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Chapter 14: the Brain and Cranial Nerves Crowther’s Tenth Martini, Chapter 14 Winter 2015 Chapter 14: The Brain and Cranial Nerves With this chapter we complete our tour of the Central Nervous System (CNS) that we began in Chapter 13 (The Spinal Cord, Spinal Nerves, and Spinal Reflexes). The 10th Martini version of chapter is VERY heavy on anatomical details, many of which we will skip. 14.0: Outline 14.1: Overview of brain anatomy The brain includes six major regions: the cerebrum, cerebellum, diencephalong, midbrain, pons, and medulla. 14.2: Protecting the brain The brain is protected physically and chemically by the cranial bones, the cranial meninges, the cerebrospinal fluid (CSF), and blood-brain barrier. 14.3: The twelve cranial nerves Cranial nerves connect directly with the brain rather than the spinal cord. The twelve cranial nerves are: olfactory (I), optic (II), oculomotor (III), trochlear (IV), trigeminal (V), abducens (VI), facial (VII), vestibulocochlear (VIII), glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII). 14.4: Example of a cranial reflex: the pupillary reflex In reaction to bright light, sensory information from cranial nerve II (optic) leads to motor output through cranial nerve III (oculomotor), contracting the pupillary muscles in the iris and thus reducing the size of the pupil. 14.5: Clinical issues Examples of brain disorders include meningitis, stroke, aphasia, seizures, Parkinson’s Disease, and Alzheimer’s Disease. 14.6: Recommended review questions 14.7: Appendix: the top 24 muscles … and their innervation 1 Crowther’s Tenth Martini, Chapter 14 Winter 2015 14.1: Overview of brain anatomy Before plunging into anatomical details, 10th Martini offers a reasonable overview of the brain in Figure 14-1 (An Introduction to Brain Structures and Functions) and the text accompanying it. According to 10th Martini, there are six major brain regions: the cerebrum, cerebellum, diencephalon, midbrain, pons, and medulla. The outline below shows how some other important structures, many of which you’ve seen in lab, fit into these six regions. The outline is not comprehensive (for example, the diencephalon includes more than just the thalamus and hypothalamus). Cerebrum o Cerebral cortex (Fig. 14-16) . Frontal lobe (Fig. 14-16a): includes motor cortex (control of voluntary muscles) and prefrontal cortex (decision-making, problem-solving) Broca’s area (Fig. 14-16b): affects speech production . Parietal lobe (Fig. 14-16a): perceives touch/pressure, taste, pain, and temperature . Temporal lobe (Fig. 14-16a): perceives smells and sounds Wernicke’s area (Fig. 14-16b): interprets language and sensory information . Occipital lobe (Fig. 14-16a): perceives visual information o Corpus callosum (Figs. 14-12, 14-15): connects the two cerebral hemispheres o Hippocampus (Fig. 14-12): a center for learning and memory o Amygdala (Fig. 14-12): influences emotions and their connections to memories Cerebellum (Fig. 14-1): subconsciously adjusts posture and movement Diencephalon (Fig. 14-5) o Thalamus (Fig. 14-12): relays and processes visual and auditory information, etc. o Hypothalamus (Fig. 14-12): regulates body temperature, heart rate, blood pressure, fluid loss, etc. (negative feedback central!) Midbrain (Fig. 14-5) o Superior colliculus (Fig. 14-5c): relays and processes visual information o Inferior colliculus (Fig. 14-5c): relays and processes auditory information o Substantia nigra (Fig. 14-9): subconscious control of muscle Pons (Fig. 14-5): relays sensory and motor information Medulla (Fig. 14-5): controls basic functions like heart rate, blood pressure, and rate of breathing Note that the groupings above are not the only possible way to think about the organization of the brain. For example, 10th Martini notes that the limbic system is defined as a group of structures that control emotion, motivation, and memory, which are split between the diencephalon (e.g., thalamus, hypothalamus) and the cerebrum (e.g., hippocampus, amygdala). 14.2: Protecting the brain The brain is protected physically and chemically by the cranial bones (discussed in Chapter 7), the cranial meninges, the cerebrospinal fluid (CSF), and blood-brain barrier. 2 Crowther’s Tenth Martini, Chapter 14 Winter 2015 You may recall from Chapter 13 or from lab that the spinal cord is covered with three layers of meninges: the dura mater (outermost), arachnoid mater (middle), and pia mater (innermost). You may also recall that CSF resides in the subarachnoid space between the arachnoid mater and pia mater. This information also holds true for the brain. In the brain, subarachnoid spaces are expanded into four ventricles, as pictured in 10th Martini Figure 14-2 (Ventricles of the Brain). The CSF cushions and supports the brain and also transports nutrients, chemical messengers, and waste products. The blood-brain barrier protects the brain in a different way: by strongly limiting the substances that can diffuse from the blood into the brain. Capillaries in the brain are lined by endothelial cells connected by tight junctions (remember these from Chapter 4?), so, to get into the brain, a substance must either pass through open channels in the endothelial cell membrane or be hydrophobic enough to pass directly through the membrane. As a company develops a new drug intended to act upon the brain, part of the development process is to make sure the drug can get through the blood-brain barrier. 14.3: The twelve cranial nerves As discussed in Chapter 13, most neural information flowing from the brain to the peripheral nervous system (PNS), or vice versa, travels through the spinal cord. The exceptions to this rule are the 12 cranial nerves, which connect directly to the brain, as shown in 10th Martini Figure 14-19 (Origins of the Cranial Nerves). The cranial nerves are numbered from 1 to 12 using Roman numerals. Roman numerals are easy to handle if you know three simple rules: I stands for 1, V stands for 5, X stands for 10, L stands for 50, etc. In general, add the values of all numerals to get the total. o For example, XXVIII = 28. If a smaller numeral comes before a larger numeral, subtract the smaller numeral from the larger numeral. o For example, IV = 4 and IX = 9. Thus the correspondence between Roman numerals and Arabic numbers is as follows. 1 2 3 4 5 6 7 8 9 10 11 12 I II III IV V VI VII VIII IX X XI XII As you can see from 10th Martini Figure 14-9, the numbering of the cranial nerves goes roughly from anterior to posterior, which cranial nerve I is most anterior, cranial nerve II is next-most anterior, etc. The exception is that cranial nerve XII is not the most posterior cranial nerve. Each cranial nerve has not only a number but a name corresponding to its function, as shown in CTM Table 14.1. Also notice that some of these nerves carry sensory information, some carry motor information, and some carry both. The nerve names and modalities (sensory/motor/both) can be remembered via menmonics such as “Oh Oh Oh, To Touch And Feel Very Good Velvet; Ah, Heaven!” and “Some Say Money Matters, But My Brother Says Big Brains Matter More,” as shown in CTM Table 14.2. 3 Crowther’s Tenth Martini, Chapter 14 Winter 2015 CTM Table 14.1: Key information about the 12 cranial nerves. (Nerves marked as M for motor are almost entirely motor but not 100% motor.) Sensory? Motor? # Name Both? Function Passage through the cranium Olfactory S Smell Olfactory foramina in cribriform plate I of ethmoid II Optic S Vision Optic canal of sphenoid III Oculomotor M Eye muscles Superior orbital fissure of sphenoid IV Trochlear M Eye muscles Superior orbital fissure of sphenoid Trigeminal B Facial senses, chewing Superior orbital fissure, foramen rotundum, and foramen ovale of V sphenoid (3 branches) VI Abducens M Eye muscles Superior orbital fissure of sphenoid Facial B Facial expressions, taste Internal acoustic meatus of temporal VII bone Vestibulocochlear S Balance and hearing Internal acoustic meatus of temporal VIII bone Glossopharyngeal B Salivary glands and taste Jugular foramen (between temporal and IX occipital bones) Vagus B Parasympathetic innervation Jugular foramen (between temporal and X of visceral organs occipital bones) Accessory M Back and neck muscles Jugular foramen (between temporal and XI occipital bones) XII Hypoglossal M Moving the tongue Hypoglossal canal of occipital bone CTM Table 14.2: Cranial nerve mnemonics. An additional mnemonic – a song to remember the cranial nerves’ functions – can be found online at http://faculty.washington.edu/crowther/Misc/Songs/cranial.shtml. I Oh → O → Olfactory Some → S → Sensory II Oh → O → Optic Say → S → Sensory III Oh, → O → Oculomotor Money → M → Motor IV To → T → Trochlear Matters → M → Motor V Touch → T → Trigeminal But → B → Both VI And → A → Abducens My → M → Motor VII Feel → F → Facial Brother → B → Both VIII Very → V → Vestibulocochlear Says → S → Sensory IX Good → G → Glossopharyngeal Big → B → Both X Velvet; → V → Vagus Brains → B → Both XI Ah, → A → Accessory Matter → M → Motor XII Heaven! → H → Hypoglossal More. → M → Motor Additionally, note that to get into and out of the brain, the cranial nerves must pass through holes in the cranium (canals, fissures, foramina, meatuses, etc.). We saw some of these holes when studying the axial skeleton in Chapter 7; now we are in a better position to see which ones go with which nerves. These holes are listed in CTM Table 14.1. In some cases, there is more than one hole for a given nerve; the olfactory foramina include many tiny holes through the cribriform plate, while the three branches of the trigeminal nerve (ophthalmic, maxillary, and mandibular) each go through separate openings. We are thus reminded that, like other nerves, cranial nerves 4 Crowther’s Tenth Martini, Chapter 14 Winter 2015 have branches and subdivisions.
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