Summary

Module 12.1: Overview of the Central Nervous System

The central nervous system performs the integrative nervous system functions.
The brain is an organ that consists mostly of nervous tissue. It is housed within the cranial cavity.
The four divisions of the brain are the cerebrum, the diencephalon, the cerebellum, and the brainstem.
The spinal cord is nervous tissue that forms an extension of the inferior brainstem.
The CNS contains both gray matter (nuclei) and white matter (myelinated axons).
The CNS develops from tissue that folds into the neural tube by the fourth week of development. The cranial end of the neural tube enlarges to form the primary brain vesicles, which enlarge further to form the secondary brain vesicles. The secondary brain vesicles develop into the four divisions of the brain.

Module 12.2: The Brain

The cerebral cortex is the cerebrum’s outer covering of gray matter. The majority of the cerebral cortex is neocortex, which functions in conscious processes such as planning of movement and interpretation of incoming sensory stimuli. The cerebral cortex has five lobes: the anterior frontal lobe, the middle parietal lobe, the posterior occipital lobe, the lateral temporal lobe, and the deep insula.
The three types of neocortex are the primary motor cortex, the primary sensory cortices, and association areas.
The basal nuclei are a group of nuclei in the cerebrum that function in movement. The three basal nuclei are the caudate nucleus, the putamen, and the globus pallidus.
The cerebral white matter may be of three types: Commissural fibers connect areas of the right and left hemispheres, projection fibers connect the outer cerebrum with other parts of the CNS, and association fibers connect gyri.
Structures of the limbic system include the gyri of the limbic lobe, the hippocampus, the amygdala, and the tracts that connect these structures with one another and with other parts of the brain. The limbic system functions in memory, learning, emotion, and behavior.
The diencephalon is the central core of the brain and is composed of four divisions, including the subthalamus and the following:
- The thalamus is the main “gateway” into the cerebral cortex, controlling the flow of information both to and from the cerebral cortex.
- The hypothalamus controls functions relating to the maintenance of homeostasis; regulates the sleep/wake cycle; regulates the autonomic nervous system; functions in the endocrine system; and plays a role in emotion, behavior, and memory.
- The epithalamus contains the pineal gland, which secretes the hormone melatonin.
The cerebellum plays a role in the coordination of movement. It is located inferior to the occipital lobe and consists of three lobes and two hemispheres, which are connected by the vermis. It has an outer layer of gray matter, the cerebellar cortex, and inner white matter, the arbor vitae.
The brainstem controls many basic involuntary processes as well as numerous reflexes, and participates in movement, sensation, and complex functions such as sleep. It houses many important nuclei and tracts of white matter.
- The midbrain is the superior portion of the brainstem. It functions in movement, sensation, and reflexes such as the startle reflex.
- The pons is the middle portion of the brainstem. The pontine nuclei play a role in movement, breathing, reflexes, and complex functions such as sleep and arousal.
- The inferiormost portion of the brainstem is the medulla oblongata. It contains the pyramids, where the corticospinal tracts decussate. It is also the site of decussation of the posterior columns.
- The reticular formation is a group of more than 100 nuclei that are found throughout all three divisions of the brainstem. Its nuclei are involved in sleep, pain transmission, motor functions, the autonomic nervous system, mood, and sensation.

Module 12.3: Homeostasis Part I: Role of the Brain in Maintenance of Homeostasis

The two systems in the body dedicated to maintaining homeostasis are the endocrine system and the nervous system.
The two structures of the nervous system most concerned with homeostasis are the hypothalamus and the reticular formation.
The homeostasis of vital functions is largely maintained by the autonomic nervous system (ANS). The hypothalamus is the main “boss” of the ANS and adjusts its output by relaying instructions to the nuclei of the reticular formation.
Respiration is regulated by neurons in the medullary reticular formation.
The hypothalamus regulates body temperature and feeding behaviors.
Sleep is the normal and reversible suspension of consciousness.
- Human sleep follows a circadian rhythm set by the hypothalamus. When light levels decrease, the suprachiasmatic nucleus (SCN) of the hypothalamus decreases the activity of the reticular formation, which “disconnects” the thalamus from the cerebral cortex and triggers sleep.
- Arousal from sleep is mediated by orexins that stimulate the neurons of the reticular formation.
- Sleep progresses through five stages, discernible by their patterns of brain activity measured on an electroencephalogram (EEG).
- Stages I, II, and III are light to moderately deep sleep stages and stage IV is deep sleep.
Rapid eye movement, or REM, sleep is characterized by rapid movements of the eyes.

Module 12.4: Higher Mental Functions

Cognition consists of processes that include social and moral behavior, intelligence, thoughts, language, problem-solving skills, and personality. These functions are carried out by the association areas, and many are lateralized to one hemisphere.
Most aspects of language are lateralized to the left cerebral hemisphere. Broca’s area and Wernicke’s area control the ability to produce and understand language, respectively.
Declarative (fact) memory is memory of things readily available to consciousness; nondeclarative memory includes skills and associations that are largely subconscious.
- Immediate memory is stored for only seconds, short-term (working) memory is stored for several minutes, and long-term memory is stored for days or longer.
- Moving the information in immediate and working memory into long-term memory is called consolidation.
- Declarative memories may be formed by long-term potentiation (LTP) that occurs within the hippocampus. Nondeclarative memories are formed by systems that do not rely on the hippocampus.
Emotion involves responses mediated by the hypothalamus and the limbic system, particularly the amygdala.

Module 12.5: Protection of the Brain

The three cranial meninges include the thick outer dura mater, the middle arachnoid mater, and the thin inner pia mater. Between the dura and the arachnoid is the subdural space; between the arachnoid and the pia is the subarachnoid space.
The cavities within the brain are the ventricles. The ventricles contain cerebrospinal fluid (CSF) and are continuous with the central canal of the spinal cord.
Cerebrospinal fluid increases the buoyancy of the brain, cushions it, and maintains a constant temperature. It is formed in the ventricles by a cluster of capillaries and ependymal cells called the choroid plexus, and is reabsorbed into the blood through the arachnoid granulations.
The blood brain barrier is a barrier between the blood in the capillaries of the brain and the extracellular fluid of nervous tissue.

Module 12.6: The Spinal Cord

The spinal cord has two main roles: It acts as a relay station to connect the brain with the rest of the body, and it carries out some processing and integration.
The spinal cord is protected by its spinal meninges. There is a fat-filled epidural space between the dura and the vertebral periosteum.
The spinal cord begins at the foramen magnum and terminates as the conus medullaris between the first and second lumbar vertebrae.
Spinal nerves project off each side of the spinal cord. The terminal bundle of spinal nerves is the cauda equina.
Spinal gray matter is surrounded by an outer sheath of spinal white matter. The spinal gray matter is divided into three regions known as horns:
- The anterior horn houses the cell bodies of lower motor neurons.
- The posterior horn contains the cell bodies of neurons that process and transmit sensory stimuli.
- The lateral horn contains cell bodies of the ANS.
There are three regions of white matter called funiculi: the posterior funiculus, the lateral funiculus, and the anterior funiculus. Each funiculus contains ascending and/or descending tracts.

Module 12.7: Sensation Part I: Role of the CNS in Sensation

Stimuli are processed in two steps: (1) The stimulus is detected by neurons of the PNS, and (2) the stimulus is interpreted by the CNS.
The general somatic senses include the tactile senses, nondiscriminative touch, pain, stretch, proprioception (joint position), and temperature. Stimuli relating to these senses are delivered to the brain via two major pathways:
- The posterior columns/medial lemniscal pathway carries stimuli concerning the tactile senses and proprioception. These tracts decussate in the brainstem. Sensory information is relayed by the thalamus to the primary somatosensory cortex and cortical association areas for interpretation.
- The anterolateral system carries pain, temperature, and nondiscriminative touch sensations. The stimuli ascend through a number of tracts, including the spinothalamic tracts, which decussate in the spinal cord, with the thalamus as the ultimate destination. The thalamus relays temperature and nondiscriminative touch stimuli to the primary somatosensory cortex, as well as pain stimuli to many areas of the brain.
Each part of the body is represented by a specific region of the primary somatosensory cortex.
Visual, auditory, and gustatory stimuli are relayed to the thalamus and then to a primary sensory cortex for interpretation.
Olfactory stimuli are first processed by the cortex of the limbic system, after which they are relayed to the thalamus, hypothalamus, and other components of the limbic system.
Vestibular stimuli are processed by multiple brainstem nuclei, the cerebellum, and areas within the cerebral cortex.

Module 12.8: Movement Part I: Role of the CNS in Voluntary Movement

Upper motor neurons reside mostly in the motor areas of the cerebral cortex. They stimulate local interneurons in the brainstem or spinal cord. The interneurons pass the message to lower motor neurons of the PNS, which innervate muscle fibers.
Axons from the cortical motor areas unite to form the corticospinal tracts, which control the muscles below the head and neck, and the corticonuclear tracts, which control muscles of the head and neck.
- The corticospinal tracts descend through the cerebral white matter into the brainstem, where the tracts decussate in the medullary pyramids. Most of the fibers then travel through the lateral funiculus of the spinal cord as the lateral corticospinal tracts.
- The corticonuclear tracts are similar to the corticospinal tracts except their axons terminate in the brainstem.
Every motion requires the selection of a motor program by the upper motor neurons of the primary motor cortex in the cerebral cortex, which plan and execute voluntary motion.
Each part of the body is represented by a specific region of the primary motor cortex.
The basal nuclei and substantia nigra inhibit inappropriate movements and initiate motion.
The cerebellum regulates ongoing movement and modifies the activity of the upper motor neurons to reduce motor error. The correction of motor error can occur over the long term as the process of motor learning.

Learning Objectives

Upon completion of this chapter, you should be able to:

Describe the structure and function of each major area of the brain.
Describe the five developmental regions of the brain, and identify the major areas of the adult brain that arise from each region.
Describe and identify the five lobes of the cerebral hemisphere, and explain how motor and sensory functions are distributed among the lobes.
Describe the structure, components, and general functions of the regions of the diencephalon, cerebellum, and brainstem.
Describe the location and functions of the limbic system and the reticular formation.
Describe the differences between the endocrine system and the nervous system in terms of how they maintain homeostasis.
Provide specific examples demonstrating how the brain responds to maintain homeostasis of regulated physiological variables in the body.
Describe the areas of the cortex responsible for cognition and language.
Discuss the concept of cerebral hemispheric specialization.
Describe the parts of the brain involved in storage of long-term memory, and discuss possible mechanisms of memory consolidation.
Describe the functions of cerebrospinal fluid as well as the details of its production, its circulation within the CNS, and its ultimate reabsorption into the bloodstream.
Describe the structural basis for and the importance of the blood brain barrier.
Identify and describe the cranial meninges, and explain their functional relationship to the brain.
Describe the gross anatomy and location of the spinal cord.
Identify and describe the anatomical features seen in a cross-sectional view of the spinal cord.
Identify and describe the spinal meninges and the spaces between and around them.
Describe the differences between ascending and descending tracts in the spinal cord.
Describe the roles of the central and peripheral nervous systems in processing sensory stimuli.
Describe the locations and functions of first-, second-, and third-order neurons in a sensory pathway.
Explain the ways in which special sensory stimuli are processed by the CNS.
Describe the locations and functions of the upper and lower motor neurons in a motor pathway.
Explain the roles of the cerebral cortex, basal nuclei, and cerebellum in movement.
Describe the overall pathway from the decision to move to the execution and monitoring of a motor program.
Explain how decussation occurs in sensory and motor pathways, and predict how brain and spinal cord injuries affect these pathways.

Tasks to Complete

Upon completion of this chapter, you will need to complete both an assignment and a quiz.