Functional Neuroanatomy and the Neurologic Examination

Functional Neuroanatomy

At its most basic level, the nervous system allows us to interact with external stimuli, serving as a bridge between the environment and our internal mental and physical worlds. In humans, there is a large evaluation step between stimulus and response, which allows for a carefully chosen (or programmed) response that is additionally influenced by an actual or perceived situational context. Using an information-processing model, we can map these concepts in three distinct steps: input of sensory information through perceptual modules, the internal integration and evaluation of this information, and the production of a response . These steps are carried out by four main anatomic systems in the brain: the thalamus , the cortex , the medial temporal lobe , and the basal ganglia ( Figure 5-1 ).

Sensory organs provide information about physical attributes of incoming information. Details of physical attributes (e.g., temperature, sound frequency, color) are conveyed through multiple segregated channels in each perceptual module. Information then passes through the thalamus, which serves as the gateway to cortical processing for all sensory data with the exception of olfaction. Specifically, it is the relay nuclei (ventral posterior lateral, medial geniculate, and lateral geniculate) that convey sensory information from the sensory organs to the appropriate area of primary sensory cortex (i.e., S1, A1, or V1) ( Figure 5-2 ).

The first step in the integration and evaluation of incoming stimuli occurs in unimodal association areas of the cortex, where physical attributes of one sensory domain are linked together. A second level of integration is reached in multimodal association areas, including regions in the parietal lobe and prefrontal cortex, which link together the physical attributes from different sensory domains. A third level of integration is provided by input from limbic and paralimbic regions of the brain, including the cingulate cortex and regions of the medial temporal lobe (hippocampus and amygdala). At this level of integration, the brain creates a representation of experience that has the spatiotemporal resolution and full complexity of the outside world, imbued with emotion and viewed in the context of prior experience. Evaluation and interpretation involve the comparison of new information with previously stored information and current expectations or desires, which allows the brain to classify information as new or old, or as threatening or non-threatening.

Based on the result of evaluation and interpretation, the brain then creates a response. The regions involved in generating, for example, motor responses, include the motor cortex, the basal ganglia, the motor nuclei of the thalamus, and the cerebellum. The basal ganglia, which include the striatum (made up of the caudate and the putamen) and the globus pallidus, are charged with integrating and coordinating this motor output. The striatum receives input from the motor cortex, projecting to the globus pallidus. The globus pallidus in turn relays the neostriatal input to the thalamus. The thalamus then projects back to the cortical areas that gave rise to the corticostriatal projections, thereby closing the cortico-striato-pallido-thalamo-cortical (CSPTC) loop. This loop is thought to be the means by which motor control is enacted; damage to regions in this loop leads to disorders, such as Parkinson's disease and Huntington's disease. In addition to motor responses, the brain generates other types of outputs, such as cognitive decision-making, emotional reactions, or socially meaningful behavioral responses. These outputs also rely on similar frontal CSPTC loops, which start in non-motor parts of the frontal cortex, such as the dorsolateral prefrontal, the medial prefrontal, or the orbitofrontal cortices. Structural or physiologic lesions to these systems lead to the cognitive, affective, or behavioral signs and symptoms that constitute neuropsychiatric syndromes.

The Neurologic Examination

The neurologic examination is a set of steps designed to probe the input, integration/evaluation, and output domains of information processing. Here, we provide an overview of the examination, using this framework. For complete details, see standard texts of neurology.