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To successfully complete this assignment first read the following exercises from the Laboratory Manual: Exercise 17: Gross Anatomy of the Brain and Cranial Nerves and Exercise 21: Human Reflex Physiology.
Student Discussion Assignment
Briefly identify and describe the structure and function of the following structures associated with the human brain:
- external structure (lobes and fissures) of the cerebral hemispheres
- ventral aspect
- cranial nerves (Table 17.1)
- internal structures (i.e., cerebrum, diencephalon, midbrain, brain stem, and cerebellum)
- Briefly identify and describe the components and structures associated with simple reflex arcs.
Write your responses in a minimum of 250 words in APA format.
As in all assignments, cite your sources in your work and provide references for the citations in APA format. Support your work, using your course lectures and textbook readings. Helpful APA guides and resources are available in the South University Online Library. Below are guides that are located in the library and can be accessed and downloaded via the South University Online Citation Resources: APA Style page. The American Psychological Association website also provides detailed guidance on formatting, citations, and references at APA Style.
To successfully complete this assignment first read the following exercises from the Laboratory Manual: Exercise 17: Gross Anatomy of the Brain and Cranial Nerves and Exercise 21: Human Reflex Physiol
When viewed alongside all Earth’s animals, humans are indeed unique, and the key to their uniqueness is found in the brain. Each of us is a reflection of our brain’s experience. If all past sensory input could mysteriously and suddenly be “erased,” we would be unable to walk, talk, or communicate in any manner. Spontaneous movement would occur, as in a fetus, but no voluntary integrated function of any type would be possible. Clearly we would cease to be the same individuals. For convenience, the nervous system is considered in terms of two principal divisions: the central nervous system and the peripheral nervous system. The central nervous system (CNS) consists of the brain and spinal cord, which primarily interpret incoming sensory information and issue instructions based on that information and on past experience. The peripheral nervous system (PNS) consists of the cranial and spinal nerves, ganglia, and sensory receptors. The PNS has two major subdivisions: the sensory portion, which consists of nerve fibers that conduct impulses from sensory receptors toward the CNS, and the motor portion, which contains nerve fibers that conduct impulses away from the CNS. The motor portion, in turn, consists of the somatic division (sometimes called the voluntary system), which controls the skeletal muscles, and the autonomic nervous system (ANS), which controls smooth and cardiac muscles and glands. In this exercise the brain (CNS) and cranial nerves (PNS) will be studied together because of their close anatomical relationship. The Human Brain During embryonic development of all vertebrates, the CNS first makes its appearance as a simple tubelike structure, the neural tube, that extends down the dorsal median plane. By the fourth week, the human brain begins to form as an expansion of the anterior or rostral end of the neural tube (the end toward the head). Shortly thereafter, constrictions appear, dividing the developing brain into three major regions—forebrain, midbrain, and hindbrain (Figure 17.1). The remainder of the neural tube becomes the spinal cord. Figure 17.1 Embryonic development of the human brain. (a) The neural tube subdivides into (b) the primary brain vesicles, which subsequently form (c) the secondary brain vesicles, which differentiate into (d) the adult brain structures. (e) The adult structures derived from the neural canal. The central canal of the neural tube, which remains continuous throughout the brain and cord, enlarges in four regions of the brain, forming chambers called ventricles (see Figure 17.8a and b, p. 289). Activity 1 Identifying External Brain Structures Identify external brain structures using the figures cited. Also use a model of the human brain and other learning aids as they are mentioned. Generally, the brain is studied in terms of four major regions: the cerebral hemispheres, diencephalon, brain stem, and cerebellum. Cerebral Hemispheres The cerebral hemispheres are the most superior portion of the brain (Figure 17.2). Their entire surface is thrown into elevated ridges of tissue called gyri that are separated by shallow grooves called sulci or deeper grooves called fissures. Many of the fissures and gyri are important anatomical landmarks. The cerebral hemispheres are divided by a single deep fissure, the longitudinal fissure. The central sulcus divides the frontal lobe from the parietal lobe, and the lateral sulcus separates the temporal lobe from the parietal lobe. The parieto-occipital sulcus on the medial surface of each hemisphere divides the occipital lobe from the parietal lobe. The sulcus is not visible externally. A fifth lobe of each cerebral hemisphere, the insula, is buried deep within the lateral sulcus, and is covered by portions of the temporal, parietal, and frontal lobes. Notice that most cerebral hemisphere lobes are named for the cranial bones that lie over them. Some important functional areas of the cerebral hemispheres have also been located (Figure 17.2d). Table 17.1 Important Functional Areas of the Cerebral Cortex Functional sensory areas Location Functions Primary somatosensory cortex Postcentral gyrus of the parietal lobe Receives information from the body’s sensory receptors in the skin and from proprioceptors in the skeletal muscles, joints, and tendons Primary visual cortex Occipital lobe Receives visual information that originates in the retina of the eye Primary auditory cortex Temporal lobe in the gyrus bordering the lateral sulcus Receives sound information from the receptors for hearing in the internal ear Olfactory cortex Medial surface of the temporal lobe, in a region called the uncus Receives information from olfactory (smell) receptors in the superior nasal cavity Functional motor areas Location Functions Primary motor cortex Precentral gyrus of the frontal lobe Conscious control of voluntary movement of skeletal muscles Broca’s area At the base of the precentral gyrus of the frontal lobe just above the superior sulcus, in only one hemisphere Controls the muscles involved in speech production and also plays a role in the planning of nonspeech motor functions The cell bodies of cerebral neurons involved in these functions are found only in the outermost gray matter of the cerebrum, the cerebral cortex. Most of the balance of cerebral tissue—the deeper cerebral white matter—is composed of myelinated fibers bundled into tracts carrying impulses to or from the cortex. Some of these functional areas are summarized in Table 17.1. Using a model of the human brain (and a preserved human brain, if available), identify the areas and structures of the cerebral hemispheres described above. Then continue using the model and preserved brain along with the figures as you read about other structures. Diencephalon The diencephalon is embryologically part of the forebrain, along with the cerebral hemispheres (see Figure 17.1). Turn the brain model so the ventral surface of the brain can be viewed. Identify the externally visible structures that mark the position of the floor of the diencephalon using Figure 17.3 as a guide. These are the olfactory bulbs (synapse point of cranial nerve I) and tracts, optic nerves (cranial nerve II), optic chiasma (where the medial fibers of the optic nerves cross over), optic tracts, pituitary gland, and mammillary bodies. Figure 17.2 External features of the cerebral hemispheres. (a) Left lateral view of the brain. (b) Superior view. (c) Photograph of the superior aspect of the human brain. (d) Functional areas of the left cerebral cortex. The olfactory cortex, which is deep within the temporal lobe on the medial hemispheric surface, is not identified. Brain Stem Continue to identify the brain stem structures—the cerebral peduncles (fiber tracts in the midbrain connecting the pons below with cerebrum above), the pons, and the medulla oblongata. Pons means “bridge,” and the pons consists primarily of motor and sensory fiber tracts connecting the brain with lower CNS centers. The lowest brain stem region, the medulla oblongata (often shortened to just medulla), is also composed primarily of fiber tracts. You can see the decussation of pyramids, a crossover point for the major motor tracts (pyramidal tracts) descending from the motor areas of the cerebrum to the spinal cord, on the medulla’s surface. The medulla also houses many vital autonomic centers involved in the control of heart rate, respiratory rhythm, and blood pressure as well as involuntary centers involved in vomiting and swallowing. Figure 17.3 Ventral (inferior) aspect of the human brain, showing the three regions of the brain stem. Only a small portion of the midbrain can be seen; the rest is surrounded by other brain regions. Cerebellum Turn the brain model so you can see the dorsal aspect. Identify the large cauliflower-shaped cerebellum, which projects dorsally from under the occipital lobe of the cerebrum. Notice that, like the cerebrum, the cerebellum has two major hemispheres and a convoluted surface (see Figure 17.6). It also has an outer cortex made up of gray matter with an inner region of white matter. Remove the cerebellum to view the corpora quadrigemina (Figure 17.4), located on the posterior aspect of the midbrain, a brain stem structure. The two superior prominences are the superior colliculi (visual reflex centers); the two smaller inferior prominences are the inferior colliculi (auditory reflex centers). Activity 2 Identifying Internal Brain Structures The deeper structures of the brain have also been well mapped. As the internal brain areas are described, identify them on the figures cited. Also, use the brain model as indicated to help you in this study. Cerebral Hemispheres Take the brain model apart so you can see a medial view of the internal brain structures (Figure 17.4). Observe the model closely to see the extent of the outer cortex (gray matter), which contains the cell bodies of cerebral neurons. The pyramidal cells of the cerebral motor cortex (studied in Exercise 15 and Figure 17.5) are representative of the neurons seen in the precentral gyrus. Observe the deeper area of white matter, which is composed of fiber tracts. The fiber tracts found in the cerebral hemisphere white matter are called association tracts if they connect two portions of the same hemisphere, projection tracts if they run between the cerebral cortex and lower brain structures or spinal cord, and commissures if they run from one hemisphere to another. Observe the large corpus callosum, the major commissure connecting the cerebral hemispheres. The corpus callosum arches above the structures of the diencephalon and roofs over the lateral ventricles. Notice also the fornix, a bandlike fiber tract concerned with olfaction as well as limbic system functions, and the membranous septum pellucidum, which separates the lateral ventricles of the cerebral hemispheres. Figure 17.4 Diencephalon and brain stem structures as seen in a medial section of the brain. (a) Photograph. (b) Diagram. In addition to the gray matter of the cerebral cortex, there are several clusters of neuron cell bodies called nuclei buried deep within the white matter of the cerebral hemispheres. One important group of cerebral nuclei, called the basal nuclei or basal ganglia, * flank the lateral and third ventricles. You can see these nuclei if you have a dissectible model or a coronally or cross-sectioned human brain slice. (Otherwise, Figure 17.5 will suffice.) * The historical term for these nuclei, basal ganglia, is misleading because ganglia are PNS structures. Although technically not the correct anatomical term, “basal ganglia” is included here because it is widely used in clinical settings. The basal nuclei are involved in regulating voluntary motor activities. The most important of them are the arching, comma-shaped caudate nucleus, the putamen, and the globus pallidus. The closely associated amygdaloid body (located at the tip of the tail of the caudate nucleus) is part of the limbic system. Figure 17.5 Basal nuclei. (a) Three-dimensional view of the basal nuclei showing their positions within the cerebrum. (b) A transverse section of the cerebrum and diencephalon showing the relationship of the basal nuclei to the thalamus and the lateral and third ventricles. The corona radiata, a spray of projection fibers coursing down from the precentral (motor) gyrus, combines with sensory fibers traveling to the sensory cortex to form a broad band of fibrous material called the internal capsule. The internal capsule passes between the thalamus and the basal nuclei and through parts of the basal nuclei, giving them a striped appearance. This is why the caudate nucleus and the putamen are sometimes referred to collectively as the striatum, or “striped body” (). Examine the relationship of the lateral ventricles and corpus callosum to the thalamus and third ventricle—from the cross-sectional viewpoint (see Figure 17.5b). Diencephalon The major internal structures of the diencephalon are the thalamus, hypothalamus, and epithalamus (see Figure 17.4). The thalamus consists of two large lobes of gray matter that laterally enclose the narrow third ventricle of the brain. A slender stalk of thalamic tissue, the interthalamic adhesion, or intermediate mass, connects the two thalamic lobes and bridges the ventricle. The thalamus is a major integrating and relay station for sensory impulses passing upward to the cortical sensory areas for localization and interpretation. Locate also the interventricular foramen, a tiny opening connecting the third ventricle with the lateral ventricle on the same side. The hypothalamus makes up the floor and the inferolateral walls of the third ventricle. It is an important autonomic center involved in regulation of body temperature, water balance, and fat and carbohydrate metabolism as well as many other activities and drives (sex, hunger, thirst). Locate again the pituitary gland, which hangs from the anterior floor of the hypothalamus by a slender stalk, the infundibulum. In life, the pituitary rests in the hypophyseal fossa of the sella turcica of the sphenoid bone. Anterior to the pituitary, identify the optic chiasma portion of the optic pathway to the brain. The mammillary bodies, relay stations for olfaction, bulge exteriorly from the floor of the hypothalamus just posterior to the pituitary gland. The epithalamus forms the roof of the third ventricle and is the most dorsal portion of the diencephalon. Important structures in the epithalamus are the pineal gland, and the choroid plexus of the third ventricle. Why this Matters Vegetative State (VS) We appreciate the thalamus more when we look at what happens when it is damaged. This is the problem that afflicts patients trapped in a vegetative state. A vegetative state (VS) is described as a disorder of consciousness that lasts more than 4 weeks in which the patient has lost awareness of self and environment. Unlike coma patients, who cannot be awakened, VS patients can be aroused—they display wakefulness without awareness. The vegetative state is clinical loss of cerebral cortex function with unaffected brain stem function. In many cases, there is no structural damage to the cerebral cortex, but its function is lost because the relay of signals to the cortex—the thalamus—is not working. Brain Stem Now trace the short midbrain from the mammillary bodies to the rounded pons below. (Continue to refer to Figure 17.4). The cerebral aqueduct is a slender canal traveling through the midbrain; it connects the third ventricle to the fourth ventricle. The cerebral peduncles and the rounded corpora quadrigemina make up the midbrain tissue anterior and posterior (respectively) to the cerebral aqueduct. Locate the hindbrain structures. Trace the rounded pons to the medulla oblongata below, and identify the fourth ventricle posterior to these structures. Attempt to identify the single median aperture and the two lateral apertures, three openings found in the walls of the fourth ventricle. These apertures serve as passageways for cerebrospinal fluid to circulate into the subarachnoid space from the fourth ventricle. Figure 17.6 Cerebellum. (a) Posterior (dorsal) view. (b) Sectioned to reveal the cerebellar cortex. (The cerebellum is sectioned frontally, and the brain stem is sectioned transversely, in this posterior view.) Cerebellum Examine the cerebellum. Notice that it is composed of two lateral hemispheres, each with three lobes (anterior, posterior, and a deep flocculonodular) connected by a midline lobe called the vermis (Figure 17.6). As in the cerebral hemispheres, the cerebellum has an outer cortical area of gray matter and an inner area of white matter. The treelike branching of the cerebellar white matter is referred to as the arbor vitae, or “tree of life.” The cerebellum controls the unconscious coordination of skeletal muscle activity along with balance and equilibrium. Meninges of the Brain The brain and spinal cord are covered and protected by three connective tissue membranes called meninges (Figure 17.7). The outermost meninx is the leathery dura mater, a double-layered membrane. One of its layers (the periosteal layer) is attached to the inner surface of the skull, forming the periosteum. The other (the meningeal layer) forms the outermost brain covering and is continuous with the dura mater of the spinal cord. Figure 17.7 Meninges of the brain. (a) Three-dimensional frontal section showing the relationship of the dura mater, arachnoid mater, and pia mater. The meningeal dura forms the falx cerebri fold, which extends into the longitudinal fissure and attaches the brain to the ethmoid bone of the skull. The superior sagittal sinus is enclosed by the dural membranes superiorly. Arachnoid villi, which return cerebrospinal fluid to the dural venous sinus, are also shown. (b) Medial view showing the position of the dural folds: the falx cerebri, tentorium cerebelli, and falx cerebelli. (c) Posterior view of the brain in place, surrounded by the dura mater. Sinuses between periosteal and meningeal dura contain venous blood. The dural layers are fused together except in three places where the inner membrane extends inward to form a septum that secures the brain to structures inside the cranial cavity. One such extension, the falx cerebri, dips into the longitudinal fissure between the cerebral hemispheres to attach to the crista galli of the ethmoid bone of the skull (Figure 17.7a). The cavity created at this point is the large superior sagittal sinus, which collects blood draining from the brain tissue. The falx cerebelli, separating the two cerebellar hemispheres, and the tentorium cerebelli, separating the cerebrum from the cerebellum below, are two other important inward folds of the inner dural membrane. The middle meninx, the weblike arachnoid mater, underlies the dura mater and is partially separated from it by the subdural space. Threadlike projections bridge the subarachnoid space to attach the arachnoid to the innermost meninx, the pia mater. The delicate pia mater is highly vascular and clings tenaciously to the surface of the brain, following its gyri. In life, the subarachnoid space is filled with cerebrospinal fluid. Specialized projections of the arachnoid tissue called arachnoid villi protrude through the dura mater. These villi allow the cerebrospinal fluid to drain back into the venous circulation via the superior sagittal sinus and other dural venous sinuses. Meningitis, inflammation of the meninges, is a serious threat to the brain because of the intimate association between the brain and meninges. Should infection spread to the neural tissue of the brain itself, life-threatening encephalitis may occur. Meningitis is often diagnosed by taking a sample of cerebrospinal fluid (via a spinal tap) from the subarachnoid space. Cerebrospinal Fluid The cerebrospinal fluid (CSF), much like plasma in composition, is continually formed by the choroid plexuses, small capillary knots hanging from the roof of the ventricles of the brain. The cerebrospinal fluid in and around the brain forms a watery cushion that protects the delicate brain tissue against blows to the head. Within the brain, the cerebrospinal fluid circulates from the two lateral ventricles (in the cerebral hemispheres) into the third ventricle via the interventricular foramina, and then through the cerebral aqueduct of the midbrain into the fourth ventricle (Figure 17.8). CSF enters the subarachnoid space through the three foramina in the walls of the fourth ventricle. There it bathes the outer surfaces of the brain and spinal cord. The fluid returns to the blood in the dural venous sinuses via the arachnoid villi. Ordinarily, cerebrospinal fluid forms and drains at a constant rate. However, under certain conditions—for example, obstructed drainage or circulation resulting from tumors or anatomical deviations—cerebrospinal fluid accumulates and exerts increasing pressure on the brain which, uncorrected, causes neurological damage in adults. In infants, hydrocephalus (literally, “water on the brain”) is indicated by a gradually enlarging head. The infant’s skull is still flexible and contains fontanelles, so it can expand to accommodate the increasing size of the brain. Cranial Nerves The cranial nerves are part of the peripheral nervous system and not part of the brain proper, but they are most appropriately identified while studying brain anatomy. The 12 pairs of cranial nerves primarily serve the head and neck. Only one pair, the vagus nerves, extends into the thoracic and abdominal cavities. All but the first two pairs (olfactory and optic nerves) arise from the brain stem and pass through foramina in the base of the skull to reach their destination. The cranial nerves are numbered consecutively, and in most cases their names reflect the major structures they control. The cranial nerves are described by name, number (Roman numeral), origin, course, and function in the list (Table 17.2). This information should be committed to memory. A mnemonic device that might be helpful for remembering the cranial nerves in order is “On occasion, our trusty truck acts funny—very good vehicle anyhow.” The first letter of each word and the “a” and “h” of the final word “anyhow” will remind you of the first letter of the cranial nerve name. Most cranial nerves are mixed nerves (containing both motor and sensory fibers). But close scrutiny of the list (Table 17.2) will reveal that two pairs of cranial nerves (optic and olfactory) are purely sensory in function. Activity 3 Identifying and Testing the Cranial Nerves Observe the ventral surface of the brain model to identify the cranial nerves. (Figure 17.9 may also aid you in this study.) Notice that the first (olfactory) cranial nerves are not visible on the model because they consist only of short axons that run from the nasal mucosa through the cribriform foramina of the ethmoid bone. (However, the synapse points of the first cranial nerves, the olfactory bulbs, are visible on the model.) Figure 17.8 Location and circulatory pattern of cerebrospinal fluid. (a, b) Brain ventricles. Regions of the large lateral ventricles are the anterior horn, posterior horn, and inferior horn. (c) Cerebrospinal fluid (CSF) flows from the lateral ventricles, through the interventricular foramina into the third ventricle, and then into the fourth ventricle via the cerebral aqueduct. Most of the CSF circulates in the subarachnoid space and returns to the dural venous sinuses via the arachnoid villi. Activity 3: Cranial Nerve Ganglia Cranial nerve ganglion Cranial nerve Site of ganglion Trigeminal Geniculate Inferior Superior Spiral Vestibular Testing cranial nerves is an important part of any neurological examination. See the last column of Table 17.2 for techniques you can use for such tests. Conduct tests of cranial nerve function following directions given in the “testing” column of the table. The results may help you understand cranial nerve function, especially as it pertains to some aspects of brain function. Several cranial nerve ganglia are named in the Activity 3 chart. Using your textbook or an appropriate reference, fill in the chart by naming the cranial nerve the ganglion is associated with and stating the ganglion location. Figure 17.9 Ventral aspect of the human brain, showing the cranial nerves. (See also Figure 17.3.) Table 17.2 The Cranial Nerves (Figure 17.9) Number and name Origin and course Function* Testing * Does not include sensory impulses from proprioceptors. I. Olfactory Fibers arise from olfactory epithelium and run through cribriform foramina of ethmoid bone to synapse in olfactory bulb. Purely sensory—carries afferent impulses associated with sense of smell. Person is asked to sniff aromatic substances, such as oil of cloves and vanilla, and to identify each. II. Optic Fibers arise from retina of eye and pass through optic canal of sphenoid bone. Fibers partially cross over at the optic chiasma and continue on to the thalamus as the optic tracts. Final fibers of this pathway travel from the thalamus to the primary visual cortex as the optic radiation. Purely sensory—carries afferent impulses associated with vision. Vision and visual field are determined with eye chart and by testing the point at which the person first sees an object (finger) moving into the visual field. Fundus of eye viewed with ophthalmoscope to detect papilledema (swelling of optic disc, or point at which optic nerve leaves the eye) and to observe blood vessels. III. Oculomotor Fibers emerge from ventral midbrain and course ventrally to enter the orbit. They exit from skull via superior orbital fissure. Primarily motor—somatic motor fibers to inferior oblique and superior, inferior, and medial rectus muscles, which direct eyeball, and to levator palpebrae muscles of the superior eyelid; parasympathetic fibers to smooth muscle controlling lens shape and pupil size. Pupils are examined for size, shape, and equality. Pupillary reflex is tested with penlight (pupils should constrict when illuminated). Convergence for near vision is tested, as is subject’s ability to follow objects with the eyes. IV. Trochlear Fibers emerge from midbrain and exit from skull via superior orbital fissure. Primarily motor—provides somatic motor fibers to superior oblique muscle that moves the eyeball. Tested in common with cranial nerve III. V. Trigeminal Fibers run from face to pons and form three divisions: mandibular division fibers pass through foramen ovale in sphenoid bone, maxillary division fibers pass via foramen rotundum in sphenoid bone, and ophthalmic division fibers pass through superior orbital fissure of sphenoid bone. Mixed—major sensory nerve of face; conducts sensory impulses from skin of face and anterior scalp, from mucosae of mouth and nose, and from surface of eyes; mandibular division also contains motor fibers that innervate muscles of mastication and muscles of floor of mouth. Sensations of pain, touch, and temperature are tested with safety pin and hot and cold probes. Corneal reflex tested with wisp of cotton. Motor branch assessed by asking person to clench the teeth, open mouth against resistance, and move jaw side to side. VI. Abducens Fibers leave inferior pons and exit from skull via superior orbital fissure. Primarily motor—carries somatic motor fibers to lateral rectus muscle that abducts the eyeball. Tested in common with cranial nerve III. VII. Facial Fibers leave pons and travel through temporal bone via internal acoustic meatus, exiting via stylomastoid foramen to reach the face. Mixed—supplies somatic motor fibers to muscles of facial expression and the posterior belly of the digastric muscle; parasympathetic motor fibers to lacrimal and salivary glands; carries sensory fibers from taste receptors of anterior tongue. Anterior two-thirds of tongue is tested for ability to taste sweet (sugar), salty, sour (vinegar), and bitter (quinine) substances. Symmetry of face is checked. Subject is asked to close eyes, smile, whistle, and so on. Tearing is assessed with ammonia fumes. VIII. Vestibulocochlear Fibers run from inner ear equilibrium and hearing apparatus, housed in temporal bone, through internal acoustic meatus to enter pons. Mostly sensory—vestibular branch transmits impulses associated with sense of equilibrium from vestibular apparatus and semicircular canals; cochlear branch transmits impulses associated with hearing from cochlea. Small motor component adjusts the sensitivity of the sensory receptors. Hearing is checked by air and bone conduction using tuning fork. IX. Glossopharyngeal Fibers emerge from medulla oblongata and leave skull via jugular foramen to run to throat. Mixed—somatic motor fibers serve pharyngeal muscles, and parasympathetic motor fibers serve salivary glands; sensory fibers carry impulses from pharynx, tonsils, posterior tongue (taste buds), and from chemoreceptors and pressure receptors of carotid artery. A tongue depressor is used to check the position of the uvula. Gag and swallowing reflexes are checked. Subject is asked to speak and cough. Posterior third of tongue may be tested for taste. X. Vagus Fibers emerge from medulla oblongata and pass through jugular foramen and descend through neck region into thorax and abdomen. Mixed—fibers carry somatic motor impulses to pharynx and larynx and sensory fibers from same structures; very large portion is composed of parasympathetic motor fibers, which supply heart and smooth muscles of abdominal visceral organs; transmits sensory impulses from viscera. As for cranial nerve IX (IX and X are tested in common, since they both innervate muscles of throat and mouth). XI. Accessory Fibers arise from the superior aspect of spinal cord, enter the skull, and then travel through jugular foramen to reach muscles of neck and back. Mixed (but primarily motor in function)—provides somatic motor fibers to sternocleidomastoid and trapezius muscles. Sternocleidomastoid and trapezius muscles are checked for strength by asking person to rotate head and shrug shoulders against resistance. XII. Hypoglossal Fibers arise from medulla oblongata and exit from skull via hypoglossal canal to travel to tongue. Mixed (but primarily motor in function)—carries somatic motor fibers to muscles of tongue. Person is asked to protrude and retract tongue. Any deviations in position are noted. Dissection The Sheep Brain The sheep brain is enough like the human brain to warrant comparison. Obtain a sheep brain, disposable gloves, dissecting tray, and instruments, and bring them to your laboratory bench. Don disposable gloves. If the dura mater is present, remove it as described here. Place the intact sheep brain ventral surface down on the dissecting pan, and observe the dura mater. Feel its consistency and note its toughness. Cut through the dura mater along the line of the longitudinal fissure (which separates the cerebral hemispheres) to enter the superior sagittal sinus. Gently force the cerebral hemispheres apart laterally to expose the corpus callosum deep to the longitudinal fissure. Carefully remove the dura mater and examine the superior surface of the brain. Notice that its surface, like that of the human brain, is thrown into convolutions (fissures and gyri). Locate the arachnoid mater, which appears on the brain surface as a delicate “cottony” material spanning the fissures. In contrast, the innermost meninx, the pia mater, closely follows the cerebral contours. Before beginning the dissection, turn your sheep brain so that you are viewing its left lateral aspect. Compare the various areas of the sheep brain (cerebrum, brain stem, cerebellum) to the photo of the human brain (Figure 17.10). Relatively speaking, which of these structures is obviously much larger in the human brain? Ventral Structures Turn the brain so that its ventral surface is uppermost. (Figure 17.11a and b show the important features of the ventral surface of the brain.) Look for the clublike olfactory bulbs anteriorly, on the inferior surface of the frontal lobes of the cerebral hemispheres. Axons of olfactory neurons run from the nasal mucosa through the cribriform foramina of the ethmoid bone to synapse with the olfactory bulbs. How does the size of these olfactory bulbs compare with those of humans? Is the sense of smell more important for protection and foraging in sheep or in humans? Figure 17.10 Photograph of lateral aspect of the human brain. The optic nerve (II) carries sensory impulses from the retina of the eye. Thus this cranial nerve is involved in the sense of vision. Identify the optic nerves, optic chiasma, and optic tracts. Figure 17.11 Intact sheep brain. (a) Photograph of ventral view. (b) Diagram of ventral view. (c) Photograph of the dorsal view. Watch a video of the Sheep Brain MasteringA&P®>Study Area>Pre-Lab Videos Posterior to the optic chiasma, two structures protrude from the ventral aspect of the hypothalamus—the infundibulum (stalk of the pituitary gland) immediately posterior to the optic chiasma and the mammillary body. Notice that the sheep’s mammillary body is a single rounded eminence. In humans it is a double structure. Identify the cerebral peduncles on the ventral aspect of the midbrain, just posterior to the mammillary body of the hypothalamus. The cerebral peduncles are fiber tracts connecting the cerebrum and medulla oblongata. Identify the large oculomotor nerves (III), which arise from the ventral midbrain surface, and the tiny trochlear nerves (IV), which can be seen at the junction of the midbrain and pons. Both of these cranial nerves provide motor fibers to extrinsic muscles of the eyeball. Move posteriorly from the midbrain to identify first the pons and then the medulla oblongata, structures composed primarily of ascending and descending fiber tracts. Return to the junction of the pons and midbrain, and proceed posteriorly to identify the following cranial nerves, all arising from the pons. Check them off as you locate them. Trigeminal nerves (V), which are involved in chewing and sensations of the head and face. Abducens nerves (VI), which abduct the eye (and thus work in conjunction with cranial nerves III and IV). Facial nerves (VII), large nerves involved in taste sensation, gland function (salivary and lacrimal glands), and facial expression. Continue posteriorly to identify and check off: Vestibulocochlear nerves (VIII), mostly sensory nerves that are involved with hearing and equilibrium. Glossopharyngeal nerves (IX), which contain motor fibers innervating throat structures and sensory fibers transmitting taste stimuli (in conjunction with cranial nerve VII). Vagus nerves (X), often called “wanderers,” which serve many organs of the head, thorax, and abdominal cavity. Accessory nerves (XI), which serve muscles of the neck, larynx, and shoulder; actually arise from the spinal cord (C1 through C5) and travel superiorly to enter the skull before running to the muscles that they serve. Hypoglossal nerves (XII), which stimulate tongue and neck muscles. It is likely that some of the cranial nerves will have been broken off during brain removal. If so, observe sheep brains of other students to identify those missing from your specimen, using your check marks as a guide. Dorsal Structures Refer to the dorsal view photograph (Figure 17.11c) as a guide in identifying the following structures. Reidentify the now exposed cerebral hemispheres. How does the depth of the fissures in the sheep’s cerebral hemispheres compare to that of the fissures in the human brain? Examine the cerebellum. Notice that, in contrast to the human cerebellum, it is not divided longitudinally, and that its fissures are oriented differently. What dural fold (falx cerebri or falx cerebelli) is missing that is present in humans? Locate the three pairs of cerebellar peduncles, fiber tracts that connect the cerebellum to other brain structures, by lifting the cerebellum dorsally away from the brain stem. The most posterior pair, the inferior cerebellar peduncles, connect the cerebellum to the medulla. The middle cerebellar peduncles attach the cerebellum to the pons, and the superior cerebellar peduncles run from the cerebellum to the midbrain. To expose the dorsal surface of the midbrain, gently separate the cerebrum and cerebellum (as shown in Figure 17.12.) Identify the corpora quadrigemina, which appear as four rounded prominences on the dorsal midbrain surface. What is the function of the corpora quadrigemina? Also locate the pineal gland, which appears as a small oval protrusion in the midline just anterior to the corpora quadrigemina. Internal Structures The internal structure of the brain can be examined only after further dissection. Place the brain ventral side down on the dissecting tray and make a cut completely through it in a superior to inferior direction. Cut through the longitudinal fissure, corpus callosum, and midline of the cerebellum. Refer to Figure 17.13 as you work. A thin nervous tissue membrane immediately ventral to the corpus callosum that separates the lateral ventricles is the septum pellucidum. If it is still intact, pierce this membrane and probe the lateral ventricle cavity. The fiber tract ventral to the septum pellucidum and anterior to the third ventricle is the fornix. Figure 17.12 Means of exposing the dorsal midbrain structures of the sheep brain. Watch a video of the Sheep Brain MasteringA&P®>Study Area>Pre-Lab Videos How does the size of the fornix in this brain compare with the size of the human fornix? Why do you suppose this is so? (Hint: What is the function of this band of fibers?) Identify the thalamus, which forms the walls of the third ventricle and is located posterior and ventral to the fornix. The interthalamic adhesion appears as an oval protrusion of the thalamic wall. The hypothalamus forms the floor of the third ventricle. Identify the optic chiasma, infundibulum, and mammillary body on its exterior surface. The pineal gland is just beneath the junction of the corpus callosum and fornix. Locate the midbrain by identifying the corpora quadrigemina that form its dorsal roof. Follow the cerebral aqueduct through the midbrain tissue to the fourth ventricle. Identify the cerebral peduncles, which form its anterior walls. Identify the pons and medulla oblongata, which lie anterior to the fourth ventricle. The medulla continues into the spinal cord without any obvious anatomical change, but the point at which the fourth ventricle narrows to a small canal is generally accepted as the beginning of the spinal cord. Identify the cerebellum posterior to the fourth ventricle. Notice its internal treelike arrangement of white matter, the arbor vitae. Figure 17.13 Photograph of median section of the sheep brain showing internal structures. Watch a video of the Sheep Brain MasteringA&P®>Study Area>Pre-Lab Videos Figure 17.14 Frontal section of a sheep brain. Major structures include the thalamus, hypothalamus, and lateral and third ventricles. Watch a video of the Sheep Brain MasteringA&P®>Study Area>Pre-Lab Videos If time allows, obtain another sheep brain and section it along the frontal plane so that the cut passes through the infundibulum. Compare your specimen with the photograph of a frontal section (Figure 17.14), and attempt to identify all the structures shown in the figure. Check with your instructor to determine if a small portion of the spinal cord from your brain specimen should be saved for spinal cord studies (Exercise 19). Otherwise, dispose of all the organic debris in the appropriate laboratory containers and clean the laboratory bench, the dissection instruments, and the tray before leaving the laboratory. Group Challenge Odd (Cranial) Nerve Out The following boxes each contain four cranial nerves. One of the listed nerves does not share a characteristic with the other three. Working in groups of three, discuss the characteristics of the four cranial nerves in each set. On a separate piece of paper, one student will record the characteristics for each nerve for the group. For each set of four nerves, discuss the possible candidates for the “odd nerve” and which characteristic it lacks based upon your notes. Once you have come to a consensus within your group, circle the cranial nerve that doesn’t belong with the others, and explain why it is singled out. What characteristic is it missing? Sometimes there may be multiple reasons why the cranial nerve doesn’t belong with the others. 1. Which is the “odd” nerve? Why is it the odd one out? Optic nerve (II) Oculomotor nerve (III) Olfactory nerve (I) Vestibulocochlear nerve (VIII) 2. Which is the “odd” nerve? Why is it the odd one out? Oculomotor nerve (III) Trochlear nerve (IV) Abducens nerve (VI) Hypoglossal nerve (XII) 3. Which is the “odd” nerve? Why is it the odd one out? Facial nerve (VII) Hypoglossal nerve (XII) Trigeminal nerve (V) Glossopharyngeal nerve (IX) 21 Reflexes are rapid, predictable, involuntary motor responses to stimuli; they are mediated over neural pathways called reflex arcs. Many of the body’s control systems are reflexes, which can be either inborn (intrinsic) or learned (acquired). Another way to categorize reflexes is into one of two large groups: autonomic reflexes and somatic reflexes. Autonomic (or visceral) reflexes are mediated through the autonomic nervous system, and we are not usually aware of them. These reflexes activate smooth muscles, cardiac muscle, and the glands of the body, and they regulate body functions such as digestion, elimination, blood pressure, salivation, and sweating. Somatic reflexes include all those reflexes that involve stimulation of skeletal muscles by the somatic division of the nervous system. Reflex testing is an important diagnostic tool for assessing the condition of the nervous system. If the spinal cord is damaged, the easily performed reflex tests can help pinpoint the area (level) of spinal cord injury. Motor nerves above the injured area may be unaffected, whereas those at or below the lesion site may be unable to participate in normal reflex activity. Components of a Reflex Arc Reflex arcs have five basic components (Figure 21.1): The receptor is the site of stimulus action. The sensory neuron transmits afferent impulses to the CNS. The integration center consists of one or more neurons in the CNS. The motor neuron conducts efferent impulses from the integration center to an effector organ. The effector, a muscle fiber or a gland cell, responds to efferent impulses by contracting or secreting, respectively. The simple patellar, or knee-jerk, reflex (Figure 21.2a) is an example of a simple, two-neuron, monosynaptic (literally, “one synapse”) reflex arc. It will be demonstrated in the laboratory. However, most reflexes are more complex and polysynaptic, involving the participation of one or more interneurons in the reflex arc pathway. An example of a polysynaptic reflex is the flexor reflex (Figure 21.2b). Because the reflex may be delayed or inhibited at the synapses, the more synapses encountered in a reflex pathway, the more time is required for the response. Reflexes of many types may be considered programmed into the neural anatomy. Many spinal reflexes, reflexes that are initiated and completed at the spinal cord level, occur without the direct involvement of higher brain centers. Generally, these reflexes are present in animals whose brains have been destroyed, provided that the spinal cord is functional. Although many spinal reflexes do not require the involvement of higher centers, the brain is “advised” of spinal cord reflex activity and may alter it by facilitating or inhibiting the reflexes. Figure 21.1 The five basic components of reflex arcs. The reflex illustrated is polysynaptic. Somatic Reflexes There are several types of somatic reflexes, including several that you will be eliciting during this laboratory session—the stretch, crossed-extensor, superficial, corneal, and gag reflexes. Some require only spinal cord activity; others require brain involvement as well. Some somatic reflexes are mediated by cranial nerves. Spinal Reflexes Stretch Reflexes Stretch reflexes are important for maintaining and adjusting muscle tone for posture, balance, and locomotion. Stretch reflexes are initiated by tapping a tendon or ligament, which stretches the muscle to which the tendon is attached (Figure 21.3). This stimulates the muscle spindles and causes reflex contraction of the stretched muscle or muscles. Branches of the afferent fibers from the muscle spindles also synapse with interneurons controlling the antagonist muscles. The inhibition of those interneurons and the antagonist muscles, called reciprocal inhibition, causes them to relax and prevents them from resisting (or reversing) the contraction of the stretched muscle. Additionally, impulses are relayed to higher brain centers (largely via the dorsal white columns) to advise of muscle length, speed of shortening, and the like—information needed to maintain muscle tone and posture. Stretch reflexes tend to be hypoactive or absent in cases of peripheral nerve damage or ventral horn disease and hyperactive in corticospinal tract lesions. They are absent in deep sedation and coma. Figure 21.2 Monosynaptic and polysynaptic reflex arcs. The integration center is in the spinal cord, and in each example the receptor and effector are in the same limb. (a) The patellar reflex, a two-neuron monosynaptic reflex. (b) A flexor reflex, an example of a polysynaptic reflex. Prepare for lab: Watch the Pre-Lab Video MasteringA&P®>Study Area>Pre-Lab Videos Activity 1 Initiating Stretch Reflexes Test the patellar, or knee-jerk, reflex by seating a subject on the laboratory bench with legs hanging free (or with knees crossed). Tap the patellar ligament sharply with the broad side of the reflex hammer just below the knee between the patella and the tibial tuberosity, as shown in Figure 21.4 . The knee-jerk response assesses the L2–L4 level of the spinal cord. Test both knees and record your observations. (Sometimes a reflex can be altered by your actions. If you encounter difficulty, consult your instructor for helpful hints.) Which muscles contracted? What nerve is carrying the afferent and efferent impulses? Test the effect of mental distraction on the patellar reflex by having the subject add a column of three-digit numbers while you test the reflex again. Is the response more or less vigorous than the first response? What are your conclusions about the effect of mental distraction on reflex activity? Now test the effect of muscular activity occurring simultaneously in other areas of the body. Have the subject clasp the edge of the laboratory bench and vigorously attempt to pull it upward with both hands. At the same time, test the patellar reflex again. Is the response more or less vigorous than the first response? Figure 21.3 The patellar (knee-jerk) reflex—a specific example of a stretch reflex. Figure 21.4 Testing the patellar reflex. The examiner supports the subject’s knee so that the subject’s muscles are relaxed, and then strikes the patellar ligament with the reflex hammer. The proper location may be ascertained by palpation of the patella. Fatigue also influences the reflex response. The subject should jog in position until she or he is very fatigued (really fatigued—no slackers). Test the patellar reflex again, and record whether it is more or less vigorous than the first response. Would you say that nervous system activity or muscle function is responsible for the changes you have just observed? Explain your reasoning. The calcaneal tendon, or ankle-jerk, reflex assesses the first two sacral segments of the spinal cord. With your shoe removed and your foot dorsiflexed slightly to increase the tension of the gastrocnemius muscle, have your partner sharply tap your calcaneal tendon with the broad side of the reflex hammer (Figure 21.5). What is the result? During walking, what is the action of the gastrocnemius? Figure 21.5 Testing the calcaneal tendon reflex. The examiner slightly dorsiflexes the subject’s ankle by supporting the foot lightly in the hand, and then taps the calcaneal tendon just above the ankle. Crossed-Extensor Reflex The crossed-extensor reflex is more complex than the stretch reflex. It consists of a flexor, or withdrawal, reflex followed by extension of the opposite limb. This reflex is quite obvious when, for example, a stranger suddenly and strongly grips one’s arm. The immediate response is to withdraw the clutched arm and push the intruder away with the other arm. The reflex is more difficult to demonstrate in a laboratory because it is anticipated, and under these conditions the extensor part of the reflex may be inhibited. Activity 2 Initiating the Crossed-Extensor Reflex The subject should sit with eyes closed and with the back of one hand resting on the laboratory bench. Obtain a sharp pencil, and suddenly prick the subject’s index finger. What are the results? Did the extensor part of this reflex occur simultaneously or more slowly than the other reflexes you have observed? What are the reasons for this? The reflexes that have been demonstrated so far—the stretch and crossed-extensor reflexes—are examples of reflexes in which the reflex pathway is mediated at the spinal cord level only. Superficial Reflexes The superficial reflexes (abdominal, cremaster, and plantar reflexes) result from pain and temperature changes. They are initiated by stimulation of receptors in the skin and mucosae. The superficial reflexes depend both on functional upper-motor pathways and on the spinal cord–level reflex arc. Since only the plantar reflex can be tested conveniently in a laboratory setting, we will use this as our example. The plantar reflex, an important neurological test, is elicited by stimulating the cutaneous receptors in the sole of the foot. In adults, stimulation of these receptors causes the toes to flex and move closer together. Damage to the corticospinal tract, however, produces Babinski’s sign, an abnormal response in which the toes flare and the great toe moves in an upward direction. In a newborn infant, it is normal to see Babinski’s sign because myelination of the nervous system is incomplete. Activity 3 Initiating the Plantar Reflex Have the subject remove a shoe and sock and lie on the cot or laboratory bench with knees slightly bent and thighs rotated so that the posterolateral side of the foot rests on the cot. Alternatively, the subject may sit up and rest the lateral surface of the foot on a chair. Draw the handle of the reflex hammer firmly along the lateral side of the exposed sole from the heel to the base of the great toe (Figure 21.6). What is the response? Is this a normal plantar reflex or a Babinski’s sign? Figure 21.6 Testing the plantar reflex. Using a moderately sharp object, the examiner strokes the lateral border of the subject’s sole, starting at the heel and continuing toward the great toe across the ball of the foot. Cranial Nerve Reflex Tests In these experiments, you will be working with your lab partner to illustrate two somatic reflexes mediated by cranial nerves. Corneal Reflex The corneal reflex is mediated through the trigeminal nerve. The absence of this reflex is an ominous sign because it often indicates damage to the brain stem resulting from compression of the brain or other trauma. Activity 4 Initiating the Corneal Reflex Stand to one side of the subject; the subject should look away from you toward the opposite wall. Wait a few seconds and then quickly, but gently, touch the subject’s cornea (on the side toward you) with a wisp of absorbent cotton. What reflexive reaction occurs when something touches the cornea? What is the function of this reflex? Gag Reflex The gag reflex tests the somatic motor responses of cranial nerves IX and X. When the oral mucosa on the side of the uvula is stroked, each side of the mucosa should rise, and the amount of elevation should be equal. The uvula is the fleshy tab hanging from the roof of the mouth. Activity 5 Initiating the Gag Reflex For this experiment, select a subject who does not have a queasy stomach, because regurgitation is a possibility. Gently stroke the oral mucosa on each side of the subject’s uvula with a tongue depressor. What happens? Discard the used tongue depressor in the disposable autoclave bag before continuing. Do not lay it on the laboratory bench at any time Autonomic Reflexes The autonomic reflexes include the pupillary, ciliospinal, and salivary reflexes, as well as many other reflexes. Work with a partner to demonstrate these four autonomic reflexes. Pupillary Reflexes There are several types of pupillary reflexes. The pupillary light reflex and the consensual reflex will be examined here. In both of these pupillary reflexes, the retina of the eye is the receptor, the optic nerve contains the afferent fibers, the oculomotor nerve contains the efferent fibers, and the smooth muscle of the iris is the effector. Absence of normal pupillary reflexes is generally a late indication of severe trauma or deterioration of the vital brain stem tissue due to metabolic imbalance. Activity 6 Initiating Pupillary Reflexes Conduct the reflex testing in an area where the lighting is relatively dim. Before beginning, obtain a metric ruler and a flashlight. Measure and record the size of the subject’s pupils as best you can. Right pupil: mm Left pupil: mm Stand to the left of the subject to conduct the testing. The subject should shield his or her right eye by holding a hand vertically between the eye and the right side of the nose. Shine a flashlight into the subject’s left eye. What is the pupillary response? Measure the size of the left pupil: mm Without moving the flashlight, observe the right pupil. Has the same type of change (called a consensual response) occurred in the right eye? Measure the size of the right pupil: mm The consensual response, or any reflex observed on one side of the body when the other side has been stimulated, is called a contralateral response. The pupillary light response, or any reflex occurring on the same side stimulated, is referred to as an ipsilateral response. What does the occurrence of a contralateral response indicate about the pathways involved? What is the function of these pupillary responses? Ciliospinal Reflex The ciliospinal reflex is another example of reflex activity in which pupillary responses can be observed. This response may initially seem a little bizarre, especially in view of the consensual reflex just demonstrated. Activity 7 Initiating the Ciliospinal Reflex While observing the subject’s eyes, gently stroke the skin (or just the hairs) on the left side of the back of the subject’s neck, close to the hairline. What is the reaction of the left pupil? The reaction of the right pupil? If you see no reaction, repeat the test using a gentle pinch in the same area. The response you should have noted—pupillary dilation—is consistent with the pupillary changes occurring when the sympathetic nervous system is stimulated. Such a response may also be elicited in a single pupil when more impulses from the sympathetic nervous system reach it for any reason. For example, when the left side of the subject’s neck was stimulated, sympathetic impulses to the left iris increased, resulting in the ipsilateral reaction of the left pupil. On the basis of your observations, would you say that the sympathetic innervation of the two irises is closely integrated? Why or why not? Salivary Reflex Unlike the other reflexes, in which the effectors were smooth or skeletal muscles, the effectors of the salivary reflex are glands. The salivary glands secrete varying amounts of saliva in response to reflex activation. Activity 8 Initiating the Salivary Reflex Obtain a small beaker, a graduated cylinder, lemon juice, and wide-range pH paper. After refraining from swallowing for 2 minutes, the subject is to expectorate (spit) the accumulated saliva into a small beaker. Using the graduated cylinder, measure the volume of the expectorated saliva and determine its pH. Volume: cc pH: Now place 2 or 3 drops of lemon juice on the subject’s tongue. Allow the lemon juice to mix with the saliva for 5 to 10 seconds, and then determine the pH of the subject’s saliva by touching a piece of pH paper to the tip of the tongue. pH: As before, the subject is to refrain from swallowing for 2 minutes. After the 2 minutes is up, again collect and measure the volume of the saliva and determine its pH. Volume: cc pH: How does the volume of saliva collected after the application of the lemon juice compare with the volume of the first saliva sample? How does the final saliva pH reading compare to the initial reading? How does the final saliva pH reading compare to that obtained 10 seconds after the application of lemon juice? Dispose of the saliva-containing beakers and the graduated cylinders in the laboratory bucket that contains bleach, and put the used pH paper into the disposable autoclave bag. Wash the bench down with 10% bleach solution before continuing. Reaction Time of Intrinsic and Learned Reflexes The time required for reaction to a stimulus depends on many factors—sensitivity of the receptors, velocity of nerve conduction, the number of neurons and synapses involved, and the speed of effector activation, to name just a few. There is no clear-cut distinction between intrinsic and learned reflexes, as most reflex actions are subject to modification by learning or conscious effort. In general, however, if the response involves a simple reflex arc, the response time is short. Learned reflexes involve a far larger number of neural pathways and many types of higher intellectual activities, including choice and decision making, which lengthens the response time. There are various ways of testing reaction time of reflexes. The following activities provide an opportunity to demonstrate the major time difference between a simple reflex arc and learned reflexes and to measure response time under various conditions.
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