lower lip
	Jaw
	Tongue	Tail of caudate nucleus
	Throat	T
	Abdomen,	hala
		m
	viscera	us

P		Palli
		du
utam		m
	en

Insula

Internal capsule

Head of caudate nucleus

Claustrum

Corticospinal tract

Medial lemniscus

Lateral spinothalamic tract

Fig. 2.19 Course of the sensory pathways by way of the thalamus and internal capsule to the cerebral cortex

Agraphesthesia: the patient cannot recognize by touch a number or letter drawn in the palm of the hand by the examiner’s finger.

Loss of two-point discrimination.

Loss of vibration sense: the patient cannot perceive the vibration of a tuning fork placed on a bone.

Positive Romberg sign: The patient cannot stand for any length of time with feet together and eyes closed without wobbling and perhaps falling over.

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