To understand neurophysiology, it is necessary to appreciate the organization of the nervous system and the gross anatomic arrangement of structures. A comprehensive presentation of neuroanatomy would be the subject of an entire text. Thus, in this chapter, the anatomy will be described briefly, as is appropriate for the physiologic context.
The nervous system is composed of two divisions: the central nervous system (CNS), which includes the brain and the spinal cord, and the peripheral nervous system (PNS), which includes sensory receptors, sensory nerves, and ganglia outside the CNS. The CNS and PNS communicate extensively with each other.
Further distinction can be made between the sensory and motor divisions of the nervous system. The sensory or afferent division brings information into the nervous system, usually beginning with events in sensory receptors in the periphery. These receptors include, but are not limited to, visual receptors, auditory receptors, chemoreceptors, and somatosensory (touch) receptors. This afferent information is then transmitted to progressively higher levels of the nervous system, and finally to the cerebral cortex. The motor or efferent division carries information out of the nervous system to the periphery. This efferent information results in contraction of skeletal muscle, smooth muscle, and cardiac muscle or secretion by endocrine and exocrine glands.
To illustrate and compare the functions of the sensory and motor divisions of the nervous system, consider an example introduced in Chapter 2: regulation of arterial blood pressure. Arterial blood pressure is sensed by baroreceptors located in the walls of the carotid sinus. This information is transmitted, via the glossopharyngeal nerve (cranial nerve IX), to the vasomotor center in the medulla of the brain stem—this is the sensory or afferent limb of blood pressure regulation. In the medulla, the sensed blood pressure is compared with a set point, and the medullary vasomotor center directs changes in sympathetic and parasympathetic outflow to the heart and blood vessels, which produce appropriate adjustments in arterial pressure—this is the motor or efferent limb of blood pressure regulation.
The CNS includes the brain and spinal cord. The organization of major structures of the CNS is shown in Figures 3-1 and 3-2. Figure 3-1 shows the structures in their correct anatomic positions. These same structures are illustrated schematically in Figure 3-2, which may prove more useful as a reference.
Figure 3–1 Midsagittal section of the brain. Relationships are shown between the lobes of the cerebral cortex, the cerebellum, the thalamus and hypothalamus, the brain stem, and the spinal cord.
Figure 3–2 Schematic diagram of the central nervous system.
The major divisions of the CNS are the spinal cord; brain stem (medulla, pons, and midbrain); cerebellum; diencephalon (thalamus and hypothalamus); and cerebral hemispheres (cerebral cortex, white matter, basal ganglia, hippocampal formation, and amygdala).
The spinal cord is the most caudal portion of the CNS, extending from the base of the skull to the first lumbar vertebra. The spinal cord is segmented, with 31 pairs of spinal nerves that contain both sensory(afferent) nerves and motor (efferent) nerves. Sensory nerves carry information to the spinal cord from the skin, joints, muscles, and visceral organs in the periphery via dorsal root and cranial nerve ganglia. Motor nerves carry information from the spinal cord to the periphery and include both somatic motor nerves, which innervate skeletal muscle, and motor nerves of the autonomic nervous system, which innervate cardiac muscle, smooth muscle, glands, and secretory cells (see Chapter 2).
Information also travels up and down within the spinal cord. Ascending pathways in the spinal cord carry sensory information from the periphery to higher levels of the CNS. Descending pathways in the spinal cord carry motor information from higher levels of the CNS to the motor nerves that innervate the periphery.
The medulla, pons, and midbrain are collectively called the brain stem. Ten of the 12 cranial nerves (CN III–XII) arise in the brain stem. They carry sensory information to the brain stem and motor information away from it. The components of the brain stem are as follows:
The medulla is the rostral extension of the spinal cord. It contains autonomic centers that regulate breathing and blood pressure, as well as the centers that coordinate swallowing, coughing, and vomiting reflexes (see Chapter 2, Fig. 2-5).
The pons is rostral to the medulla and, together with centers in the medulla, participates in balance and maintenance of posture and in regulation of breathing. In addition, the pons relays information from the cerebral hemispheres to the cerebellum.
The midbrain is rostral to the pons and participates in control of eye movements. It also contains relay nuclei of the auditory and visual systems.
The cerebellum is a foliated (“leafy”) structure that is attached to the brain stem and lies dorsal to the pons and medulla. The functions of the cerebellum are coordination of movement, planning and execution of movement, maintenance of posture, and coordination of head and eye movements. Thus, the cerebellum, conveniently positioned between the cerebral cortex and the spinal cord, integrates sensory information about position from the spinal cord, motor information from the cerebral cortex, and information about balance from the vestibular organs of the inner ear.
Thalamus and Hypothalamus
Together, the thalamus and hypothalamus form the diencephalon, which means “between brain.” The term refers to the location of the thalamus and hypothalamus between the cerebral hemispheres and the brain stem.
The thalamus processes almost all sensory information going to the cerebral cortex and almost all motor information coming from the cerebral cortex to the brain stem and spinal cord.
The hypothalamus lies ventral to the thalamus and contains centers that regulate body temperature, food intake, and water balance. The hypothalamus is also an endocrine gland that controls the hormone secretions of the pituitary gland. The hypothalamus secretes releasing hormones and release-inhibiting hormones into hypophysial portal blood that cause release (or inhibition of release) of the anterior pituitary hormones. The hypothalamus also contains the cell bodies of neurons of the posterior pituitary gland that secrete antidiuretic hormone (ADH) and oxytocin.
The cerebral hemispheres consist of the cerebral cortex, an underlying white matter, and three deep nuclei (basal ganglia, hippocampus, and amygdala). The functions of the cerebral hemispheres are perception, higher motor functions, cognition, memory, and emotion.
Cerebral cortex. The cerebral cortex is the convoluted surface of the cerebral hemispheres and consists of four lobes: frontal, parietal, temporal, and occipital. These lobes are separated by sulci or grooves. The cerebral cortex receives and processes sensory information and integrates motor functions. These sensory and motor areas of the cortex are further designated as “primary,” “secondary,” and “tertiary,” depending on how directly they deal with sensory or motor processing. The primary areas are the most direct and involve the fewest number of synapses; the tertiary areas require the most complex processing and involve the greatest number of synapses. Association areas integrate diverse information for purposeful actions. For example, the limbic association area is involved in motivation, memory, and emotions. The following examples illustrate the nomenclature: (1) The primary motor cortex contains the upper motoneurons, which project directly to the spinal cord and activate lower motoneurons that innervate skeletal muscle. (2) The primary sensory cortices consist of the primary visual cortex, primary auditory cortex, and primary somatosensory cortex and receive information from sensory receptors in the periphery, with only a few intervening synapses. (3) Secondary and tertiary sensory and motor areas surround the primary areas and are involved with more complex processing by connecting to association areas.
Basal ganglia, hippocampus, and amygdala. There are three deep nuclei of the cerebral hemispheres. The basal ganglia consist of the caudate nucleus, the putamen, and the globus pallidus. The basal ganglia receive input from all lobes of the cerebral cortex and have projections, via the thalamus, to the frontal cortex to assist in regulating movement. The hippocampus and amygdala are part of the limbic system. The hippocampus is involved in memory; the amygdala is involved with the emotions and communicates with the autonomic nervous system via the hypothalamus (e.g., effect of the emotions on heart rate, pupil size, and hypothalamic hormone secretion).