Gross regions of central nervous system

Cerebral Cortex

The cerebral cortex is a structure within the brain that plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. In dead, preserved brains, the outermost layer of the cerebrum has a grey color, hence the name grey matter. Grey matter is formed by neurons and their unmyelinated fibers, whereas the white matter below the grey matter of the cortex is formed predominantly by myelinated axons interconnecting different regions of the central nervous system.

Corpus Callosum

The corpus callosum is a structure of the mammalian brain in the longitudinal fissure that connects the left and right cerebral hemispheres. It is the largest white matter structure in the brain, consisting of 200-250 million contralateral axonal projections. It is a wide, flat bundle of axons beneath the cortex. Much of the inter-hemispheric communication in the brain is conducted across the corpus callosum.

Diencephalon

The diencephalon is the region of the brain that includes the thalamus, hypothalamus, epithalamus, prethalamus or subthalamus and pretectum. It has important autonomic and endocrine functions, and is involved in processing of information to and from the cerebral cortex.

Mesencephalon

The mesencephalon (or midbrain) is the middle of three vesicles that arise from the neural tube that forms the brain of developing animals. Caudally the mesencephalon adjoins the pons (metencephalon) and rostrally it adjoins the diencephalon. It is involved in coordination of sensory and motor function. Midbrain together with the pons and the oblongata medulla form the encephalon trunk, the midbrain is the more rostral portion of it. It contains the midbrain aqueduct and it is constituted by the cerebral peduncles and the tecto. The central cavity of the midbrain is the mesencephalic aqueduct. It is a narrow conduct originated from the neural tube cavity which communicates with the third ventricle rostrally and with the fourth ventricle caudally. The midbrain is connected with the cerebrum cranially through the cerebral peduncles, with the pons caudally, and with the cerebellum by the rostral cerebellar peduncles.

• External morphology

The ventral surface shows the cerebral peduncles, two thick tracts of fibers that descend from the cerebral cortex, and diverge rostrally to penetrate into each cerebral hemisphere. Each peduncle is composed of a ventral portion, the foot of the cerebral peduncle and a dorsal part the tegmentum, separated from each other by the substantia nigra. The foot of cerebral peduncle contains motor tracts and the tegmento that is the sector between the cerebral peduncles and the tecto, it contains also the mesencephalic aqueduct, tracts and nerve nuclei and it is covered with the midbrain tecto. There is a space between the feet of the cerebral peduncle, called interpeduncle trench containing the caudal perforated substance, with numerous vascular holes. Cerebral peduncles are related to the mammilar body rostrally. The fibres of the oculomotor nerve (III) are observed in the medial border of the cerebral peduncles and the lateral face of its related to the trochlear nerve (IV). The dorsal surface of the midbrain, the tecto is covered with the cerebral hemispheres and the rostral portion of the cerebellum; therefore these structures have to be removed to see this surface. The tecto is formed by two pairs of colliculi, the rostral and caudal ones. The rostral colliculus has a spherical shape; they are very close to each other forming a very marked grove and they are connected to the thalamus. The caudal colliculi are projected laterally; they are separated by a wide furrow and related to the trochlear nerve (IV).

• Internal morphology

The coronal section demonstrates that the midbrain is divided into three parts, from the top to the base in relation with the mesencephalic aqueduct. The top part is the tecto, where the colliculi are situated and which belong to the visual and acoustic via. Each pair of colliculi has a commisure which allows the passage of fibers from one side to another in the median plane. Each colliculus are connected to the thalamus by the geniculate bodies. Visual information reaches the rostral colliculus by the lateral geniculate body and acoustic information reaches the caudal colliculus which is then transported to the medial geniculate body. The tecto contains fibres that are involved in the coordination of movements of accommodation of the head, eye and ear; the oculomotor and trochlear origin nuclei motor nerves and the parasympathetic nucleus of the oculomotor nerve. All three are located at different levels, forming the gray substance ventral to the aqueduct where also appear the red nucleus which receives fibres from the cerebral cortex, and from where fibres to the spinal cord are originated. Sensitive fibres pass through the midbrain tecto too. The middle part is the tegmentum, where the oculomotor and red nuclei are situated. The ventral tegmental area and the substantia nigra’s nucleus appear under these nuclei. The fibres of the substantia nigra and interpeduncle nuclei are directed to the cerebrum, striatum body, cerebral cortex and hippocampus respectively.

'Bibliography' -Climent,S.et.al. Manual de Anatomía y embriología de los animales domésticos. Tomo 1. Acribia. 1998. -Gilbert. W Biología del desarrollo. 7ª ed. Ediorial Panamericana. 2005 -Noden, D.M. y de Lahunta, A. Embriología de los animales domésticos. Acribia. 1990 -Dyce, K.M.; Sack, W.O y Wensing C.J.G. Anatomía veterinaria. Editorial McGraw-Hill. Interamericana. 2ª edición. 1999 -Zuccolilli, G. Neurobiología Básica. Conceptos para medicina veterinaria. Segunda edición. Editorial de la Universidad Nacional de La Plata. Buenos Aires. Argentina. 2003 -Paxinos G y Watson C. The rat brain. Compact 3ª edición

Cerebellum

Cerebellum - "Little brain" - is a region of the brain that plays an important role in the integration of sensory perception, coordination and motor control.

Pons

What the cerebellum does is process input from other areas of the brain like the sensory receptors to provide precise timing to enable coordinated and smooth body movement.

Medulla Oblongata

Saggital section of canine brain showing gross regions