This section explains how the nervous system is organized to assess sensory input and execute motor actions. The functioning nervous system has a hierarchical organization to carry out its activities.
Incoming sensory fibers, called afferents, have their input into the spinal cord as well as the brainstem, except for the special senses of vision and olfaction (which will be discussed separately). This sensory input is processed by relay nuclei, including the thalamus, before the information is analyzed by the cortex. In the cortex, there are primary areas that receive the information, other cortical association areas that elaborate the sensory information, and still other areas that integrate the various sensory inputs.
On the motor side, the outgoing motor fibers, called efferents, originate from motor neurons in the brainstem and the spinal cord. These motor nuclei are under the control of motor centers in the brainstem and cerebral cortex. In turn, these motor areas are influenced by other cortical areas and by the basal ganglia, as well as by the cerebellum.
Simpler motor patterns are organized as reflexes. In all cases, except for the myotatic (muscle) reflex, called the stretch reflex (discussed with Figure 44), there is some processing that occurs in the CNS, involving interneurons in the spinal cord, brainstem, thalamus, or cortex.
The processing of both sensory and motor activities, beyond simple reflexes, therefore involves a series of neuronal connections, creating functional systems. These include nuclei of the CNS at the level of the spinal cord, brainstem, and thalamus. In almost all functional systems in humans, the cerebral cortex is also involved. The axonal connections between the nuclei in a functional system usually run together forming a distinct bundle of fibers, called a tract or pathway. These tracts are named according to the direction of the pathway, for example spino-thalamic, means that the pathway is going from the spinal cord to the thalamus; cortico-spinal means the pathway is going from the cortex to the spinal cord. Along their way, these axons may distribute information to several other parts of the CNS by means of axon collaterals.
In Part I of this section, we will be concerned with the sensory tracts or pathways and their connections in the CNS. Part II introduces the reticular formation, which has both sensory, motor, and other "integrative" functions. In Part III we will discuss the pathways and brain regions concerned with motor control.
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