Brain Anatomy and Function
The most notable structure of the adult human brain are the brain stem, cerebellum, and cerebrum. The lowest, most caudal portion of the brain stem, the medulla oblongata, is continuous with the spinal cord. Above it, the pons bulges prominently and is contuous with the mid-brain, into which the stemlike cerebral peduncles extend. The peduncles carry upper motor neuron fibers that originate in the cerebral cortex to the cell bodies of cranial nerves in the brain stem and to cells in the spinal cord, called lower motor neurons, which cause certain muscles to move.
Brain Stem
The brain stem contains all afferent and efferent nerve fibers between the spinal cord and the higher brain centers. Some upper motor neuron fibers cross in the brain stem, whereas others do not. Most cranial nerve to each side of the head connect evenly with both cerebral hemispheres. In contrast 80% of the fibers in the corticospinal tract from each side cross over in the medulla oblongata, so that the affected muscles are controlled primarily by one hemisphere. Thus, a unilateral (single-sided) lesion in the cerebrum or brain stem above this crossover causes weakness or paralysis on the opposite side of the body.
Cranial Nerves
The human brain has 12 paired cranial nerves. The first, 12, the olfactory and optic nerves, enter the brain above the brain stem and are actually extension of the brain. Afferent impulses originating in peripheral sensory organs form ascending fiber tracts that cross either in the spinal cord or in the brain stem on their way to synapsing in nuclei above. The largest of these nuclei is the thalamus, whose many subdivisions act as relay stations to the cerebral cortex, the primary cortical areas. Different afferent tracts and thalamic areas subserve sensations of pain; sensations of heat and cold; proprioception (the sensation of movement at joints); and special senses of sight, taste, and hearing. The other 10 cranial nerves, in descending order of location, are the oculomotor, trochlear, trigeminal, abducens, facial, acoustic, gloss pharyngeal, vegus, spinal accessory, and hypoglossal nerves.
Brain Stem
The brain stem contains all afferent and efferent nerve fibers between the spinal cord and the higher brain centers. Some upper motor neuron fibers cross in the brain stem, whereas others do not. Most cranial nerve to each side of the head connect evenly with both cerebral hemispheres. In contrast 80% of the fibers in the corticospinal tract from each side cross over in the medulla oblongata, so that the affected muscles are controlled primarily by one hemisphere. Thus, a unilateral (single-sided) lesion in the cerebrum or brain stem above this crossover causes weakness or paralysis on the opposite side of the body.
Cranial Nerves
The human brain has 12 paired cranial nerves. The first, 12, the olfactory and optic nerves, enter the brain above the brain stem and are actually extension of the brain. Afferent impulses originating in peripheral sensory organs form ascending fiber tracts that cross either in the spinal cord or in the brain stem on their way to synapsing in nuclei above. The largest of these nuclei is the thalamus, whose many subdivisions act as relay stations to the cerebral cortex, the primary cortical areas. Different afferent tracts and thalamic areas subserve sensations of pain; sensations of heat and cold; proprioception (the sensation of movement at joints); and special senses of sight, taste, and hearing. The other 10 cranial nerves, in descending order of location, are the oculomotor, trochlear, trigeminal, abducens, facial, acoustic, gloss pharyngeal, vegus, spinal accessory, and hypoglossal nerves.
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