Lois B. Laemle

The Reticular Formation and the Limbic System

The reticular formation (RF), although seemingly diffuse is actually a highly organized network of neurons distributed through the tegmental core of the brainstem. The RF extends rostrally as the intralaminar nuclei of the thalamus and caudally as the gray matter in the vicinity of the central canal of the spinal cord. The RF receives the summation of sensory information that flows into the spinal cord and brainstem via the peripheral nerves. These inputs are significant in influencing the level of arousal (i.e., the responsive tone of conscious awareness and physical performance). The RF contains assemblies of interacting neurons that integrate reflexes and basic stereotyped action patterns mediated by cranial nerve input. The basic motor responses range from facial nerve-innervated muscle responses to mechanisms of eating, drinking, and breathing. They are centrally coordinated and assembled as highly complex behaviors under voluntary control by higher cerebral motor circuits, including those of the limbic system. The precise and subtle patterns of motor behaviors mediated by cranial nerves involve the organized circuitry of the brainstem RF.

Reflexes and Muscle Tone

The spinal cord contains the local neuronal circuits that coordinate somatic reflexes. These circuits are participants in the complex voluntary movements of the body that are governed by the higher centers of the brain. One expression of somatic motor function is the effortless ease with which humans carry out the most dexterous of motor activities without a conscious awareness of joint movements and the accompanying muscle contractions synchronized with the required “relaxing” of antagonistic muscles. The quality of sequential combinations of movements is dependent on the continuous flow of visual, somatosensory, and postural (vestibular and joint senses) information that results in seemingly immediate integrated responsive actions. Although we might be consciously aware of making decisions regarding execution of the movements to accomplish the goal, we are unaware of the details that are instrumental in creating the motions, as they seem to take place automatically.

Gross Anatomy of the Brain

The average human brain weighs about 1,500 g (3 lbs) or approximately 2% of the body weight of a 150-lb adult. The brain, a gelatinous mass, is invested by a succession of three connective tissue membranes called meninges and is protected by an outer capsule of bone, the skull. The brain floats in cerebrospinal fluid (CSF), which supports it and acts as a shock absorber in rapid movements of the head. The major arteries and veins supplying the brain lie among the meninges.

Discriminative General Senses, Crude Touch, and Proprioception

Touch and the discriminative general senses (DGSs) encompass a number of sensory modalities. Touch by itself refers to crude (also called light) and movement sensation, which yields little information apart from the fact of contact with an object. The DGSs also include the following: (1) “pressure touch,” which enables an awareness of shape, size, and texture; (2) stereognosis, appreciation of an object’s three dimensionality; (3) perception of an object’s weight; (4) vibratory sense (as tested with a tuning fork); (5) position sense (awareness of body parts, especially joints); and (6) awareness of body and limb movement, including direction. The last two are often grouped as kinesthetic sense.

Motoneurons and Motor Pathways

The sensory systems create our mental images of the external world. These representations provide us with information and cues that guide the motor systems to generate movements produced by the coordinated contractions and relaxations. The motor systems are hierarchically organized in the central nervous system (CNS) as the spinal neuronal circuits that control the automatic stereotypic reflexes ( Chap. 8). Higher centers in the brainstem mediate postural controlled and rhythmic locomotor movements. The highest centers, including the motor areas of the cerebral cortex, initiate and regulate complex skilled voluntary movements.

Auditory and Vestibular Systems

The auditory and vestibular systems often are considered together because their end organs share space within the petrous portion of the temporal bone (Fig. 16.1) and both arise from the otic vesicle. They also share the VIIIth cranial nerve, albeit the two almost completely separate divisions, auditory and vestibular classified as special somatic afferent. However, the auditory system is exteroceptive, whose purpose is to transduce airborne waves in the acoustic spectrum and most importantly deliver signals to higher centers of the auditory system for perception of sounds. The vestibular system in contrast is proprioceptive. Its receptors monitor head position and movement and convey this information into the brain stem, where it is integrated into the motor systems. The vestibular system is important for maintaining equilibrium and upright posture and for control of synergistic eye movements.

Pain and Temperature

Our perception of the external world, state of attentiveness, assessment of body image, and regulation of movements are all dependent upon input from sensory systems. The sensory receptors that respond to a particular physical property or modality (e.g., temperature), together with tracts and nuclei that transmit this information to higher levels of the central nervous system (CNS), are referred to as a somatosensory system. There are four major general somatic modalities—the sensations of pain (signaling tissue damage or chemical irritation), temperature (warmth or cold), touch, (for recognition of size, shape, and texture), and proprioception (sense of static position and movements of the limbs, body, and head) (see also Chap. 10). Somatosensory receptors are located in the skin, muscles, joints, and viscera, a distribution that makes this system the largest and most varied of the sensory systems. Although primarily sensory, this system is also of importance in the control of coordinated movements by providing appropriate feedback to the somatic motor system about joint position, muscle tension, velocity of muscular contractions, and contact of the body with external surfaces (Chaps. 8 and 11). Sensations occur when stimuli interact with receptors (sensors). Sensory information then is transmitted cephalically as patterns of action potentials by individual neurons and by assemblies of neurons acting in consort. All sensory systems, regardless of modality, transmit information pertaining to the intensity, duration, and location of the stimuli that activate them. Every general sensory system processes input sequentially through: (1) primary afferent fibers, (2) relay nuclei located in the spinal cord, brainstem, and thalamus, (3) cerebral cortex. Each processing nucleus not only consolidates inputs from adjacent receptors but also integrates signals from inhibitory neurons and descending projections to transform and enhance the sensory information (Fig. 3.13).

Cranial Nerves and Chemical Senses

Twelve pairs of cranial nerves are peripheral nerves of the brain (Fig. 14.1). The olfactory and optic nerves are nerves of the cerebrum (telencephalon). The other ten pairs are nerves of the brain stem (and, in one case, of the cervical spinal cord). They supply structures of the head and neck and, in the case of the vagus nerve, structures of the trunk. Some cranial nerves contain almost exclusively afferent fibers, others almost exclusively efferent fibers, and a third group contains substantial proportions of both afferent and efferent fibers (Table 14.1). The afferent fibers arise from cell bodies located in peripheral ganglia with one exception, those that mediate unconscious proprioception; their central processes enter the brain stem and end in sensory nuclei (Figs. 14.2 and 14.5). Efferent fibers arise from cell bodies located in brain stem motor nuclei (Fig. 14.3). Cranial nerves pass to and from the cranial cavity through foramina, canals, and fissures in the skull.

Basal Ganglia and Extrapyramidal System

The cerebrum exerts control of voluntary somatic motor activity through several descending pathways. The direct pathway is via the pyramidal system, which includes the corticospinal (pyramidal) tract together with fibers that diverge from it to innervate cranial nerve motoneurons—the corticobulbar tract (Fig. 24.1). The cortical neurons that form the tract are upper motoneurons. There are two other major descending pathways that arise from the cortex, the corticorubral/rubrospinal tract and the corticoreticular/reticulospinal tract, but these are less direct. They involve a synapse in the red nucleus and in the reticular formation of the lower brainstem, respectively. Before evolution of the cerebral cortex in mammals, voluntary somatic motor activity was mainly mediated by upper motoneurons in the red nucleus and the brainstem reticular formation. These nuclei together with the corpus striatum, the major component of the basal ganglia and the most important forebrain center for motor control in premammalian vertebrates, were designated as components of the extrapyramidal system whose descending fibers pass through the tegmentum rather than the medullary pyramid. However, a dichotomy between a pyramidal and an extrapyramidal system does not really exist. The cortex which gives rise to all of these descending tract systems is anatomically and functionally inextricably interconnected with the basal ganglia.

Development and Growth of the Nervous System

Individuals are as old as their neurons in the sense that almost all neurons are generated by early postnatal life and, except for a few locations such as the olfactory bulb and subplate of portions of the cerebral cortex, are not generally replaced by new ones during a lifetime. On the other hand, development of the complex circuitry of the nervous system continues throughout life, tempered and honed by readjustments and responses to the influences and demands from both the internal and external environments.