55 3

3 Motor System

Central Components of the

Motor System and Clinical

Syndromes of Lesions Affecting

Them . . . . . . . . . . . . . . . . . . . . . . . 57

Peripheral Components of the

Motor System and Clinical

Syndromes of Lesions Affecting

Them . . . . . . . . . . . . . . . . . . . . . . . 67

Complex Clinical Syndromes due to Lesions of Specific Components of the Nervous

System . . . . . . . . . . . . . . . . . . . . . . 69

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356

3Motor System

The motor impulses for voluntary movement are mainly generated in the precentral gyrus of the frontal lobe (primary motor cortex, Brodmann area 4) and in the adjacent cortical areas (first motor neuron). They travel in the long fiber pathways (mainly the corticonuclear and corticospinal tracts/pyramidal pathway), passing through the brainstem and down the spinal cord to the anterior horn, where they make synaptic contact with the second motor neuron—usually by way of one or more intervening interneurons.

The nerve fibers emerging from area 4 and the adjacent cortical areas together make up the pyramidal tract, which is the quickest and most direct connection between the primary motor area and the motor neurons of the anterior horn. In addition, other cortical areas (especially the premotor cortex, area 6) and subcortical nuclei (especially the basal ganglia, cf. p. 330) participate in the neural control of movement. These areas form complex feedback loops with one another and with the primary motor cortex and cerebellum; they exert an influence on the anterior horn cells by way of several distinct fiber pathways in the spinal cord. Their function is mainly to modulate movement and to regulate muscle tone.

Impulses generated in the second motor neurons of the motor cranial nerve nuclei and the anterior horn of the spinal cord pass through the anterior roots, the nerve plexuses (in the cervical and lumbosacral regions), and the peripheral nerves on their way to the skeletal muscles. The impulses are conveyed to the muscle cells through the motor end plates of the neuromuscular junction.

Lesions of the first motor neuron in the brain or spinal cord usually produce spastic paresis, while lesions of the second motor neuron in the anterior horn, anterior root, peripheral nerve, or motor end plate usually produce flaccid paresis. Motor deficits rarely appear in isolation as the result of a lesion of the nervous system; they are usually accompanied by sensory, autonomic, cognitive, and/or neuropsychological deficits of various kinds, depending on the site and nature of the causative lesion.

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Central Components of the Motor System and Clinical Syndromes of Lesions Affecting Them · 57	3

Central Components of the Motor System and Clinical Syndromes of Lesions Affecting Them

The central portion of the motor system for voluntary movement consists of the primary motor cortex (area 4) and the adjacent cortical areas (particularly the premotor cortex, area 6), and the corticobulbar and corticospinal tracts to which these cortical areas give rise (Figs. 3.1 and 3.2).

Motor Cortical Areas

The primary motor cortex (precentral gyrus, Fig. 3.1) is a band of cortical tissue that lies on the opposite side of the central sulcus from the primary somatosensory cortex (in the postcentral gyrus) and, like it, extends upward and past the superomedial edge of the hemisphere onto its medial surface. The area representing the throat and larynx lies at the inferior end of the primary motor cortex; above it, in sequence, are the areas representing the face, upper limbs, trunk, and lower limbs (Fig. 3.2). This is the inverted “motor homunculus,” corresponding to the “somatosensory homunculus” of the postcentral gyrus that was discussed in Chapter 2 (see Fig. 9.19, p. 374).

Motor neurons are found not only in area 4 but also in the adjacent cortical areas. The fibers mediating fine voluntary movements, however, originate mainly in the precentral gyrus. This is the site of the characteristic, large pyramidal neurons (Betz cells), which lie in the fifth cellular layer of the cortex and send their rapidly conducting, thickly myelinated axons (Fig. 3.3) into the pyramidal tract. The pyramidal tract was once thought to be entirely composed of Betz cell axons, but it is now known that these account for only 3.4­4% of its fibers. The largest fiber contingent in fact originates from the smaller pyramidal and fusiform cells of Brodmann areas 4 and 6. Axons derived from area 4 make up about 40% of all pyramidal tract fibers; the remainder come from other frontal areas, from areas 3, 2, and 1 of the parietal somatosensory cortex (sensorimotor area), and from other areas in the parietal lobe (Fig. 3.1). The motor neurons of area 4 subserve fine, voluntary movement of the contralateral half of the body; the pyramidal tract is, accordingly, crossed (see Fig. 3.4). Direct electrical stimulation of area 4, as during a neurosurgical procedure, generally induces contraction of an individual muscle, while stimulation of area 6 induces more complex and extensive movements, e. g., of an entire upper or lower limb.

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	3	58 · 3 Motor System
	2 1		3 4 6aα	6aβ	Fig. 3.1 Primary motor
					area/precentral gyrus

8	(area 4), premotor cor-
	tex (area 6), and pre-
	frontal eye field (area
	8). For the functions of
	these areas, see text.

8

	Thorax, abdomen
Upper	Shoulder
	Thigh
limb
Hand	Leg

	Finger V		Toes
	IV		Bladder, rectum
	III		Precentral gyrus
	II
Thumb
Neck, face
Tongue		Dorsal	Vision, hearing
Jaw
			Temporopontine tract

Larynx, pharynx

		P
		alli
u		d
		um
P
tam
	en

Insula

Claustrum

Somatic sensation

Thalamus

Genu of internal capsule Head of caudate nucleus Frontopontine tract Frontothalamic tract

Lentiform nucleus

Ventral

Corticospinal tract

Fig. 3.2 Course of the pyramidal tract, upper portion: the corona radiata and internal capsule

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