Neuroanatomy PDF

522 Chapter 11 11 soap suds and water temperature), but you need not respond to all...

522 Chapter 11 11 soap suds and water temperature), but you need not respond to all of it, because your brain will monitor and alert you to dangers. You can think about other things while your hands do the dishes. Speech capitalizes on similar automaticity, as you can see in your ability to speak easily while riding a bike or while walking. You no more think of every movement of the extraordinary number of muscles contracting for the simple speech act than you do during dishwashing. This changes when the mechanism changes, however. When your mouth is numb from the dentist’s anesthesia, you become very aware of Neuroanatomy your lack of feedback from that system, and inaccurate speech results. When an individual suffers a cerebrovascular accident, the result is often a loss of previously attained automaticity in speech. When a child is born with developmental apraxia of speech, a condition that limits the ability of a child to plan articulatory function, achieving automaticity may be a lifelong struggle. Automatic functions are supported by a background tonicity, a partial contraction of musculature to maintain muscle tone. All action occurs within an environment, and the environment of your musculature is the tonic contraction of supporting muscles. As you extend your arm to reach for a coffee cup, the action of your fingers to grasp the object is supported by the rotation of your shoulder and the extension of your arm. Without these background support movements, T here is nothing in nature so awe-inspiring as the nervous system. the act of grasping would not occur in the graceful, fluid manner Despite centuries of study, humans have only begun to gain to which you are accustomed. Your body works as a unit to meet understanding of this extraordinarily complex system. There are your needs. approximately 100 billion neurons in the nervous system, and each of Voluntary functions are the domain of the cerebral cortex or these neurons may communicate directly with as many as 2,000 other cerebrum, a new structure by evolutionary standards that makes neurons, providing at least 1 trillion points of communication. As up the bulk of the human brain. It is the seat of consciousness, and before, we will discuss both the structure and function of this system. sensory information that does not reach the level of the cerebrum does not reach consciousness. The cerebrum is also the source of voluntary movement, although many lower brain centers are involved in the OVERVIEW execution of commands initiated by the cerebrum (see Figure 11-1). Movement requires coordination, and that is one of the During our discussion of human anatomy associated with speech and responsibilities of the cerebellum. Information from peripheral sensors language, we have been quite concerned with the voluntary musculature is coordinated with the motor plan of the cerebrum to provide the Anatesse Lesson and the supporting framework associated with it. Communication is, body with the ability to make finely tuned motor gestures. The output by and large, voluntary. Nonetheless, the term voluntary takes on new from the cerebrum is modified by the basal ganglia, a group of nuclei meanings when seen in the context of automaticity and background. (cell bodies) with functional unity deeply involved in background Automaticity refers to the development of patterns of responses that no movement. longer require highly specific motor control but rather are relegated to Motor commands are conveyed to the periphery for execution by automated patterns. Background activity is the muscular contraction neural pathways termed nerves or tracts. Sensory pathways transmit that supports action or movement, providing the form against which information concerning the status of the body and its environment voluntary movement is placed. Voluntary activities are generally to the brain for evaluation. This information permits the cerebrum considered to be conscious activities, but they are, in reality, largely and lower centers to act on changing conditions to protect the automated responses. To prove this to yourself, try washing the dishes systems of the body and to adjust to the body’s environment. For and actually thinking about the act of dishwashing. The movements and instance, information that tells your brain that it is cold outside will responses to this act are so automated that you probably feel you do be transmitted to the cerebrum and hypothalamus—the result will be them “without thinking.” In fact, your brain receives more than 40,000 shivering and goosebumps as well as a conscious effort to retrieve your signals from body sensors per second (including information about ski parka from the car. 521 (A) Cerebrum 524 Chapter 11 Midbrain Corpus 4th Ventrical callosum The brain can communicate with its environment only through sensors and effectors (see Tables 11-1 and 11-2). Sensors are the means by which your nervous system translates information concerning the internal and external environment into a form that is usable by the brain, whereas effectors are the means by which your body responds to changing conditions. Broadly speaking, superficial sensation (temperature, pain, touch) is sensation arising from stimulation of the Hypophysis Cerebullum surface of the body. Deep sensation includes muscle tension, muscle Thalamus length, joint position sense, muscle pain, pressure, and vibration. Pons Combined sensation integrates both multiple senses to process Brainstem Medulla stimulation. This involves integration of many pieces of sensory information to determine a quality, such as stereognosis (the ability to Spinal cord recognize the form of an object through touch). Lateral cross-section of the brain Within each of these broad classes are specific types of sensation. Somatic sense is that sensation related to pain, thermal sensation (B) (temperature sense), and mechanical stimulation. Mechanical Table 11-1. Classes of sensation. Cerebrum SUPERFICIAL SENSES DEEP SENSES Temperature Muscle length and tension Pain Joint Touch Proprioception Brainstem Muscle pain Pressure Vibration Cerebellum Table 11-2. Sense, sensor, and stimulation. SENSE SENSOR TYPE STIMULUS Pain Nerve ending Aversive stimulation Temperature Thermosensor Heat and cold Mechanical Pacinian corpuscle Light and deep stimulation pressure Pacinian corpuscle Vibration Golgi tendon organ Joint sense Muscle spindle Muscle stretch Kinesthetic sense Labyrinthine (vestibular) Motion of body hair cells Vision Photoreceptors Light stimulation Olfactory Chemoreceptors Chemical stimulation Figure 11-1. (A) Medial view of cerebrum, brainstem, and cerebellum. (B) Sagittal section Audition Labyrinthine (cochlear) Acoustical stimulation showing relationship among cerebrum, cerebellum, and brainstem. hair cells Source: Delmar/Cengage Learning Gustatory Chemoreceptors Chemical stimulation 523  Neuroanatomy 525 526 Chapter 11 Coming to Your Senses When Sensation Goes Bad We have come to know the “five senses” as stereotypes: touch, taste, vision, Sensations are processed by the nervous system as a means of relating the real hearing, and smell, thanks to Aristotle. The reality is far different, however. world with our “inner world.” These sensations are required for normal function, Physiologists now state that there are at least 21 senses, and some place the including motor coordination and protective action. When sensory systems number much higher. Here is how they come to this conclusion. fail, any valuable information required by the nervous system is lost. The most There are certainly basic senses: vision, hearing, olfaction (smell), obvious sensory loss for those of us in the fields of speech-language pathology gestation (taste), tactile sense (touch), nociception (pain), mechanoreception, and audiology is those associated with hearing and vestibular function. Prenatal thermal sense, and interoception (senses associated with internal, subconscious loss of hearing has a significant impact on the development of speech and processes such as blood pressure, CO2 in the blood, etc.). That said, each of language, whereas postnatal loss results in significant communication problems. these basic senses can be further subdivided. For instance, vision can be Vestibular dysfunction, as in that seen in Meniere’s disease (endolymphatic divided into light and color, but some say it should be divided into light, red hydrops), results in significant balance difficulties and vertigo. sense, green sense, and blue sense. This makes “sense,” in that the receptors One can also have a disruption of sensory systems related to joint for these colors are mutually exclusive. You could make the same argument perception, which can cause errors in motor execution. Similarly, a deficit in for taste. Taste consists of sweet, salty, bitter, sour, and umami. Smell is smell, sensory feedback drives the muscle spindle into hyperfunction when upper right? Wrong. We can sense over 2,000 different and highly specific smells, and motor neuron lesion limits the inhibition of that reflexive system. Diseases there is evidence that the olfactory receptors are highly specialized! such as diabetes can cause loss of pain sense, which may result in failure to Mechanoreception diversifies in the same way. We have the sense of recognize a wound until it has become seriously infected. Loss of appropriate balance and acceleration (rotational and linear) mediated by the vestibular sensory input to the cerebellum, which coordinates motor function (including mechanism, but we also have joint sense (proprioception), body movement correction of the motor plan), will result in cerebellar ataxia, which is reduction sense (kinesthesia), and muscle tension (Golgi tendon organs, GTO), and stretch of coordinated movement. (muscle spindle) senses. Finally, there are at least five basic interoceptors, including sensors for blood pressure, arterial pressure, venous pressure, oxygen content, acid balance of cerebrospinal fluid, and stomach distension! So, it looks as if Aristotle needs physical distortion of tissue. Pressure on the skin, for example, will result an update! in the distension of Pacinian corpuscles, whereas hair follicles have mechanoreceptors to let you know that something has disturbed your hair. Muscle spindle and Golgi tendon organs and the labyrinthine stimulation takes the form of light and deep pressure, vibration hair receptors of the inner ear are also mechanoreceptors. In contrast, (which is actually pressure that is perceived to change over time), and olfaction (smell) and gustation (taste) are mediated by chemoreceptors changes in joints and muscles, particularly stretch. Kinesthetic sense, because they depend on contact with molecules of the target substance. or kinesthesia, is the sense of the body in motion. Special senses are Visual stimulation by light is transduced by highly specialized those designed to transduce (change one form of energy into another) photoreceptors, and temperature sense arises from thermoreceptors. specific exteroceptive information. For example, in the visual sense, Visual and auditory sensors are also termed teleceptors because their light from external sources is transduced into electrochemical energy respective light and sound stimuli arise from a source that does not by the photoreceptors of the retina; in hearing, acoustical disturbances touch the body (olfactory sense is stimulated directly by molecules of in the air are transduced by the hair cells of the cochlea. Other special the material being sensed). senses are olfaction (sense of smell), tactile sense (sense of touch), and Another way to categorize receptors is by the region of the body gustation (sense of taste). receiving stimulation. Interoceptors monitor events within the Sensor types vary by the stimulus to which they respond. body, such as distention of the lungs during inspiration or blood acidity. Sensors communicate with the nervous system by means of dendritic Exteroceptors respond to stimuli outside of the body, such as tactile connection with bipolar first-order sensory neurons. Axons of these stimulation, audition, and vision. Contact receptors are exteroceptors first-order neurons synapse or make neurochemical connection within that respond to stimuli that touch the body (e.g., tactile, pain, deep and the central nervous system. A graded generator potential arises from light pressure, temperature). Proprioceptors are sensors that monitor adequate stimulation of the sensor, and adequate stimulation will cause change in a body’s position or the position of its parts, and these the generation of an action potential. include muscle and joint sensors, such as muscle spindles and GTOs. Receptors may be mechanoreceptors, chemoreceptors, Vestibular sense falls into this category because it provides information photoreceptors, or thermoreceptors. Mechanoreceptors respond to about the body’s position in space.  Neuroanatomy 527 528 Chapter 11 Table 11-3. Divisions of the nervous system from anatomical and physiological perspectives. Locked-in Syndrome Locked-in syndrome is a condition that typically arises because of brainstem Anatomical Divisions of Nervous System: stroke or cerebrovascular accident (CVA). The condition results in complete Central Nervous System: Cerebrum, cerebellum, brainstem, spinal cord, paralysis of all musculature except the ocular muscles for eye movement. thalamus, subthalamus, basal ganglia, etc. The condition typically has no effect on sensory or cognitive function, so the Peripheral Nervous System: Spinal nerves, cranial nerves, sensors individual is left with intact mental function but no means of communication. Functional Divisions of Nervous System: A system of eye blinks is typically established with the individual as a means Autonomic Nervous System: Involuntary bodily function of alternative communication. Sympathetic nervous system: Expends energy (e.g., vasoconstriction when frightened) Parasympathetic nervous system: Conserves energy (e.g., vasodilation upon removal of feared stimulation) Despite our rich communicative ability, we can know our Somatic Nervous System: Voluntary bodily function environment only by means of the sensory receptors of our skin, muscles, tendons, eyes, ears, and so forth. Without sensation, a perfectly functioning brain would be worthless as a communicating system. Likewise, communication requires some sort of muscular activity or as well as the sensory receptors. All the CNS components are housed glandular secretion. The absence of all motor activity would signal within bone (skull or vertebral column), whereas most of the PNS the end of communication. Fortunately, the extraordinary number of components are outside of bone. We will spend a great deal of time sensors in the human body permits us to use alternate pathways for within this organizational structure as we discuss the anatomy of the receiving communication (e.g., tactile, or touch, communication) or nervous system. for passing information to another person (e.g., use of eye-blink code). We are rarely completely cut off from communication with others. Let us now examine the components of this system. Autonomic/Somatic Nervous Systems A functional view of the nervous system categorizes the brain into DIVISIONS OF THE NERVOUS SYSTEM autonomic and somatic nervous systems (see Table 11-3). The autonomic nervous system (ANS) governs involuntary activities of The nervous system can be viewed and categorized in a number of the visceral muscles or viscera, including glandular secretions, heart ways. It is important to develop a framework for the discussion of this function, and digestive function. You have little control over what system, lest the volume of components become overwhelming. In the happens to that triple chili cheeseburger once you make the commitment overview, we discussed an informal functional view of nervous system to eat it, although you will admit that occasionally you are aware of the organization, assessing the components in terms of their relationship digestive process. to the systems of communication. We can view the nervous system as The ANS may be further divided into two subsystems. The being composed of two major components (central nervous system and subsystem that responds to stimulation through energy expenditure is peripheral nervous system) or as having two major functions (somatic called the sympathetic system or thoracolumbar system, and the system nervous system and autonomic nervous system) (see Table 11-3). We can that counters these responses is known as the parasympathetic system or also view the nervous system in developmental terms, differentiating craniosacral system. You feel the result of the sympathetic system based on embryonic structures. when you have a close call in an automobile or hear a sudden, loud noise. Sympathetic responses include vasoconstriction (constriction of blood vessels), increase in blood pressure, dilation of pupils, cardiac Central Nervous System/Peripheral acceleration, and goose bumps. If you attend for a few more seconds, you Nervous System will notice the glandular secretion of sweat under your arms. All these fall into the category of “flight, fight, or fright” responses. Your body The nervous system may be divided anatomically into central and dumps the hormone norepinephrine into your system to provide you peripheral nervous systems (see Table 11-3). The central nervous system with a means of responding to danger, although the speed of modern (CNS) includes the brain (cerebrum, cerebellum, subcortical structures, emergencies such as automobile accidents clearly outstrips the rate of brainstem) and spinal cord. The peripheral nervous system (PNS) sympathetic responses (wear your seatbelt). consists of the 12 pairs of cranial nerves and 31 pairs of spinal nerves  Neuroanatomy 529 530 Chapter 11 You may be less aware of the parasympathetic system response. This telencephalon, rhinencephalon, diencephalon, metencephalon, and system is responsible for counteracting the effects of this preparatory act, telencephalon: Gr., telos enkephalos, myelencephalon (see Table 11-4). The telencephalon refers to the because extraordinary muscular activity requires extraordinary energy. end or distant brain “extended” or “telescoped” brain and includes the cerebral hemispheres, the Parasympathetic responses include slowing of the heart rate, reduction white matter immediately beneath it, the basal ganglia, and the olfactory of blood pressure, and pupillary constriction. rhinencephalon: Gr., rhis enkephalos, tract. The rhinencephalon refers to structures within the telencephalon. The CNS component of the ANS arises from the prefrontal region nose brain The name arises from the relationship of the structures to olfaction. These of the cerebral cortex, as well as from the hypothalamus, thalamus, are parts of the brain that developed early in our evolution and include the hippocampus, brainstem, cerebellum, and spinal cord. The viscera are olfactory bulb, tract, and striae; pyriform area; intermediate olfactory area; connected to these loci of control by means of afferent (ascending, afferent: L., ad ferre, to carry toward paraterminal area; hippocampal formation; and fornix. The diencephalon typically sensory) and efferent (descending, typically motor) tracts. is the next descending level and includes the thalamus, hypothalamus, The PNS components of the ANS include paired sympathetic efferent: L., ex ferre, to carry away pituitary gland (hypophysis), and optic tract. The mesencephalon is the trunk ganglia running parallel and in close proximity to the vertebral from midbrain of the brainstem, and the metencephalon includes the pons column, plexuses (networks of nerves), and ganglia (groups of cell and cerebellum. The myelencephalon refers to the medulla oblongata, the bodies having functional unity and lying outside the CNS). lowest level of the encephalon. The term bulb or bulbar refers technically The somatic nervous system (voluntary component) is of major somatic: Gr., soma, body to the pons and medulla but is nearly always used to refer to the entire importance to speech pathology. This system involves the aspects of brainstem, including the midbrain. bodily function that are under our conscious and voluntary control, Let us examine the nervous system, beginning with the building including control of all skeletal or somatic muscles. CNS control of block of the nervous system. The basic units of the nervous system are muscles arises largely from the precentral region of the cerebral cortex, neurons, from which all larger structures arise. with neural impulses conveyed through descending motor tracts of the brainstem and spinal cord. Communication with the cranial nerves of the brainstem and with the spinal nerves of the spinal cord Table 11-4. Development and elements of the encephalon. permits activation of the periphery of the body. Likewise, the sensory component of the somatic nervous system monitors information about DEVELOPMENT OF ENCEPHALON the function of the skeletal muscles, their environment, and other Prosencephalon Telencephalon (including rhinencephalon) “nonvisceral” activities. Diencephalon The motor component of the somatic system may be Mesencephalon Mesencephalon subdivided into pyramidal and extrapyramidal systems, although Rhombencephalon Metencephalon defining all the anatomical correlates of this functional division is Myelencephalon difficult. The pyramidal system arises from pyramidal cells of the motor strip of the cerebral cortex and is largely responsible for the COMPONENTS OF LEVELS OF THE ENCEPHALON initiation of voluntary motor acts. The extrapyramidal system also Telencephalon Cerebral hemispheres arises from the cerebral cortex (mostly from the premotor region Basal ganglia of the frontal lobe) but is responsible for the background tone and Olfactory tract movement supporting the primary acts. The extrapyramidal system Rhinencephalon is referred to as the indirect system, projecting to the basal ganglia Lateral ventricle, part of third ventricle and reticular formation. Diencephalon Thalamus There is one more important way of categorizing structures within Hypothalamus the nervous system, and it is developmental in nature. The anatomical Phylogeny refers to the evolution Pituitary gland (hypophysis) and developmental organizations overlap, and both sets of terminology of a species, whereas ontogeny is Optic tract are often used together. the development of an individual Third ventricle organism. The statement that Mesencephalon Midbrain “Ontogeny recapitulates phylogeny” Cerebral aqueduct refers to the notion that structures Cerebral peduncles Development Divisions that are phylogenetically oldest tend Corpora quadrigemina to emerge earliest in the developing Metencephalon Pons During the fourth week of embryonic development, the brain organism, whereas later evolutionary Cerebellum (encephalon) is composed of the prosencephalon (forebrain), additions, such as the cerebral cortex, Portion of fourth ventricle mesencephalon (midbrain), and rhombencephalon (hindbrain). will emerge later in development. Myelencephalon Medulla oblongata As the encephalon develops, further differentiation results in the Portion of fourth ventricle and central canal  Neuroanatomy 531 532 Chapter 11 ANATOMY OF THE CNS AND PNS cell has come under close scrutiny, and it looks as if its original role as a support system for neurons grossly understates its function. Some Although it is an understatement to say that the CNS is extremely scientists have demonstrated that without glial cells the neurons would complex, it may be a comfort to realize that there is a common be virtually incapable of storing information in long-term memory. denominator to all the structures of the nervous system: All structures Glial cells are also critical players in the development of synapses. are made up of neurons. Functionally, the smallest organizational unit Astrocytes secrete thrombospondin, which supports the development of of the nervous system is also the neuron, followed in complexity by the synapses, so that without astrocytes there would be no communication spinal arc reflex and higher reflexes (see Table 11-5). The brainstem between neurons. provides the next level of complexity, followed by subcortical structures The general structure of most neurons includes the soma, or cell and the cerebellum, and finally the most complex aggregate of tissue, body; a dendrite, which transmits information toward the soma; and an the cerebral cortex. Let us begin with the discussion of the most basic axon, which transmits information away from the soma (see Table 11-6 component of the nervous system, the neuron. and Figure 11-2). Neurons respond to stimulation, and the neuron’s response is the mechanism for transmitting information through the nervous Neurons system. A neuron can have one of two types of responses: excitation or inhibition. Excitation refers to stimulation that causes an increase Overview of activity of the tissue stimulated. That is, if a neuron is stimulated, The nervous system is comprised of the communicating elements, it will increase its activity in response. It is as if you were at a traffic neurons, and support tissue, glia or glial cells. Neurons (nerve cells) signal in your car, and when the light changed, the person behind you are the functional building blocks of the nervous system and are honked. Your response to this stimulation is to take off from the light unique among tissue types in that they are communicating tissue. (excitation). Inhibition, in contrast, refers to stimulation of a neuron Their function is to transmit information. Recently, however, the glial that reduces the neuron’s output. That is, when a neuron is inhibited, it will reduce its activity. Again, using the traffic analogy, if you hear a siren while at the traffic light, you know that there is an emergency vehicle coming. The siren inhibits your activity, and you decide not Table 11-5. Hierarchical order of complexity for the structures of the nervous system. to move because of that stimulation. That is an inhibitory response. Neurons with excitatory responses give an active output when STRUCTURE FUNCTION stimulated, whereas those with inhibitory responses stop responding Glial cells Nutrients to neurons, support, phagocytosis, myelin when they are stimulated. Neuron Communicating tissue Reflexes Subconscious response to environmental stimuli Ganglia/nuclei Aggregates of cell bodies with functional unity Tracts Aggregates of axons that transmit functionally Table 11-6. Basic components of the neuron. united information; spinal cord Structures of brainstem Aggregates of ganglia, nuclei, and tracts that COMPONENT FUNCTION mediate high-level reflexes and mediate the execution of cortical commands Dendrite Receptor region Diencephalon Aggregates of nuclei and tracts that mediate sensory Soma Contains metabolic organelles information arriving at cerebrum and provide basic Axon Transmits information from neuron autonomic responses for body maintenance Hillock Generator site for action potential Cerebellum Aggregates of nuclei, specialized neurons, and Myelin sheath Insulator of axon tracts that integrate somatic and special sensory Schwann cells Form myelin in PNS information with motor planning and command Oligodendrocytes Form myelin in CNS for coordinated movement Nodes of Ranvier Permit saltatory conduction Cerebrum All conscious sensory awareness and conscious Telodendria Processes from axon motor function, including perception, awareness, motor planning and preparation, cognitive Terminal end boutons Contain synaptic vesicles function, attention, decision-making, voluntary Neurotransmitter Substance that facilitates synapse motor inhibition, language function, speech Synaptic cleft Region between pre- and postsynaptic neurons function  Neuroanatomy 533 (B) (A) Dendrites Ependymal cell Neurons Soma Astrocyte Axonal Capillary hillock Axon Myelin sheath Node of Ranvier Figure 11-2 continued. (B) The astrocyte is a glial cell that supports transport of nutrients to the neuron while shielding it from toxins via the blood-brain barrier. Source: Delmar/Cengage Learning Presynaptic neuron Direction of conduction Telodendria of nerve impulse Vesicles containing neurotransmitters Figure 11-2. (A) Schematic of basic elements of a neuron. Mitochondrion End buttons (continues) Source: Delmar/Cengage Learning Synaptic cleft Morphology Characteristics. There are several important landmarks of the neuron (see Figure 11-3). A neuron may have many dendrites, often referred to as the “dendritic tree” because it looks “bushy,” but it will typically have only one axon. The axon hillock is the junction of the axon and the soma. Many axons are covered with a white fatty wrapping called the myelin sheath. Myelin is made up of Schwann cells in the PNS and of oligodendrocytes in the CNS, but in both cases Figure 11-3. Schematic of elements of synapse. Note that the myelin serves a very important function: It speeds up neural synapse consists of the terminal conduction. This means that axons (fibers) that have myelin wrapping end bouton, synaptic cleft, and around them are capable of conducting impulses at a much greater rate postsynaptic receptor sites. Postsynaptic neuron Receptors on postsynaptic than those that do not have myelin. This will be very important when Source: Delmar/Cengage Learning membrane bound to neurotransmitter you study diseases that destroy the myelin, such as multiple sclerosis and amyotrophic lateral sclerosis. 534  Neuroanatomy 535 (A) Dendrite Myelin is segmented, so that it resembles a series of hot dog buns Dendrite linked together. The areas between the myelinated segments are known as nodes of Ranvier, and we shall see that these are important in conduction as well. If you follow the axon to its end point, you will see telodendria, which are long, thin projections. The telodendria Axodendritic have terminal (end) boutons (or buttons), and within the boutons synapse are synaptic vesicles. Synaptic vesicles contain a special chemical known as neurotransmitter substance (or simply neurotransmitter). Neurotransmitters are compounds that are responsible for activating Axon the next neuron in a chain of neurons. As we will discuss later on, Soma Axoaxonic neurotransmitter is released into the gap between two neurons (the synapse synaptic cleft). The boutons also contain mitochondria, organelles responsible for energy generation and protein development. Groups of Axosomatic cell bodies appear gray and are referred to as gray matter, whereas white synapse Axon matter refers to myelin. The synapse deserves special discussion. When a neuron is suf- Synapse is a noun, but is often used ficiently stimulated, the axon discharges neurotransmitter into the as a verb, indicating the action synaptic cleft. The neurotransmitter is like the key to your door: The of communication between two neurotransmitter released into the synaptic cleft is the one to which neurons. (B) the adjacent neuron responds. If some other class of neurotransmitter makes its way into that synaptic region, it will have no effect upon the adjacent neuron. This lock-and-key arrangement lets neurons have specific effects on some neurons while not affecting others. We speak of the neurons in a chain as being either presynaptic Dendrites or postsynaptic. Presynaptic neurons are those “upstream” from the synapse and are the ones that stimulate the postsynaptic neurons (the ones following the synapse). This makes sense when you realize that information passes in only one direction from a neuron: Information Synaptic knobs enters generally at the dendrite and exits at the axon. Neurotransmitter released into the synaptic cleft stimulates receptor sites on the postsynaptic neuron. When the postsynaptic neuron is stimulated, ion channels in its membrane open up and allow Myelin sheath ions to enter, and this leads to a discharge or “firing” of that neuron as well. Dendrites are the typical location for synapse on the receiving neurons, and these synapses are called axodendritic synapses. Synapse Telodendria may also occur on the soma: These are called axosomatic synapses and are usually inhibitory. If synapse occurs on the axons of the postsynaptic neuron, it is called an axoaxonic synapse (see Figure 11-4), and these synapses tend to be modulatory in nature. Sometimes an axon stimulates a neuron secondarily on its way to the synapse with another neuron, and this is referred to as en passant (“in passing”) synapse. Two other less common synapse formations are somatosomatic synapse, in which the soma of a neuron synapses with the soma of another neuron, and dendrodendritic synapse, in which communication is between Figure 11-4. (A) Types of synapses, including axodendritic (excitatory), axosomatic two dendrites. (inhibitory), and axoaxonal (modulatory). (B) Illustration of excitatory and inhibitory synapses on a postsynaptic neuron. Note that excitatory axons synapse on the dendrite, Morphological Differences between Neurons. There are several types while inhibitory axons synapse at the cell body. Source: Delmar/Cengage Learning or forms of neurons distributed throughout the nervous system. Monopolar (unipolar) neurons are those with a single, bifurcating process arising from the soma (see Figure 11-5). Neurons with two 536  538 Chapter 11 processes are called bipolar neurons, and multipolar neurons will have more than two processes. Sensory neurons are generally monopolar or pseudomonopolar. The exception is neurons that transmit information about smell (olfaction), hearing (audition), and vestibular senses: these are bipolar. Glial cells make up the majority of the brain tissue, providing support and nutrients to the neurons. Astrocytes appear to be largely structural, separating neurons from each other and adhering to capillaries. They appear to play a role in supplying nutrients to neurons. Oligodendrocytes are glial cells that make up the CNS myelin, and Schwann cells are glia constituting the myelin of the PNS. Although Dendrite Dendrite Dendrite Dendrite technically not neurons, glial cells are an important component of the nervous system tissue. Astrocytes provide the primary support for neurons, aid in the suspension of neurons, and transport nutrients from the capillary supply. They also provide the important blood-brain barrier, a membranous filter system that prohibits some toxins from Central axon passing from the cerebrovascular system to neurons. Soma Soma Yet another type of glial cell, microglia, performs the housekeeping Axon process known as phagocytosis. Microglia scavenge necrotic tissue formed by a lesion in the nervous system. Astrocytes will assist by Soma forming scarring around necrotic tissue, effectively isolating it from the rest of the brain tissue. Neuroscience is now taking a very hard look at the role of astrocytes. We have long known that they had an important role in Peripheral Axon support of neurons, including nutrient delivery, but only recently has axon Axon Axon evidence emerged to indicate that glial cells are critical to the creation of long-term memory, as well as to the formation of synapses. Soma Functional Differences between Neurons. There are functional differences between neurons as well. Interneurons make up the largest class of neurons in the brain. The job of interneurons is to provide Purkinje cell of cerebellum Spinal motor neuron communication between other neurons, and interneurons do not PSEUDO-UNIPOLAR UNIPOLAR BIPOLAR MULTIPOLAR NEURON exit the central nervous system. Another type of neuron is the motor NEURON NEURON NEURON neuron. Motor neurons are efferent in nature, and they are typically Figure 11-5. Types of neurons. Pseudo-unipolar and unipolar neurons are primarily somatic afferent neurons, whereas bipolar neurons mediate special senses. bipolar neurons that activate muscular or glandular response. These Multipolar neurons, which are primarily efferent, include pyramidal cells, spinal motor neurons, and Purkinje cells of the cerebellum. Source: Delmar/Cengage Learning neurons usually have long axons that are myelinated. Motor neurons are further differentiated based on size, conduction velocity (how fast they can conduct an impulse), and the degree of myelination. Generally speaking, a neuron with a wider axon and thicker myelin will have more rapid conduction of neural impulses. 537 Neuronal fibers are classified in terms of conduction velocity as being A, B, or C class fibers. The A and B fibers are myelinated. The A fibers are further broken down, based on conduction velocity, into alpha, beta, gamma, and delta fibers (see Table 11-7). Alpha motor neurons have high conduction velocities (between 50 and 120 m/s) and innervate the majority of skeletal muscle, called extrafusal muscle fibers. Slower-velocity gamma motor neurons innervate intrafusal muscle fibers within the muscle spindle, the sensory apparatus responsible for maintaining muscle length. Thus, alpha motor neurons activate the prime movers of the motor act; gamma motor neurons are responsible for maintaining muscle tone and muscle readiness for the motor act.  Neuroanatomy 539 540 Chapter 11 Table 11-7. Type A, B, and C sensory and motor fibers. The nervous system may be divided functionally as autonomic and somatic nervous systems serving involuntary and voluntary functions, respectively. FIBER VELOCITY MOTOR SENSORY CLASS (m/s) FUNCTION FUNCTION It may be divided anatomically as central and peripheral nervous systems as well. A-Alpha 50–120 Large alpha motor neurons De velopmental div isions s eparate the brain into innervating prosencephalon (which is further divided into telencephalon extrafusal muscle and diencephalon), the mesencephalon or midbrain, and the Ia 120 Primary muscle rhombencephalon, which includes the metencephalon and spindle afferents myelencephalon. Ib 120 Golgi tendon organs; Neurons are widely varied in morphology but may be broadly pressure receptors categorized as monopolor, bipolar, or multipolar. Beta II 70 Muscle spindle Neurons communicate through synapse by means of secondary afferents; neurotransmitter substance, and the response by the touch and pressure postsynaptic neuron may be excitatory or inhibitory. Gamma 40 Intrafusal muscle The size and type of axon is related to the conduction of neural of spindle impulse. Delta Glial cells provide the fatty sheath for myelinated axons, as III 15 Touch, pressure pain, well as support structure for neurons, and long term memory coolness potentiation. B 14 Smooth muscle C 2 Smooth muscle Anatesse Lesson IV 2 Pain and warmth Anatomy of the Cerebrum Data from Winans, Gilman, Manter, & Gatz, 2002. The cerebrum is the mostly highly evolved and organized structure of the human body. This is the largest structure of the nervous system, weighing approximately three pounds and made up of billions of neurons. The cerebrum is divided into grossly similar left and right Sensory alpha fibers are identified by a Roman numeral and a hemispheres and is wrapped by three meningeal linings that protect lowercase letter (type Ia, Ib, II, III, or IV fibers), reflecting a different and support the massive structure of the brain. We will discuss those classification scheme. The Ia neurons are the primary afferent meningeal linings first and then introduce you to the most important fibers from the muscle spindle, whereas the Ib neurons send sensory structure of your body. information generated at the Golgi tendon organs, sensors that respond The brain remains one of the most complex structures in the to the stretching of the tendon. Type II afferent fibers are secondary known universe. For every cubic millimeter of gray matter, there are muscle spindle afferents of the beta class and convey information from 100 billion synapses. Realize that a cubic millimeter is approximately touch and pressure receptors. The type III afferent fibers are delta class, the size of the letter “o” (lower case), if it represented a tiny globe. conducting pain, pressure, touch, and coolness sensation. Type IV fibers Similarly, the human cerebral cortex contains as many as 18 billion convey pain and warmth sense. Again, some sensory neurons are essential neurons per cubic millimeter. If you were to place the axonal fibers in for movement, and these include types Ia, Ib, and II. Types III and IV a cubic millimeter end-to-end, it would stretch 300 feet. This is some are important for transmitting other body senses (pain, temperature, very dense tissue! pressure) but are not essential to movement. In summary, the nervous system is a complex, hierarchical Meningeal Linings structure made up of neurons. The CNS is invested with a triple-layer meningeal lining serving Many motor functions become automated through practice. important protective and nutritive functions. There are three Voluntary movement, sensory awareness, and cognitive dura mater: L., tough mother meningeal linings covering the brain. The dura mater is a tough function are the domain of the cerebral cortex, although we bilayered lining, which is the most superficial of the meningeal linings are capable of sensation and response without consciousness. (see Figure 11-6). The dura mater itself is made up of two layers that The communication links of the nervous system are spinal nerves, cranial nerves, and tracts of the brainstem and spinal cord.  Neuroanatomy 541 542 Chapter 11 (A) Skull bone pia mater: L., pious mother; so lining, the pia mater. The pia mater is a thin, membranous covering Skin of scalp named because of its gentle that closely follows the contour of the brain. The major arteries and Periosteum but faithful attachment to the veins serving the surface of the brain course within this layer. cerebral cortex The function of the meningeal linings is to protect the brain, holding structures in place during movement and providing support for those structures. To provide this protection, the linings must conform to the structure of the brain. As part of this support, the dura mater takes Dura mater on four major infoldings. These infoldings of the dura separate major Arachnoid structures of the brain, providing some isolation. The four infoldings mater are the falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sella. The falx cerebri and falx cerebelli are sagittal dividers, separating Subdural space left and right structures of the brain, while the tentorium cerebelli and Pia mater diaphragma sella separate brain structures by means of a transversely Subarachnoid space posed membrane. Arachnoid The falx cerebri separates the two cerebral hemispheres with a villus vertical sheath of dura, running from the crista galli of the ethmoid to the tentorium cerebelli. The falx cerebri completely separates the two hemispheres down to the level of the corpus callosum (to be discussed). Cerebral cortex The falx cerebelli performs the same function for the cerebellum, of brain separating the left and right cerebellar hemispheres for protection and isolation (see Figure 11-7). The tentorium cerebelli is a horizontal dural shelf at the base of Blood vessel the skull that divides the cranium into superior (cerebral) and inferior (cerebellar) regions. The diaphragma sella forms a boundary between the pituitary gland and the hypothalamus and optic chiasm. The dura (B) mater encircles the cranial nerves as they exit the brainstem. Because of its placement, the tentorium cerebelli supports the cerebrum and keeps Villi its mass from compressing the cerebellum and brainstem, as would most certainly happen if the dural lining was absent. The dural shelves can be liabilities when trauma results in subdural hematoma, a release of blood (1) Dura mater through hemorrhage beneath the dura that can push on the cerebrum, (2) Arachnoid membrane causing the temporal lobe to herniate under the tentorium. Subdural Cerebral cortex (3) Pia mater hematoma may also cause a life-threatening herniation of the brainstem into the foramen magnum. Figure 11-6. (A) Meningeal linings of the brain. (B) Schematic of meningeal linings. There are meningeal linings of the spinal cord as well, paralleling Source: Delmar/Cengage Learning the structure and function of the cerebral meninges. At the foramen magnum, the meningeal linings are continuous with the spinal meningeal linings. The spinal meninges are broadly similar to those of are tightly bound together. The outer layer is more inelastic than the the brain, with some exceptions. The dura of the brain adheres to the inner layer, and meningeal arteries course through this layer. Whereas