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by Rhawn Joseph, Ph.D.
(Academic Press, New York, 2000)
THE SECONDARY MOTOR AREAS
by Rhawn Joseph, Ph.D.
FUNCTIONAL NEUROANATOMY OF THE FRONTAL SECONDARY MOTOR AREAS
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THE SECONDARY MOTOR AREAS
by Rhawn Joseph, Ph.D.
FUNCTIONAL NEUROANATOMY OF THE FRONTAL SECONDARY MOTOR AREAS
Movement and motor functioning are dependent on the functional integrity of the basal ganglia, brainstem, cerebellum, cranial nerve nuclei, the motor thalamus, spinal cord, as well as the primary, secondary and supplementary motor areas of the frontal lobes (Passingham, 1997; Schmahmann, 1997). Indeed, these areas are all interlinked and function as an integrated system in the production of movement (Mink, 1997; Mink & Thach, 1991; Parent & Hazrati 1995; Passingham, 1997).
However, as to fine motor movements including those involved in the articulation of speech, these are almost completely dependent upon the functional integrity of the primary motor areas located along the percental gyrus (area 4), and within which are represented the muscles controlling the hands, fingers, and oral laryngeal musculature. And yet, as noted above, the primary motor areas are in turn dependent upon motoric impulses which are organized in premotor and the supplementary motor cortex--the latter of which is located along the medial wall of the hemispheres. Considered from a very broad and simplistic perspective, it could be said that primary area is programmed and under the control of the secondary and supplementary motor areas as well as the "prefrontal" and other areas of the cerebrum, although neurons in the primary area also become active prior to and during movement (Passingham, 1997).
For example, Exner's writing area is in part, within areas 6 and becomes active prior to (as well as during) hand movements and appear to program hand movements, whereas the frontal eye fields (within areas 6,8,9) becomes active prior to (as well as during) eye movements and appears to program eye movements. As noted above, the primary area representing the oral-laryngeal musculature, is programmed by Broca's expressive speech area areas 45, 46 (Foerster 1936; Fox 1995; LeBlanc 1992; Petersen et al. 1988, 1989). Broca's area also becomes active prior to vocalization and during subvocalization as indicated by functional imaging.
Hence, like the sensory areas where information is generally received in the primary zones before transmission to the association areas as well as in the association areas independently of the primary areas (Zeki, 1997) motor impulses are processed and acted on in parallel (Passinghma, 1997), though in general, they generally begin their organizational journey in the supplementary motor areas (Alexander & Crutcher, 1990; Crutcher & Alexander, 1990) and/or the cingulate and other forebrain structures (Passingham, 1997; Stephan, et al., 1999), which are then transmitted to the primary regions where they are acted upon. Again, however, there are areas within the primary region which also become active prior to movement, though these same areas become increasing active during movement (Passingham, 1993).
Because, in general, the impulse to move originates outside the primary motor cortex, direct electrical stimulation of this region does not give rise to complex, coordinated, or purposeful movements (Penfield & Boldrey, 1937; Penfield & Jasper, 1954; Penfield & Rasmussen, 1950; Rothwell et al. 1987). Rather stimulation will only induce, for example, twitching of the lips, flexion or extension of a single finger joint, protrusion of the tongue or elevation of the palate. It is noteworthy that patients never claim to have willed these movements, which again suggests the will-to-move is initiated elsewhere. However, if electrical stimulation is applied when the patient is attempting to move, the result is paralysis (Penfield & Jasper, 1954; Penfield & Rasmussen, 1950). Presumably the reception of impulses-to-move (which are initiated and organized elsewhere) are blocked by primary motor electrical stimulation.
Motor functioning and movement, however, is also dependent on the parietal lobes, thalamus, basal ganglia, brainstem, cerebellum, and spinal cord, structures which are directly or indirectly interconnected (Kaas, 1993; Mink, 1997; Passingham, 1997; Schmahmann, 1997). For example, smooth, purposeful, coordinated movement requires sensory feedback which is provided by the primary and association/secondary somatosensory areas located in the parietal lobe and which contain neurons which guide the hand and arm in visual space (see chapter 20). These cortices are intimately linked with the primary motor and the motor association/secondary and supplementary motor areas as well as the basal ganglia, brainstem, and cerebellum--regions which become highly active during and often prior to movement initiation (Passingham, 1997). For example, when human subjects learn a sequence of finger movements, functional imaging reveals increased activity in the primary and premotor cortex, and the primary and association somesthetic cortex, as well as in the cerebellum, striatum, ventral thalamus and cerebellum (Passingham, 1997). In fact, most of the "motor" area also contain independent motor maps of the body, which is why, if a subject moves an arm, leg, or shoulder, each of these areas becomes active almost simultaneously--at least as demonstrated with functional imaging.
On the other hand, as based on functional imaging, it has also been shown that there is increased activation of the primary motor area when subjects actually make a movement, as compared to merely preparing to make the movement (Passingham, 1997).
Hence, although some movements are programmed in parallel by a number of motor areas simultaneously, others are programmed in temporal sequences such that the SMA programs the premotor area which acts on the primary area which becomes maximally active when movements are actually performed. IN this regard it is noteworthy that whereas stimulation of the SMA can prevent movement, stimulation of the premotor area can give rise to the illusion that a movement is about to be made, that is, of an impending movement (Penfield, 1938), whereas activation of the primary area can trigger twitching, and minor motor movements--which again suggests a hierarchical organization.
THE SECONDARY/PREMOTOR MOTOR AREAS
The SMA is directly linked with the premotor areas (area 6), which in turn appears to be dependent on the SMA for functional programming. That is, the premotor area receives impulses to move that have been organized in the SMA, processes and integrate these signals, and then acts to program the adjacent primary motor areas with which it is intimately interconnected. Both the premotor and primary motor areas are located along the lateral and medial surface of the hemisphere. Although the premotor area does not contain giant Betz cells (which are found in area 4), it contributes almost 1/3 of the fibers of the corticospinal tract.
In addition to its interconnections with the SMA and primary motor areas (Jones et. al., 1978; Jones & Powell, 1970), the premotor area receives information directly from the primary and secondary somesthetic and visual (area 17, 18, 19) cortices (Jones & Powell, 1970; Pandya & Kuypers, 1969). It is precipally concerned with the guidance and refinement of movement via the assimilation of sensory information provided by the sensory areas (Godschalk et al. 1981; Porter 1990) and interacts with the basal ganglia, motor thalamus, and SMA so as to achieve these goals (Alexander & Crutcher, 1990; Crutcher & Alexander, 1990; Mink & Thach, 1991; Parent & Hazrati 1995).
As noted, the SMA becomes activated prior to the premotor area, which becomes activate prior to the primary motor area. Hence, whereas neurons in the primary motor region become active during movement, excitation in the premotor cortex precedes cellular activation of the primary region (Weinrich et al. 1984). Moreover, cells in the premotor cortex become activated before movements are even initiated, whereas electrical stimulation of this area induces the illusion of a impending movement (Penfield, 1938). These and other findings suggest that the premotor area may be modulating and exerting controlling and integrative influences on impulses which are to be transmitted to the primary region for expression. Indeed, the premotor area appears to be highly involved in the programming of various gross and fine motor activities, and becomes highly active during the learning of new motor programs (Passingham, 1997; Porter 1990; Roland et al. 1981). Moreover, electrical stimulation elicits complex patterned movement sequences as well as stereotyped and gross motor responses such as head turning or torsion of the body (Fulton, 1934; Passingham, 1981, 1993).
Unlike the primary area, damage limited to the pre-motor cortex does not result in paralysis but disrupts fine motor functioning and dexterity, including simple activities such as finger tapping (Luria, 1980). With extensive damage fine motor skills are completely lost and phenomena such as the grasp reflex are elicited (Brodal, 1981); i.e. if the patient's hand is stimulated it will invulantarily clasp shut.
EXNER'S WRITING AREA
Exner's writing area lies within a small region along the lateral convexity, near the foot of the second frontal convolution of the left hemisphere, occupying the border regions of Broadmans areas 46, 8, 6. Although some authors have denied the existence of Exner's area, this region appears to be the final common pathway where linguistic impulses receive their final motoric stamp for the purposes of writing; i.e. the formation of graphemes and their temporal sequential expression. Thus, Exner's writing area appears to program the ajacent hand-area represented in the primary motor areas (e.g. Boroojerdi et al., 1999) so that lingusitic impulses received through Broca's area, can be integrated into hand movements so that words can be written down.
Exner's area is dependent on Broca's expressive speech area with which it maintains extensive interconnections. In fact, Exner's writing center extends to and appears to become coextensive with Broca's area (Lesser et al. 1984)--which in turn was originally a hand area--at least in primates. Broca's area possibly acts to organize and relay impulses received from the posterior language zones to Exner's area in instances where written expression is desired. Exner's area, in turn, transfers this information to the secondary and primary motor areas for final expression.
Electrical stimulation of this vicinity in awake moving patients has resulted in the arrest of ongoing motor acts, including the capacity to write or perfrom rapid alternating movements of the fingers (Lesser et al., 1984). In some instances, writing and speech arrest were noted.
Agraphia.
Lesions or seizure activity localized to this vicinity lead to deficiencies involving the elementary motoric aspects of writing; i.e. agraphia (Penfield & Roberts, 1959; Ritaccio et al. 1992; Tohgi et al. 1995). Grapheme formation becomes labored, incoordinated, and takes on a very sloppy appearance. Cursive handwriting is usually more disturbed than printing. In cases of well circumscribed lesions, usually there are no gross deficiencies of motor functioning or speech, although mild articulatory disturbances may be observed (e.g. lisping) as well as abnormalties involving fine motor control (Cf Lesser et al., 1984; Goodglass & Kaplan, 2000; Levine & Sweet, 1982.)
In cases of pure (frontal) agraphia, spelling may or may not be affected, whereas with parietal lesions spelling as well as writing is often abnormal. Rather, with left frontal lesions more frequently there are disturbances of grapheme selection such that the patient may seem to have "forgotten" how to form certain letters and/or may misplace or even add unnecessary letters when writing (Hecaen & Albert, 1978; Tohgi et al. 1995). When spelling orally or typing the ability to spell is often better preserved.
Damage localized to this vicinity can be secondary to perinatal trauma, tumors, or vascular abnormalities. Disturbances involving constructional or manipulospatial functioning are not apparent. In fact, one such patient whose damage was secondary to birth injury, although able to write or print only with great difficulty, was able to draw and paint with some professional acumen. However, his ability to copy letters was severely effected. Hence, disturbances secondary to lesions localized to Exner's area limited to abnormalities involving linguistic-symbolic grapheme motor control.
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