Cerebrum

The outermost part of the brain is the cerebral cortex, which is often referred to as the gray matter because the neurons are unmyelinated and give a gray rather than a white appearance. The outer layer of the brain is composed of fissures and grooves, or sulci, and the ridge between grooves is the gyrus. The young infant has fewer convolutional surfaces, or sulci, in the cerebral cortex and more pliable skull bones. These characteristics decrease the incidence of bruising and tearing of the cerebral cortex in the young infant with minor head trauma. The sulci of the brain deepen throughout childhood and continue to mature and change into young adulthood.

The cerebrum is the largest part of the brain and is covered by the cerebral cortex. The cerebrum is divided into two hemispheres, the left and right hemispheres, with the left hemisphere being dominant in 95% of individuals. The right hemisphere controls the functions of the left side of the body, and the left hemisphere controls the functions of the right side of the body. The hemispheres are connected by a bridge of myelinated axons, the corpus callosum, which lies between the fissures of the left and right hemispheres.¹ The corpus callosum controls and integrates motor, sensory, and higher intellectual functions. The right and left hemispheres are divided into four lobes with arbitrary borders named the same as the skull bones that cover them. Each lobe controls particular bodily functions and behaviors (Figure 19-2).

The basal ganglia are masses of myelinated axons that form a subcortical layer of white and gray matter through the central hemispheres. They border the lateral ventricle deep in the brain. The maturation of the basal ganglia occurs in early childhood, and they are involved in movement functions such as arm swinging during walking and running and throwing a ball overhand. Dysfunction of the basal ganglia results in abnormal postural movement patterns as in cerebral palsy and Huntington chorea.

Cerebellum

The cerebellum is located in the posterior cranium and has an outer layer of gray matter overlying white matter. The cerebellum maintains the body’s equilibrium and coordinates both voluntary and involuntary movements of the limbs, trunk, head, larynx, and eyes.¹ The motor cortex in the cerebrum relays signals to the cerebellum, which results in the fluid and skilled muscle movements requiring a high level of dexterity. It is the portion of the brain that processes the sensory input from the musculoskeletal system as well as from the visual, auditory, and touch receptors, and it transmits signals to the motor system to direct or correct muscle activity. Damage to the cerebellum causes ataxia (loss of coordination of motor movement, inability to perform rapid alternating movements, a wide-based gait), hypotonia, and nystagmus. In preterm infants, arrested development of the cerebellum can result in deficits in language, visual reception, and social/behavioral function.

Brainstem

The brainstem is in the central core of the brain and includes the pons, medulla oblongata, and midbrain (Figure 19-3). The pons acts as the neural transmission center from all parts of the central cortex and supports ascending and descending nerve fibers. It controls basic breathing, eating, and motor functions. Cranial nerve V (trigeminal nerve) and cranial nerve VI (abducens nerve) arise from the pons. Damage to the peripheral pons causes a loss of sensory functions of the facial and mouth area and a loss of outward or lateral motion of the eye muscles resulting in strabismus. The medulla oblongata, which lies between the pons and the cerebellum, is a continuation of the spinal cord. The medulla processes impulses from the hypoglossal, vagal, spinal accessory, glossophyarngeal, and the vestibular and acoustic cranial nerves. It also aids in the life functions of respiration and circulation, and controls involuntary reflexes such as coughing, sneezing, and yawning. Damage to the medulla causes weakness in the shoulder muscles, affects tongue muscles and salivary function, decreases gastrointestinal motility, alters swallowing and speech functions, causes nerve deafness, and diminishes cardiovascular and respiratory functions.

The diencephalon (Figure 19-4) is the extension of the brainstem and lies embedded in the cerebral cortex. It contains the thalamus, hypothalamus, pituitary gland, and the pineal gland (an endocrine gland that produces melatonin, a hormone that regulates sleep-wake cycle). Parts of the third ventricle and the nuclei of the cranial nerves also arise from the diencephalon. The midbrain contains the neural fibers that come from the spinal cord and merge into the thalamus and hypothalamus. The midbrain controls the integration of basic bodily functions.

The limbic system is the group of subcortical structures in the diencephalon including the hypothalamus and the hippocampus. This system regulates emotion and motivation and organizes memories. Aggression and fear also are regulated by the limbic system. Disruption that occurs during the development of the limbic system causes distorted perceptions and aggressive behavior. Maternal substance abuse early in pregnancy can disrupt the migration of neuron activity in the cerebral cortex, and later prenatal exposure disrupts neuronal synapses. Children exposed to illicit drug use in utero often have disturbances in memory, learning, attention span, and oppositional behavior disorders.

Spinal cord

The spinal cord is an extension of the medulla oblongata and is composed of gray and white matter extending to the lumbar region. The gray matter runs laterally along the spinal cord and protects the myelinated and unmyelinated fibers of the white matter. Proprioceptors, the specialized nerve endings in muscles, tendons, and joints, are located in the white matter and are sensitive to changes in the tension of muscles and tendons. Temperature, pain, touch, and equilibrium are transmitted through the proprioceptors to the brainstem.

The brain and spinal cord are lubricated by cerebrospinal fluid, normally a clear liquid that is formed in the ventricles of the brain. It flows from the lateral ventricles to the third and fourth ventricles through a series of foramens. The fourth ventricle, which lies in the medulla, contains three openings that allow the cerebrospinal fluid to pass into the subarachnoid space.

The brain is covered by protective layers that cushion and lubricate the outer surface (Figure 19-5). The dura mater lies just beneath the skull bone and periosteum and consists of layers of fibrous connective tissue. Adjacent to the dura mater is the arachnoid, the avascular, weblike membrane that cushions the cortex. The dura mater is separated from the arachnoid by the subdural space. The pia mater is the highly vascular area of the cortex that attaches directly to the gray matter or irregular surface of the brain. The subarachnoid area and a cushion of cerebrospinal fluid separate the arachnoid from the pia mater.

Upper and lower motor neurons

Upper motor neurons are located within the central nervous system and convey impulses from the motor areas of the cerebral cortex to the lower motor neurons in the spinal cord. They can influence the function of the lower motor neurons as evidenced in conditions such as cerebral palsy. The lower motor neurons are located primarily in the peripheral nervous system and provide pathways for nerve fibers to translate movement of the muscles into action. Muscle wasting can be the result of dysfunction in the anterior horn cells of the upper motor neurons. Dysfunction in the lower motor neurons can cause wasting of localized muscle groups and a soft rather than firm tone to the muscle mass. Acquired atrophy of the muscles accompanied by a wide-based gait and muscle weakness when arising from a sitting position is characteristic of muscular dystrophy, a developmental muscle wasting condition with onset in early childhood.

Spinal nerves

There are 32 pairs of spinal nerves that innervate the upper and lower torso, extremities, skin, and muscles (Figure 19-6). The spinal nerves form complex nerve networks called plexuses. There are four major plexuses in the peripheral nervous system—the cervical, brachial, lumbar, and sacral plexuses. The body surface that is innervated by the plexus of a spinal nerve is called a dermatome. Although dermatomes map specific segments of the body surface, spinal nerve sensation can be transmitted to adjacent dermatomes (see Figure 19-6). The sensory pathways of the spinal nerves carry sensations of touch, temperature, and pain; the motor fibers activate reflexes and impulses that control skeletal muscles and the involuntary muscles of the viscera. The spinal nerves function as part of the lower motor neurons and become dysfunctional in the presence of spinal cord lesions.

Spinal reflexes

Reflex behavior provides the major assessment of brainstem function. A reflex is an expected response between a stimulus and an elicited motor response. The reflex arc operates outside the level of conscious control and is the basic defense mechanism of the nervous system. Reflexes help the body maintain appropriate muscle tension and react to painful or harmful stimuli. A stimulus creates an impulse that is transmitted instantaneously outward by the motor neurons of the spinal cord via the spinal nerve and peripheral nervous system to produce a brisk muscle contraction (Figure 19-7).