Introduction to Human Anatomy and Physiology

Chapter 9 - Nervous System

1. What is the difference between the Central Nervous System (CNS) & the Peripheral Nervous System (PNS)?

The brain and the spinal cord are considered to be the CNS. The nerves that branch out from the CNS and send communications back & forth with other parts of the body make up the PNS.

The PNS can be further subdivided into the somatic nervous system and the autonomic nervous system. The somatic system includes nerves that connect the brain & spinal cord to receptors in the skin and skeletal muscles & is involved in conscious activities. We can think of the autonomic nervous system as an "automatic" system of nerves that connect the brain & spinal cord to various organs to control unconscious activities. It includes nerve fibers that arouse the body for stress or "fight or flight" responses and then returns the body to a resting state. (See question 4 for more information on the autonomic nervous system.)

2. Is there a difference between the terms "neuron" & "nerve" or can either word be used?

Yes, there is a difference between a neuron & a nerve. The term "neuron" refers to a single cell that has the ability to become "excited" & pass a nerve impulse along to a muscle, gland or another neuron. Figures 9.2, 9.3, 9.4 & 9.5 illustrate several features of neurons & other cells that support them.

The term "nerve" actually refers to a structure made up of the fibers of many neurons. These fibers are bundled up in connective tissue. The fibers may be carrying impulses only toward the CNS, in which case the nerve will be a sensory nerve. An example of a sensory nerve is the optic nerve which carries impulses from the retina to the brain. If a nerve contains both sensory fibers & motor fibers (impulses traveling away from the CNS), the nerve is referred to as a mixed nerve. An example of a mixed nerve is the sciatic nerve which serves the lower limb. Figure 9.14 in your textbook is an illustration of the structure of a nerve.

3. What are the main parts of the brain and their general functions?

You can refer to Chapter 9 in your textbook to view drawings of the major parts of the brain & their locations from pages 236 to 243.

a. cerebrum

In humans, the cerebrum is the largest portion of the brain. It is involved in conscious motor functions, reception & analysis of sensory input, & conscious thoughts such as judgment, memory & reasoning. It is separated into a right & a left hemisphere. Each hemisphere has 5 lobes, 4 of which are named according to the skull bone they lie under: frontal, parietal, occipital, temporal. The fifth lobe of each hemisphere is called the insula ("island"), which is folded toward the inside & covered by sections of the frontal, parietal, & temporal lobes. The outer approximately 1/4 inch of the cerebrum is the cerebral cortex, which contains nerve cell bodies. Each region of the cerebrum has very specific functions. Figure 9.24 of the textbook points out the specific functions of several parts of the cerebrum.

b. diencephalon

The diencephalon includes the thalamus, hypothalamus, & epithalamus. It also includes the fibers of the optic nerve where they are part of the optic tract & optic chiasm and the mamillary bodies. Structures in this same general area are involved with the limbic system, or "emotional brain". Here's a brief description of the functions of the thalamus, hypothalamus & epithalamus:

thalamus: is a "relay" point for in-coming (sensory) impulses forwarding those responses on to the cerebral cortex for interpretation. The thalamus has been shown to be an area of general interpretation of the quality of various sensations such as pain, touch & temperature. It also seems to be tied to the anticipation of pain, etc.

hypothalamus: links to various parts of the body & maintains homeostasis by regulating body temperature, heart rate & arterial blood pressure, hunger, thirst, production of certain hormones, water balance, & other activities.

epithalamus: includes the pineal gland which is involved in circadian rhythms (body activity cycles that occur about every 24 hours).

4. How is the autonomic nervous system (ANS) different from the somatic nervous system?

The motor division of the somatic nervous system (the fibers that carry impulses away from the CNS) is under voluntary control, but the autonomic nervous system operates at an unconscious level. In other words, you don't have to think about the activities the ANS needs to accomplish, they will occur automatically.

The ANS has 2 subdivisions. The sympathetic division prepares the body for what generally might be thought of as stress situations where energy may need to be expended for survival or emergency situations, although they may not be as extremely dire as this implies. The parasympathetic division restores the body to a more relaxed condition. Each division has a specific inhibitory or stimulating effect on the organ it innervates.

The somatic nervous system usually has just one motor neuron in a pathway from the brain or spinal cord to a skeletal muscle. The autonomic nervous system will have 2 neurons in a given pathway. (See figure 9.32 in the textbook.) These 2 neurons are referred to as the preganglionic neuron & the postganglionic neuron. The ganglion of a sympathetic fiber will be located close to the spinal cord. The ganglion of a parasympathetic fiber will be located close to the organ it will innervate & stimulate.

Another difference between the ANS & the somatic nervous system is the location of the nerve cell bodies of each of the 2 divisions. The primary somatic motor area is located in the precentral gyrus of the cerebrum. The 2 subdivisions of the ANS are located ôlowerö in the central nervous system:

Sympathetic division: neuron cell bodies are located in the gray matter of the thoracic & lumbar regions of the spinal cord.

Parasympathetic division: neuron cell bodies are located in the brain stem and in the sacral region of the spinal cord.

Figures 9.33 and 9.34 show the locations of preganglionic & postganglionic fibers of the sympathic and parasympathetic divisions and the organs they innervate.

Another difference between the somatic motor nervous system & the ANS is the neurotransmitter substances they use. The individual voluntary motor neurons of the somatic nervous system release the neurotransmitter acetylcholine at neuromuscular junctions to simulate skeletal muscles. The 2 neurons found in an ANS pathway however may differ from this pattern:

Sympathetic division: the preganglionic fiber releases ACh, but the postganglionic fiber releases NE (norepinephrine).

Parasympathetic division: the preganglionic fiber AND the postganglionic fibers both release ACh, as was seen in the somatic motor system. (See figure 9.35)

For additional Information, quizzes, case studies, hot links to related web sites, and study outlines related to this chapter, go to the "Hole's Essentials of Human Anatomy & Physiology" textbook web site.

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