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NERVOUS SYSTEM - THE BRAIN AND CRANIAL NERVES

INTRODUCTION

1. The adult brain contains almost 98 percent of the neural tissue in the body. No correlation exists between brain size and intelligence.

2. The nuclei of the brain are surrounded by tracts of white matter; the brain also contains gray matter in superficial areas of neural cortex.The brain communicates with the rest of the body over tracts from the spinal cord and via the cranial nerves.

3. There are five major divisions in the adult brain: the telencephalon (cerebrum), the diencephalon, the mesencephalon (midbrain), the metencephalon (cerebellum and pons), and the myelencephalon (medulla).

THE CRANIAL MENINGES

1. Unlike the spinal dura, the cranial dura has a central region containing blood vessels. Dense dural partitions project between the cerebral hemispheres (falx cerebri) and between the cerebral hemispheres and the cerebellum (tentorium cerebelli).

2. The brain has a very high rate of metabolism and negligible energy reserves.

3. Capillaries leaving the blood vessels of the pia mater

penetrate the neural tissue of the brain. The blood vessels also invade the nonneural roofs of the diencephalon and myelencephalon and form the choroid plexus.

CEREBROSPINAL FLUID FORMATION AND CIRCULATION

1. CSF surrounds the outer surface of the central nervous system and fills the internal canals and passageways.

2. CSF forms at the choroid plexus at rates of 45-130 ml/day. Thechemical characteristics of cerebrospinal fluid are unusual and its formation involves active secretion.

3. The brain contains a series of interconnected chambers, the ventricles. The ventricles are continuous with the central canal of the spinal cord and the subarachnoid space.

4. Cerebrospinal fluid reaches the subarachnoid space through the lateral and median aperturesin the roof of the fourth ventricle. ventricle. The fluid returns to the circulation via the arachnoid villi that project into the superior sagittal sinus .

GROSS ANATOMY AND ORGANIZATION OF THE BRAIN

The Cerebrum (Telencephalon)

The Cerebral Cortex

1. The cerebral cortex contains gyri (folds) separated by sulci (grooves). The longitudinal fissure marks the separation between the cerebral hemispheres.

2. The central sulcus separates the sensory and motor portions of the cortex and marks the boundary line between the frontal and parietal lobes. Other characteristic sulci mark the boundaries of the temporal and occipital lobes.

3. The primary motor cortex of the precentral gyrus directs the contraction of skeletal muscles under voluntary control.

4. The primary sensory cortex of the postcentral gyrus receives sensory information that will be consciously perceived.

5. Motor and sensory association areas control the activity of the motor cortex and integrate and interpret sensory data.

6. "Higher order" integrative centers receive information from many different association areas. These centers control the most complex mental activities.

The Central White Matter

1. The central white matter contains association fibers commissural fibers, and projection fibers.

The Cerebral Nuclei

1. The cerebral nuclei are located beneath the floors of the lateral ventricles. They include the caudate nucleus, the amygdaloid body, the claustrum, the putamen, and theglobus pallidus.

The Limbic System

1. The limbic system includes nuclei in the diencephalon and cerebrum. This system is concerned with emotional states and behaviors, degrees of alertness, and memory processing.

2. The cerebral components of the limbic system include the amygdaloid nucleus, cingulate gyrus,parahippocampal gyrus, and hippocampus of the cerebrum, and the fornix that begins at the hippocampus and ends at the mamillary bodies of the diencephalon.

3. The anterior nuclear group of the thalamus and the emotional centers of the hypothalamus are diencephalic contributions to the limbic system.

4. The reticular formation, the last component of the limbic system, consists of an interconnected network of nuclei that extends the length of the brain stem

The Diencephalon

1. The diencephalon provides switching and relay centers for theintegration of voluntary and involuntary activities.

2. The roof of the diencephalon contains the pineal glandand

the choroid plexus.

3. The walls of the thalamus contains four major groups of nuclei, each with specific functions.

4. The hypothalamus contains integrative centers important to the control of autonomic and endocrine functions. It also contains centers involved with sensations of thirst and hunger.

The Mesencephalon

1. The tectum contains the corpora quadrigemina consisting of the superior and inferior colliculi. These cortical areas receive a variety of sensory inputs.

2. The mesencephalon contains the red nucleus and the substantia nigra. The red nucleus adjusts muscle tone and posture; the substantia nigra helps to regulate the motor output of the cerebral nuclei. The mesencephalon also contains the headquarters of the reticular formation.

3. The cerebral peduncles contain ascending fibers bound for the thalamic nuclei, and descending motor commands originating at the cerebral cortex.

The Metencephalon

The Cerebellum

1. The cerebellum controls important postural reflexes and refines voluntary and involuntary movements of the body.

2. The cerebellar hemispheres consist of neural cortex thrown into folds (folia) comparable to those of the cerebral hemispheres.

3. The surface of the cerebellum can be divided into the anterior and posterior lobes, the vermis, and the flocculonodular lobe.

4. The cerebellum monitors all sensory and motor information reaching the brain.

The Pons

1. The cerebellar peduncles of the pons attach the cerebellum and form the pathways for afferent and efferent cerebellar tracts.

2. The pons contains nuclei associated with several cranial nerves and two respiratory centers.

The Medulla (Myelencephalon)

1. All communication between the brain and spinal cord involve the medulla as a relay stop or a thoroughfare.

2. The medulla contains theolivary nucleiand the sensory and motor nuclei associated with five of the cranial nerves. * 3. The medulla also contains reflex centers including the cardiac, vasomotor, and respiratory rhythmicity centers.

THE CRANIAL NERVES

1. There are twelve pairs of cranial nerves, each with a name that refers to its appearance or function. They are also numbered according to their order of appearance along the rostral/caudal axis of the brain.

2. The olfactory nerve (I) provides the sense of smell. It begins at the olfactory epithelium in the superior surface of the nasal chamber and ends in the olfactory bulb of the cerebrum.

3. The optic nerve(II) brings visual information from the eye to the diencephalon.

4. The oculomotor nerve (III) arises on the ventral surface ofthe mesencephalon. It controls four of the extrinsic oculomotor muscles.

5. The trochlear nerve (IV) innervates the superior oblique muscle of the eye. This nerve also has a mesencephalic origin.

6. The trigeminal nerve (V) is a mixed nerve. It innervates the orbit and skin of the forehead (ophthalmic branch), the upper lip, cheek, nose, and lower eyelid (maxillary branch) and the muscles of mastication and adjacent structures (mandibular branch). This nerve arises from the lateral surface of the pons.

7. The abducens nerve (VI) innervates the lateral rectus muscle of the eye. It emerges near the ventral boundary between the pons and medulla.

8. The facial nerve (VII) is another mixed nerve arising along the lateral surface of the pons. This nerve innervates the facial muscles and monitors the taste buds along the anterior two-thirds of the tongue.

9. The acoustic nerve (VIII) provides the sensations of hearing (cochlear branch) and equilibrium (vestibular branch).This nerve appears at the pons/medulla border just caudal to the origin of the facial nerve.

10. The glossopharyngeal nerve (IX) and the succeeding cranial nerves arise from the medulla.This is a mixed nerve servicing the posterior third of the tongue and the pharynx.

11. The vagus nerve (X), another mixed nerve, carries autonomic fibers to visceral structures as far caudad as the initial segments of the large intestine. It also has diverse sensory functions.

12.The spinal accessory nerve (XI) innervates two muscles, the sternocleidomastoid and the trapezius. It has medullary and spinal branches outside of the skull.

13.The hypoglossal nerve(XII) innervates the muscles of the tongue.

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