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Occipital lobe

Lateral sulcus

Cerebellum

Temporal lobe

Figure 21-1 Lateral view of the left cerebral hemisphere.

such as flashes of light or rainbows. Destructive lesions cause homonymous hemianopsia on the contralateral side. Central macular vision is spared.

The primary auditory cortex is located in the temporal lobe along the transverse temporal gyrus. Irritative lesions in this area produce a buzzing or ringing in the ears. Destructive lesions almost never produce deafness.

The basal ganglia are situated deep within the cerebral hemispheres. The structures constituting the basal ganglia include the caudate and lenticular nuclei, as well as the amygdala. The amygdala is part of the limbic system and is concerned with emotion. All other components are important structures in the extrapyramidal system, which is concerned with

Figure 21-2 Medial view of the right cerebral hemisphere.

Longitudinal fissure

Optic chiasm

Olfactory trac

Base of cerebral pedu

Pituitary stalk

Longitudinal fissure

Optic chiasm

Olfactory trac

Base of cerebral pedu

Pituitary stalk

Temporal lobe

Midbrain

Figure 21-3 Basal view of the cerebral hemispheres.

modulating voluntary body movements, postural changes, and autonomic integration. The basal ganglia are especially involved with fine movements of the extremities. Disturbances of the basal ganglia can result in tremors and rigid movements.

The thalamus is a large nuclear mass located on each side of the third ventricle. The thalamus is the chief sensory and motor integrating mechanism of the neuraxis. All sensory impulses, except olfactory ones, and the major output from systems that modulate and modify motor function (i.e., the cerebellum and corpus striatum) terminate in the thalamus, from which they are projected to specific areas of the cerebral cortex. The thalamus is involved in certain emotional connotations that accompany most sensory experiences. Through its connections with the hypothalamus and striatum, the thalamus can influence visceral and somatic effectors serving primarily affective reactions. Through its control of the electrical excitability of the cerebral cortex, the thalamus plays a dominant role in the maintenance and regulation of the state of consciousness, alertness, and attention. The thalamus may be the critical structure for pain perception and thermal sense, which remain after complete destruction of the primary sensory cortex. Thermal sense endows sensation with discriminative faculties and is not concerned with the recognition of crude sensory modalities.

The hypothalamus is located below the thalamus. It includes the optic chiasm and the neurohypophysis. The hypothalamus is responsible for many regulatory mechanisms, such as temperature regulation;neuroendocrine control of catecholamines, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating and luteinizing hormones, prolactin, and growth hormones; thirst; appetite; water balance; and sexual behavior.

The brain stem consists of the midbrain, pons, and medulla. Figure 21-4 shows the external anatomy of the brain stem. The brain stem is responsible for relaying all messages between the upper and lower levels of the central nervous system. Cranial nerves III to XII also arise from the brain stem. The brain stem contains the reticular formation, a network that provides for constant muscle stimulation to counteract the force of gravity. In addition to its antigravity effects, this area of the brain is essential for the control of consciousness. The neurons in the reticular activating system are capable of waking and arousing the entire brain.

Superior colliculus

Inferior colliculus

Figure 21-4 Anatomy of the brain stem.

Fasciculus gracilis Fasciculus cuneatus

Superior colliculus

Inferior colliculus

Fasciculus gracilis Fasciculus cuneatus

Pineal body

Cerebral peduncle

Trochlear nerve

Pineal body

Cerebral peduncle

Trochlear nerve

The midbrain contains the superior and inferior colliculi, the cerebral peduncles, and the motor nuclei of the trochlear (CN IV) and oculomotor (CN III) nerves. The superior colliculi are associated with the visual system, and the inferior colliculi are associated with the auditory system. The cerebral peduncles converge from the inferior aspect of the cerebral hemispheres and enter the pons. A destructive lesion of the superior colliculi causes paralysis of upward gaze. Destructive lesions of the cranial nerve nuclei produce paralysis of the affected nerve. A destructive lesion of the cerebral peduncle gives rise to spastic paralysis on the other side of the body. Destruction of other tracts in the midbrain results in rigidity and involuntary movements.

The pons lies ventral to the cerebellum and rostral to the medulla. The abducens, facial, and acoustic (and vestibular) nuclei are found in the pons, and their nerves exit through a groove that divides the pons from the medulla. The motor and sensory nuclei of the trigeminal nerve are also located in the pons. At this level, the corticospinal tracts (also known as the pyramidal tracts) have not yet crossed, and a lesion at this level produces loss of voluntary movement on the opposite side. Destructive lesions of the pons may produce a variety of clinical syndromes, such as the following:

• Contralateral hemiplegia with ipsilateral trigeminal hemiplegia (paralysis of the jaw muscles and loss of sensation over the same side of the face)

• Contralateral hemiplegia with ipsilateral facial palsy (Bell's palsy)

Contralateral hemiplegia with ipsilateral facial palsy and ipsilateral abducens palsy (paralysis of the lateral rectus muscle on the same side of the face) Contralateral hemiplegia with ipsilateral abducens palsy

• Quadriplegia and nystagmus

The medulla is the portion of the brain stem between the pons and the spinal cord. The nuclei of the hypoglossal, vagus, glossopharyngeal, and spinal accessory nerves are located in the medulla. It is within the medulla that the majority of fibers in the corticospinal tracts cross to the opposite side. Destructive lesions in the medulla produce symptoms that are referable to the tracts interrupted by the lesion. Some clinical syndromes are as follows:

• Contralateral hemiplegia with ipsilateral hypoglossal palsy*

• Ipsilateral vagal palsy{ with contralateral loss of pain and temperature sense

• Ipsilateral vagal palsy with ipsilateral spinal accessory palsy{

*Paralysis of the tongue muscles on the same side as the lesion. The tongue deviates to the side of the lesion when the patient is asked to stick out the tongue.

{Paralysis of the soft palate and difficulty speaking, termed dysarthria.

{Paralysis of the sternocleidomastoid or trapezius muscle, or both. This results in the inability to turn the head to the side opposite the lesion and to shrug the shoulder.

Anterior communicating artery

Anterior cerebral artery Internal carotid artery

Middle cerebral artery Posterior communicating artery -

Figure 21-5 Circle of Willis.

Posterior cerebral artery Basilar artery

Vertebral artery

Anterior communicating artery

Anterior cerebral artery Internal carotid artery

Middle cerebral artery Posterior communicating artery -

Figure 21-5 Circle of Willis.

Posterior cerebral artery Basilar artery

Vertebral artery

Ipsilateral vagal palsy with ipsilateral hypoglossal palsy

Ipsilateral vagal palsy, ipsilateral spinal accessory palsy, and ipsilateral hypoglossal palsy

There are many more clinical syndromes that are beyond the scope of this text. The reader is advised to review the neuroanatomy further to understand the complexities of these neurologic syndromes.

The cerebellum is located in the posterior fossa of the skull and is composed of a small midline vermis and two large lateral hemispheres. The cerebellum acts to keep the individual oriented in space and to halt or check motions. The cerebellum is also responsible for the fine movements of the hands. Essentially, the cerebellum coordinates and refines the action of muscle groups to produce steady and precise movements. Destructive lesions of the cerebellum cause swaying, staggering, intention tremors,* and inability to change movements rapidly.

Of the blood supply to the brain, 80% is through the internal carotid arteries and 20% is through the vertebral basilar arteries. Each internal carotid artery terminates at the anterior cerebral and middle cerebral arteries. The posterior cerebral artery arises from the basilar artery, which joins with the posterior communicating artery, a branch of the internal carotid artery. The two anterior cerebral arteries are joined by the anterior communicating artery. This vascular network forms the circle of Willis, located at the base of the brain. This is illustrated in Figure 21-5.

Continuous with the medulla is the spinal cord, a cylindric mass of neuronal tissue measuring 40 to 50 cm in length in adults. Its distal end attaches to the first segment of the coccyx. The spinal cord is divided into two symmetric halves by the anterior median fissure and the

*Tremors that result when the individual moves the hands to do something but that may not be present at rest.

Posterior median sulcus

Fasciculus gracilis

Posterior horn

Fasciculus cuneatus White matter Gray matter

Posterior median sulcus

Fasciculus gracilis

Posterior horn

Anterior root

Dorsal root ganglion

Anterior median fissure

Anterior root

Dorsal root ganglion

Anterior median fissure

Figure 21-6 Cross-sectional view through the spinal cord.

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