Memoria constructiva

6.3 Cerebral blood supply 6.4 Cerebral venous drainage 6.5 Brainstem

6.6 Cerebrospinal fluid 6.7 Cerebellum

6.8 Spinal cord anatomy 6.9 Division of the cord 6.10 Cord blood supply

6.1 Overview

• The cerebral hemispheres essentially constitute the developed forebrain.

• The midbrain contains an aqueduct and acts as a connection to the hindbrain (pons, medulla oblongata, and cerebellum).

• The cavity of the hindbrain is the fourth ventricle.

• The brainstem comprises the midbrain, pons, and medulla.

• The medulla passes via the skull’s foramen magnum to form the spinal cord, from which cervical nerve roots emerge.

• CSF forms within ventricular choroid plexuses and exits via the foramina in the roof of the fourth ventricle.

6.2 Cerebral hemispheres

• The cerebral hemispheres are covered by a thin outer layer of grey matter - the cerebral cortex.

Deeper within the brain are masses of grey matter such as basal nuclei and thalamus.

• The brain surface is comprised of folds (gyri) separated by grooves (sulci). The larger sulci (fissures) divide the brain into lobes (see Figure A.6.1):

■ Frontal lobe - anterior to central sulcus and above lateral sulcus

■ Parietal lobe - behind central sulcus and above lateral sulcus

■ Temporal lobe - below lateral sulcus

■ Occipital lobe - below parieto-occipital sulcus

Figure A.6.1 Lobes of the brain

6.2.1 Structural aspects of the cerebral hemispheres

• The basal nuclei (also called basal ganglia) are situated at the base of the forebrain lying on both sides of the thalamus. The basal nuclei function as a supraspinal control centre over voluntary movement and movement coordination.

• The white matter can be divided into 3 major groups:

■ Association (arcuate) fibres (forming cortical connections within the same hemisphere)

■ Commissural fibres (interconnect the 2 hemispheres)

■ Projection fibres (from the cerebral cortex to other parts of the central nervous system, e.g.

thalamus) - these projection fibres located between the basal ganglia and the thalamus are termed the internal capsule

• A thrombosis or haemorrhage occurring in the internal capsule results in a contralateral hemiparesis or hemiplegia. This is the most common site of intracerebral haemorrhage.

• The corpus callosum is a structure composed of commissural fibres, in the longitudinal fissure that connects the left and right cerebral hemispheres, and facilitates communication between the two.

6.2.2 Cortical areas

• Appreciation of the key areas within which bodily function is determined is a fundamental part of the rationale for knowledge of CNS anatomy.

• The effects of traumatic and atraumatic brain lesions can be predicted based upon a working knowledge of the likely clinical signs, and this works in reverse such that typical neurological presentations infer typical areas of central damage.

• Broca’s area lies in the inferior frontal gyrus in the left frontal lobe. It is important for speech production, language processing, and language comprehension. See Figure A.6.2.

• Wernicke’s area is located at the posterior section of the superior temporal gyrus in the left hemisphere. It is important for language comprehension, recognition, and interpretation.

• The auditory area (or cortex) is located in the floor of the lateral sulcus and on the dorsal surface of the superior temporal gyrus in the temporal lobe. It is responsible for the processing of auditory information.

• The visual area (or cortex) occupies the entire surface of the occipital lobe. It is responsible for processing visual information (see below).

Figure A.6.2 Areas of the brain

6.2.3 Visual fields and pathways

• As a part of the retina, the bipolar cells exist between photoreceptors (rod and cone cells) and ganglion cells. They act, directly or indirectly, to transmit signals from the photoreceptors to the ganglion cells.

• Retinal ganglion cells collectively transmit visual information from the retina to several regions in the brain. They do this by the defining property of a long axon extending into the brain - the optic nerve.

• The optic nerve (cranial nerve II) transmits visual information from the retina to the brain. It leaves the orbit via the optic canal and runs posteromedially toward the optic chiasm where the nasal visual field crosses over. This allows for parts of both eyes that attend to the right visual field to be processed in the left visual system in the brain, and vice versa.

• The optic chiasm is located in the bottom of the brain, immediately below the hypothalamus. The optic nerve then continues as the optic tract to the lateral geniculate nucleus.

• Most of the axons of the optic nerve terminate in the lateral geniculate nucleus, from where information is relayed to the visual cortex. This is the primary processing centre for visual information and is found in the thalamus.

• From the lateral geniculate body, fibres of the optic radiation (forming part of the internal capsule) pass to the visual cortex in the occipital lobe of the brain. See Figure A.6.3.

• The internal carotid artery supplies the optic chiasm and optic nerve. The posterior

communicating artery supplies optic chiasm and optic tract. The anterior cerebral artery supplies the optic chiasm.

Figure A.6.3 The optic pathway

6.2.4 Olfactory pathways

Knowledge not required.

6.2.5 Limbic system

Knowledge not required.

6.2.6 Ventricles

• The ventricular system is composed of 2 lateral ventricles and 2 midline ventricles, referred to as the 3^rd and 4^th ventricles.

• The chambers are connected to allow the flow of CSF via 2 interventricular foramen (referred to as the foramen of Monro) between the lateral and 3^rd ventricles; and the cerebral aqueduct (referred

to as the aqueduct of Sylvius) between the 3^rd and 4^th ventricles. See Figure A.6.4.

• The ventricles are the source of CSF. CSF is secreted by the choroid plexuses, which are vascular conglomerates of capillaries, pia, and ependyma cells. The bulk of CSF arises from the plexuses of the lateral ventricles.

• In cross-sectional radiology, the midline cavities (3^rd and 4^th ventricles and the aqueduct) are symmetrical, but the lateral ventricles (the cavities of the hemispheres) are not.

• The 2 lateral ventricles, located within the cerebrum, are relatively large and C-shaped, roughly wrapping around the dorsal aspects of the basal ganglia. Each lateral ventricle has 3 horns:

■ The anterior horn extends into the frontal lobe

■ The posterior horn extends into the occipital lobe

■ The inferior horn extends into the temporal lobe

• The body of the lateral ventricle is the central portion, just posterior to the frontal horn.

• The 3^rd ventricle is a slit-like space in the sagittal plane. It is bordered on each side by thalami, and lies directly above the hypothalamus. The 3^rd ventricle contains the choroid plexus, which is

responsible for CSF production; this in turn helps protect the brain from trauma.

• The 4^th ventricle has a characteristic diamond shape in cross-sections of the human brain. It is located within the pons or in the upper part of the medulla.

Figure A.6.4 Ventricles of the brain

6.2.7 Thalamus

• The thalamus lies between the cerebral cortex and the midbrain and is a wedge-shaped structure that surrounds the 3^rd ventricle. It consists of clusters of cell groups (nuclei).

• The thalamus is known to have multiple functions:

■ The thalamus is believed to both process and relay sensory information selectively to

various parts of the cerebral cortex

■ It also plays an important role in regulating states of sleep and wakefulness by strong reciprocal connections with the cerebral cortex that are believed to be involved with consciousness

■ The thalamus plays a major role in regulating arousal, the level of awareness, and activity

■ Damage to the thalamus can lead to permanent coma