Color Atlas of Neurology

Cortical Structures

Different areas of the cerebral cortex (neocortex) may be distinguished from one another by their histological features and neuroanatomical connections. Brodmann’s numbering scheme for cortical areas has been used for many years and will be introduced in this section.

Projection areas. By following the course of axons entering and leaving a given cortical area, one may determine the other structures to which it is connected by afferent and efferent pathways. The primary projection areas are those that receive most of their sensory impulses directly from the thalamic relay nuclei (primary somatosensory cortex; Brodman areas 1, 2, 3), the visual (area 17), or the auditory (areas 41, 42) pathways. The primary motor cortex (area 4) sends motor impulses directly down the pyramidal pathway to somatic motor neurons within brainstem and the spinal cord. The primary projection areas are somatotopically organized and serve the contralateral half of the body. Proceeding outward along the cortical surface from the primary projection areas, one encounters the secondary projection areas

(motor, areas 6, 8, 44; sensory, areas 5, 7a, 40; visual, area 18; auditory, area 42), which subserve higher functions of coordination and information processing, and the tertiary projection areas (motor, areas 9, 10, 11; sensory, areas 7b, 39; visual, areas 19, 20, 21; auditory, area 22), which are responsible for complex functions such as voluntary movement, spatial organization of sensory input, cognition, memory, language, and emotion. The two hemispheres are connected by commissural fibers, which enable bihemispheric coordination of function. The most important commissural tract is the corpus callosum; because many tasks are performed primarily by one of the two hemispheres (cerebral dominance), interruption of the corpus callosum can produce various disconnection syndromes. Total callosal transection causes splitbrain syndrome, in which the patient cannot name an object felt by the left hand when the eyes are closed, or one seen in the left visual hemifield (tactile and optic anomia), and cannot read words projected into the left visual hemifield (left hemialexia), write with the left hand (left hemiagraphia), or make pantomimic move-

ments with the left hand (left hemiapraxia). Anterior callosal lesions cause alien hand syndrome (diagonistic apraxia), in which the patient cannot coordinate the movements of the two hands. Disconnection syndromes are usually not seen in persons with congenital absence (agenesis) of the corpus callosum.

Cytoarchitecture. Most of the cerebral cortex consists of isocortex, which has six distinct cytoarchitectural layers. The Brodmann classification of cortical areas is based on distinguishing histological features of adjacent areas of isocortex.

Functional areas. The functional organization of the cerebral cortex can be studied with various techniques: direct electrical stimulation of the cortex during neurosurgical procedures, measurement of cortical electrical cortical activity (electroencephalography and evoked potentials), and measurement of regional cerebral blood flow and metabolic activity. Highly specialized areas for particular functions are found in many different parts of the brain. A lesion in one such area may produce a severe functional deficit, though partial or total recovery often occurs because adjacent uninjured areas may take over some of the function of the lost brain tissue. (The extent to which actual brain regeneration may aid functional recovery is currently unclear.) The specific anatomic patterns of functional localization in the brain are the key to understanding much of clinical neurology.

Subcortical Structures

The subcortical structures include the basal ganglia, thalamus, subthalamic nucleus, hypothalamus, red nucleus, substantia nigra, cerebellum, and brain stem, and their nerve pathways. These structures perform many different kinds of complex information processing and are anatomically and functionally interconnected with the cerebral cortex. Subcortical lesions may produce symptoms and signs resembling those of cortical lesions; special diagnostic studies may be needed for their precise localization.

I (Molecular layer)

II (Outer granule cell layer)

III (Middle pyramidal cell layer)

IV (Inner granule cell layer)

V (Large pyramidal cells)

The precise region of impaired sensation to light touch and noxious stimuli is an important clue for the clinical localization of spinal cord and peripheral nerve lesions. Reflex abnormalities and autonomic dysfunction are further ones, as discussed below (p. 40, p. 110).

Dermatomes (pp. 34, 36)

A dermatome is defined as the cutaneous area whose sensory innervation is derived from a single spinal nerve (i.e., dorsal root). The division of the skin into dermatomes reflects the segmental organization of the spinal cord and its associated nerves. Pain dermatomes are narrower, and overlap with each other less, than touch dermatomes (p. 104); thus, the level of a spinal cord lesion causing sensory impairment is easier to determine by pinprick testing than by light touch. (The opposite is true of peripheral nerve lesions.) Radicular pain is pain in the distribution of a spinal nerve root, i.e., in a dermatome; pseudoradicular pain may occupy a bandlike area but cannot be assigned to any particular dermatome. Pseudoradicular pain can be caused by tendomyosis (pain in the muscles that move a particular joint), generalized tendomyopathy or fibromyalgia, facet syndrome (inflammation of the intervertebral joints), myelogelosis (persistent muscle spasm resulting from overexertion), and other conditions. For mnemonic purposes, it is useful to know that the C2 dermatome begins in front of the ear and ends at the occipital hairline; the T1 dermatome comes to the midline of the forearm; the T4 dermatome is at the level of the nipples (which, however, belong to T5); the T10 dermatome includes the navel; the L1 dermatome is in the groin; and the S1 dermatome is at the outer edge of the foot and heel.

through the dorsal branches of the spinal nerves. Knowledge of the myotomes of each spinal nerve, and of the segment-indicating muscles

(Table 2, p. 357) in particular, enables the clinical and electromyographic localization of radicular lesions causing motor dysfunction. The seg- ment-indicating muscles are usually innervated by a single spinal nerve, or by two, though there is anatomic variation.

Plexuses (pp. 34, 36) and Peripheral

Nerves (pp. 35, 37)

The ventral branches of spinal nerves supplying the limbs join together to form the cervical (C1– C4), brachial (C5–T1), lumbar (T12–L4), and sacral plexuses (L4–S4). The brachial plexus begins as three trunks, the upper (derived from the C5 and C6 roots), middle (C7), and lower (C8, T1). These trunks split into divisions, which recombine to form the lateral (C5–C7), posterior (C5–C8), and medial (C8 and T1) cords (named by their relation to the axillary artery). The cords of the brachial plexus branch into the nerves of the upper limb (p. 35). The nerves of the anterior portion of the lower limb are derived from the lumbar plexus, which lies behind and within the psoas major muscle (p. 37); those of the posterior portion of the lower limb from the sacral plexus. The coccygeal nerve (the last spinal nerve to emerge from the sacral hiatus) joins with the S3–S5 nerves to form the coccygeal plexus, which innervates the coccygeus and the skin over the coccyx and anus (mediates the pain of coccygodynia).