Файл: Kaplan USMLE-1 (2013) - Anatomy.pdf

Chapter 6 • Head and Neck

ChapterSummary

•The neck is divided by the sternocleidomastoid muscle into an anterior and posterior triangle. The anterior triangle contains vascular structures (carotid artery and internal jugularvein), cranial nerve (CN) X, and the respiratory (trachea and larynx) and digestive (pharynx and esophagus) visceral structures. The posterior triangle contains the muscles associated with the

cervical vertebrae, CN XI, cervical plexus, and the origins ofthe brachia[ plexus.

•Many structures of the head and neck develop from the branchial (pharyngeal) apparatus. The apparatus consists of pharyngeal arches, pouches, and grooves. The grooves are composed of ectoderm, the pouches

are composed of endoderm, and the arches are composed of mesoderm and neural crest cells. The adult derivatives ofthe arches and pouches are given in Tables 111-6-1 and 111-6-2, respectively.

•The anterior 2/3 of the tongue develops from the first pharyngeal arch, and the posterior 1/3 develops from the third pharyngeal arch.

•The muscles of the tongue derive from myoblasts that migrate into the head from the occipital somites and are innervated by CN XII.

•The face develops from 5 structures derived from the first pharyngeal arch: frontonasal prominence, a pair of maxillary prominences, and a pair of mandibular prominences. The mandibular prominences form the lower jaw, the frontonasal prominence forms the forehead, and the maxillary prominences form the cheek, lateral upper lip, and the secondary palate. The mid line of the upper lip, the nasal septum, and the primary palate are formed by the medial nasal prominence. The primary and secondary palate fuse to form the definite palate.

•The floor ofthe cranial cavity is divided into the anterior, middle, and posterior cranial fossae. The openings in the skull provide for passage ofthe cranial nerves and blood vessels. These are listed in Figures 111-6-8 and 111-6-9.

•Venous return from the brain and other structures ofthe cranial vault is provided by the dural venous sinuses, which ultimately drain into the internal jugularvein at the jugular foramen. Most ofthese sinuses are located in

the folds ofthe dura mater (falx cerebri and tentorium cerebelli). The major ones are the superior and inferior sagittal and the transverse, sigmoid, and cavernous sinuses.

•The cavernous sinus is significant because CN Ill and IV and the ophthalmic and maxillary divisions of CN V course in the lateral wall ofthe cavernous sinus, and the internal carotid artery and CN VI are found in the lumen.

•The orbit contains the ocular muscles, eyeball, and transmits the optic

nerve and ophthalmic artery. CN VI innervates the lateral rectus muscle, CN IV innervates the superior oblique muscle, and the remaining muscles are innervated by CN Ill.

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Notochordal

process Neural groove

Day 18

Neural fold

Neural tube

Rostral neuropore (closes at day 25)

Failure to close results in anencephaly,

causing polyhydramios

and increased alpha­ Basal plate (motor) fetoprotein and AChE

Neural crest

Caudal neuropore (closes at 270)

Failure to close results in spina bifida

Day 22 and increased alpha­ fetoprotein and AChE

Figure IV-1-1 . Development of Nervous System

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Section IV • Neuroscience

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Chapter 1 • NervousSystem

ChapterSummary

•The peripheral nervous system (PNS) consists of 1 2 pairs of cranial nerves, 31 pairs of spinal nerves with their related sensory and motor ganglia, and the peripheral part ofthe autonomic nervous system. The afferent and efferent neurons in the PNS convey somatic and visceral (autonomic) functions to and from the central nervous system (CNS).

Cells of Nervous System

•The basic functional cell for conducting motor and sensory functions within the nervous system is the neuron. Neurons in the CNS are myelinated by oligodendrocytes, and in the PNS neurons are myelinated by Schwann cells. Oligodendrocytes myelinate multiple axons but Schwann cells myelinate only a segment of one axon.

•The skeletal motor neurons and preganglionic motor neurons in the CNS develop from the neural tube, whereas the sensory neurons and postganglionic neurons located in sensory or autonomic ganglia, respectively, in the PNS derive from neural crest cells.

•Neurulation and the development of the nervous system begin in the third week of development. As the primitive streak regresses caudally, the notochord develops in the midaxis ofthe embryo between the

buccopharyngeal membrane and the cloaca[ membrane. The appearance of the notochord then induces the ectoderm overlying the notochord to form the neural plate composed of neuroectoderm. The neural plate is wide at the cranial end and tapers caudally. By the end of the third week, the lateral margins of the neural plate thicken and become elevated to form the neural folds with the neural groove located centrally between the 2 folds. The neural folds then grow over the midline and begin to fuse to form the neural tube. Closure ofthe neural tube begins in the cervical region and continues cranially and caudally. The cephalic (cranial neuropore) and the caudal (caudal neuropore) ends ofthe neural tube close last. Failure of closure of the cranial and caudal neuropores results in anencephaly and spina bifida. Alpha-fetoprotein levels are increased with the neural tube defects. During closure of the neural tube, neural crest cells are formed from neuroectoderm at the margins of the neural folds. The neural crest cells migrate throughout the embryo and form a number of cell types.

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