Somatosensory System
There are two major somatosensory pathways running from the spinal cord to the primary sensory area (area 3 or S1) on the postcentral gyrus of the cerebral cortex (Fig. 2.1). Information about touch and proprioception is carried in the dorsal column medial lemniscus system, whereas temperature and pain information traverses the spinothalamic pathway [3].
In the touch system, fibers from first-order neurons with cell bodies in the dorsal root ganglia traverse the dorsal columns of the spinal cord (Fig. 2.1) to the dorsal column nuclei within the caudal medulla. Of these, the cuneate nucleus receives input from the upper limbs and body whereas the gracile nucleus deals with fibers from the lower limbs. Second-order neurons then project across the midline as the “sensory decussation” and ascend via the medial lemniscus to the ventral posterolateral nucleus of the thalamus. Touch information from the head follows a parallel route, with the first major relay in the principal trigeminal nucleus of the pons, with mid-pontine decussation to reach the ventral posteromedial nucleus of the thalamus via the ventral trigeminal tract. The inputs from the various regions of the body are segregated and aligned such that the pathway is somatotopically organized. This is reflected by the cortical projection (Fig. 2.1), where there is an orderly mapping of the contralateral side of the body on the cortical surface.
Fig. 2.2 Schematic cross-section of spinal cord showing the gray matter and white matter containing the major sensory (right) and motor (left) nerve tracts
Information about temperature and pain (particularly the fast component) is transmitted by the spinothalamic tract. Here, the first synaptic relay is in the gray matter of the spinal cord; projection fibers then decussate in the ventral white commissure to form the contralateral spinothalamic (or anterolateral) tract (Fig. 2.2).These reach the ventral posterolateral nucleus of the thalamus, and some fibers also project to the small thalamic intralaminar nuclei. Parallel pathways transmit information from the face, via the spinal trigeminal nucleus, to the ventral posteromedial thalamus. The pathway depicted in Figure 2.1 is also known as the neospinothalamic pathway, and there are other, phylogenetically older pathways transmitting pain information (particularly the slow, poorly localized, long-lasting component). These are the paleospinothalamic and spinoreticulothalamic pathways, which are polysynaptic and ascend through the reticular formation to nonspecific nuclei in the medial thalamus and intralaminar nuclei. They project to widespread regions of cerebral cortex, rather than being associated with the somatotopic organization of the primary sensory cortex: this may contribute to the poor ability to localize slow pain.
Motor System
There are a number of important fiber tracts (Fig. 2.2) descending from the brain to control the activity of ventral horn motoneurons supplying the skeletal muscles [2]. These can be divided into lateral and ventromedial groups. The lateral pathway comprises corticospinal and rubrospinal tracts and is primarily involved in voluntary movement, particularly of the distal muscles, under direct cortical control. The ventromedial pathways originate in the brainstem, forming reticulospinal, tectospinal and vestibulospinal tracts, which are involved in the control of posture and locomotion.
The most important component of the lateral pathway is the corticospinal tract, which originates primarily from areas 4 (primary motor cortex or MI) and 6 (supplementary motor area) of the frontal lobe, on the precentral gyrus, located across the central sulcus from the primary somatosensory cortex. The motor cortex is somatotopically organized, and axons pass through the int9rnal capsule and course through the midbrain and pons to form a pyramid-shaped tract running down the ventral surface of the medulla. At the junction with the spinal cord, the pyramidal tract decussates. Thus, as in the somatosensory system, the right motor cortex processes information for the left side of the body and vice versa. The axons from the motor cortex then group to form the lateral corticospinal tract and terminate in the dorsolateral and intermediate gray matter region of the ventral horns, where the motoneurons and interneurons that control the distal muscles are located.
The rubrospinal tract is a much smaller component of the lateral pathway and originates in the red (Latin ruber) nucleus of the midbrain. Axons decussate in the pons and join the corticospinal tract in the lateral columns of the spinal cord. The red nucleus itself receives its major input from the motor areas of cerebral cortex, which also give rise to the corticospinal tract. While the rubrospinal tract is important in many mammalian species, in man much of its function has been taken over by the corticospinal pathway.
The ventromedial pathways may be divided functionally into two groups: the tectospinal and vestibulospinal tracts control the posture of head and neck, whereas the pontine and medullary reticulospinal tracts control the posture of the trunk and limb antigravity muscles. The vestibulospinal tract originates in the medullary vestibular nuclei, which are involved with processing sensory activity from the vestibular apparatus of the inner ear. In combination with proprioceptive information about body and neck position, this pathway is importantly involved in maintaining head-body alignment to ensure that the eyes and our image of the world remain stable [4]. The tectospinal tract originates in the superior colliculus of the midbrain. This structure receives direct input from the retina as well as the visual cortex and auditory systems. It is involved with the coordination of head and eye movements and orienting responses towards stimuli [5].
The pontine reticulospinal tract acts to facilitate the antigravity reflexes of the spinal cord to aid the maintenance of a standing posture by promoting extensor activity in the lower limbs and flexor activity in the upper limbs. The medullary reticulospinal tract has an opposite action, to inhibit reflex domination of anti-gravity muscles, thus allowing greater control by lateral pathways. The balance of activity in these reticulospinal tracts is controlled by descending signals from motor cortex [2].
Fig. 2.3 Schematic of motor control showing the major cortical and subcortical regions of the central nervous system. The association and premotor areas of cerebral cortex, along with the basal ganglia, are responsible for planning and initiating voluntary movements. The motor cortex, and its direct connection with the α-motoneurons in the spinal cord, is responsible for sending the appropriate instructions for execution of the movement by the skeletal muscles. The cerebellum provides information about coordination, sequencing, and timing of complex movements. Brainstem mechanisms, along with the vestibular apparatus, superior colliculus, and older areas of the cerebellum, have a major role in the control of posture and gait
Fig. 2.4 Cross-section of the brain showing the location of the major nuclei that contribute to basal ganglia function upper limbs. The medullary reticulospinal tract has an opposite action, to inhibit reflex domination of antigravity muscles, thus allowing greater control by lateral pathways. The balance of activity in these reticulospinal tracts is controlled by descending signals from motor cortex [2].
While cortical areas 4 and 6 comprise the motor cortex, in terms of the control of voluntary movement many other areas of the cerebral cortex are involved as well as important subcortical structures such as the basal ganglia and cerebellum. The overall