General Organization

The central nervous system (CNS) includes the cerebrum, cerebellum, brain stem, and spinal cord (Fig. 1) plus a few scary-sounding structures situated between the brain stem and cerebrum; namely, the diencephalon (Which includes everything with the name "thalamus;" i.e. the thalamus, hypothalamus, epithalamus and subthalamus) and the basal ganglia (which includes the caudate nucleus, the globus pallidus, the putamen, claustrum, and amygdala). Fortunately, it is clinically unimportant to have a detailed understanding of the connections of the diencephalon and basal ganglia. You'll see why later.

Basal Ganglia Organization
Fig. 1 The central nervous system. Within the brain stem and spinal cord the superior-inferior axis is synonymous with the "rostral-caudal" axis, and the anterior-posterior axis is synonymous with the "ventral-posterior" axis.
Rostral Caudal Axis
Fig. 2 The neuron.

The basic functional unit in the CNS is the neuron (Fig. 2). Electrophysiological impulses travel down a neuron from its dendrites to the cell body and axon. Information then is chemically transmitted to other neurons via connections known as synapses. A chain of such communicating neurons is called a pathway. Within the CNS, a bundle of pathway axons is called a tract, fasciculus, peduncle, or lemniscus. Outside the CNS (i.e., in the peripheral nerves, which connect the CNS with the skin, muscles, and other organ systems), bundles of axons are called nerves. So you can immediately see the problem with neuroanatomy. There are too many names for the same thing. But the basic logic of neuroanatomy is simple. We shall try to restrict names to a minimum.

There are 31 pairs of spinal nerves and 12 pairs of cranial nerves. Note in figure 3 that cervical nerves C1-C7 exit over their corresponding vertebrae, but that thoracic nerve 1 and the remainder of the nerves exit below their correspondingly numbered vertebrae. Cervical nerve 8 is unique since there is no correspondingly numbered vertebra. Also, note that the spinal cord is shorter than the vertebral column so that the spinal nerve roots extend cau-dally when leaving the spinal cord. This disparity increases at more caudal levels of the cord. The spinal cord ends at about vertebral level L2 but nerves L2-S5 continue caudally as the cauda equina ("horse's tail") to exit by their corresponding vertebrae (Fig. 3).

Figure 4 illustrates the subdivision of the cerebrum into frontal, parietal, occipital and temporal lobes. These are further subdivided into bulges, called gyri, and indentations called sulci and fissures (small and large, respectively).

The brain stem contains three parts — the midbrain, pons and medulla (Fig. 1). The pons lies squashed against the clivus, a region of bone resembling a slide that extends to the foramen magnum, the hole at the base of the skull where the spinal cord becomes the brain stem (Fig. 5).

Sometimes the brain stem does "slide down" the clivus, herniating into the foramen magnum. This is a serious clinical condition, generally resulting from a pressure differential between cranial and spinal cavities. Many clinicians therefore are wary in removing cerebrospinal fluid during a spinal tap in patients with high intracranial pressure.

Spinalnerv Th2

cygeaf nerve6 ^ C' ^^ T' ,hor8CÍC; L' lumbar; S' sacrali Coc-

Note the close proximity of the clivus to the nasal passages. Sometimes rare invasive tumors of the nasal passages erode and break through the clivus and damage the brain stem. Pituitary tumors may be reached surgically

Central sulcus (of Rolando)

Central sulcus (of Rolando)

Rolando Sulcus

Supramarginal ■gyrus ngular gyrus

Superior temporal sulcus

Lateral fissure (of Sylvius)

Fig. 4 The cerebrum.

Lateral fissure (of Sylvius)

Supramarginal ■gyrus ngular gyrus

Superior temporal sulcus

Fig. 4 The cerebrum.

Clivus Bone Sagittal

Nasal passa

Foramen magnumj


Fig. 5 Sagittal view of the brain. CO., corpus callosum—the major connection between the two cerebral hemispheres; f, fornix; 3v, third ventricle; p, pituitary gland; 4v, fourth ventricle. Shaded areas are zones containing cerebrospinal fluid.

via the nasal passages by producing a hole in the sphenoidal bone, which houses the pituitary gland — the "transsphenoidal approach".

A spider named Willis lives on the pons and its nose fits into the pituitary fossa, but more of this later.

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  • Luukas
    Where is the epithalamus?
    8 years ago
  • Beatriz
    Where Is The Clivus In The Brain?
    7 years ago

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