The very complex neurologic system is responsible for coordinating and regulating all body functions. It consists of two structural components: the central nervous system (CNS) and the peripheral nervous system.
The CNS encompasses the brain and spinal cord, which are covered by three layers of protective meninges. The subarachnoid space surrounding the brain and spinal cord is filled with cerebrospinal fluid, which cushions the brain and spinal cord, nourishes the CNS, and removes waste materials. Electrical activity of the CNS is governed by neurons located in the sensory and motor neural pathways. The CNS contains upper motor neurons that influence lower motor neurons located mostly in the peripheral nervous system.
Located in the cranial cavity, the brain has four major divisions: the cerebrum, the diencephalon, the brainstem, and the cerebellum (Fig. 21-1).
The cerebrum is divided into the right and left cerebral hemispheres, joined by the corpus callosum-a bundle of nerve fibers responsible for communication between the hemispheres. Each hemisphere sends and receives impulses from the opposite side of the body and consists of four lobes (frontal, parietal, temporal, and occipital), which mediate higher level functions (Table 21-1). Damage to a lobe impairs its specific function.
The diencephalon lies beneath the cerebral hemispheres and consists of the thalamus and hypothalamus. Most sensory impulses travel through the thalamus, which is responsible for screening and directing impulses to specific areas in the cerebral cortex. The hypothalamus (a part of the autonomic nervous system, which, in turn, is a part of the peripheral nervous system) is responsible for regulating many body functions, including water balance, appetite, vital signs (temperature, blood pressure, pulse, and respiratory rate), sleep cycles, pain perception, and emotional status.
Located between the cerebral cortex and the spinal cord, the brainstem consists of the midbrain, pons, and medulla oblongata. The midbrain serves as a relay center for ear and eye reflexes and relays impulses between the higher cerebral centers and the lower pons, medulla, cerebellum, and spinal cord. The pons links the cerebellum to the cerebrum and the midbrain to the medulla. It is responsible for various reflex actions. The medulla oblongata contains the nuclei for cranial nerves (CNs) and has centers that control and regulate respiratory function, heart rate and force, and blood pressure.
The cerebellum, located behind the brainstem and under the cerebrum, has two hemispheres and is responsible for coordination and smoothing of voluntary movements, maintenance of equilibrium, and maintenance of muscle tone.
The spinal cord (Fig. 21-2) is located in the vertebral canal and extends from the medulla oblongata to the first lumbar vertebra. The inner part of the cord has an H-shaped appearance and is made up of two pairs of columns (dorsal and ventral) consisting of gray matter. The outer part is made up of white matter and surrounds the gray matter. The spinal cord conducts sensory impulses up ascending tracts to the brain, conducts motor impulses down descending tracts to neurons that stimulate glands and muscles throughout the body, and is responsible for simple reflex activity. The simplest stretch reflex involves one sensory neuron (afferent), one motor neuron (efferent), and one synapse, such as the knee jerk, elicited by tapping the patellar tendon. More complex reflexes involve three or more neurons.
Sensory impulses travel to the brain by way of two ascending neural pathways (the spinothalamic tract and posterior columns) (Fig. 21-3). These impulses originate in the afferent fibers of the peripheral nerves and are carried through the posterior (dorsal) root into the spinal cord. Sensations of pain, temperature, and crude and light touch travel by way of the spinothalamic tract, whereas sensations of position, vibration, and fine touch travel by way of the posterior columns. Motor impulses are conducted to the muscles by two descending neural pathways: the pyramidal (corticospinal) tract and the extrapyramidal tract (Fig. 21-4). The motor neurons of the pyramidal tract originate in the motor cortex and travel down to the medulla where they cross over to the opposite side; then, they travel down to the spinal cord where they synapse with a lower motor neuron in the anterior horn of the spinal cord. These impulses are carried to muscles and produce voluntary movements that involve skill and purpose. The extrapyramidal tract motor neurons consist of those motor neurons that originate in the motor cortex, basal ganglia, brainstem, and spinal cord outside the pyramidal tract. They travel from the frontal lobe to the pons where they cross over to the opposite side and down the spinal cord where they connect with lower motor neurons that conduct impulses to the muscles. These neurons conduct impulses related to the maintenance of muscle tone and body control.
Carrying information to and from the CNS, the peripheral nervous system consists of 12 pairs of CNs (Table 21-2) and 31 pairs of spinal nerves. Comprising 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve, the 31 pairs of spinal nerves are named after the vertebrae below each one’s exit point along the spinal cord (see Fig. 21-2). Each nerve is attached to the spinal cord by two nerve roots. The sensory (afferent) fiber enters through the dorsal (posterior) roots of the cord, whereas the motor (efferent) fiber exits through the ventral (anterior) roots of the cord. The sensory root of each spinal nerve innervates an area of the skin called a dermatome (Fig. 21-5). These nerves are categorized as two types of fibers: somatic and autonomic. Somatic fibers carry CNS impulses to voluntary skeletal muscles, whereas autonomic fibers carry CNS impulses to smooth, involuntary muscles (in the heart and glands). The somatic nervous system mediates conscious, or voluntary, activities, whereas the autonomic nervous system mediates unconscious, or involuntary, activities.