Chapter summary

Embryonic development

1. The CNS is derived from the ectoderm. During gastrulation, the notochord develops from the Chordamesodermal tissue. The notochord, in a process called primary induction, sends a signal to the overlying ectoderm to thicken, thus forming the neural plate.

2. After formation of the neural plate, its lateral edges become elevated forming the neural folds which flank the neural groove. As the neural plate begins to invaginate, the neural folds sur­round it. The lateral edges of the neural folds even­tually migrate toward the longitudinal midline of the embryo, thus forming the neural tube.

3. Failure of the tube to close at different sites results in various birth defects. Spina bifida occurs if the posterior neural tube does not close, whereas anencephaly is a lethal condition that results when the anterior neural tube fails to close. Craniora­chischisis is a failure of the entire tube to close.

Organization of the brain

Cerebral hemispheres

1. The Lelencephalic vesicles form the telencepha­lon, which consists of two cerebral hemispheres. As the brain develops, the Lelencephalic vesicles grow posteriorly and laterally until they encase the diencephalon.

2. The surface of the brain is marked by many con­volutions. The grooves are called sulci (singular: sulcus), while the ridges are called gyri (singular: gyrus).

3. The larger grooves that separate brain regions are called fissures. The longitudinal fissure sepa­rates the two cerebral hemispheres, whereas the

cerebral hemispheres are separated from the cer­ebellum by the transverse fissure, which runs perpendicular to the longitudinal fissure.

4. When viewing the cerebral hemisphere, four lobes—frontal, parietal, temporal, and occipi­tal—are visible.

Ventricles of the brain

1. The brain contains four fluid-filled chambers called Cerebroventricles.

2. The lateral ventricles are connected to the third ventricle via the interventricular foramen of Mon­roe. The third ventricle connects to the fourth ven­tricle via the mesencephalic aqueduct, also called the aqueduct of Sylvius or cerebral aqueduct.

3. CSF, formed largely in the choroid plexus located at the top of each ventricle, flows by bulk flow from the lateral ventricles to the third ventricle, to the fourth ventricle.

Cerebral cortex

1. The cerebral cortex is arranged as layers of cells that lie parallel to the surface of the brain.

2. The neocortex, which is found only in mammals and is associated with higher brain functions such as conscious behavior, is found over most of the surface of the cerebral hemisphere and consists of six layers of cells.

3. Medial to the lateral ventricles is an area of cortex named for its unique shape called the hippo­campus (Greek for "seahorse"). It is only a single cell layer.

4. Ventral and lateral to the hippocampus is the third area of cortex called the olfactory cortex (piriform, or pyriform cortex) that consists of two cell layers.

5. The primary motor cortex is the final site for cortical processing of motor commands before messages are then sent to the somatic muscles. In mammals, this area lays in the rostral region of the frontal lobes.

6. The extrapyramidal system includes all the de­scending somatic motor pathways excluding those described previously that constitute the pyrami­dal system.

Cerebral white matter

1. There are deep subcortical nuclei called the basal ganglia or nuclei.

2. The limbic system consists of a group of struc­tures located in the medial region of each cere­bral hemisphere. These structures encircle (limbus = ring or border) the brain stem. The limbic lobe of the cerebral hemisphere includes gyri surrounding the diencephalon, as well as other underlying structures. Specifically, it con­sists of three gyri. The cingulate gyrus is dorsal to the corpus callosum. The dentate gyrus and parahippocampal gyrus form the inferior and posterior portions of the limbic lobe. The limbic system is involved in emotional and behavioral patterns.

Diencephalon

1. The diencephalon consists of three paired structures—the thalamus, hypothalamus, and epithalamus.

2. The hypothalamus is found ventral to the thala­mus and forms the inferolateral walls of the third ventricle. It extends from the optic chiasm to the posterior border of the mammillary bodies. The infundibulum, which connects the hypothala­mus to the pituitary, lies between the optic chiasm and mammillary bodies.

3. The hypothalamus controls the autonomic ner­vous system, and thereby controls growth, feeding, drinking, circadian rhythms, and maternal behav­ior. Specifically, the hypothalamus plays a key role in six physiological areas.

Epithalamus

The epithalamus lies superiorly, caudally, and medi­ally relative to the other parts of the diencephalons, and thus represents a cephalad extension of the pre­tectum of the mesencephalon.

Thalamus

1. The thalamus lies dorsal to the hypothalamus and is bordered by the caudate nucleus dorsally and the internal capsule laterally. Its two halves are separated by the third ventricle.

2. The thalamus is the major relay station for sensory information generated in the periphery

and transferred for processing to the cerebral hemispheres.

Mesencephalon

The mesencephalon lies between the diencephalons and pons. The tectum forms the roof of the mesen­cephalon and contains two pairs of prominent bulges known as the corpora quadrigemina. Consisting of the superior and inferior colliculi, these nuclei process visual and auditory stimuli, respectively. The tegmentum forms the floor of the midbrain.

Metencephalon

1. The metencephalon consists of the pons and cer­ebellum. The pons links the spinal cord with the forebrain, as well as the cerebellum with the fore­brain and spinal cord. It forms part of the ante­rior surface of the fourth ventricle.

2. The cerebellum is the second largest region of the brain accounting for 10% of its total mass, but containing half of all neurons in the brain.

3. The cerebellum is important in coordinating mus­cle movement and maintaining balance. The cer­ebellum monitors all proprioceptive, visual, tactile, balance, and auditory sensory information.

Medulla oblongata

Originating from the myelencephalon, the medulla oblongata is continuous with the spinal cord. The rostral portion of the medulla contains part of the fourth ventricle, while the caudad portion contains a central canal.

Functional systems

Reticular activating system

1. The reticular formation forms the core of the brain stem tegmentum extending from the medulla oblongata through the pons and mid­brain. It consists of loosely clustered neurons and is homologous to the central gray area of the spinal cord that contains interneurons.

2. The RAS is inhibited during sleep and is thus involved in the sleep-wake cycle.

3. Sleep is categorized into two categories. REM sleep is noted for the movement of the eyes under the eyelids, and the almost complete inhi­bition of skeletal muscle tone. This stage of sleep is when most dreaming occurs. Non-REM sleep is characterized by a change in the EEG.

Cranial nerves

Cranial nerves connect directly to the brain rather than the spinal cord.

Organization of the spinal cord

1. The spinal cord is housed in the vertebral column.

2. The spinal cord terminates as a tapered structure called the conus medullaris.

3. In cross section, the spinal cord has a central core of gray matter consisting mostly of cell bodies and an outer region of white matter containing myelinated and unmyelinated nerve fibers.

4. The central gray area has two posterior horns and two anterior horns. In addition, in the tho­racic and superior lumbar regions of the spinal cord, the central gray area also has lateral horns on either side. The central gray area also has a gray commissure that connects both sides of the central gray area.

5. The white matter of the spinal cord is grouped into three white columns, also called funiculi.

Protection of the central nervous system

Meninges

1. The meninges consist of three connective tissue membranes that overlay the CNS. These mem­branes act to cover and protect the CNS, as well as encase blood vessels and help divide gross areas of the CNS.

2. The three layers, named from the outermost, include the dura mater, arachnoid mater, and pia mater.

Cerebrospinal fluid

1. A clear and colorless fluid, CSF has many func­tions: (1) It maintains a constant external envi­ronment for cells in the brain; (2) provides a route for removing harmful metabolites from the brain; (3) provides a cushion to protect the brain from trauma; (4) acts as the lymphatic system for the brain; and (5) provides a route for peptides that are released at one site and act at a distant site in the brain.

2. CSF is produced mostly from the choroid plexus. The remaining CSF is formed from the ependy­mal cells.

Blood-brain barrier

1. Neurons of the brain and spinal cord are isolated from the systemic circulation by the BBB, which prevents the movement of many molecules into the CNS.

2. The BBB is formed by the endothelial cells lining the blood capillaries. While in the periphery, these cells are fenestrated; in the CNS, these cells form tight junctions.

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References

Constantinescu, G.M. 2001. Guide to Regional Ruminant Anatomy Based on the Dissection of the Goat. Iowa State Press, Ames, Iowa.

Constantinescu, G.M. and LA. Constantinescu. 2004. Clini­cal Dissection Guide for Large Animals, Horse and Large Ruminants, 2nd edition. Iowa State Press, Ames, Iowa.

Getty, R. 1964. Atlas for Applied Veterinary Anatomy. Iowa State Press, Ames, Iowa.

King, A.S. 1987. Physiological and Clinical Anatomy of Domestic Mammals, Vol. 1, Central Nervous System. Oxford University Press, Walton Street, Oxford.