Why is so much of the cerebral cortex devoted to sensory and motor connections to the eyes?
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The cerebral cortex is the outer covering of gray matter over the hemispheres. This is typically 2- 3 mm thick, covering the gyri and sulci. Certain cortical regions have somewhat simpler functions, termed the primary cortices. They are also involved in the synthesis of movements.
Primary somatosensory cortexis located in the post central gyrus. This receives somatotopic input from the VPL and VPM of the thalamus. Histologically, this area would consist of granular cortex. The sensory homunculus includes cortical representation of the body based on the degree of sensory innervation. There are actually four submaps, one each in area 3a, 3b, 1 and 2. Very sensitive areas such as the lips and the fingertips have a huge representation. Neurons within each cortical site (particularly layer IV) are arranged in columns representing specific body regions. If a region is amputated there is reorganization with neurons responding to stimulation of adjacent body parts. This can also happen as the result of increased use of a body part. Damage to the sensory cortex results in decreased sensory thresholds, an inability to discriminate the properties of tactile stimuli or to identify objects by touch.
The secondary somatosensory cortex is in the lower parietal lobe. This receives connections from the primary sensory cortex and also less specific thalamic nuclei. This responds to sensory stimuli bilaterally, although with much less precision than the primary cortex. Nonetheless, lesions to this area may impair some elements of sensory discrimination.
The somatosensory association cortexis directly posterior to the sensory cortex in the superior parietal lobes. This receives synthesized connections from the primary and secondary sensory cortices. These neurons respond to several types of inputs and are involved in complex associations. Damage can affect the ability to recognize objects even though the objects can be felt . Cortical damage, particularly in the area of cortex where the posterior parietal lobe meets the anterior occipital and the posterior, superior temporal lobe, can cause neglect of the contralateral side of the world. This typically happens with nondominant hemisphere lesions since this hemisphere appears necessary to distribute attention to both sides of the body. The dominant hemisphere appears to only “pay attention” to the associated side of the world. Therefore, neglect usually involves the left side and can be so severe that the individual even denies that their left side belongs to them.
Primary and Secondary Motor Cortices
The primary motor cortex is in the precentral gyrus. This is the origin of most of the corticospinal tract and a large number of cortical bulbar fibers, particularly those controlling motor cranial nerves. This also has projections to the thalamus and basal ganglion. The VL of the thalamus makes significant input to this nucleus and the precentral gyrus also receives significant input from sensory cortical areas as well as from the premotor portions of the cerebral cortex. There is a very well-defined somatotopic organization of the motor cortex and this is the region of cortex from which movements can be generated by the lowest intensity of electrical stimulation. Specific movements tend to be represented (such as elbow flexion) rather than specific muscles. Lesions produce spastic contralateral weakness, which is most prominent in the distal extremities.
The premotor cortex is immediately anterior to the motor cortex and has many of the same connections as the motor cortex. However, most of its output is to the motor cortex, with a smaller output to the brain stem and the spinal cord. This region receives input from the sensory association cortex as well as feedback from the basal ganglia via the VA and VL of the thalamus. Electrical stimulation of this area tends to produce more complex movements and at a higher stimulus intensity than the simple movements from MI. Lesions produce less severe weakness but greater spasticity than patients with isolated precentral gyrus lesions.
The supplementary motor areais a part of the premotor cortex that extends onto the medial side of hemisphere. This projects to the primary motor cortex, basal ganglia, thalamus and brain stem and also has connections with the contralateral supplementary motor area. This area becomes active before movement and is felt to be involved in initiation of motion. Lesions of this area can cause inability to initiate motions, called abulia.
Eye Fields
The occipital eye fields are located in the visual association cortex. This projects to the frontal eye fields as well as to the pontine nuclei to generate smooth pursuit eye movements. Lesions will produce difficulty in fixing on a target and also will produce abnormalities in optokinetic responses.