The central nervous system (brain and spinal cord) is protected by the skull (Fig. 1-1) and vertebral column (Fig. 13-40), respectively. The peripheral nervous system (cranial nerves and spinal nerves) gets out of the bones (Fig. 13-64) (Fig. 13-84).
When the brain is viewed laterally, the three components are identifiable: the cerebrum, cerebellum, and brainstem.
< Cerebrum >
When the cerebrum is cut, it is divided into the (gray) cerebral cortex and the (white) cerebral medulla. They are also distinguished in the magnetic resonance images.
The color difference is best explained by their histology: The axon’s myelin sheath is composed of white fat. Think about the white fat in the bacon.
The cerebral cortex plays a key role in all conscious functions of the brain (Fig. 13-33). The readers of this chapter are asked to pay attention to the cerebral cortex.
Without activity of the cerebral cortex, one cannot feel, think, or move to be unconscious like in the coma.
Sleeping is a naturally recurring state of the cerebral cortex characterized by reduced consciousness.
In human, due to the big cerebral cortex, the brain is large. So, the human skull has big cranial cavity. It can be identified in a natural history museum that exhibits the skulls of various animals and primitive men.
Comparing to the cerebral medulla, the cerebral cortex should be large for containing plenty of nerve cell bodies. A nerve cell body is much larger than its axon (Fig. 13-48).
During development, the cerebral cortex should contain more and more nerve cell bodies. Because the brain is confined to the skull, the brain size is limited. Solution is that sulci create larger cerebral cortex. When looking at the brain, 2/3 of the cerebral cortex is hidden in the sulci (Fig. 13-3) (Fig. 13-18).
Fig. 13-10. Lobes of cerebrum.
The lateral sulcus and central sulcus are the first and second deepest furrow. They are border between the frontal, parietal, and temporal lobes. The occipital lobe is demarcated by a less distinct sulcus and a notch. The four lobes roughly correspond to the frontal, parietal, temporal, and occipital bones (Fig. 1-7).
Fig. 13-11. Gyri of cerebrum in lateral view (top), medial view (bottom).
Each lobe consists of the gyri, demarcated by sulci as well. For example, the frontal lobe consists of the superior, middle, and inferior frontal gyri, demarcated by the superior and inferior frontal sulci, excluding the precentral gyrus.
Each gyrus has its specific role. The postcentral and precentral gyri are the sensory cortex (Fig. 13-55) and motor cortex (Fig. 13-56), respectively. One can memorize it with a simple sentence, “Action is Anterior.” There are plenty of imprudent people like the authors, who prefer action to thinking.
< Limbic system >
Fig. 13-13. Limbic system including hippocampus.
A part of the cerebrum is the limbic system; its core is the hippocampus, a primitive cerebral cortex, located deep below the temporal horn of lateral ventricle (Fig. 13-38). The hippocampus is continuous with the parahippocampal gyrus (Fig. 13-11).
The hippocampus (Fig. 13-13) and fornix, in the superior view, resemble a seahorse.
Fig. 13-15. Circuit of impulse in limbic system.
The limbic system includes the hippocampus (Fig. 13-13), fornix (Fig. 13-14) (Fig. 13-18), mammillary body (Fig. 13-24) (Fig. 13-35), anterior nucleus (Fig. 13-22), cingulate gyrus, parahippocampal gyrus (Fig. 13-11) that form a circuit. The circuit as well as the amygdaloid nucleus (Fig. 13-19) influence the hypothalamus (Fig. 13-24).
Through the circuit, impulse is successively conveyed to carry out two cerebral functions: memory and emotion. Memory is stored in the hippocampus.
< Basal nuclei >
Fig. 13-17. Composition of basal nucleus.
Although commonly used, the term “basal ganglia” is wrong because the ganglia are situated in the peripheral nervous system (Fig. 13-54). The correct term “basal nuclei” include the corpus striatum and substantia nigra (Fig. 13-20).
Fig. 13-18. Corpus striatum (horizontal plane).
This figure basically shows the cerebral cortex (gray matter) and cerebral medulla (white matter) (Fig. 13-3). The cerebral medulla includes the external and internal capsules, both of which contain the motor and sensory nerves (Fig. 13-55) (Fig. 13-56). In the deep cerebral medulla, there is the corpus striatum (another gray matter).
A part of the corpus striatum is the lentiform nucleus that is the combination of putamen and globus pallidus (Fig. 13-17), the latter of which is relatively pale. Considering the globus pallidus is a gray matter (nucleus), the globus pallidus is still darker than the white matter such as the internal capsule.
Fig. 13-19. Corpus striatum.
During development, the putamen extends to form the caudate nucleus. The C-shaped caudate nucleus determines the C-shaped lateral ventricle (Fig. 13-18) and even the C-shaped cerebrum with the lateral sulcus (Fig. 13-10).
The curved caudate nucleus can be seen two times in the horizontal plane. Two parts of the caudate nucleus are in contact with the lateral ventricle. The internal capsule passes between the lentiform nucleus and the caudate nucleus (or the thalamus) (Fig. 13-18).
Fig. 13-20. Impulse around corpus striatum.
When the cerebral cortex decides a certain motion, its impulse arrives at the striatum (putamen, caudate nucleus) (Fig. 13-17). Simultaneously, impulse from the substantia nigra of midbrain (Fig. 13-35) arrives at the striatum. The putamen is at the center of cerebrum (Fig. 13-18); the caudate nucleus is elongated (Fig. 13-19) in order to receive impulse from the wide area of brain.
The impulse then passes the globus pallidus and thalamus (ventral anterior nucleus, ventral lateral nucleus) (Fig. 13-22) to enter the precentral gyrus, which is the motor cortex (Fig. 13-12). This pathway makes the motor nerve activated or inactivated to prevent too little or too much movement.
< Thalamus >
Fig. 13-21. Thalamus hidden by cerebrum.
Whereas the corpus striatum is a gray matter in the cerebrum (Fig. 13-19), the thalamus is a gray matter between the cerebrum and brainstem (Fig. 13-37). The thalamus cannot be seen outside due to the overgrowth of the cerebral hemisphere (Fig. 13-2).
Fig. 13-22. Nucleus of thalamus.
The thalamus is composed of several nuclei. In the central nervous system, the nucleus is a mass of nerve cell bodies (gray matter) with the same function (Fig. 13-48). Other example is the lentiform, caudate nuclei (Fig. 13-18).
Simply speaking, the thalamus is a secretary of the cerebral cortex. Various sensory nerves make a synapse in the thalamus before approaching the cerebral cortex such as the postcentral gyrus (Fig. 13-56). The nerve from the corpus striatum also makes a synapse in the thalamus before entering the precentral gyrus (Fig. 13-20).
Fig. 13-24. Thalamus, brainstem (medial view).
Below the thalamus, there is the hypothalamus which is another independent gray matter of the brain. The border between the thalamus and hypothalamus is the hypothalamic sulcus.
The hypothalamus is influenced partly by the limbic system (Fig. 13-15). The hypothalamus influences the autonomic nerve (Fig. 13-60) and endocrine glands (Fig. 9-12).
< Cerebellum >
Fig. 13-25. Cerebellum (sagittal plane).
Like the cerebrum (Fig. 13-18), the cerebellum consists of the cerebellar cortex (gray matter), cerebellar medulla (white matter), and cerebellar nuclei (gray matter). These gray matters are connected to each other to keep and activate information.
Information in the cerebellum is needed to support movement ordered by the precentral gyrus (Fig. 13-12).
Fig. 13-27. Three parts of cerebellum.
The cerebellum is divided into the small part (vestibulocerebellum) (Fig. 13-25), intermediate part (spinocerebellum), and large part (pontocerebellum).
The small vestibulocerebellum is for balance. The VESTIBulocerebellum is related with the VESTIBule of internal ear to perceive body movements (Fig. 14-45).
The spinocerebellum enables appropriate use of force. The SPINocerebellum is related with the somatic sensory nerves of SPINal nerve (Fig. 13-56).
The large pontocerebellum is for skilled movements such as writing. The PONtocerebellum is related with the PONs (Fig. 13-35) and cerebral cortex. In the nervous system, the large parts are for highly complicated functions; the cerebral cortex for consciousness and thinking is another instance (Fig. 13-5).
Both the corpus striatum and the cerebellum help motor nerves, but they are different. The corpus striatum prohibits unintentional movement (too much or too little movement) (Fig. 13-20), while the cerebellum promotes intentional movement (balance, appropriate force, or skilled movement).
< Brainstem >
Through the brainstem, the motor and sensory nerves between the cerebrum, cerebellum, and spinal cord pass (Fig. 13-55) (Fig. 13-56).
The brainstem plays another important role in the regulation of respiratory and cardiac functions.
The function of the brainstem is primitive but essential for survival.
In contrast with the cerebrum (Fig. 13-8) and cerebellum (Fig. 13-30), the human brainstem is not larger than other animals.
Fig. 13-35. Brainstem, cranial nerves
The small brainstem is subdivided into the midbrain, pons, and medulla oblongata. Among them the pons is bulging because its numerous impulses should reach the large part of the pontocerebellum (Fig. 13-27).
The brainstem is the site where most cranial nerves emerge (Fig. 13-67).
< Ventricle >
The ventricle is a cavity in the brain where the cerebrospinal fluid is produced and flows.
Fig. 13-37. Neural tube becoming brain, spinal cord.
The above figure shows the neural tube that develops into the brain and spinal cord. The neural tube has the canal that develops into the ventricles (Fig. 8-25).
Among the four serial ventricles, the first two are the lateral ventricles in the right and left cerebral hemispheres. After the development, the lateral ventricles and the cerebral hemispheres becomes much larger than in the figure above (Fig. 13-38).
The lateral ventricles are continuous with the third ventricle that is between the right and left thalami (Fig. 13-18). The aqueduct of midbrain is literally in the midbrain. The fourth ventricle is located in the pons and medulla oblongata. A supplementary ventricle, central canal is situated in the spinal cord (Fig. 13-24). (Exactly, the central canal is in the caudal medulla oblongata and the spinal cord.)
Fig. 13-38. Ventricles.
In the above figure, the left lateral ventricle is excluded. The lateral ventricle consists of a body and three horns.
From the capillaries in the lateral, third, and fourth ventricles, the plasma (Fig. 11-3) gets out to become the cerebrospinal fluid. The cerebrospinal fluid made in the lateral ventricle flows through the third ventricle, aqueduct of midbrain, and fourth ventricle (Fig. 13-37).
The pia, arachnoid, and dura maters (meninges) cover the brain and spinal cord (central nervous system) (Fig. 13-40) (Fig. 13-43).
Fig. 13-40. Spinal meninges, resultant spaces.
The dura mater is thick, the arachnoid mater is entangled like a spider’s web, and the pia mater is attached to the brain and spinal cord.
The subdural space is a potential space; its volume is close to zero unlike the above figure and Fig. 13-43. The subdural space can be increased in abnormal conditions like bleeding. On the other hand, the subarachnoid space is a real space.
The cerebrospinal fluid in the fourth ventricle (Fig. 13-24) (Fig. 13-38) exits to the subarachnoid space.
The cerebrospinal fluid surrounds and protects the soft brain and spinal cord.
Fig. 13-43. Cerebral meninges, resultant spaces.
In the brain, the dura mater constitutes dural venous sinuses. This coronal plane presents two examples, the superior and inferior sagittal sinuses. The superior sagittal sinus is in contact with the calvaria (Fig. 15-8), whereas the inferior one is not.
The cerebrospinal fluid in the subarachnoid space drains to the dural venous sinuses (such as superior sagittal sinus) through extension of the arachnoid mater. The dural “venous” sinuses are considered as “veins” (Fig. 10-33) (Fig. 10-63).
The cerebrospinal fluid flows from the capillary until it reaches a vein just like lymph (Fig. 12-10).
< Spinal cord >
Fig. 13-44. Spinal cord enlargements, cauda equina.
Medulla means the spinal cord; therefore, inferior part of the spinal cord is called the conus (cone) medullaris. Inferior part of the brain is regarded as the elongated spinal cord to be called the medulla oblongata (elongated) (Fig. 13-35).
Medulla also means the bone marrow (e.g., medullary cavity of bone) (Fig. 1-2), or the internal part of an organ (e.g., renal medulla, adrenal medulla) (Fig. 6-14) (Fig. 9-21).
At a very early stage of development, the vertebral column is as long as the spinal cord. Afterward, vertical growth of the vertebral column is faster than that of the spinal cord. Because of this discrepancy in length, the conus medullaris ends at LV1–LV2 level in adult (Fig. 1-15).
Also, lower spinal nerves from the spinal cord are lengthened to reach the corresponding intervertebral foramina (Fig. 1-14) (Fig. 13-85). These spinal nerves are collectively named the cauda (tail) equina (horse).
The cervical enlargement of the spinal cord gives off the brachial plexus (CN5–TN1) innervating the upper limb (Fig. 13-90). Likewise, the lumbosacral enlargement gives off the lumbosacral plexus (LN2–SN3) innervating the lower limb (Fig. 13-100).
The central nervous system is not repaired after disconnection. That is why the brain and spinal cord are well protected by the skull and vertebral column, respectively (Fig. 1-13) (Fig. 13-65).
< Somatic nerve >
Before discussing the peripheral nervous system, the somatic and visceral nerves are explained along with neurons.
Note that a nerve cell is synonymous with a neuron. Neurons are physically and functionally supported by neuroglias.
A neuron has one nerve cell body containing a nucleus, one or more dendrites, and one axon.
The dendrite conveys impulse to the nerve cell body, while the axon conveys impulse from the nerve cell body.
Synapse between the axon of a neuron and the dendrite of the next neuron functions with neurotransmitter. Neurotransmitter is also found at the axon ending of motor neuron, where it stimulates the muscle cell (Fig. 13-54).
Billions of neurotransmitters work constantly to keep the brain functioning, managing everything from breathing and heartbeat to learning and concentration.
Commonly, the dendrites are very short compared to the axon, and omitted.
Fig. 13-53. Development of sensory neuron.
At the early stage of development, a sensory neuron has an exceptionally long dendrite. The dendrite and axon are fused a bit to form a characteristic sensory neuron (Fig. 13-54).
Fig. 13-54. Somatic nerve.
The somatic motor nerve sends impulses to the skeletal muscle while the somatic sensory nerve receives impulses from receptors in the skin, subcutaneous tissue (Fig. 15-18), or skeletal muscle itself (Fig. 13-58). The somatic sensory nerve contains the sensory ganglion (Fig. 13-56).
From now on, the somatic motor nerve and somatic sensory nerve related with the spinal nerve (not cranial nerve) are explained.
Fig. 13-55. Somatic motor nerve.
In fact, the somatic motor nerve consists of two neurons. The upper motor neuron from the precentral gyrus (Fig. 13-12) descends, crosses the midsagittal plane at the border of the brainstem and spinal cord (Fig. 13-35). It descends again and meets the lower motor neuron. The lower motor neuron exits the spinal cord, passes the spinal nerve (anterior root) (Fig. 13-86), and controls the skeletal muscle voluntarily.
Fig. 13-56. Somatic sensory nerve.
In contrast, the somatic sensory nerve consists of three neurons. Beginning of the 1st neuron is the receptor (Fig. 15-18). The 1st neuron passes the posterior root, where it forms the spinal ganglion (Fig. 13-53) (Fig. 13-86). The 1st neuron meets the 2nd neuron at the level of the spinal cord or brainstem. The 2nd neuron crosses the midsagittal plane, ascends, and meets the 3rd neuron in the thalamus (ventral posterolateral nucleus) (Fig. 13-22). The 3rd neuron ascends to arrive at the postcentral gyrus (Fig. 13-12).
In summary, the somatic motor nerve requires two neurons; the somatic sensory nerve requires three neurons. It is because the somatic motor nerve does not make a synapse in the thalamus (Fig. 13-55) while the somatic sensory nerve does (Fig. 13-56).
Fig. 13-58. Reflex arc.
Occasionally, the 1st neuron of somatic sensory nerve (Fig. 13-56) directly meets the lower motor neuron of somatic motor nerve (Fig. 13-55) in the spinal cord. It is called the “reflex arc” because it causes “reflex” and it looks like an “arc.”
For example, the 1st neuron of somatic sensory nerve conveys information that the quadriceps femoris is lengthened by tapping the patellar ligament (Fig. 3-67). Consequently, the lower motor neuron of somatic motor nerve spontaneously generates impulse to contract the quadriceps femoris (Fig. 13-58). Because this pathway does not go to the cerebral cortex (Fig. 13-3), the reflex happens involuntarily. However, the reflex arc does not belong to the autonomic nerve (visceral motor nerve) (Fig. 13-60).
< Visceral nerve >
The visceral nerve for innervating the smooth and cardiac muscles (involuntary muscles) will be explained below.
Fig. 13-60. Visceral motor nerves (autonomic nerve), visceral sensory nerve.
Like the somatic nerve, the visceral nerve exists both in the central and peripheral nervous systems. The visceral motor nerve is called “autonomic” nerve because it “autonomically” controls the smooth and cardiac muscles.
In the peripheral nervous system, the somatic motor nerve consists of a single neuron (lower motor neuron) (Fig. 13-55). However, the visceral motor nerve is composed of two neurons. Therefore, the visceral motor nerve inevitably has the ganglion (sympathetic or parasympathetic ganglion) that is the nerve cell body of the 2nd neuron (dotted lines in the above figure).
The sympathetic nerve has the longer 2nd neuron, while the parasympathetic nerve has the shorter 2nd neuron. In other words, the sympathetic ganglion is close to the central nervous system than the parasympathetic ganglion.
The visceral sensory nerve is like the somatic sensory nerve (Fig. 13-54). Their difference is that the visceral sensory nerve delivers impulse from the receptor near the smooth and cardiac muscles. An example of impulse would be abdominal pain from the gastrointestinal tract (Fig. 13-82).
The visceral motor and sensory nerves are not discernible during dissection. Neither are the somatic motor and sensory nerves (Fig. 13-54).
The Sympathetic nerve is for Stimulated (war) state of the body, while the Parasympathetic nerve is for Peaceful state.
The sympathetic nerve is contained in TN1–LN2. (Fig. 13-85) (T1LTwo reminds us of a TILTed building in war state.) The parasympathetic nerve is contained in III, VII, IX, X (Fig. 13-35), and SN2–SN4.
Fig. 13-62. Sympathetic nerve of thorax.
In nature, TN1–LN2 are distributed only to the trunk (Fig. 13-98). To send sympathetic impulse to the head, neck, upper limb, or lower limb, the sympathetic nerve should ascend or descend through the sympathetic trunk. The sympathetic trunk includes ganglia that are the nerve cell bodies of the 2nd neurons (Fig. 13-60).
The 1st–4th thoracic ganglia in the sympathetic trunk send sympathetic impulse to the heart, lungs, and esophagus. Near each organ, the sympathetic nerve is mixed with the parasympathetic nerve to form the plexus (Fig. 13-63). The remaining (5th–12th) thoracic ganglia send it to the abdominal organs by way of the thoracic splanchnic nerves (Fig. 13-64). The thoracic splanchnic nerve must pass the diaphragm between the thoracic and abdominal cavities (Fig. 3-26).
Fig. 13-63. Parasympathetic nerve of thorax.
Meanwhile, the parasympathetic nerve of X is responsible for all the thoracic and abdominal organs (Fig. 13-79). A branch of X sends parasympathetic impulse to the heart; another sends it to the lungs. The main trunk of X accompanies the esophagus to enter the abdominal cavity through the diaphragm (Fig. 3-26).
Fig. 13-64. Sympathetic and parasympathetic nerves of abdominal and pelvic cavities.
The splanchnic nerves (sympathetic nerve) and X (parasympathetic nerve) enter the celiac, superior mesenteric, and inferior mesenteric plexuses. The nerves then travel along with the branches of the respective arteries (celiac trunk, superior mesenteric artery, and inferior mesenteric artery) (Fig. 8-32). Eventually, the sympathetic and parasympathetic impulses are appropriately sent to the abdominal organs.
The sympathetic nerve (sacral splanchnic nerve) from the sympathetic trunk (Fig. 13-62) and the parasympathetic nerve (pelvic splanchnic nerve) from SN2–SN4 are responsible for the smooth muscle in the pelvis, perineum.
Examples are the smooth muscle for erection, ejaculation of male and that for urination of both sexes.
More information on the parasympathetic nerve of III (Fig. 14-8), VII (Fig. 13-72), IX (Fig. 13-78), X (Fig. 13-79) will be given in the following subchapter “cranial nerve” and in the chapter “sensory system.”
All sympathetic and parasympathetic nerves are influenced by the hypothalamus, the headquarters of the autonomic nerve (Fig. 13-24).
< Cranial nerve >
A human has twelve pairs of cranial nerves: olfactory nerve (I), optic nerve (II), oculomotor nerve (III), trochlear nerve (IV), trigeminal nerve (V), abducens nerve (VI), facial nerve (VII), vestibulocochlear nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X), accessory nerve (XI), and hypoglossal nerve (XII).
Cranial nerves relay impulse between the brain and the head, neck. An exception is X that is also distributed to the thorax and abdomen (Fig. 13-79).
Cranial nerves emerge from the brainstem. Exceptions are I from the cerebrum, II from the thalamus (Fig. 13-68), and spinal root of IX from the spinal cord (Fig. 13-35).
I and II are often regarded as the extended brain. It is because the two nerves are enclosed by the pia mater. Remember that the pia mater covers the brain (Fig. 13-43) and spinal cord (Fig. 13-40).
I (olfactory nerve) contains sensory nerve fibers relating to smell. It originates from the olfactory receptor in the upper part of the nasal cavity (Fig. 5-3). It reaches numerous areas of the temporal lobe (Fig. 13-10). It does not pass the thalamus since it is the extension of cerebrum.
Fig. 13-68. Optic pathway.
Regarding II, the neurons from the retina’s receptors (Fig. 14-4) cross or do not cross the midsagittal plane at the optic chiasm (Fig. 13-35) and meet the next neuron at the lateral geniculate nucleus of thalamus (Fig. 13-22). The next neuron arrives at the cerebral cortex in the occipital lobe (around calcarine sulcus) (Fig. 13-11).
III, IV, VI are responsible for movement of the extraocular muscles (Fig. 14-19). III also contains the parasympathetic nerve to constrict the pupil (Fig. 14-8) and to thicken the lens (Fig. 14-9).
Fig. 13-69. Three divisions of V.
Somatic sensory nerve of V forms the trigeminal ganglion like somatic sensory nerve of the spinal nerve forms the spinal ganglion (Fig. 13-86). The trigeminal ganglion is the common ganglion of V1, V2, and V3.
Fig. 13-70. Skin areas of V1, V2, V3.
The three skin areas innervated by the ophthalmic nerve (V1) (main branch: “frontal” nerve), “maxillary” nerve (V2), and “mandibular” nerve (V3) are on the “frontal” bone, “maxilla,” and “mandible,” respectively.
V1, V2, and V3 travel through the supraorbital notch or foramen (Fig. 1-10), the infraorbital foramen, and the mental foramen, respectively (Fig. 13-69) to approach the skin (Fig. 15-18).
Fig. 13-71. V3.
A branch of V3 is the lingual nerve that receives general sense from the tongue (anterior 2/3) (Fig. 4-7). This general sense is like the pain felt when the tongue is bitten (Fig. 4-5). Another branch of V3 is the motor nerve to the masticatory muscles (Fig. 3-13) (Fig. 3-15) and the some suprahyoid muscles (Fig. 3-19).
Fig. 13-72. VII.
VII in the cranial cavity enters the temporal bone (Fig. 1-7) where it divides in a complicated manner. The parasympathetic nerve of VII makes a synapse at ganglia and stimulates the lacrimal gland (Fig. 14-28), submandibular gland (Fig. 4-10), and sublingual gland for secretion. It is crucial to recall that III, VII, IX, X include the parasympathetic nerves, an activity of which is to enhance the secretion of several exocrine glands (Fig. 9-2).
A branch of VII is to receive a special sense (taste) from the tongue (anterior 2/3) (Fig. 4-7).
A motor nerve of VII innervates the some suprahyoid muscles (Fig. 3-19) and the facial muscles (Fig. 3-9). The “facial” muscles are the nomenclatural origin of the innervating VII, “facial” nerve.
Fig. 13-74. Branches of VII in parotid gland.
Inside the parotid gland (Fig. 4-12), VII divides into five branches for lots of facial muscles (Fig. 3-9).
The vestibulocochlear nerve transmits equilibrium (balance) and sound information. The vestibular nerve traveling from the saccule, utricle, semicircular duct of the inner ear (Fig. 14-41) is closely related to the vestibulocerebellum (Fig. 13-28).
Fig. 13-75. Auditory pathway.
The first neuron of the cochlear nerve travels away from the cochlea duct of internal ear (Fig. 14-40). The second neuron crosses midsagittal plane on the pons; the third neuron arises from the inferior colliculus of midbrain (Fig. 13-24).
The fourth neuron passes from the medical geniculate nucleus (Fig. 13-22) (Fig. 13-23) to the transverse temporal gyrus that is the superior surface of the temporal lobe in the lateral sulcus (Fig. 13-11). Unlike the typical somatic sensory nerve requiring three neurons (Fig. 13-56), the auditory pathway demands four neurons.
Fig. 13-77. IX, X, XI passing through jugular foramen.
IX, X, XI are closely related with one another, so the authors call them triple X. The triple X exits the cranial cavity through the jugular foramen. Recall that the internal “jugular” vein is the biggest structure passing through the “jugular” foramen (Fig. 10-63).
Note that both IX and X have the superior and inferior ganglia individually. The four sensory ganglia are found around the jugular foramen like the spinal ganglion (Fig. 13-86) around the intervertebral foramen (Fig. 1-14).
Above and below figures showing the superior and inferior ganglia do not follow the rule that a neuron has only a nerve cell body (Fig. 13-48). It is only because the authors have always simplified the figures.
Fig. 13-78. IX.
Regarding the superior and inferior ganglia of IX, they receive sensory impulse from the “tongue” (posterior 1/3) (Fig. 4-7) and “pharynx” (Fig. 4-15). Therefore, the full name of IX is the “glossopharyngeal” nerve. The parasympathetic nerve of IX reaches and stimulates the parotid gland for secretion (Fig. 4-12).
Fig. 13-79. X.
This realistic drawing of X needs to be compared to the neuron drawing of X (Fig. 13-77).
A branch of X innervates the muscles in the soft palate (Fig. 4-18) and pharynx (Fig. 4-22). Another branch, the recurrent laryngeal nerves, hook around the subclavian artery (on the right side) or around the aortic arch (on the left side) (Fig. 10-29). The recurrent laryngeal nerve controls muscles in the larynx (Fig. 5-12).
As already explained, the parasympathetic nerve from X and the sympathetic nerve from the 1st–4th thoracic ganglia intermingle around and collectively innervate the heart, lung, and esophagus. These mixed nerves are known as the cardiac, pulmonary, and esophageal plexuses (Fig. 13-62). They are like the superior mesenteric plexus and so on (Fig. 13-64).
The parasympathetic nerve (X) in the cardiac plexus make the heart beat slow. At the same time, the parasympathetic nerve (X) in the pulmonary plexus (Fig. 13-79) contracts circular smooth muscles in the bronchi.
Repeatedly, the parasympathetic nerve of X is distributed in the thoracic and abdominal cavities.
Regarding the superior and inferior ganglia, X receives impulse from very large regions including the abdominal cavity (Fig. 13-77). The visceral sensory nerve (Fig. 13-60) of X is partly responsible for the hunger and abdominal pain.
Fig. 13-83. Spinal root of XI in posterior cervical triangle.
While the cranial root of XI becomes a part of X, the spinal root of XI innervates two muscles, related with the neck motion (Fig. 13-77). The sternocleidomastoid muscle rotates the neck (Fig. 3-17); the trapezius extends the neck with its insertion (scapula) fixed (Fig. 2-21) (Fig. 3-37).
XII controls all the tongue muscles (Fig. 4-6). IV, VI (Fig. 14-20), XI as well as XII contain only somatic motor nerves (Fig. 13-54). These four cranial nerves are even simpler than the spinal nerve. What a relief!
< Spinal nerve >
The spinal nerves exit by passing the intervertebral foramina (Fig. 1-14).
Fig. 13-85. Spinal nerves, vertebrae.
On each side, the spinal nerves are grouped into 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve. The cervical nerves are one more than the cervical vertebrae in number (Fig. 1-15).
Fig. 13-86. Somatic nerves in spinal nerve.
A spinal nerve consists of two roots, one trunk, and two rami. In this “spinal nerve“ subchapter, the somatic motor nerve and somatic sensory nerve (Fig. 13-54) are called the motor nerve and sensory nerve for simplification.
The motor nerve passes the anterior root, while the sensory nerve passes the posterior root. Say “Action is Anterior.” It reminds us that the precentral gyrus is a motor cortex (Fig. 13-12). Because the sensory nerve passes the posterior root, the spinal ganglion resides in the posterior root (Fig. 13-96).
The motor and sensory nerves coexist in the trunk of spinal nerve, anterior ramus, and posterior ramus. As the posterior ramus is only for the deep back muscles (Fig. 3-34), it is thin. Pay attention to the anterior ramus from now. The anterior rami contribute to the cervical plexus (Fig. 13-87) (Fig. 13-88), brachial plexus (Fig. 13-90), and lumbosacral plexus (Fig. 13-100) as well as the intercostal nerves (Fig. 13-96).
Fig. 13-87. Ansa cervicalis.
The anterior rami of CN1–CN5 constitute the cervical plexus. Among them, CN1–CN3 form a loop around the internal jugular vein (Fig. 10-64); from the loop, branches supply most infrahyoid muscles (Fig. 3-21).
Fig. 13-88. Phrenic nerve.
CN3–CN5 combine to become the phrenic nerve, which reaches down to the diaphragm (Fig. 3-26).
During embryonic development, the diaphragm (exactly, a portion of the diaphragm) used to be located cranial to the heart. As the head folding takes place, the diaphragm descends to be caudal to the heart (Fig. 8-30). At this time, the diaphragm drags the phrenic nerve. In principle, the nerve follows until the end (Fig. 8-37).
Tip on how to memorize the cervical plexus is introduced in the cartoon.
Fig. 13-90. Trunks, divisions, cords of brachial plexus.
The anterior rami of CN5–TN1 are source of the brachial plexus (Fig. 13-44). The five anterior rami (Fig. 13-86) unite and split repeatedly to form three trunks, six divisions, and three cords. Do you know the old film actor, Robert Taylor? The sentence “Robert Taylor Drinks Coffee” represents the “Rami, Trunks, Divisions, Cords” of the brachial plexus.
While the superior, middle, and inferior trunks are almost horizontal, the distal cords are nearly vertical. Therefore, the cords are named lateral, medial, and posterior based on their spatial relationship with the axillary artery. While the trunks are in the neck, the cords are in the axilla; the border between the neck and axilla is R1 (Fig. 10-40).
Important criterion in the brachial plexus is the divisions. Three anterior divisions constitute the lateral and medial cords for the anterior muscles such as the biceps brachii (Fig. 3-48). In contrast, three posterior divisions constitute the posterior cord for the posterior muscles such as the triceps brachii (Fig. 3-51).
Fig. 13-91. Branches of the brachial plexus.
The five distal branches (Musculocutaneous, Axillary, Median, Radial, Ulnar nerves) of the brachial plexus can be recalled by “My Aunt Mary Requires Umbrella.” She might be Mary Poppins.
The “musculocutaneous” nerve innervates the anterior arm “muscles” (Fig. 3-47) and then becomes the “cutaneous” nerve (Fig. 15-18) in the forearm. The median and ulnar nerves, which has no function in the arm, innervate the anterior forearm muscles (Fig. 3-53) and palm muscles.
The axillary nerve is responsible for the deltoid muscle (Fig. 3-40) and teres minor (Fig. 3-42), while the radial nerve is for the posterior arm muscles (Fig. 3-47) and posterior forearm muscles (Fig. 3-53). It is because the two nerves are from the posterior divisions (Fig. 13-90).
Fig. 13-92. Hand skin innervated by three nerves.
In the hand, the median, ulnar, and radial nerves equitably occupy the skin as the cutaneous nerves (Fig. 15-18).
The keenest median nerve is used for touching important things.
The skin area innervated by the ulnar nerve (Fig. 13-92) can be felt when the ulnar nerve behind the medial epicondyle of humerus (Fig. 1-22) is hit.
Dermatome is the skin area that is innervated by a spinal nerve. In the posture of fetus (Fig. 2-28), the dermatomes of upper and lower limbs can be easily drawn. It is a common sense of anatomy learners that CN5–TN1 are in the upper limb (Fig. 13-90) while LN2–SN3 are in the lower limb (Fig. 13-100).
CN5 contributes to both the cervical plexus (Fig. 13-88) and brachial plexus. TN1 contributes to the brachial plexus (Fig. 13-90), intercostal nerve (Fig. 13-96), and sympathetic nerve (Fig. 13-62). Such is common for spinal nerves located on the borderline.
Fig. 13-96. Intercostal nerve.
Intercostal nerves are the anterior rami (Fig. 13-86) and the anterior cutaneous branches of TN1–TN11.
The intercostal nerves innervate both the intercostal muscles (motor nerve) (Fig. 3-23) and the overlying skin (sensory nerve) (Fig. 15-18). The representative dermatomes of thorax and abdomen are explained below.
Fig. 13-98. Dermatomes of thorax, abdomen.
The nipple is located on the level of R4 (Fig. 15-20); therefore, the nipple is theoretically innervated by TN3 or TN4. It is actually innervated by TN4.
Skin on the xiphoid process is innervated by TN7, since R7 is directly attached to the xiphoid process. As “xiphoid” is spelled with 7 letters, the xiphoid welcomes R7 and TN7.
Below R7, the anterior intercostal spaces gradually decrease in length, so TN7–TN11 pass not only the thoracic wall (as the intercostal nerves) (Fig. 3-23) but also the abdominal wall (Fig. 3-33). Among them, TN10 is distributed to the umbilicus. This is why the umbilicus is drawn as X (10 in Roman numerals) in this book. LN1 innervates the skin on the inguinal ligament (Fig. 7-3).
In summary, dermatomes of the nipple (TN4), xiphoid process (TN7), umbilicus (TN10), and inguinal ligament (LN1) have same intervals of the spinal nerves.
Fig. 13-100. Lumbosacral plexus (medial view).
The brachial plexus innervates the upper limb including the pectoral region (Fig. 3-46), scapular region (Fig. 3-42), and even superficial back (Fig. 3-37). Similarly, the lumbosacral plexus (from the anterior rami of LN2–SN3) (Fig. 13-44) innervates the pelvis, perineum, and lower limb.
In the above figure, branches of the lumbosacral plexus are depicted separately. From LN2–LN4, the femoral nerve and the obturator nerve arise. The femoral nerve descends posterior to the inguinal ligament and then accompanies the femoral artery (Fig. 10-55). It controls the anterior thigh muscles (Fig. 3-65). The obturator nerve passes through the obturator foramen (Fig. 1-34) (Fig. 13-100) to approach the medial thigh muscles (Fig. 3-69).
The pudendal nerve (SN2–SN4) passes the pelvic cavity, gluteal region (Fig. 3-71), and finally the perineum (Fig. 1-35). For the course, the pudendal nerve is getting out through the greater sciatic foramen, and getting in through the lesser sciatic foramen (Fig. 3-63).
Fig. 13-101. Branches of pudendal nerve.
In the perineum, the pudendal nerve controls the external anal sphincter in the anal triangle (Fig. 4-36) and the external urethral sphincter (Fig. 6-23) (Fig. 7-15), bulbospongiosus, and ischiocavernosus (Fig. 7-20) (Fig. 7-39) in the urogenital triangle.
Following the meaning of the Latin word “pudendus” (embarrassing), the nerve is related to the three “embarrassing” activities: defecation, urination, and sexual intercourse. In addition, the sympathetic nerve and parasympathetic nerve are responsible for the smooth muscles in the perineum (Fig. 13-64a).
The sciatic nerve, the thickest nerve in the body, is composed of five spinal nerves (LN4–SN3); it is noteworthy considering that the whole brachial plexus is composed of the five spinal nerves (Fig. 13-90). The sciatic nerve and its branches (motor and sensory nerves) are distributed to vast regions of the lower limb: posterior thigh (Fig. 3-65), whole leg (Fig. 13-103), and whole foot (Fig. 13-104)). The sciatic nerve travels through the greater sciatic foramen with the pudendal nerve (Fig. 13-100).
Fig. 13-102. Course of sciatic nerve.
The sciatic nerve then passes midpoint of the ischial tuberosity (Fig. 1-34) and the greater trochanter (Fig. 1-41). In order not to injure the sciatic nerve, intramuscular injection to the gluteus maximus (Fig. 3-71) is conducted in the superior lateral quadrant of the gluteal region.
Fig. 13-103. Division of sciatic nerve.
Above schematic drawing indicates that the posterior, lateral, and anterior leg muscles are innervated by the tibial, superficial fibular, deep fibular and nerves, in that order. In fact, the sciatic nerve bifurcates into the tibial nerve and common fibular nerve at the posterior thigh (Fig. 3-75). The pattern of the nerve division differs from that of the artery division (Fig. 10-57).
Fig. 13-104. Sole skin innervated by two nerves.
The tibial nerve divides into the medial and lateral plantar nerves like the posterior tibial artery bifurcates into the medial and lateral plantar arteries (Fig. 10-59). The sole skin is innervated by the two nerves, the border of which passes the 4th toe. It can be said that medial and lateral plantar nerves are homologous with the median and ulnar nerves, respectively (Fig. 13-92). The deep fibular nerve, responsible not only for the anterior leg muscles (Fig. 13-103) but also for the skin of dorsum of foot, is homologous with the radial nerve (Fig. 13-92).