Human Anatomy - M.M. Kurepina, A.P. Ozhigova, A.A. Nikitina 2010

Nervous system
Brain structures involved in motor control

The Central Nervous system receives information from receptors, processes it, and generates a response. In some cases, this response is reflex in nature, manifesting as movement, posture maintenance, or, less frequently, changes in secretory activity. The initiation and execution of all movements involving skeletal Muscles are regulated by the motor centers of the CNS. Movements can be automatic, mediated by centers in the Spinal Cord and Brainstem, or voluntary, occurring under the control of higher centers (cortex, Basal Ganglia). However, with frequent repetition of the same type of movement, a program is formed in the CNS to control these movements, rendering them subconscious and automatic. Examples include certain professional skills, such as gymnastic movements, typing, etc.

At the level of the spinal cord, relatively simple reflex arcs, known as spinal Reflexes, are completed. These include, for example, the knee-jerk reflex, which is a stretch reflex. Information from Muscle and tendon receptors enters the spinal cord through the dorsal roots and synapses in the ventral horns onto motor Neurons, which form motor endings in the muscles. In other cases, one or more interneurons exist between the sensory and motor neurons, transmitting impulses to adjacent segments of the spinal cord. These reflexes are crucial for maintaining muscle length at an optimal level and the tone of muscles involved in posture maintenance. When a muscle is stretched, receptors in the muscle spindles and tendons are excited; the resulting impulse from the motor neurons causes the muscle to contract.

Impulses arriving from cutaneous receptors (pain, tactile) often excite motor neurons not only on the ipsilateral side but also on the contralateral side. This can result in a crossed extensor reflex, in which the ipsilateral limb flexes while the contralateral limb extends.

Within the spinal cord, there are ascending and descending intersegmental reflex pathways that never leave the spinal cord. They are also known as propriospinal tracts or intrinsic bundles (the segmental apparatus) of the spinal cord. They are formed by the processes of interneurons, which may be arranged in distinct clusters. These neurons are responsible for executing automatic movements, enabling complex, coordinated movements in response to signals from the periphery or other PARTS OF THE CNS.

Spinal reflexes are under the control of higher centers of the CNS (Atl. Fig. 161). Brainstem motor control centers in lower vertebrates function more or less independently, and damage to the Forebrain does not cause noticeable movement impairment. In higher vertebrates, especially primates, these centers are subordinate to the Cerebral Cortex; cortical damage leads to significant motor deficits. The Influence of the brainstem on the spinal cord is mediated via descending pathways. These originate from the motor nuclei of the Cranial Nerves (primarily the vestibular nuclei), the red Nucleus, the Midbrain tegmentum, and the reticular formation. Additionally, feedback loops involving the cortex and basal ganglia are completed at the midbrain level.

Information from cutaneous, visual, and vestibular receptors converges in the brainstem. It helps adapt rhythmic movements, such as walking, to environmental changes (uneven ground, visual stimuli).

Descending fibers from neurons of the brainstem reticular formation travel within the reticulospinal tract to neurons whose processes form the segmental apparatus of the spinal cord. Thus, the reticular formation is directly involved in the rapid control of rhythmic movements (walking, running) and posture maintenance.

The reticular formation is closely linked to the vestibular nuclear complex. Fibers from the Cerebellum also terminate on these nuclei, and the vestibulospinal tract originates from the lateral nucleus. The medial portion of its fibers reaches only the cervical spinal cord, conveying impulses to the Neck Muscles to adjust HEAD rotation based on vestibular signals. The remaining fibers of the tract transmit impulses to limb motor neurons, stimulating their extensors, which is essential for maintaining standing posture. Fibers from the medial vestibular nucleus project to the medial longitudinal fasciculus, playing a role in eye rotation in response to vestibular stimulation.

The descending rubrospinal tract originates from the red nucleus in the midbrain and reaches the motor neurons of limb flexor muscles. Thus, the vestibular nuclei and the red nucleus exert opposing effects on limb musculature, thereby facilitating their movement during walking.

The cerebellum plays an important role in maintaining balance during movement. This is made possible by its connections with the reticular formation, the red nucleus, and the lateral vestibular nucleus (Fig. 3.23). Spinal, vestibular (via mossy fibers), and visual (via climbing fibers) signals, as well as afferents from the trigeminal system, enter the cerebellar vermis. Efferent fibers project from the neurons of the cerebellar vermis to the aforementioned structures. Consequently, the cerebellar vermis regulates and coordinates movements during their execution. The cerebellar cortex is connected to the cerebral cortex via the ventrolateral Nucleus of the thalamus, participating in motor programming and the fine mechanisms of sensorimotor coordination. The cerebellum is characterized by a somatotopic Organization of projections to the cerebral cortex.

The basal ganglia are of paramount importance in organizing motor acts. Information from the cortex, thalamus, and other subcortical structures converges on these structures. Efferent pathways from the basal ganglia project to the cortex, primarily to the frontal lobe. Projections of the basal ganglia are topographically organized. The activity of neurons in specific Regions of the basal ganglia always corresponds to specific movements of particular body parts and can also determine the force, amplitude, or direction of that movement. Such regulation involves pathways running from the premotor (area 6), motor, and sensorimotor cortical areas through the putamen and the medial segment of the globus pallidus or the substantia nigra to the motor nuclei of the thalamus, and then back to area 6 of the cortex. It has been established that neurons responsible for face and Mouth movements are located in the lateral region of the substantia nigra.

Signals traveling from areas 7 and 8 to the caudate nucleus, the medial segment of the globus pallidus, and the ventrolateral part of the substantia nigra control gaze direction. Their subsequent pathway runs through the thalamus to the frontal eye field (area 8). Axons of substantia nigra neurons also project to the superior colliculus of the midbrain. The pathways controlling the movement of different parts of the trunk, head, or eyes do not overlap at the level of the basal ganglia. Their integration may occur at the cortical level. Most likely, the integrating area is area 6, which receives inputs from the motor and frontal association cortices.

The primary motor cortex is located in the precentral gyrus, including the anterior wall of the central sulcus, and corresponds to cytoarchitectonic area 4. The cellular composition of this region has several distinctive features: the internal granular layer is absent, and layer V contains large pyramidal Betz Cells. This cortical region is referred to as the agranular gigantopyramidal cortex. Regions of the motor cortex responsible for the movement of specific body parts are located in strictly defined Zones of the precentral gyrus (Fig. 3.82). Larger cortical areas regulate the movement of body parts that possess the greatest freedom of movement.

More complex motor responses also occur upon stimulation of areas 1, 2, 3, 5, and 6. For instance, stimulation of area 6 leads to Rotation of the trunk and eyes and elevation of the contralateral arm. This indicates that impulses from several motor systems converge on neurons of area 6 (Fig. 3.83). For this reason, the cortex of area 6 is called the secondary motor area. It is divided into medial (supplementary motor area) and lateral (premotor cortex) parts. Recent studies have demonstrated that in humans, area 6 plays a leading role as an associative motor field. This is supported by increased metabolic activity in this cortical zone during movement execution. Specifically, in the primary motor cortex, Blood flow increases in the representation area of the moving body part. However, an even more pronounced increase in blood flow has been observed in area 6. METABOLISM in this zone increases most when the movement requires heightened attention or when the subject is asked to imagine the movement without executing it. Most of the efferent pathways from the basal ganglia passing through the thalamus terminate in the cortex of area 6.

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Fig. 3.82. Distribution of neurons responsible for the movement of individual body parts in the precentral gyrus (motor cortex):

1 — knee; 2 — hip; 3 — trunk; 4 — shoulder; 5 — arm; 6 — elbow; 7 — wrist; 8 — hand; 9 — fingers; 10 — thumb; 11 — neck; 12 — eyebrow; 13 — eye; 14 — face; 15 — Lips; 16 — jaw; 17 — Tongue

Abundant evidence exists of extensive connections between the motor and somatosensory cortical areas. Via association fibers, neurons of the motor cortex receive signals from sensory areas that transmit information about the outcome of a motor act (obstacles to movement, required force, etc.). Some of the neurons activated in this process are corticospinal; their excitation influences the activity of spinal motor neurons. Consequently, the activity of these motor neurons depends not only on excitation propagating along spinal reflex arcs but also on the state of cortical neurons.

Axons of cortical neurons form descending corticospinal (pyramidal) tracts (Fig. 3.84). 40% of the axons in these tracts originate from Cells of the motor cortex (area 4), 20% from the somatosensory cortex (areas 3, 1, and 2), and the remaining 40% join the corticospinal pathways from other cortical areas. Only 3% of the fibers are axons of Betz cells in layer V of the cortex. All fibers of the Pyramidal Tracts terminate on neurons of the contralateral side of the Brain, i.e., they decussate. This decussation can occur at different levels of the CNS. The fibers of the pyramidal tracts pass through the internal capsule and descend into the Base of the cerebral peduncle, then into the ventral part of the Pons and Medulla Oblongata. Here, these pathways are visible on the ventral surface as paired elevations called pyramids, which gave the entire tract its name. The pyramidal tracts comprise three fiber systems: corticonuclear (corticobulbar), corticopontine, and corticospinal. Corticonuclear fibers terminate on neurons of the motor nuclei of cranial nerves (pairs III, IV, V, VI, VII, IX, X, XI, XII) on the contralateral side of the respective brainstem regions. Some fibers of the pyramidal tracts terminate on the nuclei of the pontine base, forming the corticoponto-cerebellar pathway. At the level of the pons, some fibers also terminate on neurons of the reticular formation.

Fig. 3.83. A — motor areas of the human cerebral cortex: 1 — primary somatosensory cortex; 2 — posterior parietal cortex; 3 — premotor cortex; 4 — frontal eye field; 5 — supplementary motor area; 6 — primary motor cortex; B — map of the human brain based on motor responses to electrical stimulation of various areas of the brain surface during neurosurgical operations. PMC — premotor cortex, or lateral area 6; MC — motor cortex, or area 4 (roughly corresponding to the precentral gyrus); SMA — supplementary motor area of the cortex, or medial area 6; POS — parieto-occipital sulcus; 1 — toes; 2 — FOOT; 3 — leg; 4 — thigh; 5 — abdomen; 6 — chest; 7 — scapula; 8 — shoulder; 9 — forearm; 10 — hand; 11 — digit V; 12 — digit IV; 13 — digit III; 14 — digit II; 15 — thumb; 16 — neck; 17 — face, upper part; 18 — face, lower part; 19 — tongue; 20 — Mandible; 21 — palate; 22 — Pharynx; 23 — Larynx; 24 — rotation of the head, eyes, and trunk to the opposite side; 25 — synergy of flexors and extensors of the contralateral limbs; 26 — rotation of the eyes to the opposite side; 27 — simultaneous synergy of flexors of the contralateral arm and leg with involvement of the ipsilateral leg; rotation of the head, eyes, and trunk to the opposite side; 28 — rotation of the head and eyes to the opposite side; synergy of flexors of the contralateral limbs; 29 — rotation of the eyes to the opposite side; complex visual sensations; 30 — visual sensations; 31 — central visual field; 32 — auditory sensations, rotation of the head, eyes, and trunk to the opposite side; 33 — synergy of flexors or extensors of the contralateral limbs; 34 — chewing, licking, swallowing, vocalization, hiccuping; 35 — Urinary Bladder; 36 — rectum; 1—22 — cortical areas

Fig. 3.84. Motor tracts:

A — corticospinal: 1 — motor cortex of the precentral gyrus; 2 — pyramidal cells of cortical layer V; 3 — corticospinal tract; 4 — parietopontine fibers; 5 — frontopontine fibers; 6 — longitudinal fibers in the base of the pons; 7 — pyramid; 8 — decussation of pyramids; 9 — anterior corticospinal tract; 10 — spinal cord interneurons; 11 — ventral ROOT; 12 — motor nerve endings; 13 — spinal cord; 14 — lateral corticospinal tract; 15 — medulla oblongata; 16 — Hypoglossal nerve nucleus (XI); 17 — pons; 18 — Abducens nerve nucleus; 19 — oculomotor (III) nerve nucleus; 20 — cerebral peduncles; 21 — midbrain; 22 — caudate nucleus; 23 — insular cortex; 24 — claustrum; 25 — lentiform nucleus; 26 — internal capsule; 27 — thalamus; B — vestibulospinal: 1 — IV ventricle; 2 — abducens nerve nucleus; 3 — pons; 4 — vestibular nerve; 5 — superior olivary nucleus; 6 — medial lemniscus; 7 — pyramidal tract; 8 — medulla oblongata; 9 — spinal nucleus of the Accessory nerve; 10 — decussation of pyramids; 11 — lateral vestibulospinal tract; 12 — motor endings in muscles; 13 — ventral root; 14 — anterior horn neurons; 15 — spinal cord; 16 — medial vestibulospinal tract; 17 — vestibular nuclei; 18 — facial (VII) nerve and its nucleus; 19 — spinal nucleus of the trigeminal (V) nerve; 20 — vestibular nuclei

Corticospinal fibers reach the neurons of the spinal cord. At the junction of the medulla oblongata and the spinal cord, a partial decussation of the tract fibers occurs. The larger bundle of fibers (up to 80%) crosses to the contralateral side, into the lateral funiculus of the spinal cord, where it forms the lateral pyramidal tract. The fibers of this tract synapse with interneurons of the dorsal horns of the spinal cord, which, via interneurons of the ventral horn, transmit impulses to motor neurons. The latter innervate the Muscles of the Trunk and limbs. The remaining 20% of the fibers remain in the ipsilateral anterior funiculus of the spinal cord, forming the anterior pyramidal tract. In each spinal segment, they cross to the contralateral side and synapse in the ventral horns onto interneurons and then onto motor neurons innervating the muscles of the trunk and limbs. The function of the pyramidal tracts is to control spinal motor neurons by recruiting reflex pathways at the level of the segmental apparatus. Additionally, the fibers of these tracts participate in regulating the activity of spinal cord neurons belonging to the Somatosensory system.

Brain structures that regulate motor function, but not via the pyramidal tracts, are classified as the extrapyramidal system (Atl. Fig. 161). These structures include the previously discussed red nuclei, vestibular nuclei, reticular formation, and superior colliculi. However, according to modern physiological and morphological data, these structures function cooperatively and under the control of the cerebral cortex. Furthermore, they send efferent fibers to the cortex, influencing its activity. Thus, classifying the extrapyramidal system as an independent motor control system is highly conventional.

Review Questions

1. List the BRAIN AND SPINAL cord structures involved in motor control.

2. How are posture maintenance and Eye Movements controlled during stimulation of the vestibular apparatus?

3. What is the difference between voluntary and automatic movements?

4. How do the CEREBRAL CORTEX AND basal ganglia participate in The regulation of motor activity?