Central Nervous System and Senses

Central Nervous System

The brain and spinal cord make up the central nervous system (CNS) A). The spinal cord is divided into similar segments, but is 30% shorter than the spinal column. The spinal nerves exit the spinal canal at the level of their respective vertebrae and contains the afferent somatic and visceral fibers of the dorsal root, which project to the spinal cord, and the efferent somatic (and partly autonomic) fibers of the anterior root, which project to the periphery. Thus, a nerve is a bundle of nerve fibers that has different functions and conducts impulses in different directions (^ p. 42).

Spinal cord (^ A). Viewed in cross-section, the spinal cord has a dark, butterfly-shaped inner area (gray matter) surrounded by a lighter outer area (white matter). The four wings of the gray matter are called horns (cross-section) or columns (longitudinal section). The anterior horn contains motoneurons (projecting to the muscles), the posterior horn contains interneurons. The cell bodies of most afferent fibers lie within the spinal ganglion outside the spinal cord. The white matter contains the axons of ascending and descending tracts.

Brain (^ D). The main parts of the brain are the medulla oblongata (^ D7) pons D6), mesencephalon (^ D5), cerebellum (^ E), di-encephalon and telencephalon (^ E). The medulla, pons and mesencephalon are collectively called the brain stem. It is structurally similar to the spinal cord but also contains cell bodies (nuclei) of cranial nerves, neurons controlling respiration and circulation (^ pp. 132 and 212ff.) etc. The cerebellum is an important control center for motor function (^ p. 326ff.).

Diencephalon. The thalamus (^ C6) of the diencephalon functions as a relay station for most afferents, e.g., from the eyes, ears and skin as well as from other parts of the brain. The hypothalamus (^ C9) is a higher autonomic center (^ p. 330), but it also plays a dominant role in endocrine function (^ p.266ff.) as it controls the release of hormones from the adjacent hypophysis (^ D4).

310 The telencephalon consists of the cortex and nuclei important for motor function, the basal ganglia, i.e. caudate nucleus (^ C5), putamen (^ C7), globus pallidus (^ C8), and parts of the amygdala (^ C10). The amygdaloid nucleus and cingulate gyrus (^ D2) belong to the limbic system (^ p. 330). The cerebral cortex consists of four lobes divided by fissures (sulci), e.g., the central sulcus (^ D1, E) and lateral sulcus (^ C3, E). According to Brodmann's map, the cerebral cortex is divided into histologi-cally distinct regions (^ E, italic letters) that generally have different functions (^ E). The hemispheres of the brain are closely connected by nerve fibers of the corpus callosum (^ C1, D3).

Cerebrospinal Fluid

The brain is surrounded by external and internal cerebrospinal fluid (CSF) spaces (^ B). The internal CSF spaces are called ventricles. The two lateral ventricles, I and II, (^ B, C2) are connected to the IIIrd and IVth ventricle and to the central canal of the spinal cord (^ B). Approximately 650 mL of CSF forms in the choroid plexus (^ B, C4) and drains through the arachnoid villi each day (^ B). Lesions that obstruct the drainage of CSF (e.g., brain tumors) result in cerebral compression; in children, they lead to fluid accumulation (hydro-cephalus). The blood-brain barrier and the blood-CSF barrier prevents the passage of most substances except CO2,02, water and lipophilic substances. (As an exception, the circum-ventricular organs of the brain such as the or-ganum vasculosum laminae terminalis (OVLT; ^ p. 280) and the area postrema (^ p. 238) have a less tight blood-brain barrier.) Certain substances like glucose and amino acids can cross the blood-brain barrier with the aid of carriers, whereas proteins cannot. The ability or inability of a drug to cross the blood-brain barrier is an important factor in pharma-cotherapeutics.

Anterior column

B. Cerebrospinal fluid spaces of the brain

Arachnoid villi

Choroid plexus

Arachnoid villi

Choroid plexus

Lateral ventricle (paired)

Central canal

Internal CSF spaces

IlIrd ventricle External CSF spaces

IVth ventricle

Lateral ventricle (paired)

Central canal

C. Brain: Cross-sectional view

Corpus callosum (1) Lateral ventricle (2) Lateral sulcus (3) Choroid plexus (4) Caudate nucleus (5) Thalamus (6) Putamen (7) Globus pallidus (8) Hypothalamus (9) Amygdala(10)

Iā€” D. Brain: Hemisection through middle

Plane of C.

Central sulcus (1) Cingulate gyrus (2) Corpus callosum (3)

Medulla oblongata (7)

Primary motor cortex Supplementary motor cortex ā€” Premotor cortex-

Frontal visual cortex

Prefrontal associative cortex

Broca's area c

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