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Thoracic dissection revealing vagus nerve

Anterior view

Deeper thoracic dissection revealing vagus nerve

Anterior view

cardiac muscle, and glands. Some of the autonomic nerves even rejoin the somatic pathways to supply the blood vessels

and glands of the body wall. The sympathetic pathways are primarily associated with vascular smooth muscle control, and

the parasympathetic pathways are principally responsible for the regulation and control of gut tube smooth muscle and

glands. The sympathetic nerves are depicted on the opposite page, while the vagus nerve, which carries 75% of the parasympathetic output, is shown below as it follows the derivatives of the gut tube.

38 Thyroid gland

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40 Principal bronchus

41 Lobar bronchus

42 Segmental bronchus

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44 Lung

45 Right common carotid artery

46 Left common carotid artery

47 Right subclavian artery

48 Left subclavian artery

49 Brachiocephalic artery

50 Pulmonary arteries

51 Pulmonary veins

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53 Cricothyroid muscle

54 Anterior scalene muscle

55 Ligamentum arteriosum

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Cranial nerves segregate into three distinct groups based

on associations they form during development. In number

there are twelve cranial nerves, which originate in pairs

Cranial Nerves

from a rostral to caudal sequence from the brain. The fi rst category, the special sensory cranial

nerves, are afferent pathways established between the the brain and the special sensory structures

of the nose, eye, and ear. The second category, the ventral or somitic motor cranial nerves, are

homologous with the ventral roots of the spinal nerves. They originate from the brainstem as efferent

pathways to somitic skeletal muscles within the head. The fi nal category, comprising the largest of the

Base of brain with cranial nerves

Inferior view

Special Sensory Nerves

 1 Olfactory nerve

 2 Optic nerve

 3 Vestibulocochlear nerve

Somitic Motor Nerves

 4 Occulomotor nerve

 5 Trochlear nerve

 6 Abducens nerve

 7 Hypoglossal nerve

Pharyngeal Arch Nerves

 8 Trigeminal nerve

 9 Trigeminal ganglion

10 Opthalmic branch

 11 Maxillary branch

12 Mandibular branch

13 Facial nerve

14 Glossopharyngeal nerve

15 Vagus nerve

16 Accessory nerve

Other Structures

17 Olfactory bulb

18 Optic chiasm

19 Optic tract

20 Infundibulum

21 Mammillary bodies

22 Cerebral peduncle

23 Pons

24 Cerebellum

25 Medulla oblongata

26 Spinal cord

27 Frontal lobe

28 Temporal lobe

29 Insular lobe

30 Parietal lobe

31 Occipital lobe

32 Right lateral ventricle

33 Choroid plexus

34 Falx cerebri

35 Falx cerebelli

36 Straight sinus

37 Superior sagittal sinus

38 Corpora quadrigemina

39 Pineal gland

40 Third ventricle

41 Fourth ventricle

42 Geniculate ganglion

43 Anterior cerebral artery

44 Internal carotid artery

45 Levator palpebrae superioris muscle

46 Superior rectus muscle

47 Lateral rectus muscle

48 Superior oblique muscle

49 Nasociliary nerve

50 Long ciliary nerve

51 Ciliary ganglion

52 Eye

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Intracranial dissection of cranial nerves

Posterolateral view

cranial nerves, are those cranial nerves associated with the pharyngeal arches. The dorsal or

pharyngeal arch cranial nerves are developmentally similar to the dorsal roots of the spinal

nerves. These fi ve dorsal cranial nerves form the general sensory afferent pathways from the

peripheral tissues of the head. However, because these nerve pathways coursed through the

specialized arches forming the pharyngeal wall of the foregut, they established parasympathetic efferent pathways to the glandular tissue of the gut wall, along with motor efferent pathways to the skeletal muscles derived from the pharyngeal arch tissues.

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Cranial nerves V and VII, the trigeminal and facial nerves respectively, have the most extensive

distribution to the tissues of the head. This page

Cranial Nerves

and the three pages that follow depict the peripheral distribution of many of the branches of

the trigeminal and facial nerves.

Dissection of head exposing branches of the facial nerve

Lateral view

Trigeminal Nerve

 1 Auriculotemporal nerve

 2 Supraorbital nerve

 3 Infraorbital nerve

 4 Mental nerve

 5 Maxillary branch

 6 Nerve of the pterygoid canal

 7 Pterygopalatine ganglion

 8 Nasopalatine nerve (cut)

 9 Superior posterior lateral nasal branch

10 Inferior posterior lateral nasal branch

 11 Pharyngeal branch

12 Lesser palatine nerve

13 Greater palatine nerve

Facial Nerve

14 Temporal branches

15 Zygomatic branches

16 Buccal branches

17 Mandibular branches

18 Cervical branch

Other Nerves and Structures

19 Greater occipital nerve

20 Lesser occipital nerve

21 Great auricular nerve

22 Auricularis posterior muscle

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23 Occipital belly of epicranius muscle

24 Galia aponeurotica

25 Frontal belly of epicranius muscle

26 Temporal fascia

27 Temporalis muscle

28 Orbicularis oculi muscle

29 Zygomaticus major muscle

30 Risorius muscle

31 Buccinator muscle

32 Masseter muscle

33 Posterior digastricus muscle

34 Parotid duct

35 External carotid artery

36 Submandibular gland

37 Frontal sinus

38 Cerebrum

39 Falx cerebri

40 Corpus callosum

41 Septum pellucidum

42 Thalamus

43 Midbrain

44 Pons

45 Cerebellum

46 Fourth ventricle

47 Choroid plexus

48 Medulla oblongata

49 Spinal cord

Parasagittal section and dissection of head exposing branches of the trigeminal and facial nerve

Medial view

50 Pituitary gland

51 Torus tubarius

52 Maxillary sinus

53 Middle nasal concha

54 Inferior nasal concha

55 Hard palate

56 Soft palate

57 Uvula

58 Tongue

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Cranial Nerves

 1 Nerve to temporalis muscle

 2 Buccal nerve

 3 Middle superior alveolar nerve

 4 Posterior superior alveolar nerve

 5 Lingual nerve

 6 Chorda tympani nerve

 7 Inferior alveolar nerve

 8 Nerve to mylohyoid muscle

 9 Pterygopalatine ganglion

10 Infraorbital nerve

 11 Hypoglossal nerve

12 Submandibular ganglion

13 Superior laryngeal nerve

Other Structures

14 Orbicularis oculi muscle

15 Temporal fascia

16 Temporalis muscle

Dissection of head exposing branches of the trigeminal nerve

Lateral view

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Dissection of head with mandible removed

Lateral view

17 Lateral pterygoid muscle

18 Medial pterygoid muscle

19 Buccinator muscle

20 Posterior digastricus muscle

21 Anterior digastricus muscle

22 Sternocleidomastoid muscle

23 Thyrohyoid muscle

24 Omohyoid muscle

25 Styloglossus muscle

26 Stylohyoid muscle

27 Geniohyoid muscle

28 Mylohyoid muscle

29 Superior pharyngeal constrictor

30 Inferior pharyngeal constrictor

31 Internal jugular vein

32 Common carotid artery

33 Dura mater

34 Cerebrum

35 External acoustic meatus

36 Tongue

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Sensory receptors are the transducers of the nervous system; that is,

they convert the different types of energy we experience such as

mechanical energy (touch, pressure, sound waves, etc.), thermal

Sensory Receptors

energy (heat), chemical energy (taste, smell), and electromagnetic energy (light) into the electrical energy of the nervous

impulse. They do this by facilitating the depolarization of the peripheral terminals of the sensory neurons. This initiates the

nervous impulse along the sensory neuron, and this input is carried by the sensory neuron to the processing centers of the

brain and spinal cord, which will be the topic of the next chapter.

Photomicrograph of corpuscle of touch

200x

Photomicrographs of taste bud

200x (left), 700x (right)

Photomicrograph of lamellated corpuscle

100x

 1 Epidermis

 2 Corpuscle of touch (Meissner’s)

 3 Dermis

 4 Dermal papilla

 5 Neuron

 6 Lamellated corpuscle

 7 Taste bud

 8 Taste pore

 9 Gustatory hair

10 Gustatory receptor cell

 11 Supporting cell

12 Basal cell

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While the neuronal circuitry of

the central nervous system is awe inspiring to say the least, the basic

concepts behind this complex integration and control center have a

simple design. At its simplest, the fundamental design of the central

nervous system involves two features: gray matter and white matter.

Th e gray matter centers represent the synaptic integration and control circuits; that is, these centers contain numerous highly dendritic interneurons along with the cell bodies of eff erent neurons

and axon terminals of incoming aff erent neurons, all forming a

myriad of synaptic circuits. In these gray centers input is integrated, compared, sensed, and stored to give rise to coordinated, controlled output. Th e white matter, on the other hand, represents

conduction tracts between the synaptic gray centers. Th ese white

tracts consist mainly of the myelinated axons of interneurons

relaying signals from one gray center to another.

A second simple concept to keep in mind is that the complexity of the central nervous system increases from a caudal to

cranial direction. Th ere is logic to this pattern because in the

spinal cord the gray centers primarily function as integration

networks that regulate input and output for their specifi c spinal

nerve levels. In other words, they are segmental control centers.

Input entering a spinal nerve level initiates refl exive output back

to the peripheral tissues at that same spinal level. Connecting

these segmental gray centers via interneuronal tracts leads to

greater association between neighboring levels, therefore improving integration and control. If one segmental gray center

can relay information received from its center to neighboring

centers, then there can be a greater spread of control generated in

response to local segmental input. Now take this a step further by

relaying information via white tracts from each of the segmental

control centers to higher centers. Th ese higher centers receive input from all the lower segmental centers, integrating the input to

gain a full body perspective, while generating the necessary output

signals to exert coordinated full body control. Because of this added

circuitry the cranial or brain end of the central nervous system increases in size. Th is additive accumulation of interconnected gray centers accounts for the structure of the brain and its amazing functional

properties.

Because much of the central nervous system circuitry is of a more microscopic nature and beyond the scope of this book. In this chapter we attempt to depict the basic gross anatomy of the central nervous system and its

protective coverings.

14 Central Nervous System

231

REAL ANATOMY

Find more information

about the central nervous

system in

232

 1 Dorsal horn of gray matter

 2 Lateral horn of gray matter

 3 Ventral horn of gray matter

 4 Posterior funiculus of white matter

 5 Lateral funiculus of white matter

 6 Anterior funiculus of white matter

Photomicrograph of spinal cord

50x

Extending from the brainstem is a long slender rod of nerve tissue, the spinal cord. The

cord exits the foramen magnum of the skull and descends within the vertebral canal of

the boney vertebral column. It is about 45 cm long (18 inches) and ends between the fi rst

Spinal Cord

 7 Central canal

 8 Dorsolateral fasciculus

 9 Dorsal root of spinal nerve

10 Dorsal root ganglion

 11 Ventral root of spinal nerve

12 Spinal cord

and second lumbar vertebrae. Although there are some slight regional variations, the cross-sectional anatomy of the spinal

cord is generally the same throughout its length. The gray matter of the spinal cord forms a butterfl y-shaped region in the

center of the cord that is surrounded by the white matter. As is the theme throughout the central nervous system, gray matter consists primarily of neuronal cell bodies and their dendrites, short interneurons, and glial cells. The white matter is organized into tracts, which are bundles of myelinated nerve fi bers (axons of long interneurons and sensory neurons) that

communicate between the gray circuit centers at all levels of the spinal cord and brain.

 Each side of the H-shaped gray matter of the spinal cord has a dorsal horn and a ventral horn sandwiching an intermediate gray region. Entering the dorsal horns from the dorsal rootlets are the axons of the afferent neurons, which

synapse with small interneuron pools to form segmental integration centers for that level of the body. The dorsal horn and

intermediate gray matter contain numerous small interneurons. The intermediate gray also contains, at certain levels, the

preganglionic efferent neurons of the autonomic output. The ventral horns are primarily populated by the efferent neurons

to the skeletal muscles of their respective spinal levels. The white matter tracts are grouped into columns of myelinated

axons that extend the length of the cord. Each of these tracts begins or ends within a particular area of the cord and brain,

and each is specifi c in the type of information that it transmits. Some are ascending tracts that carry signals derived from

sensory input. For example, one tract carries information derived from pain and temperature receptors, whereas another

carries information regarding touch. Other tracts are descending tracts that relay messages from the brain to motor neurons

in the ventral horn.

 Both the white and gray matter exhibit regional differences throughout the length of the spinal cord. There is relatively more white matter at the cranial end of the spinal cord than at the caudal end. Notice that the gray matter, especially

the ventral horn, is the largest at lower cervical levels and at lower lumbar-upper sacral levels. These levels correspond to

upper and lower limb anatomy respectively, where large amounts of muscle tissue require motor innervtion from the ventral

horn motor neuron pools.

13 Conus medullaris

14 Cauda equina

15 Dorsal rami of spinal nerve

16 Cerebrum

17 Cerebellum

18 First lumbar vertebra

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Cervical spinal cord

Thoracic spinal cord

Lumbar spinal cord

Sacral spinal cord

Dissection of vertebral column and skull revealing brain and spinal cord

Posterior view, with call-out of terminal end of cord

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The brain is the large, anterior-expansion of the neural tube situated within the cranium. Rapid development of

the rostral e nd of the neural tube forms three expanded regions — the prosencephalon, mesencephalon,

and rhombencephalon. The prosencephalon undergoes further development to form the telencephalon

Brain

and diencephalon, and the rhombencephalon continues to develop to form a metencephalon and myelencephalon. These

fi ve embryonic regions give rise to the brain. The telencephalon becomes the cerebrum, the diencephalon becomes the

thalamic regions, the mesencephalon becomes the midbrain, the metencephalon becomes the cerebellum and pons, and the

myelencephalon becomes the medulla oblongata. A variety of views of the full brain are depicted on this and the facing page.

 1 Spinal cord

 2 Medulla oblongata

 3 Pons

 4 Cerebellum

 5 Midbrain

 6 Diencephalon

 7 Frontal lobe of cerebrum

 8 Parietal lobe of cerebrum

 9 Occipital lobe of cerebrum

10 Temporal lobe of cerebrum

 11 Longitudinal fissure

12 Transverse fissure

13 Lateral cerebral sulcus

14 Anterior median fissure

15 Gyrus

16 Sulcus

Brain

Lateral view

17 Central sulcus

18 Precentral gyrus

19 Postcentral gyrus

20 Precentral sulcus

21 Postcentral sulcus

22 Inferior frontal gyrus

23 Superior temporal gyrus

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