While we might compare our nervous system to a computer, the nervous system is much more complex. Even so, a computer makes a good analogy. What would you say is most like the computer’s keyboard—maybe sensory receptors? What would be like the wires leading from the keyboard to the computer—maybe the nerves? Then we have the computer’s hard drive, which integrates the information sent to it from the keyboard and sends out information to a monitor screen to show us what’s going on inside the computer. The computer’s hard drive would be most like our nervous system’s integration center, or the central nervous system.
The central nervous system (CNS) includes the brain and spinal cord. The brain is the primary integrating organ for the CNS and the spinal cord is a tubular structure that courses through the vertebral column and conducts impulses between the brain and other parts of the body.
OVERVIEW OF THE BRAIN
Your brain, the part of the nervous system enabling you to understand what you are reading now, occupies your cranial vault (the cavity surrounded by the cranium). The adult brain, in males and females, weighs about 1500 grams (3-4 lbs.) and includes four major regions: The cerebrum is responsible for most of a person’s perception of stimuli and all of a person’s thoughts. The diencephalon contains several functional areas and is the major control center for the autonomic nervous system. The brain stem conducts impulses between the brain and spinal cord and is the site of many important “centers” that control various involuntary activities, such as breathing, heart rate, etc. The cerebellum helps a person maintain balance and equilibrium. See the external features of the brain in Figure 12-1.
THE CEREBRUM
The cerebrum (se-RĒ-brum; “brain”) is the largest part of the brain, making up approximately 80% of its total weight. The surface of each cerebral hemisphere displays numerous ridges called gyri (JĪ-rŭ), and each ridge is a gyrus (gyros, circle). The grooves separating the gyri are called sulci (SUL-sē), and each groove is a sulcus (“ditch”). The gyri greatly increase the brain’s surface area. As a result, more cerebral tissue can exist in the cranial cavity than would be possible if the brain’s surface were not rippled. Think about how much farther you will run around a track if you weave back and forth (making gyri patterns) as opposed to running a straight path on the inside lane. There are six major sulci (three on each side cerebral hemisphere), and they separate the cerebrum into ten cerebral lobes (five on each side):
- Central sulcus: Separates the frontal lobe from the parietal lobe. Immediately anterior to this is the precentral gyrus, which is part of the frontal lobe. Immediately posterior to the central sulcus is the postcentral gyrus, which is part of the parietal lobe.
- Parieto-occipital sulcus: Separates the parietal lobe from the occipital lobe.
- Lateral sulcus: Separates the frontal and parietal lobes from the temporal lobe. The insula (IN-sū-la; “island”) is a cerebral lobe lying deep to the frontal, parietal, and temporal lobes, and is visible in the lateral sulcus when the temporal lobe is pulled inferiorly.
The outer few millimeters (~1/8 inch) of the cerebrum consist of gray matter called the cerebral cortex (cortex, bark), and it has sensory, association, motor, and integrative areas (Figure 12-2).
Sensory Areas of the Cortex
The sensory areas of the cerebrum interpret afferent information sent in from sensory receptors in the PNS (sense literally means, “to feel”), and include:
- Gustatory cortex (GUS-ta-tor-ē; gusta, taste): located in the insula, it is responsible for the perception of taste.
- Olfactory cortex (OL-fak-tor-ē; olfact, to smell): located partly in the temporal lobes and partly in the frontal lobes, it is responsible for the perception of smell.
- Primary auditory cortex (AW-di-tor-ē; audio, to hear): located in the temporal lobes, it is responsible for the perception of sounds and their different aspects, such as its loudness, pitch (bass, treble, etc.), and location.
- Primary somatosensory cortex: located in the postcentral gyrus, it is responsible for the sense of touch, and perception of temperature and pressure. It is also responsible for spatial discrimination, the ability to know which region of the body is being stimulated.
- Primary visual cortex: located in the occipital lobe, it is responsible for the perception of light, color, and the shape and movement of objects viewed.
- Vestibular cortex: located in the insula, it is responsible for the perception of balance and equilibrium.
- Visceral sensory area (VIS-ser-al; viscera, internal organ): located in the insula, it is responsible for the perception of various sensations originating in the internal organs of the thorax and abdomen.
Association Areas of the Cortex
The association areas receive impulses from the sensory areas and determine actions to take in response.
- Auditory association area: in the temporal lobes; interprets sounds, such as recognizing a gunshot.
- Somatosensory association area: in the postcentral gyrus; interprets touch; allows a person to determine the size and texture of an object by touch, e.g., distinguishing an apple from an orange inside a bag.
- Visual association area: located in the occipital lobe; interprets visual stimuli, e.g., recognizing an apple as an apple when you see it.
Motor Areas of the Cortex
The motor areas (motor means “to move”) initiate efferent information that eventually reaches skeletal muscles to control voluntary muscle movement.
- Premotor area: located immediately anterior to the precentral gyrus in the frontal lobe; responsible for storing memory for skilled motor movement such as finger movements used when playing a piano. One region in this area, called the frontal eye field, controls learned eye movements, such as reading the words of a sentence in the proper order.
- Primary motor cortex: located in the precentral gyrus; controls voluntary muscle movements. The neurons residing in this region are called pyramidal cells, so-named because of their pyramid-like cell bodies.
Integrative Areas of the Cortex
The integrative areas receive impulses from as- sociation areas, evaluate it, and send out impulses to other areas of the brain for normal functioning in everyday life, including analyzing and solving problems.
- Motor speech area (also called the speech center or Broca’s area; BRŌ-kahz): located most often in the left frontal lobe, it controls tongue, lip, and throat movements for speech. This is an integrative area because it receives input from auditory areas to allow a person to know how loud to speak and how to put words together appropriately in conversation.
Figure 12-1. External regions of the brain
Figure 12-2. Functional aspects of the cerebrum
- General Interpretive Area (also called Wernicke’s area or the gnostic area—NOS-tik; “knowledge”): located only on one side of the brain; in most people, it is in the parietal and temporal lobes on the left side near the posterior end of the lateral sulcus. It receives input from the visual and auditory association areas, allowing a person to respond to visual and audible stimuli. For example, it enables a person to understand language and know how to respond appropriately when asked questions or given commands.
- Prefrontal cortex: located in the frontal lobes anterior to the premotor area, it is responsible for intellectual thoughts. In most people, the right prefrontal cortex is responsible for imagination, insight, forming images of sensations, artistic ability, and appreciation, whereas the left prefrontal cortex is responsible for science and math skills, analysis, reasoning, reading, writing, etc.
Impulse Pathways in the Cerebrum
White matter in the cerebrum consists primarily of myelinated axons that conduct millions of impulses through the CNS. Neurologists classify these cerebral axons (fibers) based on where they conduct impulses:
- Association fibers conduct impulses within a single hemisphere.
- Commissural fibers (kom-Ĭ-shur-al; “united”) conduct impulses from one hemisphere to the other. Most of these fibers pass through a structure called the corpus callosum (KOR-pus kŭ-LŌ-sum, meaning “thick body”). Commissural fibers enable the left and right cerebral hemispheres to communicate and work together as a single unit.
- Projection fibers conduct impulses from the cerebral hemispheres to other parts of the brain and spinal cord.
Nuclei in the Cerebrum
Several gray matter regions, called basal nuclei (formerly called basal ganglia), are found deep within each cerebral hemisphere (Figure 12-3). They constantly monitor and influence impulses sent out from the primary motor cortex and pre- frontal areas, thereby preventing unnecessary muscle movements. For example, they help coordinate the semivoluntary swinging motion of the arms when a person is walking. Names assigned to basal nuclei components include caudate nucleus, putamen, globus pallidus, lentiform nucleus and corpus striatum.
THE DIENCEPHALON
The diencephalon (dī-en-SEF-a-lon; die, through; encephalon, brain) is located superior to the mesencephalon and inferior to the corpus callosum, and includes three major regions: the epithalamus, thalamus, and hypothalamus (Figure 12-4).
Epithalamus
The epithalamus (ep-i-THAL-a-mus; epi, upon; thalamus, inner room), located inferior to the corpus callosum, forms the superior rim of the diencephalon. The pinecone shaped pineal body (PIN-e-al) projects from the posterior end of the epithalamus and is important in a person’s circadian rhythm (daily sleep-wake cycles; ser KĀ-dē-an; circa, around, dia, day). The pineal body secretes a hormone called melatonin (mel-a-TŌ-nin) that promotes drowsiness. The rate of melatonin secretion correlates positively with exposure to dim light or darkness. Prolonged exposure to low-light conditions causes excessive secretion of melatonin. The result may be seasonal affective disorder (SAD), characterized by mood swings. Treatment of SAD may involve periodic exposure to bright light for regulated periods. Prolonged exposure to light, such as when a person stays awake for extended periods during a long trip, causes less melatonin secretion. This may upset the biological clock and may cause insomnia, fatigue, etc. Jet lag is a feeling of fatigue resulting from a lack of melatonin.
Thalamus
The thalamus (THAL-a-mus; “inner room”) is the largest part of the diencephalon and is located inferior to the epithalamus. It consists of two oval shaped halves with nuclei that serve as relay centers for all sensory impulses passing from the spinal cord to the cerebrum.
Hypothalamus
The motor areas (motor means “to move”) initiate efferent information that eventually reaches skeletal muscles to control voluntary muscle movement.
- Premotor area: located immediately anterior to the precentral gyrus in the frontal lobe; responsible for storing memory for skilled motor movement such as finger movements used when playing a piano. One region in this area, called the frontal eye field, controls learned eye movements, such as reading the words of a sentence in the proper order.
- Primary motor cortex: located in the precentral gyrus; controls voluntary muscle movements. The neurons residing in this region are called pyramidal cells, so-named because of their pyramid-like cell bodies.
The hypothalamus (hī-pō-THAL-a-mus) is the lower portion of the diencephalon, and it performs numerous functions related to homeostasis. Physiologically, the hypothalamus has eight major functions:
- Controls the Autonomic Nervous System (ANS): The hypothalamus controls the ANS by regulating the activity of various centers in the brain stem, which in turn, regulate the activity of smooth muscle, cardiac muscle, and various glands. As a result, the hypothalamus can help regulate heart and breathing rates, blood pressure, activities of digestive organs, etc.
- Regulates Limbic System Output: The hypothalamus is a major component of the limbic system, or emotional part of the brain described under the heading “Functional Systems of the Brain.” In short, the hypothalamus influences motor output to skeletal muscles responsible for performing movements as- sociated with feelings of pleasure, fear, rage, hunger, satisfaction, and libido (li-BĒ-do; “sex drive”).
- Major Coordinator of the Endocrine System: One of the most important functions of the hypothalamus is its influence on the endocrine system. Suspended just below the hypothalamus is the pituitary gland, deemed the endocrine system’s “master gland” because its numerous hormones influence the activities of most of the other endocrine glands. The hypothalamus secretes hormones that, in turn, control the production of all hormones in the pituitary gland. However, two hypothalamic hormones are actually released through the pituitary gland, and these are ADH and oxytocin. ADH (antidiuretic hormone) causes the kidneys to conserve water and produce less urine; thus, more water stays in the blood preventing a decrease in blood pressure. Oxytocin (OT; ok sē-TŌ-sin, “swift birth”) stimulates contraction of smooth muscle tissue in the uterus and mammary glands (promotes childbirth and milk secretion, respectively). ADH and OT are made in cell bodies of neurons in the hypothalamus but then travel through axons that have axon terminals in the pituitary gland; thus, while these hormones enter the blood from the pituitary gland, they are actually produced in the hypothalamus.
- Controls Food Intake: The hypothalamus contains the feeding center, which responds to changes in blood glucose levels. When blood glucose levels drop below normal, the feeding center makes the person feel hungry. Taking in food counteracts this feeling by raising the blood glucose levels. When the blood glucose rises above normal, the hunger center makes the person feel satiated (no longer hungry).
- Controls Fluid Intake: The hypothalamus contains the thirst center, which responds to changes in blood osmotic pressure. The hypothalamus helps maintain blood osmotic (OP) pressure through the action of its osmoreceptors. When the blood OP rises above normal, the thirst center makes the person feel thirsty. In addition, the hypothalamus releases more ADH through the pituitary gland (mentioned earlier). By drinking more fluids and producing less urine, more water remains in the blood and lowers the blood’s OP.
- Controls Circadian Rhythm: The hypothalamus contains a person’s “biological clock,” that regulates sleep-wake cycles (circadian rhythm). A hypothalamic nucleus receives sensory input from the eyes and sets the cycle period according to the lengths of daylight and darkness in a 24-hour period. In addition, the nucleus sends impulses to the pineal gland to regulate its release of melatonin.
- Controls Body Temperature: The hypothalamus contains the body’s “thermostat,” which influences activities affecting a person’s body temperature. Setting the thermostat to a higher value promotes activities that increase body temperature, such as shivering, increased metabolism, and constriction of blood vessels in the skin to prevent heat loss. Setting the thermostat to a lower value pro- motes activities that decrease body temperature, e.g., sweating, decreased metabolism, and dilation of vessels in the skin allowing more heat loss.
Figure 12-3. Basal nuclei (frontal section through brain)
Figure 12-4. Diencephalon and brain stem
THE BRAIN STEM
The brain stem serves as a connection between the other parts of the brain and the spinal cord. From most superior to most inferior, the brain stem includes three parts: mesencephalon, pons, and medulla oblongata.
Mesencephalon
The mesencephalon (mez-en-SEF-a-lon; mes, between), or midbrain, is the most superior portion of the brain stem. The dorsal (posterior) surface of the mesencephalon displays four knoblike structures, known collectively as the corpora quadrigemina (KOR-por-a, “bodies;” kwod-ri-JEM-i-na, “four twins”); each knob is a colliculus (ko-LIK-ū-lus; “mound”). The superior colliculi (ko-LIK-ū-lī; plural for colliculus) contain nuclei responsible for movements of the eyes and head in response to visual stimuli, e.g., flinching when you detect something out of the corner of your eye. The inferior colliculi contain nuclei responsible for reflexive movement of the head in response to sound, e.g., flinching in response to a gunshot. Since the corpora quadrigemina covers the dorsal surface of the midbrain, it is referred to as the tectum (“roof”).
The basal nuclei in the cerebrum connect with a mesencephalic nucleus called the substantia nigra (sub-STAN-shē-a NĪ-gra; “black substance”), so named for its melanin content. Because of its interaction with the basal nuclei, some neurologists consider the substantia nigra as part of the basal nuclei. Neurons in the substantia nigra release the neurotransmitter dopamine, which inhibits unnecessary muscle movements. A deficiency in dopamine secretion from the substantia nigra is primarily responsible for the uncontrollable muscle movements associated with Parkinson’s disease.
The red nucleus is gray matter lying near the substantia nigra and named for its reddish appearance, which is due to iron-containing pigments. The red nucleus relays motor impulses passing from the cerebrum to the spinal cord and affects movements of the limbs. This nucleus is also part of the reticular formation, described shortly.
Pons
The pons (“bridge”) is a large bulge located immediately superior to the medulla. It contains mostly white matter and functions as a relay center for sensory and motor impulses traveling between the brain stem and cerebellum and between the brain and spinal cord. It contains the salivation center (stimulates salivary glands to release saliva) and parts of the respiratory center and reticular formation. In addition, the pons is the origin of three pairs of cranial nerves: the trigeminal, abducens, and facial, which we will describe in the next chapter.
Medulla Oblongata
The medulla oblongata (me-DŪ-la, “marrow;” ob-lon-GOT-a, “long”) is the most inferior part of the brain stem and houses nuclei responsible for various involuntary activities related to homeostasis. It is located at the level of the skull’s foramen magnum and connects the brain stem directly to the spinal cord. Axons connecting higher brain centers with the spinal cord “crossover” in the medulla; neurologists call this anatomical feature decussation (dē-ku-SĀ-shun; “crossing”). Consequently, each side of the brain monitors and controls the opposite side of the body.
Two parallel ridges called pyramids run longitudinally along the anterior portion of the medulla and contain motor tracts passing between the cerebrum and spinal cord. Inferior and lateral to the pyramids are two oval-shaped mounds called olives, which contain sensory tracts that relay impulses from stretch receptors in skeletal muscles and joints to the cerebellum. This enables the cerebellum to monitor and control motor impulses sent to skeletal muscles from the precentral gyrus. The medulla oblongata contains part of the reticular formation and includes a number of nuclei that serve as important regulatory centers:
- Cardiovascular center: regulates blood flow and blood pressure in the cardiovascular center. This center is subdivided into two centers: the cardiac center controls heart rate (how fast the heart beats) and stroke volume (how much blood the heart pumps out with each beat). The vasomotor center (vaso, vessel) helps control blood vessel diameter by regulating vasoconstriction and vasodilation.
- Respiratory center: regulates the rate and depth of breathing; part of this center is the pons.
- Deglutition center: (dē-glū-TISH-un; deglut, swallow) controls involuntary muscle movements associated with swallowing, coughing, sneezing, and hiccupping.
- Emetic center: (ē-ME-tik) controls muscle actions associated with vomiting.
CEREBELLUM
Located inferior to the occipital lobe of the cerebrum, the cerebellum (ser-e-BEL-um; “small brain”) is the second largest part of the brain. Like the cerebrum, the cerebellum contains gray matter in its cortex and white matter in its inner region. The white matter has a branching tree appearance and is so named arbor vitae (“tree of life”). The cerebellum constantly receives and trans- lates sensory information from joints, muscles, eyes, and the inner ear (which contains organs for equilibrium), and sends out impulses that help coordinate muscle movements to maintain balance and equilibrium. Depending on body position, the cerebellum sends impulses to the brain stem nuclei, which in turn modify motor impulses sent from the precentral gyrus to skeletal muscles. This constant monitoring and evaluation of sensory input and signaling to the cerebrum and brain stem makes the cerebellum crucial for maintaining coordination, posture, and balance.
FUNCTIONAL BRAIN SYSTEMS
A functional system in the brain is one that performs specific functions but has neurons that extend through multiple regions of the brain. The brain’s functional systems are the reticular formation and the limbic system.
The Reticular Formation
The reticular formation is a group of neurons that have axons extending through the entire brain stem and into the diencephalon, cerebellum, and spinal cord. The red nucleus, located in the mesencephalon, is part of the reticular formation that helps coordinate skeletal muscle movements in the limbs. The nuclei for the various centers described for the medulla oblongata are also in the reticular formation.
Part of the reticular formation in the brain stem functions in maintaining consciousness and is called the reticular activating system (RAS). The RAS monitors sensory impulses reaching the brain and sends impulses to the thalamus, which in turn relays the impulses to the cerebral cortex to maintain consciousness and alertness. The RAS filters out most of the sensory input so that we are not consciously aware of every little stimulus.
However, a person can become aware of these stimuli if reminded of them. For example, you were probably not aware of the shoe pressing on your foot until you were reminded of it just now. The RAS is important in sleep-wake cycles, where it is responsible for the arousal from sleep. Sudden trauma to the RAS, which can happen in boxing when the brain stem twists due to a hard punch to the side of the jaw, can cause unconsciousness (the person gets “knocked out”).
The Limbic System
The limbic system is a collection of nuclei located between the cerebrum and diencephalon, and they play a role in one’s emotions, behavior, motivation, memory, and regulating autonomic activities related to blood pressure and body temperature. The limbic system is also important in integrating emotions with thoughts, and for this reason, has the nickname “emotional brain.” The word limbic means “ring,” and refers to the way in which the nuclei in the cerebrum surround the superior portion of the brain stem. An alphabetized list of major structures of the limbic system, and their functions is provided below, and a depiction of their locations is shown in Figure 12-5.
- Amygdala (a-MIG-da-la; “almond”): Region responsible for a person’s expressions of fear and recognizing fear and anger in other people. It also plays a role in a person’s emotional reactions to remembered thoughts and smells.
- Cingulate gyrus (SIN-gū-lāt; “belt”): The superior portion of the limbic lobe, which consists of two gyri, the other being the parahippocampal gyrus. The cingulate gyrus is important in helping a person display emotions with different facial expressions. Its impairment can cause inappropriate emotions, lack of fear and learning difficulty. •Fornix (FOR-niks; “arch”): White matter (tracts) connecting upper and lower regions of the limbic system.
- Hippocampus (hip-ō-KAM-pus; “seahorse”): plays a role in a person’s emotional reactions to remembered thoughts and one’s ability to form new memories. •Hypothalamus: Responsible for a person’s perception of pleasure, anger, and fear, and it plays a role in the sex drive and regulating sleep-wake cycles. It also secretes hormones that affect the pituitary gland.
- Mammillary bodies: Important in spatial memories, i.e., being able to remember locations and navigate through everyday familiar locations.
- Parahippocampal gyrus: The inferior portion of the limbic lobe, and it participates in memory processing, especially related to smell.
- Thalamus: Responsible for processing information related to senses (vision, hearing, taste, and touch) and with memory and emotions.
Figure 12-5. Limbic system
BRAIN WAVES
Billions of impulses pass through the brain continuously and show up on monitoring equipment as brain waves. The graphical display of brain waves is an electroencephalogram (e-LEK-trō-en-SEF-a-lō-gram), or EEG. Reading an EEG allows evaluation of brain health and state of mind. There are four types of brain waves:
- Alpha waves are the most common when a person is awake but in a relaxed state with eyes closed. As soon as the person falls asleep or opens their eyes to focus on various tasks, the alpha waves disappear.
- Beta waves are typical in a person who is conscious, alert, thinking, and receiving sensory input.
- Theta waves are common in children but their appearance in adults suggests emotional stress or certain brain abnormalities.
- Delta waves are normally seen only when the person is asleep. The presence of delta waves in a person who is awake suggests some sort of brain abnormality.
SLEEP
Sleep is a state of unconsciousness in which ad- equate stimulation can bring about conscious- ness. In order to understand sleep, we must first understand what it means to be awake. When a person is conscious (awake), a variety of synonyms may describe that person: alert, aware, attentive, cognizant, mindful of surroundings, etc. A coma is also a state of unconsciousness, but unlike sleep, even intense stimulation will not arouse a person from a coma. While we might think the brain is not doing much during sleep, it is still active and processing information. There are two major types of sleep: non-rapid eye movement sleep and rapid eye movement sleep.
Non-rapid eye movement (NREM) sleep occurs within the first hour of unconsciousness and has four stages:
- Stage 1 NREM: eyes are closed, and the person has sporadic thoughts; the EEG is dominated by alpha waves, and immediate arousal is possible.
- Stage 2 NREM: irregular brain waves inter- mingled with sporadic short bursts of specific brain waves; arousal is more difficult than in stage 1.
- Stage 3 NREM: vital signs diminish; decrease in body temperature and blood pressure; some dreaming (nightmares); EEG is dominated by theta and delta waves; arousal is difficult.
- Stage 4 NREM: deep sleep; delta waves common; awakening causes slow responses due to minimal vital sign activity; bedwetting or sleepwalking may occur.
Rapid eye movement (REM) sleep, so named because the eyes experience a fluttering movement, occurs after stage 4 of NREM sleep. REM sleep begins about 90 minutes after NREM sleep begins. During REM sleep, almost complete paralysis of skeletal muscles occurs. With the exception of nightmares, most dreaming occurs during REM sleep.
A person’s normal sleep pattern exhibits sleep cycles, in which periods of NREM and REM sleep alternate with one another. After about the first 90 minutes of sleep, a person has experienced all stages of NREM then experiences REM for 5-10 minutes. The person then reverts to the early stages of NREM and the cycle repeats with REM occurring about 90 minutes later. While each NREM episode lasts about 90 minutes, each successive REM episode lasts longer, so that near morning, REM episodes may last an hour or so. For this reason, long dreams occur near morning.
PROTECTION OF THE BRAIN
As important as the brain is to maintaining homeostasis, it is not surprising that it is protected by a number of items, including the skull, meninges, cerebrospinal fluid, and the blood-brain-barrier.
Meninges
Protective connective tissue membranes called meninges (me-NIN-jēz; meninx, membrane) lie between the skull and the brain’s surface, and include the dura mater, arachnoid mater, and pia mater.
Dura mater (DUR-a, “tough”; MA-ter, “mother”), the most superficial and thickest meninx, is an opaque, dense connective tissue covering that resembles a leather cap. Around the brain, the dura mater exists as two layers that enclose the dural sinuses, which drain blood from the brain. Anteriorly, the dura mater lies in the longitudinal fissure where it forms the falx cerebri (FALKS, “sickle;” SER-e-brē). Anteriorly, the falx cerebri attaches firmly to the crista galli of the ethmoid bone. Posteriorly, the falx cerebri becomes the falx cerebelli (ser-e-BEL-ī), which separates the two cerebellar hemispheres. The tentorium cerebelli (ten-TOR-ē-um; “tent”) is dura mater that separates the cerebrum from the cerebellum. Dura mater does not extend into sulci.
Arachnoid mater (a-RAK-noyd; arach, spider; noid, resembles), the middle meninx, lies deep to the dura mater and is named for its spider web-like appearance. The arachnoid mater is thinner and more transparent than the dura mater and supports the large blood vessels around the brain. The subarachnoid space lies deep to the arachnoid and contains CSF. Fingerlike projections called arachnoid villi (VIL-ī; “shaggy hair”) extend into the dural sinus and drain cerebrospinal fluid into the blood.
Pia mater (PĒ-a; “gentle”), the deepest meninx, is a thin, transparent membrane that adheres to the brain’s surface. It dips into the sulci and supports small vessels and capillaries on the surface of the brain. The pia cannot be removed easily without damaging the cerebral cortex.
Cerebrospinal Fluid
A clear, watery fluid called cerebrospinal fluid (CSF) flows around the brain and through canals and four major cavities in the brain called ventricles (Figure 12-6). CSF originates as blood plasma (the liquid part of blood around the blood cells) and then squeezes out of a cluster of blood capillaries called choroid plexuses (KOR-oyd PLEK-sus-ez; chor, membrane; plexus, braid). Each brain ventricle contains a choroid plexus. CSF circulates through the CNS with the help of beating cilia on ependymal cells that cover the choroid plexuses. CSF has several important functions:
- Reduces the weight of the brain and spinal cord
- Acts as a shock absorber
- Prevents dramatic temperature changes in the CNS
- Delivers nutrients and removes waste materials from nervous tissue.
CSF returns to the blood through arachnoid villi, which penetrate into a blood-filled superior sagittal sinus along the longitudinal fissure between the two cerebral hemispheres.
Blood-Brain-Barrier
The blood-brain-barrier (BBB) is the protective barrier between blood capillaries and the brain’s tissues. Part of the BBB is attributable to tight junctions between endothelial cells that form the inner lining of the brain’s capillaries. In addition, the basement membrane on which the endothelial cells rest is thicker than that in most other capillaries; consequently, materials leaving the blood must pass through the cytoplasm of the endothelial cells. This pathway allows the endothelial cells to detoxify potentially harmful substances before they enter the brain’s tissues.
Part of the BBB includes microglial cells surrounding the vessels and neurons. They help regulate the types of materials that pass between the blood and brain tissue. The BBB does not exist in the hypothalamus or the emetic center in the medulla oblongata; therefore, these parts of the brain can monitor various blood-borne chemicals and make adjustments in the body’s metabolic activity in order to maintain homeostasis. The BBB of an infant is not well developed.
THE SPINAL CORD
The spinal cord relays impulses between the brain and most of the rest of the body. It courses through the vertebral foramina and extends from the medulla oblongata into the lumbar region of the vertebral column (Figure 12-7). The most inferior portion of the cord is a cone-shaped region, the conus medullaris (KŌ-nus, “cone;” med-u-LAR-is). The cord stops growing at about age 5, but the vertebral column continues to lengthen; thus, the conus exists at about the second lumbar vertebra (L2) in the adult and at about L3 in an infant.
The spinal cord is nearly divided into two halves by a posterior median sulcus and an anterior medial sulcus. The posterior sulcus is the deeper of the two sulci. Centrally located within the cord and running the entire length of the cord is a narrow tube, the central canal, through which CSF flows.
Support of the Spinal Cord
The same meninges that cover the brain cover the spinal cord. The pia mater covers the cord’s surface and attaches to the arachnoid and dura mater by denticulate ligaments (den-TIK-ū-lāt; “tooth-like”), which are slender extensions of the pia. CSF circulates within the subarachnoid space. The arachnoid and dura extend inferiorly from the conus medullaris to the level of the second sacral vertebra (S2). The pia mater anchors inferiorly to the coccyx by a fibrous cord called the filum terminale (FĪ-lum, “filament;” ter-mi-NA-lē, “end”).
Note:
During a spinal tap in an adult, the physician removes CSF from the subarachnoid space inferior to L2, reducing the chance of puncturing the spinal cord. Analysis of CSF can often reveal information about the health of the individual.
Spinal Cord White Matter
The nervous tissue surrounding the spinal cord’s gray matter consists of white matter arranged in regions called columns. There are three pairs of columns named according to location: posterior (dorsal) columns, anterior (ventral) columns, and lateral columns. Spinal cord columns contain tracts, or fasciculi (fa-SIK-ū-lī; fascicle, bundle), which are bundles of axons belonging to interneurons. Sensory impulses are relayed to interneurons in a posterior horn then travel to the brain through ascending tracts, whereas impulses traveling from the brain to motor neurons in the anterior horns move along descending tracts. At some point along the way, most tracts decussate (cross over) to the opposite side of the cord. Many tracts decussate in the medulla oblongata, but others decussate within the spinal cord. This explains why the right side of the brain receives impulses from and sends impulses to the left side of the body, and why the left side of the brain receives impulses from and sends impulses to the right side of the body. Major tracts in the spinal
Figure 12-6. Cavities and tubes in the CNS (a) Lateral view of brain ventricles and choroid plexuses (in red) (b) Front view (c) Yellow arrows show circulation of CSF
Figure 12-7. Spinal segments and spinal cord segments; more inferiorly they do not coincide exactly
Spinal Cord Gray Matter
Gray matter exists in the entire spinal cord as two parallel tubes extending from the medulla oblongata to the conus medullaris. Along most of their length, these two tubes of gray matter have projections called horns. When viewed in cross section, the cord’s gray matter resembles a butterfly, with the “wings” connected in the middle (at the gray commissures) and displaying three projections (horns). The gray matter is organized according to the functions of its neurons as follows:
- Posterior (dorsal) horns: contain cell bodies of interneurons that receive sensory input from somatic sensory neurons with receptors in the skin, skeletal muscles, and joints, and visceral sensory neurons with receptors in visceral organs. Specific interneurons may relay the impulses to the brain and/or other neurons in the spinal cord.
- Anterior (ventral) horns: contain cell bodies of somatic motor neurons, which receive impulses from other neurons in the spinal cord and send impulses to skeletal muscles.
- Lateral horns: contain cell bodies of visceral motor neurons, which receive impulses from other neurons in the spinal cord and send impulses to involuntary muscle tissue and glands. There are no lateral horns in the cervical region of spinal cord.
Ascending (Sensory) Tracts
Ascending tracts conduct impulses superiorly through the spinal cord’s white matter, called columns, and into the brain. These impulses originate at sensory receptors in the peripheral nervous system. The neurons that conduct the impulses into the spinal cord are first-order neurons. In turn, first-order neurons transfer impulses to second-order neurons, located inside the spinal cord. The second-order neurons conduct impulses superiorly within the cord and transfer them to either the thalamus or cerebellum. Third- order neurons, located in the thalamus but not in the cerebellum, transmit impulses to the primary somatosensory cortex. Second and third-order neurons are interneurons, whereas first-order neurons are sensory neurons.
Descending (Motor) Tracts
Descending tracts conduct impulses inferiorly through the spinal cord’s columns and into ventral horns, where they synapse with motor neurons. Both the somatic and autonomic nervous systems utilize descending tracts, but we will only consider those for the somatic nervous system (which innervate skeletal muscles). Motor impulses for the somatic nervous system always pass through two neurons within the CNS. The first neurons are located in the cerebrum and are called upper motor neurons. The second neurons are located within nuclei of the brain stem or spinal cord and are called lower motor neurons. Descending tracts utilized by upper motor neurons are located within anterior and lateral columns of the spinal cord. Major tracts in the spinal cord are listed in Table 12-1.
Table 12-1. Major tracts in the spinal cord
TOPICS TO KNOW FOR CHAPTER 12
(Central Nervous System)
alpha waves
amygdala
anterior (ventral) horns
arachnoid mater
arachnoid villi
arbor vitae
ascending tracts
association areas
association fibers
auditory association area
basal nuclei
BBB
beta waves
blood-brain-barrier
brain stem
brain waves
Broca’s area
cardiac center
cardiovascular center
caudate nucleus
central canal
central nervous system
central sulcus
cerebellum
cerebral aqueduct
cerebral cortex
cerebral hemispheres
cerebrospinal fluid
cerebrum
choroid plexuses
cingulate gyrus
circadian rhythm
columns in spinal cord
coma
commissural fibers
conus medullaris
corpora quadrigemina
corpus callosum
corpus striatum
corticobulbar tracts
corticospinal pathways
crista galli
CSF
decussation
deglutition center
delta waves
denticulate ligaments
descending tracts
diencephalon
dopamine
dura mater
dural sinuses electroencephalogram
emetic center
epithalamus
falx cerebelli
falx cerebri
fasciculus
filum terminale
first-order neuron
fornix
fourth ventricle
frontal eye field
functions of brain parts
general interpretative area
globus pallidus
gnostic area
gray matter
gustatory cortex
gyri
hippocampus
horns in spinal cord
hypothalamus
inferior colliculi
infundibulum
insula
integrative areas
intermediate mass
jet lag
lateral horns
lateral sulcus
lateral ventricles
lentiform nucleus
limbic lobe
limbic system longitudinal fissure lower motor neurons
mammillary bodies
medulla oblongata
melatonin
meninges
mesencephalic aqueduct
mesencephalon
midbrain
motor areas of cortex
motor speech area
non-rapid eye movement
NREM
nuclei in the CNS
olfactory cortex
olives
parahippocampal gyrus
parieto-occipital sulcus Parkinson’s disease
pia mater
pineal body
pituitary gland
pons
postcentral gyrus
posterior (dorsal) horns
precentral gyrus
prefrontal cortex
premotor area
primary auditory cortex
primary motor cortex
primary somatosensory cortex
primary visual cortex
projection fibers
protection of the brain
Purkinje cells
putamen
pyramidal cells
pyramids
rapid eye movement sleep
red nucleus
REM
respiratory center
reticular activating system (RAS)
reticular formation
reticulospinal tract
rubrospinal tracts
salivation center
seasonal affective disorder (SAD) second order neuron
sensory areas of cortex
septum pellucidum
sleep
sleep cycles
somatosensory association area spatial discrimination
speech center
spinal cord
spinal cord support
spinal cord white matter
spinal tap
spinocerebellar tracts spinothalamic tracts
stage 1 NREM
stage 2 NREM
stage 3 NREM
stage 4 NREM
subarachnoid space
substantia nigra
sulci
superior colliculi
tectospinal tracts
tectum
tentorium cerebelli
thalamus
theta waves
third ventricle
third-order neuron
tracts
unconsciousness
upper motor neurons
vasomotor center
vestibular cortex
vestibulospinal tracts
visceral sensory area
visual association area
Wernicke’s area
