1. Overview

• The peripheral nervous system is organized into sensory (afferent) and motor (efferent) pathways. The efferent pathways are either somatic (voluntary) or autonomic (involuntary). The autonomic system is further divided into the sympathetic and parasympathetic systems which often have opposing influences on the the muscles and glands which they influence. (Eckert, Fig. 8-6)
• The spinal cord is the principle site of nervous system integration in simple vertebrates. There is very little cephalization (concentration in the head region) of the nervous system, and much of the nervous system activity is pre-wired reflex activity. (Eckert, Fig. 8-7a, Frog). In complex vertebrates, some reflex pathways (Eckert, Fig. 8-8b) remain but there is significant development of the brain.(Eckert, Fig. 8-7b, Human).
• QUESTION: Why is the white matter in the brain and spinal cord white? Why is the gray matter gray?

2. The brain

• Vertebrate brains show three major divisions, the prosencephalon (cerebral hemispheres and thalamus), mesencephalon (tectum), and rhombencephalon (cerebellum, pons, and medulla oblongata). Notice how the relative size and complexity of these regions changes as vertebrates become more complex: fish, frog, human.
• The rhombencephalon: The medullary respiratory center generates the rhythm of breathing. Stretch receptors in the lungs send signals to the medulla and cause a reflex response that prevents overinspiration by inhibition of the respiratory muscles. This is called the Hering-Breuer reflex. The cerebellum coordinates the more automatic motor activities, including posture.
• The mesencephalon: The tectum of the mesencephlon is also known as the optic lobes or in mammals, the superior colliculus. This is where the visual input enters into the brain. In mammals, there is less use of the superior colliculus than in other vertebrates and the lateral geniculate body (LGB) of the prosencephalon becomes the primary site of optical information processing in the brain. NOTICE the typical pattern of sensory and motor nerves, as the enter the central nervous system - They cross! The visual system is a little strange, however, in that not all fibers of the optic nerve cross from one side of the body to the other. NOTICE also, the complex visual information processing, that begins in the retina itself and continues in the LGB before visual signals are finally sent to the visual cortex of the cerebrum.
• The prosencephalon: The forebrain contains the thalamus, a coordinating center for information from sensory neurons and output to motor neurons. The amygdala is the seat of the emotions and is linked to the hypothalamus, which we studied previously. The hypothalamus is not only involved with temperature regulation but also with sexual arousal, pleasure, and rage. Some of this activity is involved with control of hormone secretion as well as signaling by nerve excitation. The very most anterior end of the forebrain, is, of course, involved with the primitive sense of the sense of smell. The cerebrum is highly developed for complex information processing in mammals, particularly in humans. The cerebrum, known as the cerebral cortex in complex vertebrates, has specific regions for visual, somatosensory, motor, and olfactory inputs. (Eckert, Fig. 8-13d). QUESTION: Which is the front of the brain in this picture? Finally, let's look at maps of the somatosensory and motor (Eckert, Fig. 8-14) cortex.

3. The autonomic nervous system and the cranial nerves.

• An overview of the two divisions of the autonomic nervous system - (Eckert, Fig. 8-18a and b) Notice that the ganglia of the sympathetic nervous system lie just outside the spinal cord, while in the the parasympathetic system, connections to the neuron that innervates the target organ lie near the target organ. Also, note that the transmitters of the postgangionic neuron are different and that the receptors on the target tissues of the parasympathetic division are muscarinic Ach receptors, not nicotinic Ach receptors - (Eckert, Table 8-02)

4. Stroke.

• A stroke can be caused by by a hemorrhage or an infarct.
• Some areas of the brain are affected in particularly serious ways. Areas where many nerve fibers travel closely together may be seriously affected. The areas where brainstem is a nasty place to be affected as many basic functions for basic maintenance of the body take place in that region and many cranial nerves enter there.
• A stroke often affects the opposite side of the body from that of the location of the hemorrhage or infarct.

All text and images, not attributed to others, including course examinations and sample questions, are Copyright, 2007, Thomas J. Herbert and may not be used for any commercial purpose without the express written permission of Thomas J. Herbert.